Silver Brazing of #360 Brass

Date 11-13-2007 21:12

Two essentially cylindrical parts need to be silver brazed together, end to end. Wall is about 1/8". We are using a high-silver (~56%) alloy wire and paste flux. We've had problems with the integrity of the joints - several have failed after being dropped. Inspection of failed joint shows very little wetting of adjoining surfaces; solder seems to be primarily around outer edge. Here are my questions:

1. What should the proper technique be? I understand that you should never have to melt the solder with the torch. It should always be melted with the part which has been heated enough to melt the solder. Practically, this is difficult to do - it is quite tempting to melt the solder with the torch and watch it flow along (but maybe not <i>into</i>) the joint. I need to get some good procedures in place to help assure good connections.

2. Is there a practical way to "look" inside the finished joint? I'd like to make sure that all of the mating surfaces have been wetted, and wonder if x-ray or ultrasonic or eddy current inspection might work. I know next to nothing about any of these, including how practical they would be for only intermittent use or how appropriate they would be to find what I want to see.

I've been trying to get our local welding supplier to come to my rescue, but they seem to be much more interested in real welding than this sissy soldering stuff. Any comments or suggestions would be much appreciated.

By aevald

Date 11-14-2007 00:19

Hello sbmathias, I am certainly not a soldering expert here, however from your description of the joint failures it sounds as if your fit-up may be a bit tight and not allowing the solder to flow fully through the joint. Generally the solder will follow the flux and the heat, if the application of the flux is consistent and present on the inside and outside of the joint as well as the mating surfaces, the joint spacing is correct between the two parts, heat application is consistent and even, the solder will flow freely throughout the joint. In most instances solder of just about any type will ball and not spread if the heat is insufficient, it will also not flow evenly between two parts if the heat isn't consistent between the two or concentrated more on one piece than the other, and if the heat is too great it will wick all over the place and not be deposited in a thick enough and concentrated enough manner to properly join the two pieces. As far as testing of these parts goes, you may need to include a bit more information so that some of the many inspection folks on the forum could suggest the proper system to verify a successful joint completion. Best regards, aevald

By yorkiepap (***)

Date 11-14-2007 01:07

Hey sbmathias,
A couple of points to obtain a bit more clarity:
1) What is the material?
2) Are the 2 parts identical at the mating point?
3) Are both parts at the same temp when joining?
4) Are you using a slight rotation of each part while joining?
5) The most important....are you wetting, "tinning" so to speak, each part prior to joining?
6) Have you tried a different flux?

Many years ago, I apprenticed under an Austrian Master Gunsmith who taught me how to effortlessly join with tremendous strength with silver alloy(55/45) and the application of "wetting" each surface prior to contact. In addition, a slight "friction", or in your case, "rotation", during the mating allowed the surfaces to bond at a superior rate. Yes, the heat of the part should melt the alloy, but not be overheated and each part kept at the same temperature. Anyway, hope I gave you some insight to examine.....Good Luck...Denny

By Tomwalz (*)

Date 11-14-2007 16:10

1. The 56% silver alloy is probably AWS BAg -7 (American Welding Society Braze ((Ag for Silver)) #7.)

The braze alloy is important because you need a brazing flux. Flux absorbs oxygen and gets used up with exposure to temperature over time. Soldering flux will get used u p too fast.

This alloy is popular because it wets out well but it is a relatively weak alloy. It is designed for uses like band saw teeth on meat saws.

Silver is just part of the metallurgy.   A 50% Silver with Cadmium or a 49% Silver with Manganese would be about 40% stronger.

2. Get the copper sanded or ground so that the mating surfaces are clean and bright. Then clean with a caustic or a detergent (not a solvent) and rinse well.

3. I would preposition a fluxed braze alloy ring between the two parts and heat the whole assembly at once.

4. Use a purified Black Flux. Common flux is made to be used externally so inert materials don't matter. You are using it internally so they do matter.

5. You can pretty well tell how good the joint is by examining the outside. A lot of this is covered at    http://www.carbideprocessors.com/Brazing/book/index.htm   The free stuff is at the bottom.

If you want to send me pictures I will be happy to look at them.   I am rewriting Braze Failure Analysis and would look at your parts no charge if I could include it as a case study with or without your name as you wish.

tom