A Q&A with our resident brazing expert about combining aluminum brazing and heat treatment.
QUESTION: Can I braze 6061 in a vacuum furnace and also do solution heat treating at the same time while still operating in a vacuum? How will I be able to cool it rapidly in a vacuum? Or do I have it take it out the furnace while still hot in order to quench the parts? Can I get a T4 temper after such cooling?
ANSWER: What you are seeking to do is theoretically possible, but it requires specialized equipment to achieve while in a vacuum furnace. Please keep in mind that the brazing cycle and the heat-treat (HT) cycles have different requirements and, although they might be able to be combined to some extent, the needs of each can be quite different.
A. First of all, the brazing cycle itself can, in fact, accomplish part of the 6061’s solution heat-treating needs. Because it brings the aluminum above the 985°F (530°C) minimum temperature needed for any solution-heat-treatment process, all constituents in the 6061 aluminum are dissolved and “in solution.” This means that any and all precipitates have been completely re-dissolved into a uniform solution with all the other metallurgical ingredients in the alloy. So, depending on the length of time that the parts are held at brazing temperature, you are meeting some (or all) of the one-hour minimum holding time at 985°F/530°C (or higher) for proper solution heat treatment of the 6061. Once that “solutioning” time at temperature has been completed, the alloy is then ready for quenching, the next key part of any solution HT process for 6061.
B. But … you cannot merely quench the 6061 from its brazing temperature (1090°F/590°C) since the BFM may still be molten, and still flowing, at that high temperature. Quenching any BFM while it is still liquid can result in very poor solidification of the BFM, disturbing it sufficiently to cause weak, poor-quality joints. The BFM should never be disturbed until it has fully solidified, which means the furnace temperature has to be reduced slowly and uniformly to ensure that this occurs. Once the BFM has fully and uniformly solidified, only then can quenching commence. During that very slow cool-down, it is possible that some precipitates may form in the metal matrix, and if quenching were to take place at that point, the room-temperature properties of the 6061 may not be what is needed to properly be able to bring it to a T4 or T6 condition later on. If a BAlSi-7 BFM were used, however, it may be possible to allow proper cooling of the molten BFM and still be slightly above the 985°F/530°C temperature that could ensure that everything is still fully in solution.
C. Then, for good metallurgical properties, the 6061 is supposed to be quickly quenched from that 985°F temperature all the way back down to RT, typically in a matter of seconds. That is hard to do inside a vacuum furnace unless you have a special furnace that has internal quenching capability. To merely try to open the vacuum furnace door at 985°F, then slowly take the parts out of the furnace and move them to a quench area will yield an unknown thermal condition by the time it is cooled down to room temperature, which may (or may not) be able to successfully achieve a T4 or T6 condition via further processing.
We will conclude our discussion in part 2