The Study Of The Microstructure And Mechanical Properties Of Rare Earth Magnesium Alloy Mg-Nd-Zn-Zr Friction Stir Welds

With the extensive use of magnesium alloy in aerospace, automotive, electronics,medicine and chemical industry, how to improve its performance is a hotspot among manystudies. Rare earth magnesium alloy arises at the historic moment, attracting much attentiondue to its unique advantages. But the welding methods have not been further studied now.This paper mainly studied the friction stir welding process of the NZ20K rare earthmagnesium and the influence law of different heat treatment processes on themicrostructure evolution and performance. The results have laid the theoretical andtechnical foundation for the application of rare earth magnesium alloy.The friction stir welding processes of the NZ20K rare earth magnesium were studiedby changing the welding speed and rotational speed. The relationship between processparameters, microstructure and properties of the joint were revealed through the observationand analysis of microstructure in different regions, and testing of mechanical properties ofthe joints.Different heat treatments were carried out on the joints. Observe and analyze themicrostructure of the joints after typical aging treatment by TEM. And the mechanicalproperties of joints after different heat treatments were tested. The aging precipitation ofrare earth precipitates processes in the joints and strengthening mechanisms for joints wererevealed.The results showed that a fine joint without defect could be achieved under the rotationspeed from1100r/min to1300r/min and welding speed from100mm/min to150mm/min.The grains in NZ were fine in which second phase particles distributed uniformly. It turnedout that Nd could refine grains well. The result of XRD analysis indicated that the secondphase particles should be Mg12Nd. When the parameter was set as1300r/min of rotationspeed and120mm/min of welding speed, the joint showed the best tensile property. Thehighest tensile strength of joint at room temperature was156MPa, equivalent to75%ofbase metal and the ductility reached4.33%. The fracture mechanism was judged as brittlefracture. When the test temperature went up to200℃, the tensile strength was137MPawhich decreased by12%compared to room temperature property. However, the tensilestrength of AZ31joint at200℃was only100MPa that decreased by44%of that at roomtemperature. NZ20K showed better high temperature property because rare-earth eutectic phase Mg12Nd is stable at200℃. The Mg12Nd in joint can pin the grain boundary well andthe grain boundary sliding and dislocation glide can be obstructed a lot so that the hightemperature property was improved with much better ductility. Either the NZ20K joint orthe AZ31joint, the distribution of micro-hardness presented inverted-V pattern and thevalues differed little. The NZ owned the highest value of micro-hardness. The valuedecreased in TMAZ and HAZ. The lowest value of hardness was achieved at the boundarybetween TMAZ and HAZ.After proper heat treatment, the mechanical properties of NZ20K FSW joint could beimproved a lot. When the joint suffered aging treatment at200℃for8hours, the tensilestrength reached the highest value of184MPa which increased18%compared to that of thejoint without heat treatment. It indicated precipitated phase in weld after heat treatmentcould improve the tensile strength of joint.