Lowering contact resistance between vacu-brazed copper and tungsten

[DerekShannon]

Hi, gang–Now that we have a micro-ohmeter, we can check resistance across electrical contacts and see where arcing is likely to occur before it happens. But we still need to lower that contact resistance when we find it’s too high (in our case, we need less than 1 micro-ohm).

We’ve had success using a technique from our colleagues at the NNSS Gemini DPF, using indium wire to squish between our copper-steel contacts. But we have a remaining contact now causing problems, that between the copper cathode base and the tungsten crown insert some of you might remember us discussing on the forum earlier. Innovation can lead to complications!

These pieces were vacu-brazed together before we had the micro-ohmeter, so now that we can confirm that this area of arcing does indeed have higher contact resistance, we are hoping for a way to touch up this copper-tungsten vacu-braze connection without the delay of having it completely re-done. Plus, re-doing it would only happen on a “best effort” basis that would not guarantee no more bad areas.

We have been trying to use our indium wire as solder, but it is not sticking to the tungsten in this bad region after acetone/isopropanol cleaning. To add to the intrigue, we were able to decrease resistance in a subset of our bad regions by just cold pressing indium flat at the gap. Hm.

Our good regions are <2 micro-ohm (allowing ~1 micro-ohm across the contact, and ~1 micro-ohm for the surface resistance), while the bad region is >10 micro-ohms.

We do have a plan to eliminate this tungsten-copper contact altogether, but it is longer term.

Instead, we’re hoping some crowd sourced brilliance will get us firing Monday!

Eternal glory and maybe even a used copper gasket for the genius(es) who come(s) up with a fix?

This Practical Machinist discussion wasn’t very encouraging, but we *have* brazed tungsten and copper successfully around ~80% of the circumference, according to our resistance measurements. The 1969 paper mentioned there is also interesting, but doesn’t seem to lead to the solution we’re looking for….

[krikkitz]

This may be off base or not an option at present, but have you considered copper plating the tungsten crown?

Edited to say, I wrote the above before reviewing the references you gave, that mention copper plating and then soldering. Hindsight is a dirty nasty biatch. How much disassembly can you do? Enough to set the offending part in a bath of copper sulfate acid solution?

[Francisl]

Can you post pictures?

[Francisl]

Process Of Bonding Copper And Tungsten
This is not easy but may be a permanent solution.

[benf]

Here are a few websites I’ve come across on the subject.

CMW
scroll down to the section on Elkonite copper tungsten welding electrodes
Also see their pdfs on machining and brazing

Plansee
Component fabrication

Marketech
A longer term solution?

Perhaps just cleaning with solvents isn’t enough, oxidation needs to be removed through etching or polishing the parts to be joined?

[pulser]

Mask off parts you don’t want coated. Polyamide tape should be good.
Etch with peroxide. This assumes a good solvent clean before taping.
Sputter etch or ion mill the tungsten oxide off and deposit your favorite metal, iridium, platinum, palladium, copper using sputtering insitu. Tungsten forms an oxide readily with exposure to air.
Remove the masking tape. Now your part has a low resistance contact.
Tungsten sucks for low ohmic contacts. We use tungsten probes to aluminium and with scrubbing typically a few ohms.

[Steffen]

The two parts have axial symmetry right?

Have you considered Friction welding the two parts together?

http://www.youtube.com/watch?v=cAXZYGZ8Yrg

Apparently the friction welding process does a good job of welding differing metals together too. Here’s a video of connecting aluminum to copper:

http://www.youtube.com/watch?v=2oAWQd2_dz8

I’m no mechanical or materials engineer, but the process seems like it might be worth looking into…

Cheers,
-S

[DerekShannon]

We are chatting about all these suggestions, thanks guys. @FrancisL thanks for the reminder to get our last report on the website, it will have lots of pics.

[KeithPickering]

sy wrote: The two parts have axial symmetry right?

Have you considered Friction welding the two parts together?

Offhand I’d say that probably won’t work for W, the melting point is too high.

[Author]

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