Tig welding aluminum question

[regal2800]

When Tig welding steel or mild steel in a T joint i get the puddle going and with the torch i can drag this puddle as long as i want and fusion weld the joint. Obviously i add filler at appropriate intervals but never the less i get this puddle and it makes for seamless addition of filler. Now for some reason when welding aluminum i can only get this puddle going 50% of the time. The other 50% of the time i can get a puddle going but it never fills in at the root of the weld and kind of keyholes (without burning through the other side). I have to add filler to close in the root. The final product looks good but i know something is wrong.

I have tried everything from higher amperage, torch angle and tungsten distance from the work piece. Can anyone give me advice on what i am doing wrong?

My coupon samples are 1/8" 6061 clean. (Cleaned with acetone and stainless steel brush.)

Machine is dyn 350. 70% balance, 110 hz freq and 150 amps.

I have added 2 pics

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