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Old February 26th, 2011, 05:35 PM   #24
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1018 = Tensile Strength, Yield 380 MPa 55100 psi Steel
6061 t651 = Tensile Strength, 324 MPa, 47000 psi alu
7075 t651 = Tensile Strength, 572 MPa, 83000 psi alum

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While reading a couple of threads here and elsewhere recently the comments were made about a good hard grade of aluminum being a suitable substitute for mild steel. The aluminum mentioned specifically in one post was 6061-T6.

I've made more than a few parts out of 1018 CRS and am now wondering if the 6061 I have in stock would indeed be suitable. Some of the parts I make are tool posts and holders for small benchtop lathes.

AL 6061-T6(trash) and most low carbon merchant steels (C1015-C1020) will have very comparable yield and ultimate tensile strengths. If that is all you need to worry about, then weight and cost will be your deciding factors. However, steel has (roughly) 1X the tensile, shear, and tangent modulus of AL 6061-T6(trash) aluminum. That means that, for any given cross-section of the part, steel with be 1X more rigid than aluminum. More importantly (at least from my knothole as a design and development engineer), steel will "fight deformation" 1X harder than aluminum when you cross over the yield point as it stretches towards ultimate failure.

Additionally, steel will have a significantly greater shear stress capability than aluminum. This means that you get nearly 2X as much carrying capacity in a given thread engagement in steel as you do aluminum. This is the difference between a body-centered cubic crystal structure (LC steel) and a close packed hexagonal structure (aluminum). Anodizing aluminum will bring their properties in this regard closer (about a 2.2:1 advantage to steel). Unscaled merchant steel will have (about) 2X the (resistance to penetration) surface hardness of (6061-T6(trash)) non-anodized aluminum.

Although most people consider this counter-intuitive, aluminum will wear out (unhardened) steel in sliding wear. Bare aluminum oxidizes quite thoroughly. Aluminum oxide is one of the very common abrasives used. Anodizing will reduce this somewhat, but only somewhat. In most instances, a sulfuric anodize is really as good as a "hard (chromic) anodize." The hard chomic anodize will flake off with impacts whereas the sulfuric anodize will deform.

Weight is the final factor. 6061-T6(trash) aluminum has a density of .098 lb/in³. Most low carbon steels have a density in the .282 to .285 lb/in³ range.

My designation "T6(trash)" has to do with process specific treatments of aluminum. When it is solution heat treated to the T6 condition, most of your material properties are fully defined there. When given a stress-relief cycle after treating, it becomes "T651" temper. If you do a straightening process thereafter, it becomes a "T6511" temper. If you start with a "T6" bar or plate, it is generally worthwhile to heat it in an oven to (about) 450°F (20 minutes + 10 minutes/inch of thickness) and let it cool in the oven. Let it sit for at least 3 days before machining to allow the temper to fully reassert itself. You should not have to do this with either "T651" or "T6511" tempers unless you are trying to hold a parallelism value less than .0015 in/6 inches of length or width.

let me hardcoat anodize one for u or regular anodize.......cost would be on the house just to see how theyd turn out...(first 1) lol hell we can even lazer etch ur logo name or part# into it... pm if interested

plus its the only materialwe can hardcoat.......f.y.I Hardcoat anodize chips carbide endmills lol

just a thought

Last edited by JoshuaCripe; February 26th, 2011 at 05:45 PM.
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