In Fig. 2, you can see the results of an actual VTS in which the base metal was Inconel 600 and the BFM was BNi-2. The VTS was brazed in a vacuum furnace at 10-4 Torr and held at a temperature of 1900°F (1040°C) for 15 minutes.
Note that the BNi-2 BFM was drawn by capillary action up into the vertical joint and was able to rise to a height where the gap was 0.006" (0.15 mm) but no higher. Above that point, the gap was too large for the conditions noted in that furnace cycle.
Some people would respond to this by saying, "Great! I can braze parts together with a gap of 0.006" (0.15mm). Wow!"
No, that's WRONG! At 0.006" (0.15mm) in this actual test, capillarity could not continue beyond that gap clearance! So, if in your own samples the gaps are nominally about 0.006" (0.15mm), you will probably have some portions of the joint that are a little less than 0.006" (0.15mm) and other areas in the joint that are greater than 0.006" (0.15mm). Consequently, some of those braze gaps will not be able to fill because they are too large.
How to Properly Interpret Results of the VTSWhen you find out the maximum gap that can be tolerated in your furnace, via the VTS, for the particular base-metal/BFM combination you are using, you then know the MAX gap that your furnace will allow. Do NOT allow any of the parts you are brazing to have braze clearances that come close to that maximum amount!
RULE TO FOLLOW: Don't let any braze gaps exceed 50% of the clearance shown on the VTS for that furnace/base-metal/BFM combination! Train your people accordingly. Thus, if the VTS shows a particular BFM "breaks capillarity" at 0.006" (0.15mm), don't allow any braze assemblies into your furnace that have braze-gaps larger than 0.003" (0.08 mm). If you do this, you should NEVER have parts that do not braze fully and completely.
Making VTS Samples1. The VTS material used must be the same as the base material being brazed.
2. The brazing filler metal (BFM) must be the same as that being used in that particular furnace run.
3. The dimensions of the VTS can be changed to suit your furnace needs or to your material availability. Please note that the specimen must have a base that will keep the two vertical members stable during the entire braze run, without the need for additional support to keep the legs of the specimen in the vertical position.
4. The two vertical legs are cut so as to allow a V-shape gap to be made, starting at the base with a "zero" gap (keep all tack welds as small and as clean as possible) and then steadily opening up to a gap at the top of at least 0.012"(0.30mm) but no greater than 0.020"(0.50mm).
5. The edges of the two strips inside the vertical joint should be square and clean (not just a sheared edge), since the BFM will be rising along those inner edges of the VTS to fill the variable gap.