Some engineering thoughts on the subject.... for what it’s worth!
A 80 Steel is significantly shorter than a 80 Aluminum. The additional volume of the 80 Aluminum over the 80 Stell is all aluminum, located in the tank wall and both ends (heads). This extra aluminum is structurally required, so an aluminum tank would have comparable strength & rupture/crack resistant to steel tanks.
The design of an aluminum scuba tank and managing tank rupture/crack resistance was likely a real challenge for pressure vessel design engineers. For example, the engineers had to compensate for the inevitable nicks in the side of an aluminum tank which would create stress risers, as a deep and/or sharpe nick would reduce the structural integrity of an aluminum tank. (That’s the reason there is a nick specification for aluminum tank annual inspections.) The extra mass of aluminum in aluminum tanks is there for good reason, otherwise aluminum tanks would be lighter..... thus the flat bottom.
The perfect shape for pressure vessel is a sphere. A cylindrical shape vessel with spheres on the end, like a scuba tank, is second best and inherently the optimum shape for the steel tank. Adding a dimpled concave bottom on a steel tank is structurally superior to a flat bottom, but doable, as we often see in industrial pressure vessel applications. If I was to guess, I’d say a spherical head tank would weigh less than a dimpled concave bottom tank.
(This is all based on equivalent pressure ratings of coarse.)
