| 摘要: | 雷射熔融製程常常在工業上的許多應用領域中出現，包括雷射切割、雷射銲接、雷射表面改質， 以及最近極具發展潛力的金屬粉末雷射積層製造，然而，在各種雷射熔融製程中，因能量高度集中， 造成雷射光束照射的加熱面會有非常高的熱能輸入，促成該區快速加熱，而熱影響區在雷射照射後， 溫度快速升高及回降的過程中，與周圍低溫區產生極大的溫度梯度，非常容易造成加工過程因熱漲冷 縮的不均或不匹配，在工件結構間產生熱應力及轉化成殘留應力，進而造成工件成品變形或龜裂，降 低工件的品質及可靠性。在雷射加工過程中，溫度變化將直接影響熱應力及殘留應力的生成與分佈， 若能掌控溫度的變化，便能有效控制工件殘留應力的分佈及大小。唯有透過對殘留應力產生機制的瞭 解，才能提出有效的預測及防制技術，在實際製作生產之前，事先選定最適合的雷射熔融製程參數， 節省試產的時間與成本，大幅提升生產效率與產品可靠度。因此，從產業發展及學術研究的觀點，發 展有效率且可靠的雷射熔融製程熱應力分析及預測模型與殘留應力防制技術，是優化金屬雷射積層製 造成品及開拓其應用領域不可或缺的重要研究課題，也是本研究的主要目的。 本計畫首要目標乃是建立一套適用於金屬雷射熔融製程之高效率有限元素法（FEM）模擬技術， 以期能有效預測各式雷射熔融製造工件的熱應力及其轉化之殘留應力，並應用於評估相關防制技術的 可行性。發展有效且可靠之雷射熔融製程FEM 模型預測溫度及應力分佈，涉及複雜的多重物理現象， 且需進行必要的驗證實驗，需耗費相當時間才能獲得足夠且有效之數據樣本進行相關分析。因此，本 研究規劃以二年時間，有系統且循序漸進建立雷射熔融製程殘留應力預測及防制技術，擬進行的工作 項目依序為：定點雷射熱源FEM 模擬與實驗驗證、移動式雷射熱源FEM 模擬與實驗驗證、一維單層 金屬粉末雷射熔融製程FEM 模擬與實驗驗證、二維單層金屬粉末雷射熔融製程FEM 模擬與實驗驗 證、評估及確認具有最低殘留應力之最佳雷射熔融製程參數組合、三維多層金屬粉末雷射熔融製程 FEM 模擬與實驗驗證、評估多道雷射掃描作為雷射熔融製程殘留應力防制技術之可行性及效益。預期 本二年期計畫所衍生的研究成果，可做為國內產業及研究機構在發展高效率雷射熔融製程生產高可靠 度金屬雷射積層工件時，評估金屬雷射積層工件結構強度及變形的重要參考依據，並提供制定防制及 改善金屬雷射積層工件殘留應力技術的參考依據，進而提升金屬雷射積層工件之可靠性及耐久性，對 提升國內金屬雷射積層製造研發能量、技術水平及人才培育，亦能有所貢獻。 ;Laser melting process is commonly seen in laser-based machining, including laser cutting, laser welding, laser surface modification, and the newly developed laser additive manufacturing. In a laser melting process, heated affected zones are rapidly heated and cooled as the high-energy laser beam passes through the planned path. Consequently, large temperature gradients and thermal stresses are generated in the workpiece because of a mismatch in volume expansion and contraction. The thermal-mismatch induced stresses are eventually, to a certain extent, left as residual stresses which could reduce the structural strength and degrade the quality and reliability of the workpiece. If the variations of temperature and thermal stress can be predicted in a laser melting process, it is possible to adjust the process parameters to minimize the residual stresses in the workpiece. In this way, quality and reliability of the workpiece can be improved so as to broaden the applications of laser melting process. It is thus necessary to develop an effective model and relevant prevention technique for prediction and prevention of residual stress in a laser melting process. For this reason, the aim of this two-year study is, through finite element method (FEM) modeling, to develop an effective modeling technique for predicting the variations of temperature and thermal stress in a laser melting process by systematically characterizing the process parameters that govern the residual stress distribution. FEM models for laser melting process are to be developed in the following sequence: heat source of a fixed-point laser beam, heat source of a moving laser beam, one-dimensional laser additive manufacturing, two-dimensional laser additive manufacturing, optimization of laser process parameters for minimizing residual stress, three-dimensional additive manufacturing, and evaluation of a residual stress prevention technique using multiple laser beams. Counterpart experiments are also to be conducted to validate the FEM models developed. It is hoped that results of this study could provide useful information for laser additive manufacturing developers and makers to prevent distortion and/or early failure of workpiece made by laser melting process. |
