QUESTION: We are brazing copper to brass components using a silver brazing alloy, and we understand that it is available in many different mesh sizes. What does “mesh size” mean, and what should we be doing about it?
ANSWER: When brazing filler metal (BFM) powder is produced, it is usually via a gas-atomizing process. Most people have seen videos showing the pouring of molten steel from large pots in steel mills. The making of BFM powders begins in much the same way, in that molten metal (the composition of the BFM) is poured through an atomizing nozzle that sits on top of a tall atomizing tank. After this, a high-pressure/high-velocity inert gas hits the molten stream of metal coming through that nozzle, blasting it into billions of droplets that then cool into individual tiny particles of powder as they fall down to the bottom of that tall atomizing tank. The powder, which has a wide range of sizes, is then collected for further processing.
Since atomized powders have a wide range of particle sizes when first produced, they must therefore be “screened” through progressively finer screen sieves to create controlled ranges of particle sizes for use in many different brazing applications.
The “mesh size” of a powder is related to the size of the openings in the screen used to sieve the powders. The mesh-size number relates to the number of openings per linear inch of the screen for powders screened to either U.S. Standard Sieve or to Tyler Standard Sieve designations. Thus, a 60-mesh screen will have 60 openings per linear inch, created by weaving 60 wires per linear inch, in a manner similar to that used in a window screen.
Powder particles that are too large to go through a particular sieve size, and therefore remain on top of the screen, are given a plus (+) designation, whereas powder particles that can go through the screen are given a minus (-) designation. Thus, a -140-mesh powder represents all powder that can go through a 140-mesh screen (Fig. 1).
A chart comparing the various powder particle sizes available for use in brazing is shown in Table 1. I originally put this chart together many years ago while working for Handy & Harman, and it shows the actual dimensions of each mesh size opening.
Brazing commonly uses -140 mesh BFM powder for most applications. For specialized applications like when powder needs to be sprayed onto heat-exchanger sheets that will be laid one on top of the other for subsequent brazing, however, a much finer-mesh powder needs to be used so that the powder particle size does not interfere with good joint fit-up during assembly. As can be seen from Table 1, the finer the powder, the higher the mesh-size number, since finer powders require more wires per linear inch to screen them. Thus, a 325-mesh screen will produce much finer powder than a 140-mesh screen (Fig. 2).
Learn more in part 2, which will be posted next week.
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