| Magnesium alloys as important lightweight materials in the field of aerospace,new energy vehicles,rail transportation,etc.,compared with other metals,have a huge advantage of energy saving and emission reduction.The high-quality welding process is technical support to expand the scope of magnesium alloy engineering applications.Friction stir welding(FSW),as a non-melting advanced joining technique,is considered the most ideal welding method for magnesium alloys.However,the FSW of magnesium alloys,especially dissimilar magnesium alloys,is still immature due to the special crystal structure and nature.The mechanisms of material flow behavior,texture evolution,and the relationship between mechanical properties and microstructure are still unclear.Given this,two magnesium alloys of ZK60 and Mg-4.6Al-1.2Sn-0.7Zn(ATZ511)were chosen to study the flow characteristics and microstructure formation mechanism of dissimilar FSW of magnesium alloys.For the forming characteristics and material flow behavior,this paper analyzed the macrostructure of dissimilar welded joints,established two typical material flow models using tracer methods and combining the apparent differences of dissimilar materials,and calculated the material flow velocity around the tool pin by measuring the volume flow rate and the width of the tracer material flow trace.The results showed that within the studied parameters,the increase in welding speed led to a gradual decrease in the area of the stir zone(SZ);the increase in rotational speed caused the width of the pin-affected zone(PAZ)to increase and then decrease,while the depth of the shoulder-affected zone(SAZ)increased;the increase in the tool pin thread size led to an increase in the area of the SZ.The counterclockwise rotation of the tool caused a clear “onion ring” and a “core” in the SZ due to the alternating distribution of dissimilar materials.The clockwise rotation of the tool led to the formation of three well-defined zones in the SZ,that is,the deposition zone of the metal from the advancing side(AS),the deposition zone of the metal from the retreating side(RS),and the mixing zone of the two alloys.The counterclockwise rotation of the tool drove the thermoplastic metal in the inner rotating layer downward in a spiral pattern,while the metal in the outer deflecting layer flowed upward to fill the vacancies;conversely,the clockwise rotation of the tool drove the metal in the rotating layer to spiral upward and the metal in the outer deflected layer to flow downward.The metal flow around the tool pin underwent two stages of acceleration and deceleration,and the peak flow velocity occurred at the RS of the flow angle of about 60?.As the welding speed,rotation speed,and the size of the pin thread increase,the material flow velocity also increases.The ATZ511 with better thermoplasticity was placed on the RS to produce a higher flow velocity compared to being placed on the AS.For the microstructure evolution and texture distribution characteristics,this paper analyzed the grain boundary evolution,texture evolution,and texture orientation distribution characteristics from different aspects by combining macroscopic features and microstructure through large-area acquisition and mapping of electron backscatter diffraction data.The results showed that the deformation of ZK60 magnesium alloy by heat and force was dominated by dislocation slip and mainly experienced continuous dynamic recrystallization(DRX).ATZ511 magnesium alloy first occurred twinning and then dislocation slip,and successively experienced discontinuous DRX and continuous DRX.Grain size in the SZ was affected by both deformation breaking and grain growth,with deformation breaking leading to grain refinement,and grain growth increasing grain size.During the welding process,the strong shear plastic deformation prompted the(0001)base plan was always aligned with the shear surface,which resulted in the c-axis of the grains deflecting sequentially around the pin along the rotation direction and always remaining perpendicular to the circumferential tangent plane of the shear deformation layer.In the SZ of the welded joints,the c-axis of the grains at the “core” location was parallel to the welding direction;the c-axis of the grains surrounding the “core” was continuously deflected from the “core” to the outer side with a gradually increasing angle with the welding direction.Texture intensity has a relative relationship with strain,and locations with larger strain also have higher texture intensity.The changes in welding heat input and material flow caused the deflection of the “core” in the SZ,which in turn affected the distribution of the texture orientation.The center of the texture orientation distribution within the SZ exhibited the same deflection trend as the “core” under the influence of different process parameters.For the inhomogeneity of mechanical properties,the relationship between microhardness and microstructure,and the correspondence between tensile strain and texture orientation distribution were established by analyzing the microhardness distribution and tensile strain distribution characteristics under different process parameters,respectively.The results showed that the microhardness distribution curves in the SZ exhibited the characteristics of high in the middle and low on both sides along the transverse direction,and there was an obvious soft zone at the edge of both sides of the SZ,which corresponded to the distribution characteristics of the texture orientation.The hardness values were higher when the hardness test plane was parallel to the {0001} plane and lower when it was parallel to the {10-10}plane or the {11-20} plane.The values of microhardness in the thermo-mechanical affected zone were increased compared to the base materials on both sides due to dislocation accumulation and strain concentration.The tensile strain distribution curves of the welded joints exhibited bimodal distribution characteristics along the transverse direction.There was a significant strain on both sides of the AS and RS within the PAZ,and less strain on the SAZ and the center as well as the bottom of the PAZ.The tensile strain of the welded joint under counterclockwise rotation was characterized by a “butterfly wing” distribution on the cross-section,while the tensile strain of the welded joint under clockwise rotation was characterized by a “W” distribution in the cross-section.The regions of severe tensile deformation correlated with the regions of easy activation of base slip induced by the texture orientation,and the greater the texture intensity,the more significant this correspondence was.There was a strongly consistent positional relationship between the tensile strain distribution and the texture orientation of the welded joints.Based on the above research,this paper elucidated the complex material flow behavior during the FSW of magnesium alloys,revealed the evolution mechanism of grain boundaries and grain orientation,clarified the distribution characteristics of texture orientation in the three-dimensional space of welded joints,and established the relationship between the inhomogeneity of mechanical properties and the distribution of texture orientation.The results of the study provided theoretical support for the microstructure regulation and mechanical properties optimization of the FSW of magnesium alloys. |
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