Braze Alloy Development for Fast Epitaxial High-Temperature Brazing of Single-Crystalline Nickel-Based Superalloys

Abstract

For the repair of single-crystalline turbine components, fabricated from nickel-based superalloys, a new high-temperature brazing technology has been developed. Cracks in single-crystalline parts can be repaired by reproducing the single-crystalline microstructure over the complete gap width within very short brazing times. Nickel-manganese-based alloys were identified as systems that provide very high, epitaxial solidification rates. In contrast to commonly used braze alloys, such as nickel-boron or nickel-silicon systems, the process is not completely diffusion controlled but works with consolute systems. For brazing experiments 300-lm-wide parallel gaps as well as V-shaped gaps with a maximum width of 250 lm were used. A complete epitaxial solidification, that is, the absence of large-angle grain boundaries, could be achieved within brazing times, being up to 100 times shorter compared to commonly used transient-liquid-phase bonding technologies. To quantify the misorientation relative to the base material and the composition within and near the filled gaps, the results of the brazing experiments were visualized by means of light microscopy and scanning electron microscopy (SEM). Furthermore, electron backscatter diffraction (EBSD) analyses and energy dispersive X-ray (EDX) analyses were conducted.

Development of a Novel Ni-Based Multi-principal Element Alloy Filler Metal, Using an Alternative Melting Point Depressant

“…It is clear from binary Ni phase diagrams and the literature [14,15] that, of numerous candidate MPD elements (transition and post-transition metals, metalloids, and refractories), most would require excessive atomic percentages (compared to that of B or Si) to achieve a liquidus comparable to current B/Si-containing filler metals; in many cases 30 at. pct or more of addition would be needed.…”

“…Laux et al (2008) trialed a range of Ni-Mn (36.7 and 58.4 wt pct Mn) and Ni-Mn-Si (20wt pct Mn-2wt pct Si, 20wt pct Mn-3wt pct Si, and 25wt pct Mn-2wt pct Si) alloys for the wide-gap brazing of a Ni-7.5Co-7.0Cr-1.5-Mo-5.0W-6.5Ta-6.2Al-3.0Re (in wt pct) superalloy. [15] For constant brazing hold times of 30 minutes, brazing temperatures ranged from 1040°C for the highest Mn content (Ni-58.4Mn in wt pct) to 1260°C for the lowest Mn content (Ni-20Mn-2Si in wt pct). Evidently, such alloys often require substantial amounts of the proposed MPD, yet the liquidus temperature is often still considerably higher than for most current nickel-based brazing filler metals (which would typically be between 1000°C…”

Brazing filler metals

“…Repair of superalloy turbine blade components using filler metals with manganese additions as the melting point suppressant has been investigated [83,84]; the solubility of manganese within nickel allows for faster processing as the melting point suppressant does not have to be diffused away from the braze gap to allow solidification as a single phase, which must be done with boron, leading to longer processing times [84]. Studies using germanium instead of manganese found similar results with single phase solidification giving joints with a UTS 90% of that of the parent material when tested at 980°C [79].…”

Overview of transient liquid phase and partial transient liquid phase bonding

“…• thin foil (rolled sheet) [2-4, 7, 16-62] • amorphous foil (melt-spun) [7, • fine powders (with or without binding agent) [5,7,[15][16][17][85][86][87][88][89][90][91][92][93][94][95][96][97][98] • powder compact (made by sintering, cold isostatic pressing, etc.) [57,[99][100][101] • brazing paste [9,17,[102][103][104] • a physical vapor deposition process such as sputtering [7, 18-21, 62-64, 105] • electroplating [10, 17-19, 30, 88, 106-112] • evaporating an element out of the substrate material to create a ''glazed'' surface [113].…”