2.6 Titanium and Titanium Casting

1. Introduction
The main advantages of titanium are a low density (4500 kg/m³) compared to steel (7800 kg/m³), good corrosion resistance (it is inert to body fluids) and high strength (depending upon alloying). With a Young's modulus of 120 GPa, it is only about half a stiff as steel (E = 209 GPa).

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
Use of titanium started in the automobile industry in the early 1970s with the use of small machined parts for racing cars, such as gear linkages, where the high costs of materials and manufacture were acceptable. Early attempts at fabricating suspension components by welding at Lotus in the early 1980s were unsuccessful as the appropriate precautions were not taken and parts cracked. By the early 1990s Barnard had devised an effective method of providing an inert atmosphere while welding and fabricated suspension uprights. As the suspension upright is "unsprung weight", weight savings here are particularly worthwhile.

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 900^o 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).
Recent applications (ref. A3) also make use of the excellent corrosion resistance of titanium which means parts can be manufactured with zero corrosion allowance, giving greater weight savings than density difference and strength considerations might suggest. Silencers for some motorcycles and cars (Corvette ZO6) and exhaust pipes for some motorcycles and engine valves for the Toyota Alteeza (Japan market Lexus IS 300)