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Thread: Cast Aluminum Young's Modulus

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    Cast Aluminum Young's Modulus

    Hi all, I am working on a project where I am making a custom clamp for making cabinet carcasses. I am using cast aluminum to make the clamp, and in the interest of saving metal and time, I am designing the clamp using the equations for the deformation of a beam.

    My question for you, is how will cast alumimum's Youngs modulus (modulus of elasticity) compare to what is listed in most reference texts. I do not degass my aluminum and there usually is some mild porosity in my castings. In my mind the stiffness should be decreased by approximately whatever percentage of the casting is porosity. Ex If 1% of the cut cross section is bubbles than the beams stiffness should be 1% less than ideal. This is assuming that the bubbles are evenly distributed and quite small.

    If anybody could enlighten me on anything else that might be good to know about the effect of porosity in casting strength/stiffness, it would be much appreciated.

    MP

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    Quote Originally Posted by mpowley66 View Post
    Hi all, I am working on a project where I am making a custom clamp for making cabinet carcasses. I am using cast aluminum to make the clamp, and in the interest of saving metal and time, I am designing the clamp using the equations for the deformation of a beam.

    My question for you, is how will cast alumimum's Youngs modulus (modulus of elasticity) compare to what is listed in most reference texts. I do not degass my aluminum and there usually is some mild porosity in my castings. In my mind the stiffness should be decreased by approximately whatever percentage of the casting is porosity. Ex If 1% of the cut cross section is bubbles than the beams stiffness should be 1% less than ideal. This is assuming that the bubbles are evenly distributed and quite small.

    If anybody could enlighten me on anything else that might be good to know about the effect of porosity in casting strength/stiffness, it would be much appreciated.

    MP
    Quote the references you are using... Because there should be many different numbers because of the key variables... alloy and heat treatment.
    gas and porosity will have a LARGE effect on strength of your finished parts, and it adds unpredictability, since the exact location of a defect in relation to the location of stress applied will be critical


    V/r HT1

  3. #3
    To be honest, if your trying to do those type of calculations in a backyard cast your wasting your time.
    Even if you knew the exact properties of the alloy your using as soon as you add it to your furnace that's all going to change.
    your fuel, your mixture the temperature, time held at that temperature............
    Make it strong make it once, simples.
    good luck
    The only time You're not following your nose is when your going backward!.......Andy (ME) .
    Have you filed in "Who do you think you are?" War Grade Report

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    Senior Member Toolshed's Avatar
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    Take it from a bloke named WARGRADE....Yeah

    'nuf said
    "If you work with your hands, you’re a laborer.
    If you work with your hands and your mind, you’re a craftsman.
    If you work with your hands and your mind and your heart, you’re an artist."

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  5. #5
    Thanks everyone

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    Post Real World Mechanical Properties vs Handbook Values

    Quote Originally Posted by mpowley66 View Post
    .....how will cast alumimum's Youngs modulus (modulus of elasticity) compare to what is listed in most reference texts. ....stiffness should be decreased by approximately whatever percentage of the casting is porosity......
    MP
    That is a great question, I spent years tracking down something similar, I could never get my test bars to consistently meet the published values

    for A356-T6

    Ultimate Tensile Strength Yield Strength Elongation

    Typical Handbook 34,000 PSI 24,000 PSI 3.50 %

    Handbook values came from ALCOA during WWII and were called out as "typical"

    Results reported are actually “what a good foundry can achieve on a good day” not average and definitely not a minimum

    These results may or may not have been from separately cast test bars and not excised from actual castings

    The lawyers got involved and no company now accepts responsibility for these results due to fear of lawsuits if a design fails



    Auto designers decided to find out what they were actually getting in their parts

    This data was from excised samples from production castings made at high quality foundries, suspension parts, parts driving down the road

    Industry was surprised to see huge scatter within part and across parts

    Strength is pretty good, buy look at elongation as high as 17% and as low as zero….porosity, oxides and shrink are killers for elongation and fatigue strength

    That is why forgings are used in Aerospace, no inclusion defects....

    Values are approximate

    Ultimate Tensile Strength Yield Strength Elongation at Break
    Max 51,000 39,000 17%
    Avg 41,000 32,000 8%
    Min 36,000 25,500 0.7%

    So for design Yield strength would be very low after safety factor is allowed, how high would you go - 15,000?

    Unfortunately I don’t have YM data, but you would have to down rate, maybe around 25% of the published values?

    Good luck with the project

  7. #7
    Thanks foundry joe, I thought it would be more complicated than it first appeared. In my situation the clamp design is already a guessing game, because for starters you have to look at how hard can a human hand twist. What is the coefficient of friction on the clamp threads going to be and how will this affect the clamping force. Using approximate numbers I end up with a clamping force of easily over 2000 N if I remember correctly. In the application I am using this for that much clamping force is overkill. So then the question becomes do I design the clamp to withstand a sensible amount of force and hope the operator will have the sense not to unnesecarily reaf on it, or do I design it to be bullet proof, knowing that that extra is probably never going I be used. I am taking first year engineering course right now, so this is an interesting challenge for me.

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    Since it came up, in aviation, we require additional factors of safety with castings, due to the inconsistencies inherent in the process.

    For example, for a casting used in critical structure of a small aircraft, the requirement 23.651 would require a factor of safety of 1.5, in addition to the standard factor of safety of 1.5 for structural elements. So 2.25x calculated worst case stack up of design loads. At the least, critical castings must receive 100% visual inspection and dye penetrant inspection, plus X-ray inspection of the structurally significant internal areas, with at least one casting load tested to those factors of safety.

    That casting factor can be reduced, by testing more specimens to obtain statistical data to demonstrate the actual process used is more robust.

    Your original question was of the achieved youngs modulus, and the effect of casting defects on such. In aviation (and I would guess the automotive industry is similar), you normally assume the youngs modulus of the homogenous material, and then insert the worst case (-3 sigma) defects which your process produces into your FEM model, and assess the effect on crack initiation and propagation, rather than assuming a lower youngs modulus and designing with it.
    Last edited by rotarysmp; 01-10-2017 at 12:48 PM.

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    Hi rotarysmp

    Do you have any experience in how 3D metal printed parts are being treated in aviation? What unique defects do have have or are they considered like a forging?

  10. #10
    This is a bit off topic, but why would a forging be more structurally sound then a casting, because isn't the metal that is used in the forging cast at some point. Is it because there is better control of turbidity and inclusions in the casting of the stock used for forging?

    Forgive my ignorance, the only way to learn is by asking questions (and failure )

    - - - Updated - - -

    It would appear that given the complex nature of designing the clamp, it will be simpler just to make the clamp with a very large safety factor and avoid having to do a redesign and making new patterns. This is what others suggested, and as usual they know what they Are talking about. You know what they say, the devil is in the details.

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