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Li, Shen - 2012 - A feasibility research on friction stir welding of a new-typed lap–butt joint of dissimilar Al alloys

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  • 7/30/2019 Li, Shen - 2012 - A feasibility research on friction stir welding of a new-typed lapbutt joint of dissimilar Al alloys


    A feasibility research on friction stir welding of a new-typed lapbutt jointof dissimilar Al alloys

    Bo Li, Yifu Shen

    College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Jiangjun Road, Nanjing 210016, PR China

    a r t i c l e i n f o

    Article history:Received 28 March 2011Accepted 14 May 2011Available online 25 May 2011

    Keywords:A. Non-ferrous metals and alloysD. WeldingE. Mechanical

    a b s t r a c t

    A new-typed joint conguration, called lapbutt joint of dissimilar aluminum alloys, consisted of a singleAA6063 plate of 4 mm thick and two overlap AA5052 plates of 2 mm thick. As needed as some engineer-ing applications, the lapbutt joint was successfully welded through the friction stir welding techniqueusing a designed tool of W9Mo3Cr4 V with some geometric improvements. The optimum work condi-tions and a process map were obtained after the welding process parameter optimization experiments.The process window was built aiming to choose the suitable processing conditions to pursue the high-quality lapbutt joints. The effects of welding parameters, especially the value of pin off-set, on weld-formation, microstructures and mechanical tensile properties of the lapbutt joints of dissimilaraluminums were investigated. The parameter of pin off-set played a very important role and exertedstronger inuence upon the joint quality. Three typical micro ow-patterns of plasticized materials werefound in the weld-zones: circumuence, laminar-ow and turbulent-ow. Some weld-defects and themorphologies of onion rings were detected.

    2011 Elsevier Ltd. All rights reserved.

    1. Introduction

    Friction stir welding (FSW), as a novel solid-state joining tech-nique invented by TWI, is gaining more popularityand more exten-siveapplicationin themanufacturing sector [1,2] . Currentlythe FSWhas become an efcient option of welding method for the same anddissimilar aluminum alloys, especially those difcult or impossibleto be welded by the conventional fusion welding without any hotcracking, blowhole or distortion. The joining of similar or dissimilarmetals and alloys in FSW process is achieved by the severe plasticdeformation (SPD).The SPDcan lead to thegrain dynamic recrystal-lization which permits the ow of plasticized material occurring insolid-state [3,4] . Thus, the recrystallized,equiaxed, and usually sub-micron grains formed the weld-zoneafter being frozen [3,4] . During

    FSW process, the FSW-tool is rotated as the pin is forced into a loca-tion on thesurface of platesuntilthe shoulder comes in contact withthe base material of plates. Heating is due to a combination of friction effects and localized SPD induced in the material by toolrotation [4,5] . The heat-input of FSW produces signicant micro-structure changes, which lead to local variations in the mechanicalproperties of welds [6] . Theheat generation andthe thermal historyof FSWare stronglyrelated to process parameters andtoolgeometry

    [79] . In addition, tool design and welding variables affect materialow-patterns. And the sufcient mobility of plasticized metal-owcontributes to avoid the FSW weld-defects [1012] . Therefore, theoptimized FSW process and tool parameters are the key points forobtaining a high-quality friction stir weld.

    Apart from the most convenient joint congurations of butt andlap joints, many other types of joint designs can also be welded byFSW technique as needed as some engineering applications [2] .Fig. 1 gives some different types of FSW joints. Fratini et al.[1315] systematically investigated the FSW process on T-shapedllet joints of similar and dissimilar aluminum alloys. Cerri andLeo [16] investigated the FSW process, post-welding heat treat-ments and mechanical properties of a so-called double lap jointof dissimilar 2024-T3/7075-T6 aluminum alloys. In the present re-

    search work, the FSW technique was applied to weld a new-typed joint conguration, a called lapbutt joint ( Fig. 2a). This composite joint was composed of dissimilar AA6063/AA5052 aluminumalloys. The aim of this work is to provide a feasibility research onthe FSW for some specic engineering structures, which aresketched in Fig. 2. The FSW of the lapbutt joints is targeted byindustrial elds for structurally demanding applications to providehigh performance benets. A process including FSW-tool designand FSW-tool selection was carried out before the followingwelding parameter optimization for welding the lapbutt joints.Furthermore, the effects of FSW process parameters on weld-for-mation, microstructures and mechanical tensile properties of thecomposite joints of dissimilar aluminum alloys were investigated.

    0261-3069/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi: 10.1016/j.matdes.2011.05.033

    Corresponding author. Tel.: +86 15050597986; fax: +86 2552112626.E-mail addresses: , (Y. Shen).

    Materials and Design 34 (2012) 725731

    Contents lists available at ScienceDirect

    Materials and Design

    j ou rna l home page : ww w.e l s ev i e r. com/ loca t e / m a tdes
  • 7/30/2019 Li, Shen - 2012 - A feasibility research on friction stir welding of a new-typed lapbutt joint of dissimilar Al alloys


    2. Experimental details

    The base materials for welding are AA6063 (Al0.7%Mg0.4%Si)aluminum alloy and AA5052 (Al2.5%Mg) aluminum alloy. Thethicknesses of the AA6063 plate and the AA5052 plate are 4 mmand 2 mm respectively. The plates were friction stir welded verticalto their rolling directions. The so-called lapbutt joint of dissimilarAA6063/AA5052 aluminum alloys consists of three plates. In this joint design, one butt-side is a single AA6063 plate of 4 mm thick,and the other butt-side is two overlap AA5052 plates of 2 mmthick. According to the denitions about FSW, one butt-side iscalled the advancing side (AS), and the other butt-side is desig-nated as the retreating side (RS) [1,2] . On AS, the tangential veloc-ity direction of tool-rotation is the same as tool traveling direction,while on RS, the rotating tangential velocity of the FSW-tool op-poses in direction against its traveling speed.

    Fig. 3 shows the appearance and geometric sizes of the stir toolused in the experimental work. The tool was further designed bas-ing on the features of some conventional FSW-tools, which consistof a shoulder and a pin. On the body of this tool, as black-arrowed

    in Fig. 3, a circular-wall structure was added around the pin, andtwo circular-ring structures were added on the shoulder face.These geometric improvements were based on the considerationof that more severely plastic deformation of materials was neededto eliminate the butt-gaps and lap-gaps in the lapbutt joints. Inthe lapbutt joint conguration, an interfacial gap existed betweenthe overlap plates of AA5052, and another butt interface lay be-tween AA6063 and AA5052. Accordingly, the geometric designscould add ow paths and expand motion regions of the plasticizedmaterial during FSW on the lapbutt joints. Fig. 4 depicts the pos-sibly plasticized material ow-patterns around the stirring pin andunder the rotating shoulder of a simple tool and the designed toolused in the present work. Many experiments and computational

    simulations have provided that the simultaneous interaction of the horizontal and vertical ows of plasticized material leads tothe weld formation [1] . Therefore, the relatively more compleximprovements in geometry on the designed tool in Fig. 3 were ex-pected to benet the weld formation, moreover, raise the strengthof the lapbutt FSW joints.

    Considering that the tool material, if it is carbon steel or toolsteel, can be eroded easily while contacting aluminum especiallyin hot environments, the selected material for the tool in this studyis W9Mo3Cr4 V, which is high-alloy ledeburite steel. Its mainchemical compositions are given as in Table 1 . After heat treatmentprocedure of quench hardening in the temperature range of 12101240 C, the mechanical properties of W9Mo3Cr4 V are shown inTable 2 . The high hardness and high wear resistance can contributeto prevent the tool from abrasion during FSW process.

    Optimization of FSW process for welding the AA6063/AA5052lapbutt joints were carried out aiming to obtain a high-qualityweld. The FSW process parameters mainly include travel speed(v, mm/min), rotation speed ( n , r/min), toolspindle tilt angle

    Fig. 1. Joint congurations for FSW: (a) square butt, (b) square butt, (c) edge butt,(d) T butt joint, (e) multiple lap joint, (f) T lap joint and (g) llet joint [2] .

    Fig. 2. Sketches of the so-called lapbutt composite joint (a) and its applications onsome engineering structures (b and c).




    Fig. 3. Appearances of the designed tool used for FSW of the lapbutt joints.



    Shoulder Shoulder


    (a) (b)

    (c) (d)

    Fig. 4. Schematic diagrams of plasticized material ow-patterns around the stirringpin and under the rotating shoulder (the general directions of plastic ow wereblack-arrowed): (a) horizontal direction using the simple tool; (b) horizontaldirection using the designed tool; (c) vertical direction using the simple tool; (d)

    vertical direction using the designed tool.

    Table 1

    The chemical compositions of W9Mo3Cr4 V (wt%).

    C Si Mn Cr Mo W V















    726 B. Li, Y. Shen / Materials and Design 34 (2012) 725731

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    (a , ), shoulder press-amount ( d, mm) and pin-offset ( t , mm). Crosssections of welds were cut, ground and polished through the stan-dard procedures. And they were etched in the Poulton reagent for15 s. An optical microscope (OM) and a QUANYU200 scanning elec-tron microscope (SEM) were utilized for the related observationson microstructures and fractographies. Mechanical static tensiletests of standard weld specimens were carried out under roomtemperature. Fig. 5 shows the tensile testing specimen with its x-ture used to investigate the strength of the lapbutt joints.

    3. Results and discussion

    3.1. Results of process optimization

    After the FSW process optimization experiments on theAA6063/AA5052 lapbutt joints, the following welding parameterswere selected to obtain a high-quality lapbutt joint: v = 100 mm/min, n = 1000 r/min, a = 2 , d = 0.1 mm, t = 0.5 mm. The AS was thesingle AA6063 plate, and the two overlap plates of AA5052 wereplaced on RS. It should be indicated that the pin off-set was devi-ated to RS, the overlap plates, from the weld butt line.

    On the condition that the other optimized welding parameterswere constant, the results of FSW on the lapbutt joints showeddifferently when the xed locations of AA6063 plate and AA5052plates were different. Fig. 6a illustrates that the upper-surface for-mation of weld was discontinuous severely when the two overlapplates of AA5052 were set on AS and the other welding parameters

    adopted had been all optimized. Besides, numerous inner weld-defects such as micro-voids were found in the weld. The inherentinterfacial gap between the two overlap AA5052 plates obstructedthe uent mass transfer of the plasticized aluminum from AS to RSand from upper to bottom in the weld-zone. Recent experimentaland computational works have provided that the majority of theplasticized material ow occurs through RS and the mass transportfrom RS to the region behind the tool forms the welded joint [1,1724] . According to the characteristics and basics of material ow inFSW, it was certain that the interfacial gap, or to say, the lap inter-face between the two overlap plates of AA5052 was not conduciveto the mass transport or the heat transfer along the thickness

    direction. If the two overlap plates were placed on AS, the insuf-cient ow of plastic deformed material would break down theweld-continuity. Therefore, placing the two overlap plates on RSbenetted the weld-formation of the lapbutt joints (as shown inFig. 6b). Moreover, if the plunging pin center deviated slightly toRS, the overlap plates of AA5052, from the weld butt-line, thewelds could be formed further tightly than those when the pinwas plunged on AS of AA6063.

    The modications and optimizations of the two common FSWprocess parameters, tool rotation speed and travel speed, are nec-essary for obtaining a high-quality joint. The conventional methodsof controlled variable optimization were used in the present exper-iment to pursue the optimum working conditions. Fig. 7 depictsthe process map regarding the change of the values of rotation

    speed and travel speed, under the premise of that the other opti-mum work conditions are invariable. In the map, a process windowwas built and marked A. The groups of parameters in the processwindow could contributed to produce the defect-free lapbutt joints, while Region B in the map expressed the other groups of process parameters to generate weld-defects including voids, tun-nels, excessive ash and surface grooves. It should be noted thatthe process map is dened basing on a series of points, whichare indicated by different symbols in Fig. 7. And the boundaries be-tween the process window and the region of resultant weld-de-fects were drawn appreciatively. The boundary locations werenot absolutely strict, due to the span of parameters choice (typi-cally 200 r/min for the tool rotation speed and 20 mm/min forthe tool travel speed). In spite of that, the process window can

    be used to choose the suitable processing conditions under whichthe defect-free lapbutt joints were obtained.

    Table 2

    The mechanical properties of W9Mo3Cr4 V after quench hardening process.

    Hardness (HRC) Tensile strength, r b (MPa) Impact toughness (kJ/cm 2)

    6466 588784 3540


    5 0 mm

    2 5 mm

    1 5 mm



    4 mm




    Fig. 5. Schematic illustration of tensile testing specimen of lapbutt joint (a);appearances of tensile testing xture (b) and specimen (c).








    AS RS






    Fig. 6. Surface formation of lapbutt composite joints: (a) the two lap AA5052

    plates are placed on AS; (b) the single AA6063 plate is placed on AS.

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    Additionally, there was no evident abrasion on the tool surfaceafter the experiments in fact. This mainly beneted from the largehardness difference between the parent materials of Al alloys andthe tool material of W9Mo3Cr4 V, which can maintain the stabilityof its microstructure and properties in the hot environments. Manyresearchers had concluded that the max temperature was no morethan 450 C of 6XXX series Al alloys, and no more than 550 C of 5XXX series Al alloys, in the weld zone during FSW process [2] .Those temperatures can not strongly disorganize the tool of W9Mo3Cr4 V. And as depicted in Fig. 4, the geometric improve-ments can add the ow paths of the plasticized aluminum duringFSW, therefore the frictional resistance between the rotating tooland the plastic material-ow can be reduced.

    3.2. Effects of pin off-set variables

    Fig. 8 shows the cross-section macro-structures of the lapbutt joints under different values of the pin off-set, t , towards to RS(overlap AA5052 plates), with the same other FSW process param-eters. Fig. 8a shows the weld cross-section when t was 0 mm. Aninterfacial gap was presented on RS. As shown in Fig. 8b, when t was 0.5 mm, the mixing extent and homogenizing degree of dis-similar aluminum alloys in the weld-zone was obviously bettercomparing with the other welds with other t values. And the inter-facial gap between the two overlap plates of AA5052 on RS wasmuch smaller in size due to the suitable pressure center of tool-shoulder. However, the interface warping of the overlap platescould be presented if the pin-plunge position was improper. When

    t was 1 mm ( Fig. 8c), it was found in the top region of weld cross-section that part of the upper AA5052 plate was severely plasticdeformed, or elongated, and transferred up to AS. The migrationtraces of AA5052 towards AS were white-arrowed in Fig. 8c. How-ever, the mixing behavior between the two overlap plates of AA5052 was so poor. The failure of the lap-welding and connectionof the two AA5052 plates on RS could be judged from the distinctdividing line of the lap-interface. When t was 1.5 mm ( Fig. 8d), theinterface-warping phenomena between the two overlap plates of AA5052 became serious, and some worse weld-defects appeared.The excessive pin off-set indeed led to an abnormal deviation of the tool-shoulder pressure. It was conclude that only the appropri-ate parameter of the pin off-set, t , could contribute to obtain a bet-ter formation inner the lapbutt joint.

    Heurtier et al. identied and modeled three types of motionpattern in the weld-zones during FSW: circumventing, torsional,

    and vortex [25] . In the present research, it was observed that threetypical plasticized ow-patterns formed in some micro regions of the lapbutt joints of dissimilar aluminum alloys: circumuence(as shown in Fig. 9a), laminar-ow (as shown in Fig. 9b), and tur-bulent-ow (as shown in Fig. 9c). The circumuence is associatedwith the ow of thickness material due to the action of stirringpin. As a result of the circumuence, the typical onion rings struc-

    ture [1,2,2628] forms in the stir-nugget (SN) zone of the lapbuttweld. And the laminar-ow and turbulent-ow are both generatedby the motion of plasticized material under the rotating shoulder.The two patterns of laminar-ow and turbulent-ow can be alwaysfound near the upper-surface of the lapbutt joint. The circumu-ence and turbulent-ow can produce the regular and irregular lay-ers alternating with dissimilar aluminum alloys of AA6063 andAA5052. And the laminar-ow contributes to the mass transfersalong the thickness direction. The simultaneous interaction of thethree ow-patterns of the plasticized material results into the jointformation. However, the occurrence of turbulent-ow is the resul-tant of poor coordination between the circumuence and the lam-inar-ow. If the two adjacent ow patterns of circumuence andlaminar-ow have good compatibility and synchronicity, theweld-zone will achieve a high mixing degree contributing to agood weld formation. In Fig. 8b, the turbulent-ow pattern disap-pears, and the weld-zone shows a sufcient mixing extent and ahigh homogenizing degree between the dissimilar aluminumalloys.

    Many references [2630] have investigated the formationmechanism of onion rings structure in the stir-nugget zone. Theso-called onion rings appears as a banded texture of alternatedmaterial layers, which experiences different levels of plastic defor-mation in a weld during FSW [29] . The formation of onion rings isdue to the tool rotation and forward movement extruding thematerial around to RS of the tool [30] . Therefore, the morphologyof onion rings structure is strongly related to both the geometricnature of FSW-tool and the FSW process parameters including therotation speed and the travel speed. In this research, it was foundthat the appearances of onion rings in the lapbutt joints weredifferent according to the different values of pin off-set when theother welding parameters were invariable. The reasonable valueof pin off-set made the onion rings homogenized and more com-patible with the adjacent material in the weld-zone. Fig. 9d showsthe discontinuous onion rings due to an excessive value of pin off-set. As Fig. 9d depicts, the mass transfer along the thickness direc-tion had been broken up at the top region of the onion rings struc-ture, though the banded texture in the bottom region wasunaffected and regular.

    The mixings of dissimilar aluminum alloys in the weld-zonespresented different levels when the values of pin off-set werenot constant. It was conclude that the welding parameter of pinoff-set, t , also affected the tensile strength ( r b) of the lapbutt

    joints. Fig. 10 illustrates that the average value of r b can reach159 MPa when v = 100 mm/min, n = 1000 r/min, and t = 0.5 mm.Besides, the upper and lower limits of are r b indicated in rela-tion-curve of Fig. 10 . The sharp decline of the average r b valuein Fig. 10 was attributed to the worse weld-defects existing inthe welds, such as the large void shown in Fig. 8d. And Fig. 9eand Fig. 9f are the magnied OM images of two micro regionsin Fig. 8d. The cracks shown in Fig. 9e and f are dangerous forguaranteeing the weld-strength due to that the cracks existing in-ner welds can be regard as the inherent fracture origins. Further-more, some fracture paths (as marked in Fig. 8a and c with red 1

    dot-lines) and the fracture surfaces of the failed lapbutt joints un-

    Fig. 7. Process map of tool travel speed and rotation speed, with the constant valueof pin off-set and the other invariable work conditions: Region A is the processwindow for obtaining the defect-free lapbutt joints, and Region B consisted of theparameters contributing to generate weld-defects.

    1 For interpretation of color in Fig. 3, 5, 6 and 8, the reader is referred to the webversion of this article.

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    der different pin off-set values were detected after the mechanicaltensile tests. Fig. 11 a and Fig. 11 b are SEM images of the fracturesurfaces at A and B positions in Fig. 8a, along the fracture path.Due to the weaker connection of the surface-layer (as white-ar-rowed in Fig. 8a) and the base material below on AS, the tear frac-ture mode occurred at position A. Hence, numerous typical tearingdimples were present on the fracture surface. However, it was ob-

    served that in a majority of areas on the fracture surface generallyat B position, a number of small-sized and equiaxed dimples were

    present in Fig. 11 b shows a typical ductile type of fracture. At posi-tion C in Fig. 8c, the fractography was macro-observed by SEM asFig. 11 c, in which a dividing line of the lap interface near the upperweld-surface was black-arrowed. Fig. 11 d shows the fracture sur-face at the position 1 in the surface-layer (in Fig. 11 c), with sometear dimples, was generally smooth. In contrast, the fracture modeof the material below the dividing line (in Fig. 11 c) was a stronger

    ductile fracture type, according to the numerous ne dimples pres-ent in Fig. 11 e. It indicated that the poor material mixing behavior



    2 3





    Interfacial gap






    ASInterfacial gap

    Interfacial gap

    Interfacial gap

    Fig. 8. Cross-sections of the lapbutt joints under different pin off-sets ( t values) towards RS of overlap AA5052 plates: (a) t = 0, (b) t = 0.5 mm, (c) t = 1.0 mm and (d)t = 1.5 mm.

    (a) 200 m 200 m 200 m

    200 m

    Circumfluence Laminar-flow Turbulent-flow

    100 m100 m


    (d) (e) (f)




    Fig. 9. OM images of microstructures in the weld cross-sections (etched in the Poulton reagent): (a) position 1 in Fig. 7a; (b) position 2 in Fig. 7c; (c) position 3 in Fig. 7c;(d) position 4 in Fig. 7d; (e) position 5 in Fig. 7d; (f) position 6 in Fig. 7d.

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    and/or the insufcient mass transport led to the deterioration of the tear resistance in lapbutt joints due to the discontinuity of

    dissimilar aluminum alloys.

    3.3. Effects of rotation speed variables

    DuringFSW process with the constant other conditions, the heatgeneration rate and the power needed by FSW are not signicantlyaffected by the welding travel speed, but strongly related to therotation speed of thetool [1,5,7,31,32] . And it was alsoreported thatthe variation of tool traverse speeds do not affect thetensile proper-ties of the friction stir welds [33] . Therefore, the investigation of therelationship between rotation speed ( n) and weld strength ( r b) is

    necessary for the lapbutt joints of dissimilar aluminum alloys.

    Fig. 12 illustrates the relation-curve between n and r b, with theupper and lower limits. The curve shows an inexion point underdifferent rotation speedsand thesame other optimizedFSW param-eters. When therotationspeedwas 1000 r/min,the average value of r b achieved the max. If the value of n is too low, the insufcientheat-input will be adverse to the mixing behavior of plastic de-formed aluminums. A lapbutt joint without a good formation ora high densication degreecan not achieve a high value of weld ten-sile strength. However, the excessive heat generation due to muchhigher value of n may lead to the grain growth coarsening and themicro-structural softening in the weld-zone.

    Fig. 10. Relation-curve between the values of pin off-set and the average weldtensile strengths with the upper and lower limits.

    50 m 50 m

    300 m 50 m 50 m

    (a) (b)

    (c) (d) (e)



    Fig. 11. SEM images of fracture surfaces of the welds: (a) fracture position A in Fig. 7a; (b) fracture position B in Fig. 7a; (c) fracture position C in Fig. 7c; (d) magniedimage of area 1 in Fig. 11 c; (e) magnied image of area 2 in Fig. 11 c.

    Fig. 12. Relation-curve between the values of rotation speed and the average weldtensile strengths with the upper and lower limits.

    730 B. Li, Y. Shen / Materials and Design 34 (2012) 725731

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    4. Conclusions

    The called lapbutt joint of dissimilar AA6063/AA5052 alumi-num alloys was successfully welded through FSW technique usinga further designed tool of quench hardening W9Mo3Cr4 V withsome geometric improvements. After FSW parameter optimizationexperiments on the AA6063/AA5052 lapbutt joints, an optimum

    process window consisted of tool travel speed and rotation speedwas drawn aiming to choose the suitable processing conditions.Placing the two overlap plates of AA5052 as RS beneted the for-mation of weld. The reasonable value of pin off-set towards theRS was propitious to avoid weld-defects and hence raise weld-strength. The average tensile strength of weld achieved 159 MPaif v = 100 mm/min, n = 1000 r/min, a = 2 , d = 0.1 mm, t = 0.5 mm.Raising the mixing degree of the dissimilar Al alloys and promotingthe material plastic deformation in the weld-zone during FSW con-tributed to obtain a high-quality lapbutt joint. Three typical microow-patterns formed in the weld-zones: circumuence, laminar-ow and turbulent-ow. The simultaneous interaction of the threeow patterns of the plasticized material contributed to the nal joint formation. The feasibility research on FSW process of thenew-typed lapbutt joint of dissimilar Al alloys is signicant forsome engineering applications.


    The research works are nancially supported by the NationalNatural Science Foundation of China (Grant No. 51075205). Andthe authors acknowledge School of Material Science and Engineer-ing, Jiangsu University of Science and Technology, Zhenjiang,China.


    [1] Nandan R, DebRoy T, Bhadeshia HKDH. Recent advances in friction-stirwelding-process, weldment structure and properties. Prog Mater Sci2008;53:9801023.

    [2] Mishra RS, Ma ZY. Friction stir welding and processing. Mater Sci Eng R 2005;50:178.

    [3] Kazi SH, Murr LE. Complex ow phenomena associated with friction-stirwelding of aluminum alloys. In: Mishra RS, et al., editors. Friction stir weldingand processing, Warrendale (PA): TMS; 2001. p. 139151.

    [4] McNelley TR, Swaminathan S, Su JQ. Recrystallization mechanisms duringfriction stir welding/processing of aluminum alloys. Scripta Mater2008;58:34954.

    [5] Schneider J, Beshears R, Nunes Jr AC. Interfacial sticking and slipping in thefriction stir welding process. Mater Sci Eng A 2006;435436:297304.

    [6] Cerri E, Leo P. Warm and room temperature deformation of friction stir weldedthin aluminium sheets. Mater Des 2010;31:1392402.

    [7] Rajamanickam N, Balusamy V, Madhusudhanna Reddy G, Natarajan K. Effect of process parameters on thermal history and mechanical properties of frictionstir welds. Mater Des 2009;30:272631.

    [8] Scialpi A, De Filippis LAC, Cavaliere P. Inuence of shoulder geometry onmicrostructure and mechanical properties of friction stir welded 6082aluminium alloy. Mater Des 2007;28:11249.

    [9] Chen YC, Nakata K. Effect of tool geometry on microstructure and mechanicalproperties of friction stir lap welded magnesium alloy and steel. Mater Des2009;30:39139.

    [10] Li B, Shen YF, Hu WY. The study on defects in aluminum 2219-T6 thick buttfriction stir welds with the application of multiple nondestructive testingmethods. Mater Des 2011;32:207384.

    [11] Rajakumar S, Muralidharan C, Balasubramanian V. Inuence of friction stirwelding process and tool parameters on strength properties of AA7075-T6aluminium alloy joints. Mater Des 2011;32:53549.

    [12] Karthikeyan L, Senthil Kumar VS. Relationship between process parametersand mechanical properties of friction stir processed AA6063-T6 aluminumalloy. Mater Des 2011;32:308591.

    [13] Fratini L, Buffa G, Shivpuri R. Inuence of material characteristics onplastomechanics of the FSW process for T-joints. Mater Des 2009;30:243545.

    [14] Fratini L, Buffa G, Micari F, Shivpuri R. On the material ow in FSW of T-joints:inuence of geometrical and technological parameters. Int J Adv Manuf Technol 2009;44:5708.

    [15] Acerra F, Buffa G, Fratini L, Troiano G. On the FSW of AA2024-T4 and AA7075-T6 T-joints: an industrial case study. Int J Adv Manuf Technol2010;48:114957.

    [16] Cerri E, Leo P. Mechanical properties evolution during post-welding-heattreatments of double-lap friction stir welded joints. Mater Des2011;32:346575.

    [17] Leal RM, Loureiro A. Effect of overlap friction stir welding passes in the qualityof welds of aluminium alloys. Mater Des 2008;29:98291.

    [18] da Silva AAM, Arruti E, Janeiro G, Aldanondo E, Alvarez P, Echeverria A.Material ow and mechanical behaviour of dissimilar AA2024-T3 and AA7075-T6 aluminium alloys friction stir welds. Mater Des 2011;32:20217.

    [19] Prangnell PB, Heason CP. Grain structure formation during friction stir weldingobserved by the stop action technique. Acta Mater 2005;53:317992.

    [20] Guerra M, Schmidt C, McClure JC, Murr LE, Nunes AC. Flow patterns duringfriction stir welding. Mater Charact 2003;49:95101.

    [21] Muthukumaran S, Mukherjee SK. Multi-layered metal ow and formation of onion rings in friction stir welds. Int J Adv Manuf Technol 2008;38:6873.

    [22] Zhang HW, Zhang Z, Chen JT. 3D modelling of material ow in friction stirwelding under different process parameters. J Mater Process Technol2007;183:6270.

    [23] Hamilton C, Dymek S, Blicharski M. A model of material ow during frictionstir welding. Mater Charact 2008;59:120614.

    [24] Schmidt HNB, Dickerson TL, Hattel JH. Material ow in butt friction stir weldsin AA2024-T3. Acta Mater 2006;54:1199209.

    [25] Heurtier P, Jones MJ, Desrayaud C, Driver JH, Montheillet F, Allehaux D.Mechanical and thermal modelling of friction stir welding. J Mater ProcessTechnol 2006;171:34857.

    [26] Krishnan KN. On the formation of onion rings in friction stir welds. Mater SciEng A 2002;327:24651.

    [27] Kumar K, Kailas Satish V. The role of friction stir welding tool on material ow

    and weld formation. Mater Sci Eng A 2008;485:36774.[28] Lee WB, Yeon YM, Jung SB. The joint properties of dissimilar formed Al alloysby friction stir welding according to the xed location of materials. ScriptaMater 2003;49:4238.

    [29] Xu SW, Deng XM. A study of texture patterns in friction stir welds. Acta Mater2008;56:132641.

    [30] Chen ZW, Cui S. On the forming mechanism of banded structures in aluminiumalloy friction stir welds. Scripta Mater 2008;58:41720.

    [31] Zhang Z, Zhang HW. Numerical studies on the effect of transverse speed infriction stir welding. Mater Des 2009;30:9007.

    [32] Frigaad O, Grong O, Midling OT. A process model for friction stir welding of agehardening aluminum alloys. Metall Mater Trans A 2001;32:11892000.

    [33] Kumbhar NT, Sahoo SK, Samajdar I, Dey GK, Bhanumurthy K. Microstructureand microtextural studies of friction stir welded aluminium alloy 5052. MaterDes 2011;32:165766.

    B. Li, Y. Shen / Materials and Design 34 (2012) 725731 731