Ally welding is a pretty skilled process, as BoB says, you don't get any warning when it's going to melt, it just goes, so you need to know how much heat you're putting in or you'll either melt holes in it or just blob metal on top which will be useless. Another complicating factor is that aluminium oxidises very readily so if you don't weld it in an inert gas atmosphere (Argon is the usual gas shield) you stand a good chance of ending up with a spattered lump of alumina instead of a weld. Finally there's the Heat Affected Zone (HAZ) that develops where the recrystallised grain structure meets the original grain and that's where it tends to break, adjacent to the weld rather than through it. I'm not a welding expert but I believe if you preheat the weld zone or post heat treat the entire component you can avoid a lot of the HAZ problems - but again it's a question of knowing how hot and how long.
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