| Summary: | 碩士 === 國立成功大學 === 機械工程學系 === 107 === Magnesium and its alloys have been demonstrated to possess excellent biocompatibility, potential biodegradability and osteoconductive properties. However, due to its high reactivity and explosive characteristic, there is no magnesium implants or bone graft substitutes made from additive manufacturing nowadays. In order to develop customized biodegradable magnesium subperiosteal implants, manufacturing high density magnesium parts by metal 3D printing is just the first step. That is to say, this study is the necessary preparation for further research. For the purpose to develop techniques to fabricate dense magnesium parts by metal additive manufacturing, simultaneously avoid meaningless material waste and danger of powder explosion, using finite element heat transfer simulations to control process parameters is essential. In this study, we choose to employ selective laser melting process to complete this difficult task, cooperating with the novel volumetric heat source and three-dimensional finite element heat transfer simulations presented in our previous study. Because selective laser melting (SLM) is the procedure which deposits metal material in the way of track-by-track and layer-by-layer, the formation of single track plays an important role in production of workpieces that are dense and have no unpredictable micro holes. In this study, spherical pure magnesium powder was used to produce bulk parts and functional specimens. To acquire dense products, and prevent high energy from laser makes powder particles explode. We use three-dimensional finite element analysis with the novel volumetric heat source which takes into account the effect of the powder size distribution on the propagation of the laser energy through the depth of the metal powder layer to estimate the size of the melt pool cross-section during the SLM process. After employing process parameters which are able to produce the single scan track with the great morphology of melt pool cross-section to confirm that our simulation results have considerable reliability, we can build the process window of single track according to the results from finite element heat transfer simulations and artificial neural networks. Finally, we can choose the process parameters in the optimized region to fabricate three-dimensional cylinders, to test relative density, Vicker hardness, micro hardness and microstructure respectively. The experiment results prove that the new methodology of building process window and the special airtight technology with an argon inflation system developed by our group can successfully fabricate stable scan tracks and make oxygen content in the chamber is less than 50ppm. However, still some unknown mechanisms hinder scan tracks to bond each other. In order to manufacture totally dense parts, figure out unknown mechanisms and physical phenomena are necessary. Next step, we will focus on alloying element addition, simulation model improvement, mechanical equipment upgrade and workpiece heat treatment in pursuit of high-quality metal 3D printing parts.
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