雷射積層製造以雷射加熱熔融粉末之方式，將金屬粉末逐層熔於基板上，藉由層層疊加直至物件完成。本研究目的為探討使用金屬粉末進行雷射積層製造時，積層方向對積層物件各項性質之影響，選用之材料為AISI 420模具鋼。本研究使用選擇雷射熔融粉末床積層製造製作三種不同積層方向之拉伸試片，分別是沿著試片厚度方向積層的Group A試片、沿著試片寬度方向積層的Group B試片以及沿著試片長度方向積層的Group C試片。並對試片進行各項材料性質之量測，包括幾何形狀與尺寸、表面粗糙度、密度、硬度與殘留應力，同時藉由拉伸試驗求得積層試片之機械性質，最後進行破斷面及微結構觀察。此外，本研究透過有限元素模型模擬金屬粉末積層製造過程，並藉由與尺寸及殘留應力量測實驗結果比對，以驗證模擬的有效性。

     實驗結果顯示，對於積層試片之形狀與尺寸而言，Group A與Group C試片具備良好的尺寸精度，但Group A試片有嚴重的翹曲變形發生，而Group C試片則無此現象。而表面粗糙度與密度受到積層方向之影響較小。積層方向對硬度有一定影響，Group C試片之晶粒較其他組試片小，因此該組試片的硬度最大。由殘留應力量測結果發現，最後幾層疊加的區域會有較大之殘留應力。拉伸試驗結果顯示積層方向對拉伸試片之機械性質具有很大的影響。因為積層方向會影響晶粒成長方向，所以當拉伸方向與晶粒長向平行時，試片有較大的強度。因此，Group C試片之機械性質最佳；而Group A與Group B之性質差不多，因其晶粒成長方向皆與拉伸方向垂直。此外，由破斷面之觀察發現，破裂起源於試片內部結構之介在物以及殘留張應力最大之區域。X光繞射及微結構分析顯示本研究積層製造試片之結構組成主要為麻田散鐵，並伴隨部分殘留沃斯田鐵。

    The aim of this study is to investigate the relationship between build direction and the relevant properties of laser additive manufacturing (LAM) build of AISI 420 steel. Three build directions are considered in fabricating tensile test specimens by selective laser melting (SLM) process with a scanning pattern of alternating path. The SLM specimens are divided into three groups according to their build direction, namely Group A, Group B, and Group C. Group A is built along the thickness direction, Group B is built along the width direction, and Group C is built along the length direction. In addition, a computer-aided engineering (CAE) technique is employed to simulate the SLM process through finite element method (FEM). In order to validate the FEM model, experimental measurements of residual stress and geometry of SLM builds are carried out for comparison. Tensile properties, density, hardness, surface roughness, and microstructure are also analyzed for the given SLM builds.

    Experimental results indicate that build direction barely affects the surface roughness and density of SLM built parts. However, it has great effects on geometry, hardness, tensile properties, and microstructure. Group A specimens have good dimensional accuracy, but buckle seriously. Group C specimens have both good dimensional and geometrical accuracy. Group B specimens have the smallest hardness as they contain the largest mean crystallite size, compared to Groups A and C. Tensile test results show that Group C has the highest yield stress, ultimate tensile stress, and elongation. Fractography analysis results reveal that fracture is initiated at either inclusion or at the region with a large tensile residual stress. Optical and scanning electron micrographs indicate that grain grows along the build direction, which influences tensile properties significantly. The loading direction in tensile test is parallel to the grain growth direction of Group C, but perpendicular to that of Groups A and B. As a result, Group C has the best tensile properties. Based on XRD results, SLM specimens contain mainly martensite and retained austenite phases.

    FEM simulation of SLM process is performed for Group A, Group B, and Group C in various build directions. The FEM model is validated to be effective as it makes fair to good predictions of geometry and residual stress distribution. According to the residual stress distribution in numerical simulation, stress in the baseplate is only affected during the first-few-layer deposition. Tensile residual stress is generally located in the final top layers of SLM built part, and compressive residual stress exists in the middle SLM build. In addition, the residual normal stress in the build direction becomes larger as the height of SLM build increases.