Defect-free joining of zinc-coated steels by bifocal hybrid laser welding

Abstract

Laser welding of zinc-coated steels in an overlap setup is prone to weld defects and seam expulsion reducing in particular the properties in mechanical loading and in general the deployability of such weldments in industry. Several laser welding process technologies failed to created defect-free welds in zinc-coated steels. This paper renders the welding of zinc-coated steels by the novel technology of bifocal hybrid laser welding. The zinc-coated steels under consideration are DX56D + Z, DC04 + ZE, and HXT700D. The bifocal hybrid laser system is realised by combining an Nd:YAG laser with a high power diode laser, both of 3 kW maximum output power. The beam parameter product (BPP) of the employed Nd:YAG laser of 25 mm mrad translates with an optical system of focal length f = 150 mm into a circular focus of diameter 0.45 mm, whereas the BPP of the HPDL of 85 mm × 200 mm mrad can achieve a rectangular focus of 0.9 mm × 3.7 mm. The optical system allows the respective focal plane and relative position of the foci to be independently vertically and horizontally positioned. This paper presents research into the causes of instabilities in laser welding of zinc-coated steels. Experimental evidence is considered and presented to establish the need for an empirical process model for stable laser welding of zinc-coated steels. The increase of process robustness is discussed.