Quantifying the link between crack distribution and resistance spot weld strength reduction in liquid metal embrittlement susceptible steels

Zinc-coated advanced high-strength steels (AHSS) used in the automotive industry are vulnerable to liquid metal embrittlement (LME) during welding. In this work, a crack index is used to link LME crack distributions in resistance spot welds to the weld performance. Other commonly used LME metrics, such as maximum crack length, are unable to predict an effect on weld performance. Because both crack size and location have been established as critical characteristics for loss in weld strength, both of these factors must be taken into account when characterizing LME severity. The crack lengths measured in LME-affected welds were observed to fit a lognormal distribution; therefore, the lognormal median is used as the parameter to account for crack size. Long cracks are more likely to adversely impact mechanical performance. Number of cracks can be used to account for the probability that a crack will be found in a critical location. These parameters are multiplied together and divided by the sheet thickness to produce the crack index. A large crack indicates that LME cracks in the weld are likely to impact the mechanical performance of the weld. A simpler version of the crack index may be calculated using the normal median.