Elucidation of dissimilar material joining phenomena on steel and aluminum alloy using hot-wire laser brazing

When steels and aluminum alloys are joined by fusion welding, brittle intermetallic compounds (IMC) are generated at the interface, which causes a decrease in joint strength. Dissimilar steel/aluminum alloy joints formed via hot-wire laser brazing can stably suppress IMCs, as elucidated by visualization during brazing and cross-sectional imaging of fabricated joints. In addition, the effects of various coatings on steel plates were investigated by comparing joint cross-sections and interfaces. Zn (galvanized, galvannealed) layers, flux, and a molten pool prevented direct laser-beam irradiation of the steel surfaces. A high brazing speed could enable a high cooling rate, creating a very low peak temperature of the steel and suppressing IMC formation. The galvanized Zn coating also prevented direct contact of the aluminum alloy with the steel surface. Thus, the galvanized steel suppressed IMC formation more than non-coated steel. Because a Fe-Zn alloy coating layer was a source of Fe for IMC formation, a thicker IMC was generated on galvannealed steel.