Diffusion Brazing of Single Crystalline Nickel Base Superalloys Using Boron Free Nickel Base Braze Alloys

Abstract

Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].

The Ni-rich part of the Al–Ge–Ni phase diagram

“…The manufacturing of high temperature alloy components based on Ni-aluminides requires sophisticated joining techniques. Diffusion brazing, also called transient liquid phase joining (TLP) [1] , is one possible technique which already was applied successfully on Ni-based superalloys [2] , [3] . Possible new brazing materials should show considerable lower melting points than the substrates and thus require the presence of melting point depressant elements like e.g.…”

Phase equilibria and structural investigations in the Ni-poor part of the system Al–Ge–Ni

“…In this case diffusion brazing, also called transient liquid phase (TLP) joining [4], was successfully applied on nickel-base superalloys [5], [6], [7]. The respective brazing materials ideally provide definitely lower melting points than the substrates and possess similarities with respect to the chemical composition of the substrate.…”

Overview of transient liquid phase and partial transient liquid phase bonding

“…In general, the relationship is parabolic, yielding a minimum isothermal solidification time at a given intermediate temperature (between the melting points of the interlayer and substrate materials) [9,18,43,50,78,79,159,162]. And yet, in J Mater Sci (2011) 46:5305-5323 5309 some cases the variables of the system yield either (1) a monotonically increasing time, in which case the optimal bonding temperature is just above the interlayer's melting point, [6,9,43,114,158,162] or (2) a monotonically decreasing time, in which case the optimal bonding temperature is as high as the substrate material allows [4,104,114,135,141]. The behavior of this trend is highly system dependent and results in part from the interplay of the diffusion rate and the phase diagram of the system [19,43,50].…”

“…The bonding process is usually confined in a vacuum [3-5, 7, 12, 14-17, 20-30, 35, 38-40, 46-58, 61-63, 65, 66, 68-72, 76-83, 86-88, 93-100, 103-105, 108, 110, 112-115, 117, 119, 122-127, 130, 132-138, 140-157], although an inert atmosphere, such as argon, can be used [6,11,14,32,33,43,45,60,67,74,75,90,111,121,134,158,159]. On rare occasions, TLP bonding is performed under a different atmosphere, such as nitrogen [44], hydrogen [160], nitrogen and hydrogen [18], or open air [129].…”

“…• significant melt-back of a material with a specifically designed microstructure [7,62,141] • the formation of a thick layer of intermetallic compounds in the bond that tend to lower its strength and ductility [9,36,87,110,141,146,157,158,167] • the segregation of particles in metal matrix composites at the joint centerline, leaving a distinct weakened band in the joint microstructure [19,33,39,42,54,105].…”