Repair-brazing is an essential part of the brazing industry, and will be looked at in two common situations:
1) In-house braze repair before release of parts to customers
2) Repairs done to brazed components that have “worn out” in service
An understanding of both is very important and their differences understood. The former involves in-house QA programs designed to catch braze defects before they get out the door. The latter situation involves those types of brazed components that literally “wear out” in service from corrosion, erosion or fatigue (thermal and/or mechanical) in cyclic service. The result is surfaces that are cracked, pitted or eroded. Let’s look at these situations in more detail.
Situation 2: Repairing "worn out" partsEven though the proper BFM is selected with the end-use service conditions in mind, very severe service conditions such as high temperature, thermal cycling, shock and vibration (and thus fatigue), and corrosive environments may still damage brazed assemblies. This leads to situations where periodic braze repair procedures may be needed. Braze repairs can deal with this condition quite well as long as certain procedures are followed. These are:
A. Proper cleaning and surface prep
B. Application of BFM
C. Heating to melt and flow the BFM
D. Inspection of the part.
A. Good cleaning and surface preparation is extremely important! Cleaning may not be too difficult for the outside surfaces of a part, but it can be very difficult when trying to clean the inside surfaces of a deep, thin surface crack.
When cleaning parts, the oils and greases and dirt must first be removed using suitable water-based (or solvent-based) degreasing techniques, and then the surfaces oxides can be dealt with after that. Never reverse that sequence! Some surfaces oils/greases/fuel-residues, etc. are so tenacious (such as synthetic silicon-based lubricants when allowed to dry on parts) that they must literally be burned off at high temperatures. Be careful when doing this, so that a brazing furnace does not become contaminated by such burnoff. There are commercial furnaces manufactured just for the purpose of burning off such oily surfaces contaminants.
Next to be dealt with are the surface oxides, which can be removed by acid pickling baths, by grit blasting the surfaces or by heating in a hydrogen- or fluoride-atmosphere furnace.
Acid pickling (always match the pickling solution to the base metals being cleaned – the base-metal vendor can give guidelines for this) is always followed by a clean-water rinse, but this is not always effective for cleaning deep cracks in parts.
Grit blasting can effectively remove surface oxides, but care must be taken not to cause surface warpage from the blasting process. Many grit-blasting products are commercially available, but always follow this rule: Never use non-metallics or oxide materials for grit-blasting surfaces to be brazed! All blasting media will leave some kind of surface residue on the parts, and this will affect surface brazeability. Non-metallics will leave voids in the brazed joint, and oxide materials (aluminum-oxide grit) can render the surface totally non-brazeable. I’ve seen this happen on numerous occasions!
If possible, heating in a dry hydrogen atmosphere (-60°F dewpoint or drier) to around 2000°F can be effective at cleaning many parts. At times, oxides are extremely difficult to remove and are contained in deep cracks – aerospace vanes and blades. The best way to clean such parts is in a commercial fluoride-ion atmosphere (known in the trade as “f-cleaning”), which can strip very tenacious aluminum-oxide layers and titanium-oxide layers from the surfaces and the deep cracks to be brazed.
Note this carefully: Only after surfaces have been thoroughly stripped of all contaminants should any attempt at re-brazing the assemblies be initiated.