A20X – alloy of the future?

Materials World magazine
,
1 Nov 2009

The aerospace and automotive industries are striving to reduce their carbon footprints and their relative impact on the environment. Bill Stott, Technical Director, and John Forde, Technical and Business Manager, of Aeromet, UK consider the benefits offered by A20X.

Product design and production routes for the aerospace and automotive are being driven by both improved energy and cost efficiency. With designers striving to reduce component weight and increase operating temperatures, pressures and loads, lightweight high-strength materials are at a premium.

Aluminium alloys have played a crucial role in providing solutions to a range of design issues due to their relatively high specific strength and good corrosion resistance. Such alloys account for over 70% of commercial airliners in various forms, such as machined from solid hogouts, fabrications, forgings, and castings. However, this will reduce to 60% use on the Airbus A380 and the future trend is towards increasing use of polymer composites and titanium, with a further reduction of aluminium use to 20% on the Boeing 787 Dreamliner and Airbus A350XWB.

Historically, wrought aluminium alloys such as AA2xxx series aluminium-copper-magnesium and AA7xxx series aluminium-zinc-magnesium-copper exhibit appreciably higher properties than cast alloys including the Sophia cast D357 aluminium-silicon alloy. High strength aluminium copper based cast alloys do exist. They were some of the first casting alloys developed and are still in use today. However,difficulties associated with castability 
and homogeneity have been an obstruction to the widespread integration of these alloys.

Casting, particularly investment casting, allows near net shape components to be produced with extremely complex features with minimal machining. This gives significant cost and energy savings over multipart fabrications and machined from solid hogouts. However, as component operating parameters are being driven ever higher, the design requirements are rapidly exceeding the property envelope for current cast aluminium alloys - alternative materials and production routes have to be developed.

The efficient incorporation of cast aluminium alloys in next generation high performance parts such as aerospace fuel systems and automotive diesel engines is becoming increasingly difficult as a consequence of these increased operating requirements. In many cases, casting would be the preferred manufacturing route in terms of cost, energy efficiency and design freedom, unfortunately the move to design from solid or heavier metals is imminent.

Alloy aid?

Aeromet International plc, in collaboration with UK organisations London & Scandinavian Metallurgical Co Ltd, the University of Birmingham, Rolls-Royce Goodrich Engine Control Systems ltd, a Rolls-Royce and Goodrich Joint Venture trading as Aero Engine Controls, and Grainger & Worral Ltd have developed A20X, a novel high strength elevated temperature casting alloy. Created to provide solutions to aerospace design dilemmas, the material improves cast alloy performance and allows castings to compete with, and replace, current aluminium fabrications, forgings and hogouts, while providing a cost effective option 
to titanium in some cases.

The potential automotive applications are equally as significant with A20X providing an alternative to cast iron and steel parts for next generation ‘green’ diesel engines.

The A201 aluminium-copper (Al-Cu) based casting alloy was developed for high strength applications. Additions of magnesium and silver alter the precipitation sequence of the binary Al-Cu alloy, allowing higher tensile strengths to be achieved with relatively high retained properties at temperatures up to 200°C. Excellent properties can be achieved using this alloy system, however the relative gains come at the expense of castability and ‘real world’ component properties.

Aluminium-copper-based alloys are exceptionally prone to shrinkage porosity, hot-tearing, compositional in-homogeneity (macrosegregation) and consequently inconsistent mechanical properties, particularly ductility. These castability issues all stem from alloy solidification over a 120°C temperature range, and the resultant interdendritic feeding solidification mechanism.

The coherency point of an alloy is the point at which a stable interconnected structure is formed - this occurs at about 30% solid fraction in A201, thus the remaining 70% liquid metal must be distributed throughout the narrow interdendritic channels in the solidifying and contracting component. Consequently, complex running systems with large amounts of feed metal and/or heavily chilled sections must be used to try and circumvent these issues to ensure a sound casting. In many cases, complex castings with large section changes still remain a challenge to cast with conventional Al-Cu based alloys.

Small changes

A20X is a modified version of A201 where these issues have been eliminated through small, but significant, compositional alterations. The interdendritic feeding solidification process being replaced by a mass feeding mechanism negates the flow of interdendritic liquid. This eradicates the need for liquid metal to travel large distances to feed remote areas and 
has a significant impact on the tendency for macrosegregation, shrinkage porosity and hot tearing. Mechanical properties taken from castings are consistent and significantly improved compared to A201 (see graph).

The castability of A20X is equivalent to current aerospace and automotive casting alloys, allowing large and complex components to be cast. The modified solidification mechanism has highlighted some novel material properties, including a reduced feeding requirement in investment casting and elimination of chills in sand castings. This results in simplified mould design and increased metal yields. A20X also exhibits a fine homogenous grain structure even across thick sections, opening up possibilities to cast large blocks for direct machining, thus providing an energy and cost efficient alternative to current production routes.

Mechanical properties achieved with the A20X are comparable with the high strength AA7xxx series wrought materials, allowing castings to compete, realistically, with fabrications, forgings and machined from solid production routes. The elevated temperature properties (see graph) will help it compete widely the AA2xxx and AA7xxx series wrought alloys.

The A20X alloy provides a cast alternative to the large carbon footprinted fabrications both in terms of mechanical and elevated temperature properties, allowing designers the increased freedom inherent to the casting process to minimise component weights. Funding has recently been acquired through the Midlands Aerospace Alliance and Advantage West Midlands to further the development of A20X. With applications in almost every industrial setting, A20X may be the alloy of the future.

Further information: John Forde