臺灣博碩士論文加值系統

鋁鋰合金因具有低密度、高強度及高彈性係數的優點，在航空工業上的應用具有相當大的吸引力。鋁鋰合金的成形方式，除了傳統的擠製、鑄造、滾軋等技術可使用外，由於其具有超塑性的性質，也可以超塑性成形的技術製造零件。超塑性鋁合金於超塑性成形過程中，由於其特殊的變形機構以及成形參數的影響，變形過程材料內部會有空孔生成之情形。空孔的生成，可能導致超塑性鋁合金於成形過程發生破裂，而無法製成所需之零件形狀，且材料中存有空孔，對其本身之機械性質亦會有不良之影響。
本論文的主要工作是探討8090鋁鋰合金在多軸應變狀態下之超塑性成形特性。實驗過程中，以不同之應變速率將超塑性板片吹製於柱形之杯狀模穴中。利用不同成形階段之試件，探討8090鋁鋰合金在超塑性成形過程中，試件的變形狀態、試件厚度分佈及局部應變速率等之變化情形，以及成形參數及應力狀態在成形過程中對空孔生成之影響。實驗結果並與現有之相關理論分析加以比較。
由實驗結果顯示，柱狀杯形零件之成形過程應該分為兩個階段來討論。在變形的初期至板片接觸模底時為第一階段，在此階段板片之變形類似自由成形的狀態。當變形板片與模底面接觸後，由於模具表面摩擦力的作用，板片的變形會受到牽制，因此，由板片觸底後至完全成形為第二階段。在第一階段中，變形試片之外形可視為球體的一部份，試件之外形輪廓和其上各點位移之軌跡各自形成兩組圓形的曲線，並且於交點上互相垂直。在此階段中，變形板片由於局部應力的差別導致不同位置具有不一樣的應變速率，最後造成成形零件不同位置厚度的差別。在第一階段中，空孔量隨著應變量的增加而增加，空孔量的增加是因空孔的生成與空孔的成長所造成。在變形的第二階段，變形板片與模具間之摩擦力作用會限制板片的變薄效應，空孔量先是隨著成形時間的增加而增加。在達到某一最大值後，空孔量反而是隨著成形時間的增加而減少。在第二階段的後期中，空孔量的減少應該是燒結作用造成空孔收縮的現象，因而導致空孔量的減少。在成形過程中，較快之成形速率成形壓力大，會有較大的空孔縮減率，此一空孔收縮現象主要是受到塑性控制機制的影響。

Al-Li alloy has received great interesting in aerospace industries due to its low density, high strength and high stiffness. In addition to the traditional processes; such as, extrusion, casting, forging and rolling, can be used to make the Al-Li alloy components, superplastic forming is another choice for aerospace application. A significant problem with superplastic aluminum alloys is the internal cavity formation during superplastic deformation. The cavities could cause premature failure during superplastic forming, and the presence of these cavities in the superplastically formed parts would have a deleterious effect on any post-form applications.
The major work in this paper is to study the deformation characteristics of an 8090 Al-Li alloy under multi-axial stress state during superplastic deformation. The superplastic sheet was formed into a right cylindrical die by compressed argon gas. Gas blow forming was performed at 520°C and over a range of average strain rate from 8´10-4 s-1 to 1.8´10-4 s-1. Several interrupted tests were performed to bulge the sheets to various depths for each strain rate, the depth of the pole region, hence the strain, could then be utilized to evaluate the effect of strain on cavitation. The experimental results were quantitatively analyzed and compared with the theoretical model.
The results showed that the forming operation for forming into a right circular cylindrical die could be considered to separate into two stages. In stage I, the sheet would freely deform as part of a spherical dome until its central point touched the bottom surface of the die. In the second stage of formation, the sheet was overlaid on the bottom surface and on the sidewall surface as the deformation proceeds. After the central region of sheet was deformed into contact with the bottom surface of the die, the surface condition of the die would affect the subsequent deformation of the overlaid region. There was a stress state gradient from the pole of the dome to the edge in stage I, this effect caused difference in strain rate and thickness distribution along the central line of the formed part. In this stage; cavity volume increased exponentially with deformation. The evolution of cavity volume was due to both nucleation and growth of cavities. In the second stage; the surface friction would restrict thinning of the sheet, the cavity volume first increased and then decreased with forming time for all test strain rates. Decrease in cavity volume in the later stage could be related to the cavity shrinkage rose from sintering effect. A higher imposed pressure during forming leaded to a greater average cavity shrinkage rate. The cavity shrinkage rate revealed that the plasticity was the dominant mechanism for cavity closure.

摘要 I
ABSTRACT Ⅲ
誌謝 Ⅴ
目錄 Ⅵ
表目錄 Ⅷ
圖目錄 Ⅸ
第一章 前言
1-1 航空用材料之發展 1
1-2 研究範圍及目的 2
第二章 文獻回顧
2-1 超塑性及其原理 4
2-2 超塑性成形之特性 5
2-3 超塑性變形之空孔狀態 6
第三章 實驗之設備與程序
3-1 成形之模擬工具程式 14
3-2 實驗材料 15
3-3 實驗模具及設備 16
3-4 實驗過程 17
第四章 結果與討論
4-1 成形過程之變形狀態分析 19
4-2 成形過程厚度之分析 20
4-2-1 成形厚度之預測及假設 22
4-3 成形過程應變量之分析 23
4-3-1 應變狀態之分析 23
4-3-2 等效應變量 26
4-4 成形過程應變速率之分析 28
4-4-1 Stage I 觸底前自由成形之球形成形階段之探討 28
4-4-2 Stage II 觸底後之成形階段探討 30
4-5 空孔狀態之分析 31
4-5-1 Stage I空孔狀態之分析 32
4-5-2 Stage II空孔狀態之分析 36
第五章 結論 40
參考文獻 42

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