I'm a welder too , been doing it on pipelines, nuclear stations, oil rigs and refineries for decades, 1975 to be exact.
I repaired a right side cover last year , tig welded it using an AC plant, it has to be done like that to have half a chance of a good job, now the problems I had were I couldn't find a grade of aluminium wire that matched the casing.
It's really expensive to get spectroanalysis done on it to determine exactly what the grade is so I did a small bit with my first choice of aluminium wire and it cracked, again and again, I dug the back out and used my second selection and it cracked and had porosity all through it, which looks like tiny bubbles on the surface if you don't know what that is, so I dug it all out again and cleaned the metal right back then used a wire that is very difficult to work with which is why it was my third choice, now that did not crack but again it had porosity in abundance but I persevered and got it reasonable looking , but when I had a guy polish the casing up properly the new welded area is so much brighter than the existing casing that it stands out like a sore thumb, it would be ok to use if painting the casings were an option or it could be chromed to look the same all over, but then the other side would need done to match.
I have welded casings before and did a lot of pipe and plate aluminium so I generally know what i'm doing but that casing defied technique and perseverance and my conclusion is it's a very low grade aluminium casting with built in defects and if you can get away with not welding one that would be best, however that's not to say somebody might not get lucky and just have either a decent casting or have a matching filler wire.
I also agree with everything Bikenstein has written , he obviously knows my trade well.
