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So Larry, you take issue with the possibility that the material in question doesn't need to be cleaned prior to welding? Either I'm going blind real fast here or hallucinating because after all, I do see what I believe is "magic marker" lines right about where the toes of the weld are located, and they seem to flow right into the "cathodic etching" shown adjacent to the toes of the weld. I also see some small lines of either some sort of contamination and/or corrosion of some type going on however small in size on the surface of the base material, perpendicular to the axis of the weld. Finally, I see a bit of surface porosity towards the toes of the weld also. Am I losing my eyesight here folks, or am I on to something here? :) :) ;) Oh I almost forgot... What is that stuff oozing from the bottom toe of the weld located close to the end, just before the crater shown in the first photo?
One half of an inch thick 6061-T6 tells me right off the bat so to speak, that the oxide surface layer is going to be thicker, deeper into the material than with thinner Al of the same grade... Take the time to clean that area better and I believe you'll see some improvement. BTW, what is the gas flow rate? You might want to lower it just a little in order for the gas to stay within the corner instead of bouncing off the surfaces - out of the corner you're describing - so to speak because the flow rate is too high.
The suggestion of using a zirconiated tungsten was just that... A third alternative! I believe we were just covering most, if not all of the bases with our suggestions. :)
Respectfully,
Henry