This study was performed with the objective of obtaining a fundamental knowledge of welding phenomena and mechanisms in producing deep and narrow weld beads in Type 304 stainless steel with a high-power fiber laser beam. Welding penetrations and defects were classified in weld beads made with the laser of about 115 and 360 Pm in beam diameter at the power of 6 kW. A focused high-power fiber laser beam could produce deeply penetrated weld beads with narrow widths, and the weld penetrations and bead widths were extremely deeper and narrower, respectively, at higher laser power density with the smaller beam diameter. Molten pool and keyhole behavior, bubble/porosity generation and melt flows, resulting in characteristic weld beads and defects, were observed during welding through high-speed video camera and the X-ray transmission real-time imaging apparatus. Consequently the formation situations and mechanisms of porosity, sound weld beads and underfilled weld beads were interpreted in terms of bubbles formation from a keyhole tip at low welding speeds, the formation of a long molten pool at moderate speeds and the production of spattering and a narrow, short molten pool at high welding speeds, respectively.

High-power and high-brightness lasers such as disk and fiber lasers are recently developed to produce deep penetration welds. This study was therefore performed to produce deep-penetration weld beads by using high power disk lasers of 10 kW and 16 kW respectively or combined lasers, and a newly developed vacuum chamber with rotary pumps. It was consequently revealed that deeply-penetrated sound weld beads of 50 and 70 mm depth could be produced in Type 304 stainless steel under the conditions of 16 and 26 kW, 0.3 m/min and 0.1 kPa, respectively. Moreover, sound full-penetration weld beads without hot cracks and underfills could be made in HT980 high tensile strength steel plate of 23 mm in thickness.

Dual-phase Ni₃Al-based alloys feature extensive applicability even under high-temperature conditions. We selectively modified the microstructure of a representative dual-phase Ni₃Al-based alloy from equi-axed grains to unidirectional grains, using a kW-class high-power laser irradiation technique. On employing the laser probe to linearly scan the Ni-9 at.% Al-16 at.% V alloy specimen, the laser-irradiated region was partially molten and then immediately solidified from the two edges of the molten pool toward the center. Laser irradiation under low-speed scanning increased the molten pool width. The grains in the laser-irradiated region extended preferentially from the two edges toward the center; their crystal orientation was similar to adjacent substrate grains, suggesting epitaxial growth. Therefore, the unidirectional orientation of grains could be extended via wide-range scanning using laser irradiation. This suggests that the microstructure of the alloy can be selectively modified to unidirectional orientated grains by optimizing laser irradiation conditions, such as the power density, scanning speed, and scanning paths. The hardness of the laser-irradiated region decreased due to the phase transformation from the ordered Ni₃Al and Ni₃V phases to the disordered fcc phase. However, the hardness improved to a value comparable to that of the alloy subjected to heat treatment at 980 °C for 1 h.