Dynamic wetting and spreading mechanisms regulated by elemental Ni in a Mg/steel immiscible system during laser processing

Magnesium alloys (Mg) and steels are immiscible as they do not react with each other and are virtually insoluble in the liquid state. This feature hinders their joining either by fusion welding or brazing, thus limiting the application of this dissimilar pair in the automotive and aerospace industries. The metallurgical bonding of these dissimilar metals will only be achieved by another alloy element as transitional interlayers. The introduction of Ni is generally considered an effective way to solve the aforementioned joining difficulties. Here, Ni coating was introduced on the steel surface via electroplating, and the wetting and spreading behaviors of the Mg alloy over the Ni coated steel substrate during laser processing were investigated. The influence by the amount of Ni on the wetting and spreading behaviors was comprehensively studied for the first time by evaluating the role of the thickness of Ni coating. With the introduction of Ni, the Mg atoms were more likely to move toward the Fe substrate, being the dominant factor in the spreading process from viscosity to adsorption and desorption. However, the excessive addition of Ni transformed the dominant spreading mechanism to reaction-controlled. The analysis of the interfacial microstructure revealed that α-Mg + Mg₂Ni generated in the triple line region and played an essential role in the wetting process. Moreover, the formation of dense Mg₂Ni intermetallic compound due to excessive Ni element diminished the spreading. This paper elucidates the regulation mechanism for alloying elements in the Mg/steel immiscible system during laser processing to further expand the lightweight applications based on improved process efficiency and product quality.