Fig. 1 The joint between the two pieces has been nicely brazed, but because too much BFM was applied, and because the joint can be filled only once, all the extra BFM applied merely forms large fillets on the outside of the joint.
We return this month to our series on the essential criteria for brazing, to complete our current series on some fundamental issues about brazing filler metals (BFMs). As was discussed in my previous article on Essential Criteria for Brazing, if you are using BFM powder in your operations, it’s essential to not only control the mesh size of the BFM powder you are using (covered last time), but also to know and specify the ratio of BFM powder to the gel-binder in the brazing paste. The gel-binder is cellulose-type gel-like substance added to the powder to form a creamy blend of powder and gel that can be extruded from a cartridge through a dispensing tip. So, whether you are using BFM powder directly, or are using a pre-formed shim/ring of BFM, or BFM in paste form, you must always ask yourself this simple question: “how many times can I fill the braze joint with the BFM that I am using?” Hmmm…. The obvious answer is “one time” only! No matter how good you are, you cannot cram any more BFM into the joint after that joint has been filled by the molten filler metal ONE time!
Let me help you quantify this in a bit more detail so that you can be sure that, when you apply the BFM to the joint, you will have applied the correct amount in order to achieve this simple goal.
Shown in Fig. 1 is an example of a brazed assembly that even though well brazed, does show the results of what happens when a little too much brazing filler metal (BFM) is applied to the joint.
Notice that the joint is filled entirely, and because that joint can be filled only one time, all the extra BFM remains outside the joint in the form of large fillets.
People often apply too much BFM to the joint “to be safe” so that they won’t get blamed for not putting enough BFM on the joint to complete the joint properly. This gives rise to the expression that is heard sometimes in brazing shops: “If a little bit of BFM is good, then more is better!” That kind of thinking is wrong.
As shown in Figs. 2 and 3, the amount of BFM to apply is such that when the BFM fully melts and turns to LIQUID, there will be enough LIQUID BFM to fill about 1.5 times (150%) the AVERAGE volume of the braze-joint!