Influence of phosphorous addition and cooling rate on the liquidus temperature of hyper-eutectic Al–Si alloys

As for the Al–Si hyper-eutectic alloys with phosphorous addition, the primary Si size is dependent on the holding temperature relative to the “Critical Temperature Line for Grain Refinement”, which was discovered by the authors. If the holding temperature falls under this line, the size will be coarsened. Cooling rate is also said to be influential to the size of primary Si crystal, however, it is unknown which stage of cooling rate is preferential. The difference between the liquidus temperature measured with phosphorous addition and the liquidus temperature measured without phosphorous addition,is not clearly understood still. To understand these problems shown above, the thermo-analysis measurements were performed during the cooling of hyper-eutectic Al–Si alloys with and without phosphorous addition, under various cooling rates, in relation with liquidus temperature, primary Si size, etc. The results obtained are as follows: The two liquidus temperature lines of hyper-eutectic Al–Si alloys were obtained according to the alloys with phosphorous and without phosphorous, the latter shows fairly low temperature but the former shows almost the same temperature line with equilibrium diagram. This means that we can use the equilibrium diagram of hyper-eutectic Al–Si alloy as a “Imaginary Practical Phase Diagram of Hyper-eutectic Al–Si Alloy with phosphorus addition”. As for the relation between cooling rate and primary crystal size, the cooling rate above the liquidus temperature seems to be more influential to the primary crystal size, compared to the cooling rate between the liquidus line and eutectic line which is not influenced by phosphorous.