Titanium is a very active metal. In the liquid state, it reacts with oxygen, nitrogen, hydrogen, and carbon fairly quickly. Therefore, titanium alloy smelting must be carried out under a higher vacuum or inert gas (ar or ne) protection. The crucibles for smelting all use water-cooled copper crucibles. There are three main methods for specific smelting processes:
- (1) Non-consumable electrode arc furnace melting Alloy melting is carried out under the protection of vacuum or inert gas. This process is mainly for preparing electrodes by consumable electrode melting.
- (2) Vacuum consumable electrode arc furnace melting The consumable electrode made of titanium or titanium alloy is used as the cathode, and the water-cooled copper crucible is used as the anode. The melted 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. The bottom and the surroundings of the crucible are forced to cool, resulting in bottom-up crystallization. The molten metal in the molten pool becomes a titanium ingot after solidification.
- (3) Vacuum consumable electrode condensing shell protection melting Smelting device diagram. This furnace is developed on the basis of vacuum consumable electrode arc furnace. It is a furnace type of casting special-shaped parts that integrates melting and centrifugal casting. The biggest feature is that there is a thin layer of titanium alloy solid shell between the water-cooled copper crucible and the metal melt, the so-called condensate shell. This layer of condensate shell of the same material is used as the inner liner of the crucible to form the molten pool to store titanium liquid To avoid the contamination of the titanium alloy liquid by the crucible. After pouring, a layer of condensed shell remaining in the loss increase can be used as a crucible lining.
In recent years, with the development of technology and production needs, new methods and equipment for smelting titanium alloys and other active metals have been researched and developed, mainly including electron beam furnaces, plasma furnaces, vacuum induction furnaces, etc. application. However, from the comparison of technical and economic indicators such as power consumption, melting speed, cost, etc., consumable electrode arc furnace (including shell furnace) smelting is still the most economical and suitable smelting method. Due to the physical-chemical properties of titanium, the casting process of titanium alloys has unique requirements and characteristics regardless of the molding material rate and process. One is to require molding materials with very high refractoriness; the other is that casting must be carried out under the protection of a higher vacuum or inert gas, sometimes with centrifugal force. The material of the connecting shell is different, and the molten model shell is divided into three different systems.
- (1) Pure graphite shell system. Use graphite powder of different particle size as refractory filler and sanding material, and resin as adhesive. The shell has high strength, light weight, low cost, and a wide range of raw materials. Suitable for centrifugal or gravity casting.
- (2) Refractory metal surface shell system. It is a composite system, except that the surface layer needs to adopt a special process due to the different molding materials (tungsten powder and other refractory metals), and the back layer from the molding material to the shell making process is the same as the investment casting of cast steel.
- (3) Oxide ceramic shell system. The surface and back layer of the mold shell are made of oxide as the molding material, so the mold shell has high strength and the smallest thermal conductivity among the three mold shells. It is suitable for pouring thin-wall castings with complex shapes.
Titanium castings cast with the above three types of shell systems have little difference in chemical composition and mechanical properties; but the surface quality is significantly different. The shrinkage of the latter two types of shells is significantly smaller than that of graphite shells, so the dimensional accuracy of the castings.
