雷射熔融製程熱傳與應力分析;Thermal and Mechanical Analysis of Laser Melting Process

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Title:	雷射熔融製程熱傳與應力分析;Thermal and Mechanical Analysis of Laser Melting Process
Authors:	施宣豪;Shih, Hsuan-Hao
Contributors:	機械工程學系
Keywords:	雷射熔融;溫度分佈;應力分佈;Laser melting;Temperature distribution;Stress distribution
Date:	2017-08-23
Issue Date:	2017-10-27 16:21:16 (UTC+8)
Publisher:	國立中央大學
Abstract:	雷射積層製造以雷射熔融材料之方式，將材料熔融並固化於基板上，配合雷射路徑及材料充填之設定，逐步疊加材料並形成成品。然而，雷射使用上，功率高且作用範圍極小，造成材料在短時間內迅速加熱及冷卻，導致雷射積層工件之不均勻膨脹及收縮，過程之熱應力及冷卻後之殘留應力相繼出現，進而降低成品使用之功效。本研究仍目的仍在建立有限元素分析模型模擬金屬粉末積層製造過程，並分析成品中由溫度變化導致之熱應力及殘留應力分佈，並以改變雷射加工參數，如雷射移動速度及軌跡，降低殘留應力之大小。為建立有效之雷射熔融製程有限元素分析模型，本研究循序漸進地發展定點雷射照射、移動式雷射照射及多層粉末雷射熔融製程之熱傳及結構分析模型。並規劃對應之實驗驗證，確認有限元素分析模型之有效性，分別進行單點雷射照射及移動式雷射照射實驗，過程以熱電耦及應變規分別量測觀測點處之溫度及應變變化，並與相對應之模擬結果比較。在實驗驗證中，分別以30 W的CO2、50及80 W的光纖雷射進行單點照射鋼板之實驗，續以30 W的CO2及80 W光纖雷射進行移動式照射。在單點照射實驗中，以CO2及50 W的光纖雷射分別照射基板，觀測點之溫度變化與模擬結果吻合。而以80 W的光纖雷射進行時，可以發現多次實驗的結果，彼此間存在些許差異，不過，模擬結果與各實驗結果之趨勢仍然接近。在移動式照射實驗中，以CO2及80 W光纖雷射照射基板，量測點溫度變化與模擬結果有一致性地吻合。在以80 W光纖雷射移動照射基板的應變量測中，其模擬中在觀測點上應變的變化與實驗量測數據趨勢相符。在多層雷射積層製造之模擬中，觀察溫度及應力分佈之結果可以發現，在雷射照射之區域中，照射點本身呈現同心圓之溫度分佈，由於雷射移動之故，移動路徑上之溫度分佈也帶有尾翼，且沿雷射軌跡方向之溫度梯度明顯較大。在積層物件上表面之正向應力分佈中，可以發現材料熔融後受到壓應力之現象，隨著熱源移開且溫度開始下降後，該區域變為承受張應力並且形成殘留應力，且受溫度梯度之影響，沿雷射軌跡方向之殘留正向應力也較大。而觀察沿雷射軌跡方向縱剖面之應力分佈狀況，發現當增加積層層數，其殘留應力也會增加，另外，較大數值之von-Mises等效應力出現在積層物件中，以及接近積層區域的基板上，此較大之殘留應力出現於積層成品中，將會影響積層製造成品結構的精確度及工件使用強度。在不同的雷射參數比較中，積層材料受雷射參數影響導致最高溫度在各個比較項目中有所差異，而造成應力分佈大小改變。在以S型軌跡進行雷射加工之殘留應力，比使用單一雷射方向加工之應力小。在不同的雷射速率比較中，以較低之雷射移動速率進行之殘留應力較大。 ;The aim of this study is using finite element method (FEM) to develop a computer-aided-engineering (CAE) technique for application in simulation of various laser processes, such as fixed-point irradiation, moving irradiation, and multilayer powder deposition. Thermal and mechanical analyses are conducted through FEM modeling to calculate the distributions of temperature and stress in various laser processes. Experiments, including fixed-point laser irradiation and line segment of laser irradiation are conducted to validate the FEM models developed in this study. Furthermore, effects of laser processing parameters, such laser scanning speed and laser scanning strategies, are analyzed to find a proper process window for laser additive manufacturing (LAM) in making a build with low residual stress. In validating experiments, CO2 laser of 30 W and fiber laser of 50 W and 80 W are employed for fixed-point irradiation, while CO2 laser of 30 W and fiber laser of 80 W are used in line segment irradiation. The temperature and strain variations at selected points are measured by thermocouples and strain gages in experiment and are compared with the FEM simulations. For fixed-point laser irradiation with a 30-W CO2 laser or a 50-W fiber laser, temperature histories at measurement points concur with the simulation results. Scattering of temperature data is observed in the fixed-point irradiation tests using an 80-W fiber laser, but the temperature changes at selected positions in simulation still have a fair agreement with experimental measurements. For a line segment irradiation, the temperature variations at selected points in experiment using a 30-W CO2 laser or an 80-W fiber laser agree well with the simulations. For strain measurement in a line segment irradiation by 80-W fiber laser, the strain variation at each measurement point in simulation shows a fair agreement with the experimental results. Simulation of thermal-to-structural changes in a multilayer powder deposition is also performed. A concentric temperature distribution is observed at the location irradiated by laser on the top surface of the build. For moving laser irradiation, a tail-shape temperature distribution is found and the temperature gradient along the laser scanning direction is extremely large. Compressive stress is induced at the region right after irradiating by the laser beam, and this local stress becomes tensile one when the irradiated area cools down. As the temperature gradient along the direction of moving laser is quiet large, the residual normal stress in this direction is much larger than that in the transverse direction. Distribution of von-Mises equivalent stress indicates that residual stress increases with increasing number of deposited layers. In addition, highly residual stresses are present in the build and at the base nearby the interface between the build and base. Such residual stress may have a detrimental effect on the structural robustness and integrity of the build made by LAM. The maximum temperature during LAM process and induced residual stress are affected by several laser processing parameters. The residual stress in the scanning path of an “S” pattern is smaller than that in a unidirectional path. The residual stress in the build and base increases with a decrease in the laser scanning speed.
Appears in Collections:	[Graduate Institute of Mechanical Engineering] Electronic Thesis & Dissertation

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