Fusor Forums

It seems like everyone welds, however is there a reason I can't torch solder all the joints to the conflats and nipples? Is the temperature high enough even on the outside edges that it would simply melt the solder joints?

Hi Tyler,

This is very hard to do, and I speak from experience! When I built my first fusor, I thought I could forgo TIG welding by silver soldering my 10" flanges to my 8" Wagner hemispheres. Even with two oxypropane torches simultaneously at work keeping the metal hot, this was impossible; I could get about half of the joint hot enough for the solder to flow, while the other half stayed frozen. Thermal expansion caused all kinds of leaky, oddly-shaped joints. Flux and stray solder made a terrific mess as well. In theory, I suppose there's not a problem with this idea, but it would take a lot of uniform heat (i.e. an oven) and there might be extensive cleanup required.

I'm of the opinion that big stuff should be welded, even if you have to pay to get a pro to do it. Joints under a few inches long can be effectively silver soldered with an oxy-gas flame without too much hassle.

-Carl

When it comes to silver soldering Carl is right.
Stainless does not bind well to the hard silver solders. I.e easyflow or similar derivatives.

however stainless solders just fine with soft solder but you have to use killed spirits of salts as a flux. Easy to buy even easier to make, just dissolve pure zinc into Hydrochloric acid making sure that there is more zinc than acid ie when reaction stops there is still zinc left that is not dissolved.

Clean stainless first to remove oxide layer by abrading (fine carborundum paper), apply flux before heating and keep applying until solder starts to melt, tin all surfaces first then assemble and sweat pieces together adding more solder if necessary.
use ordinary tin lead 60/40 solder.

Do not overheat stainless while tinning or you will have to abrade the oxidised surface off and start again.

do not use electronic type solder with the rosin flux inside unless you are prepared to move all traces of rosin flux before the sweating operation.

Although you can easily soft-solder stainless with the right flux (www.mcmaster.com) it won't be strong enough. Soft solder is weak at room temperature, has a high vapor pressure even without the flux, and gets very weak very fast at temperatures you can hold in your hand without getting burned. You would not be able to torque the bolts without breaking your joint, unless you are more careful than most.

Silver solder containing nickel (there are a few types) and no Cd will work on 304 SS, but by the time you get the joint hot enough, even with an oxy-acetylene torch, you'll be in serious danger of warping. I have had that one happen -- and CF flanges don't like that one bit. You want to avoid Cd and Pb and Sb in your tank if at all possible -- high vapor pressures at fairly low temperatures. I have a mass spectrometer and have seen this problem.

TIG welding can be done so fast and so easily it's the way to go. When you look into how much all the building you may do would cost to have some pro do it -- you'll just cry and then buy a welder, or hopefully live near someone here who has one who can help. Try Grizzly tools for a pretty good one. (avoid Harbor freight -- their little inverter one stinks as I found out to my wallet's sorrow).

Good luck!

Since the joint is under compression in this application, soft solder might just work (no guarantees, because soft solder is much weaker than a silver braze).

It should be lead-free for vacuum compatibility. The Aladdin 450 brand is a good example--97% tin, 3% silver. Rosin flux is worthless on stainless steel. Nothing but liquid acid flux described by John will do the job acceptably. Immediately after soldering, wash in concentrated HCl for a few minutes or CLR cleaner for half an hour to get rid of flux and oxides. I still think an 8" joint would be challenging to keep hot enough and cool at the right rate for this to be successful. If the joint cools too slowly, the solder crystallizes, and if it cools too fast it will probably crack. Unlike the hard solder, soft silver solder has a very large volume expansion when molten. This is a source of difficulties during inspection because it can be hard to tell if the joint is solid.

-Carl

Carl is correct! Note* In real life, when I first TIG welded my old fusor III, even the TIG weld cracked on cooling in two places where I had used too low a current and too thin a weld seam. I touched it up, of course, but even the mighty TIG in "first time" hands might crack at places. Can you imagine the issues with a lesser strength joint that requires, higher overall material heating might engender?

Large SS joints are a job for TIG

Richard Hull

I have had very good results with a silver brazing rod sold by an online company called MUGGY Weld. This stuff is rated at 100k psi strength, and requires at least dull to bright red metal color to melt, so I would think it would work here. It is flux coated with a material that is especially formulated for stainless, and flowes like a dream. Muggy has welding/brazing videos on his site that can get anybody up and running fast. As to a torch, I agree that propane or oxypropane would prove inadequate. I would use a medium to heave duty oxy acetylene torch. Starter outfits usually run around 400.00 to 500.00 dollars. The muggy rod is about 10.00 a rod, and is a 56%silver alloy. I hope this helps. I am a total nubee to Fusors, but I have worked with stainless steel for several years.
Steve

Well, I guess I should read all the posts before replying... How thick is the wall typically? And what difficulties have hard silver brazing caused? I was planning on using the aforementioned Muggy rod, unless my chances of success are low. I do use it with moderately high pressure steam, but that is a different animal I suppose...
Thanks
Steve