2.6 Titanium and Titanium Casting
A difficulty when manufacturing titanium components is that it is a reactive metal, particularly at high temperatures, taking up oxygen and nitrogen from the atmosphere and hydrogen if moisture is present. The absorption of small amounts (20ppm) causes a reduction in fatigue strength. Hence where parts are manufactured by casting or welding, the environment must be purged with an inert gas, argon is normally used.
Link to Design notes on titanium and then also to Titanex.
2. Applications and Processes
More recently developments in casting have broadened the availability of cast titanium parts, but the reactivity of molten titanium means that it has to be melted under vacuum. A company in California, Coastcast, has developed a reliable casting process for titanium parts, using a selected laser sintering process to make moulds directly from rapid prototyping data. This process is type of investment casting. Because the cast parts will take up very small amounts of oxygen from the moulds, the outer surface of the casting may contain oxygen, to a depth of up to 0.5 mm, it is common to chemically mill the outer layer off castings. Due to the rapid solidification of castings, there can be small voids in the centres of components. If these voids need to be closed up hot isostatic pressing is carried out, under argon at a temperature of 900o C and a pressure of 105 MPa for 2 hours.
Applications include impellers on the cold (compressor) side of turbo chargers to
enable manufacturers of turbo diesels to meet new regulations for emissions which requires higher
pressure ratios (and operating temperatures).
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David J Grieve, 21st January 2003.