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WELDING RESEARCH OCTOBER 2002-S202 lidification process (Refs. 9–11). While in service, the brazed joint can be exposed

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Content	WELDING RESEARCH OCTOBER 2002-S202 lidification process (Refs. 9–11). While in service, the brazed joint can be exposed to elevated temperatures over a long operat- ing time, causing potential phase changes in the joint and, in particular, at the inter- face between the base and filler metals. The properties of the interface reaction products can significantly affect joint per- formance. The long-term aging of brazed joints has not been extensively studied. Shimoo et al., assessed the kinetics of solid-state reactions between Si3N4and Ni (Ref. 12). Most aging studies have examined the in- terface microstructure that develops ini- tially between the molten filler metal and the base metal during the brazing process (Refs. 13–15). Several publications de- scribe high-temperature, interface reac- tions due to diffusion bonding processes (Refs. 16–18). Although the latter investi- gations provided important insights into potential solid-state reactions, the starting microstructures and aging conditions did not adequately represent those of brazed joints. The objective of the present inves- tigation was to study filler metal/base metal interactions that can occur in the brazements of heat engines. The investi- gation examined brazed joints made be- tween a low-temperature Ag-Cu-Ti filler metal and Thermo-Span™ or Inconel™ 718 substrate alloys; data from an analo- gous investigation of a high-melting-tem- perature Au-Ni-Ti braze alloy and Thermo-Span™, as well as an AISI Type 347 stainless steel, will be reported in Part 2. Standard analytical tools were used to characterize the brazed joint microstruc- tures. The four-point bend test was used to determine the effect of microstructure on joint mechanical performance. Experimental Procedures Materials: Base Metals The two base metals selected for this study were the precipitation-hardened Thermo-Span™ (24.5Ni-29.0Co-5.5Cr- 4.8Nb-(Si, Ti, Al)-bal. Fe wt-%) and In- conel™ 718 (55Ni-21Cr-5.5 (Nb+Ti)- 3.3Mo-bal. Fe alloys (Refs. 19, 20). The Thermo-Span™ was received in the solu- tion-annealed condition [1093°C (2000°F), 1 h, air cool]; the condition of the as-received Inconel™ 718 stock was not documented. In order to assure con- sistent material properties, the Inconel™ 718 alloy was subjected to a solution an- nealing treatment. The Thermo-Span™ and Inconel™ 718 materials were then ex- posed to an aging (precipitation-anneal) heat treatment. The condition of each base metal was assessed using Rockwell C (HRC) hardness measurements. Six mea- surements were performed on three sam- ple blanks. The heat treatment schedules and hardness data follow. Thermo-Span™ (As-received HRC= 23±2, solution annealed). Solution an- nealing (at the mill): hold at 1093°C (2000°F) for 1 h; air cooling. Precipitation annealing: holding at 718°C (1324°F) for 8 h; furnace cooling at 0.015°C/s (0.027°F/s) to 621°C (1150°F); hold at 621°C (1150°F) for 8 h; air cooling. Postheat treatment HRC= 39±1. Inconel™ 718 (as-received HRC= 22.5±0.5, unknown condition). Solution annealing: hold at 954°to 1010°C (1750°to 1850°F) for 0.5 to 1 h, air cooling. Precip- itation annealing: hold at 718°C (1324°F) for 8 h, furnace cooling to 621°C (1150°F), holding for 10 h, air cooling. Postheat treatment HRC= 42±2. All brazing experiments were per- formed on base alloy surfaces that had been ground to a nominal √32 finish as de- termined by profilometer measurements. Four profilometer traces, two in one di- rection and the other two in a perpendic- ular direction, were made over a distance of 6 mm on each of duplicate samples. The arithmetic average roughness, or RA numbers (mean and ±one standard devi- ation) are shown in Table 1. The solution and aging heat treatments caused both substrate materials to have a lower surface roughness after grinding as compared to that of the as-received material. Materials: Brazing Filler Metals The brazing filler metal Cusil™ ABA (63.3Ag-35.1Cu-1.6Ti) was evaluated in this study (Ref. 21). The filler metal was in the form of 0.051-mm-thick (0.002-in.) strip. The Ag-Cu-Ti alloy has a nominal melting range of 780°to 815°C (1436°to 1499°F). The composition of the material batch used in this study was verified by atomic emission spectroscopy (AES); the chemical composition was 61.8±4.2 Cu, 35.1±0.7 Ag, and 1.7 Ti (wt-%). The chemical analysis was performed in tripli- cate; the error terms represent a 95% confidence interval. The onset (solidus) temperature of the Ag-Cu-Ti alloy was Fig. 1 — Configurations of the test specimen. Fig. 2 — Schematic diagram of the sessile drop test specimen used to assess filler metal wetting and spreading. The quantitative metric is the contact angle, θ. The symbol r is the effective spread radius assuming a circular footprint to the area of spread, and h is the height of the filler metal mound. Table 1 — Arithmetic Average Roughness (RA) of the Base Metal Specimens with a √32 Finish Base MetalConditionRA (µm) Thermo-Span™Solution treated0.19 ±0.02 Thermo-Span™Solution treated and aged0.09 ±0.02 Inconel™ 718As-received0.12 ±0.03 Inconel™ 718Solution treated and aged0.034 ±0.005 Note: Surface Profilometry Data (Med. Scan Speed) Fig. 3 — Time and temperature parameters for the Ag-Cu-Ti brazing process.
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Following Datasheets	10-2002-XIE-s (8 pages) 10-2003-COLLINS-s (8 pages) 10-2003-GOULD-s (5 pages) 10-2003-MENDEZ-s (11 pages) 10-2003-SOLOMON-s-1 (10 pages) 10-2003-VIANCO-s (10 pages) 10-2004-KIMAPONG-s (6 pages) 10-2004-MURUGANANTH-s (10 pages) 10-2004-WANG-s (5 pages) 10-2005-JENKINS-s (8 pages)
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