Yeah,
Nice early effort.
I mean, I've only started doing AC TIG on aluminum about a year and a half ago, and that only includes a handful of small projects involving, maybe, several hours of AC arc time. But I previously did a lot more DC TIG on a more basic unit, using no foot pedal at all, since one wasn't included and it wasn't so critical to use one on mild and stainless steel, I think.
Your coupon, even though it's a decent size, shows how quickly the heat in aluminum will saturate a piece like that and bonce back at you, so-to-speak, i.e., the bead profile starts out nice, maybe even too convex, but then flattens, widens, and virtually drops out at the end.
Your torch and filler technique looks good. Maybe just watching the puddle a little closer, starting off a little hotter and slower, backing off the heat a little sooner, moving a little faster a little sooner, or using a little bigger rod as filler to provide some quenching action. But, I'd probably want to use 1/16" filler on that and just have a nice, narrow bead with decent penetration. So, you may want to show us the underside of that coupon, to identify the kind of penetration profile you're going for too.
Otherwise, I'd undoubtedly want to crank up th AC frequency a bit, maybe use a Triangle wave, and just move faster. Or, maybe even stop midway, let the piece cool for 30 seconds, then pick back up on the exact same spot, which will be virtually undectable (it looks like you may have tried that, for how long?). Or, stop midway, let it cool, and restart at the other bitter end and meet in the middle, to avoid the potential dropout on the heat saturated edge. Pulse is another option in the heat management TIG tool box, but I've honestly not dabbled with pulse much at all yet.
Looks like you have a good handle on it...shouldn't take you too long. You need to be good on the pedal and a heat sink for your coupon will really help you.
