The impact of microstructure and interfacial phases on the mechanical properties and failure behavior of weld-brazed ZnAlMg coated steel

The use of weld-brazing in the automotive industry has grown significantly over the past few years in comparison to conventional fusion welding techniques, as weld-brazing reduces heat-affected zone softening, Zn burn-off, and distortion due to the lower heat input of the process. The weld-brazing process is considered an innovative non-fusion joining method used to join thin-gauge Zn-coated steels to both steels and dissimilar metals. In spite of this, the application of weld-brazing for the joining of advanced materials in the development of lightweight and crash-resistant automobiles has not been fully explored in the literature. The purpose of this study is to investigate the impact of travel speed on microstructural evolution, interfacial reactions, and mechanical properties of weld-brazed ternary ZnAlMg coated steel. Samples were fabricated from a ZnMgAl coated steel using the gas metal arc brazing (GMAB) technique where the torch travel speed was varied between 15 inch/min and 30 inch/min. It has been shown that the density of Fe-Si-Cu precipitates in the brazed beads increased with increased heat input (i.e., slower travel speeds) during the weld-brazing process. Additionally, the thickness of the interfacial region and the geometry of the beads were strongly influenced by heat input. The results of the shear-tension loading test revealed that the ductility of the brazed samples strongly depends on the microstructural and geometric characteristics of the beads. Digital image correlation (DIC) analysis of the shear-tension tests of the brazed samples demonstrated that strains were localized during loading both in the interfacial area and in the base metal region of the specimens. The results of this work expand the current state-of-the-art knowledge regarding the impact of heat input on microstructural evaluation and mechanical properties during weld-brazing of advanced ZnAlMg coated systems with the application of joining lightweight materials in the automotive industry.