Melting and Primary Fabrication

Titanium can be extracted from rutile (TiO2) ore by chemical reduction, using either the Kroll process, which uses magnesium, or by the Hunter process, which uses sodium. Tickle (TiCl4) is first produced by reacting TiO2 with coke and chlorine. Generation of TiCl4 allows purification of the titanium by distillation. Then, in the Kroll process, TiCl4 is reacted with magnesium to form a porous mass of titanium called sponge.

TiO2 + 2Cl2 + C ^ TiCl4 + CO2 TiCl4 + 2Mg ^ Ti + 2MgCl2

Both of these processes, while capable of producing high grade titanium sponge, are expensive and significantly contribute to the cost of titanium. However, recent research7 shows great promise in reducing the cost of titanium extraction. The product flow for various titanium product forms is shown in Fig. 4.8.

Due to their high reactivity with refractory lined vacuum induction furnaces, titanium alloys are made by consumable vacuum arc melting.8 Vacuum arc melting, actually called vacuum arc remelting (VAR) even though it is the initial melting operation, is used to remove hydrogen and other volatiles and to reduce alloy segregation. An electrode is made by blending small diameter (less than 0.4 in.) sponge particles with master alloy granules of the same size. Master alloy is a pre-prepared mixture that contains the alloying elements at a much higher concentration than the final alloy composition, which helps to reduce segregation and achieve the correct alloy chemistry. The blended sponge and master alloy are pressed together into briquettes and then welded together to form the electrode. Recycled titanium, or revert, can also be added to reduce costs if permitted by the specification. A typical electrode size is 14 ft long by 2 ft in diameter. The minimization of residual elements that could form refractory or high density inclusions is important in both the sponge and the electrode fabrication processes. As shown in Fig. 4.9, during melting, the electrode is the anode and the water cooled copper crucible is the cathode, i.e. the titanium electrode is consumably arc remelted in a water cooled copper crucible in a vacuum arc melt furnace. The ingot produced by this process becomes the starting electrode for additional VAR. The ingot is then VAR once or twice more or remelted using either a plasma torch or electron beam in a cold hearth furnace.

g dD Ingot

Blend Prepared Revert, Compact Weld Electrode Non-Consumable and/or Double or Triple Alloys and Ti Sponge Consumable Electrode Vacuum Melted

Furnace

Forge

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