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Title: | 高能量加工效能之分析 The Analysis on Working Efficiency of High Energy Beam Machinery |
Authors: | "Ho, Je-Ee" 何正義 |
Advisor: | 楊宏智 |
Keyword: | 移除率,脈衝雷射,電子束穿透, ablation rate,pulse laser,electron beam penetration, |
Publication Year : | 2008 |
Degree: | 博士 |
Abstract: | 材料移除率,非線性穿透機構及高深寬的比值不但為高能量精密製造重要之特徵;亦直接決定了加工品質之優劣。為了評估高能量系統其效能的變化,上述參數對加工效率的影響構成本研究的焦點項目;經本論文模式的推導,及數值差分法的計算;其對應結果並以實驗數據的比對及印證。
本論文分成三大主題首先以脈衝雷射機構分析材料移除率,期間發現瞬間的高能量束密度及雷射光波長不但主宰了離子電漿的吸收率;亦直接提供強烈的蒸發移除機制;此種現象驗證了Schmit[1] 及 Tim[2]的實驗結果。本研究另以長脈衝雷射或電子束穿透模式解釋高能量加工過程中非線性穿透的變化;一套臨界流理論(孔穴底部蒸氣壓力與表面張力的平衡)首次地被推導且合理地的闡述了最大鑚孔效率的移除模式。本論文同時推導證實材料的熔化速率低於臨界流動速度時,蒸發效應為孔穴產生的主要原因。此現象成功地評估了非線性穿透的現象及其加工效率;同時亦吻合了Allmen[8]的實驗數據及本研究中的實驗結果。 本研究最後利用電子束銲接觀察加工後的銲道變化;高深寬比的鎖孔型式存在於低傳導性的金屬如不鏽鋼304;亦隱示了入射的能量主要用於軸向的貫穿。至於高傳導性的鋁材,深寬比較低且比值未隨入射能量的增加而有明顯的變化;以上結論說明了材質中徑向的熱傳導量是不可忽略的。由上述鎖孔尺寸的觀察可推知入射能量於材料內部消散的方向性以及其能量的比例。此數量化的分析業於本研究中的實驗獲得印證。 本文引用熱傳的公式、液動的理論、相關的量子觀念並配合差分技巧、網格的配置,順利地發展一套自主性的程式;經由實驗的結果合理解釋了高能量加工的特性變化。本論文之研究成果對於更高一階的pico,femto雷射加工及電子束加工機制提供更具價值的參考資料。 Material ablation rate, nonlinear penetration behavior and high aspect ratio constitute the special characteristics of high energy beam machinery, which are believed to have significant influence on the working quality of electronic beam (EB) or laser beam (LB) machining systems. The performance of the two machining systems and their respective characteristics form the main focuses in this study. The working efficiency estimated from relative parameters had been assessed by the analytical and numerical solutions developed in this study. Experiments were devised and conducted to compare with theoretical results derived. Three main areas are covered and discussed in this research. First, an investigation of the material removal rate using an impulse laser is made. In this study the incident energy intensity and wavelength of the laser beam are found to be the active parameters, which are the keys to improve the absorption of plasma, in turn inducing a strong ablating mechanism of evaporation. Te finding is also consistent with the experimental result made by Schmidt [1] and Tim [2]. The study followed is a setup of theoretical model corresponding a continuous and long pulse of laser or an electron beam (EB) to discuss its nonlinear penetration behavior, i.e. the instantaneous increase of the drilling velocity primarily concerned during the transitional energy density region. To identify the prior parameters responsible for the formation of the keyhole, a critical flow theory dealing with the balance of the vapor pressure near cavity base and the surface tension was then proposed. The related analysis not only defined the nonlinear penetration characteristic clearly, but the determination of incident energy density for the maximum material removal efficiency was made possible. By referring to the definition of critical flow theory, the vapor pressure induced by evaporation is required to compensate and support the keyhole if the melting velocity is slower than the critical flow velocity. It is found that the surface tension will otherwise be predominated in forming a new cavity again. The continuous formation of the keyhole can be confirmed by the both penetration mechanism mentioned above, which agrees with the analytical results obtained from Allmen and experiments carrying out in this study. The thesis is concluded by investigating an electron beam welding process to examine the variation of welding channels, where a higher aspect ratio appearing on the lower thermal conductivity metal, such as stainless steel 304, in which most of the irradiative energy on the cavity base will be used to penetrate into the material. An opposite phenomena observed from the aluminum, a higher thermal conductivity metal, shows a lower aspect ratio of depth to width as a contrast, which is nearly independent with the input energy density. This study shows that the heat absorbed by material will be dissipated in all direction evenly instead of increasing the welding depth only. The above theoretical analysis associated with experimental observation had been done and both were found to be compatible. Based on the principles of heat conduction, hydrodynamics and statistic distribution, a source code invoking the numerical method including central differential scheme and non-uniform spatial deposit of nodal grids was carefully made. The analytical package developed offers an easier and effective solution, and is found a useful tool to interpret the variation of physical characteristic observed in the high energy beam environments. The conclusions derived also provides useful and valuable directions for the advanced pico-laser or femto-laser machining applications. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/37542 |
Fulltext Rights: | 有償授權 |
Appears in Collections: | 機械工程學系 |
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