Essential Criteria for Brazing: Item 4a – Brazing filler metal (BFM) powder used in making brazing-paste.

We return this month to our series on the essential criteria for brazing and will look more closely at another aspect of brazing filler metals (BFMs). If you are using BFM powder in your operations, then it will be very important for you to understand and to control the mesh size of the BFM powder you are using, since powder mesh size can affect brazing performance.

Shown in Fig. 1 is a photo of some metal powder, showing the wide range of metal particle sizes that will commonly be produced when metal is melted and then atomized into powder.

As mentioned above, a common way to create BFM powder is by melting the raw ingredients for the BFM in a large melting pot using induction heating, and then pouring the resulting liquid metal through a specialized atomization nozzle in which a very high-pressure gas will literally blast the molten metal stream into billions of tiny particles (inside an atomization chamber). The tiny particles will fall by gravity to the bottom of the chamber, solidifying along the way into solid powder particles, as shown in Figure 2.

The solidified powder particles will have a wide range of sizes, just as shown in Fig. 1. This powder will then be put through a separation-process, in which the powder will be “screened” through various screen sieves to create a controlled range of particle sizes for use in various brazing applications. Please note that the “mesh size” of a particular BFM powder is related to the size of the openings in the screen used to sieve the powders. The mesh-size number itself relates to the number of openings per linear inch of the screen (i.e., the number of wires per linear inch, per the US Std. Sieve series). Fig. 3 shows an example of such a sieve-screen, used for 140-mesh powder.

If the powder is not able to go through a particular screen because the openings in the screen are smaller than the size of the powder particle, then that large particle is called “a plus size powder” meaning that it sits on top of the screen and cannot go through. If a powder particle can go through the screen it is given a  “minus (-)” size particle designation, indicating that it can go through that screen. Thus, in Fig. 3, in which a 140-mesh screen is shown (which means that there are 140 wires per linear inch, or 140 openings per linear inch), powder that sits on top of that screen and cannot go through is known as “plus (+) 140 mesh powder”. Powder that is small enough to go through that screen is known as “minus (-) 140 mesh powder”.

The 140-mesh screen size is only one of many different sized screens that can be used to segregate different sized powder particles from one another. Table 1 shows a number of other screen sizes commonly used.

The -140 mesh powder size is a common one used for BFM powders. Notice from the chart that such a powder has a nominal particle size of approximately 0.004-inch (0.105 mm). As can be seen, such powder particles are larger in size than the recommended joint clearance for most brazing of about 0.000-0.002” (0.000-0.050mm). Putting such BFM powder INSIDE a joint would cause the gap-clearance to become too large. Therefore, standard -140 mesh powder is typically used for pastes that are applied on the outside of joints. Then, when the powder melts during brazing, the liquid BFM formed can easily be pulled into, and through, the narrow joint by capillary action.

If your process requires you to pre-place the BFM powder INSIDE a joint prior to brazing, then, looking at Table 1, it can be seen that a powder size of approximately -325 mesh or finer (i.e., -400 mesh) would be preferred. Therefore, specifying the size of the BFM powder you purchase is very important from an application perspective.

Table 2 answers another question some of you have regarding the allowed range of powder sizes for each published “mesh size”. Yes, -325 mesh powder should all go through a 140-mesh screen. But it cannot be called -140 mesh powder, since the mandatory “controlled range” of particle sizes for -325 powder is very different than that allowed for a powder that is classified as being a -140 mesh powder.

Oxidation of powder. Another important part of BFM powder selection has to do with the effect of powder-size on the oxidation of the BFM powder or paste you are using. Look at Fig. 4. Here is a challenge question for you — the two boxes in Fig. 4 have the same total volume inside those two boxes. If the left box were filled to the top with -140 mesh powder particles, and the box on the right side were filled with -325 mesh powder, and then both boxes were dumped out onto two separate large sheets, and you calculated the surface area on each powder particle, which group of powder particles would have the greater total surface area? The -140 mesh powder group, or the -325 mesh powder group?

Yes, the -325 mesh powder would have a much greater total surface area than the -140mesh particles. Again, we’re referring to the total surface area of all the particles in each of those two identical-volume boxes, one filled with the coarser -140 mesh powder, and the other box filled with the much finer, smaller -325 mesh powder.

My point is, that if you are applying a given quantity “X” of BFM paste or powder to a particular joint to be brazed, then, if that quantity “X” consists of -140 mesh powder, the total surface area of powder in that paste that will be exposed to the oxygen in the brazing atmosphere will be much less than the total surface area of powder in a similar volume of -325 mesh BFM paste/powder.

Consequently, if, for any reason, your brazing atmosphere is “marginal”, i.e., has too high a dewpoint, or a high leak-up rate in a vacuum-furnace, etc., then, because of the excess amount of moisture (thus oxygen) in such a marginal-atmosphere, you will probably find that finer mesh powders, with their much greater total surface area exposed to all that excess oxygen, will tend to resist melting properly, and perhaps merely “ball-up” into lumps rather than flow out nicely. I’ve seen this happen, whereas a much coarser powder in the same environment flowed out okay because so much less of that coarse powder’s total volume had any oxides.

This phenomenon has led to an interesting quality-control test that you can use to verify the quality of your brazing atmosphere. Put two different mesh-size powders (of the same BFM alloy) on a sheet (keep the two small piles of BFM powder well separated from each other), and run them in one of your regular brazing production runs. After the brazing cycle, compare the flowing characteristics of the two small piles of powder. They should both have flowed out nicely. BUT, if the -325 mesh powder tends to ball-up on the sheet while the -140-mesh powder flows out okay, then your furnace atmosphere is becoming marginally poor, and this test can be a quick way to catch that before it actually hurts some of your production parts.