What are Some Hard, Crack-Resistant, Low-Toxic, Low-Melting-Temp Alloys?

Discussion in 'Castings, finishing/ repair/ and patina's' started by Bayview BOOM, Dec 31, 2018.

  1. hi

    I need a low-toxicity, low-melting point metal (before casting), to cast hardware parts. The cast object will be approx 4" x 1/2" x 1/4".

    After casting, the parts must not crack under torque or impact, and should remain hard up to at least 100 deg C. I mean, daily environment of the part will never exceed 100C.

    torque: hand-tightening by a person of average strength with an average consumer wrench, as in nuts and bolts.

    impact: roughly, dropped from 20 feet while mounted to a 2 lb weight (sorry, i don't have exact force rating)

    My current plan is to make a Tin/Bismuth alloy, since low-toxicity, low-melting point. I've heard a tiny amount of silver can make this alloy less brittle. i plan to start with "pure" metal powders or ingot, and combine in the melting pot. Wondering if there's a DIY hardening process for this alloy.

    Any suggestions? Could be a different alloy, or a hardening process. I'm not committed to tin, bismuth, or silver. But need to keep things relatively low-toxic and low-temp. (home, indoor fab). Need to minimize shrinkage/expansion of cast part.

    thx!
     
    Last edited: Jan 1, 2019
  2. DavidF

    DavidF Administrator Staff Member Banner Member

    Your idea of low temperature is considerably different than most of ours. I would be looking into some of the zamack alloys or even zinc, but that's just because it's what I'm more accustomed to.
    Sounds like your planning on making some type of tool?? Not knowing what it is your trying to make and how forces are exerted on it adds to the difficulty.
    How are you planning on melting? Do you have a furnace or using other means?
    Welcome to the forum.
     
    Bayview BOOM likes this.
  3. hi, thx for reply.

    I'm making a little bracket, which will be bolted down with a nut and bolt, with a hand-wrench. Tightening the nut, and the on-going hold of the nut, will be the only force exerted on it. It will bear less than 1/2 lb weight directly (i mean an object it will support, not counting the pressure of the nut). It will be bolted to something which may be impacted as i described in the OP.

    currently planning to use the 'compact and sinter' method (aka 'solid state sintering'), where loose metal powder is packed into a mold, and the whole thing placed into an oven.

    melting point: it's my understanding that tin and bismuth are considered low-melting-point metals. When i mentioned 100C, i wasn't referring to my foundry temp-- i was referring to the environment where the part will be used after casting.

    i've read antimony can be used to harden tin, and wondering if there's a less toxic solution that could be used.

    also considering a copper-tin alloy, if low-temp is possible.

    cheers and happy new year
     
    Last edited: Dec 31, 2018
  4. DavidF

    DavidF Administrator Staff Member Banner Member

    Solid state sintering with low temp alloys that is low toxic??
    Ok, I'm assuming that this will be a production run of a part to generate an income/profit.
    I feel your over thinking and over engineering this.
    KISS. Keep it simple
     
    Bayview BOOM likes this.
  5. Simple is exactly what I'm trying to do :)

    Yes, this is small scale production of a product.

    But, not for-profit. We are a nonprofit mentoring org for disadvantaged teens. Our goal is to give our apprentices a taste of professional methods and materials, in a safe, simple way.

    Https://BayviewBOOM.org

    Compact-and-sinter appeals to me because, no exposed pots of hot molten metal. They would only handle dry, room temperature powder (in a glove box). The cool, packed mold goes into the oven, and a cool, hard metal piece comes out. Seems darned safe and simple to me :)
     
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    Ah ok I see where your going now.
    Hmm, I'm going to have to put some more thought into this now....
    What do you have on hand that you would be using to sinter the metals together and what were you planning on using for the mold?
    I'll think things over tonight and see if I can come up with something to help you out...
     
    Bayview BOOM likes this.
  7. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Think you will need some very good temp control. There is a fine line between sinter and melt, and the line gets fine the lower the melting point of the alloy.

    Best,
    Kelly
     
    Bayview BOOM likes this.
  8. DavidF

    DavidF Administrator Staff Member Banner Member

    I keep thinking back to a video I have seen of some primitive investment casting where the metal was shelled up inside the mold along with the wax part. The mold was fired and then flipped transferring the melt into the mold cavity while remaining sealed. Once cooled the mold was broken open revealing the cast part.
    Now where's that video??
     
  9. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Not sure but I want to say that was Artopsy. I remember the video because I believe it was bronze and he grabbed the shell with mits or maybe wool to flip it....:eek:

    Best,
    Kelly
     
  10. DavidF

    DavidF Administrator Staff Member Banner Member

    Nope not him, this was done outside in a foreign land. I'll search it out later tonight.
    Might have been some type of ancient gold casting???
    I've seen way too many videos over the years to remember exactly..
     
  11. Plz tell me more! What is the difference? What would result if we melt? Pethaps we can mitigate this issue with a very very slow, lower temp heating phase.

    Says here, "The temperature used for sintering is below the melting point of the major constituent of the Powder".

    That's very intriguing, as it might open us the option for harder, higher-melt-point metals.

    Says here, the lower temp metal may well melt during sintering, which isn't an issue, as long as the higher temp metal doesn't melt . Therefor, fine-temp-control might not be needed after all :)

    I've read it's possible to "dissolve" a cold metal into an already melted metal, and that "dissolve" isn't the same as "melt". Is that related to sinter-vs-melt?

    I'm very curious about bronze, since bronze is hard. but concerned about the high melting point of copper. Maybe low-temp sintering could make bronze a possibility for us :)

    I understand what makes bronze hard is the significant size difference between copper and tin atoms. The large tin atoms cause misalignment of the slip-planes of the copper atoms, causing a sort of atomic traffic jam :) Wait, I think that's about alloys, not sintering.

    My understanding is that sintering can produce a harder result even with just a single type of metal . Maybe we don't need a mix! Here's a paper about sintering bismuth by itself. Wow!
    What does TM mean?
    I don't want to use wax. The wax will make fumes when it melts, it's messy, and is only needed with a loose mold like sand. I plan to use a solid silicone mold. It will be non-shrinking platinum-cured silicone. No wax, no sand, no mess.

    Want to avoid flipping or touching the hot part mid-process. My intention is to mix the powders to desired proportion before packing into the mold. And we'll ensure the oven heats evenly all around. Not just a bottom burner.

    Hoping the smallness of our part will minimize various risks. Also hoping starting with powder might speed up the heating phase- I'm ( naively) hoping powders will fuse more easily than shot or ingot. Seems powder may fill crevices easier than a pour, with less chance of air bubbles.

    This guy is doing PM without gloves or a mask!



    Silicone molds for molten metal is an established method . Powder metallurgy is an established method . My novel idea is combining them :)

    One concern is the compacting process might deform the silicone .

    Thx!
     
    Last edited: Jan 1, 2019
  12. DavidF

    DavidF Administrator Staff Member Banner Member

    Before you go sailing any further into uncharted waters you should check the metalurgy out a bit more and try a few experiments.
    One thing I ran across while doing a quick search was that tin based alloy powders do not sinter well due to the rapid formation of oxides.
    However you could possibly melt it inside the silicone mold provided that it is gated in a way that would allow the dross to exit the part itself.
     
  13. DavidF

    DavidF Administrator Staff Member Banner Member

    TM in this case would be melting temperatures.
    So your looking for a heat of 78% of the metals melting point
     
  14. i've no intention to go into production until the process is well-tested and validated, if that's what you mean.

    can you plz link to that? I'm thinking we may get hardening using a single metal instead of mixture. Btw, not sure sintering of mixed powders would be called an "alloy", as they are not molten together.
    Ah, dross is another issue. But i believe in sintering, unlike melting, there's no dross.
    thx!
     
  15. DavidF

    DavidF Administrator Staff Member Banner Member

    I'll have to find it again, but basically what was stated is that Tin and tin based alloys rapidly form an oxide layer. This oxide layer would act as a barrier and keep the metal powders from sticking together during the sintering process.
     
  16. DavidF

    DavidF Administrator Staff Member Banner Member

  17. i'm wondering if a flux or lubricant can prevent that.
     
    Last edited: Jan 1, 2019
  18. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Inert gas sintering atmosphere.

    Best,
    Kelly
     
    Bayview BOOM likes this.
  19. they say:
    i think they mean "must be", not "can be".
    Woah, they say:
    Hah! That statement makes the previous sentence incorrect: if tin can't be sintered, then it can't be sintered at any temp. Well ok, tin's out, if we go with sintering (and if adding flux doesn't help) :D

    So, need to find a low-temp powder which doesn't have this problem. If sintering works at ~75% melting point, and if i want my max oven temp ~250c, then need to find a non-oxidized power with a melting point ~650C.

    they go on:
    unclear why they mention "previously pressed". Tin powder is previously pressed?

    Anyhoo, they don't mention bismuth. See my link above re bismuth sintering. Apparently, that can done.

    Kelly, plz see my overview here of methods to prevent/remove oxidation. Seems that neither flux nor inert gas will help, cuz according to the above paper, the tin powder is already oxidized.

    thx!
     
  20. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Inert gas or vacuum would be to prevent oxidation during sintering process. Most sintering processes use combinations of pressure and temp. A silicone mold won't stand any appreciable pressure and is a poor thermal conductor. Not quite as lofty a goal as room temperature fusion, but......good luck, let us know how you get on with it.

    Best,
    Kelly
     

Share This Page