Microstructure,Texture And Mechanical Properties Of Friction Stir Welded And Processed AZ31 Magnesium Alloys

As a new type of solid-state joining technique,friction stir welding(FSW)has been proven to be an ideal technique for welding Mg alloys.To date,the systematical and deep study on FSW of Mg alloys,especially the structure-property relationship,is still lacking.It is quite hard to achieve a joint strength equal to base materials in the FSW of wrought Mg alloys with superior performance,which gives rise to the study trend of optimizing welding parameters and developing joint-strengthening process.Besides,with the industrial applications of more complex welding structures and component shapes,new derivations of FSW like bobbin-tool friction stir welding(BTFSW)has gradually been adopted.However,the microstructure and mechanical properties of BTFSWed Mg alloys have seldomly been studied Moreover,despite the intimate correlation between twinning and the microstructure evolution as well as mechanical properties of FSW joints of Mg alloys,the detailed twinning behavior and its evolution mechanism in the stir zone(SZ)have not been fully understood.For these reasons,investigations were conducted as follows.As-extruded AZ31 sheets were selected as base material and welded in three directions aligned at angles of 0°,45°and 900°to the extrusion direction(ED),and thus different initial joint textures were prepared.It was showed that the difference in initial textures almost had no influence on the microstructure and texture evolution in the SZ,but could result in the formation of different textures in thermal-mechanically affected zone(TMAZ).In the microstructure evolution of TMAZ,extension twinning and accompanied dynamic recrystallization played a dominant role.There was also a gradual variation of texture components in the TMAZ with the location approaching to the SZ.Similar yield strength and tensile strength were obtained for joints with different welding directions.At an off-axis angle of 45° between welding direction and ED,significantly higher elongation of the joint was obtained,which can be explained by the reduced strain localization owing to the accommodation of well-deformable non-SZ regions.It was noted that taking control of initial texture was helpful for improvement of joint performance.The three FSW joints exhibited similar hardness profiles along mid-thickness of the sheets with two highs and two lows in the SZ.The whole variation trend of hardness in the SZ showed distinct texture dependence.The fracture locations of FSW joints shifted with the three welding directions,which can be attributed to the incompatible deformation of different sub-regions and the contraction twins generated at the SZ center.Besides,difference can also be observed in the low-cycle fatigue behaviors between FSW joints and base material.In order to enhance the joint efficiency,two kinds of processes were developed and their strengthening mechanisms were studied.It was found that the SZ could be turned into a two-layer structure by largely increasing the tool rotation rate during FSW.The area and the strain of the region with strong texture can thus be reduced,leading to weakened non-uniform deformation and strain localization in the joint.Therefore,the mechanical performance of joint was improved.In the region with specifically-distributed strong texture,special "concave-convex" appearance formed on the surface of tensile-deformed joint.The triangular shape of the concave regions was consistent with the distribution area of extension twins.The distribution characteristics of twins could be well explained by a simplified orientation model.In the SZ,basal slip and extension twinning played different roles in different sub-regions.With the location moving from SZ boundary on the retreating side to the SZ center,the Schmid factor(SF)for basal slip would increase first and then decrease while the SF for extension twinning would continuously decrease.During the whole tensile deformation process,the consistent lattice rotation induced by basal slip in the SZ would give rise to the continuous increase of SF for basal slip in the sub-region near the SZ boundary and the continuous decrease of SF in the sub-region near the SZ center.The second method to strengthen the joint was to apply additional pass of friction stir processing(FSP)on the original FSW weld.With optimized processing parameters,additional FSP could introduce abundant extension twins to the whole SZ.The introduction of twin lamellae exerted a grain-refinement strengthening effect and the introduction of twin orientations weakened the original strong texture in the SZ and improved the texture distribution in different sub-regions,resulting in weakened strain localization and enhanced joint performance.In the study on BTFSW of Mg alloys,it was found that the formation of the special texture distribution in the SZ can be attributed to the directional stacking of two sets of bowl-like shear layers.The texture evolution in the TMAZ was related to the material flow.With the location approaching the SZ,the basal poles of grains would gradually rotate around the sheet normal direction due to the traverse of pin and also gradually rotate to be parallel with the SZ boundary due to the lack of material in mid-thickness area of the SZ.Compared with the traditional FSW,the contribution of twinning to the texture evolution of the TMAZ was not that much in BTFSW,and thus the tilt of grain orientations was limited,which gave rise to the obvious orientation softening in the TMAZ on the advancing side.This sub-region then became a place sensitive to strain localization and crack initiation.In the study on the twinning behavior of FSPed Mg alloys under a loading state in favor of basal slip,it was found that the concentration and distribution of geometrically necessary dislocations(GNDs)could influence the local stress-strain field,and hence the twin variant selection behavior could be more likely affected by the local strain coordination instead of Schmid law.The effect of GND concentration bands on twinning was complicated GND bands could induce twinning and guide twin growth direction as well as inhibit twin growth.Long twin bands across multiple grains could frequently take place,which was ascribed to the high values of geometrical compatibility factors among adj acent grains.