QUESTION: We are induction brazing an aluminum tube into a fitting on top of a larger, thick-wall aluminum tube. We are using the induction coil design shown in Fig.  1 at the recommendation of the induction brazing company from whom we bought the induction setup. We’re having problems with overheating of the sidewalls of the larger tube, however, and the filler metal that is supposed to join the small tube into the fitting tends to stay outside of the joint and flows down the outside of the fitting. Any ideas?

ANSWER: Yes, it appears that the induction coil design you showed in your drawing (Fig. 1) may be simple for the induction company to make, but it may not be at all ideal for your brazing needs.

One of the most important parts of good induction brazing relates to coupling distance of the induction coil from the part being heated. The coupling distance is defined as the radial distance between the ID of the induction coil and the OD of the part being heated/brazed. Thus, I’ve shown in Fig. 1, in the end view, the coupling distance between the induction coil and the brazing filler metal (BFM) preformed ring and another coupling distance between the induction coil and the sidewall of the large tube into which the fitting is being brazed. Notice how wide the coupling distance is near the BFM ring and how close the coupling distance is to the side of the large tube. This is NOT good for brazing.

Here’s why. The closer (smaller) the coupling distance, the more intense the heating of the base metal being heated by the coil will be. The farther away (larger) the coupling distance, the weaker the heating of the base metals by the induction coil will be.

Next time we will conclude this discussion and provide a solution.