Laser absorption of aluminium alloy in high brightness and high power fibre laser welding

Abstract

Laser welding of aluminium alloys is difficult because of their high reflectivity and heat conductivity. However, high brightness and high power lasers such as fiber laser or disk laser can produce narrow and deep penetration welds, when highly-efficient laser absorption is expected. The objective of this research is to measure laser absorption in bead-on-plate welding of A5052 aluminium alloy with a 200 μm fine laser beam spot under a wide range of conditions such as fiber laser powers of 2–10 kW or welding speeds of 17–250 mm/s by water-calori-metric method. Furthermore, the relationship between the laser absorption and the keyhole geometry was investigated by using X-ray transmission in-site observation system or 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 93% high. Compared with focusing feature of the incident fiber laser beam, X-ray images of the keyhole shape demonstrated that the centre part of the incident beam with a bell-shape profile could be delivered directly to the tip of a keyhole. Moreover, the increase in the welding speeds from 17 to 250 mm/s reduced the absorption from 93 to 72%. The high-speed observation pictures showed that the incident fiber laser beam was partly shot on the molten pool in front of the keyhole inlet at higher welding speeds, which might be the chief cause of the decrease in the laser absorption. Consequently, the tightly-focused high-power fiber laser welding was confirmed to be a highly-efficiency process of 93–72% in laser absorption.