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  1. #1
    Join Date
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    Default Porosity in cast A390

    Trying to cast engine pistons in a high Silicon alloy.

    Got some ingots of A390, which is meant to be the best alloy for that, but all of the pours have black air pockets near the top, that seem to carry quite a way through the casting (i.e. unable to machine away to get clean metal)

    IMG_2703.jpg


    The inside and bottom of the casting are shiny and beautiful.
    IMG_2704.jpg
    They are currently being done in a primitive split die, with a side pouring tube that feeds about a third of the way up:
    IMG_2705.jpg
    All the dies are well pre-heated.




    We are at a loss as to why, have tried a few different top plates (this is the second one, with a smaller vent hole, and a taper inside to try and float any impurities up)

    Interestingly, the ingots we bought also have the same "feature":
    IMG_2707.jpgIMG_2708.jpg
    as does a random dribble that was dropped on out base plate, and I machined the top off.





    Here is the last pour from yesterday. Got the alloy extra hot - note the ripples on the surface from the oxy torch:
    IMG_2709.jpgIMG_2710.jpgIMG_2711.jpg
    (haven't dissected the casting yet, but I suspect it will have the same issues)




    So, apart from "just sand cast it", does anyone have any suggestions?

  2. #2
    Join Date
    Mar 2006
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    Waikerie SA
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    Default

    I don't preach to be an expert but try bubbling argon through your metal before pouring. Just a suggestion! Oh the thrills of casting in the backyard!

  3. #3
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    Default

    Thanks Vin. First time I have heard of that.



    A few web searches say it de-gasses the alloy, reducing hydrogen.



    I assume that is bubbled through the ladle?
    (because bubbling through the mould/die would be a challenge
    in the 10—20 seconds it takes for the metal to solidify!!!



    I only have welding gas handy (Ar/C02/O),
    but I'll look for an Argon cylinder and try to build a small lance.

  4. #4
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    Mar 2006
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    Waikerie SA
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    Default

    Yes, bubble the argon in the ladle just before pouring. Also if possible have far more metal in your ladle than you need if pouring small casts. Once again no expert, just a novice!

  5. #5
    Join Date
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    Default

    Hi Nigel,
    First of all a disclaimer in that I have never had anything to do with casting , cast iron.

    I have however had much to do with welding it.

    By welding, I mean with a water cooled oxy acetylene welding nozzle after the weldment was preheated to a suitable welding temperature with a 50mm gas line.

    To assist with the purification of the molten (cast iron) metal, we used a flux.
    The flux was borax ,potash and salt.

    The flux,purifies molten metal and floats the impurities out and up and they are removed as a crust from the cast iron when cooled.

    Porosities and voids were released and also floated out.


    1 cup of borax. …
    Pour 1 cup of potash into a measuring cup. …
    Measure 1/2 cup of table salt. …
    Mix the three ingredients together until they are fully combined.
    Pour the flux mixture into a lidded jar. …
    Use the flux dry. …
    Modify amounts used in this recipe to suit your needs.

    It needs to be applied and stirred through the molten iron.

    Do you think it is worth a try?

    Grahame

  6. #6
    Join Date
    Mar 2006
    Location
    Waikerie SA
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    Default

    Grahame is probably on the money, I add borax to both aluminum and bronze melts, I also use it when I melt silver making jewellery. once the melt is ready I scrape the dross add a small handfull of borax, stir and skim the last bit of dross before pouring. Borax can be very harsh on crucibles so use sparingly.
    Just in case you are not aware borax can be bought at woolworths!

  7. #7
    Join Date
    Mar 2014
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    South of Adelaide
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    Default

    I think you need a taller sprue to put more head pressure on the molten metal to stop the porosity forming.

  8. #8
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    Dec 2005
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    South Australia
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    Default

    Many health food or save the planet type shops sell Borax in bulk much cheaper than supermarkets

  9. #9
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    Default

    Haven't tried any more pours yet, but found someone that used to pour this material for racing cart engines.

    1) They started the day with a 20kg melt in a furnace, and used that material all day.

    2) They added Copper Phosphorous to the melt, to modify and "evenly distribute the silicon" thru the melt. That was after trying various tablet/powder products. I am still researching this:
    http://www.milward.com/pdf/8-P-CuS.pdf
    my fear is that too much would affect the strength of the alloy.

    3) They also bubble Nitrogen through, with a graphite lance for 20 minutes

    4) They advised a hotter die — approx 400°C — to help the casting cool more evenly and draw from the sprue. Advocated coatings on the sprue and die; ceramic from the Foseco company.
    Not sure it is necessary for these temporary pistons, but the final spin casting die deserves coating.

    5) Like snapatap, they also advised a much larger sprue.
    We might do this with a second sprue on top.

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