Laser absorption characteristics in high-power fibre laser welding of stainless steel

Abstract

A tightly focused high-power fibre laser beam can produce a narrower and deeper penetration weld than a conventional laser weld of wine cup shape, which is expected to improve laser absorption. The objective of this research was to assess laser absorption in a wide range of conditions such as laser powers of 2–10 kW or welding speeds of 17–250 mm/s in bead-on-plate welding of Type 304 austenitic stainless steel with a laser beam of 200 μm spot diameter by water-calorimetric method. Furthermore, the relationship between the laser absorption and the keyhole formation location to a focused laser beam was revealed by using X-ray transmission in-site observation system and high-speed video camera with diode-laser illumination. It was found that the absorption at 10 kW laser power and 17 mm/s welding speed was 89% high. Compared X-ray transmission observation images of the keyhole with the focusing feature of the fibre laser beam and the centre part of the incident beam with a bell-shaped profile were delivered directly to the tip of a keyhole. Moreover, the increase in the welding speed from 17 to 250 mm/s reduced the absorption from 89 to 65%. The high-speed observation pictures indicated that the incident fibre laser beam was partially exposed out of the keyhole inlet at higher welding speed, which led to the decrease in the laser absorption. Consequently, it was confirmed that the tightly focused high-power fibre laser welding was high-efficiency process at the maximum of about 90% in laser absorption at low welding speeds, owing to the irradiation of almost all the incident laser beam into the keyhole inlet. It was also assessed that the energy loss due to vapourization was smaller than 1%.