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. i'm confused. i thought the issue with sintering tin is that the powder is already oxidized. You can't prevent it getting oxidized during sintering with gas, because it's already oxidized. Right?
    Yes. I'm wondering if it's possible to change the ratio of pressure to heat, say if i want to sinter at low temp then increase the pressure, or if i want to sinter at low pressure then increase the temp. But, DavidF's article indicates sinter temp is always going to be about 75%, so that tradeoff might not be an option. Another adjustable variable is processing time, so maybe time vs pressure is a possible tradeoff.

    Yep, as i mentioned above. However, the silicone will be encased in a rigid shell (prolly steel), so there's really no place for the silicone to go. Deformation is still a concern, and that's all about the pressure. Maybe we can find a pressure/temperature/time tradeoff which will not result in deformation.

    Or, maybe we can simply bring the metal to full melt, and eliminate the pressure altogether. Then it's not sintering, then it's just standard casting. In which case, hardening is still the issue, in which case making an alloy is of interest.

    Good point! Might not be an issue-- the point of sintering is that it's low-temp. Note, there are thermally-conductive silicones.

    silicone mold for molten metal pours is an established, validated technique. Powder Metal compact-and-sintering with a metal mold is also an established, validated technique. Therefor, the idea to combine them can hardly be compared to room temperature fusion!

    thx, but are you locking this thread or ending the discussion?
    Last edited: Jan 1, 2019
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Not locking. Inventing a new manufacturing process to make a part that could probably be done better by existing processes, just doesn't strike me as a sensible approach. If you just want to be able to make the 4 x 1/2 x 1/4 widget in a class room environment that is safe for the young folks, why sinter?

    You could just cast the part with low temp alloy in your silicone mold without the additional complexity for no apparent gain. The low temp alloys will be comparatively expensive low performing materials. If the idea is to commercialize the widget at some point, why not use a manufacturing process that's fit for purpose, and develop a business plan designed to make a return on the investment? ....Now that's a valuable lesson to learn!

  3. thx for suggestion, but i'm seeking metals fabrication advice -- not business advice, nor educational advice, nor cost-analysis.

    Would be helpful if you could respond to my question about oxidation, as your suggestion to use an inert gas seems incorrect.

    The goal is low-toxicity, low-temp, and hardness. That's the gain sought, by sintering. Sorry if i was unclear about that.

    Would be delighted to find a "manufacturing process that's fit for purpose". Which process are you referring to, which is low-toxicity, low-temp, and gives hardness?
    Last edited: Jan 1, 2019
  4. DavidF

    DavidF Administrator Staff Member Banner Member

    Bayview, dont be so cockey! :mad:
    Bayview BOOM likes this.
  5. Which aspect of sintering do you consider unsafe?
  6. DavidF

    DavidF Administrator Staff Member Banner Member

    You seem to be misinterpreting the point he is trying to make. You also have no understanding of how sintering works.
    I suggest slowing down your fingers on the keyboard and allow your brain to catch up for awhile...
    Bayview BOOM likes this.
  7. hi Kelly et al

    i am totally grateful for your feedback. And i have much respect for folks who have deep experience with foundries and metals. I am lucky to learn from the gurus here.

  8. i get that this website is dedicated to traditional casting, not PM. So, i won't post about PM if that offends anyone.

    @Kelly, i share your concern that a silicone mold might not withstand the pressure. Even with a steel casing, the silicone may deform. So, acrylic resin is another option. Or a hard metal, which could be cast using conventional means (our youths need not get near that process).

    I just discovered "metal clay" (metal powder + water + binder), which can safely be prepared by hand :) Seems a strong candidate. This vid show someone molding the clay by hand, and sticking it in the oven. Could be ideal for us!

    (skip to 2:15)

    They're using something called "Shrinkage Stoppers", which i think just means a mold which restricts shrinkage by the rigidity of the mold. I'm guessing the result will be a more porous metal than if allowed to shrink naturally.

    To prevent oxidation, some makers place the pieces to be heated into activated carbon inside a container (developed by Bill Struve). Pretty ingenious.

    Jewelers use precious metal clay (cuz jewelry), but seems you could use any kind of metal powder. These DIY'ers are experimenting with inventing their own metal clays. Neat!
    Make your own metal clay

    Tho, clay isn't loose, like molten metal or powder, so it might not fill crevices as well.

    Last edited: Jan 4, 2019
  9. HT1

    HT1 Silver Banner Member

    take your questions here

    I think they can find you an alloy that would do what you want , though I would give up o the powdered metal idea, and simply put actual metal into a void above the the mold ... Picture an Hourglass , heat the entire mold to the pouring temp. molten metal runs down into the actual mold cavity. wait for it to cool open it up, tada finished product. if your part is simple this could be dome in a permanent mold similiar to sinker molds, just size the fill area large enough to hold the metal to fill the cavity ... QED

    V/r HT1

  10. Thx! Of the low-temp metals they list, only these seem to be low-toxicity:
    gallium, tin, zinc, bismuth.

    Of those, gallium's temp seems too low to cast anything. Leaving tin, zinc, bismuth, which are already on my short list. So that's good validation .


    Great, that is worth testing. I figure That will require a higher temp than sintering powder .

    Last edited: Jan 4, 2019
  11. HT1

    HT1 Silver Banner Member

    Go ask your Questions to a metal manufacturer, and they can give you the SDS sheets so you have documentation... not " my references seem to indicate it is safe" as a defense if someone does get hurt.

    all the powder metal processes i'm familiar with require pressure and "HIGH " temperature. Pressure will be a NOGO in a rubber mold as will high heat. additionally you have to source powdered metal, that will be difficult and expensive . think about it they have to take solid metal and grind it into powder, that has to be costly. and I have no idea where you would source it. anyone???

    if you use my hour glass concept with metal in a small shot form, calculate and weigh your material carefully , you will have no waste, and very little clean up, and no handling of molten metal hot parts, seems like just what you are looking for simplified and more cost effective. My humble educated opinion

    V/r HT1

    P.S. I would seriously consider moving away from the Rubber molds as soon as possible consider something like this https://www.ebay.com/itm/CUSTOM-MOL...m=222539311879&_trksid=p2047675.c100011.m1850
    Al Puddle likes this.
  12. Hourglass method
    thanks for that awesome suggestion, i'm seriously considering it.

    In your method, we'd have to ensure the raw metal chunks are just the right size, which i guess means cutting them down. So, there are going to be metal shavings/dust involved either way, right?

    If we source metal in power form, and prepare the mold in a glove-box, then that seems cleaner, easier, and safer than chopping up ingots with a hacksaw.

    We could also get shot, that would enable your method without any cutting. Shot for $15/lb, about half the price of powder, so that's interesting.

    i'm curious why belmont's is a different color than roto's.



    here's a good one.

    naturally. That's not my "defense if someone gets hurt", it's my assessment of materials at this early planning stage. Once we involve apprentices, we have legal advisors, liability waivers, insurance, etc. and yes we back ourselves up with legit documentation.

    High yes. But lower than melting temp, correct?
    Therefore, your recommended process of bringing the metal to full melt would require even higher temps than sintering, correct? Also, i'm choosing the lowest-temp metals available, to keep temps a low as possible, whichever casting method we use. Bismuth melting temp is 271c, so it's sintering temp should be about 211c.

    Agreed, therefor a rubber mold is a nogo for PM. A metal mold could be used for either powder sintering or hourglass-melting.

    We'd have to use a metal mold, who's melting temp is higher than the sintering temp of the PM material. How much higher? It should be a low as we can get away with safely, since we'll want to produce the metal mold ourselves.

    Would be great, if it wasn't too expensive. We can prolly source it cheaper from a from a local metallurgist, but would rather make it ourselves. (apprentices would not be involved in making the metal mold, so fewer constraints there).

    Sourcing Powder
    Yes, we have to source it, unless we make our own. So far, i haven't seen an acceptable process for making our own.

    Belmont carries Bismuth powder for ~$30/lb. If my math is correct, that's about 3" cu of bismuth. Our component is 0.5" cu, so we'd get maybe 5 parts per lb. That's about $6 per part. A bit expensive, but in the ballpark. Belmont's just the first source i looked at, so i wouldn't assume it's the cheapest.

    Metal Selection
    Bismuth is a softer metal, and the goal is to make it stronger. My understanding is high-pressure PM sintering produces a stronger, denser metal than pour-casting. Correct?

    Note, i've now rejected tin, due to the problem of tin-whiskers. We're making an electronic device, so we can't have that.

    Last edited: Jan 5, 2019
  13. HT1

    HT1 Silver Banner Member

    is there a reason you keep trying to comeback to making your own alloys ? Making alloys and casting alloys are two different processes , sometimes they overlap but not much, definitely not in modern work. if temp is a huge concern for you, , http://www.hitechalloys.com/hitechalloys_002.htm

    You can melt cerroalloys in a double boiler , they can provide you SDS, and alloy mechanical specifications, and if you need it in shot, you can make your own, by melting and drop balling into cold water.

    as to sintering being stronger, possibly in very specific alloys, processed in very specific ways, and again I suspect pressure is involved. a lot of pressure.

    is your goal a science project or a finished project ??? if it is a finished project you need to think KISS

    V/r HT1
    Al Puddle likes this.

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