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I Forge Iron

6061 - Casting

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Ok, before I get the knee jerk response that 6061 isn't good for casting I actually have something to add to the subject.  Well, I think I do.  

Yes, 6061 as it is is not great for casting, but because of my primary business I generate a lot of scrap 6061.  The first solution I found is maybe beyond the average backyard metal caster.  Its pressure die casting and fast cooling.  You still have to make some allowances in your design for shrinkage and what not because you are making a molten metal solid, but it doesn't allow for the grain structure formation that seems to make 6061 less suitable for casting.  Its an industrial process and take a little more than most people are willing to do, but I saw it as a possibility.  Not one I would likely do because of the work or capital (or a little of both involved), but a possibility.  I'm not an expert on the subject by any means.  I've just read a couple articles on it and a few published research papers that touch on it.  The steel dies would not be horrible for me to make.  Its the whole pressure casting setup that is a bit more involved.    

The other one was a bit of a surprise.  I posted in a machining group to give away a bunch of small bits to somebody who could use them for their hobby machining projects.  I sent off a stuffed full padded flat mail envelope (not even a noticeable dent in my scrap piles) of small blocks.  A few days later a fellow by the monicker of Gary H Lucas said he considered those to be melty bits.  I run into Gary on various forums and he's usually not full of horse pucky, so I explained that those were 6061 and generally not accepted to be great for metal casting.  Yep.  He agreed and then explained he adds a small amount of silicon metal and it then pours a lot like 356.  

356 aluminum is one of the most common cast aluminum alloys.  It or proprietary blends that are suspiciously similar are used for engine blocks, heads, light machine tables, and a host of other applications.  I have some remelted alloy ingots on hand that came from engine blocks melted in another small foundry.  

Now I have not tried it yet, but I did order 22 lbs of elemental silicon chunks.  I will be trying it eventually.  If I can recycle most of my scrap 6061 it will be awesome.  Silicon is not cheap, but proper 356 alloy ingots aren't either.  After I ordered the silicon (not here yet) I decided to look up the milting point of silicon metal.  Wow.  Its a lot hotter than aluminum.  2577F Beyond the heat range of the furnace I am building.  Actually converting some of the parts of a pottery kiln to my own setup with PID controls.  

So, I went on a forum Gary hangs out on regularly and asked him about alloying the silicon with the aluminum.  He claimed it was melted in at his regular 1100F aluminum pouring temperature.  I guess that could happen.  There are other metals that "blend" at lower than their melting point.  If it all works I'll report back.  If you have more knowledge on this please feel free to pile on.

From what I understand it wouldn't be good for the mountains of chips I generate due to the percentage of it that is oxidized aluminum (the skin on all aluminum exposed to air), but it would be nice to not shovel all those drops, and bad cuts into the recycle bin.  

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I really like the way you are thinking. Making something out of "waste" is how people get rich. 

I don't know much about casting, but your friend is right about the Silicon just melting. You can see this by looking at a phase diagram for the two metals. If you have about 12 percent Silicon then the melting point is 577C/1071F. But you don't have to be exactly this amount to get them to melt together, you can add in less and still get liquid because it COULD be 12% at some point in your solution.  once thoroughly mixed and solidified you would need at least that temp to melt it again.  Now this is oversimplifying things though because 6061 has many other metals in the mix and each one has an effect. 



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I was just chatting with Gary over on the CamBam forum and he confirmed something for me.  I had found an old post he had made on the subject where he talked about how much aluminum and silicon he had used to make some castings.  I quickly did the math and he only added about 4% silicon.  That brings 6061 upto about 4.4%.  356 as I mentioned earlier is 6.5-7.5%  I asked how it poured and he said he only used 4% because he only had a pound of silicon and was saving some just in case, but his 4% add poured as good as he hoped for.  I don't know what that means metallurgically, but that may be good news economically.  If I can get good castings with just 4% added that's a big deal.  Silicon metal is expensive.  Not like gold expensive of course, but for general use and playing expensive.  Around $10/lb.  More for just a couple pounds.  A little less for 20+lbs.   

I also ran across some references to using ferrosilicon alloy.  I need to read more on that because actual 356 does have some iron as well.  

My goal would be a casting with good machinability.  

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  • 11 months later...

If only I had the time to play...  Been so busy machining parts.  Right now I'm work on a 50 mold (100 parts) wholesale order, and there are a dozen custom jobs waiting.  On the other hand there is a lot more 6061 scrap in the shop to play with.  LOL.  

My son has a steady girlfriend now (he even invited her for Thanksgiving dinner), and its hard to get him to help in the shop.  He's working on pharmacy tech license, still working on his engineering degree, and of course working, so I can't begrudge him wanting to hang with his girl instead of his dad.  I did get him to haul a bunch of plywood upstairs in the shop for some shelves I want to build up there the other day.  

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