[Sandow]

So I'm trying to put a tungsten grid together for a high temperature hard vacuum application. I've tried welding pure tungsten electrodes with thin rod as fill and it sticks together just fine but it is extremely brittle. Even a few degrees of flex and the portion that has melted snaps away clean. The grid structure I'm trying to build is more or less a hollow sphere so the geometry is a bit complex and I keep snapping it to bits when I get to triple points. It doesn't really need to have much strength or corrosion resistance in the end since it will never see mechanical load and it will be in hard vacuum most of the time but it needs to be very high melting. Any suggestions?

-Sandow

welded tungsten

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[big gear head]

Will Inconel or Molybdenum not work?

[Sandow]

Inconel is too low in melting point but moly is an interesting suggestion. I've seen literature about tungsten/moly alloy welding. It was sintering plates together though. I imagine this would be tig braising rather than actually melting the base metal or is there a tungsten/moly alloy you had in mind? Braising would prevent issues with the crystalline transition but what are the properties of moly? Is it going to be brittle in of its self?

-Sandow

[Braehill]

I don't really have anything to add as far as the material but, if Inconel can't take the heat, wouldn't the filler metal melt if it was brazed?

Len


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[Sandow]

I'm missing something. Doesn't the filler metal always melt?

Inconel it tough stuff but it melts at around 2550F vs 4750 for Moly and 6150 for tungsten. There are other metals that are pretty close to tungsten but they are exotic. Pure moly wire was hard to find but what are the chances of finding Rhenium feed rods?

hmm...
http://www.ebay.com/itm/like/2220242660 ... noapp=true

Evidently I'm not the only lunatic trying to do this...
http://rhenium.com/tungsten-rhenium.html

-Sandow

[big gear head]

How hot is this going to get?

[Sandow]

Hot enough that thermal collapse is the primary failure mode even with tungsten wire as the material. There is no way to sink heat from the grid beyond radiative emission. If you picture this as more or less a lightbulb filament, that isn't too far off the mark.

-Sandow

[Poland308]

Exactly what temps is the finished product going to see?

[dynasty200sd]

I came across this bulletin from September of 1971 someone provided a link to, located on the American Welding Society's website that may be of help.

Weldability of Tungsten and Its Alloys - Tungsten and its alloys can be successfully joined by gas tungsten-arc welding, gas tungsten-arc braze welding, electron beam welding and by chemical vapor deposition

[Sandow]

Exactly what temps is the finished product going to see?

Hot.

sunshine in a jar

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For application, the reaction energetics are limited by the grids heat tolerance. The hotter it can get before it slags, the higher the forward voltages can be and the longer the reactor can be run before needing to be cycled down. There also seems to be an increase in reaction efficiency as drive voltage increases so there is every reason to make the grid as high melting as possible.

Dynasty, that is a great paper. Thanks for the link.

-Sandow

[Otto Nobedder]

I'm missing something. Doesn't the filler metal always melt?

-Sandow

Len was not referring to the brazing process. Of course the filler melts, whether soldering, brazing, or welding.
He was referring to the ultimate temperature you need this to achieve, and the fact that this will melt any filler you would braze with/any filler with a lower melt temp than the parent metal.
This must be welded with the parent metal to have a uniform failure temperature.

Steve S

[Braehill]

Thanks Steve, that's what I was referring to. I got busy and didn't get the time to respond myself, but you nailed my sediments exactly.

Len

[Poland308]

What if you heated it up enough to bend it ? To make less weld joints. With a lay out that left you only a few weld joints geometrically opposed from each other to limit the stress they caused on the others.

[GreinTime]

Sandow,

What exactly do you do for a living?

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[Sandow]

Grientime, genetics and IT mostly but I do some high vacuum repairs too.

Poland, that is a good idea. Turns out the .040" pure tungsten is ductile enough to cold bend at the angles I'd need (108 and 120). 1/16" snaps like a twig with hardly and deflection though and that is what I'd wanted to use. The ductile brittle transition point is is around 400C which is over the corrosion temps. Let me see if I can pin one end and heat it with a TIG arc to bend it...

-Sandow

[Sandow]

Yup lol

forging tungsten lol

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Clearly I need to swap over to my jumbo gas lens but that worked surprisingly well.

I've ordered some rhenium/tungsten wire so we'll see how that pans out.

-Sandow

[big gear head]

Do you have a purge box that you can put it in when you heat it?

[Sandow]

Nothing made for the purpose but I could seal off my sand blasting glove box and flood that with argon. Granted, it would be a ton of argon...

I'm going to need to make a fixture either way. Something like this maybe:

fixture

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So I'm thinking with a copper plate with a hole bored in it and lanthanated tungsten (blue) as the stops and the fulcrum. I can hold one end of the rod with a screw (black) and hit the bend point with the arc. Between the 18 cup and the copper, i'm hoping that I can complete the bend and have it all cool down fast enough that I won't need to go to a diy purge box.

-Sandow

[Poland308]

Argon will fill a box like water would. So if it's sealed and you vent at the top then you only need enough cubic ft to fill the box. Plus enough to keep it under positive pressure and to compensate for any leaks. Nice bends.

[Sandow]

So it is about 16 cubic feet which doesn't sound like much until I realized it would be 20 minutes at 45 cfh lol. Still, it sounds like it would be interesting just to try. Is there any good way of testing that your purge is adequate? When I displace oxygen while carbonating, I usually need to use about three volumes to get the job done. Are purge chambers like that too or can you get away with a single volume?

-Sandow

[LtBadd]

So it is about 16 cubic feet which doesn't sound like much until I realized it would be 20 minutes at 45 cfh lol. Still, it sounds like it would be interesting just to try. Is there any good way of testing that your purge is adequate? When I displace oxygen while carbonating, I usually need to use about three volumes to get the job done. Are purge chambers like that too or can you get away with a single volume?

-Sandow

There are weld purge monitors available, and typically it does require multiple volumes to achieve the desired atmosphere. Here is a flexible purge chamber that is sold by CK Worldwide

[big gear head]

I keep the argon flowing in my purge box while I'm welding. I have unlimited argon at work. We use argon like water. There is no telling how much we use in a day with vacuum furnaces, coating furnaces and welding all day.

You could put a piece of steel in your box and run a small hot bead on it and pull the torch away and see if the weld turns blue. That would tell you if you have enough purge. You need a small hole in the top of the box just large enough to work through.

[Sandow]

I suppose even running 45 cubic feet into it is really only $10 in argon and it will be entertaining to try if nothing else. I think I can run the leads in through the vacuum port and then seal off the seams and gaps with vinyl tape. There is already a hole at the top back that lets the air volume equilibrate between the comprised air in and the vacuum out. I think with that partially closed off, it should work ok. If is does, it isn't a bad solution for $200.

http://www.harborfreight.com/abrasive-b ... 68893.html

-Sandow