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Emerald Group Publishing Limited
Copyright © 1998, MCB UP Limited
Aerospace - where only the best will do
Aerospace where only the best will do
Keywords Aerospace industry, Alloys, Brush Wellman, Bushing
The aerospace industry is often seen as one of the pinnacles of engineering, where safety, reliability and performance are paramount. Aircraft are built to exacting standards and the quality assurance procedures are rigorous to say the least. But these days another factor is coming onto play life cycle cost. More specifically, cost-effectiveness is of interest as, if the benefits can be justified, a new material, component or assembly can still find its way into a design, even if it is not the cheapest initial option.
Copper beryllium is an alloy that possesses an unusual combination of properties which makes it well suited for use in specialist applications such as bushes and bearings in landing gear and control surface mechanisms. One alloy used is Brush Wellman's Alloy 25. This contains up to 2 per cent beryllium (by weight) and can be precipitation hardened by heat treatment to give a range of strengths. For most bushes and bearings in aircraft landing gear and control surface mechanisms, a tensile strength of 1,150MPa is used, with a yield strength of 980MPa and 4 per cent elongation at yield.
Apart from the strength, other proerties of copper beryllium which are beneficial for these high/low cycle bearing applications are the low coefficient of friction and resistance to galling. Also worth noting are the excellent machinability and corrosion resistance that is claimed.
Materials that copper beryllium has to compete with for these applications include aluminium bronze, aluminium nickel bronze and aluminium zinc bronzes. Compared with these others, copper beryllium, to the AMS 4533 aerospace material specification, has slightly higher strength and also offers better performance in terms of wear and galling resistance. It has been reported that Alloy 25 is up to four times stronger than the aluminium bronzes and up to eight times more wear resistant. This all adds up to give reduced life cycle maintenance requirements for aircraft operators. In the initial design phase, the higher strength of the copper beryllium often allows the engineers to reduce the size of the component, resulting in weight savings. Hence copper beryllium has been specified for use on state-of-the-art commercial aircraft such as the Boeing 777.
Brush Wellman has worked closely with Boeing on a number of projects and a result of this has been that two new products have been developed. Engineers at Boeing had recognised the need for a more ductile grade of copper beryllium so that, for example, bushings could be swaged in place to improve their resistance to migration and rotation. In November 1996 Boeing issued a material specification BMS 7-353 with two types defined. One of these has a UTS of 825-1,000MPa, a yield strength of at least 620MPa and a minimum of 15 per cent elongation at yield. This new Type 2 material is being used for swageable bushings on a number of new designs and modifications to existing designs.
It might be expected that, as with many specialist materials, there are drawbacks to using copper beryllium. Certainly it is not one of the cheapest materials on the market. However, its availability as hollow bar can provide savings in both the raw material cost and machining time.
So far only bushings and bearings have been discussed, but other aerospace applications also exist for copper beryllium. For high reliability electrical connectors, the conductivity, resistance to stress relaxation and corrosion resistance of copper beryllium contribute to its success. Brush Wellman supplies material for use in electrical connectors for the aerospace, automotive and telecommunications industries.
Other materials that Brush Wellman supplies to the aerospace industry include beryllium metal for structured components in the solar array deployment systems of SADMS satellites. AlBeMet is another specialist alloy which is lightweight, has high stiffness and high thermal conductivity. This alloy is used for a number of structural components in the Global Star, Hot Bird III and Eutelsat W24 satellites. ToughMet is another specialist alloy range of copper-nickel-tin spinoidal alloys for severe service applications or where long bearing/wear life is desired.
The aerospace industry is undoubtedly one of the most demanding markets and calls for specialist materials. For a supplier to be able to serve this market well requires an understanding of the market as well as a thorough knowledge of the materials involved. Brush Wellman claims to fulfil both of these criteria and says it is well placed to serve the UK and overseas aerospace markets.
For further details contact Brush Wellman Limited, Units 4 and 5 Ely Road, Theale Commercial Estate, Theale, Reading, Berkshire RG1 4BQ. Tel: +44 (0) 118 930 3733; Fax: +44 (0) 118 930 3635.