Study and Applications of Dynamic Resistance Profiles During Resistance Spot Welding of Coated Hot-Stamping Steels
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TRODUCTION
RESISTANCE spot welding (RSW) is a process of joining two or more metal sheets at discrete spots and is the most common technique employed in joining sheet metal during body-in-white (BIW) assembly of automobiles.[1,2] It is based on the Joule heating effect where heat energy is generated by passing an electrical current through the joint over a given period. The heat generated melts the material, forming a nugget at the faying surfaces of the workpieces. The quantity of heat generated (Q) can be quantified (neglecting heat losses) by the equation: Q = i2ÆRÆt, where i is the current, R is the resistance, and t is the duration of the current. During the welding process, the electrical resistance of the joint changes, which may be measured during the cycle to reveal a characteristic dynamic resistance profile with respect to a set of parameters and material. Additionally, automotive steel sheets are manufactured with different coatings, which impart characteristic resistance profile during RSW. Thus, dynamic resistance profile may be used to study the process and perform non-destructive evaluation and quality monitoring of RSW process during production.[3–5]
OSAYANDE LORD-RUFUS IGHODARO and Y. NORMAN ZHOU are with the Mechanical and Mechatronics Engineering Department, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Contact e-mail: olighoda@ uwaterloo.ca ELLIOT BIRO is with the Mechanical and Mechatronics Engineering Department, University of Waterloo, and also with ArcelorMittal Global Research, 1390 Burlington Street East, Hamilton, ON L8N 3J5, Canada. Manuscript submitted January 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
Zn coating offers protection against corrosion. Al-Si coating also serves to protect press hardened steels against decarburization and oxidation during hot stamping. These coatings on steel significantly affect RSW parameters and joint properties;[6–8] therefore, dynamic resistance profile study of the different coating systems will help foster better understanding and quality monitoring of the joining stage during RSW manufacturing processes. It can also help in the development of coating systems for the enhancement of weldability by RSW process. For example, from dynamic resistance profile the time necessary to remove the coating from the faying surfaces or the change in resistance profile with the change in coating composition may be measured; this information can be used in the development or enhancement of the coating system for specific applications. Research has been performed on the dynamic resistance of Zn-based coating steels.[3,9,10] For example, Gedeon et al.[4] identified six stages in the dynamic resistance profile for galvannealed (GA) steels and four stages for uncoated steels. But no work has been performed on the dynamic resistance profile of Al-Sicoated steel. The research presented in this paper describes the resistance response of Al-Si-coated hot-stamping steel (Usibor) during RSW process, as compared to galvannealed coating on similar
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