雷射積層製造技術是透過雷射光聚焦於金屬粉末上，利用加熱熔融的方式將粉末逐層堆疊至物件完成。本研究共製造了五組試片，選用的材料為AISI 420不銹鋼，Group S1 至Group S4皆使用帶狀掃描策略，且光斑直徑分別為0.1 mm、0.2 mm、0.3 mm、0.4 mm，Group S5的掃描策略為棋盤式，其光斑直徑與Group S1 相同 (0.1 mm)。本研究的目的為探討雷射積層製造的製程參數對積層工件各項性質的影響，包括幾何尺寸、表面粗糙度、密度、硬度與殘留應力，並進行拉伸試驗求得試片的機械性質，最後觀察試片之破斷面和微結構。此外，本研究亦利用商用有限元素軟體模擬雷射積層製造過程，並透過與尺寸及殘留應力的量測值比對，以驗證模擬的有效性。
實驗結果顯示，光斑大小對於尺寸、表面粗糙度、硬度、密度、殘留應力及機械強度皆有很大的影響，主要原因為大的光斑尺寸與小的相比，更容易產生球化現象與空孔，也會產生較大的重複加熱區域和晶粒尺寸。比較帶狀掃描策略與棋盤式掃描策略的結果顯示，除機械性質外，其他量測結果並無明顯差異，而棋盤式掃描的機械強度低於帶狀式掃描的強度，原因可由微結構觀察出棋盤式掃描之樣品在拼接的接合處有明顯的缺陷，且對應到的試片斷裂處即是發生在接合處。最後XRD與微結構分析結果顯示，本研究試片內主要組成為麻田散鐵，且有部分殘留沃斯田鐵並存。
模擬結果顯示，對於試片的殘留應力而言，最大殘留應力主要分布在積層試片的最上層，而且隨著光斑直徑的變大，殘留應力逐漸變小。此外，從殘留應力各方向分量的分布結果可發現，平行與垂直掃描方向的殘留應力分量最大值皆發生在試片的最上層，且平行掃描方向的殘留應力分量比垂直方向的分量還大，而平行積層方向的殘留應力分量在試片的最上層其數值幾乎為零，最大值主要發生在試片的中間層。

The purpose of this study is to investigate the effects of laser spot size and scanning strategy on the quality of SLM build, including deformation, mechanical properties, and residual stress. The specimens are made of AISI 420 stainless steel powder through SLM process. Five groups of specimens are fabricated. Group S1 to Group S4 are fabricated using an island scanning strategy with a laser spot size of 0.1 mm, 0.2 mm, 0.3 mm, and 0.4 mm, respectively. Scanning strategy for Group S5 is a checkerboard pattern, and its laser spot size is the same as that of Group S1 (0.1 mm). In addition, a three-dimensional FEM model is developed to simulate the SLM process. Measurements of residual stress and dimensions are performed to validate the FEM model. Dimensions of the geometry are measured using a coordinate measuring machine. Residual stress is measured using an X-ray diffraction instrument. Surface roughness, hardness, and density are also measured in this study. Moreover, fractography and microstructure are analyzed for the built parts.
Experimental results indicate that laser spot size has a great influence on the dimensions, surface roughness, hardness, density, residual stress, and mechanical strength of SLM build. The main reason is that a large laser spot size is more prone to balling phenomenon and greater porosity compared to a small one. It also produces a larger re-heated region and sub-grain size. For the given island and checkerboard scanning strategies, scanning strategy has limited effect on the properties investigated except mechanical strength. The mechanical strength of the specimens fabricated by checkerboard scanning strategy is lower than that of the island scanning strategy. The reason is that a weak texture exists at the juncture between two neighboring squares of checkerboard scanning strategy. Finally, the results of XRD analysis indicate that the SLM specimens of AISI 420 steel exhibit a high content of martensite (68%) and retained austenite (32%).
Simulation results show that the maximum residual stress is mainly distributed on the top layer of the SLM specimens. As the laser spot size increases, the residual stress becomes smaller. The maximum residual stress components parallel and perpendicular to the laser scanning direction occur at the top surface, while the maximum residual stress component in the build direction is located at the middle layer.

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