Effect of Double-Pulse Strategy on the Expulsion Formation and Peak Loads During Resistance Spot Welding of Dissimilar Thickness Ultra - High Strength MS1500 and Mild DD11 Steels

Resistance spot welding (RSW) is widely utilized in the automotive industry due to its high efficiency, flexibility, and compatibility with automation. These advantages make it a preferred method for joining advanced high-strength steels, enabling lightweight designs without compromising structural integrity. This study explores the mechanical properties of resistance spot-welded joints between ultra-high-strength MS1500 steel and hot-rolled DD11 steel, with a focus on single-pulse and double-pulse welding strategies. The effects of varying current levels were evaluated in terms of tensile-shear strength, failure energy, nugget diameter, hardness distribution, and microstructural transformations. In single-pulse welding, expulsion was observed at a current of 8.6 kA, limiting joint performance. Conversely, the double-pulse welding strategy provided enhanced control, yielding superior results. Optimal performance was achieved at a second welding current of 8.4 kA, with a tensile-shear strength increase of 11.6% and a fracture energy enhancement of 32.2% compared to single-pulse welding. Beyond 8.6 kA, expulsion caused inconsistencies in mechanical properties, highlighting the importance of current optimization in welding strategies. The findings demonstrate the effectiveness of the double-pulse welding strategy in improving the strength and quality of resistance spot-welded joints.