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41743Re: Heat treating newly cast aluminum - Now Porosity

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  • Lyle
    Mar 1 9:13 AM
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      The swirling happens when I have the air turned up too high.
      I have a MIFCO B-16 souped up with a blower from a #30 controlled with a router speed control. Even with the speed control at about 25% it gets too much air. I should have left everything alone when I cobbled it together. But the original blower is on a different homemade furnace now. I have observed the swirling on different furnaces and it seems to be a function of how close the lid is to the top of the crucible as it's caused by the swirling of the gas inside of the furnace.

      Yes, you're technically correct about the dross not being a flux, but being it serves the same purpose by protecting the melt from gas absorption, most people refer to stuff put on top for this purpose as a cover flux.

      LL
      Back to molding up cast aluminum flasks.

      --- In hobbicast@yahoogroups.com, "tmoranwms" <tmoranwms@...> wrote:
      >
      > I've never seen swirling in the melt in a crucible furnace before, but I don't know what yours looks like.
      >
      > FYI, the oxide surface is a cover but it is NOT a flux. Flux comes from the Latin "to flow", which most fluxes do (think of how hard brazing would be without a flux to make it flow ever so nicely!). Indeed, a flux is required with aluminum, to break up the oxide when dirty metal is used, or when very clean castings are required.
      >
      > Hydrogen has reasonable solubility in molten aluminum. Because aluminum is reactive, it is enough to have steam present. After all, at room temperature, with the help of a little acid or base to overcome its skin, aluminum yields hydrogen copiously!
      >
      > In the environment of a flame, there is fuel (hydrocarbons), fuel in the process of breaking up (hydrocarbon radicals, which give the characteristic green and blue hues of a clean flame), air, air in the process of breaking up (mostly as hydroxyl radicals, the same thing that's created near the ozone layer, that breaks down smog and CFCs and other pollutants), the halfway combinations of air and fuel molecules (like carbon monoxide), and finally the end products -- mostly carbon dioxide and steam, mostly towards the end of the flame where it's no longer visible. This is because radicals are so reactive and energetic, they release light when they react; when they're done, no more light is produced.
      >
      > All told, it's a very interesting mixture, and a very reactive one. This can easily be seen by heating a piece of copper metal in air and watching it oxidize: if you pass a stream of propane over the surface (without ignition), it hardly does anything; the black oxidized metal continues looking the same. You know an excess of fuel is supposed to reduce oxides, but it simply isn't hot enough to start. Now, spark the fuel into a flame and observe how the metal shimmers between matte and shiny -- the oxide is being reduced by the reactive fuel molecules in the flame.
      >
      > The more reactive the flame, the more powerful the reducing effect. Unignited propane hardly does anything; ignited, activity starts just before red heat. Oxyacetylene is active even lower.
      >
      > Aside:
      > You can even tell what kind of chemical reactions are going on inside the flame this way. In a typical neutral propane flame, the first, purplish blue cone actually oxidizes hot copper (try it!), because it's the unignited fuel-air mixture -- the fuel isn't reactive (it's not burning yet), and there's plenty of oxygen around to tarnish the metal. The next cone, the main blue-green part, is where all the action is, and where copper metal stays nice and shiny. Outside of that cone, there may be residual byproducts (like carbon monoxide, which burns with a faint, deep blue color) which are still able to reduce it, but once you get into the CO2 and H2O exhaust, it mainly acts as a shield gas rather than a reducing agent (if it's shiny, it stays shiny; if it's tarnished, it remains tarnished).
      >
      >
      > Needless to say, all these reactive molecules would love to react with aluminum, but because aluminum is so much more reactive than copper, the reaction goes the other way, with aluminum stealing the oxygen (or hydrogen). Even without an active flame, the steam in the exhaust would be more than enough for aluminum to react with.
      >
      > In short, flames and steam will both react with a disturbed aluminum surface, introducing hydrogen into the melt.
      >
      > Tim
      >
      >
      > --- In hobbicast@yahoogroups.com, "Lyle" <creepinogie@> wrote:
      > >
      > >
      > >
      > > One thing that happens to me once I get everything melted is the top of the melt swirls from the air blast. This swirling constantly breaks the top surface which acts as a cover flux. So I've learned to turn down the air once I get a melt which prevents this and also gives a reduced atmosphere. This, and not letting the melt sit too long before pouring are the two main things I do.
      > >
      > > Getting back to the original post stating "hydrogen porosity" and doing a little internet browsing has told me that molten aluminum does have an affinity for hydrogen wheras I had thought the big problem was oxygen. So, how much hydrogen is actually present in the typical gas fired furnace atmosphere? It can't really come from the air so it must come from a byproduct of combustion?
      > >
      >
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