Process optimization of aluminum/steel resistance spot welding based on dynamic resistance analysis

Because aluminum and steel have different characteristics, directly joining them one cannot obtain desired Al/steel welded joint. In order to improve the Al/steel resistance spot welding (RSW) process and achieve a goal of obtaining welded joint with satisfactory quality, this work analyzed the process using dynamic resistance variation. A whole Al/steel RSW process can be divided into three stages: no nugget stage, Al nugget stage and Al/steel double nuggets stage, according to different melting times of the parent metal sheets, which can be reflected in the dynamic resistance profile. Two optimization measures, which are increasing the roughness of contact surface between two parent metals and using spherical electrodes to replace traditional planar electrodes, are proposed. A two-dimensional (2D) numerical model was established and the calculation results showed that the optimization measures could increase the sizes of aluminum nugget and enable earlier melting of two metal sheets. In addition, a series of comparative welding experiments were conducted. Corresponding measurement and analysis results showed that the two optimization measures could improve the tensile-shear strength of the Al/steel welded joint, respectively, by 6.35% similar to 35.33% and 11.63% similar to 81.22%, with a corresponding comprehensive percentage improvement in at least 24.41%. Also, the measurers could promote the wetting and spreading level of liquid aluminum on the surface of solid steel, decrease length of cracks at the edge of the welded joint, and decrease intermetallic compound (IMC) layer thickness and make its distribution more even. This work provides information for process improvement and optimization in dissimilar metal welding.