The development of rapid prototyping (RP) methods as a manufacturing technique has been fascinating to witness. There are several strands to this: many RP techniques simply produce a facsimile of a component or assembly but which have no functional use, as the materials are very low in strength, stiffness and ductility. The materials in this category include paper and some polymers. There are other polymer materials that are well suited to the production of ‘unstressed’ components, and the range of these materials is increasing all the time with a huge number of filled and unfilled polymers now available, including those filled with glass, carbon and solid lubricants.
However, for engineers concerned with race engines, hybrid systems and transmissions, the most eagerly watched group of materials are the metals. RP metallic materials have been available for a more than a decade – I had my first laser-sintered parts made about ten years ago for an engine test rig – although their visual quality or structural integrity often left much to be desired. Now though, the range of materials has improved, as has the quality of the powdered material. During the past few years, the machinery which produces the parts has also become more capable and more affordable.
The range of metallic materials covers everything from aluminium to high-temperature superalloys. Inconel is a case in point, which is currently used for a lot of development components. Inconel 718 is used in sheet-metal form for exhaust systems, so its use in making RP exhaust components should come as no surprise. The ability to make complex designs that would be impossible to weld gives design and development engineers more avenues for investigation.
The technique of laser sintering allows us to produce complex internal structures in components, the only limiting factor being the need to consider the removal of unconsolidated powder from the component. I have seen a sectioned Inconel poppet valve with a complex ‘matrix’ inside, and there is work being done to optimise material properties for this and similar applications with a view to it becoming a viable mass-production process in years to come.
The disadvantage with the use of high-temperature materials for valve manufacture is high component mass. Where boosted engines are concerned though, especially those that have to last more than a few hundred kilometres, the mass penalty is something that has to be accepted. The hollow poppet valve with a load-bearing internal matrix is a tempting prospect, and may allow the use of Inconel valves with a much smaller weight disadvantage compared to titanium valves than has traditionally been the case.
There is one piston manufacturer who has an almost skeletal prototype Inconel piston in its display cabinet. There is no evidence to suggest that this component has ever been run in an engine, but it does show some of the design possibilities for a piston developed for high-temperature use. This idea is discussed in more detail in issue 78 (May 2014) of Race Engine Technology.
Written by Wayne Ward