Effect of heat source and Zn-coating type on the geometry and morphology of laser-brazed thin-gauge steels

Laser brazing (LB) offers significant advantages over other types of traditional seam joining methods: the ability to join dissimilar materials without significant melting of the substrates, reduced Zn burn-off in Zn-coated steels, and minimal heat-affected zone (HAZ) formation. However, the edge (or the tip) of laser-brazed beads can be one of the potential cracks initiations sites which results in catastrophic failure of the parts. The existing literature on LB offers no insights into the geometry and elemental segregation at the edge of the brazing bead as a result of the interaction between the laser beam and the substrate. To bridge this gap in knowledge, LB was conducted in the bead-on-plate configuration on two different types of Zn-coated hot-dip galvanized (GI) and galvannealed (GA) advanced high-strength steels (AHSS) using a Si-Bronze filler wire. LB provided an improved wettability because of the formation of a tail-like geometry at the bead edge of both coatings showing a high Zn concentration, which is opposed to what has been observed in similar studies on gas metal arc brazing (GMAB). LB had a significant effect on modifying the surface morphology of the GA-coating, which has previously not been reported in the literature. Similar Zn distribution of LB samples at the bead edge of both coatings clearly showed that the wettability of molten filler material in LB is improved relative to GMAB.