Yeah it's stumping me too.  Nickel based alloys like Inconel should be a walk in the park you should be doing something very wrong in order to get porosity.  Any audit I perform everything looks good, the shielding gas is clean to 4 ppm, welds are coming out bright silver or straw at worst, titanium welds are bright silver.

They flap wheel grind 2" on each side, followed by scotch bright then acetone wipe with lint free.  Welding rod is acetone wiped and put in on clean craft paper.  The shop is dirty but no cutting fluid or fluid within this building. 

All 100% argon thin wall 0.020-0.120"  Seems to affect the thicker welds more often, always caught in X-ray. About 10% of parts and when it happens it's bad.   I'm going to try welding pieces for a few weeks in an atmospheric chamber just to rule out bad shielding but I would have thought to see that if the welds were something other than bright silver.  Only other guess is that it's actual entrapped pockets of air under tack welds or some other internal pore.  The cleaning procedures appear to be quite good.

I suspect another culprit as well.

Inconel forms a passivating oxide layer when heated.  Are the parts preheated or go through any "work hardening" type processes before welding?

If the parts are not preheated, is it possible to very lightly preheat the parts before welding to drive off any moisture?

Thanks I look into this more. D17.1 doesn't mention anything.