Resistance rivet welding of multilayer dissimilar materials

Resistance rivet welding (RRW) process was applied to join three-sheet of Al/steel/steel. The formation mechanism, microstructure and mechanical performance of three-sheet dissimilar materials joints were studied in detail. The Al/steel/steel mixed nugget is produced by the fusion of Al/steel mixed and steel/steel nuggets. The microstructure of Al/steel/steel mixed nugget varies from mixed martensite and ferrite to full martensite as the increasing heat input. The ferrite contains numerous B2 structure nano-precipitates. The introduced Al element by the rivet cavity plays a significant influence on the microstructure, and the competition between ferrite and austenite stabilized elements in mixed nugget is responsible for the microstructure evolution. The phases performance of ferrite and martensite is determined. The average nano-hardness of ferrite is 2.66 GPa (271.3 HV), while that of martensite is 4.68 GPa (477.4 HV), and the yield strength for ferrite and martensite is calculated to be 475.0 MPa and 771.1 MPa, respectively. The average microhardness of the mixed nugget is 369.1 HV, 454.1 HV and 481.7 HV for welding parameters 7 kA-200 m s, 8 kA-200 m s and 9 kA-200 m s, presenting a trend of increasing with the heat input. Considering the multilayer joining containing both the Al/steel heterogeneous and steel/steel homogeneous interfaces, the comprehensive tensile-shear performance is optimal at welding parameters 8 kA-200 m s, 5334 N for Al/steel interface and 19,531 N for steel/steel interface.