Model solution to castings

Better quality and reduced costs is the aim of research into metal casting processes

Metal casting is generally a cheaper, more efficient way than machining for producing large numbers of irregularly shaped parts. But the process used, for example, to produce car crankshafts, needs to be tightly controlled for cost and quality reasons.

Computer models of the casting process developed over the past 20 years leave a lot of room for guesswork. But research sponsored by car parts supplier T&N should give the castings industry a better understanding of what happens inside the mould during the metal solidification process.

The aim is to produce castings of a consistently higher quality than possible using current analytical methods. This should cut the time and costs involved in developing new mould and die designs.

Dr Bill Griffiths, T&N senior research fellow at Umist in Manchester has been given five years and a £135,000 grant from T&N to produce an advanced analytical model. A further grant of £80,000 was awarded by the Engineering and Physical Sciences Research Council.

Part of the money was used to build a transmission ultraviolet light microscope – the only one of its kind – for metallurgical studies.

Two years into the project and Griffiths has worked out what happens in principle when a very hot liquid metal strikes the cold metal surface of the mould.

Measuring this so-called boundary process is essential to build the advanced analytical model which the metal casters can use to determine the ultimate properties of the casting.

But, says Griffiths, `it is quite difficult, technically, to measure the heat transfer coefficient [the boundary process].’

When molten liquid metal enters the die, for a short time span of around half a second there is a very high heat transfer between the liquid and the mould surface. A thin skin, about a few microns deep, forms on the casting. That is the start of metal solidification.

But, because of the high solidification rate, stresses built up inside the casting cause its surface to buckle and deform. The irregular surface conducts heat to the mould at different rates depending on how near the conducting surfaces are to each other. Further, the mould expands as the casting contracts, adding other uncertainties to the analytical model.

Griffiths is now looking to develop his theories to make them applicable to real casting situations.

By Sue Stuckey

As-cast surface of an A1-4.5wt %Cu alloy taken in polarised light. Circular features are contact areas between solidifying liquid alloy and the surface roughness peaks of the chilled surface against which the alloy was cast