Someone asked to weld up a stand using some old
aluminum tread plate. I needed to machine some fittings from new 6061-T6 bar and structural tubing, then weld them to the tread plate.
The old
aluminum tread plate was oxidized on both sides, but the smooth side looked worse to me. There was no obvious pitting on either side.
One of the machined pieces attached to the smooth side of the plate. I cleaned the plate with a flap disc followed with stainless steel wire brushing of the plate and fitting. The weld was no problem at all, and the result looked great.
The other pieces were prepped in exactly the same way, but the weld to the tread (bumpy) side of the plate was nearly impossible. I used the same machine, same settings, and same filler rod. Every time I started a puddle, it would film over with black crud, and it was downhill from there. Repeated cleaning didn't improve things. I tried different settings to no avail, and even went to a larger electrode so I could use higher current. All the changes were tried on scraps of new aluminum, and I could get a nice puddle and bead on it.
I eventually got a barely acceptable weld on the work, but I was really disappointed with the way it looked. Fortunately, the fellow was happy with it.
Any idea what the problem(s) could have been? Is something done to the tread side of the plate that would interfere with welding? Both sides looked equally shiny and clean after the prep.