Attempts at casting metal matrix composites reinforced by silicon carbide particles using rheocast technology
Paweł Darłak, Piotr Dudek Instytut Odlewnictwa, ul. Zakopiańska 73, 30-418 Kraków, Poland
Quarterly No. 4, 2008 pages 385-389
DOI:
keywords: thixotropic castings, metal matrix composites, rheocast structure
abstract The main defect in castings manufactured by the common high pressure die casting process is their porosity originating from the occluded gaseous phases. This is the reason why extensive studies have been carried out on the development of technologies that would reduce the level of these defects in castings. This publication describes the results of trials on the fabrication of castings from an AlSi9CuFe alloy-based composite containing 20 vol. % (F3D.20S) of silicon carbide particles, using the thixocasting process which enables elimination of the formation of gaseous occlusions. The aim of the present studies was determination of an effect of the mixing time and temperature on the formation of rheocast structure. The studies were carried out at the temperatures of 700, 720 and 740°C, subjecting the liquid composite to mechanical stirring for a time of 10 and 20 s. Ingots prepared from F3D.20S composite and AlSi9Mg alloy were cast on a BUHLER machine of 160T locking force, applying the operating parameters used during the manufacture of thixotropic castings from AlSi7 and AlSi9 alloys. The die designed for thixocasting had a number of channels of different cross-sections to investigate the process of die cavity filling with metal in semi-solid state. As an outcome of the research a batch of composite castings were fabricated and subjected next to mechanical tests and examinations of microstructure. The results have proved that producing a rheocast structure is quite difficult due to the properties of the liquid composite slurry. In a monolithic alloy characterised by much better castability and low viscosity, the formation of gaseous occlusions practically never takes place during the long-time mixing and at a low starting temperature. The structure of a monolithic alloy subjected to mixing within the range of solidus- liquidus temperature bears some visible traces of changes in the morphology of phase and correlates with the process parameters, i.e. the time of mixing and the starting temperature. Castings made from F3D.20S composite were characterised by some well-visible defects in the form of gaseous occlusions, formed especially in the thin-wall casting sections, where the metal flow rates exceeded the boundary values of a laminar flow. Even raising the temperature and reducing the time of mixing could not give satisfactory results, i.e. a batch of castings which would be entirely free from the structural defects. The defects directly influence the drop of casting strength. The mechanical tests as well as the examinations of casting microstructure confirmed the formation of rheocast structure in castings made from both monolithic alloys and composite materials. No immediate interrelation with the casting hardness has been reported.