Laser weldability of lap joints in zinc coated steel sheets

Summary

This paper describes a quantitative investigation of the static and fatigue strengths of laser-welded lap joints with different zinc coating weights of the steel sheets tested. The welding behaviour of the zinc coated steel sheets was observed by laser strobe. Based on the observed results, the mechanism of weld defect formation is examined. The results obtained may be summarised as follows:

1. The amount of spatter blown off the weldpool increases with increasing zinc coating weight of the steel sheets. When the steel sheets have a zinc coating weight of more than 40 g/m², some 30–40% of the total weight of molten metal is scattered as spatter.

2. Since blowholes are formed by spattering, the static and fatigue strengths of the joints decrease with an increasing zinc coating weight. For example at a zinc coating weight of 20 g/m², the joint strength is equivalent to that of uncoated steel sheets, whereas, at a zinc coating weight of 40 g/m², it is less than 50% of that of uncoated steel sheets.

3. There is a negative correlation between the amount of spatter and joint strength.

4. The gap width between the steel sheets suppresses spatter and blowholes. The zinc coating weight dependence of the critical gap width necessary to achieve complete suppression of blowholes is quantified.

5. With regard to the spatter generation process during lap welding of electrogalvanised steel sheets, the following considerations are based on the results obtained with the laser strobe. The molten metal formed at the keyhole front wall leads to the flow around the keyhole being disturbed by zinc vaporisation, with part being thrust inside the keyhole through globularisation due to surface tension. The laser beam is then incident on the molten metal droplets forced inside the keyhole, causing the surface to vaporise. Spattering is considered to be due to outward scattering of molten metal, whose driving force is the reactive force of the molten metal to vaporisation. That is to say, the driving force of the molten metal being ejected as spatter is therefore the laser beam, and the vapour pressure of the zinc only assists hot metal disturbance and spatter formation.