Mechanism of pitting-adhesion coupling in aluminum alloy resistance spot welding electrodes and optimization of asymmetric electrode configuration

In resistance spot welding (RSW) of aluminum alloys, the reduction in nugget size caused by electrode pitting and adhesion remains a critical challenge restricting welding quality. This study reveals the influence of interface contact characteristics on nugget evolution and mechanical properties by controlling electrode surface conditions (adhesion/pitting degree), and proposes an electrode optimization strategy based on curvature radius matching. The results show that when 83 % of the electrode end face is adhered without pitting, the increased contact resistance reduces heat dissipation, expands the nugget diameter, and improves the tensile shear properties by 6.3 %. However, this condition decreases welding stability and induces recrystallization of the nugget equiaxed crystals, reducing crystalline defects. When the pitting area is less than 26 %, no significant effect on mechanical properties is observed; when the pitting area exceeds 55 %, the nugget size decreases by 6.7 %, and the tensile shear properties decline by 21 %. To address pitting issues, a combined scheme of an upper electrode with a large curvature radius and a lower electrode with a small curvature radius is proposed. Experimental and simulation validations show that the upper electrode delays pitting by increasing the contact area to uniform current distribution, enhancing electrode life by 2.5 times; the lower electrode (R80) utilizes a small curvature radius to concentrate heat generation, increasing the nugget size by 6.1 % and improving tensile shear properties by 6.4 %. Additionally, this study first discovers that current aggregation at the nugget edge during welding leads to a ring-shaped pitting phenomenon, which shifts outward as the nugget expands. This research provides a universal "functional partition optimization" strategy for RSW electrode design and a theoretical basis for analyzing and optimizing electrode degradation mechanisms.