A common way to create brazing filler metal (BFM) powder is by melting the raw metallic ingredients for the BFM in a large melting pot using induction heating and then pouring that alloyed liquid metal through a specialized atomization nozzle (Fig. 1).
As the molten metal stream exits the tundish (a funnel-like refractory device that channels the molten metal into a single stream pouring out of its base), it passes through an atomizing nozzle through which a very high-pressure gas is flowing. The nozzle will literally blast the molten metal stream into millions of tiny particles inside an atomization chamber. The tiny particles will fall by gravity to the bot-tom of the chamber, solidifying along the way into solid powder particles of various sizes (Fig. 2).
Fig. 1. A typical atomization process, in which a metal is melted using induction heating and is then poured through a specialized atomization nozzle to create tiny metal droplets. These droplets solidify into metal powder particles as they fall to the bottom of the tank.
Because of the wide range of sizes of these metal powder particles, the powder needs to be run through a size-separation-process known as powder screening. In powder screening, the powder is poured through a series of screen sieves, starting with screens with large openings and proceeding sequentially through screens with smaller and smaller openings in them. The purpose of this “screening” process is to create a controlled range of BFM particle sizes for use in various brazing applications.
Fig. 2. Wide range of metal-powder particle sizes resulting from production of powder from a molten metal via an atomization process.
Please note that the mesh size of a particular BFM powder is related to the size of the openings in the sieve screens used for these powders. The mesh-size number itself relates to the number of openings per linear inch of the screen, which is the same as the number of wires per linear inch blocking the flow of powder through that screen (per the U.S. Std. Sieve series).
Figure 3 shows a laboratory wire-mesh sieve screen used for 140-mesh powder. If atomized powder is not able to go through this screen because the openings in the screen are smaller than the size of those powder particles, then those large particles will remain on top of the 140-mesh screen and would be called “plus (+)” sized powders because they sit on top of the screen and cannot go through. Pow-der particles that can go through the screen are given a “minus (-)” particle-size designation, indicating that they can go through that screen.
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. Notice in the table that the 140-mesh screen (in the U.S Std. Sieve column) is the same as a 150-mesh screen in the Tyler Std. Sieve series and that each opening in such a screen is just over 0.004 inch (0.10 mm) in size. Notice that as the mesh-size number itself gets larger (such as 325-mesh, which means that there are about 325 openings per linear inch), the size of each opening has to get smaller and smaller. For a 325-mesh screen, each opening is only 0.0015 inch (0.038 mm) in size.
Fig. 3. In a 140-mesh screen, there will be 140 wires per linear inch. Powder sitting on top of that screen is known as a “+140-mesh powder,” and powder going through that screen is called “-140 mesh powder.”
People may ask, “I understand that powders are classified by which mesh screens they can go through, but doesn’t all 325-mesh powder also go through a 140-mesh screen? Why then isn’t all that 325-mesh merely classified as a -140 mesh powder?”
Yes, all the 325-mesh powder will technically go through a 140-mesh screen, but it cannot be labeled as -140 mesh powder since each particular mesh-size designation has additional requirements that must be met for it to be properly classified by a given mesh number.
Notice in Table 2 that there are strict requirements for the amount of oversized and undersized powder particles in each mesh size group. For example, to be classified as a -140C powder (“C” stands for “course”), the screened powder can only have a maximum of 20% -325 mesh powder. Thus, 80% of powder classified as -140C-mesh must be -140 mesh/+325 mesh, meaning it should all go through the 140-mesh screen, but 80% or more must sit on top of, and not go through, a 325-mesh screen.
BFM Powder Sizes
The -140 mesh powder size (either C or F; course or fine) is the size most often used for BFM powders. Notice from the chart in Table 2 that such a powder has a nominal particle size of approximately 0.004 inch (0.10 mm). The reader needs to understand that such a powder particle size is larger than the recommended joint clearance for most brazing, which ideally calls for a clearance of about 0.000-0.002 inch (0.000-0.050 mm).
Putting such BFM powder INSIDE a joint would cause the gap clearance to become too large. There-fore, standard -140 mesh powder is typically not put directly inside a joint but is instead used in brazing pastes that are applied on the outside of joints. When the powder in that paste melts during brazing, the resultant liquid BFM 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 – a powder size of approximately -325 mesh or finer (e.g., -400 mesh) would be preferred. Please verify this for yourself by studying Table 1.
It is important to note that all BFM containers (powder or paste) should always show the particular BFM powder mesh size used in that product on their labels. If it doesn’t, you need to find out why that information is missing. Be sure your purchasing personnel always specify mesh size each time they order brazing powder or paste from their suppliers, and be sure it shows up clearly on the labeling and certifications received. Otherwise, brazing problems can occur in your shop.
Fig. 4. Two equal-size boxes with identical volumes. The one of the left is filled with -140 mesh powder, and the one on the right is filled with -325 mesh powder.
Potential Oxidation Problems
Please be aware that the finer the mesh size of the brazing powder used in your furnace brazing, the greater the risk of oxidation of that powder. This depends on the quality (dryness) of your furnace atmosphere if brazing in a regular atmosphere furnace or the leak-up rate of your furnace when vacuum brazing. Figure 4 should help you understand why this is the case.
If you were to fill a container with -140 mesh powder and then fill an identically sized container with -325 mesh powder, and then each box was dumped out onto separate large sheets and you added up the surface areas of each powder particle on each of the two sheets, which group of powder particles would have a greater cumulative total surface area (the -140 mesh powder group or the -325 mesh powder group)? The answer is the total cumulative surface area of all the -325 mesh powder particles will be far, far greater than the cumulative surface area of all the -140 mesh powder particles.
Therefore, since there will be much greater surface area exposed to the furnace atmosphere when using smaller and smaller mesh size powders, it can be understood that you might not be able to effectively braze with -325 mesh powder in a marginal furnace atmosphere (an atmosphere with too high a content of oxygen or moisture) because of the much greater total amount of surface area exposed to potential oxidation when compared to using -140 mesh powder.
If a company has a vacuum furnace with a poor leak-up rate (e.g., 40 microns per hour leak rate) or if their atmosphere furnace was operating with a dew point of about -40°F (-40°C) or wetter, they may find it difficult to braze with a fine mesh powder (such as -325 mesh powder) since that fine mesh powder exposes far more surface area to all the oxygen and moisture in that marginal atmosphere than would a coarser powder.
I’ve seen fine mesh powders ball up in a marginal atmosphere and not flow out properly, whereas a coarser powder in the same environment flowed out because it had far less surface area of BFM powder exposed to the oxygen in that marginal atmosphere.
Atmosphere QC Test for Your Furnace
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. Do NOT spread the powders too thinly on these test coupons. Place them in small piles on each sheet. Then, after the brazing cycle, compare the flowing characteristics of the two small piles of powder.
They both should have flowed out nicely. If the -325 mesh powder tends to ball up on the sheet while the -140 mesh powder flows out, however, then your furnace atmosphere is becoming marginally poor. This test can be a quick way to catch that before it actually hurts some of your production parts.