Die casting is a well-established process, able to produce complex metallic parts economically at volume. Using die casting can enable highly optimised forms for lightweighting, material reduction and part consolidation. Die casting already produces metallic parts with fine details and high strength, but semi-solid casting brings a range of similar processes that can produce very high-performance parts with wall thicknesses as low as 0.4mm and very good material properties. Thinner walls cool more rapidly, resulting in finer grain structures and stronger parts. These are highly automated processes well suited to re-shoring manufacturing closer to markets.
The die-casting process uses a permanent metal mould, referred to as the die. Molten metal is first poured into an injection cylinder, known as a shot chamber and a piston then injects it through a nozzle into the die at between 0.7MPa and 700MPa. The die has two halves, one with a fixed hole where the metal is injected and the other with a press to resist the pressure of the moulten metal being injected. For smaller parts, it is common for multiple castings or parts to be produced in a single shot.
Dies are much heavier than the parts they produce and are formed from hardened steel that is able to resist the thermal cycling. The expense of producing dies means that die-casting is not suitable for small production runs of less than a thousand and typically tens of thousands are required for die-casting to be competitive. When properly designed and operated, dies can produce hundreds of thousands of parts before there is significant wear. Die changes can be carried out in just a few hours so that a variety of different parts can be produced in batches.
UNDER PRESSURE
For high-pressure die-casting, the machines used are extremely large and heavy. They are rated by their clamping force, typically somewhere between 25 tonnes and 3,000 tonnes. Other machine specifications include the die size, piston stroke and shot pressure. Higher pressures allow rapid cycle times, thin walls and fine features.
The process is very similar to injection moulding but using metal instead of plastic. Zinc, aluminium and magnesium alloys can all be die cast, including high-performance components used in aerospace and automotive structures. Processes can be optimised to produce a fine grain structure but eliminating porosity cause by tiny air bubbles during the casting process can be a challenge. Higher pressures are required to force metal rapidly into narrow cavities to produce thin wall sections as well as to achieve rapid cycling times. However, forcing molten metal rapidly into fine features causes a lot of turbulence, trapping air and leading to porosity. These tiny voids then act as crack initiation sites, significantly reduce the strength and fatigue life of parts when compared with wrought metal.
Semi-solid casting processes are derived from die-casting but instead of injecting fully moulten metal, it is heated to just below its melting temperature and injected in a semi-solid state. This produces results which are comparable to forging, with virtually no porosity, with excellent ductility and fatigue – and with the possibility of full heat treatment and welding.
The main semi-solid casting processes are thixocasting and rheocasting for aluminium alloys – and thixomolding for magnesium alloys. Thixocasting was the first commercially available semi-solid casting process but it is expensive due to its dependence on pre-cast billets to control the process. Rheocasting reduces costs by removing this requirement, using a simple slurry making machine to supply slurry to a conventional die-casting machine. Thixomolding uses a screw conveyor to move chipped material from a hopper into a heated barrel containing a screw conveyor. This feeds the magnesium chips while mixing them to create a globular semi-solid state. Thixomolding machines resemble the injection moulding machines used for plastics – and they can operate in fully automated cells in which slurry making and casting are fully integrated.
A SLURRY STATE OF AFFAIRS
“The difference between rheocasting and high pressure diecasting is the slurry machine. It’s a slurry maker producing a slurry at the same time as the casting machine is working, meaning the casting machine is never waiting for the slurry,” explains Staffan Zetterstrom, sales and marketing manager at Comptech. “It’s a very robotic process and you can change over between traditional high pressure die-casting and rheocasting just by changing the dose and program. With rheocasting we are using standard alloys, both secondary and primaries depending on the functionality of the parts produced.”
The tools are almost exactly the same as conventional die-casting. “The only difference is the gating system. It is slightly bigger compared to conventional die-casting, otherwise the tools are exactly the same,” says Per Jansson, managing director of Comptech. “We use the same type of steel, we use the same type of hardening, and basically the same type of design of the tool.”
There are similarities and differences between rheocasting and high pressure die-casting – it’s essentially the same casting machine used but, in general, a smaller one compared to high pressure die-casting. “And the robots for metal handling are exactly the same standardised robots. The same standard furnaces as you use in high pressure die-casting,” says Jansson. “The tools look pretty much the same. The alloys are standard alloys. The difference is that you add a slurry maker, it’s the only difference. With the slurry maker there’s a wider range of possible alloys to cast.
“A wall thickness of 0.4mm and 1° draft angle is fully possible with rheocasting,” continues the MD. “However, if this was in serial production, we would recommend an average thickness of 0.8mm in order to increase the yield… In conventional casting you make a raw casting, you machine the holes and you mill all the hights. With rheocasting its possible to go directly to the final version.”
LOSE WEIGHT; GAIN PERFORMANCE
For structural lightweighting applications, the advantages of rheocasting in aluminium alloys are considerable. Thin walls, shallow draft angles and very low porosity lead to fine grain structures and full heat treatability, meaning excellent material properties. It is also possible to produce geometries with internal cavities as the relatively low shot pressures enable the use of simple sand cores for lost core processes. The lower density of magnesium allows compared to aluminium alloys can give additional lightweighting opportunities. Elektron 43 is a magnesium alloy that has obtained SAE’s Aerospace Material Specification (AMS) and is included in the aerospace Metallic Materials Properties Development and Standardization (MMPDS) handbook. Allite super magnesium is a leading proprietary alloy with extremely good structural properties for high-strength, low-weight applications. The magnesium has been in use within the aerospace and defence industries since 2006 and is now becoming more widely available.
The push for higher performance at reduced cost is related to the transition to a low-carbon economy. Advanced forms of die-casting are now driving widespread industrialisation of semi-solid casting processes. Electric vehicles require low-weight and high-strength components, with performance close to that required in aerospace. However, this must be achieved in high-volume production using highly automated processes with costs comparable to traditional automotive production. Rheocast aluminium and thixomolded magnesium alloys can deliver very high performance components that are fully recyclable from highly automated and low cost processes.