Method development for the avoidance of liquid metal embrittlement during resistance spot welding of advanced high strength steels

到目前为止，对液态金属脆化(LME)的研究主要集中在该现象的单个方面-或分析特定的影响因素、材料厚度组合(MTC)或裂纹强度。因此，一些现有的研究陈述了有争议的结果。因此，不可能确定各种调查之间的主要关联。到目前为止，还没有一种整体的方法，系统地研究LME的影响因素，方法一致。为了推进轻量化设计理念，有必要研究极限抗拉强度在1000兆帕以上的先进高强度钢(AHSS)牌号。这些高强度钢种在电极压痕区域内外都出现了LME裂纹，这对接头强度至关重要。此外，它们还显示出LME裂纹的穿透深度显著更高。为了提高加工AHSS的可信度和安全性，有必要对LME裂纹类型进行全面表征。应建立不同裂纹尺寸和几何形状的精确分类，以便能够确定可容忍/临界LME强度，以确定对受裂纹影响的点焊承载能力的影响。温度场、应力场和应变场及其相互作用的数值模拟支持实验研究。这些值不能用传统的测量技术获得。从而提高了对过程的理解，扩大了实验测试的参数范围。它允许以精细的增量复制不同类型的裂纹。没有实验散射，以及在裂纹位置和大小方面的高度灵活性，提供了任何实验设置之外的可能性。所描述的方法适用于评估不同的MTC，以及制定潜在的LME预防策略。

Until today, investigations on liquid metal embrittlement (LME) mainly focus on single aspects of the phenomenon only – or analyze specific influencing factors, material thickness combinations (MTC) or crack intensities. As a result, some of the existing studies state controversial results. Consequently, it is impossible to identify principal correlations between the various investigations. So far, a holistic approach is missing, which systematically investigates influencing factors of LME with a consistent method. Investigations on advanced high strength steel (AHSS) grades with an ultimate tensile strength above 1000 MPa are necessary to facilitate lightweight design concepts. These high strength steel grades exhibit LME cracks both inside and outside the electrode indentation area, which is critical for the joint strength. Additionally, they show a significantly higher penetration depth of LME cracks. A comprehensive characterization of LME crack types is necessary to improve confidence and security in processing AHSS. A precise classification of different crack sizes and geometries shall be established, enabling the determination of tolerable/critical LME intensities regarding effects on the load bearing capacity of crack-afflicted spot welds. Numerical simulations of temperature, stress and strain fields and their interactions support the experimental investigations. These values are not accessible with conventional measurement techniques. The simulation thereby improves the process understanding, and extends experimentally tested parameter ranges. It allows the reproducible replication of different crack types in fine increments. The absence of experimental scattering, as well as a high grade of flexibility in terms of crack position and size, provide possibilities beyond any experimental setup. The described approach is suitable for evaluation of different MTCs, and development of potential LME prevention strategies.