臺灣博碩士論文加值系統

鎂金屬及其合金已被證明具備優異的生物相容性(biocompatibility)、潛在的生物可降解性(biocompatibility)與骨傳導性(osteoconductive)。然而，由於其高活性和易爆性，目前仍沒有使用積層製造的方式所製成的鎂金屬植入物或骨移植替代物。為了開發客製化的可生物降解鎂金屬骨膜下植入物，透過金屬3D列印的方式來製作高密度鎂金屬工件僅是第一步，換言之，本篇研究是未來進一步研究的必要準備。為了開發透過金屬積層製造來製造緻密鎂工件的技術，同時避免無意義的材料浪費以及粉末爆炸的危險，使用有限元素熱傳模擬來控制製程參數是必要的。在這項研究中，我們選擇採用選擇性雷射熔融(selective laser melting)技術結合我們團隊在先前研究中所提出的新型體積熱源和三維有限元素傳熱模擬來完成這項艱鉅的任務。因為選擇性雷射熔融（SLM）是以逐道和逐層的方式來沉積金屬材料的一種製程，單道的成形在製造緻密且無不可預測之微孔洞的工件中扮演重要的腳色。在本項研究中，我們使用球型純鎂粉末來生產工件與功能性試片，為了獲得緻密的產品並防止雷射所產生的高能量使粉末顆粒爆炸，我們使用新型體積熱源進行三維有限元素分析，此熱源考慮粉末粒徑分佈以及粉層之深度改變對藉由金屬粉層傳播的雷射能量所造成的影響，用以估算選擇性雷射熔融製程中的熔池截面積尺寸。在使用能夠製造具備優異的熔池截面形貌之單到掃描製程參數以確認我們的模擬結果具有相當的可靠度之後，我們可以根據有限元素熱傳模擬與人工類神經網絡的計算結果來建立單道的製程視窗(process window)。最後，我們可以選擇製程視窗中之優化區域的製程參數來列印三維圓柱體，並分別測試相對密度、維氏硬度、微硬度以及顯微結構。實驗結果證實建立製程視窗的新方法以及由我們團隊所開發的特殊氬氣填充氣密系統可以成功地製造穩定的掃描軌道並使工作腔體中的氧氣農含量小於50 ppm。然而，仍然有一些有待瞭解的機制阻礙掃描軌道之間相互結合，為了製造出完全緻密的工件，勢必需要找出其中的未知機制與物理現象。下一步，我們將會將研究重心放在合金元素的添加、模擬模型改良、機械設備升級和工件熱處理，以追求高品質的金屬3D列印工件。

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.

中文摘要………………………………………………………………………………………………………………………………I
Abstract…………………………………………………………………………………………………………………………III
Acknowledgment………………………………………………………………………………………………………………V
Table of Contents……………………………………………………………………………………………………VI
List of Figures……………………………………………………………………………………………………VIII
List of Tables…………………………………………………………………………………………………………XII

Chapter 1. Introduction……………………………………………………………………………………………………………………1
1.1. A brief introduction to magnesium………………………………………………1
1.2. Application and research of magnesium in the field of biomedicine………………………………………………………………………………………………………………1
1.3. Comparison of magnesium alloys and other biomaterials……………………………………………………………………………………………………………………2
1.4. The necessity of novel process development………………………3
1.5. The background of selective laser melting…………………………4
1.6. Motivation and aims of the research…………………………………………5
1.7. Brief literature Review…………………………………………………………………………5

Chapter 2. Materials, methods, and systematic simulations………………………………………………………………………………………………………………………8
2.1. Materials and packed powder bed modeling……………………………8
2.2. Absorption and thermal properties of packed powder bed…10
2.3. The Volumetric heat source modeling and finite element heat transfer analysis…………………………………………………………………21
2.4. Building up the process window……………………………………………………33
2.5. Sample processing and characterization test…………………43
2.5.1 The morphology and dimension of single scan track…43
2.5.2 Multi-layer stacking of single scan track……………………43
2.5.3 Relative density and indentation test………………………………45
2.5.4 Microstructure observation……………………………………………………………46

Chapter 3. Experimental results and discussion………………………48
3.1. The morphology and dimension of single scan track……49
3.2. Multi-layer stacking of scan tracks………………………………………59
3.3. Relative density and indentation testing…………………………60
3.4. Microstructure observation………………………………………………………………65

Chapter 4. Conclusions………………………………………………………………………………………70

References………………………………………………………………………………………………………………………72

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