Relationship of laser absorption to keyhole behavior in high power fiber laser welding of stainless steel and aluminum alloy

This study was undertaken in order to assess absorption of a 10 kW fiber laser beam with tightly focused spot diameter of 200 μm in Type 304 austenitic stainless steel or A5052 aluminum alloy in bead-on-plate welding with a wide range of welding speeds from 17 to 300 mm/s, by using water-calorimetric measurement. The maximum absorptions of the steel and aluminum were 89 and 93% at the lowest speed, respectively. Their absorptions were high over 80% at less than 100 mm/s speeds and were reduced greatly by more than 20% at the highest speed. X-ray transmission in-site observation demonstrated that the center part of an incident beam with a bell-shape profile was delivered directly to a tip of a keyhole below 100 mm/s speed. High-speed video camera observation indicated that an incident beam was more partially exposed out of a keyhole inlet over 100 mm/s, which led to great decrease in laser absorption.