Hi Ryan !
For good results, boring holes are typically limited to about 1.5-2x boring bar diameter.

As such, a 3/8 boring bar is only good to about 1/2 inches.
After this it will chatter with variable results.

All lathe tools work well only upto about 1-3x their size.
Ie a 1/2 inch tool will only work well to 1-1.5 inches deep.

However..
Alu is easy.

Your problem is rake.
CCMT is 7 degrees positive and you need to maintain this, *especially* when doing deep bores.
With a -5 degrees boring bar, you are losing the benefits of the insert, and greatly increasing the problems of the overly long, slim, thin bar.

It *can* be done with a 3/8 in alu but not well, imo.
You need to have at least 7 degrees rake, even? a bit more.

If you can make (buy) another bar (any steel) of near .7 inches, and have the bit be at zero-1 degree rake, it will go vastly better.
For alu, at .75 inches or about 18 mm, I would recommend about 1000 rpm, 0.1 - 0.06 mm cuts.

-- 
-hanermo