Enhancement of wettability in AZ91/B4C system by a mechanical purification of liquid alloy

Mg-based alloys are potential candidate materials for a fabrication of lightweight boron carbide based composites through a reactive melt infiltration approach. In this paper, the effect of a mechanical purification of molten AZ91 alloy's surface on its wettability with polycrystalline B4C is experimentally evaluated for the first time. For this purpose, sessile drop experiments were performed under the same operating conditions (700 °C/5 min; Ar atmosphere), by using both the classical contact heating (CH) and the improved capillary purification (CP) procedure. It was found that the evolution of contact angle values was strongly influenced by the applied procedure. In particular, by using the classical CH procedure, the presence of a native oxide layer on the metal surface hinders the observations of melting process, resulting in a misleading conclusion that the system is non-wettable. Contrarily, during the wetting test performed by applying the CP procedure, the surface oxide layer was mechanically removed by squeezing the molten AZ91 alloy through a capillary. Accordingly, the oxide-free AZ91 drop with a regular and spherical shape was successfully obtained and dispensed on the B4C substrate. A reliable contact angle value of θ=83° was measured at the AZ91/B4C triple line at 700 °C, which in turn proves that B4C is wetted by the liquid AZ91 alloy. In contradiction to the literature, these good wetting conditions were assisted by a non-reactive wetting mechanism occurring at the AZ91/B4C interface. To succeed in the fabrication of AZ91/B4C composites by liquid metal infiltration, such experimental observations make it reasonable to expect a spontaneous infiltration process exclusively driven by capillarity, which in turn increases the efficiency of the process by the absence of reaction products that could be a potentially detrimental factor.