Gas bubble problems

Discussion in 'Castings, finishing/ repair/ and patina's' started by master53yoda, Sep 15, 2017.

  1. master53yoda

    master53yoda Copper Banner Member

    I am having very small gas bubble problems in a 1/2" by 12" flywheel casting, they are through out the part and about the size of finely ground pepper, I am using SS sand as the mold because of having steam bubble problems with green sand. Could the slower cooling of the SS mold be causing these bubbles. I did the pour using washing soda as the degassing agent and it did boil. I poured at about 1350.

    Art B
     
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Can we get a picture of the bubbles?? Washing soda?? Thought that was a flux??
     
  3. HT1

    HT1 Copper Banner Member

    Aluminum???? if they are through out the casting, you have gassy metal, if they are in certain locations or just near the surface it could be a sand issue. SS is more likely to cause gas.
    issues then some other types of sand.

    V/r HT1
     
  4. master53yoda

    master53yoda Copper Banner Member

    The metal is aluminum, the scrap was automotive cylinder heads, Very close to 356.

    I use sodium and potassium chloride for the flux, the holes are through out the metal, the circumference had almost a 1/4" machined off and they are still there. will a commercial degas tablet work better, I've never needed them before but this casting is taking a long time to cool because of no moisture in the sand close to 20 minutes before i pull it from the sand and it is still about 600 to 650F . The holes are small enough that when I have tried to get a picture and blow it up, the pinholes are out of focus. looking at them with a magnifying glass they do not have anything in the metal it is just a tiny hole. The casting weighs about 9 lbs with the sprues etc removed.

    I did melt the metal in my large furnace that has flame directly hitting the cylinder heads, once melted to about 1225 I pour it into the crucible and take the temp to 1350 in my small furnace and then stir in the flux and washing soda, it does bubble off when i do that.

    Art B
     
  5. OCD

    OCD Silver Banner Member

    Would an Alka Setlzer work as a de-gasser? [​IMG] :)
     
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    Out of curiosity, have you tried doing nothing to the melt and just pour as soon as you are able? I hear alot of chatter about degassing and the chlorine tabs seem to be the best answer. But I also hear they go bad pretty quickly (absorbing moisture??) and can do more harm than good as well. From what I know washing soda is a flux for glass and I am left uncertain as to how it is supposed to degas aluminum. If someone cares to explain my ears are open.
     
  7. master53yoda

    master53yoda Copper Banner Member

    myfordboy used washing soda for degassing, that is where i got started using it from, it seems to work ok on other castings, but they were greensand and no where near as large a surface area this wheel is poured 12" in diameter and 3/4" thick it is machined to 11.5 "and .5 "thick
     
  8. Tobho Mott

    Tobho Mott Silver Banner Member

    Myfordboy responded to another AA & THF member's claim that it doesn't work as a degasser here, with some sciencey stuff I didn't quite follow all of:
    http://www.alloyavenue.com/vb/showt...-Sodium-carbonate-to-degas-chemistry-involved

    I've only used the pool shock option myself, and that only to have tried it out. Great way to boil over a crucible! :)

    Normally I try to keep my furnace atmosphere controlled so it won't become an issue. But I've not been casting anything in aluminum where some porosity would matter anyhow really, so that is about all I know about degassing it.

    Jeff
     
  9. DavidF

    DavidF Administrator Staff Member Banner Member

    Could it be that the washing soda has absorbed moisture like the chlorine tabs do?
     
  10. Petee716

    Petee716 Silver Banner Member

    If so, can it be heated to drive out the moisture? And also, can the same be done with commercial degasser, or would they be chemically altered permenantly?

    Pete
     
  11. Jason

    Jason Silver Banner Member

    ...... I've seen Tobho's surface finish. It is EXCELLENT. Smooth as can be. I use nothing with the bronze. I've got no problems at the moment so not changing a thing.
     
  12. Tobho Mott

    Tobho Mott Silver Banner Member

    I blame my sand for that, and the giant muller at Smelko Foundry Products that made it. For all I can tell, Jason's candle holder and my other recent stuff could be full of pinholes on the inside. :) (They looked ok where the sprues were cut off at least)

    Something must be causing this porosity, nobody can doubt Art knows how to run his furnace... Maybe it really is the SS sand. Can't see how degassing could help if the hydrogen is coming from the mold itself!

    Jeff
     
  13. master53yoda

    master53yoda Copper Banner Member

    I think that the slower cool down time is the only effect that the SS bonded sand has on the casting. Also with the experimentation that i did in response to the soda question for a degasser leads me to belief that the actual turbulence that happens with the soda is actually entrained moisture in the soda...... when i allowed the soda to set on top of the crucible for a time it doesn't boil, also when i dried it in the oven it also didn't boil. I'm going to try some pool shock as mentioned, How much shockshoiuld I use in 12 lb of aluminium. One of the problems with using SS bonded sand for the entire mold is that it has about a $10 per pour cost as the sand isn't redially reusable and this casting is using 70 lbs of sand per pour. I'm hoping to get this worked out as the customer wants 10 of these if I can solve the bubble problem.
     
  14. PatJ

    PatJ Silver Banner Member

    Some of my pieces have had porosity issues and some have not.
    None of the issues were bad enough to make the parts unusable, luckily.

    I have tried the washing soda thing, and it appeared to have no effect at all on the AL melt; no gassing, no bubbling; the entire soda mass just floated to the top and had to be skimmed off.
    There was no mixing it into the melt.

    I think ironsides did a video or two on this topic; I will look for a link, but as I recall he seems to think the bubbling in the melt is more a matter of having moisture in the washing soda, not because any degassing is going on, and I tend to agree.
    (sidenote: ironsides is "luckygen1001" on YT, don't get confused)

    I have never had any bubbling when using dry washing soda; it had no effect at all.

    I am not bashing the use of washing soda, just stating what my experiences have been.
    If I am missing something I would like to know.

    Ironside's video above includes a test that shows that using washing soda significantly increases the gassing in aluminum.

    I have had some excellent AL pours with no gassing and with no flux or degassing agents added.

    I am not sure about pool shock; I have never used it due to its potential toxicity.

    I think the only sure method for degassing is to use an inert gas, and even then, it has to be done correctly.


    Here is an interesting video on casting motorcycle cylinders:

    and a crankcase pour:


    This one is pretty interesting too:


    I have not had time to study these videos closely.

    I think I can recycle SS sand by keeping the mix at 3%.
    It seems to break up much easier at that percentage.
    If nothing else, use fresh sand for the face, and recycled sand to fill the balance of the mold.
    Just don't overgass the SS sand, else it will have no strength.
    If you are using a catalyst, then that may be a different thing.

    Also, I have seen recommendations from the SS folks that SS molds can be lightly flamed with a propane flame to dry them out, and I have started doing that for small cores to ensure that they don't gas.
    I have not been doing it for larger molds, but I may start doing that.

    Here is Jeff H using I think argon to degas the AL for his motorcycle cylinder.
    I have read that if you force an inert gas into the mix with bubbles that are too coarse, then you are just entraining more air into the mix, and it does not work.
    Jeff could probably comment on the effectiveness of his argon lance, but generally the commercial lances use some method to make a very fine bubble, such as a motorized system.


    Jeff did a lot of research on sand, binders, degassing, heat treating, etc. before he cast his cylinders, and he seems to have gotten it right the first time, but he did his homework first.



    Edit:
    Here is some info I think I saw years ago; perhaps an AA guy; I can't place the name.
    It this somebody familiar? Seems that way.
    http://foundry101.com/archive.htm
    But he talks about flaming the mold, and gas in aluminum.

    Some points he makes about gas in aluminum:
    1. Don't stir the melt, it only adds gas. Skim only.
    2. Don't overheat the melt. I think masteryoda routinely superheats his melt, but I generally bring my melt up to 1350 carefully, not letting it every exceed this temperature by more than about 20 degrees.
    3. More surface area means more gas absorbed by the AL.

    I am sure there is more info on his website.
     
    Last edited: Sep 17, 2017
  15. PatJ

    PatJ Silver Banner Member

  16. Tobho Mott

    Tobho Mott Silver Banner Member

    Olfoundryman uses argon or nitrogen to degas for 7 minutes(!!) In this video, where he also talks about the importance of bubble size:



    That video turned everything I've read or seen online about degassing on its head. 7 or 15 minutes?! He sure seems.to know what he's doing in all his other videos though, so he probably does here too.

    Jeff
     
  17. DavidF

    DavidF Administrator Staff Member Banner Member

    olfoundryman is a pro at this stuff. I talk to him from time to time and have been trying to get him to join up with us, but the early days of AA left a bad taste in his mouth and he keeps himself pretty busy with his casting work. I still hope that one day he will join us, for now he said he would like to just watch us and see how things go...
     
  18. HT1

    HT1 Copper Banner Member

    I use actual calcium hypocrite (SP?) to degass aluminum, It is about 6 years past it's shelf life and is actually a puddy rather then a powder so when I have to do aluminum I put it in my special (DO NOT EAT OUT OF THIS PAN... PAN) and slowly dry it out on the burner of the BBQ GRILL. wrap it up in a piece of aluminum foil and keep it warm before plunging the heat.

    the commercial degasser tablet I used came individually wrapped and looked exactly like urinal biscuits(???) and we kept them in the plastic bucket they came in and used them for 5 years no problem... we used half a biscuit for a 70 LB crucible of aluminum, and saved the other half for next time

    V/r HT1
     
  19. PatJ

    PatJ Silver Banner Member

    I generally don't pay too much attention to many of the online casting videos, because most of them are beginners, and the advice they give may or may not be good (almost universally not good).

    This guy has it going on to be sure.
    Commercial quality, but he uses commercial grade products too (no shortcuts and home-brewed stuff), and he apparently goes by the book from start to finish.

    I watched several of his videos.
    Excellent stuff.
    I noticed that he vented his motorcycle plaque molds; I have to do the same with my castings since my sand is so fine that if I don't vent, then I end up with trapped air, which has caused bubble holes as large as a quarter, and over 0.25" deep.
    With molding sand that is porous enough, I have heard you can omit the vents, but I definitely cannot.

    Most impressive work. That surface finish is enviable.
    Tons of experience I can tell.

    Edit:
    I wonder, since he is using a crucible cover when he is degassing, could he not just pump large bubbles into the mix, since at the top of the crucible is totally enclosed with the cover, thus holding in the inert gas, so the large bubbles would only mix more inert gas into the mix?

    Edit02:
    That die-casting rig is the cat's meow; and very nice cast-in bolt holes; such a nice detail.
     
    Last edited: Sep 16, 2017
  20. crazybillybob

    crazybillybob Copper Banner Member

    I had some issues with gas when I was using SS. I found that even after gasing it with CO2 there was unset SS in the sand. I found that leaving the mold set overnight after gasing. This helped clear up the issue. I know guy bake their SS cores. But getting the whole mold in the oven is a headache!
     

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