Formation and mechanical performance of the pre-counter bore resistance rivet welded joints for thick cast aluminum and ultra-high strength steel

Abstract

Al/steel dissimilar joining represents a crucial strategy for vehicle body lightweighting. However, the increasing adoption of integrated die-casting with thick aluminum components in automotive manufacturing has introduced unprecedented challenges for Al/steel joining technologies. A novel approach to address thick cast Al to steel joining was developed through an innovative modification of resistance rivet welding (RRW). By incorporating a presetting counter bore prior to the traditional RRW process, high-performance flat surface joining between thick cast aluminum and press hardened steel (PHS) was successfully achieved, as demonstrated with 3.0 mm and 1.5 mm thickness specimens respectively. Through systematic investigation of process parameters, a comprehensive process window was established and detailed analyses of joint geometry characteristics were conducted. Microstructural characterization revealed that Al elements were uniformly distributed throughout the Al/steel mixed nugget. Under low heat input conditions, the nugget consisted of ferrite with an average grain size of 135.9 μm, while under high heat input conditions, it was composed of martensite with an average grain size of 3.9 μm. The joints exhibited mechanical performance, with a peak microhardness of 580 HV, tensile-shear strength of 10,691 N and cross-tension strength of 4057 N achieved. This study provides an effective solution for joining thick cast aluminum to steel and expanding the capabilities of dissimilar metal joining technology.