Suppression of Boride Formation in Transient Liquid Phase Bonding of Pairings of Parent Superalloy Materials with Differ
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INTRODUCTION
DUE to their exceptional properties at very high temperatures,[1,2] Ni-based superalloys are often used for blades and vanes in aeroengines and industrial gas turbines. The interest in joining these alloys has increased through the last years, because it shows the potential to simplify manufacturing of blades and vanes of modern gas turbines. With increasing size of the blades, the probability of casting defects is rising. By fabricating the blades out of several smaller castings, which are joined together afterward, this risk for casting defects could be decreased. Furthermore, due to this modular approach, it would be possible to use different materials in one blade depending on the level of stress applied or corrosion resistance needed in the various regions of one blade. An established repair technique for high temperature components is transient liquid phase (TLP) bonding. Its application is shown in several studies.[3–8] TLP bonding for Ni-based superalloys was first reported in the 1970’s SUSANNE STEUER, PostDoc, is with the Department of Materials Science and Engineering, Institute of Science and Technology of Metals, University of Erlangen-Nuremberg, Martensstr. 5, 91058 Erlangen, Germany and also with the Institut Pprime, CNRS ENSMA - Universite´ de Poitiers, Poitiers, France. Contact e-mail: [email protected] ROBERT F. SINGER, Professor, is with the Department of Materials Science and Engineering, Institute of Science and Technology of Metals, University of ErlangenNuremberg. Manuscript submitted November 15, 2013. Article published online April 16, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
by Duvall et al.[9] This method is based on the diffusion of the melting point depressant (MPD) out of the braze alloy in the parent material resulting in isothermal solidification of the molten filler alloy by increasing the melting point. The crystal orientation of the solidified joint will equal the parent material, because the crystal will grow epitaxially at the solid–liquid interface, and the nucleation of new grains in front of the solidification front is suppressed. As MPD boron or silicon is common, because they reduce the melting point by a suitable amount and diffuse rapidly in Ni-based superalloys. The disadvantage of boron as MPD is its very low solubility in nickel and Ni-based superalloys. Because of that brittle borides precipitate in the parent alloy in a band-like zone parallel to the joint during the brazing process.[6,10–16] In the literature, several studies about the precipitation as a function of the brazing temperature and time can be found.[6,11,17] At low and medium brazing temperatures, all authors observed either globular or plate-like precipitates or a mixture of both forming in the parent materials. Their relative amount depended on the alloy and the temperature. At very high brazing temperatures (>1493 K (1220 C)), a different behavior was reported: While Schnell[6] showed that boride precipitation in the single-crystal superalloy CMSX-4 can be totally avoided,
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