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Industrial Processes Title

Moulding the future

   
 
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ADVACAST brought together four companies and three university research teams from Germany, France, Greece and Portugal in the development and testing of high performance aluminium casting alloys and advanced casting processes. The project yielded a new alloy, and improvements in the SOPHIA investment casting process, which will find commercial applications in the aerospace and automotive sectors, allowing the design of lighter, resource efficient components. By the end of 1996, the new alloy was already in industrial use in the production of aircraft engines and high performance car gear boxes.


Ecological and commercial concerns are exerting continuous, and increasing, pressures on aeronautics designers and manufacturers. The ever-rising amount of affordable air travel brings with it an increase in jet engine emissions and operating cost restraints on carriers. As a result, aircraft manufacturers have made great efforts to develop cleaner, more fuel efficient engines. In fact, over the last few years, fuel-per-passenger-kilometre figures have typically fallen by 25%. However, the push is still on.
A key part of the aircraft manufacturers' strategy to meet the needs of operators is to reduce the weight of aluminium alloy engineering components, including those used in engines. Complex, stress bearing alloy components are currently produced by expensive, time consuming milling and machining techniques. Sub-components are made which are then assembled using bolts, fasteners and seals to produce the finished parts. These parts are far from ideal in terms of both their weight and materials consumption. One possible solution which has long been recognised is the use of casting rather than machining techniques to produce the complex shapes required. However, conventional casting techniques and alloys do not fully meet the need: dimensional tolerances and mechanical properties of cast parts are lacking.
Improvements in the performance of aluminium alloy precision cast components can be made both by optimising the casting process and by modifying the composition of the alloy. The BRITE-EURAM project, ADVACAST, has resulted in substantial process improvements and the development of a new aluminium alloy which demonstrates improved mechanical properties, particularly at high temperatures, to the point where industrial viability has been proven.

Process control

So-called 'investment casting' techniques involve the production of a wax model of the required shape - subtly altered to take account of mould shrinkage of alloys on cooling - which is subjected to a succession of dip coating processes to produce a ceramic shell. These processes are indeed an art in themselves, as this ceramic mould must have exactly the right mechanical and thermal conductivity properties to suite the particular shape. The ceramic shell is then heated and the molten wax allowed to drain away. Any remaining hydrocarbon contamination is removed by high temperature oxidative vaporisation, producing the finished mould.
Casting and cooling processes - carried out either under vacuum or in a nitrogen atmosphere, as molten aluminium alloys are potentially reactive in the presence of oxygen - have a significant effect on the mechanical properties of the finished part. Broadly speaking, the faster the cooling the better. Conventional investment casting techniques allow only relatively low cooling rates, and can lead to the formation of microporous structures with insufficient mechanical strength and elongation. The proprietary SOPHIA investment casting process allows higher cooling rates to be achieved, which result in better control of the solidification fronts throughout the castings, and produce finer microstructures, displaying much improved properties. Additionally, these higher cooling rates permit an additional degree of freedom in alloy composition, enabling ADVACAST to develop the ultimately successful 'A357+Cu' alloy.
As an aircraft manufacturer, project leader Daimler-Benz Aerospace (DASA) was well aware of the economic potential of investment casting technology, but was equally aware of technical shortcomings. DASA launched ADVACAST in 1991 in partnership with foundries Ciral of France (part of the Pechiney group) and Thyssen of Germany, Mirtec, a Greek SME specialised in industrial testing, computing and materials engineering consulting and the Universities of Lisbon, Munich and Patras, Greece.

Casting around

The project started with the production, by Thyssen, of wax models and ceramic shells for use in both conventional and SOPHIA processes. The casting qualities of experimental alloys and reproducibility studies were investigated at DASA, Thyssen, Ciral and Mirtec using non-destructive techniques such as X-ray and dye penetration.
The practical section of the project was supported by theoretical modelling of thermodynamic and mechanical properties at Mirtec and the academic partners.
The experimental alloys studied fell into two main groups: 'basic', modifications of the AlSi7Mg ('A357') alloy and; 'advanced', based on modified A1-Zn alloys and A1-Cu alloys ('A201' and 'A224').
Conventional and SOPHIA investment casting processes for the various alloys were optimised by Thyssen and Ciral, focusing on the need to minimise processing times, while maintaining consistent alloy performance. Samples were then subjected to an exhaustive testing programme to evaluate their ambient and high temperature tensile properties, fatigue behaviour, crack propagation characteristics, fracture toughness, creep and corrosion-related behaviour. Based on these results, the more promising formulations were selected for workshop tests which addressed the practical requirements of machining behaviour, compatibility with standard metal treatment processes such as pickling and anodising, as well as for a repair welding programme.

Improved properties

The results from the group of advanced alloys were, perhaps, a little disappointing, with no natural contender for commercialisation emerging. The basic alloys, however, were an altogether different story: an A357+Cu alloy was developed which, when compared to standard A357, displays comparable properties at ambient temperatures, but much improved tensile and creep characteristics at high temperature (200C). The new alloy is easy to cast with the SOPHIA process, and can be repair welded without degradation of properties.

New applications

The industrial partners have benefited in different but complementary ways from the project.
Daimler-Benz Aerospace has vastly increased its understanding of the use of conventional and SOPHIA castings in aerospace structures. The use of castings, for both performance and economic reasons, is expected to increase in the future, as a direct result of ADVACAST.
Mirtec is actively marketing the destructive and non-destructive testing, phase diagram calculation and thermodynamic modelling expertise acquired during the project.
Ciral and Thyssen have improved the SOPHIA process, including automated process control, to the point where it is now an industrial reality, and looks set to broaden the use of casting technology beyond its traditional aerospace industry market into areas such as the automotive sector. The industrial value of the process, using the A357+Cu alloy, has already been demonstrated. It is being successfully used in the production of components for BR 710 aero engine - where it has achieved weight reduction of 20% and a cost saving of 60% - and is producing parts for Formula 1 racing car gear boxes.

 

Project Title:  
Advanced aluminium-precision casting for integrally stiffened net-shape components.

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-4084

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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