Effect of resistance spot welding parameters on the residual stress of automotive ultra high strength steel

Abstract

Studies on residual stress effect in resistance spot welding (RSW) have not been widely explored for the new ultra-high strength steels (UHSS), such as, press-hardened steel (PHS) used for the automotive industry application. The number of PHS parts has been widely increasing in the vehicle structure, due to its ultra-high tensile strength achieved after the hot stamping process, which can reach 1500 MPa. This outstanding steel properties is function of steel grade and is a guaranty for lightweight and safety. Weldability is one of major issues to allow the large product application in the automotive industry however, the numbers of papers and works with focus on PHS weldability is still small. The residual stresses in spot welded joints play an important role in affecting the durability of the joint. It is well known that the residual stresses may reduce the mechanical properties of the welding joint and contribute to hydrogen embrittlement effect. This paper has undertaken an experimental survey of residual stress on 22MnB5 - PHS steel, using X-ray diffraction technique, before any heat treatment, i.e., without hot stamping, with variation of the main welding parameters. In general, it was found that the residual stress results showed a great influence of the resistance spot welding parameters. The residual stress measures showed levels up to 300 MPa in PHS material, which can impair the material usage.