I have made an aluminum frame in the past and got around heat treating by using large gussets where all frame members joined. It was over built and worked.

I posted pictures of my super heavy steel prototype delta trike in the recumbent forum.

Is there a safe way to low temperature weld aluminum so no heat treating is needed? What about the welding sticks that can be used with propane or butane torches.?

I need to make a lighter frame and have started one out of wood. I would go to aluminum instead if people think it's possible.

Any alternatives to expensive heat treating? Thanks Alan

5000 series does not need heat treat. Schwinn made some nice aluminum bikes out of it in the '80s.

I had thought that the 7000 series (like 7005) was formulated for this purpose, when compared to 6000 series, to eliminate the need for ovens. It age hardens after a weld is done. Andy.

Andy is right about the special 7000 series alloys. Or, make yourself an oven. Guys do that for powder coating, so why not for heat treat?

I have no experience with heat treating aluminum bike frames, but I have heard it's very difficult. I heard rumors of Klein (or was it Cannondale) who had batches of ruined frames early in their learning curve, because the alloy is so soft during the first step that the tubes sagged just from gravity. Rear trianles came out at the wrong width etc. So the frames need to be held in a jig that supports all the critical dimensions, and thus there's that much more metal (frame plus jig) to heat up. Required temperatures are high, so expect a hefty electrical bill. An oven big enough for a trike frame will be ungodly expensive.

Like I say this is just some poorly-remembered gossip, so maybe someone who actually knows something will chime in here. I do know that to bring 6061 to T6 requires solution heat treatment, quenching, and artificial aging. The first step requires nearly 1000°F, so you're probably not going to get that in a pizza oven. Then the quenching requires some quench medium, such as water, because air-cooling isn't fast enough, you will not achieve T6. Quenching should happen within 10 seconds of taking it out of the thousand degree oven. Do you really want to deal with that?

I did work in a small factory (Alpenlite) that made 6061 frames, decades ago, but they were sent out for HT, to an industrial heat-treater. Maybe look into that for your frame. Those were backpack frames not bikes, but same deal. The cheap frames were sent out without HT, just overbuilt enough to be adequate strength without it, but they had an Expedition grade frame that got HT to T6.

Welds in 6061 will naturally age to T4 eventually, but I don't know how long that takes. Hours? Months?
Yield strength of T4 is less than half that of T6.
So over-designing the frame to where T4 is strong enough might be acceptable if you don't need the lightest weight, and you aren't in a hurry to ride it. Gussets don't help if they're fully welded, because the HAZ will extend past the gusset. Maybe a gusset or doubler that extends past the weld, to support the tube beyond the HAZ?

I also heard that the 7000 alloy used in bike frames is a marketing term, it is not really in the 7000 family by the engineering definition. (AISI?) 7075 is the best-known 7000 series alloy, famous for being the strongest Al alloy there is (or right up there), so they chose the marketing term 7000 to kinda sound like that. I trust that someone who knows will correct me on that if I heard it wrong.