鑄造技術已有數千年的歷史，而隨著時代的進步，對於材料的使用環境限制更加是嚴苛，材料之特性與強度的提升需求增加，而在鑄造之凝固過程中，溫度與濃度場的變化會影響材料的顯微結構，而微結構之控制更是改善其機械性質及物理特性的關鍵所在。一般的鑄造過程是不易控制其凝固結構之形態，最多只能改變其晶粒大小，而方向性凝固之方法可使得鑄件之微結構沿著某一固定方向成長，也是單晶成長之基礎。本文以錫鉛合金為測試材料，採五種實驗模式來探討不同形狀結構之晶種對於方向性凝固微結構影響。在凝固成長過程中提供一相同成份的多晶之晶種誘發鑄件底部的枝狀晶發育時可延晶種優選方向成長，消除底部因高冷卻率形成之細小等軸晶，並獲得較佳之方向性凝固之結構。於實驗中探討不同形狀結構的晶種對其枝狀晶之優選方向控制情形、鑄件晶粒尺寸、晶體成長的束縛控制的影響及對其溫度梯度、成長速率、主枝狀晶臂間距及二次枝狀晶臂間距之間的影響。

The casting skill has been developed for several thousands years. With the progress of time, the environment of material application becomes more severe and hence the promotion requirement of material properties and strength increases. In a casting process, the temperature and concentration fields will affect the microstructures of materials and this influence is the key point of improving their mechanical and physical properties. In a general casting process, it is not easy to control the morphology of solidifying microstructures. The scheme of directional solidification can make the microstructures grow along a fixed direction and it is also the base of single-crystal growth. In this study, five experimental models are utilized to investigate the effects of different-shape seeds on the microstructures of directional solidification and lead-tin alloy is the testing material. In the solidification process, a poly-grain seed with the same initial concentration of the solidifying casting is used to induce the columnar growth at the bottom portion of the casting, which could avoid the equiaxed growth due to the high undercooling or cooling rate there. This is expected to have the better or more complete structure of directional solidification. In the experimental analysis, we studied the influences of different geometry seeds on the constrained growth, the preferential growth direction of dendrite, the grain size, the temperature gradient, the growth rate, the primary arm spacing and the secondary arm spacing.

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