Effects of laser focusing properties on weldability in high-power fiber laser welding of thick high-strength steel plate

The effect of various high-power laser-welding parameters on obtaining deep penetration welds without weld defects has been investigated. However, there are no studies on the effect of laser focusing. In this study, high-power fiber laser welding of a 12-mm-thick high-strength steel plate was performed by using two optics systems with different power density distributions and focus depths (2 or 4 mm) to investigate the effects of laser focusing properties on weldability. Full penetration welds without weld defects were obtained with the 4 mm focus depth optics system at low welding speeds of 25–50 mm/s. High-speed video and X-ray transmission images showed that the behavior of the molten pool on the top surface during laser welding was more stable for the 4 mm system than for the 2 mm system. The keyhole was stable with no large fluctuations, and no bubbles were formed in the keyhole. This result was attributed to keeping the power density within 50–120 kW/mm2 to maintain a stable keyhole shape during the welding. Consequently, it was concluded that the laser-focusing properties during laser welding of a thick steel plate affected the weldability strongly, and the optics system with long focus depth was particularly useful in producing sound welds.