Effect of Cr and Si Contents on Expulsion Phenomenon in Electric-Resistance Spot Welding of Advanced High-Strength Steels

An expulsion phenomenon occurred during resistance spot welding of advanced high-strength steel sheets, and the effects of the Cr and Si contents on the expulsion limit current (C-Exp) were investigated. The correlation between C-Exp and the steel properties (e.g., resistivity, melting point, coefficient of thermal expansion, high-temperature yield-strength and viscosity), were evaluated, as were the qualities of the alloying elements. The C-Exp decreased with increasing Si contents. This resulted in a narrow range of acceptable welding current, as well as poor suitability for spot welding. On the other hand, the effects of Cr-contents on the C-Exp, and on the acceptable range of welding current were minimal (with content of 3 wt%). Thus, the suitability for spot welding was not affected by the Cr-Contents (at the experimental concentration). As Si-Contents increased, resistivity also increased and the melting point decreased. This resulted in easy melting of the base metal at a low welding current (low heat input) and in the rapid increase of the pressure due to the increased coefficient of expansion. However, the high-temperature yield-strength of the corona bond, which blocked the expulsion of the molten melt, also decreased. Consequently, the C-Exp of steel containing additional Si was lower than that containing Cr.