My dad has that exact same machine and torch.
Tungsten turning colors while welding is not enough shielding gas. The electrode (tungsten) should stay shiny in its envelope of pure argon. You should have about 15 to 30 CFH of gas to weld. Closer to 30 for aluminum.
The old practice was to use pure tungsten for alumunim on an AC sine wave machine (you have) and other blends on an inverter welder.
With pure tungsten (green paint on end) the ball will form on its own or you can make an arc on DC+ for a second or so to form it.
Its OK to use the thorated you have. It may form a ball or not. It may grow all kinds of things off the end so it needs re sharpening more frequently. If dunked (which I do frequently) stop welding right away and replace or re sharpen the electrode.
I cut the new electrodes in half and then get a short cap for the torch. It makes things easier than waving around the long back cap.
70 amps aint jack for aluminum. I think what you are doing is heating up the whole cupon then welding it. I have a tough time on 1/4" aluminum with 200 amps. Its not quite enough unless I switch to helium.
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