Influence of Solid-State Diffusion during Equilibration on Microstructure and Fatigue Life of Superalloy Wide-Gap Brazem
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ipitation-strengthened cast nickel-based superalloys are used for manufacturing hot-section components of aero and land-based gas turbine engines, because of their remarkable high temperature mechanical strengths and hot-corrosion resistance. The demand for higher efficiencies, which require turbine engines to operate at higher temperatures, has led to increased degradation of engine components through higher levels of creep, fatigue, and oxidation. It is generally more economically attractive to repair damaged parts instead of going for a complete replacement. Traditional repair techniques, such as welding, are commonly used in the repair of many superalloy components. However, precipitationstrengthened cast nickel-based superalloys, like Inconel 738 (IN 738), are generally difficult to weld because of their high susceptibility to weld cracking.[1–6] Wide-gap brazing is an alternate technique for joining and repairing gas turbine engine components made of difficult-to-weld superalloys.[7–11] The technique often involves the use of composite powder mixture as the interlayer material. The powder mixture consists of regular brazing filler alloy powder that contains melting point-depressant (MPD) solute and an additive powder L.O. OSOBA, Lecturer, formerly with Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada, is now with Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Lagos State, Nigeria. O.A. OJO, Professor, is with the Department of Mechanical and Manufacturing Engineering, University of Manitoba. Contact e-mail: [email protected] Manuscript submitted November 25, 2012. Article published online July 9, 2013 4020—VOLUME 44A, SEPTEMBER 2013
alloy that is essentially free of the MPD solute, usually the base-alloy powder. The use of the powder mixture reduces undesirable liquid-phase erosion of the base material and also enables desirable enrichment of the joint region with the base-material alloying element for enhanced joint properties. During wide-gap brazing, the brazing powder particles (BPPs), which normally have a lower melting temperature than that of the additive powder particles (APPs), melt completely, which in turn produces solutal melting of the APPs. It is generally assumed that APPs only undergo partial melting during wide-gap brazing. The main thrust for the solutal melting of the APPs into the surrounding molten brazing filler alloy is to reduce the concentration of the MPD solute in the liquid to the equilibrium liquidus value at the brazing temperature. In analytic brazing models, this equilibration process is often assumed to exclusively involve the addition of solvent element from the solid powder particles to the surrounding liquid without concomitant transfer of the MPD solute from the liquid into the particles by solidstate diffusion within the particles.[12] This is due to the difficulty in modeling simultaneous solid-state solutetransport within APPs and their melting by the surrounding liquid. Nev
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