A process that has been used successfully for repair and material addition to components is known by several different names: diffusion brazing; liquid phase diffusion sintering; and transient liquid phase bonding. These names generally refer to a process wherein a consumable material is melted at a temperature that is less than the solidus temperature of a substrate and then is caused to solidify to become integral with the substrate. (A similar process used to join two components without melting is known as solid-state diffusion bonding.) The consumable material may typically include a melting point depressant such as boron, silicon or phosphorous to ensure that the consumable material will melt at a temperature below the melting temperature of the substrate. The work piece and consumable material are held at an elevated temperature for a sufficient interval of time to promote diffusion of the melting point depressant into the substrate material. This diffusion causes the melting temperature of the liquid to increase, resulting in the solidification of the bond line by epitaxial growth of the grains of the substrate. Prior to the onset of solidification, some local melting of the substrate will occur as the melting point depressant diffuses into the substrate. Exemplary applications of such a process to superalloy materials used in gas turbine components are described in U.S. Pat. Nos. 5,836,075; 6,193,141; and 6,508,000, all of which are incorporated by reference herein.
FIG. 1 illustrates a desirable microstructure for a transient liquid phase joint 10 joining two substrates 12, 14. A consumable insert in the form of a braze foil (not shown) containing a melting point depressant such as boron, silicon or phosphorous was located between the surfaces 12, 14 and was heated to above its liquidus temperature, which is below the solidus temperature of the substrates 12, 14. A molten region 15 is formed having a thickness T as indicated by dashed lines 16, 18. The thickness T is typically equal to the thickness t (not shown) of the consumable insert plus a portion of the thickness of the adjoining substrate surfaces (typically 25% times t) that becomes molten when its liquidus temperature is depressed as the melting point depressant diffuses into the substrate. Upon completion of the bonding heat treatment, the melting point depressant has diffused sufficiently into the substrates 12, 14 so that the grains 20, 22 of the substrate surfaces 16, 18 have grown isothermally and epitaxially to solidify the melted consumable insert. The result is bond 10 having the microstructure, grain size and mechanical properties approximating those of the substrates 12, 14 provided that the mating grains of the two surfaces are favorably oriented. If, however, the grains are somewhat misaligned, a transverse boundary 30 may exist at the bond centerline, thereby degrading the mechanical properties of the joint.
In real world applications, the epitaxial growth of grains from the substrate 12, 14 into the molten material may be retarded or disrupted by the nucleation of grains from within the molten material itself. Grain nucleation within the molten material may result from unmelted consumable insert material, substrate oxide fragments or unclean surfaces. Grain formation from within the molten region may result in a bond joint 24 having the microstructure illustrated in FIG. 2. The non-epitaxial grains 26 are smaller than the substrate grains 20, 22 and they are limited in size to the thickness T of the molten zone. The mechanical properties of such a joint 24 are inferior to those of a joint having the desired epitaxial microstructure.
To achieve epitaxial growth, prior art transient liquid phase bond processes have required the bonding surface to be clean and to exhibit a low amount of residual stress. Low residual stress may be achieved by avoiding the introduction of stress into the surface and/or by conducting a stress-relieving heat treatment prior to the bonding process. Mechanical preparation of the surface that may be necessary for cleaning or for developing a desired surface geometry is conducted with a low stress-generating process. Low stress-generating processes are known to include low stress grinding, electro-chemical machining (ECM) and electro-discharge machining (EDM).