When I have looked at braze-spatter on parts, and then evaluated how the brazing paste is being applied to the parts, I’ve come to a few conclusions about the causes of braze-spattering:
1. Braze spatter does not seem to occur when solid BFM is used (rod, wire, or preform rings) when furnace brazing (or torch or induction brazing). Thus, braze-spattering only seems to happen when brazing-paste is being used. It is important to note, and understand, that most BFM-pastes consist of a blend of a water-based gel-binder as a carrier for a BFM powder.
2. Spattering seems to be related, too, to how much gel-binder is used in the BFM paste. BFM-powder to gel-binder ratios is usually expressed as a percent by weight ratios, such as 80/20, or 90/10. This means that the paste consists of 80-percent by weight of BFM powder, and only 20-percent by weight of gel-binder, or 90-percent metal by weight, etc. Obviously, since the metal is much, much heavier than the gel binder, then an 80/20 ratio brazing paste might actually consist of almost equal volumes of powder and gel-binder in order to meet the weight-ratio requirements. If someone is using a 60/40 ratio paste, then it’s quite possible that there is a much greater volume of gel-binder than powder in that paste! All of that binder must completely volatilize if good brazing is to occur.
3. Spattering seems to be more prevalent when the thickness of the applied paste-bead is large.
4. Spattering seems to occur much more readily when very rapid heating rates are being used.
5. Spattering seems to be more prevalent if the size of the BFM powder grains is large (i.e., coarse mesh powder).
Notice that only one of the factors above relates to worker-variability, namely, item#3. Part of worker training should, in my opinion, involve training each person about the items in this article, so that the workers will not try to put on paste-beads that are too thick to properly dry, melt, and flow during the brazing process.
Let’s look at some of these factors more closely.