Tensile and fracture toughness properties of three typical aluminum casting alloys, A (0.7%Si)–T6, B (4%Si)–T6 and C (7%Si)–T6, were evaluated in both as-cast and as-forged conditions. Cast and forged alloys showed increased elongation and fracture toughness with increase in forging ratio, especially in A (0.7%Si)–T6 alloy. The tensile and yield strengths of cast and forged alloys of B (4%Si)–T6 and C (7%Si)–T6 increased with forging ratio, but decreased in 50% forging ratio. Mechanism of the increased fracture toughness are considered to be a decrease in the size of casting defects with forging ratio. On the other hand, it is also considered to change microstructures of α phase and eutectic Si particle. In the Al–Si–Mg alloys, Mg₂Si is a major precipitate, however, it is considered that the excess Si also contributes to the increase of the strength.

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Change in the morphology of dendrite primary crystals heated at a temperature of solidification range was examined in hypoeutectic Al–Si–Cu alloys. Dendritic α–Al crystals, which have been refined by the addition of grain refiner, Ti–B and cast into copper mold with the cooling rate more than 10 K/s, are easily spheroidized only by heating to semi-solid state. The size of spheroidized α–Al phases is about 100 μm in diameter under the semi-solid state of fraction solid 0.4 at 860 K. This morphology of α–Al crystals is suitable for the semi-solid squeeze casting. Semi-solid squeeze cast Al–7% Si–3%Cu alloys containing 0.1~1.0%Fe have superior in tensile properties, especially in elongation. The elongation of an as-cast alloy containing 1%Fe is 9%. The tensile properties of semi-solid squeeze cast alloys are further improved by solution heat treatment.

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The effect of strain induced to alloys prior to the semi-solid processing (SSP) on microstructures was investigated on a Al–7%Si–5%Mg alloy. The microstructural changes during heating and isothermal holding at a semi-solid state were examined on a strain-induced alloy. The strain was induced by cold-rolling with a reduction ratio of 10 to 30%. The effect of cold-rolling on producing spherical and grain-refined α phase increases with increasing the reduction ratio up to approximately 20% and almost saturates at higher reduction ratio than 20%. During heating to 580°C, the characteristic boundaries become visible on the inside of the dendrite-shaped α phase. Simultaneously, the eutectic silicon particles are partially melted and the liquid phase preferentially flows into the small capillaries of boundaries. With increasing temperature the boundaries become more distinctive and act to break down the dendrites into small grains. The boundaries are considered to be formed as a result of recrystallization. The isothermal holding at 580°C of semi-solid temperature region for up to 600 s has essentially no eflfect to increase the size of the α phase and to change morphologies.

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In this study, a forming belt was attached on the free solidified surface side of melt drag process which is a kind of single roll process. The purpose of the equipment of forming belt is the improvement of strip production in higher roll speed than conventional strip casters. The effects of some factors, such as, the forming belt position, the forming belt contacting angle, the roll speed and the puddle form type on the continuity of the strip casting, the microstructure and surface quality were investigated. Al–12 mass%Si alloy was used as the experimental material. As the result, the secondary cooling effect by the equipment of the forming belt was proved. Producing rapidly solidified strips in high roll speed up to 200 m/min became possible. And it became clear that puddle form type affects the surface quality and microstructure type. Without puddle, the forming belt forms the free solidified surface in semi-solid state and free solidified surface was improved.

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In order to improve the free solidification surface of strip, the property in case of semi-solid forming of Al–12 mass%Si strip by melt drag process with a forming roll was studied. The effect of some producing factors, such as, roll separating force, roll angle to contact and roll speed on strip property was investigated. As the result, a higher roll speed or a smaller roll angle to contact made the solid fraction of the solidifying strip between rolls lower. Considering solid fraction, the forming pattern of forming roll side surface was classified into two types. On the condition that the solid fraction between rolls is low, it was possible to produce strip with improved free solidification surface under low roll separating force (0.1 kN). On each roll speed, the roll-separating force which causes sticking between strip and roll was obtained. And the relation between roll separating force and reduction, surface quality and microstructure was made clear. From these results, a roll speed higher than 50 m/min is suited for the melt drag process applied of semi-solid forming.

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The microstructure of 6061 DC billets cast using the float were investigated by the cross sections. The very fine crystal grains are located near the center of billets and the very coarse one is segregate near the edge of float. These phenomena are related to the float shape adopted the outside diameter and the slit position. The decrease in diameter of the float increase the this tendency and in the reverse case the change of crystal grain size average as a whole in a billet. As results, the grain refining mechanism in DC casting process using the float is inferred as follow; the equiaxed crystals nucleated on the float wall separate from the wall immediately after crystallized and diffuse. The forced fluid motion flowed into mold through three slits of the float prevent equiaxed crystals from distributing to the outside of the mold, resulting in very fine crystal zone at vicinity of billet center.

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