2007 Volume 25 Issue 1 Pages 18-23
A high-power fiber laser can produce an ultra-high peak power density of MW/mm2 level corresponding to a focused electron beam, and is promising as one of the desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, fiber laser welding and simultaneous observation with high-speed video cameras and X-ray transmission real-time imaging system were performed with the objective of obtaining a fundamental knowledge of welding phenomena and defect formation mechanisms in producing deep and narrow weld beads in Type 304 stainless steel. A 6 kW fiber laser beam through φ 100 μm feeding fiber or φ 300 μm process fiber could be focused at 1.1 MW/mm2 or 120 kW/mm2 peak power density, respectively. Consequently, deeply penetrated weld beads with narrow width were formed, and fully penetrated welds in 8 mm thick plate could be produced at less than 2 and 4.5 m/min for 120 kW/mm2 and 1.1 MW/mm2, respectively. In the case of the ultra-high power density, humping weld beads were formed at more than 6 m/min. This reason was attributed to the intermittent melt ejection from a small keyhole inlet and the subsequent melt flows on the narrow molten pool surface. On the other hand, in the case of the high power density, weld fusion zones with porosity, sound partial penetration welds and underfilled weld beads with large spatters were obtained at the welding speeds below 3 m/min, between 4.5 to 8 m/min and above 10 m/min, respectively. These formation mechanisms were interpreted by considering bubbles formation from a keyhole tip leading to pores captured by the solidifying front, the formation of a long molten pool suppressing and accommodating spattering, and the production of a narrow, short molten pool causing easy generation of spattering, respectively.
