ND-YAG LASER-WELDING OF COATED SHEET STEEL

The weldability of coated sheet steels by a 2 kW Nd:YAG laser has been examined. Laser seam welds were produced in 0.75-mm thick (23 gauge) galvanized and galvannealed sheet steels in the lap-joint configuration. Three types of laser beam power output were used: continuous wave, sine-wave modulated, and square-wave modulated. The effects on weld quality of varying laser welding parameters such as welding speed, shielding gas composition, and gas flow rate were studied. Also, welds were produced with three different joint geometries: no gap between the sheets, preset gaps between the sheets, and a new joint geometry consisting of a groove projection in the top sheet of the joint. Poor-quality Nd:YAG laser seam welds were produced in these coated sheet steels when there was no gap between the sheets of the lap-joint. However, high-quality seam welds could be made at welding speeds of up to 3.0 m min-1 by using preset gaps of 0.10-0.20 mm between galvannealed sheets, and up to 2.7 m min-1 by using a gap of 0.20 mm between galvanized sheets. Welds made with larger gaps (>0.20 mm) showed excessive drop-through of the weld metal into the gap. Finally, high-quality seam welds were produced at welding speeds up to 2.7 m min-1 using the new groove-projection joint geometry.