| dc.description.abstract | 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. | en_US |