Effusion Mechanism of α-Layer Formation in Vacuum Casting of Titanium Alloys

Abstract

The formation of the so-called α-layer when casting titanium alloys significantly impairs the mechanical properties of the metal and leads to the need for additional product surface treatment. It is known that simultaneously with the change in mechanical properties, the composition of the casting surface layer is often changed, and the composition change depth, as a rule, does not coincide with the depth of α-layer formed. The reason for α-layer formation, most often, is considered to be the introduction of foreign elements into the crystal lattice of a metal casting. According to our assumption, the mechanism of α-layer formation is two-stage, and it is associated with an intense diffusive effect on the melt structure in the casting outer layer, resulting from its boiling in vacuum during pouring into porous ceramic shells (CS). The paper suggests possible mechanism for composition changing of the surface α-layer in titanium castings due to the secondary processes of effusion saturation of the metal surface with reverse vapor flows coming from the porous ceramic shell after solidification of the casting. In this case, the primary process is the saturation of the CS pores due to the effusion of metal vapors during the boiling of the poured melt due to the high intrinsic vapor pressure exceeding the pressure in the foundry furnace. Chemical interaction of the alloy component vapors with the CS material leads to the volatile components: silicon, silicon II oxide, zirconium II oxide, saturating the metal surface layer due to reverse secondary effusion. The paper presents experimental results on α-layer formation when casting BT5JI alloy in CS having different composition and porosity. The research results confirm the assumption about both the diffusion mechanism of α-layer formation and the effusion mechanism of changes in its composition.