Modelling Crack Behaviour During Formation and Growth of Liquid Metal Embrittlement

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Due to environmental challenges, regulatory bodies are requiring automakers to decrease the greenhouse gas emissions of their vehicles. One way automakers are accomplishing this is by reducing vehicle weight using ultra high strength steels (UHSS). This allows thinner gauges of steel to be used without sacrificing safety. However, UHSS is susceptible to a cracking phenomenon known as liquid metal embrittlement (LME) when joined using resistance spot welding (RSW). LME is an intergranular cracking phenomenon where liquid metal weakens the surface grains of a metal being contacted. Under tensile stress, grain boundaries in the solid metal separate, forming a crack. Much work has been done to understand LME, however, there are still many questions. Although there is some understanding of how LME cracking affects post-welded strength, there is no understanding of how material attributes, temperature and stress states exasperate LME, leading to cracking during welding. Without this understanding, LME cracking cannot be modelled nor can crack-free welding procedures be designed. The proposed research program seeks to understand the role of material composition, microstructure, and strength in the initiation and growth of LME cracking. This knowledge will be used to create a process model capable of predicting cracking to design welding parameters to minimize LME. The program seeks to understand the role of material characteristics on LME by carrying out a series of in-situ observations. Various materials will be observed when heated under tension, so that role of microstructure on crack initiation and growth may be studied. Further understanding will be gained by analyzing the cracked area using electron microscopy. This knowledge will be built on by in-situ observations of weld cross-section during welding, showing how the dynamics of welding affects crack formation and how grain texture results in cracking repeatability. With understanding of how material characteristics, stress, and temperature affect crack formation, a process model will be made to predict cracking during welding. This model will be used to develop new process parameters and machine modification to minimize LME occurrence during spot welding. The proposed research program will develop an understanding of LME that will connect the role of material characteristics such as: microstructure, grain boundary orientation, composition and strength to local weld temperature and stress to LME formation. This knowledge will be used to develop guidelines to design LME resistant steels. Furthermore, the developed welding process model will be integrated into mechanical models so that post-weld properties predictions may account to for the role of LME cracking. The results from this program will offer welding methodologies to help Canadian industries minimize LME formation, allowing automakers to produce crack-free welds in their assembly plants, expanding the use of UHSS.