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ASM International 1059-9495/$19.00

Microstructures and Mechanical Properties of Resistance Spot-Welded Steel/Aluminum Alloy Joints with Process Tapes Yingying Che, Lei Wang, Daqian Sun, Hongmei Li, and Wenhua Geng (Submitted February 5, 2018; in revised form April 21, 2018; published online September 6, 2018) The microstructure and mechanical property of resistance spot-welded joints of steel and aluminum were studied by using process tapes. The steel/aluminum joint consists mainly of aluminum nugget and interface zone. The liquid nugget solidification has an epitaxial growth characteristic, and the interface zone displays a double-layer structure, Fe2Al5 layer at steel side and Fe4Al13 layer at aluminum nugget side. Compared with joints without process tape, the joint nugget diameter and tensile shear load increased by 35.45 and 10.98%, respectively, and its indentation ratio decreased by 56.25% due to the process tapes increasing heat input, producing larger nugget diameter and continuous interface layer, and protecting against deep indentation. The joining area of intermetallic layer and aluminum nugget is weak link in the joint. It is favorable to use the process tapes for improving welding quality, raising joint strength, and energy saving. Keywords

aluminum, Fe-Al intermetallic compounds, mechanical properties, microstructures, process tapes, resistance spot welding, steel

1. Introduction With the vigorous development of the global economy, reducing fuel consumption and gas emission has become a serious challenge for the automotive industry (Ref 1). Automotive lightweight is recognized as the most direct and effective way for that above, which has been a hot innovation topic of automobile manufacturers (Ref 2). Under the premise of ensuring the body safety, replacing steel with aluminum alloy to form a hybrid structure in the non-bearing parts of the automobile is a compromise between performance and cost (Ref 3). Consequently, the joining of dissimilar metals of steel and aluminum alloy has become the urgent problem to be solved due to their significant differences in physical and thermal properties and forming brittle Fe-Al intermetallics (IMC) at the steel/aluminum interface, which seriously deteriorates the mechanical properties of welded joints (Ref 4, 5). There are some research activities on the joining of steel and aluminum in recent years. The current methods are generally solid-state welding processes, such as the diffusion welding (Ref 6, 7), friction stir welding (Ref 8), ultrasonic welding (Ref

Yingying Che, Daqian Sun, Hongmei Li, and Wenhua Geng, Key Laboratory of Automobile Materials, Ministry of Education, School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PeopleÕs Republic of China; Lei Wang, Key Laboratory of Automobile Materials, Ministry of Education, School of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, PeopleÕs Republic of China; and Changchun Automobile Industry Institute, Changchu