Modelling Crack Behaviour During Formation and Growth of Liquid Metal Embrittlement

由于环境方面的挑战，监管机构要求汽车制造商减少车辆的温室气体排放。汽车制造商实现这一目标的一种方式是使用超高强度钢(UHSS)来减轻车辆重量。这使得可以在不牺牲安全的情况下使用更薄的钢规。然而，当使用电阻点焊(RSW)连接时，UHSS容易出现一种称为液态金属脆化(LME)的开裂现象。LME是一种晶间开裂现象，即液态金属削弱了被接触金属的表面颗粒。在拉应力作用下，固态金属中的晶界分离，形成裂纹。人们已经做了很多工作来理解LME，然而，仍然有很多问题。尽管人们对LME裂纹如何影响焊接后强度有一些了解，但对于材料属性、温度和应力状态如何激化LME，导致焊接过程中出现裂纹，目前尚无了解。如果没有这种理解，LME裂纹就无法建模，也无法设计无裂纹的焊接程序。拟议的研究计划旨在了解材料成分、微观结构和强度在LME裂解的引发和发展中的作用。这些知识将被用来创建能够预测开裂的过程模型，以设计焊接参数以最大限度地减少LME。*该计划试图通过进行一系列现场观察来了解材料特性对LME的作用。在拉伸下加热时，会观察到不同的材料，从而可以研究微观组织在裂纹萌生和扩展中的作用。通过对裂纹区的电子显微镜分析，将会得到进一步的了解。这些知识将建立在焊接过程中对焊缝横截面的现场观察基础上，展示焊接动力学如何影响裂纹形成，以及颗粒织构如何导致裂纹的重复性。在了解了材料特性、应力和温度对裂纹形成的影响后，将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改进，以最大限度地减少点焊过程中LME的发生。*拟议的研究计划将加深对LME的理解，将材料特性(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成形的作用联系起来。这些知识将被用来制定设计LME耐钢的指导方针。此外，所开发的焊接过程模型将被集成到力学模型中，以便焊后性能预测可以考虑LME裂纹的作用。该项目的结果将提供焊接方法，帮助加拿大工业将LME的形成降至最低，使汽车制造商能够在其组装厂生产无裂纹焊接，扩大UHSS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.**