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

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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.