High Performance Aluminium Castings
Theory and Reality of their Production
Nick Green FICME, Professor of Casting Technology was the speaker at the March meeting of the West Midlands, Birmingham and Coventry Branch of ICME.
He is pictured on the left of the photo, alongside Die Casting Society President, Simon Hanson, and on the right Branch President Dean Millington.
Nick began by describing his early work, in the 90s, with Professor John Campbell at the University of Birmingham which included early studies using real time x-ray to study turbulent filling and oxide entrapment during casting of aluminium alloys. The detrimental effects of these oxides, which then act as voids and crack initiators in the castings was characterised through tensile testing which clearly indicated large variations, and hence uncertainty in the mechanical properties of the resulting castings. The effects of the oxide are even more clearly demonstrated by fatigue testing where the variations in fatigue life, or cycles to failure provide a very clear indication of the detrimental effects of oxide film entrainment.
It is this uncertainty that is almost the greater problem since it requires designers to include large safety factors in their designs as they cannot have confidence in the properties of castings.
This work then led on to the development, by John Campbell, of the Cosworth Process. This process, as Nick described, involves holding the bath of molten aluminium quiescently, for up to 10 hours, to enable gases to be removed and impurities to be settle, filtering the metal and then filling the mould cavity in an uphill manner, in a quiescent manner, via a pump, taking only the cleanest metal from the centre of the holding furnace.
The results obtained were astounding, with the metal cleanliness being shown to be as good as that obtained in filtered foil stock aluminium – the ultimate level in terms of metal quality. The mechanical properties, characterised by fatigue tests also demonstrated that the castings had very good reliability with consistent, good properties.
Improving the reliability of castings clearly then means that engineers and designers can reduce safety factors, reduce section thicknesses and have more confidence in the properties of castings.
Nick explained how this work is continuing at the University, in particular with the new LiME (Liquid Metals Engineering Innovative Manufacturing Research Centre) in conjunction with the Universities of Oxford and Brunel University) which will bring significant research funding to the castings industry in the UK. This work will amongst other areas, be looking at grain nucleation as well as recycling and reuse of aluminium with significant possibilities for energy savings.
This branch meeting was jointly held with The Diecastings Society, who kindly sponsored the buffet for the members of both organisations and guests attending.