Rapid prototyping

Thursday, February 09, 2012

Tags :  advanced-materials

advanced-metalsUntil relatively recently, the so-called 'rapid prototype' manufacturing methods were restricted to polymers and some other fairly 'flaky' materials - at least in terms of commercial availability. After the initial polymer materials (some of which are excellent and can be used perfectly well as test and race parts for chassis and engine use) some metal materials became available, though nothing which we might have considered useful for race engine components.

In the past few years, there have been a number of companies who have been able to offer some very 'useful' metallic materials with strengths to rival those of commercially available cast and even wrought alloys. In general, with only a few notable exceptions, the metallic parts of race engines are made from aluminium, titanium and steel. Owing to the pressure to produce ever-higher performance from an ever-decreasing engine mass, parts naturally become more stressed. Readers with an interest in this article would do well to read John Stowe's excellent article on the subject in issue 59 of Race Engine Technology.

In the initial stages of adopting a new manufacturing technology, the first use that many people will find is as a straight substitution for an existing material and manufacturing method. In general, they will produce a design very similar to that which they might have produced with conventional methods. For the 'rapid prototyped' component to be competitive on weight, it will need to have comparable strength to the part it replaces. Later, as the real benefits of the manufacturing method are taken account of, and designers feel less constrained by their experience of what can be produced conventionally, considerable mass savings are likely to be found.

However, before people are happy to use rapid prototyped metallic components, they will check the material strength and, if available, fatigue properties. Fortunately, there are now a number of materials that offer a real prospect of making top-level race engine components, even if their costs are very high at present.


advanced-metals-exhaust

There are two commercially available aluminium materials, based on aluminium-silicon alloys. Containing 10% and 12% respectively of silicon, these are similar to some high-strength casting alloys, and compare well on strength. Aluminium is of great interest, as many 'static' components are produced from this material.

Titanium Ti-6Al-4V shows great promise as a rapid prototyping material, with strength figures to rival wrought bar, but the manufacturing method has the ability to produce parts of fantastic complexity. Other titanium materials are also commercially available in suitable powder form ready for manufacture.

With two Inconel alloys and two cobalt-chromium materials, the high-strength 'superalloy' base is covered, offering the opportunity to produce hot components for exhaust system or possibly turbocharger 'hot-side' use.

High-strength steels are also very well represented; precipitation-hardening steels such as 15-5 and 17-4 offer a 'medium to high strength' option. Higher strength steels are also covered, with both hot-work tool steels such as H13 and maraging steel being commercially available.

There are three barriers to such methods becoming widely used for race engine components though, other than for very specialised parts - the cost of the components, the size of component that can be made, and sometimes the speed of the 'build'.

Fig. 1 - Exhaust production using additive manufacturing methods has been used by a number of companies

Written by Wayne Ward

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Comments

The high silicon aluminum casting alloys noted have good hot strength, but are difficult to sand cast.  Using rapid prototype plaster molds makes it easier.

As for rapid prototype of parts using laser sintering of Ti powder, the mechanical properties of the finished part are not as good as those made using wrought material.  Titanium is highly reactive with oxygen when molten.  Thus laser sintering of powder, or fusion welding, or investment casting must be done in an inert atmosphere.  

In terms of fatigue properties, a laser sintered material will not have the same high quality, refined grain microstructure a wrought/forged material has.  Thus it will have inferior fatigue properties.

For highly stressed parts like crankshafts, double vacuum melt wrought alloy steels are still the best option.   The key to maximizing the fatigue life of these materials is careful heat treatment, use of mechanical surface cold work processes such as shot peen or rolling, careful geometric design of areas with potential stress concentrations, and careful control of surface roughness.