The Effect of Zinc Coating Type on the Morphology, Joint Geometry, and Mechanical Properties of Weld-Brazed Thin-Gauge Automotive Steel

Advanced high-strength steels (AHSSs) are generally difficult to join using traditional fusion welding processes because the heat input required for steel fusion has a detrimental effect on the mechanical properties of the joint and the heat-affected zone (HAZ). An alternative non-fusion joining method called weld-brazing, has garnered serious attention in the automotive industry as it uses traditional welding heat sources to braze different types of alloys using lower melting temperature filler materials. The process offers significantly lower heat input which reduces HAZ softening while minimizing zinc burn-off and other welding defects. However, the literature available on this joining process is limited and the reported strength of these weld-brazed joints is typically very low for load-bearing applications. This study shows that weld-brazed joints can, in fact, be used for load-bearing applications by using the appropriate type of Zn-coated steel (i.e., galvanized (GI) or galvannealed (GA)) and properly controlling the joint geometry. The results showed that the type of Zn coating had a significant effect on joint strength, with the GA coating performing significantly better compared to the GI coating. A novel plasma cleaning surface treatment was used to separate the effect of root geometry from the effect of alloy composition, microstructure, and local mechanical properties of the braze at the root, showing that the root geometry and its alloying composition were the main determining factors that controlled the fracture mode and the joint strength of arc-brazed lap joints. By offering insight into the effect of two of the main factors affecting the mechanical properties of arc-brazed lap joints that have not been discussed in the existing literature, the results presented in this study are highly novel and of great relevance to the adoption of this transformative technology on a much wider scale.