Advantages, Disadvantages and Applications of Titanium Alloy Fasteners

Titanium is a very active metal. In the liquid state, it reacts very quickly with oxygen, nitrogen, hydrogen and carbon. Therefore, the smelting of titanium alloys must be carried out under the protection of higher vacuum or inert gas (ar or ne). Crucibles for smelting are all water-cooled copper crucibles. The specific smelting process mainly includes three methods: (1) Non-consumable electrode arc furnace smelting. Alloy smelting is carried out under the protection of vacuum or inert gas. The process is mainly consumable electrode smelting to prepare electrodes. (2) Vacuum consumable electrode electric arc furnace smelting 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 molten electrode enters the crucible in the form of droplets, forming a molten pool. The surface of the molten pool is heated by the electric arc and is always liquid, and the surrounding area where the bottom and the crucible are in contact is 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 condensed shell-protected smelting The smelting device is shown. This furnace is developed on the basis of vacuum consumable electrode electric arc furnace. It is a furnace type for casting special-shaped parts that combines smelting and centrifugal casting. Its biggest feature is that there is a thin solid shell of titanium alloy between the water-cooled copper crucible and the molten metal, the so-called solid shell. This solid shell of the same material is used as the lining of the crucible to form a molten pool to store titanium liquid , To avoid the crucible pollution to the titanium alloy liquid. After pouring, the layer of condensed shell left in the increased loss can be used as the lining of the crucible.

In recent years, with the development of science and technology and the needs of production, new methods and equipment for smelting titanium alloys and other active metals have been researched and developed, mainly electron beam furnaces, plasma furnaces, vacuum induction furnaces, etc., and have achieved a certain degree of application. However, from the comparison of technical and economic indicators such as power consumption, melting speed, and cost, consumable electrode electric arc furnace (including condensing shell furnace) smelting is still the most economical and applicable smelting method at present. Due to the physical-chemical properties of titanium, the casting process of titanium alloys has its own unique requirements and characteristics, whether it is the molding material rate or the process method. One is a molding material that requires a very high refractoriness; the other is that the pouring 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 fusion shell is divided into three different systems.

(1) Pure graphite shell system. Graphite powders of different particle sizes are used as refractory fillers and sanding materials, and resins are used as adhesives. The shell has high strength, light weight, low cost, and a wide range of raw materials. Suitable for centrifugal or gravity pouring.

(2) Refractory metal surface layer shell system. It is a composite system, except that the surface layer requires special processes due to different modeling materials (tungsten powder and other refractory metals), and the back layer from the modeling 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 shell are made of oxide as the modeling material, so the shell strength is high, and the thermal conductivity is the smallest among the three kinds of shells. It is suitable for pouring thin-walled castings with complex shapes.

Titanium castings poured by the above three shell systems have little difference in chemical composition and mechanical properties; but there are obvious differences in surface quality. The shrinkage rate of the latter two shells is significantly smaller than that of graphite shells, so the dimensional accuracy of the castings.
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