Flux and Degasser

Discussion in 'Sand Casting' started by Kenglaze, Nov 12, 2021.

  1. Kenglaze

    Kenglaze Lead

    Hi All,

    Still very new to this. I tried searching around for flux and degasser to add to my melted aluminum ahead of pouring. I am not doing anything special here and aren't really running into issues with the outcome of pouring aluminum that has had nothing added to it but figured it seems like an important step so I should get on it!
    A quick search for these types of products online is not helping. There seem to be specialty products and custom mixes available but the purpose and its necessity remains unclear.

    I am hoping to get some advice about what products should always be used to degas the aluminum (6061 mostly) and if flux/degasser should be added for regular run-of-the-mill stuff. What do you use normally? Do you have a link to an online supplier I can buy what ever it is from?

    Thanks,
    Ken
     
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    When talking about conditioning aluminum melts folks often use the term fluxing interchangeably with degassing but they are not the same thing. Fluxes are primarily used to clump and gather oxides and impurities (generically called drosses and the act of drossing) and provide surface barriers on top the melt, while degassers are used to liberate hydrogen gas which is soluble in molten aluminum and can cause porosity. There are home brews but here are a couple source.

    https://smallfoundrysupply.com/store/index.php/consumables/fluxes-and-degas-tablets.html
    https://www.budgetfoundrysupply.com/aluminum-flux

    Argon gas and lance is also often used for degassing. This can become a little more involved.

    http://forums.thehomefoundry.org/index.php?threads/degassing-lance.204/page-3#post-3911

    You mention 6061 which is a wrought alloy. It's not a great casting alloy. Best casting alloys are those that have already been cast, so clean recycled automotive castings, engine parts, etc are better because they contain things that make them flow and solidify better in the mold.

    If you are a hobbyist, I'd say you'd be best served by using a clay/graphite crucible, making sure you have clean casting stock of a casting alloy, and a neutral burn/flame (assuming you have a fuel fired furnace) in your furnace. If you're not sure of what some of this means, search and/or ask for more details.

    Best,
    Kelly
     
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  3. FishbonzWV

    FishbonzWV Silver Banner Member

    Welcome Ken,
    It sounds like you are trying to fix a non-existent problem. If you are getting clean pours there is no need to complicate things.
    I too thought that it had to be done because I watched a bunch of vids of others doing it. After making a mess of my crucible and seeing no improvement, I abandoned the practice and I still get good quality castings. I'm not going to flux to gain an extra tablespoon of aluminium out of the dross. If you get visible porosity then tune your burner. The KISS principle is your friend in the foundry.
     
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  4. Petee716

    Petee716 Gold Banner Member

    Great input above. I consider the need for fluxing and degassing to be kind of like distant cousins. They’re different topics that have different outcomes, and many of the causes are different as well, but the common factor to control both seems to be the combustion environment. I primarily run waste oil and I’ve found that the largest contributor to a gassy melt is running too rich. Overheating the aluminum is the other one. Castings end up with a gozillion tiny pinholes throughout the cross section as well as the surface. I understand that to be a hydrogen absorption issue.
    The need for flux (for our purposes at least) would be to minimize the amount of dross being scooped off. Running lean seems to increase dross as excess oxygen forms the oxide layer, but dirty or painted scrap is by far the biggest culprit. Lacquer coating on rims, etc. The waste seems excessive as you’re scooping the crap out of the crucible, but you can remelt and recover some of the aluminum later if you’re so inclined. You have to skim the crucible anyway, so it’s not a super huge deal. I’m more concerned about the dross getting mixed with the melt below, so I place additional metal into the crucible gently through the dross layer as opposed to plopping it in and avoid stirring or excessive agitation.

    Pete
     
  5. Melterskelter

    Melterskelter Gold Banner Member

    I melt aluminum in an electric furnace usually. I will echo some of the folks’ comments above about paying attention to melting stock and not bothering withfluxing or degassing. I buy “356” ingots that a member here makes from auto engine blocks. They are of very good quality and very convenient to use. I’ve had not trouble with gas and I do skim off a tiny bit of dross before pouring. That’s it. Dead simple and for the time and effort making patterns, flasks, and molds, I feel it’s worth it to use a casting aluminum to make castings.

    A few times I’ve melted aluminum in my diesel furnace and noted no problems with gas or dross either. Again good stock and neutral flame.

    Denis
     
  6. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    So you are probably sensing a consensus opinion here. I also use a resistive electric furnace, clean ingot, and never flux and almost never degass. To back up and get right to it, why do you think you need to degass and flux, what type of heat/burner do you have, and do you have a means to measure melt temperature?

    Best,
    Kelly
     
  7. Kenglaze

    Kenglaze Lead

    Hey All, thank you so much for the responses. Yeah I am seeing a pattern here of if it ain't broke don't fix it! hahahaha
    The only reason I was looking into it was because I see a lot of people online in videos doing it.

    For those that asked, I am using a propane fueled furnace to produce my melt with a graphite crucible. I have been pouring into green sand I made myself which is 70% Silica Sand and 30% Bentonite clay by weight. So far I have only be melting scrap 6061 aluminum because that is what I have a lot of coming from my milling machine. It will be interesting to try some scrap aluminum from previously commercially cast parts to see how that compares.

    I recently purchased a K Type thermocouple I was planning to use for measuring the melt temperature, which I have not tried using yet. Any recommendation for a target temperature or minimum temperature that should be achieved before pouring?

    The photo attached is representative of the quality I have been able to produce. Do you think an aluminum alloy meant for casting would improve the surface porosity I am seeing here even after cleanup?
     

    Attached Files:

    Last edited: Nov 13, 2021
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That is way to rich in bentonite and depending on the type of bentonite, may mean high moisture content. Do you know the mesh size of sand and whether it's Sodium or Calcium Bentonite? What was the source?

    Use a casting alloy. All else seems ok, if you have neutral burner tune. Pour temp depends on alloy but typically 1250-1350F.

    Best,
    Kelly
     
  9. Kenglaze

    Kenglaze Lead

    Yeah I was getting the impression that my green sand mixture could take more sand, which is good because I always seem to need more green sand so I should be able to dilute the clay by adding more sand.
    I did have an "incident" with one of my pours: when the first metal of a pour entered the sprue there was a pop and some molten metal came shooting back up a couple feet in the air. I am assuming that either my green sand was too wet causing some steam to be generated or that maybe my melt was too hot or something. I have since purchased a leather welding jacket, welding apron, and face shield to go along with my gloves for PPE.

    I am not positive what kind of Bentonite it is honestly, I visited a local pottery club that had bags of powdered Bentonite clay they use to adjust the mixture of glazes used for pottery. I was desperate to try my first casting so I went with what I could find that weekend and didn't want to wait for an online purchase to arrive. The sand is "Feldspar #90 Silver Sand" from P.W. Gillibrand which has a very small particle size distribution, this is something I had on-hand from an unrelated project :)

    I was considering the purchase of Petrobond as a more long term solution hoping that it might require less maintenance than the clay/sand mixture that needs to have the moisture evaluated/adjusted for each casting. Unfortunately I am still in search of a good source for Petrobond as the volume and cost of what is sold on Amazon feels like a rip off. Also annoying that it is sold by weight and all I know is the volume I need to fill so it is unclear how much I need to buy. Any idea what the density of Petrobond is so I can calculate the mass needed from the volume I have to fill? Any good online sources for something like a 5 gallon volume of this?
     
  10. Petee716

    Petee716 Gold Banner Member

    -Look up seller "petrobondforsale" on ebay. I've been buying from him for years. Price goes down with volume. Let him know you were referred from here. He's about as cheap as it gets. Petrobond is great for surface finish because of its fineness but it's not a panacea. Its absolutely worth buying but regular use will require a muller to keep it conditioned. I use both PB and greensand depending on the project. This is a very wide topic.

    -I estimate all sand at 100lbs per cubic foot. Greensand and Petrobond are a little on the fluffy side when prepared for molding, but once they're rammed, they pack right down densely.

    -Your blowback is almost certainly a moisture issue. Too much bentonite likely got you there. I mix 10% bentonite with sand by weight. (Just for reference, most mixtures you see here, whether it be sand or alloys, are almost always by weight pretty much as a rule). Water is around 5% or so.
    Google "navy foundry manual" pdf for tons of info on greensand.

    Pete
     
  11. rocco

    rocco Silver

    The bentonite you get from a pottery place is almost certainly sodium bentonite. If there's a distributor's label on your package of bentonite, they might have an MSDS posted on their website which could confirm which type you have.
     

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