If it is 6061-T6, you're gonna play hell. You'll need a helium-argon mix and then a way to preheat the aluminum prior to welding. Otherwise you'll get mega porosity.

Show us a picture of your torch so we can see cup size and how much your tungsten is extended. Also tell us what your flow rate is. I'll always use a gas lens when it's an option. I'm sure you just need some fine tuning.