Many people use silver-based brazing filler metal (BFM) when torch-brazing (i.e., flame-brazing) a variety of base metals such as copper, copper-alloys, and many types of steel (including stainless steels). Unless you are brazing pure copper to pure copper, a brazing flux should always be used. Pure copper to pure copper can be brazed using a phos-copper BFM, in which the phosphorus content of the BFM acts as a flux on the copper, and no additional paste flux is needed.
However, for all other metals a paste flux will need to be applied to the surfaces of the parts to be brazed in order to protect the components (especially the inside of the braze joint itself) from oxidation during the brazing process, since all metals tend to react with oxygen when heated, to form surface oxidation. BFM does not like to bond to, or flow over, oils, dirt or oxides on the surfaces to be brazing, and so, steps must be taken to insure that the surface is very clean prior to brazing, and then protected from oxidation during that high-temperature brazing process itself.
Shown below is a photo of several test stripes of brazing flux applied to a surface with varying degrees of surface cleanliness prior to braze, courtesy of Handy & Harman (Lucas-Milhaupt Division). Notice how the flux beads up in the two stripes on the left when heated, whereas they remain closely adherent to the surfaces on the right.
The two stripes on the LEFT are classic examples of what happens when someone tries to apply flux to a dirty or oily surface and then expects the flux to do the cleaning of the surface during heating. It is not uncommon to hear some folks say: “Oh, don’t worry about that, the flux will clean up the surface”. No, it won’t! The surfaces to be brazed must be thoroughly cleaned prior to assembly for brazing, and then the parts must be carefully handled to prevent fingerprints, airborne oils and dust, etc., from contaminating the surface prior to braze.
The two flux stripes on the RIGHT side of the photo show the effect of heating clean surfaces on which the flux was applied. Note how the flux remains in place, and does its protective job, namely keeping the surface protected from oxidation during the heating process.
When people notice that the flux wants to ball-up on the surface during heating, some folks try to drive in more heat to the surface, obviously thinking that they’ll overcome the resistance of the flux to flow by more intense heating. That won’t work! Stop heating the parts as soon as you see that the flux tells you (by beading up) that the surface is not clean. thoroughly clean the surface of whatever contaminant is there, and then re-flux and try brazing once again. Make sure that the flux, when heated, always stays closely adherent to the surface, as shown in the two stripes on the right side of the photo.