Aluminium sand casting defects.

Discussion in 'Sand Casting' started by Ironsides, Apr 7, 2024.

  1. Ironsides

    Ironsides Silver

    I don't cast a lot of aluminium but when I do there is a defect that I cannot identify so I made a video showing those defects.
  2. FishbonzWV

    FishbonzWV Silver Banner Member

    It looks like you had some die cast stock.
    I had some that cast terribly too. On a whim I added aluminium wire (98% Al) at a 50/50 ratio and got better castings. The lumpiness disappeared from the sprue/ingots and didn't see shrink in the parts.
    That saved me from throwing out about 70 pounds of die cast and a lot of aluminium wire, both useless on their own as casting stock.
    HT1 likes this.
  3. Can you get a sample tested with an XRF gun at a scrap metal dealer somewhere?. I've noticed certain alloys like pewter and zamac (zinc /aluminium) can get huge shrink defects from uneven solidification. Also oily, carbon rich scrap like pistons, cylinder heads and engine blocks seem to be a poor source of aluminium.
    Last edited: Apr 8, 2024
  4. Ironsides

    Ironsides Silver

    I do have some heavy gauge aluminium wire and it would be barely 2 kg. Getting over 200kg of wire might be a problem. When I fractured the ingot it had a great grain structure so this scrap is not suited for sand casting.
  5. Ironsides

    Ironsides Silver

    Not many scrap yards where I live and none have an XRF gun.
  6. Rocketman

    Rocketman Silver Banner Member

    Too much zinc from diecast scrap. Which has a high shrink rate. Visible in the top of sprue and ingot. There is sand wash present in the mold. The particles of sand create a hot spot, and also surface tension around the particle(s) keeps metal away from the face of the sand in these areas, limiting heat transfer. They cool last, creating an isolated shrink pocket
  7. Tobho Mott

    Tobho Mott Administrator Staff Member Banner Member

    Surprising to me that the auto engine castings gave you troubles that you didn't see when using 'wheelium'.

    The defects remind me of the time I carelessly left clumps of parting inside a mold cavity then saw similar looking rough-textured pits on the casting in those locations. But that was a one time oversight, not a persistent issue with a particular type of alloy as this seems to be.

  8. HT1

    HT1 Gold Banner Member

    definitely signs of some Zinc in there, and who knows what else!... Aluminum doesn't like Pb, so if you picked up some of that from a Babbit bearing ... could be your issue , definitely possible you got some specialty alloy auto parts that did not mix well with others , also auto parts could definitely give you some weird trace alloys from oil, grease, soot, that might poison good alloy the way Sulphur poisons Brass.
    Ive Dome small engine Blocks(lawn mower) before without issue other then them being nasty and smoky melting , but produced good alloy Cast rims are always a good bet, but you gotta watch out for the Billet rims which will be 5000 or 6000 forgings , also at least in America, everything along a road, lights signs etc.. are 6061 (required by DOT) even the cast staff , its centrifugal cast , so it doesnt need Si to fill the molds

    V/r HT1
  9. Ironsides

    Ironsides Silver

    The weird thing is some lawn mover engines do it and some don't make that defect so I stopped using them altogether. To give you an example one time I used a B&S lawn mover engine with no defects and then months later used another B&S lawn mover engine, it had those defects, that is why I use car rims.
  10. Some random small percentage of Aussie made "Rover" brand mower bases were magnesium in amongst the usual aluminium....ask me how I know :eek:
    HT1 and Petee716 like this.
  11. Petee716

    Petee716 Gold Banner Member

    No need for me to ask. All I could do was stand there and watch it burn!
    Mark's castings likes this.
  12. Ironsides

    Ironsides Silver

    Olfoundryman made a reply on my video and will share it on this thread. In regards to that BMW rim I will have to cut a sample piece off and melt it to see how it behaves when it solidifies.

    (Ah well. Now you know why I don't use scrap aluminum! I am dubious of even alloy wheels as many are not the 356 alloy that is easily castable. BMW wheels for example are NOT 356 they are a somewhat higher silicon alloy. Your problem is pure and simply the silicon content of the alloy that you are working with . That strange surface on the sprue and ingots is due to the formation of eutectic cells. This happens in alloys with silicon levels approaching 12% (356 has about 6.5%) The defects you see are the areas between the cells and they are a type of shrinkage defect - the cell grows for its center and the areas where cells meet as they grow will often show shrinkage as being the last areas to solidify they have run out of feed metal. I have seen these eutectic cells reach over 25 mm in diameter particularly on ingots that have cooled slowly. You can see these cells in my video called "Solidification Curiosity" Its the areas where the cells meet that tend to produce the defects that you have got. I do not believe that the defect has anything to do with either sand moisture or sand wash. I used this high silicon alloy for some gravity diecasting work where the rapid solidification prevents this sort of defect. It is sometimes used for sand work where its much higher fluidity is helpful for getting thin castings to run. Additions of Sr (or Na) can help as it refines the silicon structure making the cells a lot smaller. Some mould coats (e.g. Mould coat 15 by Foseco which looks like a non drying silver frost paint and indeed I have used actual silver frost paint on sand cores) can help. Moto - leave the reclaiming of scrap to those that know what they are doing and who do it large scale professionally - Just go out and buy good ingot 356 - your casting success rate will improve dramatically - scrap in scrap out... Martin PS. If you must use wheels try putting them on a Barbeque (or whatever), getting them good and hot (300 to 400 C) and then they should break up with a few easy hammer blows..... Martin)
    Chazza, HT1 and Tobho Mott like this.
  13. HT1

    HT1 Gold Banner Member

    So What Im hearing is we need to tell all the Guys trying to cast aluminum cans that they need to mix that mess half and half with a BMW rim:p

    V/r HT1

    P.S. im telling everyone I heard it from Ironsides
    Tobho Mott likes this.
  14. Chazza

    Chazza Silver

    Do you know where in Oz I can buy a 356 ingot? I have searched the net several times and found nothing,
    Cheers Charlie
  15. Chazza likes this.
  16. Chazza

    Chazza Silver

    Thanks Mark.

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