Joining dissimilar metals - Soldering on with the boiler (wah wah wah)

It would seem that the boiler will be composed of a few metals.

There are a number of possibilities. The two Auroras that I have taken apart have two different copper boilers made in different ways. The U.S. model has a hemmed and (presumably) soldered joint along the cylinder - the two ends of a sheet of copper are folded over and hooked into each other before being soldered together and flattened slightly with a hammer. This one has one semi-spherical end and flange at the other (more about this later) which are both (again presumably) soldered in place afterwards. The European model has a much neater looking seam along the length of the cylinder which looks suspiciously like a butt joint. The two ends are bossed and fit around the exterior diameter of the cylinder. All the joints look, to my inexpert eye, to be too perfect to have been made by hand. The neatness and the likely presence of a butt joint both suggest that this exotic beast may be machine brazed. Funnily enough, I don't have a custom-made brazing machine, so replicating this is looking unlikely.

Aurora 'euro' model boiler - note the perfect seams.

I have fooled around a little with some scrap copper, experimenting with the techniques to see just how hard making the boiler out of sheet copper stock would be. 

      

   


Top left to bottom right: Silvabrite silver solder from the plumbing supply store, slit and annealed copper pipe, flattened and hemmed sheets (MAPP gas in the background), joint before soldering, soldered joint before clean up, detail of the joint cut through on the band-saw (note the small void, which may or may not be a problem). 

So back to that flange on the U.S. model. The tricky part about this is that it is made of brass and a stainless steel ring ...and the flange bolts are steel. 

I have no solid theory as to why the stainless ring is there. The only plausible explanation I can come up with (other than strength) is thermal expansion: perhaps the stainless band acts to keep the expansion of the brass flange in check. Although copper and brass are soldered all the time, perhaps the size of the parts concerned makes the joint prone to failure from the thermal cycle. Certainly, the numbers for the materials lend this hypothesis some credibility.

Metal                          Thermal expansion (micro inch/(in deg F))

Copper                                  9.8
Brass C23000                         10.4
Brass C28000                         11.6
Stainless 303, 304                   9.6

Stainless 316                          8.8

But, taking it as a given that the engineers who designed the boiler knew what they were doing, ours not to reason why...

Silver brazing these parts together is possible, but exposing the stainless to high heat is potentially a problem:


While brazing can provide a stronger joint, the high brazing temperatures can be bad for stainless steel. At those temperatures, carbon in the stainless steel can form chromium carbides which takes the chromium out of solution, making the steel non-stainless near the joint. This area is prone to rust and cracking after it is in service. The problem cannot be fixed by re-passivation so it is best to avoid excessively heating the parts during the braze and keep the total time at temperature to four minutes or less.

So that means brazing the steel bolts to the brass flange, and then either shrink-fitting the stainless ring to the brass or soldering the flange to the ring and the copper boiler skin in one shot - without destroying the hemmed copper seam. Soldering the stainless to the copper (and presumably also to the brass) can be done using the silver solder and a special flux: Harris Stay-Clean® Liquid Flux - which can apparently be bought at Linde and other Canadian welding supply places, otherwise on ebay in the States. I have come across some mentions of using muriatic (i.e. hydrochloric) acid or plain old plumbing flux, but the Harris stuff is supposed to make it easier.

All this makes me wonder whether an all stainless construction (with the exception of the boiler to grouphead flange which will necessarily be brass) might not be simpler. Replacing the flange and the end cap with laser or water-jet cut stainless parts and swapping in welded stainless schedule 10 tubing for the copper might be more expensive in materials but considerably less labor. Additionally, this would eliminate the cost of the molds for the brass parts.

The only complication (other than potentially having to drill and tap stainless) is the connection between the boiler and the boiler to grouphead flange. This last part serves as a reservoir for the grouphead and is heated by conduction through the wall of the boiler. Copper has about twenty times the thermal conductivity of stainless so a 1/8" thick stainless instead of a 1/16" copper boiler wall is going to have a significantly effect on the heat transfer to the reservoir.