Aluminum is about half the weight of steel and the more common 6061-t6 is about half the strength. After welding the aluminum frame it would have to be reheat treated. Aluminum also has the bad habit of fatigue cracking after a few years of use. It can be done but has draw backs you should be aware of.
Like Bill said, fatigue in aluminum is cumulative over time. So the design needs to identify areas of stress and increase strength in those areas. Still, if built right, should last 30+ years. Gonna need a pretty large heat treat oven. 900 degrees for 10 hours and a quench should do it.
As opposed to steel which can be stressed over and over and as long as it is not pushed past its molecular elasticity, will last forever (figuratively).
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