Effects of Metallic Coatings on Resistance Spot Weldability of Hot Stamping Steel

Abstract

Advanced high strength steels (AHSS) are being increasingly incorporated into automotive body structure to reduce vehicular weight without adversely affecting crashworthiness. Ultra-high strength steel (UHSS), a sub-set of AHSS, possess excellent anti-intrusion properties which make them very suitable for fabricating crash relevant automotive structures. The steel is usually coated as a necessary preparation for further processing. During resistance spot welding, the principal joining method for body-in-white automotive structures, the contact resistance, which is principally determined by the properties of the coating, play key role in defining welding parameters, thermal profiles, microstructure and eventually, sometimes, mechanical properties of the joint. In this work, the contact resistance associated with AlSi and galvannealed coatings on Usibor 1500 hot stamping steel were determined using their dynamic resistance profiles. The results showed significant differences in the contact resistance values associated with the different coatings, and the trend in these values represented trends in current requirements for attaining acceptable nuggets sizes during resistance spot welding. Although peak load for tensile shear loading in static mode was similar for the AlSi and galvannealed coated sheets, there were significant differences in failure mode transition characteristics and energy absorption capabilities associated with the different coatings. Moreover, it was found that joints formed from dissimilar combinations of AlSi and galvannealed coated sheets exhibited poorer mechanical properties than either of AlSi or galvannealed coated sheets.