Study On Microstructure And Properties Of Resistance Spot Welding Of Rare Earth Magnesium Alloy

Vehicle lightweighting plays a key role in energy conservation,emission reduction,and fuel consumption reduction,and has now become a research hotspot in the automotive industry at home and abroad.As the lightest engineering metal and green engineering material,magnesium alloy can achieve ideal lightweight effect when used in automobile body manufacturing;In addition,as a country with large rare earth resources,rare earth as an important strategic resource makes rare earth magnesium alloy more meaningful in the research of automobile lightweight.As the most common welding method in the automotive field,resistance spot welding will also improve the research value of resistance spot welding of rare earth magnesium alloys.At present,there are few researches on resistance spot welding of rare earth magnesium alloys at home and abroad.Therefore,this paper conducts resistance spot welding experiments on rare earth magnesium alloy sheets,and studies the changes in the microstructure and mechanical properties of welded joints,which will provide an important reference value for lightweight vehicles.and guiding significance.In this paper,single-pulse welding,double-pulse welding and aging treatment experiments were carried out on the rare earth magnesium alloy of 1mm thick NDM2（independently developed by Nanchang University）.Through the above experiments,the microstructure change,failure mode,microhardness,tensile shear load and phase analysis of welded joints were studied,and the following results were obtained:（1）In the single-pulse welding experiment,the changes of the welding process on the quality characteristic parameters of the solder joint（the diameter of the solder nugget,the peak load of the solder joint,whether it is spattered or not,and the failure mode）were studied,and the optimal welding parameters were obtained:The welding current is 16.5k A,the welding time is 400ms,and the electrode pressure is 1.5k N.The peak load of the welded joint under the optimal welding process parameters is1.899k N,and the nugget diameter is 4.90mm.（2）In the double-pulse（single-pulse optimum welding current and then applying a tempering current（TC））welding process test,it was found that the tempering current can transform the columnar dendrites（CDZ）in the nugget zone（NZ）into equiaxed dendrites（EDZ）,under the double-pulse test,the peak load reaches 2.364k N under the tempering current of 11k A,which is 24.49%higher than that of the single-pulse test.（3）In the single-pulse and double-pulse welding tests,it was found under the optical microscope（OM）that twin zones and fine-grained zones were formed in the base metal zone（BM）near the heat-affected zone（HAZ）in the solder joint;under the welding heat input,the Al-RE phase and Al₁₀RE₂Mn₇phase in the nugget area were transformed into Al₂RE phase and Al₈REMn₄phase,under scanning electron microscope（SEM）,it was found that the aggregation of Al₂RE between grain boundaries could hinder the continuity of the network Mg₁₇Al₁₂phase.Compared with the single pulse,the maximum value of the microhardness of the nugget increased from 90.8HV to 95.7HV under the tempering current.In addition,the failure modes of welded joints under single and double pulses are both weld nugget pullout failure（PF）,mixed fracture under single pulse,and ductile fracture under double pulse.（4）In order to improve the mechanical properties of the welded joints,the welded joints obtained under the best single-pulse process parameters are selected for aging treatment.After aging treatment,it is found that the solder joints increase with the aging time,and the structure of the solder joints will be transformed into a cellular structure.The lamellar Mg₁₇Al₁₂phase is precipitated,and the lamellar Mg₁₇Al₁₂phase is precipitated in most of the grains in the base metal area in the solder joint,but the precipitation of the grains in the nugget area is very small.The study found that the peak load of the welded joint obtained after 8h aging treatment is the best2.145k N,which is 12.95%higher than that before aging;In addition,the organization of the heat affected zone is obviously refined with the increase of aging time;Grain precipitation layer lamellar Mg₁₇Al₁₂phase and grain refinement in heat-affected zone increase the overall microhardness value of solder joints after aging treatment;The failure modes of the welded joints under the aging treatment are all partial interface failure（PIF）,and the fractures of the welded joints are all transgranular fractures from cracks in the nugget area until they extend to the heat-affected zone.