Effect of Process Parameters and Nugget Growth Rate on Liquid Metal Embrittlement (LME) Cracking in the Resistance Spot Welding of Zinc-Coated Steels

Abstract

Although the influence of weld process variables on LME cracking was known to be significant, limited studies have been conducted on the effect of process variables with systematic approaches in the equivalent nugget growth behavior and heat input. This study aimed to identify the effect of weld process variables on LME sensitivity with the equivalent nugget diameter and underlying mechanism with induced tensile stress for cracking. Among the welds with equivalent nugget diameters in the combination of different welding current and time, higher LME sensitivity was observed with the high welding current and short welding time combination than that with the low welding current and long welding time combination for the equivalent nugget diameter. Because a high current and short time combination resulted in faster weld nugget growth than the low welding current and long welding time combination, it rapidly increased the surface temperature along with the cooling from the electrode. These combined effects induced a higher thermal gradient and thermally induced tensile stress on the weld surface, satisfying the conditions of the LME cracking. The simulation results also confirmed that the critical weld cycle time of the LME cracking (<i>t_c</i>), which is the cross point between the nugget growth diameter and a contact diameter of the electrode, could be different with the combination of the weld process variables with the equivalent weld nugget size. Therefore, <i>tc</i> can be applied for the sensitivity index of LME cracking of the resistance spot weldment considering complex weld variables.