The principle of a new process to produce the semi-solid slurry of hypereutectic Al-Si alloy containing fine primary silicon crystals has been proposed: the process was named “slurry-melt mixing (SMM)” process. In this process, a phosphorus-added hypereutectic Al-Si alloy melt (second-alloy melt) was mixed with a semi-solid slurry of a hypoeutectic Al-Si alloy (first-alloy slurry) under a furnace-cooling atmosphere. As the two kinds of alloys were brought into contact with each other with agitation, the second-alloy melt was rapidly quenched to precipitate numerous fine primary silicon crystals, while the equiaxed alpha solid solution in the first-alloy slurry was remelted absorbing the latent heat of fusion. And the enthalpy balance of the two alloys led the mixed alloy to a stable solid-liquid coexisting state.
Al-20 mass%Si alloy slurry was produced by the SMM process with the combination of an Al-8 mass%Si alloy and a phosphorus-added Al-32 mass%Si alloy. The arithmetic mean of area-equivalent diameters of the primary silicon crystals in the SMM slurry was 27 μm. The fluidity of the SMM slurry could be kept for about 1000 s because of the low cooling rate of the atmosphere. Sodium-addition into the SMM slurry makes it possible to achieve the modification of the eutectic silicon crystals without the coarsening of the primary silicon crystals.
