Learn to Trust Capillary Action

This rule of applying from one side and inspecting on the other obviously depends on capillary action being able to effectively do its job when the BFM melts and begins to flow. Good capillary action will depend on having proper gap clearance, clean surfaces inside the joint and proper heating of the joint. If such conditions exist, then learn to trust capillary action to do its job!

Obviously, proper brazing also depends on applying the correct amount of BFM so that there is just enough BFM to fill the entire joint but not so much that excessive run-off of extra BFM occurs onto other surfaces near the brazed joint.

Fig. 6. By burying a preform ring inside an assembly, the BFM will flow from the inside of the part to the outside of the joint, allowing for easy inspection.

At which end of the joint should the BFM be placed?

Since you should feed the BFM from only one location and then inspect at the other end/location I recommend that you try to place the BFM in such a way that when it flows to the other side of the joint the side to which it flows should be easy to inspect. Thus, it makes sense to place a preform ring inside a part and allow the BFM to flow to the outside, where it can be easily inspected (Fig. 6).

Buried preforms should always be solid rings of BFM, never paste, because if BFM paste were to be used the binders in that paste would volatilize into huge amounts of gas inside the part. This would possibly distort the fit-up of the parts or push one component completely out of the joint.

What about clad sheet? Many aluminum base-metals already have a cladding of BFM applied to their surfaces during original production in the mill. Therefore, the BFM is al-ready inside the joint when that clad sheet of BFM is used, and the rule of “feed from one side and inspect on the other” does not apply.

Clad materials also have the advantage of being solid coatings. Therefore, outgassing of BFM components should not be a problem. In a similar manner, when you separately place solid BFM foil or sheet into a joint (creating a sandwich-like assembly) any concerns about outgassing of volatiles are eliminated because those materials are all solid.