Status of Analysis and Countermeasures
Abstract
During spot welding, some zinc coated Advanced High-strength steels (AHSS) are prone to sufer from Liquid Metal Embrittlement (LME) The zinc present at the surface of the steel melts due to its low melting point (420 °C) Under certain conditions (temperature, stress, time, etc ), a rapid penetration of the liquid zinc along the grain boundaries may cause embrittlement Cracks are then observed at diferent locations of the spot welds Avoiding LME or, at least, quantifying its impact on the behavior of the assembly becomes one of the main objectives of steel manufacturers and carmakers
LME cracking depends on the material and the welding conditions One material which could be considered as sensitive to LME in certain conditions could also be considered as non-sensitive in other conditions Furthermore, LME is a scattered phenomenon, the presence, occurrence and size of the cracks could vary from one spot weld to another The current paper gives an overview of analysis and countermeasures particularly on:
- Some testing conditions during welding processes that produce LME cracks with increasing severity of certain parameters (such as welding current, gap between sheets, misalignment, electrodes shape/diameter),
- Mitigation of cracking sensitivity by the type of stack-up (2 and 3 sheets in homogeneous and heterogeneous confgurations) and thus allowing access to certain part designs where the most severe confgurations can be ruled out,
- Efect of such cracks on the mechanical performances of spot-welds When these cracks occur on the surface of the spotwelds, they are not on a transfer path of force and therefore do not show a decrease in performance during static, dynamic and fatigue loading.
