The liquid metal embrittlement during the resistance spot welding of high-strength steel sheets.

The car manufacturers always try to decrease the weight of the vehicle and in this manner fuel utilization and carbon dioxide emissions. Especially in the auto-body, lightweight material development is continuously on progress, however, the occupant safety must be ensured in all conditions. These necessities request high-quality steels with good forming and crash attributes, as well as adequate joining methods. In the manufacturing of vehicles, advanced high strength steels (AHSS) are used in an effort to reduce body-in-white weight while at the same time accomplishing an enhancement in structural reinforcement, and Resistance Spot Welding (RSW) is the main joining technique. During the resistance spot welding of zinc-coated AHSS (including the case of dissimilar joining between zinc-coated AHSS and other low alloy sheet steels), liquid zinc leaks between the grain boundaries in the heat-affected zone (HAZ), causing the liquid metal embrittlement (LME) phenomenon. This contributes to the AHSS cracking and final failure. Despite exploration for over a century, the processes behind the LME are not yet fully understood. The targets of this investigation are to reproduce the LME of RSW joints performed on AHSS in a laboratory environment, to develop methods for the metallographic assessment of the ensuing cracks, to illustrate the cracking morphology, and to recognize the weld microstructures which are vulnerable to LME, while taking into consideration different welding parameters.