Tungsten grid construction

For posts specifically relating to fusor design, construction, and operation.
Post Reply
Alex Cordesman
Posts: 12
Joined: Tue Dec 01, 2015 2:57 pm
Real name: Alex Cordesman
Location: Central VA

Tungsten grid construction

Post by Alex Cordesman »

I was looking at some of the grid designs and was wondering with tungsten grids how they were being put together. For most other materials I'd just TIG it all together and be done with it. Are those of you that are using tungsten soldering/brazing it together with a dissimilar metal or are you TIG welding the tungsten together. If the later, are you simply using a thick electrode in the torch and welding DC? Can green electrodes be used as the fill rod or is it easier to just autogenously weld? FWIW, I'd just try it rather than asking but I've never had a reason to buy green electrodes.

On a different note also regarding tungsten, isn't tungsten at more of a risk for becoming radioactive after neutron exposure than lighter elements? For instance copper doesn't have a radiological isotope with a half-life longer than 24 hrs. It looked like quite a number of the enriched isotopes have moderate half-lives (60 to 150 days) which begs the question of disposal... How does one go about properly disposing of potentially radiological material?

-Alex
Always assume that it will explode.
– Astronautics, issue 38, October 1937.
User avatar
Bob Reite
Posts: 576
Joined: Sun Aug 25, 2013 9:03 pm
Real name: Bob Reite
Location: Wilkes Barre/Scranton area

Re: Tungsten grid construction

Post by Bob Reite »

At the rate we are generating neutrons so far, there is almost no chance of activating tungsten enough to detect any radioactivity at all! We have to try for easy materials like silver to activate, and silver stays radioactive barely long enough to measure.

As far as building tungsten wire grids, I just used crimp connectors. My next grid experiment will use tungsten wire press fit into graphite end pieces. In both cases, no welding or brazing required.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
Alex Cordesman
Posts: 12
Joined: Tue Dec 01, 2015 2:57 pm
Real name: Alex Cordesman
Location: Central VA

Re: Tungsten grid construction

Post by Alex Cordesman »

I didn't think the neutron density was high enough to be a huge problem but it is far enough outside my bailiwick that I wanted to float the question.

I found some discussion on welding tungsten on a different forum and it seems fairly doable. With a ceriated 1/8 electrode in the torch running DCEN the tungsten on the receiving end should melt much sooner than on the torch end. I've ordered some mixed diameter green (pure tungsten) electrodes and will try welding a few .040" rods to the end of a 1/8 rod as I would for the start of pentagonal geodesic sphere. If it works, I'll post pics and the settings that panned out. If it doesn't I'll let you all know what went wrong.

-Alex
Always assume that it will explode.
– Astronautics, issue 38, October 1937.
Alex Cordesman
Posts: 12
Joined: Tue Dec 01, 2015 2:57 pm
Real name: Alex Cordesman
Location: Central VA

Re: Tungsten grid construction

Post by Alex Cordesman »

IMG_5220.JPG
Yup, autogenous welding of tungsten is pretty straight forward. Pretty much the brightest thing I've ever welded but a fairly easy to control puddle. It was mechanically weak though and when I flexed the joint it snapped clean at the weld point after less than 10 degrees of deflection. I'm guessing that if it is annealed after the weld is done that it might be more resilient but that is a swag. I'm not sure that matters much as far as we are concerned though. It isn't like we are dealing with large mechanical stresses in operation.

I'm going to try some more complex geometry and I'll post pics when I do. In the mean time, if anyone wants a tungsten grid stuck together and it isn't crazy complex geometry I'm happy to help as long as you supply the materials.

-Alex
Always assume that it will explode.
– Astronautics, issue 38, October 1937.
Roberto Ferrari
Posts: 364
Joined: Tue Mar 02, 2004 12:21 pm
Real name: Roberto Ferrari
Location: Argentina
Contact:

Re: Tungsten grid construction

Post by Roberto Ferrari »

Alex
Tungsten is a complex material. Remember it is prepared by pulvimetallurgy. It develops a fibrous structure that is lost when heated.
Intermediating with a nickel sheet will be helpful.
After bringing it to high temps, it becomes fragile. Besides, oxidises easily when heated in air so is advisable to heat or braze or spot welding it in an inert or even reductive atmosphere (90% Ar-10% H2 is very good).
References: Tungsten – its metallurgy, properties and applications. By Colin Smithells, Chem.Publ. Co, 1953.
Good luck!
Alex Cordesman
Posts: 12
Joined: Tue Dec 01, 2015 2:57 pm
Real name: Alex Cordesman
Location: Central VA

Re: Tungsten grid construction

Post by Alex Cordesman »

I don't think oxygen exposure is an issue since I was using a jumbo gas lens and a 10 second post flow. Coloration is a good indication since tungsten will blue quickly in air. There is a small spot an inch from the weld that was outside the gas shielding. Since the break when it was flexed was at the melted/unmelted transition, the stresses at the transition point were still high. It is why I was thinking annealing was probably the answer. My furnace doesn't get hot enough to get to tungsten annealing temps and I can't purge the oxygen out anyway so it is a bit of a mute point.

I have nickel fill rod and could certainly TIG braze with nickel fill but that would leave the joint melting point at less than half of the base metal.

I don't really think the mechanical strength of the welds is a big deal given the application. The forces it will experience aren't very directional and they are low magnitude. It isn't like the weld was so brittle that it couldn't be handled, I was trying to break it when I flexed it.

-Alex
Always assume that it will explode.
– Astronautics, issue 38, October 1937.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Tungsten grid construction

Post by Richard Hull »

I have mentioned this before. Many metals, especially sintered metals, and tungsten in particular, are subject to hydrogen embrittlement. This is a terrible thing for structural strength but a get gift to fusor grids.

Put together under a bit of bending stress, a finshed grid of Tungsten, tantalum, titanium or hafnium will embrittle rapidly under deuteron bombardment this will relieve all stresses in the curved circles such that they suffer no tendency to open up if a weld or braze lets go. The entire struture is frozen forever, stress free. Nautrally it will now break like glass if stressed mechanically. Part of the stress relief early on is due to heat annealing as well, of course, assuming it reaches annealing temperatures. It can still melt, of course, under intense heat via bombardment currents. Continuous heating and cooling can also work harden many metals. Many factors enter into the equation.

Stainless steel grids will embrittle also but take a bit longer. The upshot is that if you make a grid of the above metals, all flexibilty and stresses will go away if operated in a deuterium environment. This makes all structural issues and worries about weldment release a virtual non-issue.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Brian_Neumeyer
Posts: 21
Joined: Tue Jan 13, 2015 1:25 am
Real name: Brian_Neumeyer

Re: Tungsten grid construction

Post by Brian_Neumeyer »

Alex, when we made our 304 SS grid we made a wire jig that formed a ring that has four "connection" points on it. Of the three rings, one had 4 female, one had 2 female 2 male, and the last had 4 male, if that makes sense. I think the picture might be a better reference.

With this method there was no welding required, the only limit is if you can bend the tungsten wire to a ID radius equal to its diameter. In our case 1/32" diameter 304 SS worked, I'll get back to you on tungsten.
Tom Bales
Posts: 10
Joined: Wed Sep 16, 2015 10:55 am
Real name: Tom Bales

Re: Tungsten grid construction

Post by Tom Bales »

We've had good luck making cathode grids of an alloy of 20% Tantalum/80% Tungsten (Ta20-W), which is available in 0.5mm diameter from a seller on *bay. We form the wire into circles, trim them for a short overlap, and then resistance weld the overlaps between pure copper electrodes (Unitek-Miyachi capacitive-discharge resistance welder). Once you have circles, it's easy to resistance-weld them into a sphere, since those welds are "cross-wire" welds. Then, zap on a stalk, and you're all set. These electrodes are happy to heat up to white heat and throw quite a bright light when the current gets out of control. No problems with embrittlement, and only very slight erosion, so far.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Tungsten grid construction

Post by Richard Hull »

As I noted embrittlement is no problem with a fusor grid, it is a Godsend. The Ti-W will embrittle rapidly over time in real operation in the deuterium environment. It is a given.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Post Reply

Return to “Fusor Construction & Operation (& FAQs)”