Titanium and titanium alloys powders
Titanium possesses an exclusive combination of low density, good mechanical properties at room and elevated temperatures and good corrosion resistance. Titanium and its alloys are used extensively in aerospace,biomedical industries and in other industrial applications. However, titanium ingots are expensive to produce and fabricate. Another problem is connected with segregation in highly alloyed titanium materials. The high cooling rate that PM technology makes possible ensures signifi cantly greater amounts of alloying elements remain in the solid solution compared with ingot metallurgy and thus can substantially infl uence material phase composition and properties. At the same time, PM processing permits articles close to their final shape and thus reduces material loss and accordingly lowers the production cost. Production of titanium powders
Many processes of titanium and titanium alloy powders manufacture are known. Currently, chemical reduction, hydrogenation/dehydrogenation, gas atomization, plasma-rotating electrode and plasma atomization processes are mainly used for commercial production. a) Chemical reduction
Generally, the initial source of titanium powder is ` sponge fines ’ , so-called owing to their appearance. Sponge fines are nodular particles of pure titanium produced during the chemical reduction pro-cess from titanium tetrachloride. The commercial manufacture of titanium metal includes the chlorination of natural or synthetically produced rutile, TiO2. The obtained titanium tetrachloride is reduced to metallic titanium by the sodium reduction method known as the Hunter process, the magnesium reduction method known as the Hunter process, the magnesium reduction method known as the Kroll process and the calcium hydride reduction process used in Russia. In reduction processes of titanium tetrachloride, most of the residual chloride is removed by vacuum distillation or by water leaching. The final sponge fi nes typically contain from 0.12 to 0.15 wt% Cl. Sodium Reduction Method : Characteristic sponge fines manufactured by the Hunter process are shown in Figure 1 .
Fig.1 Sponge fines produced by the sodium process
Magnesium Reduction Method : The titanium sponge produced is the sinter of crystals up to several centimeters in size. It is crushed to a required size. The riddlings (particles fi ner than 2 mm) are 3–5% of the mass of the sponge produced. They are used in powder metallurgy, in anticorrosive coatings, for production of titanium carbides, nitrides and other compounds. The yield of powder fractions of magnesium-thermal titanium essentially increases if vacuum separation of the reaction mass by hydrometallurgical processing is used. However, the quality of powders in this case is impaired. Titanium powders can be produced by continuous reduction of TiCl 4 with gaseous magnesium in a stream of argon. In this process, the titanium powder and magnesium chloride are continuously carried out of the reaction zone. To produce titanium powder, mechanical crushing of titanium sponge is used. However, in this case,there are some diffi culties connected with the plasticity of high-purity titanium, therefore titanium is preliminarily oxidized or hydrogenated to embrittle it. Such a method is especially interesting for crushing titanium alloys, Ti–Al alloys in particular, produced by aluminothermic reduction of titanium dioxide. The hydride/dehydride (HDH) process : is based on the reversible interaction of titanium and hydrogen. Titanium has a very high affinity for hydrogen and is easily hydrogenated by heating titanium in a hydrogen atmosphere. Hydrogenated titanium is very brittle and can be crushed to a fine powder. The hydrogen can then be simply removed by heating the powder in a dynamic vacuum. The minimum hydrogenation temperature for commercially pure titanium is about 400 °C (750 °F) at 0.007 MPa (1 psi) positive...