硬質合金在工業上得到了廣泛的應用，包括刀具、鑽具與拉模。它們通常是由液相燒結技術製造的，不過這個過程非常耗時。此外，這種傳統技術還需要模具，而且難以製造客製化的工件。為了解決上述問題，可以以擇區鐳射熔融方法替代之。這項新技術利用鐳射光束進行粉末層的熔融。它在構建複雜形狀，特別是內部部件方面具有優勢。然而，未完全熔融、粗糙的表面和裂縫等宏觀缺陷對積層部件造成了負面影響。不完整熔融以及裂縫會導致機械性能劣化，而不平整的表面則會降低外觀的品質。因此本研究的主要議題是透過操作各種策略，用以抑制宏觀缺陷，進而提升撓曲強度與磨耗阻抗等機械性質。

Cemented carbides are extensively applied in the industry nowadays, including cutting tools, drilling tools and drawing dies. They are conventionally fabricated by the liquid phase sintering process, which is extremely time-consuming. Additionally, molds are required for this traditional technique, and it has difficulties manufacturing customized work-pieces. To solve problems above, the selective laser melting process can substitute for the sintering. The laser beam is utilized to fuse powder layers in this novel technology. It is assigned with superiority in building complex shapes, especially inner parts. However, macroscopic defects such as the inadequate fusion, rugged surface and cracks will cause negative influences on the printed work-piece. The inadequate fusion site and cracks are prone to bring about deteriorated mechanical properties, while the rugged surface reduces the quality of exterior appearances. As a consequence in this study, the main issue is placed on the suppression of macroscopic defects by operating a variety of strategies, in order to enhance mechanical properties such as flexural strength and abrasion resistance.

Abstract………………………………………………………………………………I
摘要…………………………………………………………………………………II
致謝…………………………………………………………………………………III
Table of contents……………………………………………………………………IV
List of figures………………………………………………………………………VIII
List of tables………………………………………………………………………XIII
1. Introduction………………………………………………………………………01
2. Literature review…………………………………………………………………02
2.1. Tungsten carbide-cobalt…………………………………………………02
2.1.1. Processing…………………………………………………………02
2.1.2. Structures…………………………………………………………04
2.1.3. Properties…………………………………………………………06
2.1.4. Applications………………………………………………………08
2.2. Selective laser melting……………………………………………………10
2.2.1. Operation principle………………………………………………10
2.2.2. WC-Co composite…………………………………………………12
2.2.3. Residual stress……………………………………………………14
2.2.4. Part distortion……………………………………………………16
2.2.5. Insufficient fusion…………………………………………………17
2.2.6. Surface roughness…………………………………………………19
2.2.7. Cracks formation……………………………………………………21
2.2.7. Support structures…………………………………………………22
2.2.8. Repeated scanning…………………………………………………24
3. Materials and methods…………………………………………………………26
3.1. Experimental procedures…………………………………………………26
3.2. Powder information………………………………………………………27
3.3. Instrument settings………………………………………………………29
3.4. Parameter windows………………………………………………………30
3.4.1. Low laser power and scan speed…………………………………30
3.4.2. High laser power and scan speed…………………………………30
3.5. Strategies optimization……………………………………………………31
3.5.1. Initial sample (sample A) …………………………………………31
3.5.2. Side length (sample B) ……………………………………………31
3.5.3. Intermediate layers (sample C) …………………………………31
3.5.4. Substrate preheating (sample D) …………………………………32
3.5.5. Scanning patterns (sample E1 and E2) …………………………32
3.5.6. Rotation angle (sample F) …………………………………………32
3.5.7. Blocks overlap (sample G) ………………………………………33
3.5.8. Substrate materials (sample H) …………………………………33
3.5.9. Repeated scanning (sample I) ……………………………………33
3.6. Measurement………………………………………………………………34
3.6.1. Image analysis software……………………………………………34
3.6.2. Vickers hardness…………………………………………………34
3.6.3. Flexural strength…………………………………………………35
3.6.4. Abrasion resistance………………………………………………35
3.7. Work-pieces manufacturing………………………………………………35
3.7.1. Low laser power and scan speed……………………………………35
3.7.2. High laser power and scan speed……………………………………36
3.7.3. Integration of parameters……………………………………………36
3.7.4. Support structures…………………………………………………36
3.7.5. Conductive blocks…………………………………………………37
4. Results and discussions…………………………………………………………37
4.1. Processing maps……………………………………………………………37
4.1.1. Low laser power and scan speed……………………………………37
4.1.2. High laser power and scan speed……………………………………40
4.2. Structural analysis…………………………………………………………42
4.2.1. Longitudinal sections………………………………………………42
4.2.2. Microstructures……………………………………………………43
4.2.3. Lack of fusion………………………………………………………46
4.2.4. Flatness index………………………………………………………55
4.2.5. Total crack length……………………………………………………56
4.3. Gauged characteristics……………………………………………………58
4.3.1. Vickers hardness…………………………………………………58
4.3.2. Flexural strength…………………………………………………59
4.3.3. Abrasion resistance………………………………………………60
4.4. Laser printed work-pieces…………………………………………………61
4.4.1. Low laser power and scan speed……………………………………61
4.4.2. High laser power and scan speed……………………………………61
4.4.3. Integration of parameters……………………………………………62
4.4.4. Support structures…………………………………………………63
4.4.5. Conductive blocks…………………………………………………64
5. Conclusions………………………………………………………………………65
6. Future work………………………………………………………………………67
7. References…………………………………………………………………………68

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