Special casting process in which aluminum is not processed in a completely liquid, as in conventional casting processes, but in a semi-solid or semi-liquid (semi-solid process) state (Fig. 1, Salzburger Aluminium Group).
The principle underlying the rheo process, the thixotropy of light metal melts, was discovered in the 1970s at the Massachusetts Institute of Technology.
In contrast to conventional die casting, in which dendritic solidified α-aluminum grains are characteristic of the microstructure, round grains (so-called globulites, Fig. 3), form during rheocasting through an upstream controlled cooling of the melt and a stirring movement (Fig. 2). These promote the flowability when filling the mold and the backfeed during solidification. Since the aluminum melt to be processed has solidified to a large extent even before the mold is filled, solidification cavity and distortion can be minimized. The high viscosity of the melt also prevents turbulence and thus gas inclusions, which, as pores, make up the largest and most harmful proportion of casting defects in conventional casting processes.
Due to the good mold filling and solidification properties, both thin-walled and thick-walled structures can be realized in one component, that means the material only has to be used where it is important for the functionality of the component. Together with the good mechanical properties, considerable lightweight construction potential can be realized (Fig. 4,Salzburger Aluminium Group).
The partially liquid melt is processed in a cold chamber die casting machine, which enables high output and thus economic efficiency. Mainly conventional hypereutectic aluminum alloys from the AlSiMg group are processed.
Due to the low pore volume fraction, blistering can be prevented in a subsequent solution annealing process. In contrast to conventional die casting part, these parts are T 6 heat treatable.
The technological advantages that can be derived from the improved casting quality using rheocasting are primarily the significantly better mechanical properties (strength and elongation). The mechanical parameters that can be achieved thus reach that of cast iron or aluminum forgings. Compared to cast iron, there is considerable lightweight construction potential, which approximately corresponds to the density advantage of aluminum, i.e. more than 60% weight savings.
In addition to the advantageous mechanical properties and good dimensional stability, the improved casting quality during rheocasting and, above all, the low proportion of pores has a positive effect on the tightness and weldability of cast components.
This means that thin-walled, helium-pressure-weldable cast components for pressure vessels in air suspension trolleys can be manufactured (Fig. 6, Salzburger Aluminium Group). The same application is also conceivable for battery trays, air conditioning compressors and heat exchangers. Components subject to thermal stress also benefit from the higher thermal conductivity of rheocasting compared to die casting.
In addition to the lightweight construction potential through rheocasting, which has a positive effect on the CO2 balance of the components in use, another aspect is the good recyclability of the process, which underlines the sustainability. The use of secondary aluminum, which can be melted down again during the rheocasting process, only requires about 5% of the energy required to produce primarily manufactured aluminum. Appropriate melt treatment enables this feedstock to be further processed into highly stressed components without sacrificing casting quality.
An application example from the commercial vehicle industry is shown in Fig. 7, Salzburger Aluminium Group. A cabin holder, which was manufactured according to the rheocasting process and replaces an existing cast iron component, achieves weight savings for two built-in parts per truck of 12 kg.
Applications can be found in the truck chassis, which is still dominated by cast iron. The lightweight design potential of the classic two-axle semi-trailer was 120 kg. This corresponds to a saving potential of 120 l fuel or 0.3 t CO2 per year and truck when driving. In addition, the payload is increased for weight-sensitive transports. This means that more freight can be transported with fewer trips, which reduces both CO2 emissions and traffic volume. Fig. 8 (Salzburger Aluminium Group) shows further examples of rheocasting components.