4130 Cromoly CrMo
.283 lb/in(cubed)
tensel strength=179000psi
-dense, stiff, inexpensive, readily avaliable
-high endurance strength
-excellent fatigue strength

6061 Al
.098 lb/in(cubed)
tensel strength=43000
-no fatigue or endurance strength
-inexpensive
-loses strength during welding, requires artificial aging

3/25 Ti
.160lb/in(cubed)
tensel strength=125000psi
-highest corrosion resistance
-very high strength to weight ratio
-infinite fatigue strength
-acid etch process

Thats all I know...