The form of BFM used in vacuum brazing of aluminum is not that critical. You can quite successfully use paste, solid-metal preform rings, sheets, clad-sheet, etc. when vacuum brazing aluminum. Obviously, with paste BFMs the binders must be completely volatilized during brazing so as not to leave any residues. But please know that all these different forms of BFM can work very well in a vacuum brazing environment. The choice is yours.
IMPORTANT: The shaded area on the left side of Table 2 shows recommended temps for brazing a specific aluminum alloy, whereas the shaded areas on the right show suggested, optimal brazing temp-ranges for a particular BFM. Try to match appropriate “ranges” on the left with those on the right. Based on what you see in the chart — which BAlSi-alloy would you recommend for brazing 6061?
Violation#11 – Aluminum tube joint-length is too long for BFM flow. Remember the brazing rule specified in “Violation#6 that states that you should feed the BFM from one end of the joint and inspect the joint at the other end, thus requiring the BFM to flow through the entire length of the joint to ensure its completeness. But aluminum-based BFMs don’t really flow long distances through brazed joints, but instead, prefer to quickly alloy with the base metal, thus limiting capillary flow through the joint. Some of you may recall from my seminars that instead of designing joint lengths of about 3T-to-6T (where T is the thickness of the thinner of the two members being joined) which applies to most metals, aluminum is different due to the extreme reactivity of the aluminum BFM and base metal due to their closeness in melting temps. Thus, for an aluminum joint design we recommend an overlap of only about 1-3T instead. Notice that the length of the joint (0.500” +/- 0.005”) is far more than the recommended 1-3T joint-overlap for aluminum tubing, and thus, it may be very difficult for the BFM-ring at the right side of the joint to melt and flow out all the way to the left side of the joint. If there were an end-use service condition that required such a long joint for the stability of the tube in the fitting, then brazing the full length of the joint may be incomplete.
1. Machine a tubular extension. It would probably have been wiser to merely machine a long tubular extension as part of the base (and then braze a cap on it), rather than brazing that long hollow cylinder into the fitting. A small BFM ring could then have been placed on top of that tubular extension to braze a cap on the end, which would have allowed the 1-3T guideline to be easily met, and would have easily sealed the top of the tube, as needed. Although not specified on the drawing, it would be assumed that the assembly would be furnace brazed in the vertical position, making such a braze quite simple.
2. Lack of a vent-hole in joint. As metals are heated during brazing, the metals expand, and so does any trapped air inside the assembly. In vacuum brazing, the expanding air inside a system is usually able to escape from the inner areas, either through the joint clearance-gaps or through the metal walls of the parts that make up the assembly. We have seen this happen in vacuum brazing applications as well as in atmosphere furnace brazing. At brazing temp, the atoms of the metals have usually opened up enough so that the air-atoms trapped in the assembly can literally make their way out of the assembly through the wide-open lattice-structure of the metal at brazing temp since the atoms in air (oxygen, nitrogen, etc.) are so tiny.
IMPORTANT NOTE: Lack of vent-hole IS very important, however, in torch and induction brazing, especially when flux is used, but is not nearly as critical in most vacuum brazing applications, depending on the heating rate used! If heated too rapidly, trapped air might expand fast enough to disturb part fit-up. But, with slow heating, the air can usually escape without any real issues. If you feel this may be a problem, then put a vent-hole in your product design to prevent trapped air problems.
3. Gap clearance not specified. It is not the purpose of the drawing to go into detail on all of the sizes and dimensions of each of the parts. The drawing was only intended to include items that demonstrated brazing errors in design or brazing-procedures. Obviously, gap dimensions would need to be shown on the drawing.
4. AMS 2678. Does this spec have a Class B? What does the spec say re voids in the joint?
So there you have it! I’ve shown you eleven (11) items on the Challenge-Drawing that are not good from a brazing perspective. I hope many of you found the drawing an interesting challenge. Feel free to contact me and let me know your further thoughts about what you found on the drawing.