In rheocasting, aluminium is processed in a partially liquid, partially solid state – as a so-called slurry. Yizumi’s Rheo series is specifically designed for this semi-solid processing. At the heart of the machine concept is the combination of very slow die filling followed by the application of high pressure.
Low filling speed, high pressure
Whereas the high filling speed used in conventional die casting can lead to turbulent flow and thus to gas entrapment, the semi-solid slurry in the Rheo machine is intended to flow through the cavity as laminarly as possible. The high pressure subsequently applied is also intended to improve feeding during solidification and reduce shrinkage, particularly in thicker-walled areas.
The technology is therefore aimed primarily at components for which porosity, shrinkage or subsequent heat treatment place particular demands on the casting process. According to Yizumi, the combination of reduced gas entrainment and high pressure also allows T6 heat treatment of the aluminium components produced.
Closed-loop control for the injection process
A key task of the machine is to translate the properties of the semi-solid material into reproducible process parameters. Speed, pressure and position must be coordinated during injection.
For this purpose, the Rheo series is equipped with a closed-loop injection system. Up to ten stages can be programmed for the low-speed phase. According to the manufacturer, the maximum injection speed is up to five metres per second. The system operates at pressures of up to 21 MPa and combines low-speed control with a servo-controlled high-pressure process. The machines are currently offered with clamping forces ranging from 3,800 to 25,000 kN; according to Yizumi, larger versions are also possible. The machines are based on the manufacturer’s Leap technology platform and Orca control system.
For the application planned for Jiangxi Chicheng, Yizumi specifies a casting pressure of more than 200 MPa and a pressure build-up time of five milliseconds. The manufacturer’s aim is to enable stable production of complex aluminium components even at very low filling speeds.
Applications for thick-walled and heat-treated components
According to Yizumi, the Rheo series has already been tested for a range of different components. These include an automotive component more than 20 millimetres thick and brake calipers requiring a high degree of leak-tightness. Both examples subsequently underwent T6 heat treatment. For one tested component, the manufacturer reports tensile strengths of more than 320 MPa and yield strengths of more than 260 MPa after heat treatment. The values were determined under specific test conditions and therefore cannot simply be transferred to other components or applications.
In addition to die casting, Yizumi also identifies low-pressure casting, gravity casting and squeeze casting as possible fields of application in which rheocasting may provide an alternative depending on component requirements. In the case of Jiangxi Chicheng, this is now becoming concrete: the company produces lightweight components for engines, transmissions, compressors, commercial vehicles and chassis systems, among other applications. It intends to convert products previously manufactured using low-pressure casting to semi-solid technology using the Rheo machine, and to use it to develop and manufa8cture new products in series.
