Question: I just read the two blogs on the Vertical Test Specimen (VTS) that you published in this column back on May 7, 2009, and on May 14, 2009. But it did not explain how to use solid brazing foils in such a test. I can understand how to use the VTS for brazing paste, but how does one use brazing foils on such a test specimen?
Let me answer that question after a quick review of what the VTS is, as shown once again in the photograph linked here (Fig. 2 in Part 2 of the previous presentation) and the diagram linked here (this is in part 1).
It is important to understand that a correct brazing clearance (distance between the two faying surfaces inside a joint being brazed) is essential between the mating surfaces of each and every brazed assembly that a brazing shop is producing for their customers. Correct clearances will allow proper flow of a brazing filler metal (BFM) by capillary action through a braze joint. If the brazing clearances are too big, then the BFM may merely drain through the gap and not remain in the joint. Additionally, if the gap is too big to be filled, additional brazing cycles may be required in order to add additional BFM to the joint to try to get it to fill completely (which is an inefficient and wasteful practice).
The VTS was developed a number of years ago so that companies could know exactly what brazing clearances/gaps their brazing furnaces would be capable of successfully filling the first time the part is run through it. It can be used in any furnace – batch-type or continuous – with any atmosphere (including vacuum) and with any base metal and BFM combination. The shape of the VTS is such that when the filler metal is placed on the VTS at the base of the vertical "V" (where the gap clearance is "zero"), the BFM will melt during the brazing cycle and be drawn by capillary action vertically up the gap between the two angled legs of the specimen. The BFM flow will stop (capillarity is broken) when the gap between the legs becomes too great for the BFM to rise any further, according to the particular conditions of that furnace cycle (time, temperature, ramp rates, etc.), atmosphere quality and BFM interaction with the base metal in that particular furnace run. Yes, these variables will be different each different furnace run and can vary significantly from furnace to furnace in the same shop, even when those furnaces are each the same Model number! Each furnace has its own personality and capability.
Applying the BFM to the VTS
Normally, people use these VTS samples in furnaces where BFM paste is used. It makes the application of the BFM at the base of the VTS very simple.
NOTE: Always be generous with the amount of BFM applied. You want to have excess BFM in the joint area so that there is a good reservoir from which to draw all the BFM needed to fill the joint as high as capillary will take it. You don’t want to starve the joint!
Applying BFM Foil
BFM foil (rolled sheet, or amorphous foil, etc.) should be cut into strips and placed into the joint at the base, as shown in Figure 1. Foil can also be cut into strips that are laid along side the vertical arms of the VTS. They do NOT have to be tack welded in place or attached with brazing cements (adhesives). They can be laid along the base of the VTS as shown and a number of the strips placed in the V-groove itself. Once again, the idea is to apply more than enough BFM to insure that when the foil melts, there will be sufficient molten BFM to fill the vertical gap as high as the furnace conditions will allow.