Effect of upsetting force on microstructure of welds in resistance spot welding of 400 MPa ultra-fine grain steel

The ultra-fine grain(UFG) steel is welded by using resistance spot welding technique with or without requirement of upsetting force.Metallographic inspection shows that the grain size of weld nugget is larger than that of the base metal and the microstructure is altered significantly.In addition,contracting defects such as air holes can be found in the nugget center.The experiments show that the defects can be effectively avoided by the technique of adding upsetting force during the nugget cooling and crystallizing processes.In tensile shear tests,the welding joint starts to crack from the inner edge of the corona bond.The results of micro-hardness tests show that the newly born martensite structure dramatically improves the hardness of the joint.Under the interactions between residual stresses and regenerated fine grains,the micro-hardness of the heat-affected zone(HAZ) is lower than that of the nugget,but evidently higher than that of the base metal.