Johan Löthman, Erik Tolf, Joakim Hedegård, Tomas Forsman

Abstract

Laser beam welding using two laser beams has become an emerging welding technique in recent years. In this study the twin-spot welding was created by beam splitting of a 3kW Nd:YAG beam into two beams of varying power, interdistance and beam positioning. Experimental results indicated that twin-spot welding could significantly improve the weld quality of zero gap lap joints of thick zinc coated C-Mn steel sheets. For 20 μm zinc coated DOGAL 350 YP (t=1.0mm), the amount of porosity and spatter was reduced, reaching weldment quality of weld class B (according to SS ISO 13919-1). In some cases a minimum weldment width of 0.9 × “sheet thickness" was difficult to achieve in the tandem configuration of the two laser beams. In these cases the weldment width could be achieved by an angular offset of the two laser spots of up to 10° relative to each other.

The theory proposed suggests that the keyhole should be enlarged in the welding direction to reduce the upstream pressure of vaporising zinc from the lap joint interface. Thereby closure of the keyhole and ejections of molten metal is avoided. A minimum vapour column width and a maximum molten metal width surrounding the column in a keyhole should be established for successful welding. In butt joints of 2 mm DOCOL 600 DP sheets, the twin-spot technology was also used to investigate improved gap bridging ability. Gaps of up to 0.2 mm could be bridged with single-spot welding. For twin-spot welding, the gap bridging ability could be increased to 0.6 mm at low welding speeds..