Filling cavities on a cast alu surface to obtain perfect flateness

Discussion in 'Materials' started by maxime.levesque, Feb 5, 2011.

  1. maxime.levesque
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    maxime.levesque New Member

    Hi, I have an aluminuim surface that is supposed to be planar,
    (perfectly flat), but has imperfections, i.e. cavities of 1 to 2 millimeters
    deep, of about 1 to 4 centimeters circumference (we're talking
    of very irregular puddle shapes here).

    The cavities originate from a poor casting technique.

    This problem would be easily solved if there existed an epoxy that could
    fill the cavities, I could then resand the finished surface and
    make it perfectly flat. I have been reading about problems with
    bonding to aluminium. So it seems that it might not be easy.

    The finished surface need only to withstand mechanical presure
    and not break, so there is no tensile strenght required here, only
    resistance to compression. I am hoping that this can put less
    constraints in the choice of material. The other constraint is that
    the filler should not be elastic. Esthetics doesn't matter here,
    so I don't care of how the finish looks like.

    BTW, the simple solution of just sanding the surface until flat is not
    feasable, because the total thikness of the plate needs to remain the
    same.


    Any advice ?
     
  2. wardd
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    wardd Senior Member

    best way would be gouge out and fill with weld

    could you attach a sheet of aluminum with fasteners or plug welds?
     
  3. maxime.levesque
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    maxime.levesque New Member

    Thanks for the quick answer !
    Pardon my poor english but I don't know what gouge means ! I googled
    and found that it might be a form of carving, if that's the case, then shoudn't the
    gouging be done *after* the welding ?

    The sheet is not an option, because the surface is *mostly* correct, so if I add
    a sheet it will thicken the whole thing...
     
  4. wardd
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    wardd Senior Member

    gouging would be opening up the cavities enough to get a solid weld in

    some cavities may be in the form of bubbles with small openings at the surface and difficult to fill with weld

    if it was cast in sand there may be sand in the cavities

    anyway if the part can stand the heat of welding without deforming that would be best

    my opinion
     
  5. michael pierzga
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    michael pierzga Senior Member

    Ive used "etch primer" in the past with good results for bonding epoxy to sandblasted and mechanically abraded Aluminum plate.

    Aluminum casting is a different alloy...contact an Epoxy manufactures technical department for bonding..etching... advice.

    For aluminium plate Ive used ....

    West system 860 Aluminum Etch
     
  6. PAR
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    PAR Yacht Designer/Builder

    Sand the aluminum then immediately wipe with acetone. When it flashes off (a minute or two), then immediately apply straight epoxy. Some will have you scrub the surface with a wire brush as the epoxy goes on, but if you follow my description you'll get a great bond. If you flood coat the area, you can have a nearly perfect surface before sanding fair.
     
  7. michael pierzga
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    michael pierzga Senior Member

    Wrong Par...The original poster is not working with flat surface plate... its a casting, he can not abrade the surface of the pit. READ... the description of his project...he is working with a pitted casting.
     
  8. PAR
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    PAR Yacht Designer/Builder

    I'm not going to get into it with you Michael, it's clear you have a limited skill set. Yes, you can abate a pitted surface, once again proving how little you know, in spite of your insistence otherwise. Just because you don't know how to do it, doesn't mean it can't be done, it just means you don't know how, which is like most of your posts here.
     
  9. War Whoop
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    War Whoop Senior Member

  10. Ad Hoc
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    Ad Hoc Naval Architect

    I would recommend PAR's suggestion too, simple, neat.

    I wouldn't recommend welding, especially if you don't know what grade of ally it is. You need to be very careful welding cast ally, such as knowing what grade of ally it is, otherwise you'll cause more problems than you think you're solving.
     
  11. PAR
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    PAR Yacht Designer/Builder

    You can also use one of a number of tools, to get down into the pitted areas, such as a needle sander.

    Some years ago I played with using uniform length steel filings (1/16") and a magnet to polarize the metal filings as the epoxy setup. Naturally, density issues cropped up and I wasn't interested in experimenting further, but preliminary test results hand some huge material properties changes, over the usual set of filler materials. My intent at the time was two fold, the first to dramatically improve compression strength and secondly as a potential anti fouling coating (of course steel wouldn't be the metal used) with considerable penetration resistance as well.
     
  12. Landlubber
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    Landlubber Senior Member

    ...i would go the Devcon way, have done it before using Devcon on cast iron machinery, and of course we use it for bedding.....it will do the job well..(Note: Devcon is an epoxy product with fine metalic particles, the alloy version will be the one to use).
     
  13. maxime.levesque
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    maxime.levesque New Member

    Thank you all for the advices, this is a very helpful cummunity. I will try the epoxy filling,
    since I am not equiped for welding, if I get bit by oxidation problem, the fill will just come
    off and I'll start over with a more careful surface preparation.
    Cheersi !
     
  14. bntii
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    bntii Senior Member

    Seconded.
    That or just pick up a tube of JB weld at the local hardware store.
    Skim it in neat with a pallet knife & there will be hardly any sanding to do.
     

  15. PAR
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    PAR Yacht Designer/Builder

    "JB Weld" is metal filled epoxy, Bntii.
     
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