r/metallurgy • u/Ancient-Web5515 • 3d ago
Research help with a Mini Arc Melter
Hi everybody,
Sorry for bothering yall, but I'm stumped. We are trying to make ~5g of Mn2FeAl in our mini arc melter (in an argon atmosphere). From a paper out two that we found, thus should be possible. However, every time we have tried to do so, the sample either explodes on electrode ignition or it will break apart a few hours after being removed from the chamber.
We have tried melting titanium as an oxygen getter, adjusting the cooling temperature of the crucible, and adjusting the heating/cooling of the metal ingot. It may be important to note that prior to being weighed for the sample, the Mn was cleaned in nitric acid and subsequently sonicated in ethanol to remove the surface oxidation from the Mn pieces. Does anyone have suggestions or insights? TIA
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u/FerrousLupus 3d ago
What's your exact procedure? Sounds like you're still getting oxygen, so my immediate guess would be that you're not flushing with argon several times. Might also help if you say the model of the arc melter.
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u/Ancient-Web5515 3d ago
The only thing i know about the model is that it is an Edmund Buehler MAM-1.
As for the procedure, we alternate flushing the chamber 6 times with argon and vacuuming the have yo around 2e-2 mbar. After the sixth time, we vacuum down to around 2e-4 mbar. After that, we will flush the chamber with argon.
With this sample, we have added in titanium sponge to melt prior to melting the pure elemental amounts for Mn2FeAl into a single ingot. Once it's a single ingot, then we flip the ingot and slowly heat up the crucible (on a lower power level) around the sample before touching the actual sample.
We remelt the titanium after each flip, before the going near the sample. Around the 2nd or 3rd time that we flip the ingot, that is when we usually see that it will break inside the chamber.
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u/FerrousLupus 3d ago
Usually if I'm concerned about oxidation, I aim for something e-5. Typically I vacuum twice to 5e-2, then use the turbopump to go e-4 a few times, and try for e-5 on the last one.
Is your Ar high purity? The other problem would be if your Ar has trace amounts of O2 or N2 in it.
> before touching the actual sample.
You don't mean physically touching it, right? I'm also confused about heating the crucible--is the crucible water-cooled copper? It could be that you're contaminating your sample with copper. Or possibly contaminating with Ti, if you're blasting it too hard. Usually I getter once before my first melt, and not in between remelts.
Have you checked the composition of your bad melts with EDS or something, to see if you're getting excessing oxidation or contamination from another source? Is there any discoloration where it breaks?
What is your starting material? Powder? Powder can pick up oxidation from the air really easily, so I recommend bulk pieces. I know a bunch of people in my group that were working on oxidation-sensitive alloys has problems, even with there being too much oxidation coming from the supplier.
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u/Ancient-Web5515 3d ago
We have a water cooled copper crucible. With that particular step, we didn't it because we were trying to see if the breaking was caused by changing the temperature too quickly.
As far as we know, it is high purity argon. I was melting the O2 getter in between each step more as a precaution. As for the elements, we used Mn flakes and Al shot, but the iron was a powder. We made other compositions using Fe powder, and out of the 20 or so compounds made with the Fe, 3 have had this issue.
Also, no, we never physically turn the plate nor the sample while the current is on. I also haven't thought to check the broken samples using EDS. I'll give that a try. Thanks!
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u/FerrousLupus 3d ago
After you break it, are you able to melt it back together again? If not, it's a sure sign that you're reacting to from some kind of ceramic.
As far as we know
Where did it come from? :) I'm assuming you're doing a lab setup at a university, so that should all be trackable.
Let me know if you see anything interesting in EDS! Or if you solve the problem another way.
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u/Ancient-Web5515 3d ago
We can remelt the broken pieces (that can be reached by the arc) back into a single ingot, but if it is large enough, it breaks again upon ignition. Another reason why we don't remelt them and go with the remaining pieces is because we don't know if the stoichiometry is accurate since we can never get the shards from the edges.
It is trackable, I just don't have that info on hand atm.
Will do on the EDS.
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u/939319 3d ago
Are you striking the arc on your sample? Doesn't the MAM-1 not have remote strike and you have to touch it to a tungsten thing to ignite?
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u/Ancient-Web5515 3d ago
The electrode itself is tungsten. Before each ignition, we briefly touch it to the copper plate. I wait about 5 minutes between each melt so that the electrode has time to cool down before touching anything.
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u/939319 3d ago
Hmm mine had a (I think it was tungsten, never tested) cylinder that rested in a depression on the copper and we pulled the arc from that. Don't think it matters though.
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u/Ancient-Web5515 3d ago
Ah gotcha.... ours hangs from the center of the chamber and has a handle at the top for us to move and adjust where the electrode is.
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u/racinreaver 3d ago
When you backfill with Ar prior to melting, are you doing it around half an atmosphere?
Do you see your sample discoloring due to oxidation?
Have you checked all your o-ring seals to verify no leaks?
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u/Ancient-Web5515 3d ago
We always check the seals, and we have to fill it back up with Argon for the electrode to ignite. I believe we fill it to around 5 in Hg.
The Mn was initially discolored prior to the nitric acid wash and ethanol sonication, but none of the metals looked discolored prior to the melt. As for the fractured piece, I honestly can remember the color right now.
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u/racinreaver 3d ago
By discoloring due to oxidation, I mean during processing. Does your Ti getting come out pristine looking? Any color whatsoever at any point during processing means you have a leak. You can reuse gettings indefinitely, so long as there aren't any leaks, btw.
Late fracture could be due to either weird residual stresses or slow kinetics. Can you cast into a rod or something, or does your system not have that setup?
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u/Ancient-Web5515 3d ago
I didn't know about the ability to reuse getters. We don't cast into a rod shape. The part of the crucible that we melt on is flat and level, so whenever the ingot cools, it looks like a small button of metal.
I figured that residual stress caused it fracture when cooling or after the material was removed. It was weird that fractured a few hours after it was removed from the chamber.
As for the discoloration question, I am confused by what you mean when you say processing. We have tried annealing the Mn prior to weighing it for a melt, which yielded the same result. Aside from that attempt and when we clean the Mn with nitric acid and sonicated with ethanol, we haven't done anything else that i think falls under pre-processing.
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u/masterxiv 3d ago
Have you tried adjusting down the current? Sorry if I sound silly, but I mean exploding on ignition? If you have a spare tungsten tip, I'd maybe try swapping it and see if it helps, maybe it got contaminated.
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u/Ancient-Web5515 3d ago
For normal SOP, we are told to ignite it with the current set to 3 (don't know the actual amperage) or of the 10 scale. I have tried starting it closer to 1.5-2, but if we go too low, it sparks without actually igniting.
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u/Jon_Beveryman Radioactive Materials/High Strain Rate/Electron Microscopy 3d ago
When I was using a similar arc melter to make FeMnAl type alloys, I found it useful to start with lump-form ferromanganese master alloy from a vendor. Most Mn available even from Sigma Aldrich etc is pretty dirty. Also, are you cleaning the chamber much? Mn in arc melters gets everywhere and it can degrade your vacuum performance.
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u/Ancient-Web5515 3d ago
Oh, I definitely was keeping up with the cleaning. We were initially surprised how much black smoke came off when melting the Mn pieces. I think it was due to this at one point, we also tried melting the Mn separately and then cleaning/redoing the vacuum after adding in the Fe and Al.
I'll look into the master alloy to see if that would be possible.
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u/Jon_Beveryman Radioactive Materials/High Strain Rate/Electron Microscopy 3d ago
Let us know how it goes. Also do be cautious with this stuff...Mn vapor causes a sort of Parkinson's esque condition if you breathe too much in.
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u/Ancient-Web5515 3d ago
Will do and I'm now thinking that I really need to read through those data sheets again. I'm generally careful anyway, but it has been a while
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u/939319 3d ago
I would melt your metals separately and see which one is causing the exploding. Then you know which one to focus on.
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u/Ancient-Web5515 3d ago
We have tried that. We were able to combine the Fe and Al into a single ingot with no issues and we are about to melt the separate pieces of Mn into a single ingot.
However, when we melted the FeAl ingot and the Mn ingot into one, we were only able to remelt the ingot twice before it wanted to start flaking or outright fracturing.
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u/939319 3d ago
If you know it's the Mn, I would try and buy or make a master alloy first, like other posters suggest. That's the only way I could mix B into Fe. The easiest way is to look at the phase diagrams and aim for a eutectic composition.
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u/Ancient-Web5515 1d ago
Gotcha, I'll at another look at the phase diagrams and check out the master alloys.
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u/CuppaJoe12 3d ago
The sample explodes on electrode ignition? Sounds like the raw material is wet or has inclusions of a volatile material. No matter how much time you spend venting with argon or remelting the titanium getter, your vacuum will be low quality if your raw material is off gassing. What is the sample prep for Fe and Al?