Enhancing coupling efficiency in laser keyhole welding of copper using femtosecond laser surface modification

Abstract

To improve laser beam coupling to the copper surface for the formation of a keyhole, a method of changing the surface reflectance of copper using femtosecond laser patterning is proposed in this study. For 2 kW fiber laser welding of copper, a 2 mm ? 5 mm pattern was fabricated on the copper surface at the welding start point, taking a maximum of 27 s to produce a suitable pattern. The surface reflectance of copper was subsequently reduced from 97% to as low as 3% (for a 1?m wavelength) by femtosecond laser patterning based on laser-induced periodic surface structures and their variants. Moreover, when a low reflectance pattern was employed, it was possible to increase welding speed by up to 300%, and once the melt pool was fully established, its size remained relatively stable for the length of the weld line. However, owing to melt pool fluctuations and the large conductive heat loss of copper, the depth and width of the fusion zone decreased and the laser coupling to the copper surface was lost in several cases. This problem was overcome by placing the patterns at regular intervals along the welding direction, indicating that this method has long weld-length applications.