The Interfacial Reaction Law And Micromechanism Between Titanium Alloy Melts And Ceramic Mould

The interfacial reaction between the molten titanium alloys and molds is always the main factor that affects the qualities and mechanical properties of titanium alloy castings in the investment casting. The wettability of titanium alloys melt on ceramic materials affects directly the interfacial reactivity and the surface quality of castings. Therefore, combined the interfacial reaction with wettability, this paper presents a new three-stage mechanism of interfacial reaction, that is, non-reactive wetting during initial stage, reactive wetting, and chemical reaction stage in order to investigate the interfacial reaction law and micromechanism between titanium alloy melts and molds.Avoiding defects in the existing methods used to measure the contact angle, an induction melting drop method is developed and studied for the high-activity alloys. By means of the contact angle testing device, the wetting law and mechanism between molten Ti, Ti6Al4V and TiAl alloys and molds of ZrO₂(Y₂O₃ stabilized), ZrO₂(CaO stabilized) and MgO are investigated in the gravity and electromagnetic field, respectively. The results show that in the electromagnetic field, the wetting process between titanium alloy melts and ceramic materials includes first stage nonreactive wetting, reactive wetting, and final equilibrium stage. And the higher the molding materials chemical stability, the longer the first stage time. Compared with the wettability of molten Ti, Ti6Al4V and TiAl alloys on the ceramic substrates in the gravity field, the contact angles for the liquid metal/ceramic substrates decrease greatly in the electromagnetic field. And based on the thermodynamics theory and combined with the wetting equation for the reactive wetting system, the minimum contact angle equation of the titanium alloy melt/molds system has been developed in the electromagnetic field.In the gravity and electromagnetic field, the thermodynamics and dynamics of interfacial reaction between titanium alloy melts and ceramic molds have been investigated in the experiments. After interfacial reaction, by means of observing the microstructure near the metal at the interface, it has been found that the Y₂O₃ has the most stability, and then ZrO₂(Y₂O₃ stabilized) and ZrO₂(CaO stabilized) against titanium melt. In the gravity and electromagnetic field, the interaction products nucleate at individual position. And with the interfacial reaction proceeding, the number of nucleating position increases and the interation products grow up and merge into each other. Nevertheless, in the electromagnetic field, the interaction products micrographic is different from that in the gravity field. And the interfacial reaction in the electromagnetic field is more intense than that in the gravity field.Based on the conservation laws of energy and mass, taking account of the effects of the electromagnetic field and chemical reaction between liquid titanium alloy and ceramic mold on the concentration field and the temperature field, a comprehensive model for numerical simulation of heat and mass transfer has been established so as to study the interfacial reaction between liquid titanium alloy and ceramic mold. With the proposed model, numerical simulations are preformed to investigate the effects of mold preheating temperature, pouring temperature and contacting time on the oxygen concentrations and reactive layer thickness in metal. The calculated results reveal that both the oxygen concentration and the thickness of reactive layer in metal are found to increase with increasing the contacting time, the preheating temperature, and the pouring temperature. The increasing of aluminum content can reduce the reaction of titanium alloy melt with molding materials. Compared to the interfacial reaction in the gravity field, the interfacial reaction in the electromagnetic field is more intense and the reactive layer thickness in casting is larger. And the simulated results are in good agreement with experimental results.