Good brazing depends on the ability of capillary action to draw the molten brazing filler metal (BFM) in all directions throughout the joint being brazed, either vertically or horizontally. In many brazed joints there is often the need for the BFM to flow upwards against gravity, and if the gap clearance is properly made, this should not be a problem. However, if the gap-clearance between the faying surfaces in a joint to be brazed is too wide, then gravity will dominate the situation, and capillary action may not occur. Therefore, it is very important for all brazing shops to know the “capillary-capability” of each of their furnaces they use for brazing. This is not hard to do.
Optimal gap clearance varies according to the combination of base metal and BFM used; e.g., copper BFMs prefer tighter joint clearances than for aluminum BFMs. Also, depending on the reactivity of the BFM with the base metal being joined, you may find that a given BFM will prefer either a tight gap or a wide gap. NOTE: By reactivity I mean the ability of the BFM to alloy with, and diffuse into, the base metal. You will also see that this reactivity will affect the distance that a BFM is able to flow into a joint being brazed.
The AWS Brazing Handbook talks about the acceptable gap clearances needed for each family of BFM. Most BFMs require fairly tight braze gap clearances for effective brazing to occur, usually in the range from 0.000″-to-0.005″ (0.000-to-0.010 mm). But in standard commercial brazing, the gap clearances faced by shop personnel often exceed this, and the furnace brazers are left wondering if such gaps can be brazed. So, they may attempt to fill the gap with enough BFM for the job, and hope for the best, only learning if the process is successful by examining parts when they come out of the furnace. If they “guessed wrong”, and the gaps prove to be too big for them to braze, they may then have to add more BFM to the joint and send the part through the furnace a second time (or even a third time) until they can finally get the braze completed.
Such frustration can be avoided by learning how to determine, in advance, what are the gap clearances that can be handled by your furnace for any given base-metal/BFM you wish to use. This is accomplished by using a “Vertical Capillary Test Specimen” (VCTS) in each of your furnaces.
Figure 1. shows the VCTS, which was developed by the late Robert Peaslee, and has been used successfully by many shops to learn exactly what their furnaces are capable of regarding gap-clearances. Please note that the dimensions shown in Fig. 1 are those that we have used, but many different dimensions may prove more practical for your own shop. Instead of only 6-inches for the height of each leg, you may find 8-inch lengths to work for you, etc. However, it is best to have longer lengths for this test, rather than short lengths. You’ll see why later in this article.
Note that the VCTS consists of two vertical pieces of metal (and should be the same metals you will be brazing in your furnace), which are tack-welded to a stable base, with zero-clearance at the base, and a wide-gap at the top of the two vertical legs. A weld at the top of the two pieces holds the gap open (top-gaps can range from about 0.012″-to- 0.020″ (0.30-to-0.50 mm).
The specific BFM you will be using in your furnace run is placed at the base of the vertical joint, and the part is run through the proposed brazing cycle for the materials to be brazed. Let me remind the reader again, that the vertical legs need to be made from the same metals that you intend to braze, and the BFM is the same that you will be using. For many job shops, this may mean that you will be making VCTSs from a variety of base metals over time, and run them with a variety of BFM combinations. You may also be using different metals for each vertical leg (such as one copper leg, and one stainless leg, etc.). Since this is only a test specimen, it is not necessary to put a minimum amount of BFM at the base of the joint. Instead, place a goodly amount of BFM at the base, to be sure there is some excess (the VCTS will use only as much of the BFM as it needs). You do not want to put on too little, thereby starving the joint of what it needs, which will give faulty, incorrect, test results.
When the VCTS has gone through a furnace braze cycle, as shown in Fig. 2, you will note that the BFM has been pulled upwards by capillary action part way up the length of the two vertical legs of the specimen. Measure the width of the gap at the point where the capillary action stopped. This is known as the “break-capillary” point, and represents the maximum gap-width that the BFM can bridge in your specific furnace under the conditions found in that furnace at the time of the test. As you see in Fig. 2, capillary action was able to draw the BNi-2 BFM upwards until the gap reached a width of 0.006″ (0.015 mm). Beyond this point, as the gap gets wider and wider, gravity predominates over capillary action, preventing the BFM from rising any further.