Warm and Room Temperature Deformation of Friction Stir Welded Thin

Materials and Design 31 (2010) 1392–1402

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Materials and Design

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Warm and room temperature deformation of friction stir welded thinaluminium sheets

E. Cerri *, P. LeoDept. of Innovation Engineering, University of Salento, via per Arnesano, 73100 Lecce, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 June 2009Accepted 28 August 2009Available online 2 September 2009

Keywords:Warm temperature deformationTensile testMicrohardnessFSWAluminium alloys

0261-3069/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.08.044

* Corresponding author. Tel.: +39 0832 297324; faxE-mail address: [email protected] (E. C

Friction stir welding is a welding solid state process of large potential advantages for aerospace and auto-motive industries dealing with light alloys. The metal to be welded is not melted and this avoids weldingdefects such as cracks and porosity. Moreover, there is no significant deterioration in mechanical prop-erties due to phase transformations in the joint and low-cost and high-quality joints can be producedeven from heat-treatable aluminium alloys, notably difficult to weld. In this study, very thin rolled sheets(0.8 mm in thickness) of 2024T3 and 6082T6 were friction stir welded, parallel to the rolling directions,obtaining similar joints (2024T3–2024T3 and 6082T6–6082T6) and dissimilar joints (6082T6–2024T3).Tensile tests at temperatures and strain rates of 170–230 �C and 10�3–10�5 s�1 respectively wereperformed on the thin joints. The flow stress decreased with increasing temperature and decreasingstrain rate. The ductility was quite independent from temperature and strain rate. The tensile stress–strain curves of the thin dissimilar joints placed at an intermediate level between the high strength2024T3–2024T3 and low strength 6082T6–6082T6 flow curves. The fracture occurred in the middle ofthe stir zone for all the investigated joints and was of ductile type. Microhardness profiles were slightlymodified by straining.

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1. Introduction

Friction stir welding (FSW) is a solid state joining method par-ticularly suited for aluminium alloys, which are difficult to be fu-sion welded without hot cracking, porosity or distortion. Duringwelding, the material is frictionally heated to a temperature atwhich it becomes more plastic. The heat of friction and plastic flowarising from the rotating tool produce significant microstructurechanges, which lead to local variations in the mechanical proper-ties of the weld [1–5]. FSW is being targeted by the industry forstructural demanding applications to provide high-performancebenefits [6]. The FSW zone consists of a stir zone or nugget, a ther-momechanically affected zone (TMAZ) and a heat affected zone(HAZ). The grain structure in the stir zone is very fine and equiaxedcausing a higher mechanical strength and ductility [7–13].

In FSW the work piece does not reach the melting point and themechanical properties, such as ductility and strength of the weldedzone, are much higher compared to the traditional techniques [14].The very attractive mechanical properties are principally due to thestrong grain refining effect of the process. The data available in lit-erature demonstrated that the mean grain sizes resulting from theFSW process are at least 10 times smaller than those measured in

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the undeformed parent material [15,16]. In short, the results ofthese studies suggest that FSW can cause grain refinement throughsevere plastic deformation. In particular, a single pass of FSW pro-duces an estimated effective strain >40 [17] resulting in a mean0.5–10 mm grain size structure.

The strong plastic deformation of the material during weldingoperation and the advantage that an optimized process reachestemperature close to 400 �C in the nugget, lead to a strong recrys-tallized structure with high performances in terms of strength andpotential superplastic properties. From this point of view, someauthors demonstrated that the microstructure in the stir zoneevolves through a continuous dynamic recrystallization process[10]. Actually, FSW is mainly used for joining similar materials,even if few systematic studies have been performed to observethe effect of combination of alloys with different composition. Thisis a core demand of aircraft and car industries to substitute the tra-ditional joining technologies with low costs and high efficiencyones such as FSW in the future advanced design.

Recent literature reports examples of FSW of dissimilar alumin-ium alloy sheets [18–20], as well as aluminium–steel [21,22], alu-minium–magnesium [23] and aluminium–silver joints [24]. FSWof very thin sheets needs to be explored as well as the mechanicalproperties of the resulting joints. FSW of very thin sheets is ex-pected to have many difficulties due, for example, to the highamount of heat lost towards the backing plate, to the necessity

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E. Cerri, P. Leo / Materials and Design 31 (2010) 1392–1402 1393

of using tools without particular features (because of small probedimensions) and to forging the material. Moreover, for micro-sheetjoining techniques the anisotropic effects must be considered sincethey increase dramatically due to specimens dimensions.

The aim of this work is to study the deformation behaviour atwarm and room temperatures of FSW aluminium alloy sheets0.8 mm thick, combined in similar and dissimilar butt joints. Thejoints will be characterized by metallography, microhardness mea-surements, tensile tests at room and warm temperature of defor-mation and X-rays diffractometry to verify their stability andperformances.

2. Experimental procedures

2.1. Materials and welding conditions

The chemical compositions of the aluminium alloys used in thisstudy are illustrated in Table 1. Rolled sheets of 2024T3 and6082T6 aluminium alloys 0.8 mm thick, have been friction stirwelded by a cylindrical non-threaded probe of 1.7 mm in diameterand 0.6 mm in height. The tool shoulder was 6 mm in diameter.The welding tool was made of 56NiCrMoV7 tool steel because ofits good strength and stability in the process. Base materials werewelded parallel to the rolling directions with a weld and rotationalspeed of 762 mm/min, 2085 rpm for 6082T6–6082T6 and 6082T6–2024T3 (dissimilar) joints, and 460 mm/min, 1800 rpm for2024T3–2024T3 joints, respectively. The welding parameters wereoptimized taking into account the mechanical properties of thesofter alloy. The used parameters represent the best combinationthat maximizes the transversal strength of the 2024T3 and6082T6 joints and were experimentally identified. Moreover, thedissimilar joint was produced with the high strength alloy(2024T3) positioned on the retrieving side and the 6082T6 (thesofter one) on the advancing side, based on the idea that the micro-structure and the mechanical properties of the stirred zone aregoverned by the retrieving side of the joint.

2.2. Metallography

After FSW process, the joints were sectioned perpendicular tothe welding direction by an electro-discharge cutting device formetallographic investigations. Metallography of the joints andbase materials were performed by a Nikon Epiphot equipped withthe image analyzer. For light microscopy, samples were mechani-cally ground and polished up to 1 lm diamond paste. Chemicaletching was performed by Keller’s reagent and if anodized, a solu-tion of 5% HBF4 in methanol was used to reveal grains by polarizedlight. The grain size and microstructural features were statisticallyanalyzed by considering at least 300 features for calculating meansize and standard deviation. The statistical analysis was performedby using the linear intercept method. The lines were imaged inindependent directions and, usually, vertical and horizontalstraight lines at regular intervals have been considered.

2.3. Mechanical tests

After metallographic investigations, microhardness profileswere measured in the cross section of the thin welded joints by

Table 1Chemical composition (% in weight) of the base materials.

Cu Mg Mn Fe Si Zn Cr Ti Al

6082T6 0.08 0.78 0.48 0.39 0.95 0.04 0.03 0.05 rem2024T3 4.67 1.34 0.63 0.25 0.15 0.02 0.01 0.06 rem

applying a load of 500 g for 15s. The surfaces were electropolishedbefore microhardness measurements. Specimens for tensile tests,with a gauge length of 30 mm, a width of 10 mm and a thicknessof 0.8 mm according to UNI EN 1002/5 (top of Fig. 1a), weremachined from the FSW sheets in order to have the loading axisnormal to the welding direction (example in Fig. 1a). An exten-someter was employed to measure the elongation and it wasplaced in order to have the mid line of the weld correspondentto the mid point of the extensometer. A cross-sectional view ofa tensile sample is reported in Fig. 1b. The tests were performedat 170, 200 and 230 �C and nominal strain rates of 10�3, 10�4,

Fig. 1. Geometry of specimens used for tensile tests showing the scheme accordingto UNI EN 1002/5 (top) and real samples (bottom) and (b) cross-sectional view of asample after rupture.

1394 E. Cerri, P. Leo / Materials and Design 31 (2010) 1392–1402

5 � 10�5 s�1 to study the warm temperature deformation behav-iour of the similar and dissimilar joints. A set of tests was alsorun at room temperature at a nominal initial strain rate of10�4 s�1. Three tests were run at least for each experimental con-dition to assure measurements accuracy. All the tensile tests wereperformed by an Instron 4485 equipped with a cylindrical furnacefor warm temperature deformation tests. The true stress, r (MPa),and true strain, e were calculated according to the followingformula:

r ¼ P=A0ðeþ 1Þ and e� lnðeþ 1Þ

where P is the tensile load (N), A0 is the original cross sectional areaof the specimen (mm2) and e is the engineering strain.

X-rays diffraction was performed on several strained samples tostudy lattice parameter and the presence of precipitate in strainedregions.

a

Base material 2024T3

c

HAZ

e

Nugget-TM

Fig. 2. Optical micrographs of the 2024T3–2024T3 joint showing (a) the base material,side), (e) the zone between the TMAZ and the nugget (advancing side), (b) statistical di

3. Results and discussion

3.1. Microstructure of the joints

Optical micrographs of characteristic zones of friction stirwelded joints are illustrated in Figs. 2–4. Fig. 2 refers to 2024T3–2024T3 joint. Fig. 2a shows the microstructure of the base materi-als, that consists of equiaxed grains of (13 ± 5) m measured in theshort transverse rolling plane (Fig. 2b). The grain size in the HAZresults substantially similar to the base material (Fig. 2c), whilethe microstructure in the TMAZ is quite different. The grains areplastically deformed as a consequence of the stirring occurring inthe retrieving (Fig. 2d) and in the advancing side (Fig. 2e) of thenugget. No recrystallized grains are found in the TMAZ of the sim-ilar 2024T3 joint. In the 2024T3, the high density of particles ofAlMnCu and AlCuMg type permits large resistance to recrystallisa-tion, as shown by TEM [25].

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nt

equivalent diameter [μm]

base material 2024-2024

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Retriet. TMAZ Nugget

AZ Advanc.

(c) the HAZ, (d) the transaction zone between the TMAZ and the nugget (retrievingstribution of equivalent grain size in the base material.

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6082T6-6082T6 nugget

f

a

6082T6 Base material

d

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HAZ

c

e

Nugget

Fig. 3. Optical micrographs of the 6082T6–6082T6 joint showing (a) the base material, (c) the HAZ, (d) the transaction zone between the TMAZ and the nugget (retrievingside), (e) the nugget of the weld. Statistical distributions of equivalent grain size (b) in the base material and (f) in the nugget.


Fig. 3 shows the characteristic zones of the 60682T6–6082T6joint. The base material (Fig. 3a) consists of equiaxed grainsof (14 ± 5) l measured in the short transverse rolling plane(Fig. 3b). The HAZ (Fig. 3c) is very similar to the base materialand no substantial difference in grain size is encountered. The bor-der between the TMAZ and the nugget is reported in Fig. 3d show-ing how the plastic deformation induced by the rotational tool hasaffected the morphology of the grains. When the stir zone is ap-proached, the grains are completely recrystallized (Fig. 3e) withan equivalent grain dimension of 2.5–3 lm (Fig. 3f).

Fig. 4a shows a composition picture of the dissimilar joint(6082T6–2024T3) with the retrieving side on the 2024T3. Themicrographs illustrate the base material, the HAZ, TMAZ and thenugget (the stirred zone) where the two aluminium alloys intersecteach other during the severe plastic deformation. This puzzle ofphotos clearly shows that the very thin joints are of good quality

because no defects or porosity have been detected nor in the2024T3 or in the 6082T6 type joints. Fig. 4b is again a compositionof the other side, i.e. of the advancing side. The contrast may be dif-ferent for the two materials respect to Fig. 4a because of the use ofdifferent chemical etchants and/or anodisation to reveal the twoaluminium alloys. The pictures demonstrate the complete welda-bility of these two alloys at the solid state. Fig. 4c and d illustratethe HAZ and the TMAZ of the advancing side, showing no differ-ence in dimensions respect to 6082T6 in the similar joint.

3.2. Hardness profiles

The differences in microstructure provide a tool to understandthe hardness profiles measured across the three welds. Fig. 5shows the hardness profile across the welds, starting 20 mm apartfrom the centre of the weld, in the advancing and retrieving

Fig. 4. Optical micrographs of the 6082T6–2024T3 dissimilar joint showing (a) the retrieving side of the joint from the 2024T3 to the stir zone, (b) the advancing side of thejoint from the 6082T6 to the stir zone, (c) the HAZ and (d) TMAZ in the advancing side of the joint.


directions. The profiles substantially differ in the joints, accordingto the thermal treatments and microstructure modifications thatoccurred during the FSW process [26]. The hardness profile ofthe 2024T3–2024T3 weld (Fig. 5a) reports values in the range of130–140 HV for the stir zone. These fluctuations should be dueto onion ring structure of the nugget and local associate precipitatedistribution which influence microhardness measurements frompoint to point [27,28]. The microhardness is slightly higher in thenugget respect to the THAZ and HAZ because of the finer grain sizepresents in the nugget. It increases to 140 HV at 17–18 mm fromthe centre of the nugget, both in the advancing and retrieving side.The microstructure modification in the stir zone respect to the basematerial as a consequence of the FSW process can also be shown byX-rays analysis. In fact, X-rays diffraction peaks of Al (Fig. 5b) showvery different grain orientation distributions in the stir zone and inthe base material (at a distance of 50 mm from the nugget centre).In particular, after FSW, the microstructure (grains of Al) of thenugget shows an intensification of the (1 1 1) and (2 2 0) peaksfor planes parallel to the surface of the nugget, while in the2024T3 base material their intensity was much lower. Moreover,

no significant shift in the lattice parameter of the Al cell was veri-fied in the nugget respect to the base material.

The HV profile for the 6082T6–6082T6 (Fig. 5c) shows less scat-tered values respect to the 2024T3 FSW joint, but a drop of almost30 HV in the stir zone is visible. The starting condition of the 6082sheet is T6 and during the FSW process, the temperature in thenugget may even reach 350–450 �C [10,29], depending on experi-mental conditions. From a microstructure point of view, this canlead to an evolution of precipitates towards coarsening or dissolu-tion (partly solubilisation) and create a decrease in hardness, evenif the grains are refined by the process [30]. Moreover, the FSWprocess determines a substantial change in Al lattice plane dis-tances of the nugget respect to the base material as shown inFig. 5d by the Al peak shift. In fact, the initial state of the sheetwas T6 meaning a state of structure relaxation much higher if com-pared with the state of stress of the stir zone. For the 2024T3 sim-ilar FSW sheets, this was not the case because of the T3 initial stateof the joints. X-rays diffractometry also shows an increment ofgrain preferential orientations in the stir zone respect to the basematerial, as for the 2024T3 similar FSW sheets. The welding

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6082-6082

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Cps

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6082-6082 T6

a b

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Fig. 5. Microhardness profiles of the (a) 2024T3–2024T3 joint, (c) 6082T6–6082T6 joint and of (e) 6082T6–2024T3 dissimilar joint in the as-received state, (b) and (d) X-raysdiffractometry showing variations in Al peak intensity of the base materials and the nuggets.


process introduces or modifies texture in the stir zone respect tothe initial state of the materials and but the study of these aspectsis beyond the aim of the paper.

Fig. 5e reports the hardness profile of the dissimilar joint(6082T6–2024T3) that looks like a mixture, in the nugget zone,of the 2024T3 in the retrieving side and the 6082T6 in the advanc-ing side.

3.3. Mechanical properties at room and warm temperature ofdeformation

The evaluation of tensile properties at room temperature of theFSW joints is reported in Fig. 6. The elastic part of the stress–straincurves exactly overlap showing a good correspondence in elasticmodulus even for the dissimilar joint (24–25 GPa). The plastic

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.070

100

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300

400

500

true

stre

ss [M

Pa]

true strain

RT2024-2024T3

6082-6082T6

6082-2024

Fig. 6. Stress–strain curves for similar and dissimilar joints at room temperature.


behaviour is illustrated by strain hardening occurring in all thejoints. The 2024T3–2024T3 weld reports the higher strength(450 MPa) and ductility (true strain 0.065), while the 6082T6–

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fsw fsw + tensile tested

2024 retreating

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160


HV 0,

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2024-2024T3


a

c

Fig. 7. Comparison of microhardness profiles in the joints before and after tensile tests2024T3.

6082T6 reaches 230 MPa in strength and 0.03 in true strain. Thedissimilar joint shows a medium strength of 260 MPa but has thelower ductility.

Microhardness measurements have been performed on thejoint samples also after straining (Fig. 7). The strain hardening con-tributes to hardness curves in all the investigated alloys, even if thepercentage of contribution is different when comparing the nug-gets and the TMAZs [26]. In the 6082T6–6082T6 joint (Fig. 7b),the strain hardening increment is max in the nugget zone thatwas the softer one before deformation. In the 2024T3–2024T3 de-formed joint (Fig. 7a), the hardness slightly increases in the nuggetand in the TMAZ respect to the unstrained sample. The dissimilarjoint shows a hardness trend that is a mixing of the two alloys de-scribed upon (Fig. 7c).

Several tensile tests have been performed at temperatures inthe range of 170–230 �C to study the mechanical response of thethin joints at warm temperatures of deformation. Some of theexperimental stress–strain curves are illustrated in Fig. 8 for FSWjoints deformed at 170 �C and 10�3 s�1, 230 �C and 10�3 s�1,170 �C and 10�5 s�1. The tensile curves of the dissimilar jointsare higher than the 6082–6082T6 curves but they remain softerthan the harder joints (2024–2024T3), at all temperatures andstrain rates investigated. Moreover, the ductility of the dissimilarjoints is very poor compared to the elongation of 2024–2024T3

5 10 15 20

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eld center [mm]

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6082-6082T6

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170°C, 10-3 s-1

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True

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170°C, 10-5 s-1

2024T3-2024T3

6082T6-6082T6

6082T6-2024T3

a b

c

Fig. 8. Stress–strain curves at warm temperature of deformation for all the type of investigated joints at (a) 170 �C, 10�3s�1, (b) 230 �C, 10�3 s�1 and (c) 170 �C, 10�5 s�1.


and 6082–6082T6 joints, at every experimental condition and evenif compared to other results [31].

A complete evaluation of tensile test results is illustrated inFig. 9 at all temperatures and strain rates investigated. The valuesof the Yield Strength (YS), Ultimate Tensile Strength (UTS) and truestrain e are reported in plot columns in Fig. 9a for the 6082T6–6082T6, in Fig. 9b for the 2024T3–2024T3 and in Fig. 9c 6082T6–2024T3. The strength values in Fig. 9a demonstrate that there isa trend for the UTS to slightly decrease with increasing tempera-ture and, keeping constant the temperature, they decrease withstrain rate. The YS is not very influenced by deformation processvariables as also the strain e does. For the 6082T6–6082T6 thinjoints, the UTS follows the classical trend of slightly decreasingwith strain rate or with increasing temperature of deformation.The YS shows some scatter respect to the behaviour describedfor the UTS and the strain e remains almost constant and indepen-dent from temperature and strain rate of deformation. The thin dis-similar joints (Fig. 9c) present values of UTS and YS quite similarbecause of fracture occurred after few percentage of elongation.The strain is in fact rather poor and mostly constant.

In order to have indications on the fracture mechanisms,microhardness have been measured on the longitudinal sectionsof a set of warm deformed samples. The results have been re-ported in Fig. 10. Fig. 10a shows the hardness as a function of dis-tance from fracture surface for the thin 6082T6 similar jointsdeformed at 230 �C, 10�3 and 10�5 s�1, while Fig. 10b illustratesthe measurements for the thin dissimilar joint compared with

the 2024T3–2024T3 one, tensile tested at 230 �C and 10�4 s�1.In the first graph, the hardness maintains higher values at thelower strain rate of deformation. The value of strain is very similarand there is a drop in hardness just close to the fracture surface.The idea is that the longer exposure (occurring at a strain rate of10�5 respect to 10�4 s�1) at 230 �C during straining enhanceshardening precipitation of incoherent phases (stable) that have amore homogeneous effect on strain hardening distribution[26,30]. The fracture develops in the middle of joint. In Fig. 10bthe differences are very light when considering the 2024T3 sideof the weld for the similar and dissimilar joints. The elongationof the dissimilar joint is half of the 2024T3–2024T3. The fracturealways occurred in the middle.

The mode of fracture of tensile tested samples is shown inFig. 11. Fig. 11a is a micrograph of the fractured zone. The ruptureoccurred in the middle of the stir zone after a light necking. Thisbehaviour occurred in all the investigated samples and suggestedthat the stir zone has a lower plastic deformation capability thanthe rest of the sample. For more accurate examinations of fracturesurfaces, tensile samples were cut close to the fracture surface, inthe direction perpendicular to the tensile axis, for scanning elec-tron microscopy observations. Fig. 11b is a fracture surface of thedissimilar joint tensile tested at 230 �C, 10�4 s�1, showing a ductilemode of fracture, as found in all the condition investigated. In thissample, moreover, layers of different thickness (Fig. 11c) are visibledue to the presence of the two aluminium alloys in the stir zone,together with fine dimples.

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Fig. 9. Tensile properties (Yield Strength, Ultimate Tensile Stress and true strain e) of the joints at warm temperature of deformation. (a) 2024T3–2024T3 similar joint, (b)6082T6–6082T6 similar joint and (c) 6082T6–2024T3 dissimilar joint.


Fig. 11. Mode of fracture of the 6082T6–2024T3 FSW joint tensile tested at 230 �C, 10�4 s�1. (a) Low magnification picture showing the fracture occurring in the middle of thestir zone. (b) SEM picture of fracture surface in the stir zone and (c) high magnification micrograph showing fine dimples.

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distance from fracture surface, mm

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tensile tested at 230°C

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distance from fracture surface, mm

2024-2024T3 dissimilar

tensile tested at 230°C, 10-4 s-1

a b

Fig. 10. Comparison of microhardness profiles in the joints before and after tensile tests performed at warm temperature for (a) 6082T6–6082T6 deformed at 230 �C at strainrates of 10�3 and 10�4 s�1, (b) 2024T3–2024T3 strained at 230 �C and 10�4 s�1.


4. Conclusions

Friction stir welded joints of 2024T3–2024T3, 6082T6–6082T6 and 6082T6–2024T3 of very thin thickness (0.8mm) were investigated at room and warm temperatures of

deformation by tensile tests and microhardness measure-ments in a range of 170–230 �C and 10�3–5 � 10�5 s�1. Themicrostructure evolution was studied by light microscopy, SEMand X-rays. The following conclusions were drawn from theresults.


– All the thin FSW joints showed the capability to undertake ten-sile stress at room temperature and at warm temperatures ofdeformation. The flow stress curves of the thin dissimilar jointswere in between the similar 6082T6–6082T6 and 2024T6–2024T6 joints at the investigated experimental conditions.

– The stress decreased with increasing temperature and decreas-ing strain rate. The ductility of the thin joints was quite indepen-dent by temperature and strain rate.

– Substantial modifications in grain size resulted during the FSWprocess as evidenced by optical microscopy, while X-rays dif-fractometry showed the variation of lattice plane distances inthe stir zone. The nuggets, initially constituted by recrystallizedgrains with a slight different lattice parameter respect to thebase material, showed microstructure modification duringwarm tensile straining, as confirmed by microhardness.

– Tensile specimen fractured in the middle of the stir zone afterstraining at room temperature or at warm temperature of defor-mation in a ductile mode. Anyway, even if the ductility wascomparable in different specimens, the microhardness valuesfall down at different distances from the rupture point. Thiscan be attributed to microvoids formation and coalescence,depending on particle type they encountered.

Acknowledgement

The authors would like to thank Ing. P.P. De Marco and Ms.Todisco for precious help in laboratory techniques.

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Warm and Room Temperature Deformation of Friction Stir Welded Thin

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