Rapidly Solidified Aluminium-High Silicon Alloys

Aluminium silicon alloys are the most used raw material for automotive applications. One of the main limitations on using aluminium high silicon alloys is the formation of coarse brittle phases under conventional solidification conditions. However, rapid solidification processing (RS) is very effective in limiting the coarsening of primary silicon due to high cooling rate. Aluminium high silicon alloys can be processed by different RS methods. However, melt spinning is the most familiar of all RS commercial methods. Rapid solidification is a material production route. It does not give the final shape. Rapidly solidified materials require consolidation after solidification. Moreover, consolidation requires mechanical and/or thermal treatment. These treatments may degrade the microstructure. Also hot extrusion may be used as a final step, which may cause deterioration of the initially useful properties due to hot working. In general, although rapid solidification may suppress the coarsening of Si during solidification, the Si particulates may still coarsen during the subsequent thermo-mechanical treatment due to heat effect during consolidation and extrusion. In the present work characterisation of the material at various stages of production has been carried out and understanding of the metallurgical phenomena involved. Several alloy variants were examined. Coarsening behaviour of the Silicon particles and hardness tests were carried out. Wear properties of the alloys have been studied under dry sliding conditions against a steel ball bearing. Analyses and observation of the wear surfaces and wear mechanisms of these alloys are discussed. Wear of RS materials was compared with piston material A390 made by casting. Degassing and HIPping at 400 °C for 12 hrs prior to HIPping seems to be the optimum the degassing is effective whilst avoiding excessive coarsening of the silicon particles. Alloy number RSP461 (Al 21Si 3.9Cu 1.2Mg 2.4Fe 1.4Ni 0.4Zr) shows excellent wear resistance due to its fine microstructure.