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Old 06-12-2010, 03:01 PM   #1
Twmaster
 
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Default Hard aluminum Vs. mild steel?

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.

Some illumination would be great.

Thanks!



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Old 06-12-2010, 04:26 PM   #2
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Default Re: Hard aluminum Vs. mild steel?

Quote:
Originally Posted by Twmaster
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(xxx) 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) 3X the tensile, shear, and tangent modulus of AL 6061-T6(xxx) aluminum. That means that, for any given cross-section of the part, steel with be 3X more rigid than aluminum. More importantly (at least from my knothole as a design and development engineer), steel will "fight deformation" 3X 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 5X 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) 4X the (resistance to penetration) surface hardness of (6061-T6(xxx)) 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(xxx) 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(xxx)" 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.

Does this help?


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Old 06-12-2010, 06:43 PM   #3
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Default Re: Hard aluminum Vs. mild steel?

Quote:
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.
TW:
A2ZCNC http://www.a2zcnc.com/machinetools.asp produces commercial tool posts and holders for small bench lathes. They are a scaled down version of the Aloris. I copied the specs below . As you can see they use 6061 -T6. I have one of there tool holder sets and love it . I have made aluminum tool holders for it and plan on making more holders and tool posts like it, Out of aluminum . In my humble opinion use what you have and move forward . It is a proven design and a proven material. Anodizing would be a nice addition but not mandatory.
Tin
Quote:
# Quick Change Tool Post Two dovetails so you can switch from turning to boring/facing without any tools or changes to your setup
# QCTP machined from billet 6061 T6 Aluminum
# Hard Anodized for durability and appearance
# No tools required to change bits
# Versions to fit Sherline, Taig, Unimat, Atlas and 7x10-12, 9x20 and most small lathes
# Additional tool holders available
# Inline handle for added strength
# Optional 1/2" boring holder available

Additional Holders & Accessories:
Standard stick tool holder
3/8" boring bar holder
1/2" boring bar holder
1" x 1" silver blank holder
cut off blade holder
USA made .040 x 1/2" cut off blade
See a review at www.Mini-Lathe.com
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Old 06-13-2010, 12:29 AM   #4
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Default Re: Hard aluminum Vs. mild steel?

In my opinion - 6061 is rather soft - I would like to build a toolpost out of 7075.

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Old 06-13-2010, 01:02 AM   #5
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Default Re: Hard aluminum Vs. mild steel?

I beg to differ 1018 shear strength is not 3x 6061 t651

1018 = Tensile Strength, Yield 380 MPa 55100 psi
6061 t651 = Tensile Strength, 324 MPa, 47000 psi
7075 t651 = Tensile Strength, 572 MPa, 83000 psi

3X ?

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Old 06-13-2010, 02:20 AM   #6
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Default Re: Hard aluminum Vs. mild steel?

Mike, have a look at the Speedy Metals website. They have material properties charts for most
everything they sell.
For instance, 6061/T651 is shown at 45k (psi) tensile and 40k yield.
1018 CRS shows just about double on both those numbers.
7075 shows numbers about equivalent to 1018, and has a similar Brinell hardness. That last
part may make a difference to you if surface deformations are an issue over much softer 6011
when choosing an aluminum alloy over steel for something like a tool post.

I know you have a market in mind for some of the things you make. Material may be a consideration
purely from a marketing viewpoint, all other things being equal, including end price. For instance,
I would buy a tool post made of steel before I would buy one made from aluminum. Even if the
seller put in goonball kickers like "aircraft aluminum" or the common "billet" misnomer.
That's just me, of course.

Maybe trying a poll here would give you some good info from an actual market group. People
here buy this kind of product. Asking "Would you rather have product X in steel or aluminum?"
may give you some insight for the buyer preference.

I understand you want to use stock you have on hand. That makes sense. It would be prudent
to give it a good, hard workout if it's something like a tool post. Threaded holes in it are going to
see a lot of use, and tool slots take a lot of pressure.

Dean

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Old 06-13-2010, 06:02 AM   #7
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Default Re: Hard aluminum Vs. mild steel?

I worked with aluminum aircraft for most of my adult life and have to say there are good things and bad things about 6061-T6. Very seldom will it be used for a structural part on an aircraft unless it must be welded or is used to carry fluids. It is not used for structural elements. There are many other alloys far superior. That said, I have been using quite a bit of it for my projects lately although nothing I do is industrial or life sustaining. All metals have good and bad properties and the best bet is to research and see which is the best match for the application at hand. You want large and light? Use aluminum. Building a steam locomotive? Steel would probably be my choice but you can bet every pound saved by using aluminum in non-critical areas would go a long way towards getting a ton of freight 450 miles on a gallon of fuel. There is a reason aluminum is one of the most used metals on earth (and not just because of beer cans).

Bottom line, use what meets the needs of the design. My 2 cents.

Steve C.

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Old 06-13-2010, 05:13 PM   #8
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Default Re: Hard aluminum Vs. mild steel?

Quote:
Originally Posted by doc1955
I beg to differ 1018 shear strength is not 3x 6061 t651

1018 = Tensile Strength, Yield 380 MPa 55100 psi
6061 t651 = Tensile Strength, 324 MPa, 47000 psi
7075 t651 = Tensile Strength, 572 MPa, 83000 psi

3X ?

Doc, you are citing typical Tensile Yield Stresses (engineers must design to minimums), not Allowable Shear Stresses. Aluminum has a more pronounced "grain" than steel and is more sensitive to cross-grain loads. This is especially true when it comes to inter-crystalline loading known as "shear." The question was posed as to the relative functionality of low carbon steel versus 6061-T6 aluminum. 7075-T6(xxx) and 2024-T4(xxx) aluminums have a modified close-packed hexagonal crystal structure that is closer to the body centered cubic structure of steel and, hence, have higher relative shear strength values.

Aerospace engineering manuals have good information, but the presentation assumes a rather broad background in metallurgy. The old American Society for Tooling and Manufacturing Engineering (ASTME -- the forerunner of the Society for Manufacturing Engineering -- SME) had really good explanations as to the different types of stress in their Manual of Blanking and Forming Dies. Finding a copy is hard (try used engineering book stores), but definitely worth the effort if this interests you.

One of my engineering professors used to say, "You need to go back to the beginning of a technology before the priesthood was established as that was the time when people were communicating information rather than proving why their need to be priests." This is why the old texts tend to be so good.
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Old 06-14-2010, 12:36 AM   #9
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Default Re: Hard aluminum Vs. mild steel?

Lew: while your dissertation here is informative lets not forget the original question can 6061 T-6 be used successfully for tool holders for a small bench lathe. The answer is yes. will other material be stronger and last longer again yes.
Here are a few I made the factory made one is in the middle.

TW not building a space shuttle nor is he attempting to launch himself in a rocket from his Texas ranch. He just wants make a few tool holders for a hobby lathe.
The old Chinese proverb comes to mind " Those who say it can not be done please do not disturb those who are doing it.

Tin

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Old 06-14-2010, 01:17 AM   #10
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Default Re: Hard aluminum Vs. mild steel?

Quote:
Originally Posted by Tin Falcon

The old Chinese proverb comes to mind " Those who say it can not be done please do not disturb those who are doing it.
I like this and have to second it!


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