Modelling Crack Behaviour During Formation and Growth of Liquid Metal Embrittlement

由于环境挑战，监管机构要求汽车制造商减少其车辆的温室气体排放。汽车制造商实现这一目标的一种方法是使用超高强度钢（UHSS）来减轻车辆重量。这样就可以在不牺牲安全性的情况下使用更薄的钢规。然而，当使用电阻点焊（RSW）连接时，超高压ss容易受到称为液态金属脆化（LME）的开裂现象的影响。LME是一种晶间裂纹现象，液态金属使与之接触的金属表面晶粒变弱。在拉伸应力作用下，固体金属中的晶界分离，形成裂纹。人们已经做了很多工作来理解LME，然而，仍然存在许多问题。虽然人们对LME裂纹如何影响焊后强度有了一定的了解，但对材料属性、温度和应力状态如何加剧LME，导致焊接过程中的裂纹却没有了解。如果不了解这一点，就无法模拟LME裂纹，也无法设计无裂纹焊接程序。拟议的研究计划旨在了解材料成分，微观结构和强度在LME裂纹的产生和发展中的作用。这些知识将用于创建能够预测裂纹的过程模型，以设计焊接参数以最小化LME。该计划旨在通过开展一系列现场观测，了解材料特性对LME的作用。在拉伸下加热时观察到各种材料，从而研究微观组织对裂纹萌生和扩展的作用。利用电子显微镜分析裂纹区域将获得进一步的了解。这些知识将建立在焊接过程中焊缝截面的现场观察基础上，展示焊接动力学如何影响裂纹的形成以及晶粒结构如何导致裂纹的可重复性。了解了材料特性、应力和温度如何影响裂纹的形成，就可以建立一个过程模型来预测焊接过程中的裂纹。该模型将用于开发新的工艺参数和机器修改，以尽量减少点焊过程中的LME发生。拟议的研究计划将发展对LME的理解，将材料特性的作用联系起来，例如：微观结构、晶界取向、成分和强度与局部焊接温度和应力对LME形成的影响。这些知识将用于制定抗LME钢的设计指南。此外，开发的焊接过程模型将集成到力学模型中，以便焊后性能预测可以考虑LME裂纹的作用。该项目的成果将提供焊接方法，帮助加拿大工业最大限度地减少LME的形成，使汽车制造商能够在装配厂生产无裂纹的焊缝，扩大超高压焊接技术的使用。

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.