Per unit of, say, tinsel strength, what's the relative cost of stainless steel verses aluminum?
Say you wanted a rust-resistant bar to withstand the pull of 1,000 kilograms. If the SS costs $10, but how much would the Al cost? (The weight of the bar and other factors are not important. Assume common building-material alloys of each, and assume some commonly available stock)
Is the "pull" pure tension, or bending?
- Nehmo -
- Ian Stirling -
- Nehmo - I meant tension as a basis for comparison. I don't have a immediate practical application.
It depends. SS can be cheaper, or Al can be cheaper. Temperature, vibration, load cycling, ... all come into it. Steels tend to be elastic and suffer no damage from strains below a certain load. Aluminium tends to eventually fatigue and crack, so more attention has to be paid to type of service.
Taking 304 SS and 6061-T6 aluminum as common alloys, the price per pound of 304 is typically 1.5X the price of 6061. The specific gravity of SS is also about 3X that of aluminum, so unless the SS has at least 4.5X the yield strength, the aluminum will be more economical.
The yield strength of 304 SS can be anywhere from 30,000 psi to 150,000 psi, depending on how much it's been cold worked. Yield for 6061-T6 is 40,000 psi, so it would seem that the aluminum is a more economical choice.
Welding will affect the mechanical properties of both materials, and as Ian pointed out, there may be other reasons to rule out or derate one material or the other.
Ned Simmons
- Ned Simmons -
- Nehmo - I've been wondering why SS isn't used more in building construction. Al is used frequently as a roofing material, but seldom is SS. (There is a SS shingle roof on the Kemper Museum here in Kansas City
- Ned Simmons -
- Nehmo - I now see some Al fences sprouting up, but I've never seen a SS one.
- Ned Simmons -
- Nehmo - So what's the comparative difference? Does Al weld better than SS?
1) It's more expensive as a base metal. 2) It's not available in large structural sections. 3) You can't cut it effectively with a gas torch - either plasma or mechanical only. 4) Welding isn't much harder, but different.
AL is harder to weld. Al is also more flexible per unit strength than steel (take the ratio of E to Fy). Many designs are governed by serviceability - deflection, sway, etc. The extra stiffness of steel helps.
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