We had the same problems for years with a particular brand of wire in 1/16" and 5/64" dia, The process that puts the flux in the wire was faulty. The wire started as a flat piece of material. As it was feed thru the forming process to make it a round wire the flux was deposited onto the partially formed wire. There problem was that the flux was not being distributed evenly causing air pockets and voids after forming it into a round product.The settling is what gave us the worm hole problem. The distributor of the wire we were using found the problem after opening up a section of wire and finding small voids in the powdered flux. At that time we changed brands and had no more such problems.
This was something that was a major concern to us ,as the welding process was for over the road and off road trailer chassis with GVW weight ratings to 80,000 pounds.
PowerTig Micro 185 SOLD GREAT MACHINE
Jet 1340 Lathe
Jet 20" Drill Press
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Forward 12,000 lb 4 Post Lift