Brush up on your Al welding, yep that is the key, wire brushing the oxide off befroe welding.
Before welding is cleaning. Remove all trace of any possible oils with acetone, then remove surface oxide with a clean stainless steel wire brush. Manual wire brushing takes some elbow grease, and light power brushing may be ok, but heavy agressive power brushing can form oxide from the excessive friction heating.
For balance, you want the majority of the cycle on DCEN for good penetration, and just enough DCEP for a narrow band of cleaning along the weld toes.
High frequency adjustment is not important. If the AC arc is stable during welding, the HF is doing it's job. Without HF, a conventional power supply cannot sustain an arc as AC cycles from DCEN to DCEP and passes through zero amps.
Some things to think about.
Aluminum transfers heat so rapidly, that you need to hit it hard with high current, get some melt on both pieces, jab some filler in to get a puddle, and go, go, go. If you piddle around in forming a puddle, a bunch of heat just spreads out though the workpiece, and when you finally get a melt on the joint edges, they ball up away from each other, and leave you with a big hole.
A tight arc and a torch angle close to perpendicular help to deliver the maximum concentration of the arc power to the workpiece. Excessive arc length or torch angle reduce the efficiency of arc energy transfer and can cause loss of shield gas coverage.
Keeping the filler within the torch shield gas envelope, that is not pulling the filler back too much during the dip/retract motion, will help reduce oxidation of the filler and introduction of that oxide into the puddle.
