以製程圖(Processing Map)分析製程條件對Ti-6Al-4V(ELI)熱加工性及微結構的影響

Abstract

Ti-6Al-4V是最廣泛使用的鈦合金，它優異的機械性質例如高強度重量比､優異的破裂韌性､高耐腐蝕性。主要的應用從飛機結構元件､渦輪盤及葉片到化學製程設備。ELI (extra low interstitials)等級的合金已經特別使用在對裂縫敏感性要求非常高的應用上，此種等級的合金有優異的抗腐蝕性，在海軍的應用上是非常關鍵的。 本研究主要是利用製程圖來分析Ti-6Al-4V ELI的熱加工性，尤其是材料在均溫性不同下對熱加工性的影響。此外，經由金相組織的觀察可驗證出製程圖在不穩定區的可靠性。實驗項目包括︰均溫性測試､壓縮試驗､金相組織觀察。 實驗結果顯示︰根據DMM model所得的製程圖與試件實際的微結構相吻合。因此，以製程圖來求出Ti-6Al-4V(ELI)的最佳成形條件是可行且有效的模擬方式。恆溫鍛造在應變量0.3時的最佳成形條件是(1) 900℃附近和應變率0.01 s-1，效率有最大值70%。(2) 950℃且應變率3 s-1，有相對較高的效率(≒45%)。(3) 1100℃且應變率0.01 s-1，有相對較高的效率 (≒45%)。非恆溫鍛造則是在(1) 900℃附近和應變率0.01 s-1，效率有最大值75%。(2) 900℃附近應變率3s-1，有相對較高的效率(≒60%)。(3) 1100℃且應變率0.01 s-1，相對較高的效率 (≒50%)。

Ti-6Ai-4V is one of the most widely used titanium alloy due to its superior mechanical properties such as high specific strength, excellent fracture toughness, and good corrosion resistance. The application of this alloy spans from airframe structural components, turbine disks and blades to chemical processing equipment. The ELI (extra low interstitials) grade of this alloy has been particularly used in fracture critical environment. This grade also has excellent corrosion resistance and has potential for critical Navy application. This research was to explore the workability of Ti-6Al-4V ELI through processing map in different thermal gradients. Besides, microscopic observations can be verified through the reliability of the processing map. Series of experiments including thermal gradients testing, compression testing, OM etc., were conducted. The results of this study showed that the developed processing map is consistent with the observed microstructure. Thus, processing map is an effective approach to determine the optimization of hot forming processes in Ti-6Al-4V ELI. According to the developed processing map, the optimal regions at a true strain of 0.3 for isothermal forging are at (1) around 900℃and 0.01s-1 with 70% efficiency, (2) around 950℃and 3s-1 with 45% efficiency, and (3) around 1100℃and 0.01s-1 with 45% efficiency. The optimal regions for non-isothermal forging are at (1) around 900℃and 0.01s-1 with 75% efficiency, (2) around 900℃and 3 s-1 with 60% efficiency, and (3) around 1100℃and 0.01s-1 with 50% efficiency.