1. Surface contamination
2. Base metal and brazing filler-metal (BFM) constituents
3. Brazing methods/temperatures used
4. Poor joint fitup
The first three items listed above can often result in gas bubbles being formed in brazed joints. Such gas-bubble voids will usually try to form in spherical shape as they move through a joint. The "rounded" edges of such bubble-voids can often be clearly seen in cross-section photomicrographs of brazed joints, especially under high magnification. The inside surfaces of a bubble-void will often appear "clean" or "shiny" as well.
Let's briefly look a little more closely at these sources of voids in brazed joints, starting in this Part 1, with a look at surface contamination. In subsequent postings, we'll look at each of the others.
1. Surface Contamination
It is not uncommon to hear some brazers say, "Don't worry about surface contamination – the furnace will take care of it," or "Don't worry about surface contamination – just put more flux on the part to take care of it." Both statements are dangerous and can lead to weakened joints and, in many cases, to failed joints.
Oils, greases, lubricants, dirt, etc. left on the surface of assembled parts in a brazement often contain ingredients that will tend to volatilize and outgas at the elevated temperatures involved in brazing. These "gases" will attempt to expand and move out of the confining braze joint, and, as long as there is an "escape path" open to the outside atmosphere for these expanding gases, they will not tend to be a problem during brazing. Since contaminants on the outside surface of parts being brazed are readily open to the atmosphere, such contaminants may completely volatilize, causing no problems to the brazement.
When the inside surfaces (faying surfaces) of a brazement are contaminated and not cleaned off prior to assembly, however, it frequently becomes impossible for those contaminants to volatilize (turn to a gas). Even if much of it could volatilize, those gases usually find it difficult, if not impossible, to make way to the outside of the joint where they can finally be released to the atmosphere. They will become entrapped (as bubbles) inside the brazed joint when the joint finally solidifies.
Additionally, be aware that surface contaminants and oxides inside a joint that do not volatilize can ruin a braze! Since molten brazing filler metal (BFM) does not want to bond to (or flow over) oils, dirt, greases or oxides on the faying surfaces inside a joint, the BFM may not be able to penetrate through the braze joint at all when surfaces have not been adequately cleaned prior to joint assembly.
Thus, it is very important that all surfaces to be brazed must be cleaned PRIOR to assembly for braze. Surfaces must then be handled with gloved hands (so that they are not recontaminated by the brazer's fingers and hands). These precautions will minimize any voids in the brazed joint.