MANAGER’S SUMMARY ZCA-15, “Zinc Alloy Properties for Market Development and Support” 2011 Final Report Issued August 2011 The subject report describes results for ZA-8 of the same designed experiment that was car- ried out during 2010 for Alloys 2, 3 and 5. In particular, different aging conditions, die tem- peratures, casting wall thicknesses, gate velocities and test temperatures were used to de- termine the range of mechanical properties, and stability with both natural and artificial ag- ing. Because of limited funds, not all experimental conditions were realized with the ZA-8 al- loy; however, enough samples have been cast to allow the complete test matrix to be ac- complished if funds become available in the future. The work confirms past results showing that ZA-8 has superior strength compared with the Zamak composition alloys and that the nature of changes in strength with natural aging is the same as the Zamak alloys. Fatigue tests are repeated from the 2010 report. Creep testing is also reported for all 4 alloys, greatly extending the information reported in the 2010 report. Both room temperature and 85 o C test- ing are reported for various aging conditions. Of significance is the observation is that very lit- tle secondary (steady-state creep) is observed in these alloys, it is mostly primary and some tertiary creep.. Observations on how ageing affects creep by influencing diffusion through the microstructure are also made.
Transcript
MANAGER’S SUMMARY
ZCA-15, “Zinc Alloy Properties for Market Development and Support”
2011 Final Report
Issued August 2011
The subject report describes results for ZA-8 of the same designed experiment that was car-ried out during 2010 for Alloys 2, 3 and 5. In particular, different aging conditions, die tem-peratures, casting wall thicknesses, gate velocities and test temperatures were used to de-termine the range of mechanical properties, and stability with both natural and artificial ag-ing. Because of limited funds, not all experimental conditions were realized with the ZA-8 al-loy; however, enough samples have been cast to allow the complete test matrix to be ac-complished if funds become available in the future. The work confirms past results showing that ZA-8 has superior strength compared with the Zamak composition alloys and that the nature of changes in strength with natural aging is the same as the Zamak alloys. Fatigue tests are repeated from the 2010 report. Creep testing is also reported for all 4 alloys, greatly extending the information reported in the 2010 report. Both room temperature and 85oC test-ing are reported for various aging conditions. Of significance is the observation is that very lit-tle secondary (steady-state creep) is observed in these alloys, it is mostly primary and some tertiary creep.. Observations on how ageing affects creep by influencing diffusion through the microstructure are also made.
Ageing of Zinc Alloys
Report by
Aalen University of Applied Sciences
Gießerei Technologie Aalen – GTA
Prof. Dr.-Ing. Lothar H. Kallien
Dipl.-Phys. Walter Leis Dipl.-Ing. Blanka Fiala
2
Content
1 Introduction 5
2 Results 5
2.1 State of the knowledge 5
2.1.1 Chemical composition of the die casting alloys 5 2.1.2 Zinc die casting alloys 6 2.1.3 Advantages of zinc alloys 7 2.1.4 Usage of Zinc 7 2.1.5 Literature 8
3.5 Comparison between ZP0400, ZP0410, ZP0430 and ZP0810 47
4 Summary 48
5 Literature 50
4
1 Introduction Hot chamber die casting is a highly productive production technology for zinc parts of highest quality. Main customers are manufactures of cars, furniture and other mechanical parts. Al-though zinc alloys have a rather high density of 6,7 g/cm³ they are used in automotive appli-cations due to their high mechanical properties, thin wall thicknesses and their plating prop-erties. Zinc parts can be recycled 100%.
The low melting temperature of 390°C leads to an increased creep rate. In addition, zinc al-loys loose mechanical properties over time due to ageing effects. The natural ageing is typi-cally compensated by artificial ageing. The parts are tested typically between -35°C and +85°C as these are the temperatures in automotive applications.
Until today only few statistical reliable data has been published on these temperate ranges.
2 Results Goal of the research was the investigation of the material properties of zinc alloys under natural and artificial ageing to find a correlation between artificial and natural ageing. As zinc die castings are produced under a variety of production conditions it was necessary to pro-duce test castings using different parameters under extremely controlled conditions.
2.1 State of the knowledge
2.1.1 Chemical composition of the die casting alloys The chemical composition of the zinc alloys influences the fluidity, the mechanical properties, the feeding properties and the structure. For most of the parts ZP0410 (Z410) with 4% alu-minum and 1% copper is used. Besides the alloys ZP0430 (Z430) and ZP0400 (Z400) are used. All of these alloys are hypoeutectic alloys with small amounts of Pb, Cd, Sn, Fe, Ni and Si. The melting temperatures are below 400°C. Additionally the alloy ZP0810 a hypereutectic alloy with 8% of aluminum would be tested.
5
RT: very low solubility of aluminum
4 weight % Aluminum
α: hexagonale structure
Eutectikum 382°C
β, β´: kfz structure
8 weight % Aluminum
Figure 1: Phase diagram zinc - aluminum [Gott07]
The phase diagram of aluminum-zinc, Figure 1 shows an extremely low solubility of zinc in aluminum and vice versa
Z430
Z410
Z400
Figure 2: Ternary system Zn – Al – Cu and positions of zinc die casting alloys [cos96]
Copper which is typically used between 0,3 and 3% increases the solubility of Aluminum in zinc, Figure 2, and therefore increases the strengthening effect.
2.1.2 Zinc die casting alloys Worldwide only four alloys are used which are described in the European Norm EN12844 Table 1 shows the alloy elements in these alloys. In Table 15 the measured composition of the alloys used in this research are shown.
6
Table 1: Specification of zinc die casting alloys after EN12844 [data09]
alloy ZP3 ZP5 ZP2 ZP8
ZP0400 ZP0410 ZP0430 ZP0810
ZnAl4 ZnAl4Cu1 ZnAl4Cu3 ZnAl8Cu1 max 4,3 4,3 4,3 8,8
aluminum % min 3,7 3,7 3,7 8 max 0,1 1,2 3,3 1,3
copper % min 0,7 2,7 0,8 max 0,05 0,05 0,05 0,03
magnesium % min 0,025 0,025 0,025 0,015
lead % max 0,005 0,005 0,005 0,006 cadmium % max 0,005 0,005 0,005 0,006 tin % max 0,002 0,002 0,002 0,003 iron % max 0,05 0,05 0,05 0,06 nickel % max 0,02 0,02 0,02 0,02 silicon % max 0,03 0,03 0,03 0,045 zinc rest rest rest rest
2.1.3 Advantages of zinc alloys A big advantage of zinc alloys is the low melting temperature near 400°C. As a result cooling and cycle times are very short. In addition die life of die casting dies lies in the range of one million and more shots exceeding the values for aluminum by the factor of ten.
As an disadvantage the creep already starts at room temperature at loads exceeding 50 MPa. The very high mechanical properties diminish within one year and also measured data of parts change.
2.1.4 Usage of Zinc In 2007 more than 11 million tons of zinc have been used worldwide, most of it for corrosion protection, zinc die castings and alloying of copper.
Figure 3: Usage of zinc [ilzro09]
The usage of zinc in automotive application increases and a passenger car already includes more than 10 kg of zinc [press09].
7
2.1.5 Literature
Until today there are few data available for ageing of zinc alloys most of them deal with measure changes, Figure 4. In [op71] describes the solubility of Al in Zn and the eutectoid phase change of the β-phase into β´-Phase. The addition of copper increases the solubility of Al. The measure changes describe [hae88] [gop89], especially Z430 contracts more than Z400 or Z 410. [hae88] [geb42]. To get around the measure changes the automotive industry suggests to heat treat the parts at 105°C for 24 hours.
Figure 4: Dimensional stability of zinc die casting parts at RT [joh82]
Zinc alloys change also their mechanical properties with time, [klr83], [sch91]. A comparison between artificial and natural ageing is given in [klein84]. An improved creep resistance is based on the copper rich ε-phase, however the production conditions under which the specimens have been produced are not available in the literature [sch95], [bir95].
2.1.5.1 Ageing processes
Basically all alloys undergo ageing as the solubility in the liquid phase differs from the and the solid phase. Zinc alloys age already at room temperature as diffusion at room tempera-ture is relatively high due to the low melting point of zinc.
8
Figure 5: Natural ageing (left) and artificial ageing (right) as a function of copper content
2.1.6 Creep processes Creep is plastic deformation under load caused by atom movements without concentration differences. Creep is also evident at pure metals. A typical creep curve is depicted in Figure 6.
Z
Deh εnung
eit t
stationäres iechenKr
Z
Deh εnung
eit t
stationäres iechenKr
Z
Deh εnungstrain
IIIIIIII
I I stationäres
iechenKrsteady state
creep
eit ttime t Figure 6: Typical creep curve
Technical important is the phase of stationary creep. The creep rate is a function of the me-chanical load and the temperature and the diffusion. Gott07 provides the following equation:
TkQn
eG
A ⋅−
⋅⎟⎠⎞
⎜⎝⎛ σ
⋅s =ε& A: coefficient of creep in 1/s (%/h) ( 1 )
G: shear modulus in MPa
9
For the most materials creep is related to the solidus temperature Tl:
0,6 < T / Tl < 1 fast creep
0,3 - 0,4 < T / Tl < 0,6 slow creep
0 < T / Tl < 0,3 no creep
2.1.6.1 Comparison Al and Zn
The low melting point of Zn of 420°C (693 K) causes slow creep at 0°C which increases at room temperature, Figure 7 shows the differences between temperature an creep for Al and Zn.
- 60°C
130°C
0,3
0,6
T / TS
Zink
- 273°C0
0°C0,40,3
0,6
10°C
290°C
T / TS
Alum
0
inium
- 273°C
100°C0,4- 60°C
130°C
0,3
0,6
T / TS
Zink
- 273°C0
0°C0,4- 60°C
130°C
0,3
0,6
T / TS
Zink
- 273°C0
0°C0,40,3
0,6
10°C
290°C
T / TS
Alum
0
inium
- 273°C
100°C10°C
290°C
0,40,3
0,6
T / TS
Alum
0
inium
- 273°C
100°C0,4
Figure 7: Homologue temperature for zinc and aluminum
Creep is proportional to the mechanical load even when the diffusion is only caused along grain bounderies. Then equation ( 2 ) describes the creep rate:
TkQ
eG
A ⋅−
⋅⎟⎠⎞
⎜⎝⎛ σ
⋅s =ε& ( 2 )
3 Experiments
3.1 Experimental Program The experiments have been carried out for ZP0400, ZP0410, ZP0430 and ZP0810 according to DIN EN 12844.
To analyze the mechanical properties die cast plates have been cast with thicknesses of 0,8 mm, 1,5 mm und 3 mm using a hot chamber die casting machine Type DAW 80 by Frech.
From these plates probes have been machined for the tensile test according to DIN 50125. A variety of casting parameters have been used to cover all different die cast parts.
Sensors for pressures and temperatures within the machine and within the die have been used to control the process during test part production.
10
The following material properties have been investigated using as cast probes, artificially aged probes and naturally aged probes:
yield strength, tensile strength, elongation and Young’s modulus At testing temperatures -35°C, RT (room temperature) and +85°C Creep rate at RT and 85°C fatigue data at RT density hardness creep behaviour
3 die temperatures
3 gate velocities
3 wall thicknesses
3 alloys
3 testing temperatures
11 ageing conditions
3 die temperatures
3 testing temperatures
11 ageing conditions
3 wall thicknesses
3 gate velocities
3 alloys4 alloys
Figure 8: Varied process parameters
Process parameters have been varied:
wall thickness 0,8 mm; 1,5 mm und 3,0 mm, gate velocity 25 m/s, 40 m/s und 55 m/s die temperatures of 120°C, 160°C and 200°C.
The influence of the natural ageing has been tested after 3 and 6 weeks, 3 and 6 months and after 1 year. After 2 years a last measurement will be conducted.
The artificial ageing was carried out using a 24 hour tempering at 65°C, 85°C and 105°C.
11
3.1.1 DOE To reduce the amount of run DOE was used using Design-Expert 7 by Stat-Ease. The design of the plan included center point conditions 1,5 mm wall thicknesses, 160°C die temperature and 40 m/s gate velocity. The plan is shown in Table 2.
Table 2: Plan of the production parameters for Z410
The solidification time is calculated after Chworinoff´s equation with the following values:
Table 7: Calculated solidification time
solidification time in ms thickness in mm TF = 120°C TF = 160°C TF = 200°C
0,8 16 19 23 1,5 57 68 83 3,0 228 270 330
After production the plates were frozen to -20°C.
60
10
15
150 Figure 13: Geometry of the specimens for static tensile tests
16
3.3 Ageing 3.3.1 Artificial ageing Five specimen have been aged at +65°C, +85°C und +105°C for 24 hours.
Figure 14: Heat treatment furnace
3.3.2 Natural Ageing Natural Ageing was performed at room temperature in a climate controlled room where also the measuring took place.
Figure 15: Boxes for the specimens for natural ageing (23°C, room air conditioned)
3.4 Mechanical data
3.4.1 Tensile testing 3.4.1.1 Test equipment
The tensile test machine is a 100 kN Universal "Schenck" with modern computerized control.
17
Figure 16: Upgraded tensile testing machine
The specimens have been tested under as cast conditions, naturally aged condition and arti-ficially aged condition under three test temperatures, Figure 17 and Figure 18.
Figure 17: Fixed specimen with extensiometer, test at RT (left) and test at +85°C with climatic
chamber (right)
18
Figure 18: Test at -35°C (left) and cooling device with pressure controlled CO2 (right)
The tests at +85°C and -35°C is executed in a climate chamber. The cooling of the probes is performed using CO2-Gas. The control was done using a thermocouple.
3.4.1.2 Stress - Strain Diagram
More than 3000 specimen have been tested. For each probe the Stress-Strain-diagram is available indicating the test conditions and the production conditions, Figure 19.
19
As cast -RT - thickness 3 mm
Figure 19: Stress-strain-curve run 23 (3 mm; 160°C; 40 m/s) measured in as cast condition at RT (23°C)
As Zn alloys show no yield strength data are provided for the elongation of 0,2%
Porosity influences the elongation properties which lead to a high variation of the elongation data between 2% and 12%. The tensile strength data show a standard variation of only 5%.
Gusszustand Probendicke 3,0 mm
KaltverfestigungEinfluss innerer
Fehler
Bruchdehnustark abhängig von
inneren Fehlern
ng ist
Gusszustand Probendicke 3,0 mmAs cast condition - thickness 3 mm Zugfestigkeit ist
enig abhängig on inneren
wvFehlern
Zugfestigkeit ist enig abhängig on inneren
wvFehlern
tensile strength hardly depends from internal
defects
Kaltverfestigung
Bruchdehnustark abhängig von
inneren Fehlern
ng ist fracture elongdepending from internal
defects
ation is strain %
Einfluss innerer Fehlerinfluence of
internal defectscold strengthening
strain %
Figure 20: Correlation between tensile strength and fracture elongation
The ageing behavior can therefore be checked using the tensile test data. Using a test tem-perature of -35°C the tensile strength and the yield strength increase by 5%. The fracture elongation data are lower.
20
Figure 21: Correlation between tensile strength and fracture elongation
At +85°C the stress-strain curve shows data depicted in figure 21. The data are 20% lower compared to room temperature. Elongation is in the range of 20%.
As cast condition - (-35°C) thickness 3 mm
As cast condition - (+85°C) thickness 3 mm
strain %
strain %
Figure 22: Stress-strain-curve run 26 (3mm; 200°C; 55 m/s) measured in as cast condition at +85°C
3.4.1.3 Young’s modulus
The data for Young’s modulus is calculated out of the stress strain curves using software. The ageing shows no influence on the Young’s modulus, neither do the production condi-tions. The main influence is the test temperature, Table 8.
21
Table 8: Young’s modulus as function of testing temperature and ageing conditions
Young’s modulus in GPa
+23°C +85°C -35°C
as cast condition 90 ± 8 82 ± 8 92 ± 8
1 year natural ageing 88 ± 8 78 ± 8 90 ± 8
3.4.1.4 Tensile strength Rm and Yield Strength Rp0,2
Both tensile strength and yield strength depend on the production data. Highest values could be achieved with thin specimen in as cast condition and low die temperature measured at -35°C testing temperature.
Design-Expert® Softw are
Rm Gusszustand
Design-Expert® Softw are
Rm zustand Guss446.25446.25
259.4
X1 = A: WandstärkeX2 = B: Formtemperatur
Actual FactorsC: Geschw indigkeit = 40D: Prüftemperatur = 25
0.8 1.4
1.9 2.5
3.0 120
140
160
180
200
260
290
320
350
380
410
Rm
Gus
szus
tand
A: Wandstärke B: Formtemperatur
259.4
X1 = B: FormtemperaturX2 = C: Geschw indigkeit
Actual FactorsA: Wandstärke = 1.54D: Prüftemperatur = 25
120 140
160 180
200 20
30
40
50
60
330
340
350
360
370
380
390
Rm
Gus
szus
tand
B: Formtemperatur C: Geschwindigkeit
Figure 23: Tensile strength as a function of wall thickness and die temperature (left) and as a func-
tion of die temperature and gate velocity (right) as cast condition
The influence of wall thickness die temperature gate velocity is linear according to the Design Expert. The largest impact has the wall thickness, followed by the die temperature and the gate velocity.
Design-Expert® Softw are
Rm Gusszustand446.25
259.4
X1 = A: WandstärkeX2 = D: Prüftemperatur
Actual FactorsB: Formtemperatur = 160C: Geschw indigkeit = 40
0.5 1.0
1.5 2.0
2.5 3.0
-35
-5
25
55
85
220
270
320
370
420
Rm
Gus
szus
tand
A: Wandstärke D: Prüftemperatur
Figure 24: Tensile strength as a function of wall thickness and test temperature as cast condition
22
The influence of the testing temperature on the measured mechanical data is not linear, Figure 24.
Actual FactorsC: Geschw indigkeit = 40D: Prüftemperatur = 25
219
X1 = A: WandstärkeX2 = D: Prüftemperatur
Actual FactorsB: Formtemperatur = 160C: Geschw indigkeit = 40
0.5 1.0
1.5 2.0
2.5 3.0 120
140
160
180
200
220
270
320
370
420
Rm
105
°, 2
4h
A: Wandstärke B: Formtemperatur
0.5 1.0
1.5 2.0
2.5 3.0
-35
-5
25
55
85
220
270
320
370
420
Rm
105
°, 2
4h
A: Wandstärke D: Prüftemperatur
Figure 25: Tensile strength as a function of wall thickness and die temperature (left) and as a func-
tion of testing temperature and wall thickness (right) artificially aged at 105°C for 24 hours
Artificial ageing leads to mechanical properties corresponding to a one year natural ageing. Figure 25 shows the influence of the wall thickness and the die temperature on the mechani-cal properties after artificial ageing (105°C, 24 hours).
Actual FactorsB: Formtemperatur = 160C: Geschw indigkeit = 40
0.5 1.0
1.5 2.0
2.5 3.0
-35
-5
25
55
85
220
270
320
370
420
Rm
105
°, 2
4h
A: Wandstärke D: Prüftemperatur
Figure 26: Tensile strength as a function of wall thickness and testing temperature, artificially aged
at 105°C for 24 hours
23
3.4.2 Static testing Z400, Z430 and ZA8
Table 9, Table 10 and Table 11 show the mechanical data for the three other alloys as con-cluded so far. Statistic correlation will be performed as soon as all data have been achieved.
3.4.3 Creep testing The creep testing equipment is shown in Figure 27 and is performed as a function of time and temperature according to DIN 50118. The system allows the measurement of changes in specimen’s length within a resolution of 1 µm up to 20% of the original length.
27
Figure 27: Creep testing equipment with 12 temperature controlled test stations
3.4.3.1 Results of Creep testing
For the creep tests stresses between 40 MPa and 100 MPa have been used at room tem-perature. At testing temperature of +85°C stresses between 12 MPa und 50 MPa have been applied to the specimen.
Typically for all alloys measured in this research project is a very long period of primary creep. The end of primary creep can be estimated between 1 and 2 % of creep strain. After primary creep there is only tertiary creep to observe because the stress is growing during the plastic deformation under creep by reduction of the cross section of the specimens.
y = 0,0110x
y = 0,0527x0,5849
0,4394
y = 0,0173x0,5703
y = 0,0373x0,5508
0,0
0,4
0,8
1,2
1,6
2,0
0 200 400 600 800 1000 1200 1400 1600 1800 2000time in h
Figure 29: Creep behaviour for Z400 after artificial ageing (105°C / 24 h), log scale
Flat lines means mathematically that the behaviour can be described by a potential function. The potential law for each stress is included in Figure 28.
y = 0,0139x0,3827
y = 0,0207x0,4909
y = 0,0335x0,5526
0,0
0,4
0,8
1,2
1,6
2,0
0 200 400 600 800 1000 1200 1400 1600
cree
p st
rain
in %
1800 2000time in h
88,7 MPA 65,4 MPA 40,7 MPA
Z 410 1,5 mm105°C / 24 h
Figure 30: Creep behaviour for Z410 after artifical ageing (105°C / 24 h), linear scale
29
0,01
0,10
1,00
10,00
1 10 100 1.000
cree
p st
rain
in %
10.000time in h88,7 MPA 65,4 MPA 40,7 MPA
Z 410 1,5 mm105°C / 24 h
Figure 31: Creep behaviour for Z410 after artificial ageing (105°C / 24 h), log scale
Figure 32: Creep elongation as a function of time and stress of Z410 at RT
y = 0,0139x0,4064
y = 0,0365x0,5072
y = 0,0142x0,548
0,0
0,4
0,8
1,2
1,6
2,0
0 200 400 600 800 1000 1200 1400 1600 1800 2000time in h
cree
p st
rain
in %
Z430 1,5 mm 105°C / 24 h
Z430-91,3 MPA Z430-67,6 MPA Z430-42,3 MPA
Figure 33: Creep behaviour for Z430 after artifical ageing (105°C / 24 h), linear scale
30
0,01
0,10
1,00
10,00
1 10 100 1.000 time in h
cree
p st
rain
in %
Z430 1,5 mm 105°C / 24 h
10.000
Z430-91,3 MPA Z430-67,6 MPA Z430-42,3 MPA
160°C / 40 m/s
Figure 34: Creep behaviour for Z430 after artificial ageing (105°C / 24 h), log scale
y = 0,0190x0,3548
y = 0,0657x0,5360
y = 0,0269x0
0,0
0,4
0,8
1,2
1,6
2,0
0 200 400 600 800 1000 1200 1400 1600
cree
p st
rain
in %
ZA8 Umicore 6 years
105°C / 24 h
,5053
1800 2000time in h
ZA8-93,8 MPa ZA8-68,1 MPa ZA8-42,5 MPa
Figure 35: Creep behaviour for ZA8 naturally aged over 6 years after artifical ageing (105°C / 24 h), linear scale
31
0,01
0,10
1,00
10,00
1 10 100 1.000 time
cree
p st
rain
in %
ZA8 Umicore 6 years
105°C / 24 h
10.000 in h
ZA8-93,8 MPa ZA8-68,1 MPa ZA8-42,5 MPa
Figure 36: Creep behaviour for ZA8 naturally aged over 6 years after artificial ageing (105°C / 24 h), log scale
Influence of testing temperature
Creep is a thermal activated process which can be expressed by Arrhenius law. The activa-tion energy for the creep behaviour is the activation energy for self diffusion of zinc with a value of ca. 94 kJ / mol. The activation energy leads to an increase of the creep rate of 25°C to 85°C by the factor of 700, Figure 37 and Figure 38.
0,0
0,4
0,8
1,2
1,6
2,0
0 40 80 120 160
cree
p st
rain
in %
200time in h
testing temperatur +85°C
51 MPa
24,5 MPa37 MPa
Figure 37: Creep elongation as a function of time and stress of Z410 at +85°C
According to this finding the creep rate can be calculated between 0°C und 100°C and stresses between 10 MPa und 100 MPa using the following equation:
KmolTRkJ94
n e ⋅⋅⋅−
⋅σs A ⋅=ε& ( 3 )
32
0,00001
0,0001
0,001
0,01
0,1
10 Stress in
Cre
ep ra
te in
%/h
o
100 MPa
RT Z105°
410 3 mm C 24 hours
85°C Z410 3mm 105°C 24 hours
Factor 70094 kJ / mol
Figure 38: Creep rate as a function of testing temperature
The stress exponent is strongly depending from the actual creep elongation and lies between 4 and 7, Figure 39. The stress exponent can be calculated from the potential law which is depicted in the creep behaviour lines ( as an example in Figure 28) by deviation. The devia-tion of a creep lines means the creep rate as a function of the creep elongation.
Example for Z400 with 68 MPa:
057031
0173,0y
⎟⎠⎞
⎜⎝⎛
4297,0t01,0 −⋅=
5703,0 tt0173,0y =⇒⋅=
15703,0t5703,0173,0y −⋅⋅=&
⎟⎟⎠
⎞⎜⎜⎝
⎛σ
=log
ylogn&
from diagram like Figure 38
Stress exponent vs Creep elongation
y = 5,45x0,16
y = 6,88x0,16
0
2
4
6
8
10
0 0,5 1 1,5 2 2,5
creep elongation in %
stre
ss e
xpon
ent
Z430 105°C / 24 h
Z410 105°C / 24h
Z410 Z430 Figure 39: Stress exponent n as a function of creep elongation
33
Influence of artificial ageing time
Some specimen has been averaged at 105°C up to 1000 hours.
Creep rate vs Ageing time
0,00001
0,00010
0,00100
0,01000
0,10000
1 10 100 1.000
cree
p ra
te in
%/h
10.000ageing time in h
Umi 21 yearsstress 76 MPa
artificially aged at 105°C
thickness 1,5 mm
thickness 3,0 mm
1 % per year
creep rate calculated at 1 % creep strain
Figure 40: Creep rate versus ageing time for Z410 as a function of wall thickness
While during natural aging the static properties (tensile strength and yield strength) have a limit of decrease after one year the creep strength decreases continuously over 20 years or longer or at elevated temperatures at 100°C over 1 year because the diffusion process in the α-phase will also reduce the creep strength but the diffusion length in the α-phase is more than factor 10 higher and therefore the diffusion time is more than factor 100 longer. The loss of creep strength is near factor 50 higher after a heat treatment at 105°C over 1000 hours than over 24 hours and also after natural ageing over 20 years. A greater wall thickness will also reduce the creep rate with his bigger grain size in the micro structure.
3.4.3.2 Creep rate of Z400, Z410, Z430 and ZA8
Figure 41 shows the creep data of the four alloys in comparison. The copper content influ-ences the creep rate by a factor of 4, between Z410 and Z430 there is only a small difference near factor two. The alloy ZA8 lies between Z400 and Z410 but the specimens had an addi-tional natural ageing process of 6 years.
Creep rate Z00, Z410, Z430 and ZA8
y = 2E-17x7,3231
y = 6E-16x6,1826
y = 3E-17x6,9683
y = 3E-18x7,6678
0,000001
0,000010
0,000100
0,001000
0,010000
10 100stress in MPa
cree
p ra
te %
/h a
t 1%
cre
ep e
long
atio
n o
Z400
Z410Z430
1% per month (730 h)
1% per year (8760 h)
50
thickness 1,5 mm artificial aged 105°C / 24 h
room temperatur tested ZA8 6 years naturally aged
stress exponent (average): 7
Figure 41: Stress exponent for secondary creep of Z400, Z410, Z430 and ZA8 calculated
For fatigue testing a resonance testing machine by Russenberger & Müller, Typ Mikroton 654 with maximal 20 kN load has been used. The test frequency was 150 Hz. The specimen is shown in Figure 42.
10
10
20
120
Figure 42: Shape of the utilized specimens for fatigue tests
The tests have been conducted at R = -1 (tension-compression) and R = 0 (tension). As small probes up to 2 mm break under compression for the probes with 0,8 mm und 1,5 mm only tension has been tested. The data vary extremely heavily with internal defects, much more than the static data. Therefore all specimen have been X-rayed before testing.
3.4.4.2 Results of the fatigue for Z410
The zinc alloy does not show a sharp edge between time strength and permanent strength. For 10 million cycles one can assume 85 MPa, after artificial ageing the value drops down to 80 MPa.
35
1,750
1,800
1,850
1,900
1,950
2,000
2,050
2,100
2,150
10.000 100.000 1.000.000 10.000.000
Numb
Stre
ss a
mpl
itude
σa M
Pa
100.000.000
er of cycles
60
80
75
70
65
10095
9085
140
130
120
110
Run 25: 3 - 120 - 25over aged 16 hours at 150°C
Run 26: 3 - 200 - 55as cast condition
Figure 44: S/N-curves (extended fatigue test) at R = -1 (compression and tension) of Z410 speci-mens in as cast condition and after artificial ageing (over ageing)
For Z 400 the lower tensile data lead to a smaller decline of the slope and the higher ductility leads to a higher permanent time value.
The sensitivity for combination of static and dynamic loads can be calculated from values at R = -1 and R = 0 and has the value of 0,45.
1,700
1,750
1,800
1,850
1,900
1,950
2,000
2,050
2,100
10.000 100.000 1.000.000 10.000.000Number
Stre
ssam
plitu
de σ
a MP
a
100.000.000 of cycles
60
8075
70
65
100959085
55
120
110
Umicore Z410: 3,0 mm natural aged in 20 years
Run 50 / Z400: 3,0 mm artificial aged at 105°C/24hR = -1
Z410
fractured specimens
not fractured specimens
Figure 45: S/N-curves at R = -1 of artificially aged Z400 in comparison of 20 year naturally aged Z410 specimens from Umicore
36
3.4.5 Hardness Hardness has been tested for all specimen and all ageing conditions. The values for as cast probes are in the range of 110 HB. One can detect that the gate velocity has no influence on the hardness.
0,8 mm /120°C
0,8 mm /160°C
0,8 mm /200°C 1,5 mm /
120°C 1,5 mm /160°C
1,5 mm /200°C 3 mm /
120°C 3 mm /160°C
3 mm /200°C
vA 25 m/svA 40 m/s
vA 55 m/s0
20
40
60
80
100
120
140
Har
dnes
s in
HB
Figure 46: Hardness as a function of processing parameters of Z410 as cast condition, thickness
and die temperature show strong influence on hardness, gate velocity does not A cold die leads to high hardness data, probes under 0,8 mm show deflection and cannot tested properly.
Hardness Z410natural ageing
100
104
108
112
116
120
1 180Ageing time in days
Har
dnes
s H
B
360
Figure 47: Hardness as a function of natural ageing time
The comparison of the hardness data for Z400, Z430 and ZA8 as cast as well as ZA8 artifi-cial aged is depicted in Figure 48 and Figure 49.
37
120 °C 160 °C 200 °C
0,8 mm1,5 mm
3 mm
0
20
40
60
80
100
120
140
Die temperature
Z4
Wall thickness
30 as cast condition
1
120 °C 160 °C 200 °C
0,8 mm1,5 mm
3 mm
120
100
0
20
40
60
80
40
Die temperatureWall
thickness
Z400 as cast condition
Figure 48: Hardness of Z400 (left) and Z430 (right) as cast condition
Out of DOE one can calculate the influence of the copper content on the hardness Figure 50.
120 °C 160 °C 200 °C
0,8 mm1,5 mm
3 mm
wall thickness
ZA8 as cast
0
20
40
60
80
100
120
140
Har
dnes
s [H
B]
die temperature
0,8 mm1,5 mm
3 mm
120 °C 160 °C 200 °C
0
20
40
60
80
100
120
140
Har
dnes
s [H
B]
die temperature
wall thickness
ZA8 artificial aged
Figure 49: Hardness of ZA8 as cast (left) and artificial aged (right)
100
105
110
115
120
125
130
135
0,02 0,85
Copper content in %
Har
dnes
s in
HB
Z430
2,95
Z400
Z410
38
Figure 50: Hardness (average values) as a function of copper content
Figure 51: Hardness (average values) as a function of aluminum content
3.4.6 Density and porosity Density and porosity have been tested according to Archimedes through weight in air and in water. Figure 52 shows the influence of die temperature wall thickness and gate velocity on the density. The wall thickness shows a strong influence. The porosity varies 0,5% und 2%, Table 13.
0,8 mm /120°C
0,8 mm /160°C
0,8 mm /200°C
1,5 mm /120°C
1,5 mm /160°C
1,5 mm /200°C
3 mm /120°C
3 mm /160°C
3 mm /200°C
vA 25 m/svA 40 m/s
vA 55 m/s6
6,1
6,2
6,3
6,4
6,5
6,6
6,7
6,8
6,9
7
Den
sity
in g
/cm
³
Figure 52: Density of the parts as a function of processing parameters
39
Table 13: Average values of density and porosity of Z410
wall thickness in mm
0,8 1,5 3,0
density in g/cm³ 6,54 ± 0,06 6,61 ± 0,02 6,64 ± 0,01
porosity in % 1,97 ± 0,94 1,02 ± 0,26 0,48 ± 0,13
3.4.7 Ageing Behaviour
3.4.7.1 Natural ageing
Tensile strength and yield strength decrease with ageing, the elongation increases. Data have been achieved for Z410 for up to 1 year so far, for Z400 until 2 months and Z430 until 4 months. For Z410 the drop in tensile strength is shown in Figure 53. This behavior is similar for all al-loys.
Z410
Figure 53: Decrease of tensile strength at RT as a function of time and wall thickness through the natural ageing process
The drop in strength should follow according the 2. Fick Law. However there are differences based upon the following facts: The error function is described using a diffusion coefficient which is independent from
the concentration, this is not the case.
Not only aluminum but also copper diffuses.
40
200
220
240
260
280
300
320
340
360
380
400
420
0,8 / -35 1,5 / -35 3,0 / -35 0,8 mm 1,5 mm 3,0 mm 0,8 / 85
Figure 56: Tensile strength as a function of testing temperature and wall thickness through the natu-
ral ageing process of Z430
Figure 54 shows the ageing behaviour of Z410, Figure 55 of Z400 and Figure 56 of Z430 as a function of testing temperature, wall thickness, die temperature and gate velocity. Figure 57 shows the same influences on the yield strength for Z410.
Figure 57: Yield strength as a function of testing temperature and wall thickness through the natural ageing process of Z410
42
3.4.7.2 Artificial ageing
Artificial ageing was performed for all alloys:
65°C for 24 hours
85°C for 24 hours
105°C for 24 hours
The tensile strength after artificial ageing for Z410 is shown in Figure 58, for Z400 in Figure 59 and for Z430 in Figure 59 as a function of testing temperature wall thickness at center-point-conditions and Figure 61 depicts the change of the yield strength after artificial ageing for Z410
200
220
240
260
280
300
320
340
360
380
400
420
0,8 mm 1,5 mm 3,0 mm 0,8 mm 1,5 mm 3,0 mm 0,8 mm
Tens
ile s
tren
gth
Rm
in M
Pa
Z410
1,5 mm 3,0 mmas cast 65°C/24h 85°C/24h 105°C/24h
mean values
Figure 58: Tensile strength as a function of testing temperature and wall thickness through the artifi-
Figure 61: Yield strength as a function of testing temperature and wall thickness through the artificial
ageing processes of Z410
Figure 62 shows that for Z410 artificial ageing at 65°C for 24 hours equals a natural ageing of 45 days. Ageing at 85°C for 24 hours equals approx. 120 days and 105°C for 24 hours equals natural ageing of 1 year.
44
: Artificial Ageing
Figure 62: Ageing behaviour (decrease of tensile strength) in comparison of natural and artificial ageing of Z410 as a function of wall thickness
If depicted in a diagram temperature 1/T against the log of time one achieves the activation energy Figure 63 as -Q/k.
Arrheniusauftragung
y = -8062,8x + 27,314
0,00
1,00
2,00
3,00
4,00
5,00
6,00
0,00250 0,00270 0,00290 0,00310 0,00330
ln (t
)
0,00350
1/T in 1/K
23°C65°C85°C105°C
Figure 63: Arrhenius plot of time ln(t) versus influence of temperature 1/T during ageing for calcula-
tion of the activation energy
With this slope in Figure 63 one achieves the activation energy for ageing for Z410:
Q = 8062 · 8,31 J/mol = 67 kJ/mol
Figure 64 shows the time which is necessary at a certain temperature for artificial ageing for Z410 to achieve natural ageing of 1 year contents numeric data for this. As a result the stor-age at -20°C of the probes prevents natural ageing for 2 years.
45
0
30
60
90
120
150
180
210
240
270
300
330
360
0 20 40 60 80
Zeit
in T
agen
100 120
Temperatur in °C
Aktivierungsene67 kJ/m
Z410
rgie ol
Figure 64: Required time as a function of temperature for artificial ageing to build up a natural age-
ing of 1 year at RT of Z410
Table 14: Correlation of time and temperature for the ageing behaviour of Z410
temperature in °C -20 0 23 50 80 105 120
time 100 years 10 years 1 year 37 days 4 days 1 day 0,4 days
Umicore supplied probes out of Z410 which have been aged for 20 years Figure 65.
200
225
250
275
300
325
350
375
400
1
Tens
ile s
tren
gth
Rm
in M
Pa
as cast 3 weeks 6 weeks 3 month 6 month 1 year 2 years 20 years
Mean values at RTthickness 1,5 mm
ageing condition
Umicore
Figure 65: Additional value of tensile strength (average value of 25 specimens with 1,5 mm wall thickness, unknown process parameters) after 20 years natural ageing (Umicore)
These probes fit very nicely into the test data and show that after 1 year natural ageing is more or less finished.
46
3.5 Comparison between ZP0400, ZP0410, ZP0430 and ZP0810 Table 15 depicts the chemical composition of the 4 alloys. As discussed in 2.1 and shown in Figure 5 copper increases the strength and increases the solubility of Al. Figure 66 shows the direct comparison for all 4 alloys and a wall thickness of 1,5 mm for center-point-conditions (die temperature 160°C, gate velocity 40 m/s) for tensile strength at room tem-perature.
Figure 66: Ageing behaviour (decrease of tensile strength) in comparison of natural and artificial ageing of Z400, Z410, Z430 and ZA8 as a function of time
Figure 66 shows that all alloys age. Copper increases the strength and decelerates the age-ing behaviour.
Related to the as cast strength after artificial ageing of at 105°C for 24 hours Z400 shows 83%, Z410 83%, Z430 88% and ZA8 86% of the original strength.
Table 15: Chemical composition of the used alloys (weight %)
The copper free alloy ages according to Figure 66 much faster which is in correlation to lit-erature data [op71].
47
80
85
90
95
100
105
110
115
0 0,5 1 1,5 2
Ku2,5 3
Proz
entu
ale
Fest
igke
it Prozentuale Zugfestigkeit nac
künstlicher Alterungkünstlih t nach
cher Alterung
tensile strength as a function of copper
content in %
pfergehalt in Gew.%copper content
Figure 67: Influence of copper on tensile strength (referring to Z410 as 100%) at RT after artificial ageing 24 hours / 105°C
Figure 67: Influence of copper on tensile strength (referring to Z410 as 100%) at RT after artificial ageing 24 hours / 105°C
4 Summary 4 Summary Compared to other die casting alloys zinc-alloys gain the highest mechanical values. The low melting temperature allows high production rates using hot chamber technology and die lives exceed 1.000.000 shots. Zinc alloys can be cast in extremely small wall thicknesses down to 0,5 mm or less. However, the low liquidus temperature leads to ageing phenomena depend-ing on time, changes in measures and creep under load.
Compared to other die casting alloys zinc-alloys gain the highest mechanical values. The low melting temperature allows high production rates using hot chamber technology and die lives exceed 1.000.000 shots. Zinc alloys can be cast in extremely small wall thicknesses down to 0,5 mm or less. However, the low liquidus temperature leads to ageing phenomena depend-ing on time, changes in measures and creep under load.
The results show that all phenomena are thermally activated and follow an Arrhenius law. The activation energy however is different: for ZP0410 and ageing it is approx. 67 kJ/mol and for creep it is approx. 94 kJ/mol.
The results show that all phenomena are thermally activated and follow an Arrhenius law. The activation energy however is different: for ZP0410 and ageing it is approx. 67 kJ/mol and for creep it is approx. 94 kJ/mol.
The maximal solubility of aluminum in zinc is 0,05 weight % at room temperature. Ageing is based upon the segregation of aluminum into cubic face centered phase at room tempera-ture which leads also to the measure changes as the centered cubic face structure has a smaller lattice constant. Copper is basically responsible for the higher mechanical properties but copper also segregates at lower temperatures.
The maximal solubility of aluminum in zinc is 0,05 weight % at room temperature. Ageing is based upon the segregation of aluminum into cubic face centered phase at room tempera-ture which leads also to the measure changes as the centered cubic face structure has a smaller lattice constant. Copper is basically responsible for the higher mechanical properties but copper also segregates at lower temperatures.
Ageing is based upon diffusion. Micro structural investigations using TEM show that ageing is based upon segregation. Aluminum segregates completely. Copper increases the solubility of Aluminum and increases the strength. The measurement shows a reduction of tensile strength after one year of up to 16% at the Z410 alloy. The strongest effect among the tested parameters is the wall thickness which strongly influences the cooling rate and the structure of the material. The influence of the gate velocity and the die temperature is much lower and only changes the mechanical properties by 3-4%. Using an artificial ageing of 105°C and 24 hours all processes influencing the mechanical properties are terminated and the material properties are stable over time.
Ageing is based upon diffusion. Micro structural investigations using TEM show that ageing is based upon segregation. Aluminum segregates completely. Copper increases the solubility of Aluminum and increases the strength. The measurement shows a reduction of tensile strength after one year of up to 16% at the Z410 alloy. The strongest effect among the tested parameters is the wall thickness which strongly influences the cooling rate and the structure of the material. The influence of the gate velocity and the die temperature is much lower and only changes the mechanical properties by 3-4%. Using an artificial ageing of 105°C and 24 hours all processes influencing the mechanical properties are terminated and the material properties are stable over time.
48
The creep behaviour is a self diffusion process which can be described using diffusion kinet-ics. The production parameters only have small influence on the creep behaviour. Overage-ing of the material at 105°C and 1000 hours or at higher temperatures over 150°C and 15 hours or longer increases the creep rate by the factor of 4 – 50. All data have been gained using DOE and statistical analysis.
The creep behaviour is a self diffusion process which can be described using diffusion kinet-ics. The production parameters only have small influence on the creep behaviour. Overage-ing of the material at 105°C and 1000 hours or at higher temperatures over 150°C and 15 hours or longer increases the creep rate by the factor of 4 – 50. All data have been gained using DOE and statistical analysis.
Artificial ageing is always necessary when using zinc alloys to finish the diffusion processes. The artificial ageing at 105°C for 24 hours should be used. Temperatures over 120°C must be avoided as other phase transformations will take place.
Conclusions:
The ageing behaviour of zinc die casting alloys is activated at room temperature and caused by the low solubility of aluminum in zinc at room temperature
Ageing is diffusion controlled. The diffusion process starts immediately after ejection out of the die
For Z410 ageing at room temperature is finished after 1 year, for Z400 half a year to a one year, for Z430 after 2 years (expected) and for ZA8 more than two years (ex-pected)
The drop in tensile strength and yield strength after completed ageing is ~15%
Natural ageing can be simulated by an artificial ageing at 105°C for 24 hours
The mechanical properties of zinc die casting alloys after ageing are high compared to aluminum- and magnesium alloys
The creep behaviour of zinc die casting alloys is caused by self diffusion of zinc and is thermally activated according to Arrhenius law
Creep in zinc die casting alloys is a function of time, the creep rate decreases with time when stress is constant. Up to 1 or 2% of creep elongation no secondary creep could be detected. There for Norton´s law is not the best fit characterize the creep behaviour of zinc die casting parts.
49
50
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