A Comparative Study of Weldable Current Range on AC and MFDC Resistance Spot Welding for 440 MPa Grade Steel Sheet

Abstract

This paper presents a comparative study of the AC and MFDC resistance spot welding process with consideration of sheet thickness. The previous studies have confirmed that there is difference in the optimum welding current and expulsion current with AC and MFDC. The aim of this study was revealing the effect of sheet thickness on weldable current range and expulsion behavior for AC and MFDC welding processes. The optimum welding current of AC was lower (1.6 kA) than MFDC welding process in 0.8 mm sheet thickness. Early nugget growth being caused by the peak current of AC developed weld interface deformation, which resulted in suppressing the growth of corona bond and occurrence of low current expulsion. The resistance spot welding for thicker sheet (1.4 mm) required lower current of 0.6 kA for the expulsion on the MFDC welding process. The growth of contact diameter (size of corona bond) and button diameter was linear up to the expulsion current with MFDC welding process. Therefore, more attention is required when the AC and MFDC resistance spot welding process is applied for different thickness of steel sheet combination for automotive application.

Four Types of LME Cracks in RSW of Zn-Coated AHSS

“…Resistance spot welding is a fusion welding process used to join steel sheets in automobile production lines in which the heat required for melting is produced by passing a high current through small spots using copper electrodes (Refs. [7][8][9][10]. In RSW, LME cracks can significantly deteriorate the spot weldability of Zn-coated AHSS (Refs.…”

Liquid metal embrittlement during the resistance spot welding of galvannealed steels: synergy of liquid Zn, α-Fe(Zn) and tensile stress

“…In the automobile industry, resistance spot welding (RSW) is a fusion welding process that is used to assemble steel sheets; in this process, the heat required for melting is produced by passing high current through copper electrodes [4][5][6]. However, Zn coating is prone to liquid metal embrittlement (LME) cracking during RSW [7][8][9][10][11].…”