I used to be pretty handy with a welder myself. Personally, I think I'd leave it the way it is.
But if you choose to have it fixed, here's the way I would do it if I were the welder (this is only my semi-experienced opinion):
I wouldn't build it up. I just have a funny feeling about dumping that much heat and material in a casing like that. Things move in funny ways when you add enough heat to melt the metal. Instead, I would mill that area as little as possible, but enough to make it flat. But when doing this, it's important not to make a sharp edge in the corner as this creates a spot that can crack easy under stress. Then I'd machine up the mating part that will end up welded in that spot. I would give both the mating part, and the case a good 45o edge so that when they mate they form a "V". This allows the weld to fill it in nicely and gives it more strength. Once I'm happy with the way it fits, it's welding time. I would use a TIG welder. And for aluminum, I've always had the welder set on AC (as opposed to DC). Aluminum is more difficult to weld than steel because it conducts heat away much faster. Another complication is that cast metals are porous and tend to "spit" and "pop" when melted. To minimize this "spitting," I'd probably lay the weld down in small strips and build it up that way.
You can imagine, what I just described might cost a good chunk of money. Machining the casing and matching part takes time.
It would be cheaper to just build it up with weld. And I suppose I could do that too, but it would involve adding material in small strips little by little. I would probably try a test run just to see how it goes. But then again, I'd be welding on your case with no guarantees for that test spot.
I have never welded something like that before. I think a really important question to ask the welder is if they have. I would find someone that has actually welded something like that before.
Anyway, I hope that might help you when talking to the guy that welds it.
