Question: We furnace-braze a cylindrical bar into a mating hole in a larger cylindrical part using AMS 4774 silver-based brazing filler metal (BFM), which contains Ag-Cu-Sn-Ni. When we machine away the braze-fillet from the edge of the joint for fit-up purposes, we often see from one to three voids remaining in the joint around the circumference. We reject these parts. What is causing these voids, and what can we do to prevent this from happening?
Answer: First of all, I suggest that perhaps you put a recess (chamfer) into the ID of the outer cylindrical part so that the braze-fillet can fill the joint and the recess, thus forming no external fillet. This can then eliminate the need to machine an external fillet away from the assembly after brazing is done. Having to machine away fillets is a time-consuming, costly affair, and it is not surprising that voids are uncovered by such machining operations.
Please note that all braze joints will contain some voids, and a few of these internal voids can easily become exposed in any given 360-degree circle machined into the fillet around the part. It’s not really surprising. The question is, of course, what is the source of the voids. It can come from surface contamination, from dirty/oily hands, trapped gases in the fillet, from outgassing of any metallic elements in the (BFM) and even outgassing from constituents in the base metal. Therefore, it is very important that parts be cleaned thoroughly PRIOR to assembly using clean gloves. Do not follow the oft-repeated, “Oh, don’t worry about pre-cleaning the parts. The furnace will take care of that.” I’ve seen too many shops have problems with their brazements due to improper cleaning beforehand or handling parts (and BFM materials) with dirty hands. Remember, even if you wash your hands, your skin still immediately begins to release oils, perspiration, acids, etc. onto its surface. So, please, ALWAYS wear clean gloves when handling clean parts for brazing.
Normally, when silver brazing by hand, I like to recommend that the joint be “wiped” (i.e. twisted slightly back and forth so as to help expel any voids from the joint). You can’t do that in a furnace-braze. So, my strongest recommendation is to get away from machining if at all possible. If parts are designed to mate-up to each other, leaving no room for a braze-fillet, then don’t create an external fillet to begin with. Instead, use the recess-design described above so that any voids present inside the joint will stay inside the joint and not become exposed through machining processes.
Keeping internal voids to a minimum is not difficult, as described above. If you ignore these guidelines, however, you may find that you have enough voids in the joint to cause leakers in your assembly, which is highly undesirable. These result in lost production, scrapped parts and, if the customer receives any of those parts, it may affect your reputation as a good supplier of brazed assemblies.