Study of solidification cracking during laser welding in advanced high strength steels

Abstract

Preventing solidification cracking is an essential prerequisite for the safety of a welded structure. An undetected solidification crack has the potential to cause premature failure during service. Two conditions generated by a weld thermal cycle are responsible for the initiation of solidification cracks. The first is the presence of excessive stresses/strains imposed on the solidifying weld metal and the second is the existence of a weak solidifying microstructure. For more than five decades, weld solidification cracking has been a subject of considerable interest. Cracking has been observed in various alloys, used in a wide range of engineering applications. Despite achieving a better understanding over this period, an accurate prediction of the occurrence of solidification cracking under a specific set of conditions remains difficult. An alloy with a high susceptibility to solidification cracking can still exhibit good weldability upon selection of appropriate welding conditions. Conversely, an alloy with supposedly high resistance to cracking, can still fail when subjected to inappropriate welding conditions.

The objective of the research work reported in this dissertation is to study and elucidate the solidification cracking phenomenon in two popular and commercially available automotive sheet steels, namely transformation-induced plasticity (TRIP) and dual phase (DP) steels. In particular, the effect of restraint (strain imposed), shape of the weld pool, solidification morphology, segregation, solidification temperature range, dendrite coherency and interdendritic liquid feeding on susceptibility to solidification cracking is considered.