Improving the Resistance Spot Welding Performance of Al-Si-Coated Press-Hardened 22MnB5 Steel through Preheating Treatment

This study introduces a novel approach to optimize the weldability of Al-Si-coated press-hardened 22MnB5 steel, focusing on the effects of preheating treatment on the coating microstructure. Despite extensive research on heat treatment during hot stamping, the impact of preheating prior to resistance spot welding on the coating structure and welding quality remains unexplored. By analyzing the evolution of coating microstructure at various preheating temperatures and durations, unique phase transformations within the Al.Si coating are revealed, with altering composition and sheet resistance. Below 750 °C, limited Fe diffusion retains a high-Al.Si layer (0.0031 Ω). As preheating temperature increases, the diffusion rate of Fe atoms within the coating gradually increases, transitioning the coating structure to a three-layer α-Fe/Fe₂Al₅/α-Fe, with a resistance value of 0.6312 Ω, 200 times higher than that without preheating. Tensile shear load increases with preheating temperature up to 950 °C, where optimized nugget size and martensitic transformation enhance joint strength. However, excessive preheating (1050 °C) causes nugget expulsion, reducing performance. This study not only advances the understanding of preheating treatment in Al.Si-coated steels, but also provides a strategy for enhancing weldability in lightweight automotive manufacturing.