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I've started playing around with brazing pieces of extruded aluminium sections to join them together for strong joins as an alternative to bolting them.

The pieces are made of 6082-T6 aluminium alloy. I've found that the heating needed to braze softens the aluminium enormously, to the point where 3mm thick sections can be effortlessly deformed with pliers.

I've done a lot of searching and I need to either naturally or artificially age-harden it (precipitation hardening). But how long it takes to naturally harden, or the precise process needed to artificially harden it (temperature & time) escapes me. Also, quenching might help, but I haven't found any definitive answers.

How can I re-harden the alloy?

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  • $\begingroup$ Have you tried quench hardening a sample? That might tell you more than we can. $\endgroup$
    – grfrazee
    May 3 '16 at 13:15
  • $\begingroup$ @grfrazee thanks - may I ask why I am unlikely to get an answer here? Is it because it's an unusual alloy? (I thought it was a common one) $\endgroup$
    – CL22
    May 4 '16 at 5:13
  • $\begingroup$ In my experience, doing something yourself answers a lot more questions than asking them. While I'm sure someone here will be able to give you a technical answer, the answer you find yourself through experimentation will likely be more pertinent. $\endgroup$
    – grfrazee
    May 4 '16 at 12:15
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I found an article that discusses the heat treatment of 6082: http://www.journalamme.org/papers_vol32_2/3224.pdf

In the interest of having a standalone answer, here's a summary:

You'll be able to recover quite a lot of strength by aging between 130 C and 190 C. The authors recommend treating at 190 C for 6 hours.

Some overaging will occur at 190 if you let it sit too long, but the effect is minimal, even at 20 hours.

Higher temperatures can be used (e.g. 1 hour at 220 C) but the risk of overaging increases a lot. Personally, I'd avoid that unless I was pretty confident that my oven would maintain a constant temperature and not swing +/- 10 degrees around the set point.

Edit: Starrise points out solutionizing is a potential step. You'd want to get the part above 575 C for a while and then quench before aging. That would get the entire part to a nice, uniform state. The only downside is a potential for warping when you quench.

If solutionizing isn't an option, postweld aging will still buy back a bit of strength, but it won't be nearly as good as getting true T6.

If you're only able to age, Maxal actually recommends starting with T4 and then aging to T6 after welding. That'll get the smallest possible reduction in strength.

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    $\begingroup$ My answer was going to basically be "stick it in the oven to age harden it." I can see that's nicely covered. The only trick is to age it you need to solutionize first, which is typically a higher temperature than commercial ovens can (safely or reliably) go. I'd add a bit about that in your answer (paper says 575 C for 4 hours, then quench, presumably in a bath of room-temperature water) $\endgroup$
    – wwarriner
    May 4 '16 at 16:46
  • $\begingroup$ Phase diagram says 577 C so that sounds right. $\endgroup$
    – wwarriner
    May 4 '16 at 16:49
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    $\begingroup$ That's a good point, I was only thinking that the heat affected zone had lost its precipitate (had been raised above that temp). But the should also be parts that simply overaged etc. at lower temps. Edits on the way. $\endgroup$
    – Dan
    May 4 '16 at 16:55
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While brazing can be useful for ad-hoc repairs of aluminium AC TIG or MIG welding is vastly preferable for structural applications and in the long run is likely to be less hassle than re heat treating it.

The advantage of TIG welding in this context is twofold: a) you can use filler metal which is as strong or stronger than the base metal and with an alloy composition optimized for welding; b) you can keep the heat input into the metal to a minimum as the arc is a much higher temperature than a brazing flame and much more concentrated so the heat goes into he weld pool only and not the bulk material. Also TIG welds are cleaned by the arc itself rather than requiring fluxing, which itself requires more heat put into the base metal.

From personal experience you can TIG weld aluminum extrusions with little noticeable loss of tensile strength, this is in the application of structural racking for commercial vehicles. I also have expedience of annealing and cold working extruded aluminium for similar applications.

Obviously your mileage may vary depending on application but I would certainly recommend looking seriously at TIG welding for this sort of application

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  • $\begingroup$ Why is it preferable? I don't see the difference - they both heat the base materials and join using a filler material? Except brazing doesn't need a protective atmosphere like argon, simply flux - I'm not saying you're wrong at all of course... it's just a good-to-know $\endgroup$
    – CL22
    May 4 '16 at 22:21
  • $\begingroup$ Fair question...see edit to my answer for details $\endgroup$ May 4 '16 at 22:26

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