Research ArticleInvestigations on the Hot Stamping of AW-7921-T4 Alloy Sheet
M Kumar1 and N G Ross2
1LKR Leichtmetallkompetenzzentrum Ranshofen GmbH Austrian Institute of Technology Lamprechtshausnerstrasse 61 Postfach 265282 Ranshofen Austria2Robinson Research Institute Victoria University of Wellington 69 Gracefield Road PO Box 33-436 PetoneLower Hutt 5046 New Zealand
Correspondence should be addressed to M Kumar mkmiitrgmailcom
Received 23 August 2016 Revised 24 November 2016 Accepted 26 January 2017 Published 26 February 2017
Academic Editor Gianfranco Palumbo
Copyright copy 2017 M Kumar and N G Ross This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited
AW-7xxx alloys have been nowadays considered for greater light weighting potential in automotive industry due to its higherstrength compared to AW-5xxx and AW-6xxx alloys However due to their lower formability the forming processes are still indevelopment This paper investigates one such forming process called hot stamping The investigation started by carrying outhot tensile testing of an AW-7xxx alloy that is AW-7921 at temperatures between 350∘C and 475∘C to measure the strength andformability Formability was found to improve with increasing temperature and was sensitive to the strain rate Dynamic recoveryis considered as usual reason for the formability improvement However examining the precipitation states of the as-receivedcondition and after hot stamping using differential scanning calorimetry (DSC) the dissolution of precipitates was also believedto contribute to this increase in formability Following solution heat treatment there was no precipitation during cooling acrossthe cooling rates investigated (5ndash10∘Cs) Samples taken from parts hot stamped at 10 and 20mm sminus1 had similar yield strengths A3-step paint baking heat treatment yielded a higher postpaint baking strength than a single step treatment
1 Introduction
B-pillars and other key automotive parts demand a highstrength-to-weight ratio to satisfy the roof crush and sideimpact standards while keeping the weight down Highstrength AW-7xxx alloys are ideal for satisfying this criterionand consequently are the focus of many investigations [1ndash3]Due to poor formability at room temperatureAW-7xxx alloysare mostly used in the aircraft industryproduction and up tothis time have only found limited use for automotive parts
The limited cold formability of AW-7xxx sheet can beenhanced by either forming the material in the W- and O-tempers or forming at warm temperatures [4ndash8] Formingwith W- and O-temper requires costly heat treatment stepsto produce distortion-free parts Additionally the gain informability from warm forming is modest compared to thesubstantial additional complexity of heating the tooling andblank In contrast a hot stamping process does not requirealloys with particular tempers and can form parts at least asfast as cold forming In this forming process the aluminium
achieves significantly higher ductility than in the warmforming processThis extends the application of hot stampingto the forming of parts with complex geometries
Investigation into the hot stamping technology for form-ing aluminium sheet has only been carried out by a fewresearchers [9ndash11] In a series of tensile tests between 350∘Cand 493∘C the ductility of AA2024 sheet was shown byWanget al [9] to increase with temperature up to 450∘C withfurther increase in temperature causing a sharp decrease inductility For the AA6082 alloyMohamed et al [10] found thelargest drawing depth andmost uniform part thickness whenhot stamping the alloy would be achieved with a formingrate of approximately 021m sminus1 Using tensile and Nakazimatests at elevated temperatures Bariani et al [11] found theoptimum hot stamping formability of AA5083 sheet is at450∘C and successfully produced an underengine cover partin an industrial hot stamping plant
In the hot stamping process of aluminium alloys the sheetis heated up to near the solution heat treatment temperaturein the furnace To reduce heat loss once heated it is transferred
HindawiAdvances in Materials Science and EngineeringVolume 2017 Article ID 7679219 10 pageshttpsdoiorg10115520177679219
2 Advances in Materials Science and Engineering
SheetSolutionizing Storage
Paint bakingForming in the watercooled die (RT)
Figure 1 A simple schematic presentation of process steps in the hot stamping process chain
Table 1 Chemical composition (in wt)
Al Si Fe Cu Mn Mg Zn Ni Cr Ti ZrRest 011 024 013 006 263 728 001 002 005 011
as quickly as possible to the water cooled forming-dies Quickforming of the sheet between the dies minimizes the heatloss and ensures a better quenching rate of the heated sheetbetween the forming-dies For age-hardenable aluminiumalloys the quenching rate should be fast enough to suppressany precipitation reactions These conditions are essential toform good quality parts
The hot stamping process chain consists of the followingmajor steps solution heat treatment of the sheet and quicktransfer into the press forming and die quenching storageand finally paint baking as shown in Figure 1 Consideringthis process chain the usage of new AW-7xxx sheet as astructural material depends (in addition to the formability)on the age hardening response of the alloy during theforming process chain In particular it is important to achieveadequate strength during the paint baking step in order toproduce a good quality product
The main objective of the current work is to investigatethe hot stamping of AW-7921-T4 sheet Consideration ismade of the processing parameters related to the transfer andquenching of the sheet and how these influence the evolutionof the microstructure and the mechanical properties in thestamped partThis is done by studying the quench sensitivityforming precipitation and age hardening behaviour of theAW-7921-T4 sheet with the help of dilatometry (DIL) deepdrawing differential scanning calorimetry (DSC) and ten-sion and hardness tests
2 Experimental
21 Materials An AW-7921 sheet in T4 condition with athickness of 2mm has been used in this work (Table 1)
22 Tension Test Tensile samples were machined from the2mm thick AW-7921-T4 sheet in the direction of rollingThetests were performed using an 805 AD Bahr deformationdilatometer Details of the tensile sample geometry anddeformation dilatometer were given in [6] Each tensilesample was heated to the test temperature in 6 s and soakedfor 4 s before each tensile test as shown in Figure 2(a)
Tension tests were performed at temperatures rangingfrom 350∘C to 475∘C and at strain rates between 001 and 1 sminus1Additional tests were also performed at RT 170 and 230∘Cat 1 sminus1 Tests were repeated at least three times to ensurereproducibility Important parameters from the flow curve
measured during the tension test were determined and aredefined as follows
Yield stress stress at 02 offset true strainPeak stress true stress at maximum loadElongation at fracture engineering fracture strainread from the engineering stress-strain flow curveStrain rate sensitivity 119898 [120597 ln120590120597 ln 120576]
120576 where 120590 is
the peak stress
23 Dilatometry (DIL) DIL samples (2mm times 4mm times10mm) were solution-treated in the dilatometer at 480∘C for30min and then continuously cooled to room temperatureat various cooling rates between 1 and 3000Kminminus1 usingHe gas During continuous cooling the change in length ofthe samples was recorded by the dilatometer Finally thedilatometry (DIL) curves were numerically differentiated andplotted as 119889Δ119871119889119879 versus T
24 Deep Drawing a ldquoSmileyrdquo Shaped Part Thedeep drawingof a ldquosmileyrdquo shape part is a simulative test used to evaluatehot stamping characteristics In the current work deep draw-ing tests were carried out on rectangular 300mm times 400mmAW-7921-T4 blanks in a 16MNhydraulic press (Figure 2(b))MultidrawDrylube C1 lubricant was applied to the tool Eachblank was heated in a furnace to a temperature near thesolution heat treatment temperature (470∘C) before beingtransferred to the tool During transfer the blank lost someheat (sim20∘C) as measured by the thermocouple during thetrials After inserting the blank into the tool the die wasclosed the blank holding pressure is applied and the punchrose up to a height of 50mm into the die The total time forthe deep drawing process took approximately 35 s as shownin Figure 2(c) Test parameters for deep drawing are listed inTable 2
25 Tension Test of Sample from the Side-Wall Section of Deep-Drawn ldquoSmileyrdquo Prototypes Tensile samples of gauge length25mm and width 8mm were machined according to theEuropean Standard EN ISO 6892-1Bn from the wall-sectionof the deep-drawn smiley form as shown in Figure 2(d)to evaluate the postforming strength Some tensile sampleswere also further heat treated to simulate the conventionalautomotive paint baking heat treatments One-step paint
A to B transfer to the press + die closingB to C simultaneous forming + die quenchingC to D die opening + water quenching
0 10 20 30 40
Tem
pera
ture
(∘C)
(c)
Top view
Side view
(d)
Figure 2 (a) Schematic of the tension tests procedure (b) Hydraulic press with smiley form die (c) schematic diagram showing a typicalthermomechanical history for the deep drawing process and (d) the smiley form prototype after deep drawing The location from wheresamples were taken is also shown in (d)
Table 2 Test parameters for smiley shape deep drawing
Parameter ValueSheet thickness 2mmLubricant Multidraw Drylube C1Deep drawing depth 50mmBlank holding force 75 kNPunch speed 10 and 20mm sminus1
baking (PB) was simulated by 185∘C for 25min and three-step paint baking (3-SPB) was simulated with successive20min heat treatments at 180∘C 160∘C and 140∘C Themechanical properties for these samples were measured atroom temperature (RT) in a ZWICKZ100 at an initial strainrate of 2 times 10minus3 sminus1 In industrial practice hot stamped
parts are stored before assembling into the space-frameTherefore to simulate this hardness measurements weremade at various storage times from 1 to 14 days on the hotstamped ldquosmileyrdquo prototype part using theHB110-10methodon a Brinell hardness tester
26 Differential Scanning Calorimetry (DSC) DSC sampleswere prepared from AW-7921-T4 sheet in the following con-ditions as-received T4-condition W-temper hot stampedand stored T6-temper and simulated 1- and 3-step paintbaked (SPB) These samples were heated up to 480∘C in aNetzsch-DSC 204 F1 with a heating rate of 10 Kminminus1 Theendothermic and exothermic peaks in the resulting temper-ature versus heat flow plots correspond to the dissolutionand formation of precipitate respectively Differences in theprecipitation state of the sheet during the hot stampingprocessing chain were then concluded from these results
4 Advances in Materials Science and Engineering
0
100
200
300
400
500
600
0
20
40
60
80
True
stre
ss (M
Pa)
True strain
Strain rate = 001 sminus1
00 02 04 06
00 02 04 06
350∘C375∘C400∘C
425∘C450∘C475∘C
(a)
0
100
200
300
400
500
600
True
stre
ss (M
Pa)
True strain00 02 04 06
Strain rate = 1 sminus1
170∘C200∘C230∘C
RT 350∘C375∘C400∘C
425∘C450∘C475∘C
(b)
Figure 3 Hot flow curves of the as-received AW-7921-T4 sheet at strain rates of (a) 001 and (b) 1 sminus1
3 Results
31 Flow Curves at Elevated Temperature Figure 3 showsthe flow curves from the dilatometer measurements of theas-received AW-7921-T4 sheet The measured flow curves attemperatures between 350 and 475∘Care found to be sensitiveto the strain rate as can be seen by comparing Figures 4(a)and 4(b) At any given test temperature strain rate of 001 sminus1the strain-hardening and true stress are considerably lower atthe strain rate of 001 sminus1 than 1 sminus1 Additionally at the 1 sminus1strain rate the true stress and strain-hardening at a strain rateof 1 sminus1 decrease faster with increasing test temperature fromRT to 350∘C than from 350 to 475∘C At temperaturesge 350∘Cfor this strain rate the true stress after initial yielding reacheda steady state While at 001 sminus1 the true stress after the peakstress decreases rapidly as indicated by the shape of the flowcurves at 350 and 400∘C This generally occurs due to fastergrowth and coalescence of voids with decreasing strain rates[12 13]
Yield stress peak stress yield stresspeak stress ratioelongation at fracture and the strain rate sensitivity (119898) wereextracted from the flow curves and are plotted in Figure 4Maximum errors in the measured values were between 1and 2 As can be seen in Figure 4(a) the yield stress andpeak stress both decrease with increasing temperature anddecreasing strain rate
Formability in sheet materials is determined by mea-suring the strain-hardening (yield stresspeak stress ratio)and strain rate sensitivity (119898) from the flow curves Onlyminor changes were observed in the yieldpeak stress ratioat the strain rate of 001 sminus1 with increasing temperaturefrom 350∘C to 475∘C and it approaches sim1 (Figure 4(b))
A continuous increase in yield stresspeak stress ratio withincreasing temperature occurs for the case of 01 sminus1 At 1 sminus1the yield stresspeak stress ratio first decreases from 350∘C to400∘C but from 400∘C this value increases
The effect of strain rate on elongation at fracture isshown in Figure 4(c) Elongation at fracture increases withdecreasing strain rates It was found to be more than 082(the maximum strain measurement limit of the deformationdilatometer) from 400∘C onwards at both 001 and 01 sminus1Strain rate sensitivity m is an important material propertyin evaluating the formability of a sheet metal As shown inFigure 4(d) 119898 has a positive value and is increasing withincreasing temperature
32 Precipitation during Cooling of Solution Heat-TreatedAA7921-T4 Sheet Figure 5(a) shows the differential dilatom-etry (DIL) curves (119889Δ119871119889119879 versus T) for the AA7921 alongwith a baseline from 995 pure aluminium samples (brokengreen line AC) Pure aluminium is used for the baselinebecause there is no precipitation during cooling and thereforeyields a straight line From this line the starting and finishingtemperatures for the precipitation reactions in AA7921 maybe determined
The comparison of the DIL curves in Figure 5(a) obtainedat different cooling rates clearly shows a deviation in AA7921compared to the baseline Similar to the baseline 119889Δ119871119889119879value of theAA7921 alloy decreases on cooling down to 423∘C(point D) with a cooling rate of 1 Kminminus1 A deviation withrespect to the baseline then occurs in the form of fastercontraction (increase in 119889Δ119871119889119879) with a peak (B) at 370∘COn further cooling below 370∘C the 119889Δ119871119889119879 value starts todecrease and amount of decrement slows downThe start and
Advances in Materials Science and Engineering 5
0
25
50
75
100
125Pe
ak st
ress
(MPa
)
Peak stress (MPa)
0
20
40
60
80
100
120
Yiel
d str
ess (
MPa
)
Yield stress (MPa)001 sminus101 sminus11 sminus1
001 sminus101 sminus11 sminus1
Deformation temperature (∘C)300 350 400 450 500
(a)
080
085
090
095
100
105
Yiel
d str
ess
peak
stre
ss
350 400 450 500
001 sminus101 sminus11 sminus1
Temperature (∘C)
(b)
04
05
06
07
08
09
Temperature (∘C)
Elon
gatio
n at
frac
ture
120576f
300 350 400 450 500
001 sminus101 sminus11 sminus1
(c)
00
01
02
03
Measured at peak stress
Temperature (∘C)
Stra
in ra
te se
nsiti
vity
m
300 350 400 450 500
(d)
Figure 4 (a) Peak and yield stress (b) yield stresspeak stress ratio (c) elongation at fracture and (d) strain rate sensitivity of AW-7921-T4at temperatures between 350 and 475∘C and at strain rates between 001 and 1 sminus1 The dashed line in (c) indicates the measurement limit forthe deformation dilatometer
finish temperatures for this peak are 423∘C and 200∘C Asindicated by arrows the temperature and height of the peakB decrease with increasing cooling rate
Precipitation during cooling is generally described bycontinuous cooling transformation (CCT) diagram A sim-ulated CCT diagram for 05 transformation fraction usingJMATPRO software is shown in Figure 5(b) In this CCTdiagram it is shown that the precipitation of stable 120578-phase (MGZN2) and T-phase (T-ALCUMGZN) has finishedaround 200∘CTherefore at cooling rates ge5K sminus1 the precip-itation of these two phases will be suppressed
33 Hardness Figure 6(a) shows the hardness of the speci-mens taken from the AA7921 sheet in different heat-treatedor hot stamped conditions A solution heat treatment (SHT)at 480∘C for 30min was applied to the as-received AA7921sheet followed by water quenching (WQ) to room temper-ature Thereafter the hardness was measured with respect tostorage (natural ageing) timeThe hardness of the SHT +WQsample increased from 80 to 150HB due to storage for 336 h(two weeks)
In the ldquosmileyrdquo prototype part hot stamped at twodifferent speeds (10 and 20mm sminus1) stamping speed producesonly a minor difference in the hardness measured ndash between
6 Advances in Materials Science and Engineering
020
025
030
035
040
F
C
A
E
D
B
Temperature (∘C)
1Kmin5Kmin20Kmin60Kmin
5Ks10Ks20Ks50Ks
dΔLd
T
150 200 250 300 350 400 450
(a)
0
100
200
300
400
Time (s)
MGZN2T_ALCUMGZNETA_PRIMET_PRIMEGP
Tem
pera
ture
(∘C)
1Kmin5Kmin
20Kmin60Kmin5Ks10Ks20Ks50Ks
100 101 102 103 104 105 106 107 108
(b)
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
Figure 1 A simple schematic presentation of process steps in the hot stamping process chain
Table 1 Chemical composition (in wt)
Al Si Fe Cu Mn Mg Zn Ni Cr Ti ZrRest 011 024 013 006 263 728 001 002 005 011
as quickly as possible to the water cooled forming-dies Quickforming of the sheet between the dies minimizes the heatloss and ensures a better quenching rate of the heated sheetbetween the forming-dies For age-hardenable aluminiumalloys the quenching rate should be fast enough to suppressany precipitation reactions These conditions are essential toform good quality parts
The hot stamping process chain consists of the followingmajor steps solution heat treatment of the sheet and quicktransfer into the press forming and die quenching storageand finally paint baking as shown in Figure 1 Consideringthis process chain the usage of new AW-7xxx sheet as astructural material depends (in addition to the formability)on the age hardening response of the alloy during theforming process chain In particular it is important to achieveadequate strength during the paint baking step in order toproduce a good quality product
The main objective of the current work is to investigatethe hot stamping of AW-7921-T4 sheet Consideration ismade of the processing parameters related to the transfer andquenching of the sheet and how these influence the evolutionof the microstructure and the mechanical properties in thestamped partThis is done by studying the quench sensitivityforming precipitation and age hardening behaviour of theAW-7921-T4 sheet with the help of dilatometry (DIL) deepdrawing differential scanning calorimetry (DSC) and ten-sion and hardness tests
2 Experimental
21 Materials An AW-7921 sheet in T4 condition with athickness of 2mm has been used in this work (Table 1)
22 Tension Test Tensile samples were machined from the2mm thick AW-7921-T4 sheet in the direction of rollingThetests were performed using an 805 AD Bahr deformationdilatometer Details of the tensile sample geometry anddeformation dilatometer were given in [6] Each tensilesample was heated to the test temperature in 6 s and soakedfor 4 s before each tensile test as shown in Figure 2(a)
Tension tests were performed at temperatures rangingfrom 350∘C to 475∘C and at strain rates between 001 and 1 sminus1Additional tests were also performed at RT 170 and 230∘Cat 1 sminus1 Tests were repeated at least three times to ensurereproducibility Important parameters from the flow curve
measured during the tension test were determined and aredefined as follows
Yield stress stress at 02 offset true strainPeak stress true stress at maximum loadElongation at fracture engineering fracture strainread from the engineering stress-strain flow curveStrain rate sensitivity 119898 [120597 ln120590120597 ln 120576]
120576 where 120590 is
the peak stress
23 Dilatometry (DIL) DIL samples (2mm times 4mm times10mm) were solution-treated in the dilatometer at 480∘C for30min and then continuously cooled to room temperatureat various cooling rates between 1 and 3000Kminminus1 usingHe gas During continuous cooling the change in length ofthe samples was recorded by the dilatometer Finally thedilatometry (DIL) curves were numerically differentiated andplotted as 119889Δ119871119889119879 versus T
24 Deep Drawing a ldquoSmileyrdquo Shaped Part Thedeep drawingof a ldquosmileyrdquo shape part is a simulative test used to evaluatehot stamping characteristics In the current work deep draw-ing tests were carried out on rectangular 300mm times 400mmAW-7921-T4 blanks in a 16MNhydraulic press (Figure 2(b))MultidrawDrylube C1 lubricant was applied to the tool Eachblank was heated in a furnace to a temperature near thesolution heat treatment temperature (470∘C) before beingtransferred to the tool During transfer the blank lost someheat (sim20∘C) as measured by the thermocouple during thetrials After inserting the blank into the tool the die wasclosed the blank holding pressure is applied and the punchrose up to a height of 50mm into the die The total time forthe deep drawing process took approximately 35 s as shownin Figure 2(c) Test parameters for deep drawing are listed inTable 2
25 Tension Test of Sample from the Side-Wall Section of Deep-Drawn ldquoSmileyrdquo Prototypes Tensile samples of gauge length25mm and width 8mm were machined according to theEuropean Standard EN ISO 6892-1Bn from the wall-sectionof the deep-drawn smiley form as shown in Figure 2(d)to evaluate the postforming strength Some tensile sampleswere also further heat treated to simulate the conventionalautomotive paint baking heat treatments One-step paint
A to B transfer to the press + die closingB to C simultaneous forming + die quenchingC to D die opening + water quenching
0 10 20 30 40
Tem
pera
ture
(∘C)
(c)
Top view
Side view
(d)
Figure 2 (a) Schematic of the tension tests procedure (b) Hydraulic press with smiley form die (c) schematic diagram showing a typicalthermomechanical history for the deep drawing process and (d) the smiley form prototype after deep drawing The location from wheresamples were taken is also shown in (d)
Table 2 Test parameters for smiley shape deep drawing
Parameter ValueSheet thickness 2mmLubricant Multidraw Drylube C1Deep drawing depth 50mmBlank holding force 75 kNPunch speed 10 and 20mm sminus1
baking (PB) was simulated by 185∘C for 25min and three-step paint baking (3-SPB) was simulated with successive20min heat treatments at 180∘C 160∘C and 140∘C Themechanical properties for these samples were measured atroom temperature (RT) in a ZWICKZ100 at an initial strainrate of 2 times 10minus3 sminus1 In industrial practice hot stamped
parts are stored before assembling into the space-frameTherefore to simulate this hardness measurements weremade at various storage times from 1 to 14 days on the hotstamped ldquosmileyrdquo prototype part using theHB110-10methodon a Brinell hardness tester
26 Differential Scanning Calorimetry (DSC) DSC sampleswere prepared from AW-7921-T4 sheet in the following con-ditions as-received T4-condition W-temper hot stampedand stored T6-temper and simulated 1- and 3-step paintbaked (SPB) These samples were heated up to 480∘C in aNetzsch-DSC 204 F1 with a heating rate of 10 Kminminus1 Theendothermic and exothermic peaks in the resulting temper-ature versus heat flow plots correspond to the dissolutionand formation of precipitate respectively Differences in theprecipitation state of the sheet during the hot stampingprocessing chain were then concluded from these results
4 Advances in Materials Science and Engineering
0
100
200
300
400
500
600
0
20
40
60
80
True
stre
ss (M
Pa)
True strain
Strain rate = 001 sminus1
00 02 04 06
00 02 04 06
350∘C375∘C400∘C
425∘C450∘C475∘C
(a)
0
100
200
300
400
500
600
True
stre
ss (M
Pa)
True strain00 02 04 06
Strain rate = 1 sminus1
170∘C200∘C230∘C
RT 350∘C375∘C400∘C
425∘C450∘C475∘C
(b)
Figure 3 Hot flow curves of the as-received AW-7921-T4 sheet at strain rates of (a) 001 and (b) 1 sminus1
3 Results
31 Flow Curves at Elevated Temperature Figure 3 showsthe flow curves from the dilatometer measurements of theas-received AW-7921-T4 sheet The measured flow curves attemperatures between 350 and 475∘Care found to be sensitiveto the strain rate as can be seen by comparing Figures 4(a)and 4(b) At any given test temperature strain rate of 001 sminus1the strain-hardening and true stress are considerably lower atthe strain rate of 001 sminus1 than 1 sminus1 Additionally at the 1 sminus1strain rate the true stress and strain-hardening at a strain rateof 1 sminus1 decrease faster with increasing test temperature fromRT to 350∘C than from 350 to 475∘C At temperaturesge 350∘Cfor this strain rate the true stress after initial yielding reacheda steady state While at 001 sminus1 the true stress after the peakstress decreases rapidly as indicated by the shape of the flowcurves at 350 and 400∘C This generally occurs due to fastergrowth and coalescence of voids with decreasing strain rates[12 13]
Yield stress peak stress yield stresspeak stress ratioelongation at fracture and the strain rate sensitivity (119898) wereextracted from the flow curves and are plotted in Figure 4Maximum errors in the measured values were between 1and 2 As can be seen in Figure 4(a) the yield stress andpeak stress both decrease with increasing temperature anddecreasing strain rate
Formability in sheet materials is determined by mea-suring the strain-hardening (yield stresspeak stress ratio)and strain rate sensitivity (119898) from the flow curves Onlyminor changes were observed in the yieldpeak stress ratioat the strain rate of 001 sminus1 with increasing temperaturefrom 350∘C to 475∘C and it approaches sim1 (Figure 4(b))
A continuous increase in yield stresspeak stress ratio withincreasing temperature occurs for the case of 01 sminus1 At 1 sminus1the yield stresspeak stress ratio first decreases from 350∘C to400∘C but from 400∘C this value increases
The effect of strain rate on elongation at fracture isshown in Figure 4(c) Elongation at fracture increases withdecreasing strain rates It was found to be more than 082(the maximum strain measurement limit of the deformationdilatometer) from 400∘C onwards at both 001 and 01 sminus1Strain rate sensitivity m is an important material propertyin evaluating the formability of a sheet metal As shown inFigure 4(d) 119898 has a positive value and is increasing withincreasing temperature
32 Precipitation during Cooling of Solution Heat-TreatedAA7921-T4 Sheet Figure 5(a) shows the differential dilatom-etry (DIL) curves (119889Δ119871119889119879 versus T) for the AA7921 alongwith a baseline from 995 pure aluminium samples (brokengreen line AC) Pure aluminium is used for the baselinebecause there is no precipitation during cooling and thereforeyields a straight line From this line the starting and finishingtemperatures for the precipitation reactions in AA7921 maybe determined
The comparison of the DIL curves in Figure 5(a) obtainedat different cooling rates clearly shows a deviation in AA7921compared to the baseline Similar to the baseline 119889Δ119871119889119879value of theAA7921 alloy decreases on cooling down to 423∘C(point D) with a cooling rate of 1 Kminminus1 A deviation withrespect to the baseline then occurs in the form of fastercontraction (increase in 119889Δ119871119889119879) with a peak (B) at 370∘COn further cooling below 370∘C the 119889Δ119871119889119879 value starts todecrease and amount of decrement slows downThe start and
Advances in Materials Science and Engineering 5
0
25
50
75
100
125Pe
ak st
ress
(MPa
)
Peak stress (MPa)
0
20
40
60
80
100
120
Yiel
d str
ess (
MPa
)
Yield stress (MPa)001 sminus101 sminus11 sminus1
001 sminus101 sminus11 sminus1
Deformation temperature (∘C)300 350 400 450 500
(a)
080
085
090
095
100
105
Yiel
d str
ess
peak
stre
ss
350 400 450 500
001 sminus101 sminus11 sminus1
Temperature (∘C)
(b)
04
05
06
07
08
09
Temperature (∘C)
Elon
gatio
n at
frac
ture
120576f
300 350 400 450 500
001 sminus101 sminus11 sminus1
(c)
00
01
02
03
Measured at peak stress
Temperature (∘C)
Stra
in ra
te se
nsiti
vity
m
300 350 400 450 500
(d)
Figure 4 (a) Peak and yield stress (b) yield stresspeak stress ratio (c) elongation at fracture and (d) strain rate sensitivity of AW-7921-T4at temperatures between 350 and 475∘C and at strain rates between 001 and 1 sminus1 The dashed line in (c) indicates the measurement limit forthe deformation dilatometer
finish temperatures for this peak are 423∘C and 200∘C Asindicated by arrows the temperature and height of the peakB decrease with increasing cooling rate
Precipitation during cooling is generally described bycontinuous cooling transformation (CCT) diagram A sim-ulated CCT diagram for 05 transformation fraction usingJMATPRO software is shown in Figure 5(b) In this CCTdiagram it is shown that the precipitation of stable 120578-phase (MGZN2) and T-phase (T-ALCUMGZN) has finishedaround 200∘CTherefore at cooling rates ge5K sminus1 the precip-itation of these two phases will be suppressed
33 Hardness Figure 6(a) shows the hardness of the speci-mens taken from the AA7921 sheet in different heat-treatedor hot stamped conditions A solution heat treatment (SHT)at 480∘C for 30min was applied to the as-received AA7921sheet followed by water quenching (WQ) to room temper-ature Thereafter the hardness was measured with respect tostorage (natural ageing) timeThe hardness of the SHT +WQsample increased from 80 to 150HB due to storage for 336 h(two weeks)
In the ldquosmileyrdquo prototype part hot stamped at twodifferent speeds (10 and 20mm sminus1) stamping speed producesonly a minor difference in the hardness measured ndash between
6 Advances in Materials Science and Engineering
020
025
030
035
040
F
C
A
E
D
B
Temperature (∘C)
1Kmin5Kmin20Kmin60Kmin
5Ks10Ks20Ks50Ks
dΔLd
T
150 200 250 300 350 400 450
(a)
0
100
200
300
400
Time (s)
MGZN2T_ALCUMGZNETA_PRIMET_PRIMEGP
Tem
pera
ture
(∘C)
1Kmin5Kmin
20Kmin60Kmin5Ks10Ks20Ks50Ks
100 101 102 103 104 105 106 107 108
(b)
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
A to B transfer to the press + die closingB to C simultaneous forming + die quenchingC to D die opening + water quenching
0 10 20 30 40
Tem
pera
ture
(∘C)
(c)
Top view
Side view
(d)
Figure 2 (a) Schematic of the tension tests procedure (b) Hydraulic press with smiley form die (c) schematic diagram showing a typicalthermomechanical history for the deep drawing process and (d) the smiley form prototype after deep drawing The location from wheresamples were taken is also shown in (d)
Table 2 Test parameters for smiley shape deep drawing
Parameter ValueSheet thickness 2mmLubricant Multidraw Drylube C1Deep drawing depth 50mmBlank holding force 75 kNPunch speed 10 and 20mm sminus1
baking (PB) was simulated by 185∘C for 25min and three-step paint baking (3-SPB) was simulated with successive20min heat treatments at 180∘C 160∘C and 140∘C Themechanical properties for these samples were measured atroom temperature (RT) in a ZWICKZ100 at an initial strainrate of 2 times 10minus3 sminus1 In industrial practice hot stamped
parts are stored before assembling into the space-frameTherefore to simulate this hardness measurements weremade at various storage times from 1 to 14 days on the hotstamped ldquosmileyrdquo prototype part using theHB110-10methodon a Brinell hardness tester
26 Differential Scanning Calorimetry (DSC) DSC sampleswere prepared from AW-7921-T4 sheet in the following con-ditions as-received T4-condition W-temper hot stampedand stored T6-temper and simulated 1- and 3-step paintbaked (SPB) These samples were heated up to 480∘C in aNetzsch-DSC 204 F1 with a heating rate of 10 Kminminus1 Theendothermic and exothermic peaks in the resulting temper-ature versus heat flow plots correspond to the dissolutionand formation of precipitate respectively Differences in theprecipitation state of the sheet during the hot stampingprocessing chain were then concluded from these results
4 Advances in Materials Science and Engineering
0
100
200
300
400
500
600
0
20
40
60
80
True
stre
ss (M
Pa)
True strain
Strain rate = 001 sminus1
00 02 04 06
00 02 04 06
350∘C375∘C400∘C
425∘C450∘C475∘C
(a)
0
100
200
300
400
500
600
True
stre
ss (M
Pa)
True strain00 02 04 06
Strain rate = 1 sminus1
170∘C200∘C230∘C
RT 350∘C375∘C400∘C
425∘C450∘C475∘C
(b)
Figure 3 Hot flow curves of the as-received AW-7921-T4 sheet at strain rates of (a) 001 and (b) 1 sminus1
3 Results
31 Flow Curves at Elevated Temperature Figure 3 showsthe flow curves from the dilatometer measurements of theas-received AW-7921-T4 sheet The measured flow curves attemperatures between 350 and 475∘Care found to be sensitiveto the strain rate as can be seen by comparing Figures 4(a)and 4(b) At any given test temperature strain rate of 001 sminus1the strain-hardening and true stress are considerably lower atthe strain rate of 001 sminus1 than 1 sminus1 Additionally at the 1 sminus1strain rate the true stress and strain-hardening at a strain rateof 1 sminus1 decrease faster with increasing test temperature fromRT to 350∘C than from 350 to 475∘C At temperaturesge 350∘Cfor this strain rate the true stress after initial yielding reacheda steady state While at 001 sminus1 the true stress after the peakstress decreases rapidly as indicated by the shape of the flowcurves at 350 and 400∘C This generally occurs due to fastergrowth and coalescence of voids with decreasing strain rates[12 13]
Yield stress peak stress yield stresspeak stress ratioelongation at fracture and the strain rate sensitivity (119898) wereextracted from the flow curves and are plotted in Figure 4Maximum errors in the measured values were between 1and 2 As can be seen in Figure 4(a) the yield stress andpeak stress both decrease with increasing temperature anddecreasing strain rate
Formability in sheet materials is determined by mea-suring the strain-hardening (yield stresspeak stress ratio)and strain rate sensitivity (119898) from the flow curves Onlyminor changes were observed in the yieldpeak stress ratioat the strain rate of 001 sminus1 with increasing temperaturefrom 350∘C to 475∘C and it approaches sim1 (Figure 4(b))
A continuous increase in yield stresspeak stress ratio withincreasing temperature occurs for the case of 01 sminus1 At 1 sminus1the yield stresspeak stress ratio first decreases from 350∘C to400∘C but from 400∘C this value increases
The effect of strain rate on elongation at fracture isshown in Figure 4(c) Elongation at fracture increases withdecreasing strain rates It was found to be more than 082(the maximum strain measurement limit of the deformationdilatometer) from 400∘C onwards at both 001 and 01 sminus1Strain rate sensitivity m is an important material propertyin evaluating the formability of a sheet metal As shown inFigure 4(d) 119898 has a positive value and is increasing withincreasing temperature
32 Precipitation during Cooling of Solution Heat-TreatedAA7921-T4 Sheet Figure 5(a) shows the differential dilatom-etry (DIL) curves (119889Δ119871119889119879 versus T) for the AA7921 alongwith a baseline from 995 pure aluminium samples (brokengreen line AC) Pure aluminium is used for the baselinebecause there is no precipitation during cooling and thereforeyields a straight line From this line the starting and finishingtemperatures for the precipitation reactions in AA7921 maybe determined
The comparison of the DIL curves in Figure 5(a) obtainedat different cooling rates clearly shows a deviation in AA7921compared to the baseline Similar to the baseline 119889Δ119871119889119879value of theAA7921 alloy decreases on cooling down to 423∘C(point D) with a cooling rate of 1 Kminminus1 A deviation withrespect to the baseline then occurs in the form of fastercontraction (increase in 119889Δ119871119889119879) with a peak (B) at 370∘COn further cooling below 370∘C the 119889Δ119871119889119879 value starts todecrease and amount of decrement slows downThe start and
Advances in Materials Science and Engineering 5
0
25
50
75
100
125Pe
ak st
ress
(MPa
)
Peak stress (MPa)
0
20
40
60
80
100
120
Yiel
d str
ess (
MPa
)
Yield stress (MPa)001 sminus101 sminus11 sminus1
001 sminus101 sminus11 sminus1
Deformation temperature (∘C)300 350 400 450 500
(a)
080
085
090
095
100
105
Yiel
d str
ess
peak
stre
ss
350 400 450 500
001 sminus101 sminus11 sminus1
Temperature (∘C)
(b)
04
05
06
07
08
09
Temperature (∘C)
Elon
gatio
n at
frac
ture
120576f
300 350 400 450 500
001 sminus101 sminus11 sminus1
(c)
00
01
02
03
Measured at peak stress
Temperature (∘C)
Stra
in ra
te se
nsiti
vity
m
300 350 400 450 500
(d)
Figure 4 (a) Peak and yield stress (b) yield stresspeak stress ratio (c) elongation at fracture and (d) strain rate sensitivity of AW-7921-T4at temperatures between 350 and 475∘C and at strain rates between 001 and 1 sminus1 The dashed line in (c) indicates the measurement limit forthe deformation dilatometer
finish temperatures for this peak are 423∘C and 200∘C Asindicated by arrows the temperature and height of the peakB decrease with increasing cooling rate
Precipitation during cooling is generally described bycontinuous cooling transformation (CCT) diagram A sim-ulated CCT diagram for 05 transformation fraction usingJMATPRO software is shown in Figure 5(b) In this CCTdiagram it is shown that the precipitation of stable 120578-phase (MGZN2) and T-phase (T-ALCUMGZN) has finishedaround 200∘CTherefore at cooling rates ge5K sminus1 the precip-itation of these two phases will be suppressed
33 Hardness Figure 6(a) shows the hardness of the speci-mens taken from the AA7921 sheet in different heat-treatedor hot stamped conditions A solution heat treatment (SHT)at 480∘C for 30min was applied to the as-received AA7921sheet followed by water quenching (WQ) to room temper-ature Thereafter the hardness was measured with respect tostorage (natural ageing) timeThe hardness of the SHT +WQsample increased from 80 to 150HB due to storage for 336 h(two weeks)
In the ldquosmileyrdquo prototype part hot stamped at twodifferent speeds (10 and 20mm sminus1) stamping speed producesonly a minor difference in the hardness measured ndash between
6 Advances in Materials Science and Engineering
020
025
030
035
040
F
C
A
E
D
B
Temperature (∘C)
1Kmin5Kmin20Kmin60Kmin
5Ks10Ks20Ks50Ks
dΔLd
T
150 200 250 300 350 400 450
(a)
0
100
200
300
400
Time (s)
MGZN2T_ALCUMGZNETA_PRIMET_PRIMEGP
Tem
pera
ture
(∘C)
1Kmin5Kmin
20Kmin60Kmin5Ks10Ks20Ks50Ks
100 101 102 103 104 105 106 107 108
(b)
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
Figure 3 Hot flow curves of the as-received AW-7921-T4 sheet at strain rates of (a) 001 and (b) 1 sminus1
3 Results
31 Flow Curves at Elevated Temperature Figure 3 showsthe flow curves from the dilatometer measurements of theas-received AW-7921-T4 sheet The measured flow curves attemperatures between 350 and 475∘Care found to be sensitiveto the strain rate as can be seen by comparing Figures 4(a)and 4(b) At any given test temperature strain rate of 001 sminus1the strain-hardening and true stress are considerably lower atthe strain rate of 001 sminus1 than 1 sminus1 Additionally at the 1 sminus1strain rate the true stress and strain-hardening at a strain rateof 1 sminus1 decrease faster with increasing test temperature fromRT to 350∘C than from 350 to 475∘C At temperaturesge 350∘Cfor this strain rate the true stress after initial yielding reacheda steady state While at 001 sminus1 the true stress after the peakstress decreases rapidly as indicated by the shape of the flowcurves at 350 and 400∘C This generally occurs due to fastergrowth and coalescence of voids with decreasing strain rates[12 13]
Yield stress peak stress yield stresspeak stress ratioelongation at fracture and the strain rate sensitivity (119898) wereextracted from the flow curves and are plotted in Figure 4Maximum errors in the measured values were between 1and 2 As can be seen in Figure 4(a) the yield stress andpeak stress both decrease with increasing temperature anddecreasing strain rate
Formability in sheet materials is determined by mea-suring the strain-hardening (yield stresspeak stress ratio)and strain rate sensitivity (119898) from the flow curves Onlyminor changes were observed in the yieldpeak stress ratioat the strain rate of 001 sminus1 with increasing temperaturefrom 350∘C to 475∘C and it approaches sim1 (Figure 4(b))
A continuous increase in yield stresspeak stress ratio withincreasing temperature occurs for the case of 01 sminus1 At 1 sminus1the yield stresspeak stress ratio first decreases from 350∘C to400∘C but from 400∘C this value increases
The effect of strain rate on elongation at fracture isshown in Figure 4(c) Elongation at fracture increases withdecreasing strain rates It was found to be more than 082(the maximum strain measurement limit of the deformationdilatometer) from 400∘C onwards at both 001 and 01 sminus1Strain rate sensitivity m is an important material propertyin evaluating the formability of a sheet metal As shown inFigure 4(d) 119898 has a positive value and is increasing withincreasing temperature
32 Precipitation during Cooling of Solution Heat-TreatedAA7921-T4 Sheet Figure 5(a) shows the differential dilatom-etry (DIL) curves (119889Δ119871119889119879 versus T) for the AA7921 alongwith a baseline from 995 pure aluminium samples (brokengreen line AC) Pure aluminium is used for the baselinebecause there is no precipitation during cooling and thereforeyields a straight line From this line the starting and finishingtemperatures for the precipitation reactions in AA7921 maybe determined
The comparison of the DIL curves in Figure 5(a) obtainedat different cooling rates clearly shows a deviation in AA7921compared to the baseline Similar to the baseline 119889Δ119871119889119879value of theAA7921 alloy decreases on cooling down to 423∘C(point D) with a cooling rate of 1 Kminminus1 A deviation withrespect to the baseline then occurs in the form of fastercontraction (increase in 119889Δ119871119889119879) with a peak (B) at 370∘COn further cooling below 370∘C the 119889Δ119871119889119879 value starts todecrease and amount of decrement slows downThe start and
Advances in Materials Science and Engineering 5
0
25
50
75
100
125Pe
ak st
ress
(MPa
)
Peak stress (MPa)
0
20
40
60
80
100
120
Yiel
d str
ess (
MPa
)
Yield stress (MPa)001 sminus101 sminus11 sminus1
001 sminus101 sminus11 sminus1
Deformation temperature (∘C)300 350 400 450 500
(a)
080
085
090
095
100
105
Yiel
d str
ess
peak
stre
ss
350 400 450 500
001 sminus101 sminus11 sminus1
Temperature (∘C)
(b)
04
05
06
07
08
09
Temperature (∘C)
Elon
gatio
n at
frac
ture
120576f
300 350 400 450 500
001 sminus101 sminus11 sminus1
(c)
00
01
02
03
Measured at peak stress
Temperature (∘C)
Stra
in ra
te se
nsiti
vity
m
300 350 400 450 500
(d)
Figure 4 (a) Peak and yield stress (b) yield stresspeak stress ratio (c) elongation at fracture and (d) strain rate sensitivity of AW-7921-T4at temperatures between 350 and 475∘C and at strain rates between 001 and 1 sminus1 The dashed line in (c) indicates the measurement limit forthe deformation dilatometer
finish temperatures for this peak are 423∘C and 200∘C Asindicated by arrows the temperature and height of the peakB decrease with increasing cooling rate
Precipitation during cooling is generally described bycontinuous cooling transformation (CCT) diagram A sim-ulated CCT diagram for 05 transformation fraction usingJMATPRO software is shown in Figure 5(b) In this CCTdiagram it is shown that the precipitation of stable 120578-phase (MGZN2) and T-phase (T-ALCUMGZN) has finishedaround 200∘CTherefore at cooling rates ge5K sminus1 the precip-itation of these two phases will be suppressed
33 Hardness Figure 6(a) shows the hardness of the speci-mens taken from the AA7921 sheet in different heat-treatedor hot stamped conditions A solution heat treatment (SHT)at 480∘C for 30min was applied to the as-received AA7921sheet followed by water quenching (WQ) to room temper-ature Thereafter the hardness was measured with respect tostorage (natural ageing) timeThe hardness of the SHT +WQsample increased from 80 to 150HB due to storage for 336 h(two weeks)
In the ldquosmileyrdquo prototype part hot stamped at twodifferent speeds (10 and 20mm sminus1) stamping speed producesonly a minor difference in the hardness measured ndash between
6 Advances in Materials Science and Engineering
020
025
030
035
040
F
C
A
E
D
B
Temperature (∘C)
1Kmin5Kmin20Kmin60Kmin
5Ks10Ks20Ks50Ks
dΔLd
T
150 200 250 300 350 400 450
(a)
0
100
200
300
400
Time (s)
MGZN2T_ALCUMGZNETA_PRIMET_PRIMEGP
Tem
pera
ture
(∘C)
1Kmin5Kmin
20Kmin60Kmin5Ks10Ks20Ks50Ks
100 101 102 103 104 105 106 107 108
(b)
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
Figure 4 (a) Peak and yield stress (b) yield stresspeak stress ratio (c) elongation at fracture and (d) strain rate sensitivity of AW-7921-T4at temperatures between 350 and 475∘C and at strain rates between 001 and 1 sminus1 The dashed line in (c) indicates the measurement limit forthe deformation dilatometer
finish temperatures for this peak are 423∘C and 200∘C Asindicated by arrows the temperature and height of the peakB decrease with increasing cooling rate
Precipitation during cooling is generally described bycontinuous cooling transformation (CCT) diagram A sim-ulated CCT diagram for 05 transformation fraction usingJMATPRO software is shown in Figure 5(b) In this CCTdiagram it is shown that the precipitation of stable 120578-phase (MGZN2) and T-phase (T-ALCUMGZN) has finishedaround 200∘CTherefore at cooling rates ge5K sminus1 the precip-itation of these two phases will be suppressed
33 Hardness Figure 6(a) shows the hardness of the speci-mens taken from the AA7921 sheet in different heat-treatedor hot stamped conditions A solution heat treatment (SHT)at 480∘C for 30min was applied to the as-received AA7921sheet followed by water quenching (WQ) to room temper-ature Thereafter the hardness was measured with respect tostorage (natural ageing) timeThe hardness of the SHT +WQsample increased from 80 to 150HB due to storage for 336 h(two weeks)
In the ldquosmileyrdquo prototype part hot stamped at twodifferent speeds (10 and 20mm sminus1) stamping speed producesonly a minor difference in the hardness measured ndash between
6 Advances in Materials Science and Engineering
020
025
030
035
040
F
C
A
E
D
B
Temperature (∘C)
1Kmin5Kmin20Kmin60Kmin
5Ks10Ks20Ks50Ks
dΔLd
T
150 200 250 300 350 400 450
(a)
0
100
200
300
400
Time (s)
MGZN2T_ALCUMGZNETA_PRIMET_PRIMEGP
Tem
pera
ture
(∘C)
1Kmin5Kmin
20Kmin60Kmin5Ks10Ks20Ks50Ks
100 101 102 103 104 105 106 107 108
(b)
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
Figure 5 (a) DIL curves for AA7921measured at various cooling rates between 1 Kminminus1 and 50K sminus1 (b) SimulatedCCTdiagramofAA7921from JMATPRO software
Figure 6 (a) Hardness of the side-wall ldquosmileyrdquo prototype samples following 2 weeks of storage as well as after subsequent 1-step and 3-steppaint baking HF + DQ refers to hot forming and subsequent die quenching (b) DSC curves of AA7921 in W-temper T4-temper and hotstamping followed by 2 weeks of natural ageing and after two different subsequent paint baking procedures (1-step and 3-step paint baking(1- and 3-SPB)) These DSC curves compared with the DSC curve of AA7921 in the T6 condition
Advances in Materials Science and Engineering 7
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
1ndash3HBndash following the 336 h (2weeks) storage It is interestingto note that after two weeks of storage the hardness of thehot stamped samples is significantly lowermdashby 19HBmdashthanthe SHT + WQ sample The speed of hot stamping has nosignificant influence on hardness of the samples after hotstamping and two weeks storage as well as after subsequentone-step paint baking However when a three-SPB is appliedthe three-SPB instead of one-SPB the hardness of the samplehot stamped at 20mmsminus1 increases to 165 which is 7HBhigher than the sample hot stamped at 10mm sminus1 It isimportant to note here that the single step paint baking heattreatment is different to the 3-step process and that the latteris not a continuation of the former
34 Characterization of Tempers by DSC The DSC resultswere analysed based on a dilatometry technique investigationof the precipitation kinetics during nonisothermal heatingof various heat-treated Al-Zn-Mg alloy samples [14] In thatinvestigation it was found that the interval temperatures20ndash120∘C 120ndash250∘C and 150ndash300∘C correspond respec-tively to the formation of GP Zones 1205781015840 and 120578 phases andthe interval temperatures 50ndash150∘C 200ndash250∘C and 300ndash350∘Ccorrespond respectively to their dissolutionHoweverformation and dissolution interval temperatures depend onalloy composition heating rate and the initial temper of thematerial
Figure 6(b) shows DSC curves of the AW-7921 sheet invarious heat-treated or thermomechanically treated condi-tions A total of 7 peaks appear in the DSC curves Onheating the SHT + WQ sample that is AA-7921-W-temperan exothermic peak (Peak 1) appears at approximately 66∘Crelating to the formation of GP ZonesThismeans the samplewas in a supersaturated solid solution condition prior to thetest At 140∘C theseGPZones dissolve as indicated by peak 2aon further heating phases 1205781015840 and 120578 and T precipitate (peaks 45 and 6) And finally these three phases dissolve as shownby the broad overlapping endothermic peaks 7(andashc) between290 and 400∘C
The formation peak for GP Zones is not present for theother samples However their dissolution is seen in the curveof the hot stamped + two weeks of natural ageing sample andthe T4-temper (SHT + WQ + two weeks of natural ageing)sample as indicated by the endothermic peaks (2b) and (2c)around 121∘C and 130∘C respectively It shows that GP Zoneswere already present in the sheet before the test and thesewould have formed during storage (natural ageing) Thereare endothermic peaks present at 193∘C (3a) 207∘C (3b) and203∘C (3c) for the T6-temper sample and the hot stamped 1-and 3-SPB samples (Figure 6(b))This shows the endothermicpeak 3 (dissolution of 1205781015840) shifts towards lower temperatures inthe order of 1-SPB 3-SPB and then T6 heat treatment Theseresults from theDSC investigation provide information aboutthe precipitatemicrostructure present in the heat-treated hotstamped and paint baked conditions as listed in Table 3
35 Hot Stamping Process Chain The process steps in the hotstamping process chain have been described in the beginning(Figure 1) It can be expected that process steps such aspreheating forming and storage and finally the paint baking
Table 3 Condition and precipitate microstructure for AW-7921sheet for the various tempers as discerned from the DSC measure-ments
Temper Precipitate microstructureSHT +WQ W-temper Zn and Mg solutesSHT +WQ + 2-week storage T4-temper GP ZoneHF + DQ 10mms + two-week storage GP Zone1-SPB 1205781015840 precipitate3-SPB 1205781015840 precipitateT6 1205781015840 precipitate
0
100
200
300
400
500
600
Yiel
d str
engt
h (M
Pa)
W-te
mpe
r
T4-te
mpe
r
Hot
stam
ping
and
stora
ge
1-ste
p pa
int
baki
ng
baki
ng3-
step
pain
t
T6-te
mpe
r
20m
ms
10m
ms
20m
ms
10m
ms
20m
ms
10m
ms
Figure 7 Effect of process steps on the yield strength of AW-7921sheet in the warm forming process chain The speeds shown are therates for hot forming All measurements made at RT except for thepreheat treatment measurements These were measured at 230∘C
treatment would alter themechanical properties of the sheetFigure 7 shows these changes by comparing the effects ofthe process steps on the yield strength of the sheet Asexpected the SHT + WQ considerably decrease the yieldstrength of the as-received AA7921 sheet (Figure 7) After hotstamping this sheet (for both the 10mm sminus1 and 20mm sminus1)and storing it for two weeks the yield strength increases toapproximately 350MPa It is clear from Figure 7 that thedevelopment of yield strength during paint baking dependson the paint baking heat treatment procedure Followinga 1-SPB treatment the yield strength of hot stamped sheet(10mm sminus1 and 20mm sminus1) with two weeks storage increasesup to 450MPa while the yield strength increases up to480MPawith the 3-SPBHowever the final yield strengthwasstill lower than the theoretical maximum of the T6-condition(520MPa)
4 Discussions
Fine dispersed metastable precipitates are generally respon-sible for strength in age-hardenable aluminium alloys for
8 Advances in Materials Science and Engineering
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
example GP Zones in T4 and 1205781015840 precipitates in T6 tempersThesemetastable precipitates form in a sequence Loffler et al[15] reviewed the sequence of precipitation inAW-7xxx alloysand have observed following precipitation sequence that is120572-supersaturated solid solution-Alrarr GP Zonesrarr 1205781015840 rarr 120578-MgZn
2or T-phase
Zn and Mg are the main alloying elements for AA7921alloy having atomic diameters 0266 and 032 nm which arerespectively 12 larger and 7 smaller than the atomicdiameter of Al These solute atoms form a substitution solidsolution when dissolved into the Al-matrix The structure ofthe solid solution remains the same as that of the Al-matrix(ie face-centred cubic) with a lattice parameter for the Al-matrix of 0405 nm and unit cell volume of 00664 nm3 Forthe 1205781015840 (ETA PRIME) and 120578 (MGZN2) phases the unit cellvolumes are 0202 nm3 (lattice parameters of 1205781015840 phase a =0521 nm c = 086 nm) and 0299 nm3 (lattice parametersof 120578 phase a = 0496 nm c = 1403 nm) [16] Thereforeprecipitation of these phases during continuous cooling willbring additional changes in alloy volume in addition to thechanges caused by temperature This leads to the nonlinearchange of 119889Δ119871119889119879 seen in Figure 3(b) [17]
Solution heat-treated state after quenching in waterconsists of solute atoms and quenched-in-vacancies Thisstate is called W-temper Since the concentrations of soluteatoms and vacancies are more than the equilibrium valueW-temper readily decomposes during continuous heatingor during natural ageing During decomposition super-saturated solute atoms and quenched-in-vacancies clustertogether and precipitates as GP Zones as reflected in the DSCheat flow signal by a broad exothermic peak at temperature of66∘C during continuous heating Meanwhile during naturalageing decomposition is reflected by the broad endothermicpeak (GP Zone dissolution) around 130∘C (Figure 6(b)) Thisstate is called T4-temper
It is clear that GP Zones precipitate during naturalageing and cause increase in hardness from W-temper toT4-temper (Figure 6(a)) There is a small difference in thehardness of T4-temper and hot formed and die quenchedparts after two weeks This is related to the introduction ofdislocations and quenched-in-vacancies into the parts afterhot forming and subsequent die quenching (HF + DQ)Since dislocations act as vacancy sinks a slowing down ofprecipitation kinetics occurs due to progressive annihilationof quenched-in vacancies on dislocations [18]
Dynamicmechanisms such as strain-hardening dynamicrecovery dynamic precipitation andor coarsening of pre-cipitates generally occur during hot deformation of precip-itation hardening aluminium alloys Additionally diffusioncontrolled growth ofmicrovoidscracks is also reflected in theflow behaviour at slower strain rates The DSC investigationabove confirmed that the AW-7921 sheet in the T4 tempercontains GP Zones (Figure 6(b)) This phase partially dis-solves during deformation at elevated temperature as shownby [19 20]The amount of dissolution of precipitates increaseswith increasing temperature This dissolution explains thedecrease in yield stress of AW-7921-T4 sheet with increasingtemperature up to 475∘C
The decrease in the yield stress and peak stress of AW-7921-T4 with increasing temperature may be respectivelyattributed to increasing dissolution of hardening GP Zoneprecipitates and dynamic recovery Dynamic recovery occursduring deformation at elevated temperatures Generallyit leads to steady state deformation due to annihilationof dislocations by cross-slip and climb supported by theapplied stress and increased diffusion [13 21] The dynamicrecovery effect increases with increasing temperature dueto diminished strain-hardening through the annihilation ofthe accumulated dislocations This is observed in the tensilebehaviour (Figure 4(b)) As temperature is increased the ratioof the yield stress to the peak stress approaches 1 and indicatesreducing strain-hardening
The strain rate effects on yield and peak stress in thecurrent work can be understood in terms of the timeuntil fracture at the test temperature Decreasing strain rateincreases deformation periods and this is believed to leadto more dynamic recovery and dissolution of GP Zoneprecipitates during the test Higher strain rates increase thestrain-hardening capacity and results in an increase in peakstress as shown in Figures 4(a) and 4(b) respectively It isbelieved that a slower strain rate allows much more timefor dynamic recovery than higher strain rate due to longerexposure at the test temperatureThis leads to the lower yieldstrength at 001 sminus1 compared to 01 and 1 sminus1
Formability of AW-7921-T4 sheet can be related to theelongation at fracture and strain rate sensitivity m The pos-itive strain rate sensitivity increases deformation resistanceand the local deformation slows down With the increase ofstrain rate sensitivity the transfer and diffusion capability ofnecking increase [13] Therefore increasing strain rate sensi-tivity along with dynamic recovery seems to be responsiblefor the increase in elongation at fracture with increasingtemperature
The decrease in yield strength (sim28) from the initial T4-temper condition to the hot stamped and stored conditionis negligible for both forming speeds (Figure 7) After asubsequent 1-SPB heat treatment the yield strength of hotstamped and stored parts increases by sim29 due to theformation of hardening 1205781015840 precipitates However when usingthe 3-SPB instead of the 1-SPB the yield strength increasesby sim37 The hot stamped and stored part exhibited thehighest yield strength after the 3-step paint baking treatment(sim480MPa) but this is still only 92 of the T6-temperyield strength (sim520MPa) Therefore there is a need forfurther optimisation of the paint bake treatment to achievethe peak-aged strength in the completed part Alternativelya heat treatment to stabilize the GP Zones may be givenafter forming allowing them to act as nucleation sites for 1205781015840precipitates
5 Conclusions
This work has investigated the hot stamping behaviour ofAW-7921 sheet on the formability and final mechanicalproperties as the sheet passes through the process chain Ithas shown the importance of considering the whole processchain and the effects of the thermomechanical processing
Advances in Materials Science and Engineering 9
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
contained therein This consideration is important to ensure(1) the best conditions for forming the part and (2) the highestpossible strength is achieved in the final product The mainfindings are as follows
(1) The cooling rate during hot forming and die quench-ing should be at least between 5 and 10K sminus1
(2) Tensile test results show that the AW-7921 alloy is sen-sitive to temperature and strain rate The YS and UTSdecrease with increasing temperature and decreasingstrain rate Strain rate sensitivity m and elonga-tion at fracture increase with increasing temperatureand decreasing strain rate This is due to dynamicrecovery and dissolution of hardening precipitatesfor instance GP Zones
(3) The yield strength of AW-7921 samples increases inthe order of 1-SPB 3-SPB and T6-temper This isrelated to the increasing stability of the 1205781015840 precipitatespresent in the samples
(4) Forming speeds of 10 and 20mm sminus1 produced almostidentical yield strengths in the hot stamped parts
(5) The final hot stamped component following a 3-steppaint bake process exhibited yield strength of 92achievable with a T6-temper while for 1 step paintbaking the yield strength was 84
Disclosure
The current address of M Kumar is as follows Ebner Indus-trieofenbau GmbH Ebner-Platz 1 4060 Leonding Austria
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
The authors would like to thank the European RegionalDevelopment Fund (EFRE) and the State of Upper Austria forsponsoring the project ForMAT in the framework of the EU-Programme Regio 13 The authorsrsquo grateful thanks go to theirtechnicians in particular Anton Hinterberger and ChristianHaslinger for their support throughout the research work
References
[1] N R Harrison and S G Luckey ldquoHot stamping of a B-pillar outer from high strength aluminum sheet AA7075rdquo SAEInternational Journal of Materials andManufacturing vol 7 no3 pp 567ndash573 2014
[2] J Dorr Semi-Hot and Hot Forming of Conventional and HighStrength Aluminum Alloys Forming in Car Body EngineeringACI Bad Nauheim Germany 2011
[3] R Kelsch ldquoAluminium in the car bodymdashnew forming conceptsfor performance improvement and spread of scoperdquo in Strate-gies in Car Body Engineering ACI Bad Nauheim Germany2013
[4] M Kumar and N G Ross ldquoInfluence of temper on theperformance of a high-strength Al-Zn-Mg alloy sheet in thewarm forming processing chainrdquo Journal ofMaterials ProcessingTechnology vol 231 pp 189ndash198 2016
[5] E Ceretti C Contri and C Giardini ldquoTube-hydroformingexperiments on an Al 7003 extruded tuberdquo Journal of MaterialsProcessing Technology vol 177 no 1ndash3 pp 672ndash675 2006
[6] M Kumar N Sotirov and C M Chimani ldquoInvestigations onwarm forming of AW-7020-T6 alloy sheetrdquo Journal of MaterialsProcessing Technology vol 214 no 8 pp 1769ndash1776 2014
[7] H Wang Y-B Luo P Friedman M-H Chen and L GaoldquoWarm forming behavior of high strength aluminum alloyAA7075rdquo Transactions of Nonferrous Metals Society of Chinavol 22 no 1 pp 1ndash7 2012
[8] M-Y Lee S-M Sohn C-Y Kang D-W Suh and S-Y LeeldquoEffects of pre-treatment conditions on warm hydroformabilityof 7075 aluminum tubesrdquo Journal of Materials Processing Tech-nology vol 155-156 no 1ndash3 pp 1337ndash1343 2004
[9] L Wang M Strangwood D Balint J Lin and T A DeanldquoFormability and failure mechanisms of AA2024 under hotforming conditionsrdquo Materials Science and Engineering A vol528 no 6 pp 2648ndash2656 2011
[10] M S Mohamed A D Foster J Lin D S Balint and T ADean ldquoInvestigation of deformation and failure features in hotstamping of AA6082 experimentation andmodellingrdquo Interna-tional Journal of Machine Tools amp Manufacture vol 53 no 1pp 27ndash38 2012
[11] P F Bariani S Bruschi A Ghiotti and F Michieletto ldquoHotstamping of AA5083 aluminium alloy sheetsrdquo CIRP AnnalsmdashManufacturing Technology vol 62 no 1 pp 251ndash254 2013
[12] M Kumar Precipitation kinetics in thermo-mechanical formingof aluminium alloys [PhD thesis] TU Wien Vienna Austria2011
[13] M Zhou Y Lin J Deng and Y Jiang ldquoHot tensile deformationbehaviors and constitutive model of an AlndashZnndashMgndashCu alloyrdquoMaterials amp Design vol 59 pp 141ndash150 2014
[14] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharacter-ization of precipitation and phase transformations in AlndashZnndashMg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[15] H Loffler I Kovacs and J Lendvai ldquoDecomposition processesin Al-Zn-Mg alloysrdquo Journal of Materials Science vol 18 no 8pp 2215ndash2240 1983
[16] L Hadjadj R Amira D Hamana and A Mosbah ldquoCharac-terization of precipitation and phase transformations in Al-Zn-Mg alloy by the differential dilatometryrdquo Journal of Alloys andCompounds vol 462 no 1-2 pp 279ndash283 2008
[17] M Kumar N Ross and I Baumgartner ldquoDevelopment of acontinuous cooling transformation diagram for an Al-Zn-Mgalloy using dilatometryrdquoMaterials Science Forum vol 828-829pp 188ndash193 2015
[18] S Ceresara and P Fiorini ldquoResistometric investigation of theageing process after quenching and cold-work in Al-Zn-Mgalloysrdquo Materials Science and Engineering vol 10 pp 205ndash2101972
[19] T Marlaud A Deschamps F Bley W Lefebvre and B BarouxldquoEvolution of precipitate microstructures during the retrogres-sion and re-ageing heat treatment of an Al-Zn-Mg-Cu alloyrdquoActa Materialia vol 58 no 14 pp 4814ndash4826 2010
10 Advances in Materials Science and Engineering
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
[20] M Kumar C Poletti and H P Degischer ldquoPrecipitation kinet-ics in warm forming of AW-7020 alloyrdquo Materials Science andEngineering A vol 561 pp 362ndash370 2013
[21] M Kumar C Poletti and H P Degischer ldquoPrecipitationkinetics in warm forming of AW-7020 alloyrdquo Materials Scienceand Engineering A vol 561 pp 362ndash370 2013
