New Boron and Silicon Free Single Crystal-Diffusion Brazing Alloys

Abstract

New diffusion brazing alloys for single crystalline component repair processes were developed and tested. Germanium was used as the melting point depressing element in these binary brazing alloys with germanium contents between 20 to 23 wt.%. Microstructural analysis has shown that the formation of a single crystalline joint was achieved after extended brazing cycles with these brazing alloys. No secondary phases other than the desired γ´ precipitates were detected within the brazing zone. This result was shown for two parent materials, PWA 1483 and René N5, a first and a second generation superalloy, respectively. The solidification mechanisms and kinetics were examined and show rather distinct deviations from Transient Liquid Phase Bonding theory for binary systems due to the multicomponent diffusion in the present system. Mechanical testing was performed at room temperature via nanoindentation and at elevated temperatures by hot tensile tests. The nanoindentation results show minor differences in hardness and elastic modulus between brazing joints and parent material. Ultimate tensile strengths of more than 90 % compared to the parent material were obtained in tensile testing at 980 °C.

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].…”

“…Specifically, this process was patented by Paulonis et al in 1971 [1] to overcome deficiencies of then current bonding techniques in joining Ni-based superalloys [2][3][4][5][6]. TLP bonding's main advantage is that resulting bonds have a higher melting point than the bonding temperature.…”

“…A pressure is usually applied to the bonding assembly to keep the substrates aligned and to promote bonding; specific pressures are categorized in Table 1 Occasionally, the substrates are held a fixed distance apart rather than applying a pressure, but this tends to cause porosity in the final bond [6,19,43].…”

“…Bond setup usually consists of placing a thin interlayer between the substrates, but the interlayer material is occasionally placed outside the joint to flow in by capillarity [6,7,[9][10][11][12][13][14][15] as is done in many brazing processes. The interlayer material can be in many different formats:…”