Jeff,
Your "title" states Insane Bicycle Mechanic. Time to step it up and learn how to weld aluminum. That is an oxy/acetylene welded frame. Most likely is 6061 Allow. Yes, it can be heat treated, but aluminum will age harden.
I have ton books on the subject, but WWII aircraft used the same material. TIG didn't exhaust, so you so "Rosy the Riveter" also oxy acetylene welding aluminum. That frame tells a story about metal fatigue (number stress cycles repeated time over time). Aluminum is an excellent material, but that happens. I can be repaired. If you want to learn, I'd recommend taking the frame, yourself, and heading down to Northern California to (yes, of all names) the Tinman! Kent White still teaches classes. I've taken lessons from him and had a blast. I have Parkinson's, so you should see his eyes light up when I was holding a torch!
Oh, it was the HAZ that started the crack, but the change in thickness of the thick weld to the thinner tube. It's a stress riser, so the minute bending concentrated in a smaller area. A friend had an aluminum trailer, he was quite proud of it, and I commented "when you see a crack appear HERE" call me. Six years later he called and thought I had jinked him as the crack started just where I said it would. The manufacturer had double platted the side with an abrupt change in thickness. So 1/4" plate on top of another 1/4" plate. Material changing from 1/4" to 1/2" meant stress concentrated in a small area. My recommendation was that top plate taper with a double taper like a > shape. 4" high, reduce over 12". He still thinks I was a jink, nope just stayed awake in my engineering classes.