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Material choices - part 3

con-rodsThis month, we continue to examine material choices. In last month’s article, we looked at some of the reasons why people choose titanium as a con rod material. This month we shall carry on discussing titanium and some more closely allied materials.

One point that was covered last month was the effect that the use of titanium has on the proportion of the externally applied loads that the con rod bolts experience. This is affected not only by the magnitude of the external loads (in this case, we are really talking of the ‘inertia’ loads that we see at TDC on the exhaust stroke on a four-stroke engine), but by the material choices and geometry of the fastener and con rod. The proportion of the external load experienced by the fastener is controlled by the ‘load coefficient’ which is influenced by the stiffness of the con rod and the bolt. If we use a less dense con rod, and design it properly, we can expect that the external loads will be higher. However, in choosing titanium as a con rod material, we increase the load coefficient, thereby increasing the proportion of the external load that the con rod bolt will have to cope with. We must be careful of this point when choosing our con rod material.

However, the low modulus of titanium in comparison with the commonly-used bolt materials means that the distribution of the load along the threads is much more even than with a steel con rod. For metric threads and identical nut and bolt materials, around 85% of the load is taken by the first four threads, in a nut with ten threads. If the nut material (in this case the ‘nut’ is the con rod) is of a lower modulus than the bolt, this situation is much improved with a lower proportion of the load being taken by the first thread, thereby causing a much lower stress concentration at this point.

One material which we have heard much of in connection with valves, and comparatively little regarding con rods are the titanium aluminide (TiAl) materials. Now banned in Formula One under rule 5.13.1(c) which bans all intermetallic materials (or which titanium aluminide is one), which are defined as having an intermetallic content above 50% by volume. If you have read it, clearly the rule was written by a materials expert, or with the aid of one, but it hasn’t stopped titanium aluminide materials being developed which have close to, but less than 50% of intermetallics. These materials are aimed at use as valves and con rods mainly in Formula One (when a new set of engine rules finally arrives). However outside of Formula One, where technical rules are less restrictive, titanium aluminide materials have been successfully used as con rods in racing conditions as was confirmed when I recently spoke to an engineer from the company involved in the design and manufacture of these parts.

In comparison with titanium, TiAl has a lower density (less than 4 g/cc) and higher modulus (varies from around 160 GPa to around 175 GPa). In fact, commercially available TiAl materials, which are being considered for possible series production parts for the automotive industry in the long term, not only contravene the above Formula One regulation, but also the specific modulus regulation which imposes a maximum ratio of elastic modulus to density of 40 GPa/(g/cc). It is a shame that Formula One is not in a position to develop materials technology which will benefit road car engines by making them more efficient. The advantages of lightweight rotating and reciprocating components in terms of reduced friction are well-known.

Written by Wayne Ward.

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