Ideas and suggestions for silver soldering this assembly

We have job that requires silver soldering eight small steel (mostly 1018) assemblies together. (Sorry can’t post a picture because of a NDA)

I can describe it here as, the main body is a piece of one inch square tubing with .125 walls and just over 1.5 inches long. Now set this piece of tubing up on end. 3/16 of an inch down from the top end of that tube will be a .125 inch thick, 1.75 inch square and flange silver soldered. (With an appropriately sized one inch square hole in the flange of course.)

We plan on machining light .002 or .003 grooves and pockets on the mating surfaces inside the boarders to provide space for fill of the solder

At the bottom of the square tube are two rectangles, .062 inches thick by .375 wide and 2.375 long these also get silver soldered. They are running parallel and on edge, along the two opposite sides of bottom of the square tube outward to the side at right angle from the square tubing.

The challenges are:

1. The surface of the square tube and the soldered 3/8 x 2 3/8 bottom rectangles must be within .005 parallel to the top face of the flange and within .005 square to the to the side of the tube.

2. We would like to do this without re-machining, but will if we have to.


We are trying to decide:

1. The best flux
2. The best rod


Our process document will read:

a. Wearing nitrile gloves completely clean all surfaces with Acetone.

b. All parts must be handled with clean, nitrile gloves or tools, and placed on surfaces that are free and clean of oils or other materials that will cause contamination of part.

c. Full immersion etch with the phosphoric acid-based cleaner, Henkel Metalprep 79 per Henkel’s technical sheet.

d. Flush with water then place on a pre-cleaned oven surface, heat to 130F until thoroughly dry. Allow to cool for handling.

e. Heat the appropriate number of parts that will fit in the oven and wet bond the mating surfaces until all are completed. (We have a small but very good PLC controlled oven)

f. Make simple fixturing to hold the parts together.

g. Flux and lay silver solder along the upper facing joints then bring up to temperature and allowing the solder to flow and wick.

h. Cool and inspect parts then apply protective coal and package.




The challenges are:

1. The surface of the square tube and the soldered 3/8 x 2 3/8 bottom rectangles must be within .005 parallel to the top face of the flange and within .005 square to the to the side of the tube.

2. We would like to do this without re-machining, but will if we have to.

In your opinions and experiences, what are?

1. The best flux?

2. The best rod?

3. Will our process work?

a. If we have holes or misconceptions in our process, what are they?

We do not claim to experts in silver soldering by this method and we are looking for peer review and comments.


Steve

In my experience tolerance is incredibly repeatable for silver soldering. Problem is as the jigs gets hot if there different materials to the parts expansion throws things off. Equaly a big heavy jig takes a long time to heat up, jig and work matching in mass - cross sectional areas helps a lot! as does leaving it as open as possible to aid heating times.

Prep with just bog standard phosphoric works fine on steel for me. Parts cleaned that shift need to be soldered that shift though ;-) Trying to store cleaned parts especially in steel did not help my results.

As to the solder look into the solder loaded fluxes, the manufacturers spec joint gaps carefully follow there advise and save on your own experiments. Pre cutting lumps of solder also works, but needs careful control on size. Look into some good application gear for the fluxs - especially the solder loaded ones.

Post heat treat they will need cleaning before inspection. Flux residues look messy and depending on grade can cause corrosion latter in storage as there often corrosive hygroscopic salts.

Don't discount the idea of going to a inert atmosphere in the soldering "kiln" Can really help if your using the higher temp solders by reducing scaling, does not need to be fancy, just a really low flow rate of nitrogen worked wonders for me.