Mixing extruder makes ‘impossible’ alloys
Tom Shelley reports on a manufacturing technique that allows the casting of light metal components from alloys and mixtures normally impossible to cast
By mixing shearing ingredients or alloys with a large freezing range in a twin screw ‘Slurry Maker’ it is possible to cast alloys that are normally regarded as either ‘difficult’ or ‘impossible’ to make.
Alloy mixtures such as lead and aluminium, which are useful for bearings, can be cast with ease, as can aluminium alloys normally restricted to ‘wrought’ applications including most of those with the best mechanical properties. The method has also been applied to metal matrix composites made up of solid ceramic particles in a light alloy matrix.
Most of metallurgy is devoted to producing alloys that are essentially composites made up of different phases – usually something hard in a matrix of something softer – and thinking up ways of dispersing particles, plates or needles of the hard phase in the soft phase in order to improve mechanical properties.
Aluminium and magnesium alloys pose a particular problem because there is usually a wide temperature range during which they are part solid and part liquid. Conventional casting methods result in hard particles or needles that are either too large, or poorly distributed, interspersed with contraction cracks. In addition, even with alloys that lend themselves to easy die casting, the quality of parts made from them is limited by the presence of porosity.
A team at the Brunel Centre for Advanced Solidification Technology – BCAST – led by Professor Zhongyun Fan, has now developed a suite of processes it calls Rheocasting. One variant, Direct Chill Rheocasting (DCRC) is being developed with partners including Innoval, Novelis, Alcan, Luxfer Gas Cylinders and Zyomax.
Professor Fan says: “We are mainly working with alloys whose liquid phases are immiscible, and difficult to cast alloys with freezing ranges of up to 400ºC. An example is aluminium lead alloys for bearing materials in cars and electrical contacts. We can make alloys with second phase particles from 1 to 10 microns across.”
The screws inside the barrel are co-rotating, fully intermeshing and self wiping. The screws have specially designed profiles to achieve a high shear rate and a high degree of turbulence. The space for liquids or semi solids is divided into a number of ‘C’ shaped pockets. Due to the rotation of the screws, an existing pocket near one screw reduces its volume from a maximum to a minimum while at the same time, a new pocket is created near the other screw. The content of the decreasing pockets is forced through the narrow spaces between the screws and the barrel into the increasing pockets after which the process repeats as the screws rotate.
BCAST has two machines: one for working with magnesium alloys, the other for aluminium alloys.
Karen Roberts, assistant director of BCAST, tells us: “Magnesium is not a problem. The Slurry Maker screws can be made of quite conventional extruder screw materials, working under a protective gas. But because of the affinity of aluminium for iron, we had to develop an appropriate alloy for the aluminium machine.”
After two years use, the screws were still intact and looked “pretty good”, she says, adding: “The only problem is that the alloy is a little expensive so we are looking for something cheaper.”
During casting, a predetermined amount of molten alloy from the melting furnace is fed into the slurry maker. The temperature is set in order to produce a particular solid fraction. Residence time is about 30 seconds, after which the exit valve is opened and the slurry run into a conventional die casting machine. Roberts says: “We think our twin screw will be effective because it can just function as an add-on and does not require any fundamental change in existing casting machine technology.”
The way of making components from alloy mixtures that cannot be cast conventionally currently depends on the rapid solidification of powders followed by expensive compaction processes or by developing complex routes to metal matrix composites. Improvements in mechanical properties relative to alloy parts made by conventional High Pressure Die Casting can be dramatic (see Design Pointers box). Since around 85% of pressure die cast components are used in motor vehicles, the bottom line consequence is components with greater mechanical performance per unit weight and lighter and more fuel efficient vehicles.
Studies have shown that both iron and high-silicon alloys can be rheo-diecast to produce components with refined, uniform, defect-free microstructures. Much of the work has been with conventional aluminium casting alloys – A357 and A380 with 1.57% iron, a newly developed cast and a commercially available wrought alloy, 2014, comprising 0.7% Si, 4.9% copper and 0.8% magnesium.
Prof Fan’s team has developed a number of variants of the basic process. He says: “Rheo die-casting has now reached an industrial scale thanks to DTI support. We can produce parts of up to 10kg. We are working with a German company and with the DTI – with whom we have produced a magnesium car seat frame for Jaguar and an aluminium engine mounting bracket for a Range Rover.”
Other processes include the Direct Chill process to produce billets and slabs. A “thin” DC process can be used to produce a near net shape product to reduce the need for thermo-mechanical processing. A horizontal casting process means that there is no need to dig casting pits and casting can be continuous and not limited by pit depth. It is possible to rheo cast one aluminium alloy onto the surface of another. It is also possible to perform Rheo Extrusion and Twin Roll Rheo Casting or TRRC. The latter comprises twin screw slurry makers, a slurry accumulator and a standard twin roll caster. The function of the accumulator is to bridge the gap between the batch processing of the twin screw slurry makers and the continuous processing of the twin roll caster.
While rheo die casting is ready for exploitation, all the other processes are described by Prof Fan as “still in development”.
