Abstract
In the current work, 4 mm thick HSLA steel plates were joined by Nd:YAG laser welding. The scanning speed of the laser was varied in the range of 1.5–3.5 m/min while maintaining constant laser power. The microstructure evolution was observed using conventional and advanced microscopic techniques. The geometry of the fusion zone resembled a classic hour glass shape with a slightly extended width at the top surface of the joint. The microstructure of the fusion zone was composed of both lath-and tempered martensite for all welding conditions evaluated. The faster cooling rate associated with laser welding induced significant phase transformation. A reduction in size of the prior austenite grains was apparent at higher scanning speeds. Irregular and planar dislocations were observed adjacent to lath martensite and along sub–grain boundaries. Martensite formation in the fusion and heat affected zones implied significant local strengthening. This did however not imply that all joints failed in the base metal during mechanical testing, possibly due to the presence of strain localization.