Hard aluminum Vs. mild steel?

Discussion in 'Metals' started by Twmaster, Jun 12, 2010.

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  1. Jun 12, 2010 #1

    Twmaster

    Twmaster

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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.

    Some illumination would be great.

    Thanks!
     
  2. Jun 12, 2010 #2
    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?
     
  3. Jun 12, 2010 #3

    Tin Falcon

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    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
     
  4. Jun 13, 2010 #4

    kf2qd

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    In my opinion - 6061 is rather soft - I would like to build a toolpost out of 7075.
     
  5. Jun 13, 2010 #5

    doc1955

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    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 ?

     
  6. Jun 13, 2010 #6

    Deanofid

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    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
     
  7. Jun 13, 2010 #7

    walnotr

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    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.
     
  8. Jun 13, 2010 #8
    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.
     
  9. Jun 14, 2010 #9

    Tin Falcon

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    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.
    [​IMG]
    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
     
  10. Jun 14, 2010 #10

    doc1955

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    I like this and have to second it!
     
  11. Jun 14, 2010 #11

    steamer

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    7075 T6 is what I made the con rods in my engine out of and it has held up fine.

    I would think for home shop, any of the solution heat treated aluminums ( The "T" portion of the number) would work fine in a toolholder application....not so much from the strength perspective which is irrelevent here. But from the wear perspective.

    A toolpost will fail from being not rigid enough to give a good finish, long before EVER reaching its tensile or yield strength. What may defeat these parts, long term, is wear. In industry....absolutely, they'll wear out in short order.....in the home shop...not so much.....tools YOU make tend to get pampered more than others.....and the lathes that are in use are low power compared to industry.

    Have at it...post pictures!

    Dave
     
  12. Jun 14, 2010 #12
    Tin,

    If you go back to my original posting, my statement was that, if ultimate and yield tensile strengths are your only criteria, then cost and weight are the factors determining which you use. However, aluminum and steel are not equal in all areas and there are real and valid differences between them. It is also important to consider the finish you are going to give them. It is important that those in the "trade" know more than merely the ultimate and yield tensile strengths (most of my metallurgy was learned as an apprentice machinist).

    My "database entries" for metals include: minimum allowable ultimate and yield tensile stress, minimum allowable shear stress, tensile modulus, bulk modulus, shear modulus, tangent or secant modulus, elongation, impact strength, fatigue factors, coefficient of thermal expansion, density, and machinability. While the tensile or secant modulus rarely impact "home shop" types, they should be understood by all who work in the "trade." Knowledge and skill are what set us apart.
     
  13. Jun 14, 2010 #13

    Cedge

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    Hey Lew....
    I've been watching your comments to various threads and I'm curious. Do you now or have you ever worked for Microsoft's tech support? I ask because in each post you've furnished highly technical and excruciatingly correct information which contributes almost nothing to the original poster's questions. While expertise is to be valued, I've yet to see anything you've made with all that knowledge.

    This forum is of, by and for amateur hobby machinists who build small marvels with limited abilities, knowledge and unlimited determination. We most often get our mystery metals from scrap yards and lucky finds. In short, we work with what we have and do minor miracles with it. Show us your bona fides in some real work before you begin lording your superior knowledge over us lesser mortals. Whats in YOUR shop?....eh?

    Steve
     
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  14. Jun 14, 2010 #14

    doc1955

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    I can't help but second this!
     
  15. Jun 14, 2010 #15
    No, I've only worked for Microsoft doing mechanical and manufacturing design engineering.

    Let's see, I started my apprenticeship as a tool & die maker in 1967. I hold my journeyman's rating through the German Machinist's Guild (passed it on my fifth try). I have worked on a contract basis most of my life and done work for more than 350 different companies and government agencies. If the schedule does not change, there's a Mars lander that will be launched in late-August or early-September which will be steered between here and Mars using a fuel pump system I designed and built back in 2006-2007. I did most of the hand work developing stratofied charge fuel systems while at the Vehicle Research Institute (DoT) back in the mid-1970's.

    The question posed here was, "Is aluminum equivalent to steel property-wise." Too many answers focused solely on yield and ultimate tensile strength. This was something I thought needed clarification. I find it interesting that you claim I am "lording my superior knowledge over (you) lessor mortals." I think of more along the lines of sharing those things I have been lucky enough to find courtesy of those who shared their knowledge and skills with me.

    A) I have not yet figured out (not really having the time) how to post pictures. It seems I need to post them someplace else and give a "link" to them. B) A significant portion of the work I do is either "proprietary" or "classified." I have posted a couple of (PDF) "project documents" and reference documents in the files section here. I try to be a "giving" member of the community. Among my recent projects are: designing and building automated and semi-automated tooling for the production of solar power cells; designing and building a laser scanning system for medical analysis; designing and building an alternative system for dialysis; and designing and building a tidal power generating system. That covers most of the past six months. If you would rather that I leave this forum, just say so.
     
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  16. Jun 15, 2010 #16

    dsquire

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    Lew

    I would very much hope that you do not leave this forum or the HMEM group. I have learned so much from reading your posts and want to continue to read and learn.

    If a person only wants a yes or no answer than they may not want to learn. If they do want to learn and that is why most people come to this forum then they will appreciate the detailed information that you have given.

    I look forward to your continued participation in these forums. :bow: :bow:

    cheers :)

    Don
     
  17. Jun 15, 2010 #17

    cidrontmg

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    I second that.
     
  18. Jun 15, 2010 #18

    capjak

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    Hi Lew,

    Please stay with us. A good number of your posts are way over my head but there is always some bit of information that I can use. If some people don't care to read your posts, all that they have to do is use the scroll wheel on the mouse or the back button in the upper left corner of the screen.

    Jack
     
  19. Jun 15, 2010 #19

    Tin Falcon

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    Lew we are asking that you consider the audience and post in clear concise language. We want you to encourage the members in there endeavors. We want to see pictures of your home shop models and projects . we are not looking for you divulge trade secrets or jeopardize national security. Normally photos are posted to a hosting sight like photobucket then a link is posted in the thread on this forum.
    Tin
     
  20. Jun 15, 2010 #20

    Cedge

    Cedge

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    Lew
    Staying or leaving is a choice you'll have to make all by yourself. I'm sure you're quite knowledgeable, but the condescending tone is both abrasive and to some extent insulting to those of us who are building at the best of our ability. Take off the starched shirt and tie....get your hands dirty. It's about the only road to earning your chops with this group.

    I'm sure the projects were both impressive and important, but here it's model engineering, where the numbers get so small that the tables aren't a whole lot of help. The stresses are such that we can cheat the numbers and still come out on the winning sideof the event. Just unwind a bit and be "one of the guys" for god sake. We don't care what letters you string behind your name and titles are just in the way here.

    Steve
     

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