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    Please use this identifier to cite or link to this item: http://ccur.lib.ccu.edu.tw/handle/A095B0000Q/271

    Title: 利用鋁合金冷作平面應變擠製探討摩擦界面硬化層現象;Mechanical Properties of a Harden Layer Generated in Cold Plane Strain Extrusion of Aluminum Alloy 6061
    Authors: 陳政宇;CHEN, JHENG-YU
    Contributors: 機械工程系研究所
    Keywords: 鋁合金;冷作擠製;應變率強度因子;平面應變;硬化層;奈米壓痕量測;cold extrusion;strain rate intensity factor;plane strain;harden layer;aluminum alloy;nano-indentation
    Date: 2017
    Issue Date: 2019-07-17
    Publisher: 機械工程系研究所
    Abstract: 在金屬成形製程中由於加工時材料表面承受極高應力及應變率下,摩擦界面處通常會發生微觀結構的重排,導致在金屬表面層上產生晶粒細化與硬化層。相較於典型的塑性變形層,摩擦界面處之薄層的機械性質變化的梯度極高,因此長久以來一直是理論發展上的一個瓶頸,需要發展一套在理論及實驗上皆能應用的新方法。另一方面,機械零件的成形製程中材料的微觀結構表面層對產品品質有顯著的影響,因此發展可預測金屬成型中的摩擦界面材料特性方法是力學與機械加工的重要課題。 本研究以應變率強度因素為理論基礎,因為等效應變率梯度在靠近摩擦表面非常高與機械性質變化所展示的高梯度之實際現象亟合。由於應變率強度因素是奇異項的係數,有限元素法的套裝軟體並不適用,所以本研究團隊發展特徵值數值分析方法來計算特徵值應變率強度因素,在特徵值坐標系中進行數值分析可以避開應變率強度因素的奇異點,建立應變率強度因素的數值運算法和摩擦界面處薄剪流層內材料機械性質的本構方程式,目前可以求得平面應變下塑性加工的材料的應力應變。 此論文的目標是發展一套實驗流程以平面應變為條件進行模具設計,並選取鋁合金為加工材料,在不須施加加工溫度的條件下探討不同平面應變時所產生硬化層的關聯性以驗證應變率強度因素的數值分析方法,進而開發出能夠預測材料在密集應變後表面硬化層厚度的方法。實驗主要是透過量測材料的硬度和平均晶粒尺寸來探討整個製程設計對應變率強度因素的影響,以金相實驗來觀察成品晶粒分布情況,再藉由奈米壓痕試驗來量測邊界硬化層的厚度分布,而實驗結果將提供足夠的數據來估算應變率強度因素的大小。最後將實驗數據結合應變率與硬化層厚度的的數值計算方法來估算應變率強度因子(D)值,並利用此應變率強度因子計算出鋁合金在擠製加工後的硬化層厚度分佈,進而驗證此數值分析理論。
    Intensive microstructural rearrangements usually occur near frictional interfaces in metal forming processes, it is because the surface of the material under extreme stress and strain rate. These rearrangements result in the generation of fine grain and hardened layers. One of the main contributory mechanisms responsible for the generation of these layers is severe plastic deformation. As compared to typical processes of severe plastic deformation, an exciting challenge of describing the evolution of material properties near frictional surfaces is that the gradient of these properties is extremely high at some distance from the surface. This suggests that existing approaches to study material behavior in deformation processes can be inadequate and calls for new approaches in theoretical and experimental research. On the other hand, the microstructure of material in narrow sub-surface layers has a significant effect of the performance of structures and machine parts produced by metal forming processes. Therefore, it is of great practical importance to develop an efficient approach to predict the evolution of material properties in narrow layers near frictional interfaces in metal forming processes. The theoretical basis is the strain rate intensity factor, found that the gradient of the equivalent strain rate is very high near such surfaces, which is in qualitative agreement with experimental results that demonstrate a high gradient of material properties. Since the strain rate intensity factor is the coefficient of a singular term, commercial finite element packages are not capable to provide the solution for this quantity. The numerical code to be developed is based on the method of characteristics. The expression for the strain rate intensity factor in the characteristic coordinates should allow one to eliminate the singularity in the numerical code, and our series of endeavors is the development of a numerical method for calculating the strain rate intensity factor in plane strain condition and the development of new constitutive equations for predicting the evolution of material properties within the thin shear layer or hardened layer near friction surfaces. The new constitutive equations will connect the strain rate intensity factor and parameters characterizing material properties within the fine grain or hardened layer. This dissertation develops a experimental processes to conduct cold extrusion of aluminum alloy under plane strain condition, which could deform the metal under plane strain without any temperature elevation, investigates the relationship between the harden layer produced by different strain with aluminum alloy is discussed, verifies strain rate intensity numerical analysis method, and develops a mechanistic approach to describe the mechanical behavior of the affected layer near the frictional surface. The mechanical properties were investigated with nano-indentation and metallographic experiments. Finally, the result obtained for the material coefficient α show that the theory based on the strain rate intensity factor(D) is applicable for describing the formation of the harden layer. Using this theory it is possible to determine the thickness of the harden layer in aluminum alloy extrusion processes and verified the numerical analysis theoretical.
    Appears in Collections:[機械工程學系] 學位論文

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