Wide Gap Brazing of Inconel 738LC Nickel-Based Superalloy: Metallurgical and Mechanical Characteristics
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DUCTION
DURING gas turbine operations, the hot section components, including the stationary and rotary parts are subjected to thermal and mechanical fatigue, oxidation, creep, hot corrosion, and foreign objects damage.[1–3] Nickel-based superalloys are used at high operating temperatures of the turbine, due to their corrosion resistance and high strength. In modern gas turbine engines, the use of higher gas pressures and operating temperatures are required to achieve higher efficiency and output.[4] Since Ni-based superalloys contain a large amount of expensive refractory elements, as well as are manufactured by sophisticated casting processes, replacement of parts increases overall operating cost. Hence, the repair of damaged parts, instead of full replacement, could be very cost-effective.[5–7] H. ALINAGHIAN, A. FARZADI, and P. MARASHI are with the Department of Materials and Metallurgical Engineering, Amirkabir University of Technology (Polytechnic of Tehran), No. 350, Hafez Ave, PO Box: 15875-4413, 15916-34311, Tehran, Iran. Contact e-mail: [email protected] M. POURANVARI is with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave, 14588-89694, Tehran, Iran Manuscript submitted May 4, 2020; accepted September 10, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Inconel 738LC is one of the most practical casting nickel-based superalloys, which is used in landing and aircraft turbine blades and nozzle vanes. It is strengthened by both solid solution and precipitation-hardening mechanisms. Various techniques, such as fusion welding processes, conventional and diffusion brazing, have been employed to repair the hot section components. The high content of Al and Ti (Al + Ti > 6 wt. pct) makes Inconel 738LC very difficult to repair using fusion welding. This is due to the higher susceptibility to solidification cracking in both fusion zone and heat-affected zone (HAZ) during welding or post-weld heat treatment.[8] Thus, brazing is an attractive alternative repairing process for these components.[9] A large gap width can lead to the formation of brittle eutectic boride or silicide chains at the centerline of the brazing zone (BZ). These brittle phases severely deteriorate the mechanical integrity of the joint.[10–13] Thus, a gap width or clearance below 200 lm is typically essential. The gap width of more than 200 lm is classified into the wide gap brazing (WGB) regime. Huang and Miglietti[14] described the WGB process and its mechanism. In the WGB process, a mixture of an additive filler alloy or high melting particles (HMPs) and a braze alloy or low melting particles (LMPs) are commonly used. Boron (B) and silicon (Si) can be used as melting point depressants (MPDs) for the
nickel-based braze alloys to reduce the liquidus temperature of LMPs. HMPs can act as an additional diffusion sink for the MPDs. They also provide the capillary action for the braze alloy. Moreover, HMPs enhance the mechanical properties by impeding the formation of brittle boride or silicide. The mechanical p
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