If the situation requires that the higher-expansion material be the inner member, and cracking does occur as a result, it may be possible to repair cracks by applying a lower-melting BFM over them and rebrazing the assembly at the lower temperature required to melt this second BFM. Because the first BFM will not remelt, the cracks may be healed by this technique if the parts are very clean before rebrazing and the cracks have not been contaminated. This repair technique will work only if the cracks are discovered during visual inspection of the parts. If cracks are discovered only because a penetrant inspection has been performed, it will be virtually impossible to repair them because the penetrant compounds may not be removed adequately from the cracks.
Even when the proper BFM is selected with the end-use service conditions in mind, very severe service conditions (such as high temperature, thermal cycling, shock, vibration, and thus fatigue) and corrosive environments still may damage brazed assemblies, leading to situations in which periodic braze repair procedures are needed.
Figure 3 shows a cross section of what a damaged surface may look like, in which surface cracks have developed in service. Brazing can repair this condition quite well as long as certain procedures are followed. These include proper cleaning and surface prep and application of the BFM. Then, the heat cycle melts and flows the BFM, and finally the part is inspected.
Good cleaning and surface preparation are extremely important. Cleaning may not be too difficult on the outside surfaces of a part but can be very difficult on the inside surfaces of a deep, thin surface crack. First, the oil, grease, and dirt must be removed with suitable water- or solvent-based degreasing techniques, and then the surface oxides can be dealt with. This sequence never should be reversed.
Some surface oils, greases, and fuel residues are so tenacious (such as synthetic silicon-based lubricants allowed to dry on parts) that they must be burned off at high temperatures. Care must be taken so that the brazing furnace does not become contaminated by burnoff. Certain commercial furnaces are manufactured just for the purpose of burning off such oily surface contaminants.
Next, the surface oxides can be removed by acid pickling baths, grit blasting the surfaces, or heating in a hydrogen- or fluoride-atmosphere furnace. Acid pickling always should be followed by a clean-water rinse. The pickling solution must be matched to the base metals being cleaned. The base metal vendor can provide guidelines for this.
Grit blasting can remove surface oxides effectively, but surface warping from the blasting process should be avoided. Many grit-blasting products are available, but nonmetallics or oxide materials never should be used for grit blasting surfaces to be brazed. All blasting media will leave some kind of surface residue on the parts, which will affect surface braze ability. Nonmetallics leave voids in the brazed joint, and oxide materials (aluminum oxide grit) can render the surface totally nonbrazeable.
Heating in a dry hydrogen atmosphere (-60 degrees F dew point or drier) to about 2,000 degrees can be effective for cleaning many parts. However, if the oxides are extremely difficult to remove and are contained in deep cracks (such as aerospace vanes and blades), then the best way to clean them is in a commercial fluoride-ion atmosphere (commonly known as “f-cleaning”), which can strip away very tenacious aluminum- and titanium-oxide layers. Only after surfaces have been stripped thoroughly of all contaminants should any attempt at rebrazing the assemblies be made.