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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 ºC 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, i.e., 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. 1. 緒 言 過共晶 Al-Si 合金は，Al 母相中に初晶の Si 粒子が分散する組織を通して Si の低密度，高耐摩耗性，低熱膨張 係数，高温強度などの優れた特性の活用が期待されている金属基複合材料の一つである (1) ．しかし，過共晶 Al-Si 合金の実用化は，硬い Si 粒子の分散が被削性の悪化を生じ，さらには工具摩耗を招く欠点が克服されていないた め妨げられている．このような材料固有の欠点を克服し，優れた特性を活用する方法の一つとして，傾斜機能材 料(以下 FGM という)化が考えられる (2) ．FGM は材料設計の概念に基づき，機能の要請に沿って組織構成を傾 斜分布させ，機能の活用を図る材料である．本研究で取り上げる過共晶 Al-Si 合金では，母相 Al の特性の組成傾 斜化による利用を通して，分散する Si 粒子のもたらす難加工性という欠点の克服を目指している． 提案されている FGM の作製方法には，様々な手法がある (2) ．私達は金属基の FGM 素材を安価に大量に製造す ることが可能な遠心力法を提案し，装置の開発をすすめるとともに，作製した素材を用いて FGM 実用化のため の研究を行ってきている (3)~(7) ．遠心鋳造法の原理に基づく遠心力法は，溶湯中の構成要素が密度の違いによって

A novel technique to characterize the transition phenomenon from solid to melt of Al-Al3Ni functionally graded material (FGM) through a wavelet analysis for the development of a thixoforming system is investigated. Identification of an optimum semi-solid condition for thixoforming is necessary not only for the construction of a system but also the fabrication of a near-net-shape product with fine microstructure. An online wavelet analysis system using Haar’s wavelet function, which is applied for its simplicity compared with Daubechies’ wavelet function, is developed to find the optimum operating condition. A thixoforming system, which is constructed adapting a threshold value as an index, monitors successfully a discontinuity of deformation of Al-Al3Ni FGM with the temperature rise. Thus, the timing of an operation is not at pre-fixed temperature but at the time when the index related to a wavelet function is satisfied. The concept is confirmed to be suitable from the micro-structural observation of the Al-Al3Ni FGM product, because the product under the optimum condition is found to have refined Al3Ni grains, which change from coarse grains and are expected to improve the mechanical properties.

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.

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