Also note welding aluminum is more difficult than steel, and 7075 can't be welded at all. Welding will reduce the strength of aluminum in the heat affected zone significantly. 6065 can be heat treated after welding to restore its strength, but you'd have to find an oven big enough to fit your whole weldment. It's easier to just design the part with a large safety margin, but doing so will eat into the weight advantage of aluminum.
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