Laser beam welding of lap joints of dissimilar materials

Abstract

This investigation has been concerned with CO2 laser welding of austenitic stainless steel in lap joints with Zn-coated carbon steel. The focus was made on weld joint quality in terms of weld profile, porosity in the weld zone and Liquid Metal Embrittlement (LME) cracking of the austenitic stainless steel base metal. The influence of type and flow rate of shielding gas, gap between the sheets, and zinc removal prior to welding was clarified. Complete weld joint penetration with acceptable weld profile free from pores can be made without a gap between the sheets providing a relatively high flow rate, 30 l/min of argon or helium is used as a shielding gas. Welds made at higher flow rates showed unacceptable weld profile. Shielding gas flow rate can be reduced to 15 l/min by using a preset gap of 0.025 - 0.05 mm between the sheets. Welds made by larger gap showed excessive drop-through of the weld metal into the gap. However, LME cracking induced in the austenitic stainless steel base metal by molten zinc cannot be avoided, regardless of the type and flow rate of shielding gas and gap distance between the sheets. The only way to produce high quality lap welds of these dissimilar materials to avoid both porosity in the weld zone and LME cracking in the austenitic stainless steel is the complete removal of zinc coating from the joint area prior to welding.