Understanding of LME Cracking Phenomenon in Spot Welding and Crack Prediction Using FE Analysis 2022-01-0328

The application of high-strength steel sheets to car bodies is expanding to improve the crashworthiness and achieve weight reduction [1, 2]. Conversely, in recent years, the occurrence of liquid metal embrittlement (LME) cracks has been discussed in resistance spot welding using a Zn-based coated high-strength steel [3-5]. This study examined the factors causing LME cracks and identified the locations of LME cracks found in resistance spot welds using a Zn-coated high-strength steel sheet. Furthermore, through an analytical approach using a scanning electron microscopy (SEM) and transmission electron microscopy (TEM), for a joint with an LME crack, it was found that (1) grain boundary fracture occurred at LME crack portion and its fracture surface was covered with Zn, (2) Zn penetrated into prior-austenite grain boundaries near the LME crack, and (3) Zn concentration decreased toward the tip of the Zn-penetrated site. From these results, it was possible to see that the concentration of Zn in the prior-austenite grain boundary in the spot weld increased during welding, the concentration exceeded the threshold, and the grain boundary melted. Additionally, the influence of the gap (also known as clearance) between the lower sheet and electrode before spot welding, as an assumed industrial noise, on the LME crack was investigated. The maximum length of an ‘inner crack’ at the outer-edge of the corona-bond was found at a certain gap. In the case of a high current, an ‘outer crack’ also occurred at the indentation center, in addition to the inner crack. Finite element (FE) analysis revealed that high tensile stress occurred at the site where the LME crack was expected, at a higher temperature over the melting point of Zn after the release of the electrode, under conditions of longer LME cracks. Utilizing FE analysis led to the prediction of the occurrence of LME cracks and a deeper understanding of LME phenomenon.