2012--Quantitative investigation

Quantitative investigation of the tensile plastic deformation characteristic and microstructure for friction stir welded 2024aluminum alloyZ.L.Hu a ,b ,⁎,X.S.Wang a ,S.J.Yuan a ,⁎⁎a National Key Laboratory of Precision Hot Processing of Metals,Harbin Institute of Technology,Harbin 150001,China bState Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology,Harbin 150001,ChinaA R T I C L E D A T AA B S T R A C TArticle history:Received 10June 2012Received in revised form 15August 2012Accepted 21August 2012The effect of the microstructure heterogeneity on the tensile plastic deformation characteristic of friction stir welded (FSW)2024aluminum alloy was investigated for the potential applications on light weight design of vehicles.The microstructure characteristics of the FSW joints,such as the grain structure,dislocation density and the distribution of precipitation,were studied by electron backscattered diffraction (EBSD)and transmission electron microscopy (TEM).The tensile deformation characteristic of the FSW joints was examined using the automatic strain measuring system (ASAME)by mapping the global and local strain distribution,and then was analyzed by mechanics calculation.It is found that the tensile deformation of the FSW joints is highly heterogeneous leading to a sig-nificant decrease in global ductility.The FSW joints mainly contain two typical deformation zones,which show great effect on the regional inhomogeneous deformation.One is the nugget zone (NZ)with a region of 8mm in width,and the other is part of the BM with a region of 10mm in width.The BM of the joints is the weakest region where the strain localizes early and this localization extends until fracture with a strain over 30%,while the strain in the NZ is only 4%.Differences in regional strain of FSW joints,which are essen-tially controlled by grain structure,the distribution of precipitation and dislocation density,result in decrease on the overall mechanical properties.©2012Elsevier Inc.All rights reserved.Keywords:Friction stir welding Aluminum alloys Microstructure Plastic deformation1.IntroductionWith the development of aerospace industry,the demand for producing lightweight,fuel-efficient vehicles has been greatly increased.In particular,aluminum alloys show a huge poten-tial for lightweight design of vehicles [1].Despite various ad-vantages associated with the use of aluminum alloys,joining aluminum alloys,especially for the high-strength aluminum alloys (2XXX series),are still challenging due to the less familiar weldability.Friction stir welding (FSW)has been proved to be an efficient,cheap and reliable joining method for the aluminum alloys [2].It shows more obvious advantages for the high-strength aluminum alloys which are widely used in the aircraft industry compared with conventional techniques [3,4].Currently,many studies have been performed to study the FSW mechanisms responsible for the formation of welds [5,6],microstructural refinement [7],effects of parameters on resultant microstruc-ture and final mechanical properties [8–10].FSW involves complex heat flow,material movement and plasticM A T E R I A L S C H A R A C T E R I Z A T I O N 73(2012)114–123⁎Corresponding author at:National Key Laboratory of Precision Hot Processing of Metals,Harbin Institute of Technology,Harbin 150001,China.Tel.:+8645186418776;fax:+8645186415754.⁎⁎Corresponding author.E-mail addresses:zhilihuhit@ (Z.L.Hu),syuan@ (S.J.Yuan).1044-5803/$–see front matter ©2012Elsevier Inc.All rights reserved./10.1016/j.matchar.2012.08.007A v a i l a b l e o n l i n e a t w w w.s c i e n c e d i r e c t.c o mw w w.e l s e v i e r.c o m /l o c a t e /m a t c h a rdeformation.The heat is generated by friction between the tool shoulder and the top of the sheets,and material movement by the rotation of the pin tool[11].These thermo-mechanical con-ditions vary across the joint,introducing a corresponding variation of the microstructure and mechanical properties. Mahoney et al.[12]investigated the effect of FSW on room-temperature tensile properties of7075Al-T651.Samples tested in transverse direction showed a significant reduction in both strength and ductility.Genevois et al.[13]determined the ten-sile properties at different locations of the FSW2024Al welds using mini tensile specimens.The tensile properties of the various regions of the2024-T351and2024-T6welds were very heterogeneous.The2024welds showed a very high hardening rate and a low ductility,which were clearly the result of strain localization in the weak zones of the weld.Recently,studies were conducted by Chen et al.[14]to examine the FSW char-acteristics of different heat-treated-state2219aluminum alloy plates.The experimental results indicated that the base ma-terial condition had a significant effect on weld morphologies, weld defects and mechanical properties of joints.From the investigations mentioned above,it should be observed that even if a qualitative description of the micro-structure and strength of the different zones for FSW joint is available[13,15,16].However,very little has been done on plastic deformation characteristic of FSW metals.The ductil-ity and fracture resistance of FSW joints are more difficult to address than the local microstructure and hardness[17–20]. Lockwood and Reynolds[18,19]studied the local and global mechanical responses of friction stir welded AA-2024exper-imentally and modeled using the finite element technique. Nearly plane stress conditions were found to exist in the specimen as demonstrated by the correspondence between experimental results and the predictions of the2-D models. Simar[17,20]developed a novel microstructure-based strain-hardening model for precipitate hardening materials,involv-ing a model which predicted precipitates evolution,yield strength and strain hardening.The main variations in the tensile properties of the welds were explained by differences in the strain-hardening behaviors which controlled the strength and resistance to plastic localization.Since FSW is a favorable joining process for the production of lightweight components,it is crucial to clarify the relation-ship between the microstructure and the resulting spatial distribution plastic deformation behaviors to determine the forming process in sheet materials.In addition,it is known that the microstructure and mechanical properties of the FSW joint are related not only with weld process parameters, but also with the alloy types and heat treatment condition [14,21].Normally,aluminum alloy blanks used for joining are applied in the hardened conditions(T3,T4and T6tempers). The published data is scarce when it comes to O temper. Currently,the FSW welds are usually used in the as-welded condition.Nearly all the investigations on FSW focus on the fundamental understanding of the welding process and the structure properties of the welded joints,which aim to reduce defects and improve strength of welds,and consequently avoid deformation and fracture during in-service.However, to expand the application of FSW to new engineering areas, such as in plastic processing field,the situation is exactly the opposite.The plasticity of FSW joints is the greatest concern.If the base material is in O condition,the blanks show the highest plasticity than that in other tempers.Therefore,the post-weld forming operation can be performed much more easily[22],and more complex shape parts can be obtained. Furthermore,starting from O temper aluminum alloy blanks, the strength of the weld is improved and the weld is stronger than the BM[23,24].Thus FSW welds no longer fail at the weld and are able to take full advantage of the plasticity of the base metal.In addition,the post-weld heat treatment(PWHT), which has been deeply investigated[23–25],can restore the mechanical properties of the joints successfully after the forming operation of the FSW welds.In the present paper,2024Al\O rolled plates are FSW and the microstructure characteristics of the FSW joints,such as the grain structure,dislocation density and the distribution of precipitation,are studied by electron backscattered diffraction (EBSD)and transmission electron microscopy(TEM).The ten-sile deformation characteristic of the FSW joints is examined using the automatic strain measuring system(ASAME)by mapping the global and local strain distribution,and then is analyzed by mechanics calculation.2.Experimental ProcedureSome5mm-thick Al2024-O rolled plates were friction stir welded vertical to the rolling direction with a travel speed,a rotational speed and a shoulder diameter of,respectively,100–300mm/min,600rpm and14mm.The friction stir pin used was of diameter5mm and height4.9mm.The schematic of the friction stir welding and the configuration of FSW plates for tensile test are shown in Fig.1.After welding,the specimens were cut perpendicular to the welding direction,and were electrolytically polished in 50ml HClO4+450ml C2H5OH solution at253K(−20°C),then were analyzed by electron back-scattered diffraction(EBSD). The EBSD measurements were carried out in an FEI Quanta 200FEG scanning electron microscope(SEM)equipped with an Oxford Inca System,operated at20kV.The step size for analysis was0.3μm.The foil specimens for TEM examination were prepared by jet eletropolishing in a100ml HNO3+400ml CH3OH solution at248K.These thin foils were observed at 200kV using a FEI Tecnai G2F30transmission electron mi-croscopy(TEM).The Vickers micro-hardness test was carried out along the centerlines of the cross-sections with a distance between neighboring measured points of1mm under a load of100g for10s.The tensile tests were performed at a crosshead speed of2mm/min using an Instron-1186testing machine.Tensile specimens were machined from the nugget zone in two directions,parallel(longitudinal)and normal (transverse)to the weld.The configuration of the tensile specimens was shown in the Fig.1.An axial extensometer with25mm gage length was attached to the test specimens at the gauge section.The strain analysis of each specimen was made by the ASAME automatic strain measuring system which was performed on the top surface of the weld.The tensile properties of the joints were evaluated using three tensile specimens cut from the same joint.115M A T E R I A L S C H A R A C T E R I Z A T I O N73(2012)114–1233.Results and Discussion3.1.MicrostructuresA FSW weld is divided into four regions based on microstruc-tural characterization of grains and precipitates,such as nugget zone (NZ),thermo-mechanically affected zone (TMAZ),heat-affected zone (HAZ)and base material (BM)[4].Fig.2shows the OIM (Orientation Imaging Microscopy)images for the various zones of the FSW joint.The BM of FSW joints exhibits a typical strip-like rolling structure containing coarse and non-uniform grains with an average grain size of ~200μm as shown in Fig.2(a).By comparison,the uniform fine-equiaxed grains about 2μm in size are observed in the NZ of FSW joints for the high weld speed (Fig.2(b)and (c)),which are nearly 100times smaller than those of the BM.The generation of these fine grains ismainly attributed to intense plastic deformation and frictional heating during FSW,which lead to dynamic recrystallization and grain refinement [11].Then it follows the TMAZ,which is unique to the FSW process and generally defined as a transition zone between the HAZ and the NZ [26].The TMAZ is characterized by a highly deformed structure,and a visible rotation and elongation of the initial grain in an upward flowing pattern around the NZ can be observed in Fig.2(b).Although the TMAZ undergoes plastic deformation,the heat and deformation strain are proved to be insufficient to totally recrystallization [27].Therefore,the effect of the grain refinement in the zone is not obvious as that in the NZ (Fig.2(c)),and a distinct interface between the NZ and the TMAZ can be obtained.Fig.2(d)shows the OIM (Orientation Imaging Microscopy)image for the NZ of the FSW joint at low weld speed.It is found that the NZ shows similar microstructure with that of thehighFig.1–The schematic of the friction stir welding technique and the configuration of FSW plates for tensile test(mm).Fig.2–EBSD showing the microstructure of the FSW joints:(a)the BM,(b)the transition zone included NZ and the TMAZ for the high weld speed,(c)the NZ for the high weld speed,(d)the transition zone included NZ and the TMAZ for low weld speed.116M A T E R I A L S C H A R A C T E R I Z A T I O N 73(2012)114–123weld speed,which differs only in the grain size.The fine-equiaxed grains in the NZ for the low weld speed is about 3μm in size,which is larger than that for the high weld speed.This is because that more energy for grain growth is generated by low weld speed.Moreover,as compared to the joints with high weld speed,it is noted that the deformation of the grains in the TMAZ for low weld speed is not so severe.The grains are also much larger and the interface between the NZ and the TMAZ are clearer,which indicate that the partially recrystallised may occur near the NZ.It is because that low weld speed means more dwell time during FSW,which generates more heat to soft adjacent material and results in severe deforma-tion.The sufficient energy and deform strain lead to partially recrystallised near the NZ,and the FSW joint for a low weld speed shows a smaller region of the TMAZ compared with that for a high weld speed.3.2.Tensile PropertiesThe tensile properties of FSW joints are shown in Fig.3.The elongation,yield strength and tensile strength of the BM,are 22.2%,88MPa and 207MPa respectively.By comparison,lon-gitudinal tensile specimens of FSW joints show a significant increase in both tensile and yield strength,while the ductility is unaffected.When it comes to the transverse tensile speci-mens,the FSW joint possesses the same tensile strength as the BM no matter FSW at high or low speed,and the yield strength is changed between 85and 92MPa (Fig.3).However,the fracture elongation of FSW joint is 12.5%for high weld speed,which is a 44%reduction compared with that of the BM.The decrease in elongation of the joint is more obvious for the low weld speed which shows an even lower fracture elongation as seen in Fig.3.In the text,the fracture elongationmeans the elongation of the gage measured just after fracture,which is an important index for plasticity evaluation.As mentioned above,longitudinal tensile specimens con-tain only recrystallized grains from the NZ.It is well known that the refinement of the grain size can lead to an increase in the strength of the material [28].The uniform fine-equiaxed grains generated in the NZ during FSW can be explained that the longitudinal tensile specimens of FSW joints show a sig-nificant increase in both tensile and yield strength.In ad-dition,the NZ shows a T4temper due to dissolution during welding of the O temper and natural aging.The T4temper is stronger than the O temper.However,transverse tensile specimens contain all four zones,i.e.,BM,HAZ,TMAZ and NZ.The observed ductility is an average strain over the gage length including various zones which have different resistances to deformation due to differences in grain size and precipitate distribution.When a tensile load is applied to the joint,the stress and strain concentration takes place in the lower-strength part or region of the joint,and consequently,the joints fail in this region.As shown in Fig.3,the transverse tensile specimens of FSW joints show the same tensile strength as the BM.Therefore,the BM is the lowest-strength region of the joint,and a fracture occurs in the BM.When FSW welds are used as blank for plastic processing,they always contain all four zones (BM,HAZ,TMAZ and NZ)which just like a transverse tensile sample.Therefore,the mechanical properties of whole joint are more important than those of the single zone.The plastic deformation character-istic of the inhomogenous joint is crucial to determine the forming process in sheet materials.In view of this reason,the relationship between the microstructure and the resulting plastic deformation behaviors of the transverse tensile sam-ples is investigated.Fig.4reveals the tensile strain distribution of the transverse tensile specimens by the ASAME.Since all the specimens are fracture at the BM on the advancing side,only the retreating sides of FSW joints are investigated which contain most part of the specimens.It is seen that the plastic strain concentrates in the BM on the advancing side with local strain exceeding 30%for the specimens at low weld speed.However,the strains in the NZ and BM on the retreating side are about 2%and 8%,respectively,showing that the NZ and BM of joint are not deformed fully.A similar phenomenon is also observed for the FSW joints with high weld speed.The joints also fail at the BM on the advancing side with a local strain over 30%,but the average strains in the NZ and BM on the retreating side are about 4%and 10%,respectively.It indicates that regional deformation of FSW joints is highly inhomoge-neous.This is mainly because that the strength of the NZ is much higher than that of the BM due to the grain strengthen-ing effect.Thus the weld exhibits small deformation during tensile test.The width of the deformation zones is much less than the gage length since the NZ deforms little,therefore,the elongation of the FSW joints decreases compared with that of the BM.It can be concluded that the highly inhomogeneous deformation throughout the joint,caused by heterogeneous microstructure,is the reason for the decrease in elongation of FSW joints compared with that of the BM.Moreover,since the more plastic strain occurs in the NZ and BM for the FSW joints with high weld speed,the elongation of the joints is larger than that of the FSW joint with low weld speed as shown in Fig.3.Fig.3–Comparison of the tensile properties for the BM and the FSW joints.The σb ,σs and δindicate the tensile strength,yield strength and the fracture elongation.117M A T E R I A L S C H A R A C T E R I Z A T I O N 73(2012)114–1233.3.Tensile Deformation Characteristic of FSW JointsIn order to investigate tensile deformation characteristic of the FSW joints,the distribution of plastic deformation strain across the different region during the tensile test is quantita-tively evaluated by ASAME.Fig.5shows the strain distribution in the gauge section of the high weld speed FSW joints for various levels of global elongation (2%,5%,7%and 10%,respectively).Occurrence of plastic deformation and exten-sion of plastic region for the FSW joint are investigated.The plastic strain is uniform across the FSW joints at very early stage (global elongation of 2%),except the NZ which shows lowest strain with a region of 8mm.As the global elongation is 5%,no obviously change of the strain can be observed in the NZ,which still shows the lowest value of the joints.The yield strength of the nugget is not yet reached while the other zones of the weld are already plastic deformation.Deforma-tion on both sides of the NZ remains roughly symmetrical with a local strain range from 6-8%.It is indicated that the FSW joint is under a uniform deformation stage until a global elongation of 5%.However,this stage is broken as the global elongation increases to 7%.The strain localization occurs in the BM on advancing side with a local strain over 11.4%,while that on retreating side is only 8.5%.This difference of the strain between two sides of the FSW joints becomes larger when the global elongation of the joint reaches 10%.Local necking can be observed with a plastic strain of 23%on advancing side.Simultaneously the local strain on retreating side is only 10.5%and the lowest strain is still in the NZ with a strain of 2.5–3.3%.It is found that the FSW joints mainly contain two typical zones.One is the NZ with a region of 8mm,and the other one is part of the BM with a region of 10mm.The HAZ and TMAZ are part of the weld,which are only the transient zone for the compatible deformation as seen in Fig.5.These two zones show great effect on the plastic deformation behaviors of the FSW joints,and result in a highly regional inhomogeneous deformation.The observed plastic deformation behavior of the FSW joints is closely related to the micro-hardness,which is quite clearly reflect the microstructural changes taking place throughout the joint.Fig.6shows the micro-hardness profile across the joints at three different depths (1.5,2.5and 3.5mm)for the high weld speed.The micro-hardness in the NZ is much higher than that in the BM regardless of the depths of the joints,while it shows a sharp decrease in the TMAZ,as seen in Fig.6.The micro-hardness profiles of the FSW joints show an asymmetric W-type distribution.The micro-hardness of the joints on retreating side is slightly higher thanthatFig.4–Distributions of the strain of (a)FSW joint with low weld speed,(b)FSW joint with high weld speed.The base material,HAZ,TMAZ and nugget is noted “BM,”“H,”“T ”and “NZ,”respectively.Fig.5–Strain distribution in the gauge section of the high weld speed FSW joints for various levels of global elongation (2%,5%,7%and 10%).The HAZ,TMAZ and nugget is noted “H,”“T ”and “NZ,”respectively.Fig.6–Distributions of microhardness along the transverse section of the FSW joints at three different depths (1.5,2.5and 3.5mm).118M A T E R I A L S C H A R A C T E R I Z A T I O N 73(2012)114–123on advancing side.Published measurements,comparing the temperature difference on the advancing and retreating side of the weld,show that the weld is hotter on the advancing side at least for cold welding conditions[29,30].Nandan and DebRoy et al[31]concluded that it was mainly because that more heat was generated in the advancing side due to more serious deformation between the tool and workpiece on the advancing side.As a result,the grain and precipitate on the advancing side are coarsened which lead to asymmetrical micro-hardness distribution.Thus the joints fail at the advanc-ing side.Moreover,the micro-hardness for the top of the NZ is significantly harder than that for the bottom.It is mainly because that the upper surface experiences extreme defor-mation and frictional heating during FSW[29,32],thereby resulting in generation of more fine recrystallized grains than that of the bottom.According to the Hall–Petch relationship, finer grains lead to higher micro-hardness.3.4.Microstructure Governing Deformation CharacteristicThe mechanical properties of the precipitation-hardening aluminum alloys greatly depend on the grain size and pre-cipitate distribution[28].Fig.7displays the TEM images in the four regions of the FSW joints with high weld speed.A low density of dislocations and few precipitates are shown in the BM of the FSW joint(Fig.7(a)).The Energy Dispersive Spectrometry(EDS)analysis shows that these precipitate particles are compounds of Al\Cu\Mg.For2024Al alloys, the main strengthening phases are metastable phases of S (Al2CuMg),which are rod-like or lath-like structure coherent/ semi-coherent with the substrate.According to the literatures these compounds in the text are most likely Al2CuMg[13,28].A similar phenomenon is also observed for the HAZ of the FSW joints except that the density of the precipitates is slight higher than that of the BM(Fig.7(b)).It is known that the strength of the precipitation-hardening aluminum alloys greatly depends on the precipitate distribution[16,28].It is easy to understand that the strength of the HAZ is higher than the BM.Therefore,the Al2024-O joints show the same tensile strength as the BM during transverse tensile test and the joints fail in the BM.The results are consistent with the reference [33],and the same phenomenon can also obtained in the Al 2219-O[14]and Al6061-O[22].It is reported that precipitate strengthened alloys show a worsening of mechanical properties in the weld zone because of the dissolution and growth of strengthening precipitates during the welding thermal cycle[13,14,34].However,the 2024-O FSW joint shows a contrary result,and the weld is significantly strengthened during FSW.It is mainly because that not only the grain sizes but also the precipitates are greatly refined in the weld.The TMAZ of the joint,as char-acterized by a highly deformed structure,exhibits a high den-sity of dislocation including the homogenouslydistributed Fig.7–The TEM images in the four regions of the FSW joints with high weld speed,(a)BM,(b)HAZ,(c)TMAZ and(d)NZ.119M A T E R I A L S C H A R A C T E R I Z A T I O N73(2012)114–123precipitates (Fig.7(c)).The NZ shows higher density disloca-tion and finer dispersed precipitates (Fig.7(d))compared with those of the BM,which is mainly due to the intense plastic deformation and frictional heating [35].Some previous studies [36–38]have actually shown that dynamic recrystallization often leaves some grains with a high density of dislocations in the stir zone.This situation suggests that the dislocation den-sity of the stir zone is higher than that of annealed bined with the fine grains as seen in Fig.2(c)),it can be understood that the strength and micro-hardness of the NZ are significantly higher than that of the BM.In addition,since the material is used in the O state and no reduction of the precipitation hardening phenomena is expected after FSW.The fine-equiaxed grains and high den-sity of dislocation play more important role to determine the tensile deformation characteristic of the joints,and the effect of the precipitations distribution is smaller in magnitude.In the tensile test,the stress and strain concentration takes place in the lower-strength part or region of the joint.3.5.Mechanical Governing Deformation CharacteristicHighly inhomogeneous microstructure of the joints inevitably leads to heterogeneous deformation as show in Fig.5,which results in a significant decrease of the joint elongation during transverse tensile test.To gain more insight into the tensile deformation characteristic of FSW joints,a simple analytical model of the plane strain is developed.Fig.8shows the schematic of a specimen under transverse tensile test,with a gauge length of L 0,width of W and thickness of t,respectively.The weld with a width of B is harder than the BM.The relative thickness of the weld (B/W)is denoted by H.It is assumed that the weld is characterized by elastic operation,and the BM is in the plastic deformation which is in the threshold state at the moment of the loss of the general plastic stability.The material of the weld and BM is assumed to be isotropic and incom-pressible.The coordinate system is introduced in accordance with the Fig.9,which is an element at the middle of the weld.When the FSW joint is elongated along the Y direction,the weld is stretched in the length (in the Y direction)and becomes narrow in the width (in the X direction).The deformation of the weld in X direction will be constrained by the adjacent BM,thus shear stresses τare generated in the cross-section of the weld (in the X direction)as shown in Fig.9.The microstructure and mechanical property of the FSW weld are inhomogeneous.However,implementation of all theweld details in the model makes it very complicated.Besides that,the values of a large number of model parameters (the mechanical and geometrical properties)of the zones of FSW blanks are very difficult to determine experimentally.There-fore,in this study,the measured property of the FSW joints is applied for both the weld and its neighbor (or the HAZ)as one average property for simplicity.The method is common for FEM simulations of the FSW joints [39,40].According to contact strengthening theory [41],tangential stresses arise at contact surfaces as a result of constraining plastic strains for the weld metal from the direction of neighboring areas of the welded joint.This tangential stresses reach the maximum value in the limiting condition and can be considered constant throughout the thickness of the layer [42].In the study,the tangential stresses τincrease with the increase of the tensile stress (σy ),which reach the maximum value when the tensile stress (σy )is the tensile strength of the BM (σB ).The tangential stresses τcan be written as follow:τ¼K σy ð1ÞK ¼σB =σWð2Þwhere K is the strength ratio of the BM and the weld,σB and σW are the tensile strength of the BM and weld respectively.By taking equilibrium of forces at the X direction,the following equations are derived:σx þd σx ðÞBt −σx Bt þ2τt dx ¼0ð3ÞAfter substituting Eq.(1)into Eq.(3),we have d σx þ2K σy =B ÀÁdx ¼0ð4ÞFrom the Von Mises yielding criterion,the tensile strength σy can be obtained:σy −σx ¼2=ffiffiffi3p σs ;d σx ¼d σyð5ÞUsing Eqs.(4)and (5),the following equations are derived:d σy =σy ¼−2K σy =B ÀÁdxð6ÞAfter integration differential condition of Eq.(6),the tensile strength σy can be obtained:σy ¼exp C −2K Bxð7Þwhere C is the constant ofintegration.Fig.8–Schematic of a specimen under transverse tensiletest.Fig.9–The stresses acting on an element of the weld.120M A T E R I A L S C H A R A C T E R I Z A T I O N 73(2012)114–123。