Failure mechanism of laser welds of DP980 and HSLA450 steels in formability testing

In this paper, dual phase (DP) steel with banded martensite was investigated with YAG laser welding process in terms of formability by use of Limited Dome Height (LDH) testing method. One high strength low alloy (HSLA) steel with equiaxial ferrite matrix was included for comparison. The failure mode and dome height at fracture were examined on parent metal and welded blanks respectively. It was found that the failure mode of DP parent metal was determined by its rolling direction. The voids initiated between ferrite/martensite interfaces due to decohesion between two phases and propagated preferentially along the interface. For DP welded blanks, the failure occurred within the softened zone regardless of the orientation between weld and rolling direction. The voids formed at the interfaces between ferrite and tempered martensite. Significant dome height decrease from the parent metal was attributed to the softened zone formation. In comparison, HSLA and its welded blanks exhibited almost the similar dome height. The failure of welded blanks initiated from the hardened fusion zone and propagated perpendicular to the unaffected base metal.