Metallurgy and Friction Stir Welding
Anne DenquinOnera, Châtillon, France
Department of Metallic Structures and Materials
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Basics of the FSW Process
6056-T78 FSW (from Gallais et al., Met Trans (2007) 38A)
Microstructural and relatedhardness evolution resultingfrom thermal cycle and strainencountered during Friction
Stir Welding
From Mishra and Ma,. Materials Science and
Engineering R 50 (2005)TMAZ
Tmax
Base Metal
BaseMetal HAZ HAZ
Welding Direction
εεεεmax
Nugget
TMAZ
HAZ Base
Metal
TMAZ
HAZBase
Metal
Hardness(Hv)
Nugget
2.5mm
NuggetTMAZ TMAZ
HAZBase MetalHAZ
Base Metal
Typical temperaturedomain for Al alloys :200°C (HAZ) to 500°C (Nugget)
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Process window for FSW of Al alloys
Weld quality as function of feed ratio, FSW of AA6061(Dubourg, et al. 6th Int. FSW Symp., St Sauveur, Canada, 2006).
General trends : • Peak temperature increases withrotation speed (ω) and decreases with feedrate (V/ ω)• Welding speed (V) plays a roleon rate of heating
V/ω ����
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The undermatching of FS welds
• The weak zone of the weld is governed by the microstructural evolution through the weld
In the case of aluminium alloys, two extreme cases have to beconsidered :
• precipitation-hardenable alloys(2XXX, 6XXX and 7XXX alloys)
• solid-solution or work-hardened alloys(5XXX alloys)
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Heat treatable alloys : the case of precipitationhardened alloys
FSW of 6056-T78 : Weakest zone near the TMAZ/HAZ interf ace due to thecoarsening of initial precipitates (Denquin et al., Mater. Sci. Forum 3 (2002)).
250nm
Base material
200nm
HAZ
Coarsening of the initial hardening precipitation
TMAZ
200nm
LHZ
Weld nugget :No precipitationGP zones
Coarsening of the initial hardening precipitation
Coarsening anddissolution of theinitial hardening
precipitation
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( from Russell and Shercliff, 1rst Int. FSW Symp., Thousand Oaks (1999)
T6, T7Hv
Dissolution
TransformationCoalescence
distance from the weld centre line
After FS welding
NuggetT6, T7
Hv
distance from the weld centre line
Natural ageing
Weak hardness zone : Coarse precipitationLow supersaturation
Heat treatable alloys : the case of precipitationhardened alloys
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Heat treatable alloys : the case of naturally agedalloys
70
80
90
100
110
-25 -20 -15 -10 -5 0 5 10 15 20 25
Distance from weld centre line (mm)
HV
(10
0g)
HAZ
weld nugget
HAZ
base material H
DA
Z
LHZ
HD
AZ
LHZ
LHZ : Lowest Hardness Zone
FSW of 6056-T4 : Weakest zone near the TMAZ/HAZ interfa ce due to coarse andintense heterogeneous precipitation (Denquin et al., Mater. Sci. Forum 3 (2002)).
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Heat treatable alloys : the case of naturally agedalloys
Effect of thermal cycle
Effect of nucleation sites density
Effect of dynamicrecovery
Effect of dynamicrecristallisation
Weldnugget
TMAZ HAZ
Distance from the weld center line
Density of dislocations
time
solvus temperature of G.P zones
solvus temperature of precipitates
Heterogeneous precipitation
Weld nuggetHDAZ
LHZ
HAZTem
pera
ture
Coarse and intense heterogeneous precipitation in theWeak hardness zone (LHZ)� no hardening potential left in this zone
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Heat treatable alloys : the case of naturally agedalloys
60
70
80
90
100
110
120
-25 -20 -15 -10 -5 0 5 10 15 20 25
Distance from the weld centre line (mm)
HV
(10
0g)
HAZWeld
nugget HAZBase
Material
HD
AZ
HD
AZ
Base material
LHZ
LHZ
FSW+natural ageing
FSW+T78 PWAHT
TM
S F
allM
eetin
g, N
ov. 2
001,
Indi
anap
olis
Influence of a PWHT : no solid solution left in the wea kest zone = no hardening(Denquin et al., Mater. Sci. Forum 3 (2002)).
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Heat treatable alloys : Influence of composition
High trempabilityalloy
6063-T5 FSW alloy :Influence of a post-weld heattreatment (from Sato et al. Metall. Mater. Trans., 30A (1999), 3125)
6056-T4 FSW alloy :Influence of a post-weld heattreatment ( (left : after naturalageing, right : after post-weld heattreatment)
60
70
80
90
100
110
120
-20 -10 0 10 20
T4-W-T78
d (mm)
T4-W-NAHv
base
nugget
Low trempabilityalloy
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Tensile behaviour of FS welds
(Mishra and Ma, Materials Science and Engineering R 50 (2005))
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rupture
0
4
8
12
16
60
80
100
120
-20 -10 0 10 20
stri
ctio
n (%
)
Hv
distance à la ligne de soudure (mm)
HAZ
Tensile behaviour of FS welds Rupture zone
Similar behaviour reported for FS welds of 2024-T3 (Biallas et al., MP Materialprüfung, 42 (2000) 6) and 7075-T6 (Mahoney et al., Metall. Mater. Trans. A 29 (1998)).
6056-T4 FSW +T78Joint efficiency : 78%Elongation to rupture : 2,2%
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0.2% weld plastic strength ≈ (σ0.2) weak zone
Partitioning of plastic deformation � ultimate properties of the weld
� σ0.2<(σm)weak zone � yielding of the weak zones, TMAZ and weld nugget
� strain hardening capability of the weak zones + stress triaxiality
� macroscopic elongation to rupture of the weld
� Preferential deformation of the weak zone until rupture
Local to global tensile properties
0
50
100
150
200
250
300
350
400
Str
ess
(MP
a)
0
10
20
30
40
50
60
70
80
90
100
Elo
ngat
ion
(%)
R0,2
Rm
A%
Weld nugget
TM
AZ
TM
AZ
HAZHAZ
Results obtained from tensilemicro-specimens extractedfrom the FS 6056-T78 weld(Denquin, et al., Welding in the world (2002))
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Non heat-treatable Al alloys
The overall behaviour is governed by the relative strengtheni ngcontributions from grain boundaries, particles and substruc ture .
5083 H19 (work hardened)(from Peel et al., Acta Materialia 51 (2003))
5251 O (solid solution hardened)(from Genevois et al., 5th Int. FSW Symp., Metz, 2004)
1 : transition zone, 2 : TMAZ, 3 : Nugget
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Tensile behaviour of FS welds
(Mishra and Ma, Materials Science and Engineering R 50 (2005))
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Fracture toughness of FS welds
FS welds present higher fracture toughness than the corresponding parent metals
• ���� by fine grain structure and small particles (nugget zon e), • ���� through low yield stress and high ratio of high-angle boun daries
• ���� PFZ and caoarsened particles (HAZ/TMAZ of some welds)
(from Dawes et al., 2nd Int. FSW Symp., Göteborg, 2000)
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Fatigue properties of FS welds : S-N curves
(From Magnusson et al.,2nd symp. on FSW, 2000)
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Fatigue properties of FS welds : defects influence
Joint Line Remnant(JLR)
Cross section of 2198 FS welds after chemical and electr ochemical etching to reveal oxide line and gap-type defect
(FRAE MASAE project, 2011)
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Careful attention must be paid to pre-welding operatio ns
Fatigue properties of FS welds : defects influence
(From T. Le Jolu, PhD thesis, 2011)
Defects detrimental to fatigue behavior result from mat erial flow in the nugget because of bad positioning of coupons be fore welding
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Fatigue behaviour of FS welds : fatigue crack propagation
Influence of residual stresses on fatigue crack propa gation (similar behaviourobtained for 2024-T3 alloy),
(from Dalle Donne et al., 2nd Int. FSW Symp., Göteborg, 2000).
Taking into accountcompressive residual stresses at the crack tip region in the FSW weldsthrough the effective stress intensity factor range ∆Keff
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Corrosion Properties of FS welds
Corrosion attack of FSW 7075Al-T651following extended exposure to a solution of 4 M NaCl–0.5M KHO3–0.1 M HNO3 diluted to 10%
(after Lumsden et al., Corrosion 55 (1999)).
FS welds are susceptible to intergranular attack in th e HAZ, TMAZ and in the weld nugget
(From Mishra and Ma / Materials Science and Engineering R 50 (2005))
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Corrosion Properties of FS welds
Precipitate microstructures in the grain interior and a long grain boundaries in a 7050 T6 FSW : (a) parent material, (b) HAZ, (c) TMA Z I, (d) TMAZ II,
(from Su et al., Acta Mater. 51 (2003))
Large influence of microstructural evolution on corrosion behavior of FS welds
(a) (b)
(c) (d)
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How to optimise friction stir welds ?
• Precipitation evolution resulting from the thermal and mechanicalcycles during FS processing plays a major role on :
• Static properties (structural behaviour, forming operations)• Corrosion resistance
� Optimization through process parameters (but staying insidethe process windows)
� Optimisation through post weld thermal or thermo-mechanical treatment (hardening heat treatment possible but may bedetrimental to elongation of the weld, stretching may favour hardeningprecipitation…)
• Fatigue properties are governed by : • Residual stresses (may be detrimental for thick products)• Weld asperities or defects (such as shoulder trace, root flaw or GAP-typedefect)
� Optimisation through process parameters, pre or postwelding operation
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How to optimise friction stir welds ?
� Further work needed : � To clarify influence of various microstructural zones on :
• Fracture thoughness• Corrosion behaviour
� To understand defect formation in the nugget, in relation with material flow
� Optimising behavior of structural panels :� Modeling the influence of FSW process on in-service joint performance :
coupling thermal, microstructural, strength and strain hardening modelsCf PhD of C. Gallais, Aude Simar (Progress in Matrials Science, 2012)
� Welding of dissimilar aluminium alloys