一般的鑄造過程不易控制凝固結構之形態，最多只能改變晶粒大小，而方向性凝固與單晶成長是屬於難度高之鑄造技術，需使用適當的鑄造機制，來控制凝固結構形態，其中以單晶之製程最為困難，方向性凝固則次之，不過方向性凝固是單晶製程的基礎。針對方向性凝固之研究，本文設計三種不同實驗模式來作探討，模式A是採用五種不同初始濃度，模式B與C是改變模式A之凝固機構的加熱方式與升降平台之下降速度。在實驗研究中，以錫鉛合金為測試材料，液態時完全融合，固體與液體具有溶解度差異，於偏析之作用使得凝固過程中最後有共晶結構之形成，使用光學顯微鏡觀察凝固顯微結構與利用熱偶線與溫測擷取系統量測鑄件軸向之溫度分布，探討本方向性凝固設備之溫度梯度變化、成長速率的熱傳環境及初始濃度之凝固控制參數與凝固顯微結構晶粒數、晶粒形態之間的相關性，由其分析結果，進一步掌握方向性凝固之控制機制。

The morphology of solidification microstructure is difficult to control in a casting process, in which only the grain size can be easily changed. To perform advanced casting techniques such as directional solidification and the growth of single crystal, a proper casting mechanism is needed to control solidifying microstructure. Among these solidification processes, the growth of single crystal is the most difficult one and directional solidification is the second. The fabricating technique of single crystal is based on that of directional solidification. To obtain directional solidification, in this paper, three different experiment models were designed. Sn-Pb alloy is used as the testing material. Five different initial concentrations of lead (2.5, 5, 10, 20, 38 wt.% Pb) are used in Model A. Models B and C have different heating settings and going-down speeds of the descending platform from those of Model A. An optical microscope is utilized to observe the solidifying microstructures and thermal couples are used to measure the temperatures along the axial direction of the cylindrical casting. The relationship between the solidification parameters (temperature gradient, growth rate and initial concentration) and microstructures was investigated for these three models. From the analysis results, the controlled mechanism of directional solidification can be further grasped.

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