The interesting issues with aluminium casting

Discussion in 'Sand Casting' started by Mark's castings, Oct 14, 2018.

  1. Melting Aluminium and then sand casting it has a few peculiar issues that I've noticed and speculated about in the past, these seem to be related to physical properties and also the chemical properties of the element. It chemically reacts with oxides and reduces them to elemental form. This happens to other nearby substances such as silicon in the sand, iron in home-made steel crucibles and likely a whole raft of other materials in the hot crucible that aren't readily apparent.

    My personal experience has been some perfectly usable castings that appear on first glance to have some visible porosity but on closer examination have grains of sand deep inside the casting in areas exposed by machining: the ultimate high silicon alloy. There was a simple reason for this, recycling runners with sand stuck all over the surface: aluminium has a specific gravity of 2.7 (2.7 times that of water) and silica sand (crystalline quartz) has a density of 2.65..... this means that sand is only going to very slowly float to the surface of the melt if at all. I foolishly assumed it would go straight to the surface and be skimmed off and my machined castings of recycled runners proved otherwise.

    I have read of stories (can't find them online at the moment) of Ford in World War Two making aero engines and having excess silicon in their castings which was traced back to sand off recycled runners getting into the melt: the molten aluminium chemically reacted with the sand to form aluminium oxide dross and metallic silicon. There are Youtube videos on sand/aluminium thermite mixes generating lots of heat and lumps of silicon so it's no stretch of the imagination to see it happening in the crucible.

    Aluminium reduces just about any oxide to it's elemental form and plain vanilla thermite for welding railway lines uses iron oxide as an ingredient, this means that a rusty steel crucible pot of molten aluminium is going to reduce some of the iron oxide back into iron and contaminate your melt with iron (Making lots of oxide dross in the process). This contaminated alloy has poorer machining properties and strength compared to iron free metal. Any iron oxide flakes have a specific gravity of 5.4-5.7 so will sink to the bottom of your pot and not be skimmed from the melt.

    So in view of these double whammys from iron oxide and sand, skimming is just not going to cut it. I'm not sure what can be done apart from using the right crucibles and cleaning your aluminium input, skimming should get some surface crud and hopefully aluminium's inability to "wet" dross allows it to ride on the surface tension of the melt like a steel needle on pure water. Maybe bubbling argon can carry contaminants to the surface to sit on the surface tension?. My foundry friend keeps telling me that "Casting hygiene is crucial" and I'm beginning to see why (The hard way of course!).

    Sand embedded deep in a recycled aluminium runner casting:
    [​IMG]


    Identical casting in LG2 bronze from recycled runners:
    [​IMG]
     
    Last edited: Oct 21, 2018
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    A lot to chew on there Mark. As everyone knows, I’m a lost foam caster. I haven’t had much problem with sand inclusions of late. I coat everything in drywall mud and use foil sprues and together they seem to keep the loose unbound mold sand stable while the molten metal displaces the foam pattern. It seems to be excellent barrier and does well in this regard. If I don’t coat the pattern and/or sprue I do get sand imbedded in the surface of the casting. So in addition to achieving good finishes, coatings help keep cutting tools in shape during post machining because as you mention, sand is hard on cutting tools. We’ll need to get some of the sand casters to weigh in but I always attributed sand inclusions to composition of facing sand and runner design. High velocity feeds and impingement can often be a culprit.

    On the subject of impurities in the melt, things that are soluble and/or alloy in the melt are best dealt with by avoiding exposure to the melt. For those things that are not soluble, the observation about specific gravity and the expectation of buoyancy is interesting. For instance, at 4gm/cm3, why does Al2O3 float on the melt since aluminum is ~2.7gm/cm3? When I re-melt sprue stock from lost foam, sometimes it still has some residual drywall mud (gypsum). That ends up deposited on the side walls of my crucible. It comes off with light brushing.

    I’ve been in the practice of immediately placing the crucible back into the furnace and letting both cool slowly. My furnace is pretty well insulated and it can take a whole day. However, I think may try to add a little clean up with my skimming spoon to that procedure because the thin layer of aluminum that is left seems to be more like Al2O3 /dross on the crucible wall after it ages in a hot furnace for hours. Sometimes (not always) it sticks to the crucible walls with no hope of removal without a melt. Other times I can get my finger nail under a piece and pull the whole skin and small heal out in one piece. I like having a nice clean crucible when charging it with good metal.

    Best,
    Kelly
     
  3. Very interesting discussion. I'm using a 304 SS crucible which does not produce rust but does produce a black scale. I'm guessing it is iron oxide, however most of the sacle is on the outside so that part won't get into the melt. But you're making me rethink using a stainless steel crucible.

    Kelly, it seems logical the heat in an unfired furnace would oxidize the surface of the aluminum on the sidewalls. I assume that's what you're thinking?
     
  4. Regarding a stainless crucible: A friend of mine build a soldering machine for a radiator repair shop out of 316 and 304 stainless. He had a lot of problems with a stainless pump impellor and anywhere the molten solder was in contact as the tin-lead solder was eroding and dissolving the stainless steel in contact with the flow. I read on another forum that the correct material for molten solder is cast iron as the exposed graphite protects the underlying iron. That may not work for us as we scrape the dross out of the crucible and disturb the graphite layer.

    Yesterday, I was actually discussing making a stainless steel skimmer rather than the usual rusty bit of steel (stainless fabrication shop is next door with free scrap). Pete, my foundry guru said they had tried it over the years in a couple of foundries but they did not last long with molten bronze contact and rusty steel lasted a lot longer.
     
    Last edited: Oct 21, 2018
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Yes. I only do so because I presumed it was easier on the crucible.

    Regarding SS in molten metal contact, I’ve been told it is more aggressively reduced by aluminum than oxidized carbon steel but OIF seems to do ok with it. As far as metals go, many report longest life with cast iron in molten aluminum service.

    Best,
    Kelly
     
  6. I would not have made a SS crucible but I did it through ignorance before reading about the issues. I've not done much melting lately as I'm busy fixing an old car. But I do have over two hundred melts in my 4" 304 pipe crucible. All I can guess is there is a hard layer of something in side which won't come out, I figure it is a liner. I scrape and poke it while hot and get slots of junk out but the liner persists. The wall is thinning but I think it is external scaling. I have more pipe and will build a new one when this one gives out.

    You sure wouldn't think molten tin would attack stainless steel.

    A cast iron aluminum crucible sounds like a great first cast iron pour.:eek:
     
  7. DavidF

    DavidF Administrator Staff Member Banner Member

    When I first started casting my first (pot) was a can out of a submersible trash pump. It was thin walled stainless and held up for a very long time until it finally burned through where the burner impinged on it.
    Id sure like like to know what grade stainless that it was made from.....
     

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