Investigation on reactive wetting during investment casting of magnesium alloy AZ91

Abstract

During investment casting of Magnesium alloys, the melt contacts with ceramic mould after pouring. The surface quality of the casting is directly affected by the wettability between the melt and ceramic mould material as the Mg melt is reactive to oxygen and moulds are mainly made of ceramic oxides. As the melt wets the wall of ceramic mould, the melt can infiltrate into ceramic mould through the capillaries on the mould surface and then metal can penetrate in the mould that may results in poor surface finish on the surface of the casting. When the temperature of casting and alloy composition fulfill the thermodynamic conditions of interfacial reaction, reaction occurs between melted metal and ceramic materials, leading to development of reactive layers on the surface of casting and deteriorating the surface quality of the casting. Therefore, there is great interest in the industry to minimize or elimination of interface reaction. Ceramic oxides with higher thermal stability found to resist the reactions to some extent. Use of protective gas also helps to overcome the reaction. Wettability has significant effect on the resultant interfacial properties of casting at high temperature liquid metal/solid ceramic systems. The present research focused on mould materials and mould development for suppression of mould-metal reaction in investment castings of magnesium alloy AZ91. An attempt was made to evaluate wetting kinetics for two different mould materials which are Zircon Flour and Fused Alumina. XRD analysis was done on the surface of Mg AZ91 and mould material to find the reaction products for the casting with both mould materials. Fused Alumina was found more suitable over conventional zircon flour for suppressing reaction based on the contact angle and surface roughness.