Optimization of aluminum-to-steel friction stir lap welding for the fabrication of high-integrity structural components

Abstract

Friction stir lap welding is a solid-state joining technique used to effectively fabricate high-quality lightweight and energy-efficient hybrid structures by overlapping thin parts of Al and other similar or dissimilar alloys. In this study, friction stir lap welding between 6061-T651 Al and JAC270 45/45 galvanized steel sheets was investigated, considering both single-pass and double-pass protocols to optimize the process and quality of the resulting welds. The single-pass welding was first studied by conducting experiments at constant tool rotation and traverse speeds (1200 RPM and 1 mm/s) and various tool plunge depths (0.05, 0.2, and 0.4 mm). The microstructures and Al-steel interfaces of the welds were characterized using microscopy and energy-dispersive X-ray spectroscopy, and the weld strength was evaluated by lap shear tensile testing. The defects and intermetallic compounds that form at the Al-steel interfaces were identified as the limiting factors for the weld strength. The intermediate tool plunge depth of 0.2 mm resulted in the highest weld strength due to the least amount of interfacial defects and intermetallic compounds amongst the studied single-pass welds. A double-pass welding method was then developed to optimize the strength of the Al-steel lap welds. The selection of the processing conditions for the double-pass welding was guided by the behavior and properties of the single-pass welds. The double-pass welding successfully eliminated the defects and intermetallic compounds from critical locations, creating a robust Al-steel interfacial bond and significantly improving the weld strength. The knowledge and findings in this study can be applied to optimize the design, material integration, and mechanical properties of lap welds for high-integrity structural components.