SMIM Showdown! Lets figure this out once and for all!

[JET]

Actually aluminum welds are not as strong as the base metal, that is true for steel though. Aluminum degrades when welded, that is why the heat treatment is important. Most shops don't have enough experience with metallurgy and things like this get overlooked. Most manifolds are made of 6061 aluminum, which has a minimum tolerance of 40k psi, but when welded the tolerance drops to 24k psi. A T6 heat treatment will bring that back up some, but to get back to the original state it must be solution heat treated.

I do agree that aluminum is a PITA to weld, it is much harder than steel or stainless. Welding inside and out usually adds enough material that even at the lower strength it is fine.