Alloy selection matters
Common structural and sheet alloys differ significantly in weldability, formability and strength after welding. If your design is not locked, ask fabricators for alloy suggestions — this is one of the easiest places to reduce cost without redesigning the part.
Distortion control
Aluminum weldments distort more readily than steel. Experienced shops plan fixture strategy, weld sequence and post-weld straightening before they quote. Ask how they intend to hold your critical dimensions.