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| Content | WELDING RESEARCH -S201WELDING JOURNAL ABSTRACT. The effects of aging were ex- amined for brazed joints made with 63.3Ag-35.1Cu-1.6Ti filler metal and Thermo-Span™ (24.5Ni-29.0Co-5.5Cr- 4.8Nb-(Si, Ti, Al)-bal. Fe) and Inconel™ 718 (55Ni-21Cr-5.5 (Nb+Ti)-3.3Mo-bal. Fe) base metals. In a companion study, the aging of 81Au-17.5Ni-1.5Ti brazed joints made of Thermo-Span™ and AISI Type 347 stainless steel (18Cr-11Ni-2Mn-1Si- (Ta, Nb)-0.08C-bal. Fe) was examined, the results of which will be presented in Part 2. Excellent wetting and spreading was shown by the Ag-Cu-Ti filler metal on both substrates, as determined by contact angle measurements. The Thermo-Span™/Ag- Cu-Ti couple interface was comprised of two sublayers having the same composi- tion, 90[(Fe, Ni, Co, Cu)2(Nb, Ti, Si, Cr)] 10 Ag, which were separated by a Ag-rich layer. Aging reduced the interface struc- ture to a single phase having the composi- tion (Fe, Ni, Co, Cu)2(Nb, Ti, Si, Cr). The interface reaction zone in the as-fabri- cated Inconel™ 718/Ag-Cu-Ti couples contained two sublayers having the composition (Fe,Ni,Cu)3(Ti,Cr,Nb,Mo)2and (Fe,Ni,Cu)2(Ti,Cr,Nb,Mo). Solid- state aging caused the overall reaction layer to thicken and the composition of the second sublayer to change to (Fe,Ni,Cu)7(Ti,Cr,Nb,Mo)3. The bend bar fracture strengths measured for Thermo-Span™/Ag-Cu-Ti and Inconel™ 718/Ag-Cu-Ti couples were not signifi- cantly affected by the solid-state aging processes. Introduction Advanced heat engines are being de- veloped that operate at higher combustion temperatures for improved fuel efficiency. Ceramic components and, in particular, engineered ceramics such as silicon ni- tride (Si3N4) and partially stabilized zirco- nia (PSZ) can provide the necessary phys- ical and mechanical properties that will allow for higher operating temperatures in future power plants (Refs. 1–3). However, the construction of mono- lithic engine structures from engineered ceramics is not presently feasible. Metal and alloy parts are still required for many components, not only because they have suitable physical and mechanical proper- ties but also because they support cost- effective manufacturing of the engine unit. Consequently, the use of metal and ce- ramic components in advanced heat en- gines will require the development of suit- able joining techniques for all three categories: 1) metal-to-metal, 2) ceramic- to-ceramic, and 3) metal-to-ceramic. The foremost challenge of making metal-to-ceramic joints rests with accom- modating the thermal expansion mis- match between metal substrate materials and the engineered ceramics, and mini- mizing the resulting residual stresses. Ce- ramics and glasses tend to have relatively low thermal expansion coefficients. For example, Al2O3and the engineered ce- ramic Si3N4have thermal expansion coef- ficients of 7–9 ppm/°C (4–5 ppm/°F) and 3.2–3.5 ppm/°C (1.8–1.9 ppm/°F), respec- tively (Refs. 4–6). Several metal alloys have been developed to have a reduced thermal expansion coefficient; the tradi- tional trade names and compositions in- clude Kovar™ (Fe-29Ni-17Co), Invar 36™ (Fe-36Ni), and Alloy 42™ (Fe-40.5Ni) having coefficients of 6.2 ppm/°C (3.4 ppm/°F) averaged over 25°to 500°C (77° to 932°F), 7.2 ppm/°C (3.4 ppm/°F) for 25° to 371°C (77°to 700°F), and 8.1 ppm/°C (4.5 ppm/°F) for 25°to 500°C (77°to 932°F), respectively (Refs. 7, 8). The mechanical performance of the brazed joint also depends strongly on its microstructure. The joint has three major components: 1) the filler metal region, 2) the interface(s) between the filler metal and the base metal, and 3) the base metal. The postprocess microstructure of the joint is a function of the filler metal and base metal compositions, as well as the so- SUPPLEMENT TO THE WELDING JOURNAL,OCTOBER 2002 Sponsored by the American Welding Society and the Welding Research Council Aging of Brazed Joints — Interface Reactions in Base Metal/Filler Metal Couples — Part 1: Low-Temperature Ag-Cu-Ti Filler Metal A study of filler metal/base metal interactions was conducted on brazements of alloys used in advanced heat engines BY P. T. VIANCO, F. M. HOSKING, J. J. STEPHENS, C. A. WALKER, M. K. NEILSEN, S. J. GLASS, AND S. L. MONROE KEY WORDS Long-Term Aging Brazing Brazement Ceramic Heat Engines Inconel™ Thermo-Span™ P. T. VIANCO (ptvianc@sandia.gov), F. M. HOSKING, J. J. STEPHENS, C. A. WALKER, M. K. NEILSEN, S. J. GLASS, and S. L. MON- ROE are with Sandia National Laboratories, Al- buquerque, N.Mex. |
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