Brazing is a well understood subject. After all there is evidence of brazing in the region/country of Thrace (now modern-day Bulgaria) between 4600--4500 B.C. (Before Christ) Mesopotamia, 3000--2500 B.C., then Greece, 2500--2200 B.C., and Ancient Egypt.
Diffusion Brazing (DB), also referred to sometimes as Transient Liquid Phase (TLP) Bonding is a metal joining process that produces joints with high mechanical integrity, such as tensile, creep strength and fatigue strength; as a result, the joint is of exceptional quality. The joint is created through the process of isothermal solidification of a melting point depressant (MPD) rich interlayer, for Ni- and Co-based superalloys, this MPD is Boron (B). The advantage of using diffusion brazing over conventional welding processes is that the microstructure and properties in the joint are very similar to those of the base metal. As a result, the DB process for the last 4 decades, finds good application is the gas turbine industry. It does not matter if the gas turbine industry is for aviation-flight turbines, land-based turbines for power generation, aeroderivative turbines or steam turbine industry, the DB process can be utilized.
Components such as vanes/nozzles, combustion chambers, liners and transition pieces degrade in service as a result of thermal fatigue cracking, craze cracking, oxidation, erosion, corrosion and foreign object damage (FOD). These components are very expensive and hence the industry demands a repair for them, in lieu of new part purchase. Welding can and still is utilized, but the weld cracking, rework, distortion and long cycle times for implementing the repair makes the DB process more popular and attractive. These components are often cast from either Co-based solid solution superalloys such as FSX-414, or Ni-based gamma prime precipitation strengthened superalloys such as IN738 and MarM247. The nominal composition of FSX-414 is Co-29.5Cr-10.5Ni-7W-2Fe (max)-0.25C-0.012B. IN738 and MarM247 have nominal compositions of Ni-0.001B-0.17C-8.5Co-16Cr-1.7Mo-3.4Al-2.6W-1.7Ta-2Nb-3.4Ti-0.1Zr and Ni-0.001B-0.15C-10Co-8.25Cr-1.5Hf-0.7Mo-5.5Al-10W-3Ta-1Ti-0.05Zr-0.5Fe, respectively.
Typically, the braze materials utilized for the component repairs are Ni and Co-based braze filler containing B and/or Si as the melt point depressants. An example of each filler metal can be found in 2 Oerlikon Metco products, namely Amdry 400 of composition Co-0.4C-19Cr-8Si-4W-17Ni and Amdry D-15 of composition Ni-15.3Cr-10.3Co-3.5Ta-3.5Al-2.3B. These melt point depressants, when repairing wide cracks (greater than 1mm) typically found on industrial gas turbine engine components, using conventional brazing, form brittle intermetallic boride and silicide phases, effecting the mechanical properties, like low cycle fatigue and thermal fatigue properties of the braze repair. As a consequence, a diffusion brazing process was developed to repair the above-mentioned damage. As a means of qualifying the diffusion braze repair, both metallurgical and mechanical property evaluations were carried out. The metallurgical evaluation consisted of optical and scanning electron microscopy. The diffusion brazed area consisted of a fine-grained equiaxed structure, with carbide and boride phases dispersed both intergranularly and intragranularly. The mechanical evaluations were tensile tests at both room temperature and elevated temperature, stress rupture tests from 760oC—1093oC, and finally low cycle fatigue (LCF) tests, the latter being one of the most important and severe tests to conduct, since the cracks being repaired are thermal fatigue driven. The mechanical test results were equivalent to that of the base metals properties, and significantly superior to the properties of weld filler metals.