Effect of the Near-Net Shape Forming on Silicon Morphology in an Al–Si Functionally Graded Material Generated by the Centrifugal Method

Abstract

A work toward practical usage of hypereutectic Al–25 mass% Si alloy, which exhibits superior properties, as a functionally graded material (FGM) was done. The Al–Si FGM, which is based on the concept of overcoming the limitations imposed by the presence of a hard silicon phase in an aluminum matrix, was generated by a vacuum centrifugal method as a thick-walled tube. Grain coarsening, which is the primary disadvantage of the centrifugal method, was observed. The fraction of silicon phase in the tube unexpectedly varied from greater than 60 mass% at the outer surface to 15 mass% at the inner surface because of the greater density of molten silicon compared to that of the eutectic melt. Thus, the outer region of the tube was lighter than the inner region after solidification. FGM billets for near-net shape forming were machined from the thick-walled tube and were formed into an Al–Si FGM cup using a backward extruding. The products of the FGM cup were successfully manufactured in the temperature range from 853 K (580 oC) to 863 K (590 oC) through visco-plastic deformation. The fraction of silicon phase in the FGM cup varied from greater than 70 mass% Si at the formed cup bottom region to less than 15 mass% Si at the cup wall region. Coarse silicon particles were refined irrespective of the pre-existence of elongated spindle-shaped particles under some experimental conditions. The optimum operating conditions were inferred to be high-speed operation at approximately 853 K (580 °C), which was just above the melting point of the eutectic Al–Si alloy.