應用加工性模擬、微量元素添加與ECAE以改善輕合金熱加工性之研究

Abstract

本研究應用加工性模擬、微量元素添加與等通道轉角擠型（Equal Channel Angular Extrusion, ECAE）等方式來改善輕合金之熱加工性。研究的材料為輕合金中較難加工的Ti-6Al-4V ELI（Extra Low Interstitials）鈦合金及AZ31鎂合金。為了獲得符合醫療植入物規格要求的Ti-6Al-4V ELI合金鍛造製程條件，選用能夠反應材料微結構動態改變的製程圖模擬來改善其熱加工性，方法是以能量消耗效率圖獲得能夠產生微細均勻的等軸α/β結構之製程條件，並以不穩定性圖避開會產生不穩定或有缺陷微結構的製程條件，最後獲得了所需要的材料微結構。在研究中也針對製程圖模擬方法有疑慮的地方進行探討。結果顯示初始微結構對於製程圖有明顯的影響，製作製程圖之壓縮測試所使用的原材料必須與所模擬材料的初始微結構狀態一致。以製程圖模擬多道次加工時，每一個道次都必須以前一個道次加工後的材料製作新的製程圖，才能精確呈現材料在變形中的動態變化。研究中也針對鎂合金AZ31的熱加工延性進行探討。AZ31的熱加工延性最主要是受到晶粒大小影響。本研究提出以添加微量Zr、Sc元素，配合原存在AZ31中的Mn元素進行改質，在輥軋與ECAE加工後可以將平均晶粒細化到2.47μm，其300℃熱延性在10-3 s-1的應變率下達到324%。改質後合金在190℃-300℃間退火都可產生微米尺度的Zr與奈米尺度的Mn-Sc均勻析出物，此析出物在TEM觀察下有抑制晶界移動的效果，而且在長時間加熱下亦能夠抑制晶粒成長。創新的改質AZ31合金具有微細晶粒與高溫穩定性，非常適合應用在工業產品大量生產上。

Both titanium alloys and magnesium alloys have less hot workability than other loght-weight alloys. This research aimed to improve the hot workability of selected titanium alloy and magnesium alloy through workability simulation, alloying and equal channel angular extrusion (ECAE) process. To achieve the specification of forged Ti-6Al-4V ELI for medical implant, a uniformly distributed □/□ microstructure must be obtained after hot forging process. Hot workability was simulated through processing map to demonstrate the dynamic behavior of material during forging process. The optimum process conditions were then adapted from the power dissipation map and the instability map. The results were confirmed via the microstructure that hot deformed by the optimum process conditions. The results also indicated that the processing map was influenced by the initial microstructure before deformation. The application of processing map for multi-passes deformation must be modified by using multi-maps according to the passes design. This research also studied the improvement of hot ductility of AZ31 magnesium alloy. Grain size in the microstructure dominates the ductility of AZ31. This work presented a novel approach for enhancing the grain refinement of magnesium alloy by adding Sc and Zr. Rolling and ECAE were performed to refine the grains. Experimental results indicated that adding 0.03~0.06 wt% of Sc and 0.05~0.15 wt% of Zr reduced the mean grain size of AZ31 to 2.47 □m. The ductility of AZ31-0.15Zr-0.06Sc can be increased to 324 % at 300℃. Results of this study demonstrated that both Sc and Zr are important in controlling grain size and ductility.