| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 107 === 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.
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