Wetting of galvanized steel by Al 4043 alloys in the first cycle of CMT process

Zn coating is essential for joining Al to galvanized steel by cold metal transfer (CMT) method; however the role of Zn on the wetting mechanism is still ambiguous. We recorded the first cycle for the molten metal transferring process using a high speed video camera, and further analyzed the wetting behavior and the interfacial microstructures. The results indicated that the significant contribution of Zn coating for the wetting is the reserved metallic characteristic of the galvanized steel surface after Zn evaporation. The residual liquid Zn film after evaporation could not improve the final wettability, and also should not be a driving force for spreading but may be a factor for contact angle hysteresis. Finally, a Zn rich zone was formed by the different affinity of Al-Fe, Zn-Fe and Al-Zn in thermodynamically, i.e., high affinity of Al-Fe and relative weak affinity of Zn-Fe and Zn-Al, and then the residual liquid Zn film was driven by the moving of molten Al alloys.