Weldability and Keyhole Behavior in Remote Welding of three Zn-Coated Steel Sheets

Abstract

Zinc-coated steels are widely used automobile bodies. Laser welding, which offers much advantage over the conventional welding with MAG, CO2, arc, etc. in terms of improved weld quality, high-speed and easy automation, has been developed for cars. However, in laser lap welding of zinc-coated steel sheets without gaps, defects such as underfilled beads or porosity are easily formed due to the higher pressure of zinc vapor trapped in the molten pool because of the lower boiling point of zinc (1180 K) with respect to the melting point of steel (Fe, 1803 K). Laser lap welding results of two Zn-coated steel sheets have reported. However, there are not enough data for welding of three Zn-coated steel sheets. Therefore, to understand laser lap weldability of three Zn-coated steel sheets, lap welding of two or three sheets with and without gaps was performed, and then molten pool motions, spattering and keyhole behavior during welding were observed by high speed video cameras and X-ray transmission real-time imaging apparatus. Lap welding of three steel sheets was difficult but acceptably good welds were produced in sheets with the upper and lower gaps of 0.1 and 0.1mm, 0.1 and 0.2mm or 0.2 and 0.1mm, respectively. Bubble generation leading to porosity formation was observed, and it was confirmed that welding phenomena were different depending upon the gap levels.