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Re: [hobbicast] Re: Alloying

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  • mikey98118
    Thanks, Tim. Your phase diagram description was a treat to read. Mikey ... From: tmoranwms To: hobbicast@yahoogroups.com Sent: Fri, 01
    Message 1 of 23 , Mar 2, 2013
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      Thanks, Tim.
      Your phase diagram description was a treat to read.
      Mikey

      ----- Original Message -----
      From: tmoranwms <tmoranwms@...>
      To: hobbicast@yahoogroups.com
      Sent: Fri, 01 Mar 2013 17:10:23 -0000 (UTC)
      Subject: [hobbicast] Re: Alloying













      You can find the phase diagram here:
      http://antipasto.union.edu/engineering/Archives/SeniorProjects/2006/ME.2006/agostina/photos%20from%20project/photopages/photo01.htm


      On the left is 100% Al, on the right, 100% Cu.



      If you follow the topmost curve, bounding the liquid area, you are following the liquidus, meaning that, above that temperature, everything is liquid. The line just below that curve is the solidus (melting point), where one phase (a solid solution or eutectic) melts, and acts as a solvent to dissolve progressively more of the higher-melting phases, until everything goes into solution (at the liquidus). So if you have a 10% Cu alloy (over on the left), starting from room temperature, it's composed of two things: nearly pure aluminum, and theta (Al2Cu, a brittle intermetallic phase). As temperature rises, it passes 548C, and a little bit melts. Now in this triangular region, there is no theta (it formed a solution with some of the aluminum), only aluminum crystals floating in solution. It has a goopy, slushy consistency. Finally, at, oh, 640C or so, the whole thing is molten.



      If you made a 50% master alloy, it would be almost pure theta, which is very brittle, and has an acicular crystal habit (the crystals that form on top of the ingot, as it cools, which have a distinctive rodlike appearance, not like the angular lumpy crystals pure 1099 aluminum makes on freezing).



      As copper content continues to rise (or aluminum falls), melting point rises steeply as a number of higher intermetallic phases form. The extent of these intermetallics, and their stability, is complicated, and since they are all very brittle, they are of no commercial importance. (Regarding stability, there's a large beta region around 88% Cu and above 567C, but it doesn't remain stable, we say it decomposes below 567C into other stuff. Alpha 2 apparently re-forms at the same percentage, at much lower temperatures. It's the same proportion of atoms, yet there are two different crystal forms, and one range where neither is stable! Sometimes, chemistry is bizarre like this.)



      Up past 92% Cu or so, a solid solution is formed, meaning aluminum is freely dissolved in the copper, whether liquid or solid. This is similar to brass, which is (for the most part) a solid solution between zinc and copper. Properties change slowly with respect to alloy in these forms, so for example, 10% zinc still looks kind of coppery, 20% looks golden, and 30% looks brassy. Aluminum is a stronger alloying agent in copper, so over the 5-10% aluminum range, color goes from coppery to gold to brass in an analogous way.



      Aluminum bronzes are usually around 10% Al. This is on the edge of the solid solution, which means some intermetallics will be present. In small quantities, these hard components lock the crystal structure, hardening it, while reducing ductility (they're brittle!).



      As for making alloys, copper dissolves in aluminum very easily, but getting it to make contact is the hard part. Both are coated in an oxide layer; submerging copper metal in an aluminum melt is only likely to make metal-liquid contact at a few points. Imagine dissolving a block of salt in water, except the salt was wrapped with duct tape. Not going to go very fast, right? If you had a solvent to disrupt that layer (for duct tape, paint stripper probably works well, but for metals, you need a flux), it would dissolve easily. Still, verifying that it's fully dissolved isn't as easy as looking inside the glass of water and seeing that all the salt dissolved. Better safe than sorry.



      To make copper-aluminum master alloy, I suggest melting a sample of copper, then adding an equal weight of aluminum (preferably aluminum wire, which is 1099 alloy, or something similar). Either do it gradually so the temperature remains high (and everything remains molten), or add it all and bring it back up to temperature before pouring. Failure to remelt will likely result in a frozen heel of intermetallics -- not so handy for consistency, or for breaking things up later (some of those higher intermetallics, like gamma, may still be brittle, but they can be very strong!).



      Tim



      --- In hobbicast@yahoogroups.com, michael.a.porter@... wrote:


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      > Howard,


      > That's an interesting supposition. "At some point" covers a lot of ground; I think you'd need to run an experiment to prove or disprove. An experiment, would probably be cheaper than asking a metallurgist.


      > Mikey


      >


      > ----- Original Message -----


      > From: StoneTool


      > To: hobbicast@yahoogroups.com


      > Sent: Fri, 01 Mar 2013 16:01:05 -0000 (UTC)


      > Subject: Re: [hobbicast] Re: Alloying


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      > Nelson:


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      > Aluminum bronze is an alloy I am familiar with, and excellent


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      > bearing material with good useful properties. My question here is, if


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      > you keep feeding copper into molten aluminum without significantly


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      > raising the melt temperature, at some point your alloy will become a


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      > copper alloy rather than an aluminum alloy.... Presumably as you pass


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      > 50%. I find it difficult to believe that the melt temp wouldn't have


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      > to increase considerably as this point is approached and passed........


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      > Howard


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      > On 02/28/2013 09:10 PM, Nelson Collar wrote:


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      > > No because the aluminum does not have the same properties that tin has. Aluminum is nothing but dirt-sand.


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      > > Nelson Collar


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      > > ________________________________


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      > > From: Robert Broughtonbroughton%40rocketmail.com" target=_blank>r.broughton@...>


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      > > To: "hobbicast@...@yahoogroups.com>


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      > > Sent: Thursday, February 28, 2013 8:43 PM


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      > > Subject: Re: [hobbicast] Re: Alloying


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      > > There are different types of bronzes with differing levels of copper and some other metal or silicon content. However on all of them, the copper is the predominate (over 60 and usually over 90 percent) component in the alloy mix. If you are only adding 6 to 10% copper, it would be more of an aluminum alloy than a bronze. However, if you switched it around so you had 90% copper and 10 percent Al, then it would be an aluminum bronze.


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      > > From: Nelson Collar nel2lar@...>


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      > > To: "hobbicast@...@yahoogroups.com>


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      > > Sent: Thursday, February 28, 2013 6:20 PM


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      > > Subject: Re: [hobbicast] Re: Alloying


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      > > Howard


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      > > It may seem hard to believe but it is true. And no it does not become a bronze. It is a aluminum alloy with copper. After the aluminum is melted with no unmelted pieces in it, place the copper (I use from 6 to 10% copper) it will melt almost on contact. When it comes to remelting it, it does not act like the aluminum you melted in the first melt. The copper makes a very hard set and is a lot nicer to machine.


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