메뉴 건너뛰기
.. 내서재 .. 알림
소속 기관/학교 인증
인증하면 논문, 학술자료 등을  무료로 열람할 수 있어요.
한국대학교, 누리자동차, 시립도서관 등 나의 기관을 확인해보세요
(국내 대학 90% 이상 구독 중)
로그인 회원가입 고객센터 ENG
주제분류

추천
검색

논문 기본 정보

자료유형
학위논문
저자정보

Guo Xian (부산대학교 )

지도교수
강남현
발행연도
2023
저작권
부산대학교 논문은 저작권에 의해 보호받습니다.

이용수0

표지
AI에게 요청하기
추천
검색

이 논문의 연구 히스토리 (3)

초록· 키워드

오류제보하기
Wire-arc additive manufacturing (WAAM) combines an electric arc as a heat source and a wire as feedstock to build a layer-by-layer component. In WAAM, process parameters are important characteristic, which can affect the mechanical properties and microstructure of the deposited material. This paper demonstrates improvements in specimens made of Ti-6Al-4V manufactured by WAAM treated by various methods (heat input, deposition speed and cryogenic vaporised Ar shielding/cooling).
Heat input: A high heat input (106 J/m, Specimen H) produced a columnar grain exhibiting a large anisotropic tensile strength. However, a low heat input (5×105 J/m, Specimen L) transformed the columnar grains to equiaxed grains owing to the accelerated solidification rates. The thermal history of the WAAM deposits was simulated using the finite-element method. The faster cooling rates in the solidification range (1600–1660 °C) in Specimen L resulted in a larger fraction of the equiaxed grains and significant reduction of tensile strength anisotropy. Meanwhile, due to more thermal cycles and rapid cooling rates during the secondary-α transformation below the β-transus temperature (700–1006 °C), Specimen H produced larger amounts of nitrogen, α’ martensite, and fine-secondary α with a tangled dislocation, thereby exhibiting a larger tensile strength and hardness than Specimen L.
Deposition speed: Equiaxed prior-β grains developed in the faster deposition speed specimen (F) and mixture prior-β grains (equiaxed and columnar) took shape in the middle and slow deposition speed specimen (M and S), which is consistent with the anisotropic property as observed in the fast deposition speed specimen with lower anisotropic tensile strength than others. Meanwhile, simulation results demonstrated that a faster deposition speed led to a lower temperature gradient (G) and a higher solidification rate (R) resulted in a larger number of equiaxed grains compared to the slower deposition speed specimen.
Cryogenic vaporised Ar shielding/cooling: The cooling time of cryogenic vaporised Ar shielding/cooling of Ti-6Al-4V deposits was ~10 times faster than that of normal Ar gas shielding/cooling. The columnar β grains in rapid cooling condition were observed to be in the longitudinal direction of the deposits same as that in natural cooling condition. Under the rapid cooling, a narrow heat-affected zone band (~70 μm) and a wide vanadium-segregation band was observed, along with a reduced aspect ratio of the α plate, which led to improved isotropic-tensile properties along the deposition and building directions. Rapid cooling condition also retained a significant amount of O and N, which enhanced its hardness and tensile strength. Thus, cryogenic vaporised Ar shielding/cooling can produce Ti-6Al-4V wire-arc additive manufacturing deposits with better tensile properties compared to normal Ar gas shielding.

목차

Chapter 1 Introduction 1
1.1 Wire-Arc Additive Manufacturing (WAAM) background 1
1.2 Previous studies and insufficient points 7
1.3 Research goals of this study 10
Chapter 2 Theoretical background 11
2.1 Metallurgy of titanium alloy 11
2.1.1 Alloy of Titanium 11
2.1.2 α+β Alloys 12
2.2 Typical phase transformations in Ti-6Al-4V 13
2.3 Solidification theory 19
Chapter 3 Experimental and modeling methods 21
3.1 Experimental 21
3.1.1 WAAM specimens with different heat input 21
3.1.2 WAAM specimens with different travel speed 27
3.1.3 WAAM specimens with different rapid cooling 30
3.2 Numerical model of thermal analysis- 36
Chapter 4 Effect of heat input on microstructure and mechanical property 41
4.1 Results and discussion 41
4.1.1 Solidification morphology for various heat inputs 41
4.1.2 Microstructure for various heat inputs 45
4.1.3 Anisotropic tensile strength for various heat inputs 49
4.1.4 Thermal simulation for various heat inputs 51
4.1.5 Effect of heat input on grain morphology 53
4.1.5.1 Temperature gradient () simulated for various heat inputs 53
4.1.5.2 Cooling rates simulated for various heat inputs 55
4.1.5.3 Solidification rates for various heat inputs 57
4.1.6 Effect of heat input on microstructure- 59
4.1.6.1 Precise characterisation of fine secondary α 59
4.1.6.2 Mechanism of microstructural evolution of secondary α 63
4.2 Conclusion 70
Chapter 5 Effect of deposition speed on microstructure and mechanical property 71
5.1 Results and discussions 71
5.1.1 Grain structure and anisotropic tensile strength at different deposition speeds 71
5.1.2 Thermal history simulation results with different deposition speed 75
5.2 Conclusion 81
Chapter 6 Effect of interpass cooling rate on microstructure and mechanical property 82
6.1 Result and discussion 82
6.1.1 Solidification morphology (prior-β grains) for various shielding and cooling methods 82
6.1.2 HAZ and segregation bands for various shielding and cooling methods 84
6.1.3 Microstructural characteristics for various shielding and cooling methods 90
6.1.3.1 Top region of specimens N and R 91
6.1.4 Oxygen and nitrogen contents for various shielding and cooling methods 103
6.1.5 Mechanical property for various shielding and cooling methods 106
6.2 Conclusion 112
Chapter 7 Future works 113
7.1 In-depth investigation of globularization α evolution mechanism and heat treatments 114
7.2 Validation of Oxygen and Nitrogen contents and its mechanism 115
7.3 Process design for microstructure and property control 116
7.4 Strategies and processes for high quality and residual stress in Wire Arc Additive Manufacturing 117
References 118

최근 본 자료

전체보기

댓글(0)

0