Direct Modeling of Liquid Metal Embrittlement in Resistance Spot Welding of GEN3 Steels

It is well known that some metals that are ductile in air can become brittle when exposed to certain liquids and this phenomenon has been experimentally assessed for over a century. When in contact with liquid metals such as Hg, Ga, Bi and Zn, ductile metals such as Al, Ni, Fe and Cu are prone to brittle fracture along the grain boundaries at low stress levels. The specific embrittlement caused by a liquid metal in contact with a solid metal is called liquid metal embrittlement (LME) in the literature. To develop a numerical model capable of indicating the LME onset during resistance spot welding simulation we leveraged experimental data to create an LME indicator to be used in finite element simulations. The concept behind the LME indication model is to use force-displacement curves obtained during thermomechanical Gleeble experiments to capture the stress level at which the force dropped below a certain threshold in the loading phase. These tests were carried out for both uncoated and coated conditions so one can assess the relative material weakening caused by the LME effects on the substrate. The data points for the coated material were curve-fitted on Excel to obtain an equation representing the overall trend across the relevant temperature range. This equation is used first for a direct observation in the results visualization to understand how the process evolves, e.g., points in the model can be tracked to obtain a time series of the LME indicator. To facilitate the understanding and have a clear picture of the LME onset and severity at the end of the spot welding process, the fitted equation was coded in a subroutine that captures the max LME value in the model throughout all time steps.