니켈기 초내열합금 첨가금속분말이 첨가된 wide-gap 브레이징부의 미세조직 및 기계적 성질 = Microstructure and mechanical properties of wide-gap region brazed with Ni-based superalloy additive powder

다국어 초록 (Multilingual Abstract)

One of the most common types of service damage experienced by turbine hot section components is cracking due to thermal fatigue. Brazing has been extensively applied to join the new parts and repair cracks in high temperature structural components such as aero engines and gas turbines.
Unlike conventional brazing where only the filler metal is used, wide-gap brazing uses a mixture of filler metal and additive powder. During wide-gap brazing, the additive powders do not melt, thus providing necessary capillary force to attract and retain the molten filler metal. Ni-based filler metals containing B and/or Si as a melting point depressant are extensively used for the joining or repair of Ni-based superalloys. However, the effects of additive powder on the microstructural evolutions and mechanical properties in the wide-gap brazed region have not been discussed systematically.
In this study, the microstructures and mechanical properties in the wide-gap region brazed with Ni-based IN738 superalloy additive powder and filler metal was investigated.
BNi-3 and DF 4B alloy powder were chosen as filler metal powder to study the effect of chemical composition on microstructural evolution in the wide-gap region. The effect and behavior of IN738 additive powder in the wide-gap brazed region was investigated with various conditions such as brazing temperature and holding time. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr for various brazing conditions.
In the case of the wide-gap brazing using BNi-3 filler metal powder, the wide-gap region had a microstructure consisting of primary Ni3B phase, binary eutectic of Ni3B-Ni solid solution, and ternary eutectic of Ni3B-Ni solid solution-Ni3Si. However, that region brazed with the IN738 additive powder and BNi-3 filler metal powder consisted of the IN738 additive, binary eutectic of Ni3B-Ni solid solution, and (Cr, W)B. After wide-gap brazing, Si was detected in the additive powder and the ternary eutectic structure could not observed in the wide-gap brazed region. This indicates that Si in the filler metal diffused into the additive powder. Therefore, the ternary eutectic of Ni3B-Ni solid solution-Ni3Si did not form in the wide-gap region brazed with additive powder.
In the case of the wide-gap brazing using DF 4B filler metal, the microstructure of the region brazed DF 4B filler metal was more complicated than that of BNi-3 filler metal. It was observed that the microstructure of the region consisted mainly of a primary Ni solid solution, a Ni solid solution-CrB eutectic colony and a Ni solid solution-Ni3B eutectic structure, and the cuboidal-shaped γ' precipitated in the Ni solid solution. EBSD pattern analysis revealed that the CrB and Ni3B phases both had an orthorhombic structure, with lattice parameters of a=0.297, b=0.786 and c=0.293 nm, and of a=0.439, b=0.522 and c=0.662 nm, respectively. Furthermore, the cuboidal-shaped phase precipitated in the Ni solid solution was confirmed to be Ni3Al (γ') which had cubic structure with a lattice parameter of a=0.357 nm. However, that region brazed with IN738 additive powder and DF 4B filler metal powder had a microstructure consisting of Ni solid solution + γ' and (Cr, W)2B. Moreover, the eutectic colony and binary eutectic structure could not observed in the region brazed with additive powder.
From the observation of the wide gap region brazed with additive powder, it is believed that IN738 additive powder added in the wide-gap brazed region prevent the formation of primary phase and eutectic structure acting as a nucleation site of molten filler metal and a sink for melting point depressant. Therefore, the amount of the eutectic and Cr boride decreased with increasing additive powder content (wt. %) in the wide-gap brazed region.
In the wide-gap region brazed with IN738 additive powder and BNi-3 filler metal, as the brazing temperature was increased from 1100°C to 1230°C, the radius of IN738 additive powder increased from 44μm to 70.9 μm. Also, the radius of IN738 additive increased from 37.5 μm to 90.5 μm with increasing brazing time from 1hr to 30hr.
It was established that B in the filler metal affects the growth of IN738 additive powder reaction layer kinetics. An activation energy corresponding to the diffusion of B in Ni was calculated for the growth of the reaction layer. The activation energy of the reaction layer was estimated to be 385.7 kJᆞmol-1.
The room temperature fracture strength of the region brazed with IN738 additive powder and BNi-3 filler metal powder (623~687 MPa) was higher than that of the region brazed with only BNi-3 filler metal powder metal powder (390 MPa), and the maximum fracture strength of the wide-gap brazed region was the powder mixture of 70 wt. % additive and 30 wt. % filler metal powder. However, the variations of the fracture strength were little as the amount of additive increased. The fracture strength of the wide-gap region brazed with IN738 additive powder and BNi-3 filler metal depends on the Ni3B-Ni eutectic and (Cr, W)B in the wide-gap brazed region. However, there was no relations between the amounts of eutectic structure and (Cr, W)B, and the fracture strength of the wide-gap brazed region.
The room temperature fracture strength of the region brazed with IN738 additive powder (536~832 MPa) was higher than that of the region brazed with only DF 4B filler metal powder (419MPa). The fracture strength of the wide-gap region brazed with 60 wt.% IN738 additive and 40 wt.% DF 4B powder at 1230ºC for 30 hr was as high as 846 MPa at room temperature. It was found that the (Cr, W)2B and pores in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.

목차 (Table of Contents)

• 목 차
• 제 1 장. 서 론
• 참고 문헌
• 제 2 장. 이론적 배경
• 2-1. 가스 터빈의 손상 원인 및 기구
• 2-2. Ni기 초내열 합금
• 2-3. 접합 공정의 분류
• 2-3-1. 고상확산접합.
• 2-3-2. 브레이징
• 2-3-3. 천이액상확산접합.
• 2-4. Ni 기 초내열합금의 정비기술개발에 관한 연구동향
• 2-4-1. 용접정비.
• 2-4-2. 블레이드의 표면균열 정비.
• 참고 문헌
• 제 3 장. 실험 방법.
• 3-1. 모재, 삽입금속 및 첨가금속 분말
• 3-2. Wide-gap 브레이징 공정
• 3-3. Wide-gap 브레이징부의 미세조직 분석
• 3-4. Wide-gap 브레이징부의 기계적 특성 평가.
• 제 4 장. 첨가금속분말이 혼합된 wide-gap 브레이징부의 미세조직
• 4-1. 서 론
• 4-2. 실험 방법
• 4-2-1. 미세조직 분석
• 4-2-2. DSC 분석
• 4-2-3. 생성상의 EBSD 분석
• 4-3. 실험 결과 및 고찰
• 4-3-1. 삽입금속분말에 따른 wide-gap 브레이징부의 미세 조직
• 4-3-1-1. BNi-3 삽입금속분말의 미세 조직 거동
• 4-3-1-2. DF 4B 삽입금속분말의 미세조직 거동.
• 4-3-2. Wide-gap 브레이징부의 미세조직 거동에 미치는 첨가금속분말의 영향
• 4-3-2-1. IN738 첨가금속분말과 BNi-3 삽입금속분말을 혼합한 경우
• 4-3-2-2. IN738 첨가금속분말과 DF 4B 삽입금속분말을 혼합한 경우
• 4-3-3. 공정 온도 및 유지 시간에 따른 IN738 첨가금속분말의 거동
• 4-4. 결 론
• 참고문헌
• 제 5 장. Wide-gap 브레이징부의 기계적 특성에 미치는 공정 변수의 영향
• 5-1. 서 론
• 5-2. 실험방법
• 5-2-1. Wide-gap 브레이징 공정
• 5-2-2. 기계적 특성 평가
• 5-2-3. 파면 관찰.
• 5-2-4. Wide-gap 브레이징부의 경도 시험
• 5-2-5. 고온 물성 평가
• 5-3. 실험결과 및 고찰
• 5-3-1. 첨가금속분말과 삽입금속분말의 혼합비에 따른 wide-gap 브레이징부의 기계적 특성
• 5-3-2. Wide-gap 브레이징부의 기계적 특성에 미치는 공정 온도 및 시간의 영향
• 5-3-3. Wide-gap 브레이징부의 고온 크리프 특성
• 5-4. 결 론
• 참고문헌
• 제 6 장. 총괄 결론
• List of Tables
• Table 2. 1 Damage and repairing method of gas turbine component[1].
• Table 2. 2 The proposed equations for calculating isothermal solidification time(tf).
• Table 3. 1 Chemical composition of base metal, additive powder and filler metal powder.
• Table 3. 2 Solidus and liquidus temperature of the additive and filler metal powders used in this study.
• Table 3. 3 Wide-gap brazing condition used in this study.
• Table 4. 1 Results of AES composition analysis of the wide-gap region brazed with only BNi-3 filler metal powder.
• Table 4. 2 Results of chemical composition analysis of the phases in the wide-gap region brazed with DF 4B filler metal powder.
• Table 4. 3 Results of AES composition analysis of the wide-gap region brazed with
• wt. % additive powder and 40 wt. % BNi-3 filler metal powder.
• Table 4. 4 Results of AES composition analysis of the phases in the wide-gap region brazed with 60 wt.% IN738 additive powder and 40 wt.% DF 4B filler metal powder
• [at.%].
• Table 5. 1 Results of EDS composition analysis of phase in the tensile-fractured surface of the region brazed with 60 wt. % additive powder and 40 wt.% filler metal powder.
• List of Figures
• Fig. 2.1. Various examples of crack in gas turbine blade during service; (a) erosion, (b) thermal fatigue, (c) creep and (d) oxidation.
• Fig. 2. 2. Relationship of inter atomic force and distance.
• Fig. 2. 3. Schematic of diffusion bonding.
• Fig. 2. 4. Schematic illustration of diffusion bonding process.
• Fig. 2. 5. Minimum bonding temperature of typical materials for diffusion bonding.
• Fig. 2. 6. Mechanism of brazing process [3].
• Fig. 2. 7. Equilibrium configuration of a liquid droplet on a solid substrate for conditions of (a) wetting, and (b) non-wetting.
• Fig. 2. 8. Mechanism of isothermal solidification during transient liquid phase bonding.
• Fig. 2. 9. Initial concentration profile of alloying elements of the brazing area; (a) element when Cb > Ci and (b) element when Cb < Ci.
• Fig. 2. 10. Weldability diagram for a number of Ni-based superalloy and related materials.
• Fig. 2. 11. Schematic of diffusion brazing repair process.
• Fig. 3. 1. Morphology of additive and filler metal powder used in this study; (a) IN
• additive powder, (b) BNi-3 filler metal powder and (c) DF 4B filler metal powder.
• Fig. 3. 2. Size distribution of additive and filler metal powder by laser particle analyzer used in this study; (a) IN738 additive powder, (b) BNi-3 filler metal and (c) DF 4B filler metal powder.
• Fig. 3. 3. Schematic of wide-gap brazing specimen used in this study.
• Fig. 3.4. Results of DSC curves of (a) IN738 additive powder, (b) BNi-3 filler metal and (c) DF 4B filler metal showing melting and solidification reactions.
• Fig. 3. 5. Schematic of wide-gap brazing process used in this study.
• Fig. 4. 1. Microstructures of the additive powder and filler metal powder used in this study; (a) IN738 additive powder, (b) BNi-3 filler metal and (c) DF 4B filler metal powder.
• Fig. 4. 2. Microstructure of the region brazed with only BNi-3 filler metal powder showing primary phase (P), binary eutectic (B) which consists of B1 and B2, ternary eutectic (T1, T2, and T3) and m which precipitates in Ni; (a) Low magnification (X300) and (b) high magnification (X1,000).
• Fig. 4. 3. Microstructures and results of AES element mapping of B and Si of the region brazed with only BNi-3 filler metal and auger spectrum for the primary phase(P), Binary eutectic (B1) and ternary eutectic (T3).
• Fig. 4. 4. SE image and auger spectrum of the phase marked as T1 in the region brazed only BNi-3 filler metal at 1200ºC for 1 hr.
• Fig. 4. 5. The partial liquidus projection of Ni-Si-B ternary equilibrium phase diagram [22].
• Fig. 4. 6. Experimental EBSD pattern obtained from the phase marked as p in Fig. 2 (b); (a) EBSD pattern of the phase marked as P, (b) pattern simulated by the parameters for Ni3B from an ICDD card, (c) EBSD pattern of the phase marked as T3, and (d) pattern simulated by the parameters for Ni3Si from an ICDD card.
• Fig. 4. 7. EBSD phase map of the Ni3B phase in the region brazed with only BNi-3 filler metal powder at 1200°C for 1 hr and followed by furnace cooling.
• Fig. 4. 8. Microstructure of the region brazed with BNi-3 filler metal; (a) tilted SE image, (b) image quality, (c) EBSD orientation index map and (d) color code for inverse pole figure.
• Fig. 4. 9. Schematic illustrations of solidification sequence of BNi-3 filler metal powder.
• Fig. 4. 10. Optical micrographs of solidified DF 4B filler metal powder showing the eutectic colony and eutectic structure; (a) low magnification, (b) high magnification of the region marked as a in Fig. 1 (a), and (c) high magnification of the region marked as b in Fig. 1 (a).
• Fig. 4. 11. X-ray diffraction pattern for the wide-gap region brazed with DF 4B filler metal powder at 1230ºC for 1hr showing the Ni and Ni3B peaks.
• Fig. 4. 12. Microstructure of the wide-gap region brazed with DF 4B filler metal powder at 1230ºC for 1hr; (a) low magnification (X180), (b) high magnification (X10,000) of the phase marked as C in (a), and (c) high magnification (X500) of the phase marked as E in (a).
• Fig. 4. 13. SE image and AES element mapping results of eutectic colony in the wide-gap region brazed with DF 4B filler metal powder; (a) SE image, (b) Ni, (c) Cr, and (d) B.
• Fig. 4. 14. SE image and auger spectrum of the eutectic colony marked as C in Fig. 4. 12(a) showing c1 contained Cr and B, and c2 contained Ni, Al and Cr, respectively.
• Fig. 4. 15. SE image and auger spectrum of the phase marked as E in Fig. 4. 12 (c).
• Fig. 4. 16. Experimental EBSD pattern obtained from the phase marked as c1, γ' and in Fig. 2 (b); (a) EBSD pattern of the phase marked as c1, (b) pattern simulated by the parameters for CrB from an ICDD card, (c) EBSD pattern of the phase marked as γ', (d) pattern simulated by the parameters for Ni3Al(γ') from an ICDD card, (e) EBSD pattern of the phase marked as e1 and (f) pattern simulated by the parameters for Ni3B from an ICDD card.
• Fig. 4. 17. EBSD phase map for the CrB and γ'(Ni3Al) phase in the region brazed with only DF 4B filler metal powder at 1230ºC for 1hr and followed by furnace cooling.
• Fig. 4. 18. Optical micrograph(X500) of water quenched DF 4B filler metal from 1230ºC to room temperature showing dendritic structure.
• Fig. 4. 19. Microstructure of DF 4B filler metal powder after quenching from 1230 ºC to room temperature showing the (a) Ni solid solution/CrB colony and (b) N
• solid solution-Ni3B eutectic structure.
• Fig. 4. 20. SE image and AES element mapping results of the wide-gap region brazed with 60 wt.% additive powder and 40 wt.% filler metal powder; (a) SEM image, (b) Ni, (c) Cr and (d) B.
• Fig. 4. 21. High magnification (X2,000) and AES element mapping of an area marked as c in Fig. 4. 20(a) which consist of c1 and c2; (a) SEM image, (b) Ni and (c) B.
• Fig. 4. 22. Binary alloy phase diagram of (a) Ni-B[33] and (b) Ni-Si[34].
• Fig. 4. 23. DSC curves of BNi-3 filler metal powder and powder mixture of IN738 additive and BNi-3 filler metal showing (a) melting and (b) solidification curve.
• Fig. 4. 24. Variations of the microstructure of the region brazed with different additive powder contents(wt.%); (a) BNi-3 only, (b) 50wt.% additive＋50 wt.% BNi-3, (c) 60wt.% additive＋40 wt.% BNi-3, and (d) 70wt.% additive＋30 wt.% BNi-3 filler metal powder.
• Fig. 4. 25. Melting and solidification behavior of powder mixture of 60 wt.% IN
• additive powder and 40 w.% BNi-3 filler metal by In-situ ESEM(environmental scanning electron microscope); (a) 14ºC, (b) 424 ºC, (c) 691 ºC, (d) 795 ºC, (e) 950ºC and (f) 951 ºC.
• Fig. 4. 26. Variations of IN738 additive powder size before and after heating cycle; (a) before heating and (b) after cooling in environmental scanning electron microscopy.
• Fig. 4. 27. Schematic illustration of solidification sequence of the wide-gap region brazed with additive powder and filler metal powder.
• Fig. 4. 28. SEM and AES element mapping of wide-gap region brazed with 60 wt.% IN
• additive powder and 40 wt.% DF 4B filler metal powder at 1200ºC for 1 hr; (a) SE image, (b) Ni, (c) Cr and (d) B.
• Fig. 4. 29. High magnification of the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt. % DF 4B filler metal powder at 1230ºC for 1hr; (a) wide-gap brazed region(X1,000) and the region marked as a in Fig. 4. 28 (a).
• Fig. 4. 30. Microstructure of the region brazed with different additive contents(wt.%) at 1230°C for 1 hr; (a)DF 4B only, (b) 50wt.% additive＋50 wt.% DF 4B, (c) 60wt.% additive＋40 wt.% DF 4B, and (d) 70wt.% additive＋30 wt.% DF 4B.
• Fig. 4. 31. Ni-Cr-B ternary equilibrium phase diagram.
• Fig. 4. 32. Microstructures of the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt. % BNi-3 filler metal powder for various brazing temperature; (a) 1100 ºC, (b) 1150 ºC and (c) 1200 ºC holding for 10 hrs and then furnace cooling.
• Fig. 4. 33. Change of area fraction (%) in the wide-gap brazed region with various brazing process temperature.
• Fig. 4. 34. Microstructures of the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt. % BNi-3 filler metal powder for various brazing time (hr) at 1200 ºC; (a) 5 hr, (b) 10 hr and (c) 30 hr and then furnace cooling.
• Fig. 4. 35. Variations of IN738 additive powder size(μm) with various brazing temperature
• Fig. 4. 36. Variations of IN738 additive powder size (μm) with various brazing times at 1200ºC.
• Fig. 4. 37. Change of the radius (μm) of the IN738 additive powder with various brazing temperature.
• Fig. 4. 38. Change of the diameter (d) of the IN738 additive powder with various brazing time at 1200 ºC.
• Fig. 4. 39. DSC curve of the powder mixture of 60 wt.% IN738 additive powder and
• wt.% BNi-3 filler metal powder with various holding time.
• Fig. 4. 40. Reaction layer thickness as a function of time for the IN738 additive powder with brazing process time.
• Fig. 4. 41. Arrhenius plots corresponding to the reaction layers for IN738 additive powder.
• Fig. 5. 1. A schematic of tensile test specimen used in this study.
• Fig. 5. 2. A schematic of creep test specimen used in this study.
• Fig. 5. 3. Change of fracture strength of the wide-gap region brazed with powder mixing ratio of additive (IN738) to filler metal powder (BNi-3) (wt.%) at room temperature and 900ºC.
• Fig. 5. 4. Change of fracture strength of the wide-gap region brazed with powder mixing ratio of additive (IN738) to filler metal powder (DF 4B) (wt.%) at room temperature and 900ºC.
• Fig. 5. 5. Fractographs of wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt. % BNi-3 filler metal powder showing cracks initiated at (a) (Cr, W)B, (b) Ni3B-Ni eutectic after tensile test at room temperature, (c) (Cr, W)B and (d) Ni3B-Ni eutectic after tensile test at 900 ºC.
• Fig. 5. 6. Crack propagation path near the fractured-surface after tensile test at room temperature brazed with 50 wt.% IN738 additive and 50 wt.% BNi-3 filler metal powder.
• Fig. 5. 7. Fractographs of the wide-gap region brazed with different additive powder
• contents (wt.%) after tensile test at room temperature showing the cracks
• initiated at the pore and (Cr, W)2B; (a) 70 wt.% additive, (b) 60 wt.% additive and (c) 50 wt.% additive.
• Fig. 5. 8. Fractographs and EDS composition analysis of wide-gap region brazed with
• wt. % IN738 additive powder and 40 wt. % DF 4B filler metal powder showing cracks initiated at (Cr, W)2B (a) room temperature and (b) 900ºC.
• Fig. 5. 9. Variations of area fraction (%) of (Cr, W)2B and pore in the wide-gap region brazed with powder mixing ratios of additive powder(IN738) to filler metal powder(DF 4B) (wt.%).
• Fig. 5. 10. Cracks around HRC indentation showing their propagation paths in the wide-gap region brazed with different powder mixing ratios of additive powder to filler metal powder; (a) IN738(70) : BNi-3(30) and (b) IN738(50) : BNi-3(50).
• Fig. 5. 11. Cracks around HRC indentation showing their propagation paths in the wide-gap region brazed with different powder mixing ratios of additive powder to filler metal powder; (a) IN738(70) : DF 4B(30) and (b) IN738(50) : DF 4B(50).
• Fig. 5. 12. Change of the fracture strength of the wide-gap region brazed with 60 wt.% IN738 additive powder and 40 wt. % BNi-3 filler metal powder for various brazing temperature tested at room temperature.
• Fig. 5. 13. Change of the room temperature fracture strength of the wide-gap region brazed with 60 wt.% IN738 additive powder and 40 wt. % BNi-3 filler metal powder for various brazing time (hr).
• Fig. 5. 14. Change of area fraction (%) in the wide-gap region brazed with 60 wt.% IN
• additive powder and 40 wt. % BNi-3 filler metal powder for various brazing process temperature.
• Fig. 5. 15. Change of area fraction (%) in the wide-gap wide-gap region brazed with
• wt.% IN738 additive powder and 40 wt. % BNi-3 filler metal powder for various brazing time (hr).
• Fig. 5. 16. Variations of element in the wide-gap region brazed with 60 wt.% IN738 additive powder and 40 wt. % BNi-3 filler metal powder at 1200ºC with various brazing times (hr).
• Fig. 5. 17. Variations of microvickers hardness value with various brazing times in the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt.% BNi-3 filler metal powder.
• Fig. 5. 18. Cracks in the wide-gap region brazed with powder mixture of 60 wt.% IN
• additive powder and 40 wt.% BNi-3 filler metal powder at 1200ºC for 30 hr. after tensile test at room temperature.
• Fig. 5. 19. Change of the fracture strength of the wide-gap region brazed with 60 wt.% IN738 additive powder and 40 wt. % DF 4B filler metal powder for various brazing temperature.
• Fig. 5. 20. Pore in the wide-gap region brazed with 60 wt.% IN738 additive powder and
• wt.% DF 4B filler metal powder at (a) 1170ºC and (b) 1200 ºC.
• Fig. 5. 21. Variations of the fracture strength of the the wide-gap region brazed with
• wt.% IN738 additive powder and 40 wt. % DF 4B filler metal powder for various brazing time (hr).
• Fig. 5. 22. Variations of microvickers hardness value with various brazing times in the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt.% DF 4B filler metal powder.
• Fig. 5. 23. Variations of elongation(%) with various brazing times in the wide-gap region brazed with 60 wt. % IN738 additive powder and 40 wt.% DF 4B filler metal powder.
• Fig. 5. 24. Cracks in the wide-gap region brazed with powder mixture of 60 wt.% IN
• additive powder and 40 wt.% DF 4B filler metal powder at 1200ºC for 30 hr. after tensile test at room temperature.
• Fig. 5. 25. Creep-rupture curve of wide-gap region brazed with various powder mixing ratios of additive powder to filler metal powder content (wt.%) at 950°C under 116 MPa; (a) IN738 additive and BNi-3 filler metal and (b) IN738 additive and DF 4B metal powder.
• Fig. 5. 26. Crack near the fractured specimen in the region brazed with various powder mixing ratios of additive powder to filler metal powders; (a) and (b) IN738(70) : BNi-3(30), and (c) and (d) IN738(50) : BNi-3(50).