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Improved titanium alloys (1)

advanced-metalsIt is a fact that development of titanium alloys is driven by the substantial needs of the aerospace industry, for improved properties or lower costs. The use of titanium is widespread in aircraft, both military and commercial, and with great emphasis being placed on reducing mass, the development of better alloys allows parts currently made in titanium to be even lighter or for steel parts to be replaced with titanium. The development and use of surface engineering processes goes hand in hand with new alloy development in allowing the replacement of steel.

The Ti-6Al-4V alloy is used widely in motorsport for fasteners, con rods and various other components. In fact it enjoys a virtual monopoly in the manufacture of mainstream titanium race components, with the exception of poppet valves, for which there are a number of alloys used specifically for them.

In the aerospace industry there is a much wider range of titanium alloys used for structural parts such as landing gear, and these alloys would make good candidates for a number of race engine applications, especially con rods. These alloys have disadvantages though, such as higher material costs due to the increased quantity of alloying elements and the lack of availability compared with Ti-6Al-4V. In aerospace, the higher cost can often be justified by the reduction in mass gained from their use, especially where they can replace steel components.

Imbued with greater tensile properties and fatigue strength than Ti-6Al-4V, they offer a real advantage when seeking to minimise mass. In decreasing the mass of a con rod, there are further advantages in that the crankshaft can also be made correspondingly lighter.

Ti 10-2-3 is an alloy containing 10% vanadium, 2% iron and 3% aluminium, and has found widespread use in the aerospace sector for a number of years. In billet form it has a 24% higher yield strength and 22% higher ultimate tensile strength compared with Ti-6Al-4V, and the advantage in these properties is increased in the forged state, with both strength measures being 27% higher compared with Ti-6Al-4V. In terms of fatigue strength, it enjoys a large advantage - according to published data sheets, axial fatigue amplitudes are about 50% greater for Ti 10-2-3.

Compared with Ti-6Al-4V, however, Ti 10-2-3 has lower elongation and fracture toughness in the forged state. Its elastic modulus is also towards the lower end of the range that we would typically find for Ti-6Al-4V.

Of course, for a con rod, strength isn't everything, and designing a con rod with the correct overall stiffness (and the correct stiffness in certain critical areas) plays a large part in the successful application of a new material. There are certainly areas on an optimised race con rod, however, where fatigue strength will be the limiting factor, and alloys such as 10-2-3 should therefore provide an advantage in terms of mass reduction.

In aerospace applications it is used at temperatures of up to 260 C (500 F), so its temperature capabilities are sufficient for all of the applications where Ti-6Al-4V is currently used.

Fig. 1 - The Boeing 777 is reported to use Ti 10-2-3 landing gear parts, which were formerly manufactured from steel (Courtesy of Boeing)

Written by Wayne Ward

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