harbor freight tig torch

If that's the torch I'm thinking about, it is only a 90 amp torch. It does have a gas valve, but it has mini DINSE connectors. Those connectors are what their little inverter uses, but wouldn't fit your Hobart. You can get adapters for that, though.

The major problems here are that aluminum is best welded with an AC power source, and that requires continuous HF. Your Hobart doesn't have HF. You could buy an HF adapter to go between the welder and the torch, but those aren't cheap (Miller wants $795 for theirs).

For DC welding aluminum (which is possible, though not recommened), a 90 amp torch is too small for all but the thinnest work. You really want a hefty water cooled torch for DC welding aluminum. (You have to DC weld aluminum electrode positive, and that throws 70% of the heat into the tungsten instead of into the work, so the torch gets very hot.)

You'd have to scratch start, and that'll contaminate the tungsten on aluminum. A copper strike block could be used, but that's one more hassle.

The lack of a foot pedal to vary the current as you weld, and the lack of HF, will be other drawbacks to using the Hobart as a power source. Those are tolerable when welding steel, but would be serious problems when trying to weld aluminum.

I'd recommend a Miller 180SD or a Lincoln SW175 as the minimum machines for welding aluminum. They have all the features you need. Street prices on either run around $1500. Either one would also replace your Hobart as a stick machine. I have the 180SD. It is a sweet machine.

There are also some inverter machines which have a form of HF, and can do squarewave AC. They're more expensive, but add other features such as pulsing and sequencing.

Gary