Abstract: Front wall beams and the support of A-Pillar are usually hot-stamped parts in side impact of vehicle, and front panel is made of mild steel with excellent formability by stamping, hence the area has formed a typical three-sheet resistance spot welded joints of unequal thickness and strength. In order to improve the overall load bearing capacity of spot welding joints, the paper investigates the influence of different lapping orders on overall mechanical properties of welded joints. Based on three kinds of material with unequal thickness and strength， B1500HS-1.4mm, B1500HS-1.6mm and DC06-0.8mm, the mechanical properties of different interfaces of two welded joints (strong-strong-soft and strong-soft-strong) under different lapping orders were compared. The nugget forming process, microstructure of nugget, fusion zone size and microhardness were also studied and analyzed. By investigating microstructure formation of nugget process of welded joints under different lapping orders, this paper reveals that the joint of strong-soft-strong can increase the bearing capacity. Finally, the article gives a reasonable design of strong-soft-strong lap joint in side impact of A-pillar and improves overall crashworthiness of vehicle.

Key words: resistance spot welding; three-sheet; lapping order; mechanical properties