Laser absorption in high-power fiber laser welding of stainless steel and aluminum alloy

High power and high brightness fiber lasers have been developed, and it is revealed that deep and narrow weld beads can be produced with an increase in the laser power and/or its density. High absorptivity of laser energy is expected, but there are few systematic experimental researches of laser absorption in conjunction with a beam diameter and a keyhole diameter and behavior. Therefore, this study was performed by utilizing the calorimeter method and the observation technique of welding phenomena to obtain a fundamental knowledge of fiber laser absorption in stainless steel and aluminum alloy. The laser absorption was measured under various welding conditions. It was confirmed that laser absorption of fiber laser was higher than that of YAG laser. It was found that the absorption of 10 kW fiber laser was about 90 % at 1 m/min, and decreased with an increase in the welding speed. In order to investigate the relationship between the absorption and the keyhole inlet, the keyhole behavior in the molten pool was observed by X-ray transmission imaging system and a high-speed video camera. It was found that the laser beam didn't completely enter into the keyhole at high welding speeds, and the laser absorption decreased because the keyhole position shifted backward from the laser irradiation point. The laser absorption of aluminum alloy was lower than that of stainless steel at low powers, but was higher than that of stainless steel at higher powers. Such higher absorption was attributed to the location and the size of a laser beam against a keyhole inlet.