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.
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
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
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
(200°C). The new alloy is easy to cast with the SOPHIA process,
and can be repair welded without degradation of properties.
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.