What is the thickness of the aluminum base material you are trying to weld?   The thicker you go in aluminum, the more difficulty you will find in welding with GTAW.  In the corners you will find that the heat from the arc is not as intense as on a fit up with both surfaces laying flat, as in a butt weld.  Fillet welds will react the same way.  When you get into the corners, or doing fillets, you probably will have to crank up the current.  Along with this you may have to use a larger tungsten to accomodate the increased current.  Shielding gas with some He may help you by contributing to a hotter arc.