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Making a Vacuum Furnace

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krfkeith

Thu Apr 25 2013, 11:21PM

Registered Member #10924 Joined: Mon Mar 11 2013, 04:02AM
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Posts: 5

My goal is very simple: I want to cast Titanium, and perhaps some other exotic metals, by myself. This has actually been done before, well, sort of. Theo Gray wrote a short piece about making metallic titanium using a thermite reduction, which, while interesting, didn't really produce anything that one might call "useful." So why Titanium? Well, why not Titanium? In all seriousness, Titanium even aside, having a vacuum furnace would open a lot of doors to do stuff I couldn't otherwise do.

I plan to use an induction furnace to melt down the charge. That being said, induction furnaces have been done to death, so I don't feel the need to rehash that here, thus why I didn't post this in the HV forum. That being said, if there are any caveats to look to look out for with induction furnaces, in the specific context of operating in a vacuum, feel free to mention so.

Now onto the fun part. In the cursory research I've done, most vacuum furnaces seem to work by using a crucible with a hole in the bottom of it. A rod, with a bulb in the end, is used to stop the hole. When the metal is fully melted, the plug is released, allowing the molten metal to flow into the mold.

Anyone have any idea on how I should go about doing this?

IamSmooth

Thu Apr 25 2013, 11:39PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I suggest going to a blacksmith forum. There should be someone there that has dealt with this. They have sections on induction furnaces and exotic metals.

Ash Small

Fri Apr 26 2013, 08:47AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Another way is to 'invert' the whole charge/mold assembly, although the method described by IAS may be more practical.

I'm very interested in this subject too, so please post as much detail as possible.

The vacuum induction furnaces I've worked on in the past all have the ability to 'purge' with argon, this is important as it enables removal of all the oxygen/nitrogen in the chamber. Also, I think some just melt the charge above the molds and allow the mold to fill by gravity (this was for casting ingots of nickel/cobalt super-alloys)

Vacuum furnaces are also useful for casting other metals, eg aluminium, because, the 'melt' is completely de-gassed, thus avoiding any pourosity in the subsequent castings.

Also, the eddy currents do a very good job of 'stirring' alloys.

I'm sure you're already aware that if molten titanium (or even hot titanium) comes into contact with air/oxygen it will explode

krfkeith

Sat Apr 27 2013, 04:48AM

Registered Member #10924 Joined: Mon Mar 11 2013, 04:02AM
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Posts: 5

I've thought of another problem. I'd like to use metal powders, for one thing so I can make it easier to load the right amount of metal.

Here's the problem, wouldn't air bubble get trapped in the powder? That seems like a nightmare to deal with. Moreover, wouldn't the higher surface area mean more out-gassing? How is this dealt with in industry?

klugesmith

Sat Apr 27 2013, 12:24PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

How would trapped air bubbles occur in a vacuum?

The high surface area of metal powders means more issues with surface "stuff". You might need a flux to be sure the particles fuse properly. Fluxes are an important ingredient in vacuum brazing processes.

As for outgassing, isn't that a vacuum pump capacity thing? How low a vacuum do you need? In an article about "specialty metals" history, I read about postwar development of high-capacity oil diffusion pumps. They enabled e-beam melters in place of vacuum arc melters (no mention there about induction heating). Would like to hear more about Ash's experiences.

One thing industry does with metal powders is sinter them without re-melting. I learned that beryllium, for example, wants a grain size too small to get by casting or working. Refining may involve vacuum melting into nuggets, but those are mechanically pulverized before hot pressing into near-net shapes. Consequently the lowest cost grade contains up to 2% BeO.

What size furnace are you thinking of? Could you use a tube of borosilicate glass or fused quartz, as used in tube furnaces, with your IH work coil on the outside?

Ash Small

Sun Apr 28 2013, 01:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

klugesmith wrote ...

Would like to hear more about Ash's experiences.

Anything in particular?

I did hear that modern 'commercial' induction furnaces tend to run 'straight off the mains', ie 120/240V @ 50/60Hz, so they must have HUGE capacitors to get a resonant frequency that low.

EDIT: or maybe three phase? I don't know.

coillah

Tue Apr 30 2013, 06:45PM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

I work on vacuum control systems. If you need help with the vacuum aspect I am happy to try and help.

Do you already know what ultimate pressure you need to get to? Also, what does your budget look like?

Conundrum

Tue Apr 30 2013, 08:53PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

I have a spare induction cooker here plus a couple of boards.

Any use?
I think the frequency is 65 kHz FWIW.

Ash Small

Sun May 12 2013, 07:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

"Operating frequencies range from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required. Generally, the smaller the volume of the melts, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. For the same conductivity, the higher frequencies have a shallow skin depthâ€”that is less penetration into the melt. Lower frequencies can generate stirring or turbulence in the metal."

I suppose this is obvious really, as the size of the melt increases, the frequency must decrease.

Billybobjoe

Sun May 12 2013, 09:34PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

I've made a small vacuum kiln successfully about a year ago. Main goal was to degas sterling silver.

The chamber is a stainless steel fire extinguisher body that I welded to a stainless plate/flange. This arrangement sits on an aluminum plate with an o-ring groove machined into it.

I didn't do induction heating. It uses a Kanthal metal heating element (1260C max temp I believe) that I wound out of bulk wire, controlled by a variac. This element was set into a soft firebrick shell. Temp is monitored with a type K thermocouple. Electrical feedthroughs were made with Hysol 1C epoxy. I didn't do induction heating for simplicity, but I plan to implement it in the future. You will probably need it if you want to reach Ti temperatures.

I pump it down to about 60 micron (limit of my rotary vane pump) and then backfill with argon. I'm having trouble finding any more pics, or any of the outer chamber, but of course I can answer any specific questions. I also have a baseplate with homemade high voltage feedthroughs for plasma experiments.

I just do melting, no pouring, but it would definitely be possible with a rotary motion vacuum feedthrough. Or use a linear motion one to slide a plate away from the bottom of the crucible and dump the melt.

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