In 1991 I was sudying for my CWI and working at an aerospace company that welded almost nothing but ducting for various aircraft/aerospace applications (Boeing). We welded a lot of Inco 718 as well, but most of the welding related problems were IJP or tungsten inclusions. Porosity was never a major problem to my knowledge.
Some situations actually required the electrode to extend well beyond the nozzle's end. There was no other way to obtain acces to the joint. All of the purge's, dams and trailing cups were home made. But the parts we welded were tested by radiography daily, so we had to be on our toes. I am also curious about porosity just being at the surface, have any of these parts been X-rayed (if the part geometry will lend itself to this method) to confirm if the discontinuities are only present on the surface? The cleaning materials may have something to do with your problem...
I did the same welds for another company in the 80's using simple welding power sources (no pulsing, squarewave, etc.) and porosity was not a problem here either, only the discontinuities mentioned above. If you have more info it might help to solve your problem. There are also some very experienced TIG welders that use this forum as well, maybe they can help.
Recommended shielding gas is argon for thin parts and manual welding; helium, or mixture of helium and argon for thicker parts. Additions of O2, CO2, and N2 can cause porosity. Small quantities of H2 (about 5%) may be added to argon for single-pass welds. The H2 addition produces a hotter arc and a smoother bead surface in single-pass welds. However, H2 may cause porosity in multiple-pass welds with some alloys.
And adequate quantity of shielding gas must be delivered to the weld zone at all times during welding, and it must be delivered with very little turbulences. For your job 4 or 5 L/min with gas lens nozzle is ideal. Gas nozzle diameter should be 4 times of tungsten electrode diameter to deliver shielding gas to weld zone at low velocity to avoid turbulence. Too large gas lens causes porosity.
The welding torch should be held with the work at 0 degrees and the travel angle of nearly 0 degrees. If the drag angle is more than 35 degrees, air may be drawn into the shielding gas and cause porosity.
The electrode extension beyond the gas nozzle should be short but appropriate for the joint design. For example 5mm maximum is used for butt joints in thin base metal, whereas up to 13mm may be required for some fillet welds.
The shortest possible arc length must be maintained when filler metal is not added. The arc length should not exceed 1mm; 0.5mm is preferable. Excessive arc length causes porosity.
Best regards
We have been doing some reading in the handbooks and we now believe that the cleaning of the material probably is a major contributor to the surface porosity. We are not required to xray, Class B parts only need pressure test and FPI. Appreciate the input.
Alan
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