Precision casting refers to the molten metal cast into the shell made of refractory material, after the solidification of the metal stripping sand to obtain the desired alloy products. Precision casting technology can be used to cast complex castings that are close to the final shape of the parts. However, because the shell is in direct contact with the alloy melt when casting, the selection of shell material is the key to ensure the casting quality for metals with high melting point and strong chemical activity such as zirconium and zirconium alloys. Zirconium and zirconium alloy melting point is (1852±10)℃, and high temperature chemical activity, melting state can react with most refractory materials, resulting in the formation of a layer of pollution on the alloy surface. Therefore, choosing refractory materials that do not react with zirconium melt as shell for precision casting is the key to obtain high quality zirconium castings. The refractory materials used in zirconium and zirconium alloy precision casting mainly include graphite, refractory metal, refractory compound, oxide and so on.
Mo, Ta, W, Nb and other refractory metals with high melting point have better stability to titanium and zirconium active metals, and can be used to make the surface layer of precision casting shell. In the process of shell preparation, a layer of tungsten or an oxide layer of tungsten is generally formed on the ceramic surface, so as to reduce the reaction between molten metal and shell. At present, the shell made by this process is mainly used in the precision casting of titanium and titanium alloy. Basche penetrates the ceramic shell with tungsten compounds and then calcines the tungsten compounds in a reductive atmosphere, thereby coating the refractory surface with tungsten and thereby avoiding the reaction of titanium with the refractory at high temperatures. However, this type of tungsten shell also has some disadvantages. For example, the solvent dewaxing method must be used to make the shell, which will cause harm to the environment and human health. In addition, the moulds deposited on the shell surface after dewaxing tend to react with the metal, forming pores on the casting surface.
Refractory compounds include carbide, boride, sulfide, nitride and so on. Because these refractory compounds are easily oxidized in the air, roasting is performed in a vacuum when used as shell materials. BN is a special refractory with excellent performance. However, when Gao Yonghui et al. studied the interface reaction between BN and TiNi alloy, they found that elements B and N diffused seriously in TiNi melt, which would lead to alloy embrittlement after mixing. Indacochea zirconium, Mcdeavitt through melting in ZrC, HfC, ZrN refractory compound tablet on the wettability of the experiment, studies the interface reaction between them, the results show that ZrN between Zr and react to generate a new phase, and other refractory compounds molten although there was no obvious transition zone between zirconium, but the element diffusion. Therefore, whether refractory metals and compounds can be used in precision casting of zirconium alloys needs further study.
