Wide Gap TLP Bonding a Single-Crystal Superalloy: Evolution of the L/S Interface Morphology and Formation of the Isolate
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SINGLE-CRYSTAL turbine blades are widely used to increase the thermal efficiency of the advanced aeroengines in the past decades.[1–4] Traditionally, single crystals are directionally solidified in a vacuum directional furnace along [001] direction exhibiting the most excellent mechanical properties. Although it is now possible to obtain the desirable single-crystal superalloy turbine blades, problems still persist such as the formation stray grains, low-angle grain boundaries, freckles during directional solidification when the geometry of the turbine blade is complex, or high volume of the high refractory elements are added to the alloys. These unexpected defects undoubtedly decrease the yields, which means high costs. Nowadays, casting and bonding the separate single-crystal superalloy segments together using transient liquid phase (TLP) bonding has become an effective method to produce turbine blades with low cost and high yield.[5] TLP bonding was developed by Duvall[6] to bond heat-resistant materials and has been applied to bond TiAl, NiAl, and superalloys by Gale et al.[6–9] These materials usually contain many c¢ forming elements, such as Al and Ti, which could lead to the formation of cracks during fusion welding. When TLP bonding is employed, such cracking problems can be avoided. There is also no need of high-pressure during TLP N.C. SHENG, Doctoral Candidate, J.D. LIU, Associate Professor, T. JIN and X.F. SUN, Professors, and Z.Q. HU, Professor and Academician, are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, 110016 Shenyang, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 31, 2012. Article published online November 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
bonding that is exerted, 50MPa in bonding TiAl alloys,[10] during solid diffusion bonding. The preparation of the faying surfaces is relatively simple as well; the surface is polished to 1000# before bonding in our experiment. Usually, TLP bonding involves the system that is eutectic or peritectic where there is a lower liquidus during alloying[10]; B, which ideally forms eutectic with Ni, is usually used as a melting point depressant element (MPD) in TLP bonding Ni-base superalloys. When bonding single-crystal superalloys, a sandwiched assembly is prepared by inserting a commercial Ni-Cr-B interlayer between two pieces of base metal, and then the assembly is TLP bonded in a vacuum atmosphere heating furnace. Conventionally, the TLP bonding process is separated into four sequential stages: interlayer melting, dissolution of the substrate and homogenization of the interlayer liquid, isothermal solidification, and the post–heat-treatment process.[11,12] The modeling of the TLP bonding has been reviewed by Zhou et al.,[12] and the traditional models predict that the four stages of the TLP bonding happen successively. But there are still some deviations from the classic models when different systems are bonded. Different from the previous views that the isothermal solidification
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