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Characterization of Inconel 625 Superalloy Braze Joints using 4 Different Brazes Ryan Kraft, and Rajiv Asthana, University of Wisconsin-Stout Our research investigates how the process of brazing modifies the original braze composition and braze metallurgy in the vicinity of joined interfaces and how braze alloys modulate the structure, composition and metallurgical adhesion to substrates. Developing and demonstrating methods to braze high-temperature alloys are the practical goals of our broader research effort. Objective and Significance Demonstrate vacuum brazing of Inconel 625 using four brazes of different compositions. Characterize joint microstructure using optical microscopy and scanning electron microscopy, and hardness distribution using the Knoop test. Heat treat vacuum-brazed joints in air at two-thirds of the absolute melting point of braze to simulate service conditions and characterize microstructure and hardness. Heat treat unbonded braze foils and compare through-thickness braze microstructure, composition, and hardness with microstructure, composition, and hardness of braze foils in joints. Analyze metallurgical changes that occur in brazing and subsequent heat treatment, and compare and contrast effectiveness of different braze compositions to join Inconel 625. Research Approach Inconel 625 (melting point: 1350C) is a general purpose nickel-chromium- molybdenum alloy in the family of Inconel alloys that were developed for elevated temperature use. Inconel 625 has good corrosion and oxidation resistance which makes it suitable for harsh environment applications. Inconel alloys are used in a wide variety of applications, from gas turbine blades and heat exchangers to NASCAR exhaust systems and X-15 rocket planes. In applications, Inconel must be joined to itself or to other alloys to make complex assemblies. Background 1.Cut Inconel on high-speed precision saw, lay braze foil at mating surface, apply a load (0.30-0.40 N) during brazing. 2.Heat under vacuum (~10 -6 torr) to 15-20 C above braze T L. After 5 (or 30) min. soak, slowly cool. 3.Heat treat joints in air at two-thirds of absolute braze melting temperature for 45 min. completed at UW-Stout. 4.Mount joints in epoxy, grind, polish and examine joint microstructure using optical microscopy (OM) and scanning electron microscopy. 5.Measure Knoop hardness (Buehler Micromet 2001 Unit, 100 g load, 10 s). Experimental Procedure Composition and Properties of Brazes [a] Morgan Advanced Ceramics, [b] Honeywell, Inc., T L : liquidus temperature, T S : solidus temperature, E: Youngs modulus, YS: yield strength, CTE: coefficient of thermal expansion, K: thermal conductivity, El: percent elongation. BrazeComp., %T L, KT S, KE, GPa YS, MPa CTE, x10 - 6 K -1 K, W/m.K%El MBF-30 [b] Ni-4.61Si-2.8B- 0.02Fe-0.02Co 12571327-- MBF-20 [b] Ni-6.48Cr-3.13Fe- 4.38Si-3.13B 12421297-- TiCuSil [a] 68.8Ag-26.7Cu- 4.5Ti 117310538529218.521928 Cusil- ABA [a] 63Ag-35.3Cu- 1.75Ti 108810538327118.518042 Inconel Braze Inconel Binary Phase Diagrams of Major Braze Constituents Cu-Ag System (base alloy for Cusil-ABA and Ticusil) Ni-Cr System (base alloy for MBF-20) Sample IDSubstratesBraze alloy Braze Temperature(C) Time(min)Weight(g) R2Inconel/inconelMBF-201045540 R4Inconel/inconelMBF-301045540 R9Inconel/inconelCusil-ABA835540 R19Inconel/inconelTicusil9205110 Inconel/Inconel Joint with MBF-20 Braze (R4, 1045 C, 5 min) Joint Microstructure vs. Knoop Hardness Profile The microstructure of the MBF-20/Inconel braze joint contained multiple layers within the reaction layer. The hardness of the inner brazed region was very high, while the other layers were softer than the surrounding Inconel substrate. 100m KHN = 281 KHN = 321 KHN = 1197 KHN = 501 Results and Data Inconel/Inconel Joint with Ticusil Braze (R19, 920 C, 5 min) Inconel/Inconel Joint with Cusil-ABA Braze (R9, 835 C, 5 min) Joint Microstructure showing eutectic structure Knoop Hardness profile across braze joint Reaction Layer 100 m KHN = 335 KHN = 87 Continuing Research -Complete microstructure characterization using scanning electron microscopy and elemental analysis of brazed joints using energy dispersive spectroscopy -Complete polishing, microscopy, elemental analysis and microhardness of unbonded braze foils (heat treated and non-heat treated). Compare and contrast with the behavior of heat treated and non heat treated braze foils within joints. -Measure shear strength of joint. Acknowledgement NASA Glenn Research Center, Ceramics Branch, for the materials used in the study and for use of high vacuum brazing furnace. The Cusil ABA/Inconel reaction layer was very narrow. Although the region is visible no hardness data could be obtained. Sample and Identification Inconel/Inconel Joint with MBF-30 Braze (R2, 1045 C, 5 min) Joint Microstructure Knoop Hardness profile across braze joint It appears that the microstructure of the Ticusil/Inconel interface contains a reaction layer extending from the brazed region into the Inconel substrate along the Inconels grain boundaries. The reaction layer was much harder than the braze region or Inconel substrate. l 100 m KHN = 660 KHN = 335 KHN = 121 Reaction Layer Joint Microstructure vs. Knoop Hardness Profile The microstructure of the Inconel MBF-20 braze joint show many layers within the reaction layer, similar to that of MFB-30. The hardness of the braze was lower than that of the substrates. KHN = 415 KHN = 252 KHN = 278 100 m