Metallurgy and Friction Stir Welding

Anne DenquinOnera, Châtillon, France

Department of Metallic Structures and Materials

2

Dire

ctio

n -

Con

fére

nce

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)

3

Dire

ctio

n -

Con

fére

nce

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 ����

4

Dire

ctio

n -

Con

fére

nce

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)

5

Dire

ctio

n -

Con

fére

nce

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

6

Dire

ctio

n -

Con

fére

nce

( 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

7

Dire

ctio

n -

Con

fére

nce

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)).

8

Dire

ctio

n -

Con

fére

nce

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

9

Dire

ctio

n -

Con

fére

nce

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)).

10

Dire

ctio

n -

Con

fére

nce

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

11

Dire

ctio

n -

Con

fére

nce

Tensile behaviour of FS welds

(Mishra and Ma, Materials Science and Engineering R 50 (2005))

12

Dire

ctio

n -

Con

fére

nce

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%

13

Dire

ctio

n -

Con

fére

nce

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))

14

Dire

ctio

n -

Con

fére

nce

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

15

Dire

ctio

n -

Con

fére

nce

Tensile behaviour of FS welds

(Mishra and Ma, Materials Science and Engineering R 50 (2005))

16

Dire

ctio

n -

Con

fére

nce

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)

17

Dire

ctio

n -

Con

fére

nce

Fatigue properties of FS welds : S-N curves

(From Magnusson et al.,2nd symp. on FSW, 2000)

18

Dire

ctio

n -

Con

fére

nce

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)

19

Dire

ctio

n -

Con

fére

nce

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

20

Dire

ctio

n -

Con

fére

nce

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

21

Dire

ctio

n -

Con

fére

nce

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))

22

Dire

ctio

n -

Con

fére

nce

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)

23

Dire

ctio

n -

Con

fére

nce

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

24

Dire

ctio

n -

Con

fére

nce

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