Effects On Properties Of Friction Stir Welded Joints Of Magnesium Alloys After Welding

Magnesium alloy is a kind of light alloy with small density,high strength and stiffness,and can withstand huge impact force.At present,magnesium alloys have been playing an important role in the fields of transportation,aerospace and defense electronics industry.Due to the low melting point of magnesium alloy,the traditional welding method is easy to cause defects such as porosity,which is not suitable for the welding of magnesium alloy.Friction stir welding?FSW?is a solid-phase welding technology invented in the 1990s,which can avoid the defects caused by fusion welding.Therefore,friction stir welding is very suitable for the welding of magnesium alloys.The effects of heat treatment and laser treatment on microstructure and mechanical properties of magnesium alloys were analyzed.First,the optimal friction stir welding parameters for different kinds of magnesium alloys were obtained by welding the same and different magnesium alloys respectively.Second,different kinds of welded joints were heat treated and laser treated respectively,and microstructure analysis,hardness analysis,tensile test,fracture analysis,XRD,SEM and EDS analysis were carried out.Compared with untreated joints and original plates,the effects of post-welding heat treatment and laser treatment on friction stir welding joints were studied.The experimental results show that the grain size of the unwelded joints varies greatly.The grain size is fine and uniform,and there are a lot of the second phase in the zone.The grains in the thermal influence zone were elongated and stratified obviously,the grains were partially grown,and the number of the second phase was less than that in the nuclear zone.There is no obvious difference between grain and base material in heat affected zone.After heat treatment,the second phase in the weld core area grew obviously and the number increased sharply.The microstructure of the thermodynamic influence zone becomes uniform,the second phase grows obviously,and the quantity does not change obviously.After laser treatment,in the part of laser remelting,the microstructure turns into fine dendrites,the second phase still exists,and a large number of new dispersion strengthening phases are generated at the dendrite boundary.Compared with the joints without laser remelting,the microstructure of the welded core area and the microstructure of the thermally affected area have no obvious change.According to the phase composition of the analyte,the two existing phase ii in the magnesium alloy base metal are white short rod Al₈Mn₅ phase and dark black island?-Mg₁₇?Al,Zn?₁₂ phase,no new phase is generated after heat treatment,and gray?-Mg₁₇Al₁₂ phase is generated after laser treatment.Through the comparison of mechanical properties,the mechanical properties of the welded joints are lower than the mechanical properties of the base metal,and the mechanical properties of the joints after heat treatment and laser treatment have increased to different degrees.