7075 (and those similar to it) are the alloys used for most ARs and I would think most handguns.
It is one of the strongest aluminum alloys and will precipitation harden and allow higher yield strengths, but also makes it hard to weld. They can be welded, but only in heavier sections and for low stress applications like molds etc. 7075 is also used a lot in airplanes. Notice how the wings and fuselage are riveted and not welded. 2024 is also a very high strength alloy, but it suffers from the same welding problems.

6061 hardens and is weldable, but will not give the yield strengths of 7075 and probably would not work for a frame unless thicker sections were used.

Interestingly some Scanadium alloys are weldable and some are not, but probably the weld will not be as strong as base metal.

Aluminum alloy is not as strong or flexible as steel and is much more apt to stress crack. Its main claim to fame is that it is about 1/3 the weight of steel. Titanium is also about 1/3 the weight of steel and nearly as strong, but. it will not take repeated flexing as well as steel. For flat out tough steel especially that alloyed with nickel is very hard to beat.

A pretty neat metal is aluminum bronze. Strong, good wear resistance, but it is heavy. Might make a WOW S&W frame.

I would love to be able to sent S&W a Damascus billet and have it forged and machine it into a frame (not the cylinder) and use a barrel liner. Be beautiful and strong. Real good Damascus barrels were not that week, but getting perfect forge welds using the process needed to make tubes was difficult. Then some cheap barrels were actually woven wire that was silver brazed.

1911 slides made from damascus look here:
damascus 1911 frame - Google Search