We conclude our discussion on this topic. Read part 1 here.
Note: Please be aware that all BFM powders are sensitive to the presence of oxygen in the brazing environment, whether you are brazing with a torch or a brazing furnace. All metals, when they are heated, can react with oxygen to form surface oxides on the metal. If the BFM powder oxidizes, it will not be able to melt and flow by capillary action into the joint that is being brazed. Therefore, adequate steps must always be taken to prevent oxidation from occurring during the brazing process. This can be accomplished via the use of a brazing flux when torch-brazing or by excluding air altogether when vacuum-furnace brazing, etc.
It should be noted that the finer the mesh size of the powder, the greater its sensitivity to furnace atmosphere oxidation. If you were to fill a given volume of space with -140 mesh powder and then fill an identical volume of space with -325 mesh powder, the total surface area of all the powder particles in that volume of space will be far, far greater for the -325 mesh powder than for the -140 mesh powder. Since there will be much greater surface area exposed to the furnace atmosphere, it can be understood that in a marginal vacuum-furnace atmosphere you might not be able to effectively braze with -325 mesh powder because of the greater amount of surface area exposed to oxidation, even though you might have been able to braze adequately with -140 mesh powder.
Recommendation: Check your inventory of brazing powder or brazing paste. Does the container specify the particular mesh size for that product? It should always do so! If it doesn’t, you need to find out why that information is missing.
Be sure your purchasing personnel are always specifying mesh size each time they order from their suppliers. Otherwise, brazing problems can occur in the shop. As an example of this, there was a brazing job shop that did not specify mesh size when ordering their brazing powders, and they received different mesh sizes from their supplier over time, depending on what mesh-size powder the supplier had in stock at the time they received the orders from that brazing shop. The brazing shop was upset when they got varying brazing results when using the different batches of BFM they were receiving and complained to their supplier about the “bad” BFM they had received. When the BFM supplier explained the difference between mesh sizes, as far as surface area is concerned, and how finer mesh powders are much more sensitive to marginal atmospheric conditions, the customer learned – by experience – that they needed to use a coarser-mesh powder, such as the -140 mesh, to get better results in their particular vacuum furnace. They needed to be sure to consistently specify that mesh size on all their subsequent orders.
Thus, if a company has a vacuum furnace with a poor leak-up rate (40 microns per hour leak rate, for example), they may find it impossible to braze with -325 mesh powder because of the very high total surface area of the fine powder that is being exposed to all the oxygen and moisture in that marginal atmosphere resulting from the high leak-up rate. To greatly reduce the amount of BFM surface area exposed to the atmosphere, they need to go to much coarser-powder mesh size. Obviously, they also need to find the source of their furnace leaks and fix them!
If you are torch brazing, then the brazing flux you use should take care of this, since the paste flux is a strong oxygen “getter;” that is, it will preferentially absorb oxygen from the air so that the BFM powder remains protected from oxidation and will be able to melt and flow as desired.
Conclusion: Be sure you know the mesh size of the BFM powder you are using. It is, in fact, an important piece of information that you must have in order to effectively control your brazing process.