| 摘要 |
Al base-Si hypereutectic alloys can exhibit problems of variable and unwanted microstructure throughout the section of the article being cast. This problem is overcome by controlled cooling of the mould in critical areas, to remove or prevent excessive accumulation of heat energy and avoid the formation of intense convection currents in still molten alloy. Consequently, the necessary coupled growth of the Al-Si eutectic is promoted and the resultant microstructure is substantially free of primary Si. The 3HA and modified 3HA alloys of the applicant are considered on the basis of their wear resistance and improved machineability for automotive applications, such as engine blocks and cylinder heads. A first feature of the controlled cooling procedure is supply of coolant to regions in the mould, above and extending from the gate, such that solidification progresses uniformly from the remote regions of the mould spaces towards the gate to give a substantially uniform microstructure throughout the casting. Various coolants may be used, so that temperature in the vicinity of the gate is 50 - 75 DEG C above those at the extremities. A second feature of the controlled cooling procedure consists of supplying melt to the mould cavity through a plurality of gates, spaced relative to one another in the critical control region of the mould. The result of using such a plurality of gates, is that the energy accumulated is more widely distributed in a plurality of critical control regions and gate regions and temperature differentials between these. |
