Can aluminium be heat treated twice?

It depends on what aluminium the frame is made of. Each type has its own alloys and heat treatment, or temper. In my previous life as an aircraft mechanic I did lots of stuff with aluminium and somewhere in my study books I have lists of the types of alu and the treatment processes. I’m buggered if I can recall it all now, but for example the most common alu alloy is 6061 T6 It’s a 6000 series aluminium alloy, with alloys like copper, magnesium and zinc. T6 is the heat treatment. I can’t recall offhand the details of the treatment involved but it’s basically heating and then cooling at a specific rate. Ageing then completes the treatment. You can repeat the process after more welding or repairs. The point with 6061 is that is that you can re-weld it, but the whole piece needs to be heat treated again to regain full strength - a structure can lose about 45% of its strength when its heated, so it must be re-treated to T6 spec.

There are other grades of aluminium alloy which need other T heat treatments, but some of them can’t be repaired by welding again. I must dig my books out again and have a read.

Steel alloys are also heat treated, too. I remember that dear old Reynolds 531 and the later 753 were both identical in composition (manganese-molybdenum steel) but given a different temper. 753 was given a harder temper and so the tubing could be made a lot thinner than 531 and so it was lighter. It was a bastard to braze, though.