稀有金属(英文版) 2019,38(11),1070-1077

Microstructure and mechanical properties of friction stir welding joint during post weld heat treatment with different zigzag lines

Zhi-Li Hu Qiu Pang Ming-Liang Dai

Hubei Key Laboratory of Advanced Technology of Automobile Components,Wuhan University of Technology

Hubei Collaborative Innovation Center for Automotive Components Technology,Wuhan University of Technology

School of Mechanical and Electrical Engineering,Wuhan Donghu University

作者简介：*Qiu Pang e-mail:pqiuhit@126.com;

收稿日期：18 October 2017

基金：financially supported by the National Natural Science Foundation of China(Nos.51775397 and 51405358);the China Automobile Industry Innovation and Development Joint Fund(No.U1564202);the Program of the Ministry of Education of China for Introducing Talents of Discipline to Universities(No.B17034);the Open Fund of Shanghai Key Laboratory of Digital Manufacture for Thin-Walled Structures(No.2017002);

Microstructure and mechanical properties of friction stir welding joint during post weld heat treatment with different zigzag lines

Zhi-Li Hu Qiu Pang Ming-Liang Dai

Hubei Key Laboratory of Advanced Technology of Automobile Components,Wuhan University of Technology

Hubei Collaborative Innovation Center for Automotive Components Technology,Wuhan University of Technology

School of Mechanical and Electrical Engineering,Wuhan Donghu University

Abstract：

Zigzag line is a common defect in friction stir welding(FSW) joint.The formation mechanism of the zigzag line in Al-Cu alloy FSW joint and its influence on the microstructure and mechanical properties during post weld heat treatment(PWHT) were studied by scanning electron microscopy(SEM),microhardness and tensile tests.It is found that the occurrence of zigzag line for PWHT joint is determined by PWHT process which in nature depends on residual stress and thermal stress of FSW joint.The optimization of PWHT process to reduce the residual and thermal stress can trigger for the deterioration of mechanical properties of PWHT joints with zigzag line.No obvious decrease of tensile properties is observed for T6-450 and T6-495 joints although zigzag line appears in the weld.PWHT determines the sizes of zigzag line cracks and consequently determines the fracture location and characteristics of FSW joint.

Keyword：

Friction stir welding; Post weld heat treatment; Zigzag line; Mechanical properties;

Received： 18 October 2017

1 Introduction

Friction stir welding (FSW),as a relatively new solid-state joining technique,exhibits many attractive advantages compared to fusion welding techniques [ 1, 2, 3] .Therefore,it has been widely studied on the optimization of the welding process to avoid welding defects and obtain high-quality joints for aluminum alloys [ 4, 5, 6, 7, 8] .Among various defects of the FSW joints,zigzag line has been proved to be a common and serious defect,which attracts a growing scientific interest [ 9, 10, 11, 12] .Most of the investigations on zigzag line of FSW joints are based on the reports of Liu et al. [ 10] ,Okamura et al. [ 11] and Sato et al. [ 12] .These investigations are reported that the zigzag line which initials form the bottom and run across the weld is mainly caused by the fragment of oxide layer in the base material surface during FSW.Optimization of FSW process,such as increasing the plunging and rotating speed of the welding tool,can prevent the formation of the zigzag line by improving welding heat input to break up of oxide layer in the contacting surfaces [ 13, 14, 15] .No welding defect and zigzag line can be detected in the etched cross-sectional weld of the as-welded joint with high heat input.However,the zigzag line,disappearing in the weld of as-welded joint,is still observed in the weld for the PWHT joints [ 16, 17, 18] .Formation mechanism of the zigzag line in FSW joint is still unclear,especially for PWHT joint.

Moreover,until now a large number of experimental studies have shown that the zigzag line hardly influences the mechanical properties of FSW 6xxx alloy weld [ 10, 19] and the tensile fracture behavior of FSWed 2024A1-T351joints [ 20] .However,it is evident that kissing bond severely affects the mechanical properties of the welded joint [ 15, 18, 21, 22, 23, 24, 25] ,especially for the fatigue lives and strength of the FSW 2198A1 [ 22] and 7475Al joints [ 23] .Compared with fatigue lives and strength of the sound welds,FSW joints with oxide array are much shorter and always below 50%those of contrast [ 23] .The effect of zigzag line on mechanical properties of as-welded joint is still controversial.But it is consistent with the opinion that the mechanical properties of PWHT weld can be seriously deteriorated due to the zigzag line [ 10, 15, 16, 18] .The mechanical properties of the heat-treated joint,especially the elongation,are considerably deteriorated with the fracture being along zigzag line after PWHT.It is noted that aluminum alloy is very prone to be oxidized and form compact Al₂O₃ layer on the surface [ 16] .Therefore,it is difficult to completely eliminate zigzag line in the weld just by clearing the butt surfaces to remove Al₂O₃ layer before FSW,although many papers report that the entrapped oxides are eliminated by proper cleaning prior to welding,and via weld tool position and welding parameters.The zigzag line,disappearing in the weld of as-welded joint,is always observed for PWHT joints [ 18, 26] .

Since the formation mechanism of zigzag line in FSW joint during PWHT is not clear,it is important to study the origin and evolution of zigzag line in FSW joint during PWHT and investigate the effect of zigzag line on the microstructure and mechanical properties,finally find the methods to reduce the damage of the mechanical properties of the joints caused by zigzag line.

2 Experimental

The 2024-O aluminum alloy plates were provided in the dimensions of 380.00 mm×75.00 mm×2.88 mm.The plates were FSW vertical to the rolling direction with the travel speed and rotational speed of 100-300 mm·min^-1and 600 r·min^-1,respectively.The FSW tool had shoulder diameter of 14 mm and cylindrical tapered pin diameter of6 mm at top and 4 mm at bottom,2.88 mm in length.It should be noted that all the samples were directly welded without any removing surface oxide layer before FSW.After welding,the joints were cross-sectioned perpendicular to the weld direction for metallographic and mechanical property tests.The cross sections were polished and etched with Keller's reagent,then were observed by the Axio Imager Vario Zeiss optical microscopy (OM).The microhardness test was carried out along the centerlines of the cross sections by HVS-1000 under a load of 0.98 N for10 s.The tensile specimens were performed at a crosshead speed of 2 mm·min^-1 using an Instron-1186 testing machine.The specimens were pided into two parts,one part for as-welded and the other for PWHT.In order to study the origin and evolution of zigzag line in FSW joint during PWHT,five PWHT processes were used:(1) T6heat treatment which included solution treatment at 495℃for 40 min,and quenching in cool water,then followed by artificial aging at 190℃for 6 h (abbreviated as T6);(2) O heat treatment which included heating to 400℃for120 min,then cooling in the furnace to room temperature(abbreviated as O);(3) O heat treatment and then T6 heat treatment with air quenching (abbreviated as O+T6A);(4) O heat treatment and then T6 heat treatment with water quenching (abbreviated as O+T6 W);(5) new T6 heat treatment which included solution treatment at 450℃for40 min,and quenching in cool water,then followed by artificial aging at 190℃for 6 h (abbreviated as new T6).After PWHT,the specimens were cut perpendicular to the welding direction and were electrolytically polished in50 ml HClO₄+150 ml C₂H₅OH solution,then were observed under a ULTRA PLUS-43-13 Zeiss field emission scanning electron microscope (FESEM).

3 Results and discussion

3.1 Microstructural features of zigzag line in FSW joint

The effect of welding heat input on zigzag line of aswelded joint is shown in Fig.1.As reported by previous studies [ 10, 11, 12] ,zigzag line apparently crosses through the central of the weld after etching with relatively lower welding heat input (Fig.la).But it disappears for higher heat input (Fig.lb).According to the study of Okamura et al. [ 11] and Sato et al. [ 12] ,the oxide layer is broken by the stirring effect during FSW,and the higher welding heat input always leads to better effect of the scattering and dilute distribution of the oxide particles.Therefore,zigzag line also exists actually,but it becomes not so obviously.

The influence of welding heat input on zigzag line during PWHT is illustrated in Fig.2.After PWHT,all the joints regardless of the level of heat input show zigzag line(Fig.2a,b),which may not be found in the as-welded joint(Fig.lb).It is noted that PWHT joint with higher welding heat input exhibits a better suppressing effect on zigzag line and abnormal grain growth (AGG).Zigzag line and AGG in the weld are more obvious for the joint with low welding heat input after PWHT at 450℃.However,zigzag line becomes more obvious as PWHT temperature increases from 450 to 495℃.The difference of zigzag line and AGG in the weld is eliminated for two joints after PWHT at 495℃regardless of the level of heat input.It is proved that PWHT temperature determines the occurrence of zigzag line defects for PWHT joint.

It is well known that thermal cycling during welding inevitably leads to a certain degree of residual stress in FSW joint.The residual stress distribution of as-welded joints exhibits an M-shape,and the lateral residual stress in the weld nugget is subjected to tensile stress [ 27] .As mentioned above,the oxide layer is broken by the stirring effect during FSW and always distributes at the grain boundary.The linear expansion coefficient of oxide is smaller than that of aluminum matrix.The difference in thermal expansion coefficient between these oxide particles and base metal (BM) tends to lead to great thermal stress during subsequent heat treatment,as shown in Fig.3 (σmeans stress andεmeans strain).When the thermal expansion force combined with the residual tensile stress is greater than the bonding strength of the particles andmatrix,the crack is generated which is called stress relief cracking (SRC).Numerous micro-cavities and cracks are detected along zigzag line with the width of～3μm.These cavities and cracks have formed a discontinuous zigzag line (Fig.4).A high oxygen content is detected on the inclusions by energy dispersive spectroscopy (EDS)analyses (Point A in Fig.2d).This indicates that these inclusions are associated with the oxide layer of the initial butt surface.The thermal stress is closely related to the heat treatment temperature.The thermal stress of the joints increases with PWHT temperature increasing,and the zigzag line is more obvious after T6 treatment at 495℃.

Fig.1 OM images for zigzag line of as-welded joint with a low heat input (300 mm·min^-1,600 r·min^-1) and b high heat input (100 mm·min^-1,600 r·min^-1)

Fig.2 OM images for zigzag line of PWHT joint with low heat input (300 mm·min^-1,600 r·min^-1) and high heat input (100 mm·min^-1,600r·min^-1):a T6-450 joint with low heat input,b T6-450 joint with high heat input,c T6-495 joint with low heat input and d T6-495 joint with high heat input

Fig.3 Formation mechanism of zigzag line in FSW joint during PWHT

Fig.4 a SEM image showing zigzag line of PWHT (495℃) joints with high heat input as marked A in Fig.2d and b corresponding enlarged image;c EDS result of selected particle

Since the zigzag line of the joint shows a strong correlation with the residual stress and thermal stress during PWHT,the effect of PWHT process on zigzag line of joint was investigated,as exhibited in Fig.5.It is interesting that no zigzag line can be observed in the weld for O heat treatment joint,as shown in Fig.5a,which proves that annealing can remove the residual tensile stress in the weld and helps to suppress the generation of the zigzag line.However,an obvious zigzag line crosses through the weld for O+T6 W heat treatment joint (Fig.5b).Rapid cooling during water quenching leads to great thermal stress,resulting in the formation of zigzag line (quenching cracks).It can be observed that zigzag line only appears in the bottom of the weld for O+T6A heat treatment joint(Fig.5c),and zigzag line is significantly weakened compared to that of O+T6 W heat treatment joint.This phenomenon proves that quenching cracks are the main factors for the formation of zigzag line in PWHT joints.In conclusion,it is clear that zigzag line is the fragment of oxide layer in the base material surface,which is broken by the stirring effect during FSW and may not be found in the as-welded joint.Residual stress of as-welded joints and thermal stress producing by subsequent heat treatment cause the crack which appears as a shape of zigzag line in the weld.Therefore,the optimization of heat treatment process is the trigger for zigzag line in PWHT joints,such as adding a pre-annealing treatment or adopting a gentle quenching rate to reduce the thermal stress.

Fig.5 SEM images for zigzag line of FSW joint with different PWHT processes:a joint after annealing at 400℃,b joint after annealing at400℃and solution at 495℃with water quenching and c joint after annealing at 400℃and solution at 495℃with air quenching

3.2 Effect of zigzag line on mechanical property

Effect of zigzag line on microhardness of as-welded and PWHT joints is presented in Fig.6.As mentioned above,the high weld heat input shows a relatively good inhibition effect on zigzag line for both as-welded and PWHT joints.However,the microhardness values of joints show little relevance with the occurrence of zigzag line.The fluctuation trend of the microhardness is basically the same for corresponding joints.The microhardness of as-welded joint shows W type from the advancing side (AS) to the retreating side (RS),and the maximum value for both joints is about HV 120 regardless of whether or not zigzag line appears.The microhardness of PWHT joint shows a straight-line type and exhibits a dependence on PWHT temperature.The maximum value of T6-450 joints is about HV 120,while that of T6-495 joints is about HV 140.It is worth noting that the crack width of zigzag line is only～3μm even for T6-495 joints,while the measuring area of microhardness is relatively larger,which is difficult to reflect the effect of zigzag line on microhardness.

The tensile properties of joints and BM are summarized in Fig.7.Zigzag line shows little influence on tensile properties of as-welded joints,whose tensile strength is the same with BM and fracture at BM.However,zigzag line shows significant influence on tensile properties of PWHT joints.T6-450 joint with high welding heat input exhibits a better suppressing effect on zigzag line and consequently shows slightly better tensile properties compared to T6-450joint with low welding heat input.Zigzag line in the weld for T6-450 joints is not very serious,and reheat cracking is small during PWHT;therefore,the difference of tensile properties is not obvious for T6-450 joints.As reported,the tensile properties of the joint deteriorate dramatically for the low welding heat input,whose tensile strength is only339 MPa,and elongation is 0.6%.However,it is interesting that no obvious deterioration of the tensile properties appears for the joint with high welding heat input,whose tensile strength increases to 409 MPa,and elongation is 5.1%.This is slightly different from the study of Liu et al. [ 10] and Ren et al. [ 17] .It is reported that zigzag line seriously damages tensile properties of the joint after PWHT.In fact,it is proved that the effect of zigzag line on tensile properties of PWHT joints depends on PWHT process,especially PWHT temperature.When temperature reaches a certain value,residual stress and thermal stress cause zigzag line to become cracks which significantly affect tensile properties of the joints.Therefore,it is the size of the crack that determines the mechanical properties of the joint,rather than the occurrence of zigzag line in the weld.

Fig.7 Tensile properties of joints and BM,where Js-L indicating FSW joints at v=100 mm·min^-1 and Js-H indicating FSW joints at v=300 mm·min^-1

The precipitate distribution also provides key information on strength of the joints,as shown in Fig.8.There are two different particles in nugget zone (NZ),marked by A and B in Fig.8d.EDS analysis indicates that they are composed of Al-Cu-Mg and Al-Cu-Fe-Mn-Si,respectively.The rod-or lath-shaped Al-Cu-Mg compounds with a size of 100-300 nm are most likely fine S precipitates (Al₂CuMg),and coarse intermetallic phases with a size ranging from 1 to 5μm are insoluble particles(Cu,Fe,Mn) Al₆ [ 28] .A large number of coarse insoluble particles (Cu,Fe,Mn) Al₆ are observed in as-weld joints,and S precipitates are dissolved and slightly coarsen.By comparison,more fine S precipitates and less (Cu,Fe,Mn)Al₆ particles are observed in PWHT joints,especially for T6-495 joints.Consequently,the microhardness and tensile strength values are the highest for T6-495 joints.

Fig.6 Effect of zigzag line on microhardness of as-welded and PWHT joints with a high weld heat input and b low weld heat input

Fig.8 Back scattered SEM images of FSW joints with high heat input at different PWHT processes:a,d as-weld;b,e PWHT (450℃);c,f PWHT (495℃);EDS spectrum of g Particle A and h Particle B in d

3.3 Effect of zigzag line on fracture characteristic

Effects of zigzag line on fracture location of PWHT joints are illustrated in Fig.9.T6-450 joints with a slight zigzag line exhibit 45°shear fracture near or at the interface between NZ and thermo-mechanically affected zone(TMAZ)(Fig.9a,b) rather than zigzag line in the weld.After PWHT at 450℃,the transition zone between NZ and TMAZ for two joints is clearly visible,which causes a weak zone.Therefore,T6-450 joints are fractured at the interface rather than the zigzag line in the weld,and consequently,the mechanical properties of the joint are close to those of BM.However,T6-495 joints with zigzag line are fractured at NZ.The distinct zigzag line runs through the whole weld zone from the top to the bottom for two joints (Fig.2c,d),which contain cavities and cracks,and then,the fracture occurs in NZ along zigzag line(Fig.9c).Thus,mechanical properties of the joint,especially the elongation,show a significant decline compared to those of BM.It is interesting that the joint with low heat input fracture completely follows the path of zigzag line.But the fracture of the joint with high heat input is totally different.The fracture originates from the zigzag line at the bottom of the weld and then expands along a straight line to the top of the weld (Fig.9d),which is similar to the fracture characteristic of PWHT joint with no zigzag line defect as reported by Liu et al. [ 10] and Ren et al. [ 16, 17] .Therefore,the mechanical properties of the joints show slightly decrease compared to those of BM.It is further proved that the exacerbation effect of zigzag line on tensile properties of PWHT joints is determined by the residual stress and thermal stress during PWHT.

Fig.9 OM images for effect of zigzag line (Z line) on fracture of PWHT joints:a T6-450 joint with high heat input,b T6-450 joint with low heat input,c T6-495 joint with low heat input and d T6-495 joint with high heat input

Fig.10 SEM images showing fracture surfaces of FSW joints with zigzag line:a as-welded,b T6-450 joint with high heat input and c T6-495joint with low heat input

The effect of zigzag line on fracture surface for the FSW joints is shown in Fig.10.SEM photographs are taken from the center of the fractures.Larger dimples and tearing ridges are observed on the fracture surfaces of as-welded joint,indicating extensive plastic deformation (Fig.10a),consequently resulting in the relatively higher elongation of aswelded joints compared to that of PWHT joints.The depth and width of the dimples of T6-495 joint are far smaller than those of T6-450 joint (Fig.10b,c),which is a typical brittle fracture charac teristic.Frac ture of T6-495joint shows a large area of smooth surface and a small amount of weak connection of small dimples,which leads to a small plastic zone ahead of the crack,and consequently results in the signific ant decrease for the elongation and strength.

4 Conclusion

The occurrence of zigzag line for PWHT joint is determined by PWHT process which in nature depends on residual stress and thermal stress of the joint.No obvious zigzag line can be observed in the weld after annealing at400℃.However,residual and thermal stress causes cracks which appear as zigzag line in the weld for the joints during T6 treatment at 450 and 495℃.The thermal and residual stress of the joints increases with PWHT temperature increasing,and the zigzag line is more obvious after water quenching compared to air quenching.

The mechanical properties of the PWHT joints depend on PWHT process.No obvious deterioration of the tensile properties is observed for the joints with high welding heat input after T6 treatment at 450 and 495℃although zigzag line appears in the weld of the joints.Fracture mechanism involves the～3-μm-width zigzag line cracks created by residual stress and thermal stress during PWHT,which acts as initiation site of the final fracture for the T6-495 joints with zigzag line.However,for the T6-495 joints with zigzag line,the transition zone between NZ and TMAZ is clearly visible and becomes a weak zone,consequently fractures at the interface rather than the zigzag line.

参考文献

[1] Heidarzadeh A,Saeid T.Correlation between process parameters,grain size and hardness of friction-stir-welded Cu-Zn alloys.Rare Met 2018;37(5):628.

[2] Wang XS,Hu ZL,Yuan SJ,Hua L.Influence of tube spinning on formability of friction stir welded aluminum alloy tubes for hydroforming application.Mater Sci Eng,A.2014;607:245.

[3] Xu HB,Sun HB,Yang H,Chi LY,Chen J.Micro structure and properties of joint for stirring brazing of dissimilar Al/Mg alloy during heating processes.Rare Met 2015;34(4):245.

[4] Xu HB,Tang S,Xi HF,Gao PY,Li MY.Joint interface during arc milling brazing of aluminum alloy to low carbon steel with cutter milling at various rotation speeds.Rare Met.2017;36(11):872.

[5] Hu ZL,Wang XS,Pang Q,Huang F,Qin XP.The effect of post-processing on tensile property and microstructure evolution of friction stir welding aluminum alloy joint.Mater Charact.2015;99:180.

[6] Zhang J,Luo GQ,Shen Q,Zhang LM,Huang ZJ.Characterization of diffusion-bonded joint between A1 and Mg using a Ni interlayer.Rare Met.2016;35(7):537.

[7] Hu ZL,Yuan SJ,Wang XS,Liu G,Liu HJ.Microstructure and mechanical properties of Al-Cu-Mg alloy tube fabricated by friction stir welding and tube spinning.Scripta Mater.2012;66(7):427.

[8] Fanelli P,Evangelisti A,Salvini P,Vivio F.Modelling and characterization of structural behaviour of Al open-cell foams.Mater Des.2017;114:167.

[9] Nandan R,DebRoy T,Bhadeshia HKDH.Recent advances in friction-stir welding-process,weldment structure and properties.Prog Mater Sci.2008;53(6):980.

[10] Liu HJ,Chen YC,Feng JC.Effect of zigzag line on the mechanical properties of friction stir welded joints of an Al-Cu alloy.Scripta Mater.2006;55:231.

[11] Okamura H,Aota K,Sakamoto M,Ezumi M,Ikeuchi K.Behavior of oxide during friction stir welding of aluminum alloy and its influence on mechanical properties.J Jpn Weld Soc.2001;19(3):446.

[12] Sato YS,Yamashita F,Sugiura Y,Park SHC,Kokawa H.FIB-assisted TEM study of an oxide array in the root of a friction stir welded aluminium alloy.Scr Mater.2004;50:365.

[13] Sato YS,Takauchi H,Park SH,Kokawa H.Characteristics of the kissing-bond in friction stir welded Al alloy 1050.Mater Sci Eng A.2005;405:333.

[14] Khodir SA,Shibayanagi T.Friction stir welding of dissimilar AA2024 and AA7075 aluminum alloys.Mater Sci Eng B.2008;148(1):82.

[15] Liu ZL,Cui HT,Ji SD,Xu M,Meng X.Improving joint features and mechanical properties of pinless fiction stir welding of alcald 2A12-T4 aluminum alloy.J Mater Sci Technol.2016;32(12):1372.

[16] Ren SR,Ma ZY,Chen LQ.Effect of initial butt surface on tensile properties and fracture behavior of friction stir welded Al-Zn-Mg-Cu alloy.Mater Sci Eng A.2008;479:293.

[17] Ren SR,Ma ZY,Chen LQ,Zhang Y.Effects of post weld heat treatment and second welding on tensile properties of frictionstir welded 7075-T651 aluminum alloy.Acta Metall Sin.2007;43(3):225.

[18] Cabibbo M,Forcellese A,Simoncini M,Pieralisi M,Ciccarelli D.Effect of welding motion and pre-/post-annealing of friction stir welded AA5754 joints.Mater Des.2016;93:146.

[19] Cui L,Yang X,Zhou G,Xu X,Shen Z.Characteristics of defects and tensile behaviors on friction stir welded AA6061-T4T-joints.Mater Sci Eng A.2012;543:58.

[20] Zhang Z,Xiao BL,Ma ZY.Effect of segregation of secondary phase particles and"S"line on tensile fracture behavior of friction stir-welded 2024A1-T351 joints.Metall Mater Trans A.2013;44(9):4081.

[21] Khan NZ,Siddiquee AN,Khan ZA,Shihab SK.Investigations on tunneling and kissing bond defects in FSW joints for dissimilar aluminum alloys.J Alloy Compd.2015;648:360.

[22] Jolu TL,Morgeneyer TF,Denquin A,Gourgues-Lorenzon AF.Fatigue lifetime and tearing resistance of AA2198 Al-Cu-Li alloy friction stir welds:effect of defects.Int J Fatigue.2015;70:463.

[23] Kadlec M,Ruzek R,Novakova L.Mechanical behaviour of AA7475 friction stir welds with the kissing bond defect.Int J Fatigue.2015;74:7.

[24] Esmaily M,Svensson JE,Johansson LG.Technical note:a major loss in tensile strength of friction stir welded aluminum alloy joints resulting from atmospheric corrosion.Corrosion.2016;72(12):1587.

[25] Ji SD,Meng XC,Ma L,Gao SS.Effect of groove distribution in shoulder on formation,macrostructures,and mechanical properties of pinless friction stir welding of 6061-0 aluminum alloy.Int J Adv Manuf Tech.2016;87(9-12):3051.

[26] Safarkhanian MA,Goodarzi M,Boutorabi SMA.Effect of abnormal grain growth on tensile strength of Al-Cu-Mg alloy friction stir welded joints.J Mater Sci.2009;44(20):5452.

[27] Han WT,Wan FR,Li G,Dong CL,Tong JH.Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets.Sci Technol Weld Joint.2011;16(5):453.

[28] Hu ZL,Yuan SJ,Wang XS,Liu G,Huang YX.Effect of post-weld heat treatment on the microstructure and plastic deformation behavior of friction stir welded 2024.Mater Des.2011;32:5055.