| Mg alloys have significant advantages in the competition of lightweight materials due to their advantages such as light weight,higher specific strength and stiffness,well electromagnetic shielding characteristic and shock absorption performance.To expand their application in lightweight filed,it is necessary and important to achieve their reliable jointing.Friction stir welding(FSW)which is a new type of thermal solid-state joining technology can effectively avoid the solidification problems associated with fusion welding,and has great attractive for the welding of lightweight alloys such as Mg and Al alloys.In this paper,commercially extruded AM60 and AZ31 Mg alloys were selected to carry out the FSW of dissimilar Mg alloys.Electron backscatter diffraction technique was used to analyze the texture evolution of the micro-regions of the dissimilar FSW Mg joints and to figure out the influence of the micro-regions texture on the mechanical properties,deformation and fracture behavior of the joints.The main results were as follows:(1)Both material placement positions(AM60 or AZ31 was located on the advancing side)were applied to weld these two metal sheets,and achieved a decent welding,implying that these two metals had a good weldability.Significant grain and texture evolution were observed in the joint.The grain size of AM60 metal in the nugget zone(NZ)was larger than that of the AZ31 metal,but the grain size in the NZ was smaller than that of AM60 base material(BM)as a whole.Obviously,there was a strong texture in the micro area of the NZ,and its distribution was symmetrical from the advancing side to the retreating side;the texture intensity of the thermo-mechanically affected zone(TMAZ)was obviously weaker than that in the NZ.Material arrangement of BMs had little influence on the joint mechanical properties and failure locations.Because the grains in the area near the NZ interface was the soft direction for basal slip and extension twinning,the strain localization of the joint during loading deformation was serious,causing the result that the mechanical properties of the joint were far less than that of BM.The different deformation mechanisms,significant grain size gradient,high dislocation and strain concentration on both sides of the NZ interface made the joint fracture tend to be near the NZ interface.Additionally,there was an obvious tensile anisotropy in the AM60/AZ31 FSW Mg joint.The comprehensive tensile properties of joint along the three directions increased in the order: transverse direction(TD),45°direction and welding direction(WD).This was because the fluctuation of the Schmid factor(SF)for the basal slip and extension twinning in the above three directions decreased in the order.In addition,the difference in fracture behavior between TD and45° tensile specimens was the result of the difference in grain size and SF fluctuations in the joint.(2)Process parameters can largely affect the size of the stirring zone,the morphology of the joint interface,texture distribution,grain size and tensile properties.The grain size of each area in joint under the high rotation speed(HRS,1600 rpm)was significantly higher than that subjected to the low rotation speed(LRS,800 rpm)joint,and the difference in grain size was more significant on the AM60 side.A NZ with smaller grain size,larger stirring zone size and higher interface angle can be obtained at high rotation speed(1600 rpm)and high welding speed(600 mm/Min)(HRS-HWS).Although welding parameters had little effect on the distribution of texture in the joint,the texture intensity of the joint was different.Specifically,the HRS exhibited an enhanced texture intensity,while the HRS-HWS joint showed a weakened texture intensity.In addition,welding parameters had a large influence on the mechanical properties of the joints.The comprehensive tensile properties of HRS-HWS and HRS joints were higher than those of LRS joint.The coarse grains in the HRS joint promoted the activation of basal slip and twinning in the regions far away from the NZ interface;meanwhile,the HRS joint had lower concentration degree of the strain and dislocation near the NZ boundary.These factors led to better mechanical properties of the HRS joint.The fine grain,high TMAZ/NZ interface angle and the change in the activation ability of the basal slip and extension twinning in HRS-HWS joint were responsible for the highest strength and better plasticity.(3)Further microstructure observation showed that there was a triple junction region(TJR)with significant texture fluctuations and grain size differences in the upper area of the weld.During the transverse tensile process,there was significant surface embossed phenomenon on the normal-transverse plane along the WD direction,and the joint displayed a C-shaped fracture along the NZ center to the interfaces of NZ and TJR.During the tensile process,cracks initiated at the bottom of NZ-center due to the appearance of compression twins and double twins as well as the higher concentration degree of strain and dislocation.Cracks tended to develop toward the AM60 side due to its larger grain size.Further,near the interface of NZ was the soft direction for basal slip and extension twinning,deformation was severe.At the same time,the complex texture distribution of the TJR in joint resulted in incompatible deformation between the micro-regions,making the final termination of the crack locate near the NZ interface and the micro-region interface of TJR.(4)Based on the above research,it was proposed to use arc partial remelting to control the weld microstructure of FSW joints.There was a fusion weld zone in both the upper and lower parts of the NZ in the FSW joint after the arc heating.Further,the grain size in the NZ-middle grew to a certain extent.The texture in the NZ-middle zone was significantly weakened after the arc.However,the texture and grain size near the NZ interface showed no significant change.The tensile strength and yield strength of the two joints were similar,but the joint elongation after arc heating was largely improved.The weakened strain localization of joint contributed to the increase of the SF values(basal slip and extension twinning)caused by the weakening of the texture,the growth of the grain size and the annihilation of the initial residual dislocation in the NZ-middle. |
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