We began this discussion in early November, and we conclude it here.
As you can see in the drawing in Fig. 1, the sidewall of the larger aluminum tube is very close to the induction coil, thus being heated much more aggressively than the BFM ring sitting on top of the heavy fitting. Not only does the BFM ring have a much larger coupling distance than the sidewall of the large tubing, but that BFM ring is sitting on top of a fairly massive aluminum fitting, which will draw away any heat that should be going to melt the BFM ring!
Therefore, as you have already experienced, not only are you seeing damage to the side walls of some of the heavy aluminum tubes, but you are seeing the BFM rings merely softening and flowing over the outside of the fitting itself rather than being drawn into the fitting by capillary action.
SOLUTION: I would suggest that you consider having the coil redesigned as shown in Fig. 2. The coil in Fig. 2 goes around a portion of the upper tube, then around the fitting, and finally has only a slight positioning above the top of the large tube.
VERY IMPORTANT: It is the job of the induction machine people to properly design, and supply, the correct induction coil that will correctly heat the joint so as to cause the BFM, when melted, to want to flow down into the joint by capillary action. To achieve this, the tighter coupling distance should be for the loop of the induction coil that goes around the fitting itself. A wider coupling distance should be around the small tubing, and then only a small induction field should be heating the big tube itself. If these coupling distances are properly designed as I’ve suggested, the small tube will heat up and conduct its heat rapidly into the fitting. The tighter coupling distance around the fitting itself will cause it to heat up more rapidly, and the combo of those two coils will cause the capillary space in the joint to heat up adequately to become the source of heat that will cause the BFM ring to melt and pull into the joint.
CAUTION: Do not heat the BFM ring directly! You do not want the BFM to become hot and melt before the actual joint (inside the fitting) has come up to temperature.
CONCLUSION: One very important part of induction heating is the coupling distance between the ID of the coils and the OD of the workpiece. Know what it is, control it and use it to properly heat the braze joint so that the molten BFM is drawn into the joint by capillary action.