Laser Powder Bed Fusion으로 제조된 Haynes 230 적층소재의 결함 형성 및 기계적 물성 연구 = Defect Formation and Mechanical Properties of Laser Powder Bed Fusion Processed Haynes 230 Superalloy

다국어 초록 (Multilingual Abstract)

In this study, a Ni–Cr–W–Mo solid–solution strengthened superalloy, Haynes 230, was fabricated by the Laser Powder Bed Fusion (LPBF) process, and the mechanisms of defect formation and their influence on the mechanical properties were systematically investigated. The primary defects were classified as keyholes, gas pores, lack–of–fusion regions, and solidification cracks. As the volumetric energy density increased, the liquid film at the end of solidification persisted longer, leading to a higher susceptibility to cracking. EBSD and EPMA analyses revealed that cracks preferentially propagated along high–angle grain boundaries, where Cr–W–Mo–rich film–like carbides and boron segregation promoted intergranular embrittlement and facilitated crack initiation under thermal stress. Room–temperature tensile tests indicated that the defect morphology, scale, and build orientation directly affected the mechanical behavior, and cracks had a more detrimental effect on strength than pores. In the in–situ tensile test, blunting behavior was observed at the crack tip, where localized stress was relaxed and plastic deformation was distributed into the surrounding matrix, indicating that microcrack propagation could be suppressed by the matrix’s deformation capacity. The mini tensile test confirmed that the size effect became more pronounced as the specimen volume decreased. At elevated temperature (1093 °C), the overall strength decreased compared to that at room temperature; however, the LPBF–processed alloy still exhibited higher strength than its wrought counterpart, which was attributed to the strengthening effect of fine grains and residual dislocation networks. This study experimentally verified the influence of crack size, distribution, and volumetric margin on the mechanical integrity of LPBF–processed Haynes 230 and elucidated the microstructural deformation mechanism that allows structural soundness to be maintained even in the presence of microcracks.

목차 (Table of Contents)

• Ⅰ. 서론 １
• 1.1 연구의 배경 및 필요성 １
• 1.2 연구의 목적 및 구성 ２
• Ⅱ. 이론적 배경 ５
• 2.1 Ni 계 초내열합금(Nickel–Based Superalloy) ５
• 2.1.1 Haynes 230 ５
• 2.1.2 고용강화(Solid Solution Strengthening) ６
• 2.2 적층 제조 공정 ７
• 2.2.1 Laser Powder Bed Fusion (LPBF) 공정 ８
• 2.2.2 공정 변수 및 VED 개념 ９
• 2.3 LPBF 공정에서의 열응력 및 균열 유도 메커니즘 １０
• Ⅲ. 실험 방법 １２
• 3.1 Haynes 230 소재 분석 및 시편 제작 １２
• 3.2 미세조직 및 석출상 분석 １５
• 3.3 기계적 특성 평가 １７
• 3.3.1 상온 인장시험 １７
• 3.3.2 고온 인장시험 １８
• Ⅳ. 결과 및 고찰 ２０
• 4.1 주요 결함 및 생성 메커니즘 ２０
• 4.1.1 Crack 의 생성 원인 ２５
• 4.1.2 결함의 정성적∙정량적 분석 ３３
• 4.2 기계적 물성 분석 ３５
• 4.2.1 상온 인장시험 ３５
• 4.2.1.1 봉상 인장시험 ３５
• 4.2.1.2 In–situ 인장시험 ４１
• 4.2.1.3 미소 인장시험 ４６
• 4.2.2 고온 인장시험 ５０
• Ⅴ. 결론 ５６
• 참고 문헌 ６０