Semi-Solid Forming of Al-Si Functionally Graded Material and Refinement of Si Particles

概要

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For the utilization of a superior nature of hypereutectic Al-Si alloy, Al-Si functionally graded material (FGM) of thick walled tube is manufactured by the vacuum centrifugal method using hypereutectic Al-25 mass% Si alloy. Then near-net-shape product of Al-Si FGM cup is produced from an FGM billet, which is machined from the FGM tube, by a backward extruding under semi-melt condition. FGM cups are obtained successfully at a molten metal and solid Si coexisting temperature range from 580 to 590°C through a visco-plastic deformation. The morphology and distribution of Si grains in both Al-Si FGM tube and Al-Si FGM cups are examined. The fraction of Si phase in the FGM tube is varied from over 60 mass% Si at outer surface to 15 mass% Si at inner surface, ii.e./i, the outer region of Al-Si FGM tube is lighter than inner region owing to the increase of density of liquid Al-Si alloy with increasing Si content. On the other hand, the fraction of Si phase in the FGM cup is varied from over 70 mass% Si at the bottom region to less than 15 mass% Si at the cup wall region. The fine grain structure was formed regardless of the existence of initial elongated large bulky Si particles at some tested temperature. The optimum semi-solid forming condition produces a significant interaction with both plastic flow and viscous flow resistances and the most preferable temperature is inferred to be just over the eutectic melting point of around 580°C.