| 6061 aluminum alloy belongs to the heat treatable high-strength aluminum alloy.It has been widely used in aviation,aerospace,automotive,marine applications and many other fields for its low density,good specific strength,corrosion-resistance and weldability.As a solid-state joining process,Friction stir welding showed great advantage in the welding of aluminum alloy and help to realize the light weight of vehicle by means of using aluminum alloy structures in the design and manufacture.This paper investigated the influence of friction stir welding parameters on grain morphology and orientation,grain boundaries(GBs)characteristics,dislocation density and precipitates of different regions of the butt joints of 6061-T6 aluminum alloy by many analytical testing techniques,such as OM,SEM,EDS,DSC and TEM.The relationships between welding parameters,microstructure evolution of different regions of the joint and mechanical properties were discussed.Thus,the essence of the influence of welding parameters on the properties of the joint was clarified.The joint was composed of four areas:base material(BM),heat affected zone(HAZ),thermo-mechanically affected zone(TMAZ)and nugget zone(NZ).Geometric dynamic recrystallization(GDRX)and continuous dynamic recrystallization(CDRX)occur in NZ simultaneously.With the increase of heat input,CDRX and GDRX were both enhanced.At high welding speed,the increase of rotation speed mainly enhances the possibility of GDRX in NZ,which gives rise to the formation of high angle grain boundaries(HAGBs);At low welding speed,the increase of rotation speed mainly enhances the possibility of CDRX in NZ and gives rise to the formation of low angle grain boundaries(LAGBs).Besides,with the decrease of welding speed,the dislocation density would decrease due to the increased ability of dynamic recovery and CDRX,which results in the decrease of hardness in NZ.The shear and recrystallization texture developed in NZ.The type and the intensity distribution of the texture components in NZ were closely related to the strain state of the material.Large strains are experienced under high welding speed and high rotation speed.The formation of incoherent ?3 GBs in the NZ is due to GDRX,rather than the occurrence of twin deformation.At the same time,the grain boundary migration rate of some CSL GBs represented by ?3 GBs is low and the increase of its length percent can refine the grain so as to improve the hardness in the NZ.The ?" phase basically disappeared;meanwhile,two kinds of small overaged precipitates exist in NZ simultaneously,namely ?'/Q' phases and ?/Q phases,and their orientation relationships with A1 matrix can be changed by decreasing the welding parameters,due to the change of shear plane of shear texture and dislocation motion in dynamic recovery and continuous dynamic recrystallization.The solid solubility of A1 matrix increases due to the large amount of precipitate dissolution in NZ.Then,the NZ can be re-aged under the thermal cycle of welding,so nanoscale dispersion of solute atomic clusters in the A1 matrix appeared after FSW.Here,the clusters have some characteristics as followings:the unit cell is in coherent with Al matrix and incorporates an anti-phase boundary(APB)along one lattice direction;the lattice parameters can be described as:a = b = 0.404 nm,c>1.212 nm;the clusters contain Al,Mg,Si and Cu atoms.Compared with 6061 parent materials after solid solution,the solid solubility of Si atoms in NZ is still relatively low,so the solute atomic clusters will preferentially change to ?' phase by further heating.The increase of rotation speed can promote the precipitation of solute atomic clusters,thus increasing the hardness of NZ.At the same time,high rotation speed will promote the breaking down or dissolution of the overaged precipitates in NZ;furthermore,u phases or dispersoids like a-Al-(Mn,Cr,Fe)-Si begin to form by heterogeneous nucleation on these tiny overaged precipitates or grow independently after the welding speed decreases to some extent.The flow mode of the material in NZ behind the welding tool can be concluded as:most material mainly enters the NZ counterclockwise from RS,meanwhile a small amount of material enters the NZ clockwise from AS.The material from AS will expand toward the center of NZ with the decrease of welding speed or expand upward with the decrease of rotation speed.When using the welding tool with thread attached,the increase of rotation speed will push more material into the upper area of NZ from RS,which finally results in the enhancement of downward flow of material from the upper area to AS so as to accelerate the plastic flow of the material.The heat input of the friction stir welding is composed of the heat generated at the tool-workpiece interface and by plastic deformation.Maintaining the same welding speed,the interfacial heat will decrease with the increase of the rotation speed;however,the heat produced by plastic deformation will increase by increasing the rotation speed.When welding 6061-T6 aluminum alloy plates with a moderate thickness using a threaded welding tool,the total heat input will increase due to the increase of the heat by plastic deformation with the increase of the rotation speed.When welding speed is low,the trend above will be more obvious.In addition,reducing the welding speed will increase the inhomogeneity of heat input of the joint.Dynamic recovery and partially dynamic recrystallization occur in TMAZ,so its length percentage of LAGBs is higher than other regions of the joint and its grain size is similar to the NZ.The way of plastic deformation is slightly different between AS and RS of TMAZ.AS is mainly affected by shear force applied by rotation tool.RS is mainly under the pressure applied by a large amount of metal flow,so the rolling texture developed.The reduction of the rotation speed will change the type of the rolling texture component of RS and change the shear stress state to the rolling stress state in AS.The P" phase was also basically disappeared in TMAZ and ?'/Q' phases developed.The dislocation density is the main factor that affects the hardness of TMAZ,which will increase with the increase of welding speed or rotation speed.The plastic deformation in TMAZ can cause the breakdown of metastable phases to some extent.The size of ?'/Q' phases can obtain a continuous increase at low welding speed and low rotation speed.HAZ,which is between BM and TMAZ,was only subjected to thermal cycle.Under the effect of static recovery,the grain size increased slightly and the dislocation density decreased to some extent.When the welding speed is high,the dislocation density of the whole joint is high,and large quantity of ?" phases still remain in HAZ,so the hardness discrepancy between NZ,TMAZ and HAZ is small.At the same time,the discrepancy of the hardness between AS and RS of HAZ is also indistinctive.As for the low welding speed,the dislocation density of the whole joint is relative low,and needle shaped ?" phases were replaced gradually by rod shaped ?'/Q' phases in HAZ;meanwhile,the size of ?'/Q' phases can also obtain a continuous increase as the heat input increases due to the absent of the stirring effect.After increasing the rotation speed,the hardness of HAZ will experience a further decrease mainly because of the decrease of dislocation density.The tensile tests of FSW joints show that the best mechanical property can be obtained under high welding speed and moderate rotation speed.The ductile fractures were obtained in HAZ close to BM for all welding parameters.The tensile strength is generally related to the lowest value of the hardness in HAZ.Meanwhile,the elongation is related to the range of softening regions including NZ,TMAZ and HAZ. |
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