Properties and applications of titanium
Titanium is a strange material, not least for the myths surrounding it. It is often described as being incredibly hard, or incredibly strong, when the truth is that it is neither. It can be impressively strong compared to its density, but it does not stand out in this respect. It has been very expensive, and to the man on the street it has the reputation of being out of reach. It is, however, no more expensive per unit mass than many grades of steel or aluminium. It is widely used in motorsport, but sparingly in production cars and motorcycles.
Leaving ill-informed opinion aside and concentrating on the mechanical and physical aspects of titanium alloys, it is still a strange material. Even though the alloys used for motorsport components are relatively strong, their behaviour when subjected to even modest levels of surface stress is extremely poor. For example, it is impossible to use titanium in almost any application where sliding is involved. Despite its strength when loaded in tension or bending, we cannot use titanium or its alloys for sliding applications unless we take measures to improve the surface through the use of hard engineering coatings such as chromium nitride (CrN), titanium nitride (TiN), diamond-like carbon (DLC) or a metallic coating such as molybdenum.
Titanium also has notably low thermal conductivity, and its ability to insulate components from the effects of heat gives it some applications in engines, where the material is allowed. Titanium, in being viewed (wrongly, in my opinion) as an exotic material, is banned in many governing bodies’ regulations for race engines. It can be used to provide an insulating ‘gasket’ for some applications, and titanium fasteners reduce heat transfer through joints compared to steel and other materials. Even though we might see the fasteners as representing a small cross-sectional area compared to the rest of the joint, the heat transfer can be significant, and measurable reductions in component temperatures can be measured by changing from steel fasteners to titanium.
The combination of thermal conductivity, density and specific heat capacity mean titanium has a very low thermal diffusivity, which is the measure of a material’s ability to conduct heat divided by its ability to store heat. Materials with low thermal diffusivity show high temperature gradients in response to local heating, as the transfer of heat from ‘hotspots’ to cooler areas is poor. The thermal diffusivity of titanium is less than 50% that of steel, and about 10% that of aluminium.
Titanium, while unremarkable in terms of strength and stiffness, has a low ratio of stiffness to strength. In some circumstances, notably in the case of fasteners, this is a very attractive combination. For a given service load on a joint, a reduction in fastener stiffness leads to reduction in cyclic load borne by the fastener. This is why, in highly stressed fasteners such as cylinder head studs or con rod bolts, the shanks are waisted. In some cases, changing from steel fasteners to titanium can also be accompanied by a reduction in the dimensions of the fasteners without a loss of durability.
The density of titanium compared to steel means that, for an equivalent component mass, more titanium can be used. For a component such as a con rod, this represents an opportunity to improve the bending and torsional stiffness. The actual deflection in normal operation may not be a concern, but stiffness has an important influence on vibration and resonance, and the increased stiffness of a titanium component may be significant in this respect.
Written by Wayne Ward