FAQ - Joining metals - A critical fusor construction element

If you have a question about this topic, the answer is probably in here!
Post Reply
User avatar
Richard Hull
Posts: 12435
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

FAQ - Joining metals - A critical fusor construction element

Post by Richard Hull » Thu Feb 15, 2018 12:18 am

The following is a quick rinse in what methods are sometimes used to join metals in fusor construction. The mere assembly and joining of components to a fusor is just part of the big picture here. However you assemble and attach items to a fusor, it must also be vacuum tight and vacuum friendly. This is a critical dual consideration here.

How do I put a fusor together from parts that will hold and work well?

1. Soft Soldering- A tin-lead or tin-silver based joining technique. Used extensively in electronics and plumbing. Flows and wets to fill small open areas and voids between joints. The weakest of all molten metal joins.
Disadvantages - Requires both join pieces to be heated to full solder melting point, (400 deg+). Typically, softens and fails structurally above 350 deg F.

2. Silver Soldering - A form of brazing using a special silver-copper alloy. Used almost exclusively by the Jewelry business or assembly of bright, white metals. The joint is very strong.
Disadvantages - Both pieces at the joint must be heated to near red heat! (900+ deg F). Will not flow to fill voids between joints. Surfaces to be joined must be close fitting or in contact. Weakens and fails near 800 deg F. A paste flux must be used and cleaned off the joint after brazing.

3. Classic Brazing - A brass/broze alloy. Used extensively where a good strong join is needed that is stronger than silver soldering. Used in some auto body work.
Disadvantages - Both pieces at the joint must be heated to near red heat! (1100+ deg F). Will not flow to fill voids between joints. Surfaces to be joined must be close fitting or in contact. Weakens and fails near 900 deg F. A paste flux must be used and cleaned off the joint after brazing.

4. True fusion welding - In this form of joining the two metals to be joined are fully melted and fused together. There are several forms of fusion welding. Fusion welding is the strongest of all metal-to-metal joints...

A. Gas Welding - Usually oxy-actyline torch welding, most often using a similar metal welding rod as filler.
Disadvantages - the welding must typically see both items to be welded heated to over 1500++ degrees F. Expensive welding setup required.

B. MIG Arc Welding - Metal Inert Gas welding. A reel of wire of the same metal to be welded is fed into a shielded gas electrical arc melting the two metals to be joined at the joint and filling the joint with molten wire metal. This typically heats to melting temperature only the joint area and only moderately heats the surrounding metal. Good for joining items where you don't want to over heat other parts of the metals to be joined.
Typically used on sheet metal joints and materials not much more than 1/8" thick. Automotive industry uses this form a lot.
Disadvanges - Expensive electrical/electronic welder demanded. Usually leaves a bulging bead of filler metal at the joint. (can be ground down)

C. TIG Arc Welding - Tungsten inert Gas welding. This is the cleanest form of welding solid joints. Typically, no feeder wire is used. The gas shielded arc literally puddles the two metals together in a smooth uniform joint. Well worked joints are prepared with excess metal lips that can melt into a smooth, non-bulging, non-concave joint that needs no cleaning or further attention. Typically used on pieces under 1/4 inch thick. Used extensively in the assembly of large medical items or scientific professional assemblies.
Disadvantages: Very expensive electronic controlled welder is the norm. Typically requires careful planning and preparation of pieces to be welded for excellent finished results expected of such TIG welds.

D. Resistance Welding - often called "spot welding". In this form form of welding two pointed electrodes of special alloy are mashed together, under pressure, between the thin pieces to be welded. Hundreds of amperes of current flow through the electrodes on opposite sides of the joint and force the two items to melt and fusion weld in a spot the size of the electrodes. The actual weld process takes a tiny fraction of a second. In the finest small item spot welders, the weld is an electronically controlled capacitive discharge. This is the lowest temperature fusion weld possible as the surrounding metal remains cool. This is the finest way to join two pieces of sheet metal or wire where other forms of fusion welding would melt the entire assembly.
Disadvantages - Rare and expensive welders are the norm in this form of welding.

E. Common arc welding - Un-shielded Arc Welding. uses a filler rod as the electrode that is covered with a flux that goes molten and shields the metal weldment as it cools from oxidation. Used extensively in massive constructions such as huge mutli-ton vehicle assembly and repair, ship building, etc.. 1 inch thick plate is often welded this way with giant kiloamp arc welders.
Disadvatages - A nasty finished weldment with a huge bulging seam that always requires at least hammer and wire brush techniques to clean the ugly flux covered seam. Grinding with a power grinder is often employed to clean away spelter balls in and around the weldment.


For fusor use

Method #1 is not used inside a fusor, ever!
If the fusor is not going to reach a shell temperature of over 200 deg F. method #1 can be used on the external shell and its external attachments. Still, its use around any item that is to be heated while in use is just not recommended.

Method #2 can be used internally, but not to attach or assemble a grid as the grid can glow red hot under power use, weakening and failing joints here. I have used this method successfully in demo fusor grids where a lot of power is not applied or allowed to heat the grid to red heat. Only high silver content, Jewelry alloys should be used. 60%++. https://www.riogrande.com/category/tool ... ing/solder
I recommend the "easy 650 paste" High melting point.

Method #3 is not recommended as many brazing alloys contain zinc that is not vacuum friendly. Stick with method #2 using silver if you must braze.

Method #4A is not recommended for fusor work. Too much heating of often delicate components is bad.

Method 4B is a viable method if you have a MIG welder. Post weld cleanup is usually demanded. Not recommended unless you are a near expert user of this method.

Method 4C is the ultimate and desired method for assembling a stainless steel fusor and all of its attachments. As close to professional assembly as possible for the amateur. TIG is the way to go.

Method 4D is the absolute best method for assembling a wire or thin metal grid. It is also used to weld the grid to a threaded bolt.

Method 4E is just too crude for fusor assembly. However, if you are extremely proficient in this form of welding and have the welder, it could be used. Definitely not recommended at all.


I hope this settles this question, but due to the total lack of newbie FAQ reading I am sure this effort will rarely be used for this type of information.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Posts: 624
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: FAQ - Joining metals - A critical fusor construction element

Post by ian_krase » Thu Feb 15, 2018 7:18 am

Let me give my practical comments on this:

- Soft soldering works great on copper and copper alloys, not so great on steel, and requires specialized (noxious) fluxes on aluminum and stainless. That said, I assembled my first "real" vacuum equipment using soft solder and it is still going strong.

- Silver soldering (which is often called silver brazing) is a great method for the fusioneer who has a small budget and not much space. You can do a fair amount of useful-for-vacuum work using an inexpensive propane or Mapp Gas torch. The brazing rods you want are at least 40 percent silver. My preferred flux is the black kind, Harris Stay-Silv Paste. I both paint the flux on the workpiece and get a thin layer on the brazing rod and pre-"toast" it -- it will first bubble as water boils off, then foam as it heats up, then when getting close to red hot temperature it actually melts.

- You can go a long way with one or two "sticks" of silver brazing rod (it may be rolled into coils instead) which will run you roughly 30 to 50 USD if I remember correctly. Harris Safety-Silv is probably good. You probably want to avoid cadmium, IMO.

- Silver soldering works great on stainless, brass, copper, and steel -- totally different from soft soldering -- and as long as your parts fit closely ("socket weld" fittings, threads of any kind, etc) it will "automagically" create a vacuum tight seal as long as you use enough, flux enough, and get every part properly heated. I have even made leak-tight seals on large diameter parts (4 inch conflat!) which were too large to heat to red hot at the same time.

- Personally, I think these are both lowball temperatures for silver and brass brazing. I usually find that silver doesn't reliably flow until at least *something* is slightly red-hot and brass needs significant red-hot-ness or more.

- Besides being expensive, gas welding setups are a bit on the dangerous side w/r/t torch "pop", oxygen that can make mildly flammable stuff ultra-flammable, volatile acyetylene, etc. OTOH, it is a very versatile welding setup for small work. IMO, you should only consider a gas welding setup (as opposed to a Mapp Gass torch and a bucket of silver brazing compound) if you can be taught the basics of how to use it safely by somebody who knows how.

- MIG and common arc (aka Stick) welding are both quite messy both for your workpiece (if not cleaned up) and for your work area. I don't see either of them as the best option for vacuum work.

A few other items should be mentioned:

- There is a special method for brazing metals to ceramics using a very special brazing compound that contains titanium. This is how the best high voltage feedthroughs are made. Sadly, I don't know too much about this method. It requires a high vacuum furnace -- possibly one could get away with a quartz tube and/or induction heaters. I don't think I've ever heard of anybody DIYing this.

- Metal can be bonded to glass using metal-to-glass seals like found in light bulbs and electron tubes. These are very finicky and require skill and the matching of very specific types of metal to types of glass (such as Kovar to borosilicate or Titanium to soda lime glass) to get right; they're a subject on their own. OTOH, if you don't need it to be vacuum-tight or totally absolutely crack-proof, you can get away with some very messy blobbed-together glass bonds.

- There are special solder glasses that melt at low temperature and can be used as a "glue" to seal together systems, esp. those partially or wholly made of glass. These are almost always lead-containing.

- If one can machine mounting features or otherwise assemble something, one should not overlook seals made with O-rings even for permanent assemblies. They are quite good enough for fusors as long as they stay cool.

- Also, one should not forget Hysol 1C -- but be careful as differences in thermal expansion can crack this epoxy. And it doesn't like heat or plasma.

- Indium metal can be used in two ways: as a crush gasket similar to but softer than the copper gaskets used in Conflat fittings, or as a kind of solder for soldering glass to metal. Unfortunately, Indium melts at a low temperature and can't be used at high temperatures.

Post Reply