Mechanism of Zn Coating on the Wettability, Spreadability, and Microstructure of Al/Steel with the Laser Welding–Brazing Method

The wettability and spreadability of the molten Al drop on the surface of bare steel and galvanized (GI) steel was studied. It was expected that the role of Zn coating during the laser welding–brazing process of Al/steel could be understood through this study. The bare steel without coating and the GI steel with different Zn coating thicknesses of 10 and 20 μm were used. Different laser power was considered. The welding time was 1000 ms. Deposited filler metal ER4043 was applied, and the feeding rate was 5 m/min. The transferring, wetting, and spreading process of the molten drop on the steel surface was observed and recorded by a high-speed video camera. The temperature field was measured by an infrared thermometer. The microstructure of the joint was observed and analyzed after welding. The GI steel with a 10-μm coating improved the wettability of Al/steel compared with the bare steel, but the GI steel with a 20-μm coating deteriorated the wettability and the weld appearance compared with the GI steel with a 10-μm coating. The evaporation of Zn coating could protect the steel surface from oxidation and absorb the excessive heat input. The formation of the brazing interface and Zn-rich zones was revealed based on the adsorption values of Al-Si/Fe and Al-Zn/Fe systems.