Titanium and titanium alloy melting and casting process
Titanium is a very reactive metal. In the liquid state, it reacts very quickly with oxygen, nitrogen, hydrogen, and carbon. Therefore, titanium alloy melting must be carried out under the protection of higher vacuum or inert gas (argon or neon). The crucibles used for smelting are all water-cooled copper crucibles. There are three main methods for specific smelting process: (1) Non-consumable electrode electric arc furnace smelting Alloy smelting is carried out under the protection of vacuum or inert gas. The process mainly prepares electrodes by smelting consumable electrodes. (2) Vacuum consumable electrode electric arc furnace smelting uses a consumable electrode made of titanium or titanium alloy as the cathode, and a water-cooled copper crucible as the anode. The molten electrode enters the crucible in the form of droplets, forming a molten pool. The surface of the molten pool is heated by the arc and is always in a liquid state, and the bottom and periphery of the crucible are forced to cool, resulting in bottom-up crystallization. The molten metal in the molten pool solidifies and becomes a titanium ingot. (3) Schematic diagram of the vacuum consumable electrode coagulation shell protection smelting smelting device. This melting furnace is developed on the basis of vacuum consumable electrode electric arc furnace. It is a furnace type for casting special-shaped parts combined with smelting and centrifugal casting. Its biggest feature is that there is a solid thin shell of titanium alloy between the water-cooled copper crucible and the metal melt, which is the so-called solidified shell. This layer of solidified shell of the same material acts as the inner lining of the crucible, forming a molten pool to store the titanium liquid. , to avoid the pollution of the crucible to the titanium alloy liquid. After pouring, a solidified shell left in the waste can continue to be used as a crucible lining.
In recent years, with the development of science and technology and the needs of production, new methods and equipment for smelting active metals such as titanium alloys have been researched and developed, mainly including electron beam furnaces, plasma furnaces, and vacuum induction furnaces. , and has been applied to a certain extent. However, from the comparison of technical and economic indicators such as power consumption, melting speed, and cost, consumable electrode electric arc furnace (including shell furnace) smelting is still the most economical and applicable smelting method. Due to the physical and chemical properties of titanium, the casting process of titanium alloys has its unique requirements and characteristics in terms of the amount of molding materials and process methods. One is the molding material that requires very high refractoriness; the other is that the pouring must be carried out under the protection of a relatively high degree of vacuum or inert gas, and sometimes centrifugal force is also required. The shell material is different, and the fusion shell is divided into three different systems.
(1) Pure graphite shell system. Graphite powder with different particle sizes is used as refractory filler and sanding material, and resin is used as binder. The casing has high strength, light weight, low cost and wide source of raw materials. Suitable for centrifugal or gravity pouring.
(2) Refractory metal surface shell system. It is a composite system, except that the surface layer requires a special process due to different molding materials (refractory metals such as tungsten powder), while the back layer is the same as cast steel from molding materials to investment casting. Shell making process.
(3) Oxide ceramic shell system. Both the surface layer and the back layer of the mold shell use oxide as the molding material, so the mold shell has high strength and the thermal conductivity is the smallest among the three mold shells, which is suitable for casting thin-walled castings with complex shapes.
The titanium castings cast by the above three shell systems have little difference in chemical composition and mechanical properties; but the surface quality is significantly different, and the shrinkage rate of the latter two shells is significantly smaller than that of graphite shells, so the dimensional accuracy of the castings is poor.
