Wetting Mechanism in Steel-Ni-Mg Alloy Reactive System During Laser Brazing Process

There is much interest in joining light-weight magnesium alloys to steel sheet for automotive and other applications. However, Mg does not wet or bond to steel. The reason can be attributed to the large difference in their melting temperatures and nearly zero solubility of magnesium in iron. The only way to enhance the wettability of Mg and Fe is to introduce another element, which can interact and bond with both of them, between them as an intermediate element (interlayer). In this context, Ni has been observed to react and bond well with the steel and magnesium alloy. The aim of the present study was to identify the conditions required for the reactive wetting of the Ni electro-deposited steel by molten AZ92 magnesium alloy to improve wettability during the laser brazing process. The mechanism of the wetting and the driving force for spreading in steel-Ni-Mg alloy reactive system will be reported. One of the major outcomes of this study was that in steel-Ni-Mg alloy system, wetting occurs with extensive chemical reaction and the formation of new solid compounds in the form of solid solution and intermetallic compound at spreading molten Mg alloy-steel substrate. Hence, the nature of spreading in this system is influenced by the reaction between molten magnesium alloy and Ni coating layer on the steel. The final degree of the wetting and the spreading behaviour are controlled by the compound formed at the interface and not by the parent base metals.