1. Surface contamination (discussed in Part 1)
2. Base metal and brazing filler-metal (BFM) constituents
3. Brazing methods/temperatures used
4. Poor joint fitup
Let's briefly look a little more closely at the second source of these voids in brazed joints, base metal and brazing filler-metal constituents. In the next posting, we'll look at brazing methods/temperatures used.
2. Base Metal and BFM ConstituentsMany base metals and BFMs have constituents in them that can easily volatilize when heated to brazing temperatures. Zinc, cadmium and lead are three such metals that will, in fact, outgas readily during any kind of brazing process, and proper precautions should be observed when such metals are used in brazing.
Lead may be found in some steels or brasses to enhance the machinability of those base metals. If such metals are then used as part of a brazement, however, the lead will quickly outgas, forming bubbles in the brazed joint and perhaps leaving holes in the base metal (such holes might result in leak paths through the metal, hurting hermeticity of the brazed assembly). This can obviously become a problem where leak-tight hermetic-seals are required for brazements in service.
Zinc and cadmium are used in certain BFMs as temperature depressants to help lower brazing temperatures and also to enhance the flowability of these BFMs on certain base metals. Zinc and cadmium may also be found as platings on some metal parts that are to be brazed. Zinc and cadmium, like lead, will readily outgas upon heating to brazing temperatures, and this will be seen as bubbles in the braze joint or as fumes in the workzone of the brazers. Neither situation is desirable.
Please bear in mind that because such outgassing of these three metals will, in fact, occur during heating to braze temps, they should NEVER be used in any vacuum brazing environment as they will contaminate the vacuum furnace and its pumping system, perhaps to the point of rendering the furnace non-usable! I strongly recommend that metals and BFMs containing these three elements be limited to flame brazing and induction brazing and that they not be used in any kind of furnace brazements because of the potential for contamination of furnace surfaces. Even with flame (torch) brazing or induction brazing, proper venting of the brazer's breathing zone must be done so that the brazer does not have to breathe the fumes being generated.