The processing and properties of aluminium-silicon alloys containing graphite

Cast aluminium-silicon alloys containing graphite particles are reported to offer the potential for use as bearing and cylinder block materials due to low friction and wear characteristics, good thermal conductivity and resistance to corrosion. However, a fundamental problem exists in dispersing graphite particles because liquid aluminium alloys do not wet graphite and so particles are rejected by the melt. The first section consists of a literature survey which examines the potential for graphite aluminium alloys, and reviews various methods for dispersing graphite particles. The potential for the use of recently developed methods of casting semi-solid alloys as a means of retaining graphite particles in melts is then considered. The second section reports on an investigation into the production of aluminium-silicon alloys which contain graphite particles and an evaluation of the mechanical and tribological properties of the composites produced with various silicon and graphite contents. Processing semisolid alloys produced an even dispersion of graphite particles without segregation, agglomeration or rejection. Solidification under pressure (squeeze casting) was used to optimise the mechanical properties of the composites. The evaluation of mechanical and tribological properties showed that strength and ductility were reduced in alloys which contained graphite. Wear tests showed that graphite additions provided increased load carrying capacity, reduced coefficient of friction, reduced steady running temperature and reduced damage to mating components. There was no evidence of a deterioration in corrosion from corrosion tests and machinability tests showed that machinability was greatly improved by the addition of graphite particles.