Necessity of Sliding during Brazing.

when you do this the melted silver absorbs oxygen from the air and when it cools this is forced out of solution and makes the blow holes. An inert gas stream welding (TIG)method would be best

TIG welding is out of question, as this an Electrical Contact Brazing application.

The possibility of silver absorbing the oxygen from air is less as Flux is used to inhibit any air contact.

I see. Strange where the trapped air/gas comes from to form the blow holes unless it is void space in the setup and the sliding action opens pathways for any trapped air to exit. Pressure to squeeze this out or instead of using foil use particulate silver in flux paste with no voids at all.

Like this. http://www.fusion-inc.com/filler3.htm

possibly they might send you a sample if they see business

Would a rotating motion serve this slide functions and have zero lateral motions. A button, being round, would not care what it's radial position was

sounds like you are using an incorrect technique for the brazing operation. you need clean metal to begin with and clean filler metal. if the contacts have been oiled up or burned th einclusions will cause bubbling in the weld. So will too much oxygen in the gas mix. the gas should be just carbeurising with the heating never touching the points of the internal flames. The metal should suck itself into the joint, do not place the filler between the jointing surfaces as this locks in dirt and scale.

You should not need to slide the joint when brazing. If the metal is not wetting to the surfaces properly it indicates a problem with the material matching, surface cleanliness or temperature. Do you use a flux when brazing. If you already do, you might need to consider using a shielding atmosphere during the braze.

Whether sliding action helps in wetting of the contact surface or is it helping in removing any gases generated during brazing? In the latter case you should ideally slide the contact sideways (equal to half its width) for removing any trapped gases, but this reduces amount of filler matl between the joint as it spreads over the slided surface.

I personally confirmed that without sliding blowholes are increased, but with sliding (2-3 mm sideways) they are actually reduced.

Pls give some logical explanation for sliding action.

We are following practice for preparing brazing joint.

-acid cleaning of copper contact

-applying flux onto the surface

-placing silver foil (filler metal) on it

-again applying flux on the foil

-putting Rectangular silver contact button on it

-heat the assy & as the filler melts, apply some pressure on the contact button, slide it & then hold it in location till it solidifies.

I'm not sure about what flux you would use as we do our brazing (copper to ceramic) in a nitrogen/hydrogen atmosphere but I would have thought that a silver/copper eutectic brazing alloy would give you a better result. Cleaning is of paramount importance as any surface residue will affect the joint. You must make sure that any cleaning agents used do not themselves leave any residue but the flux should do the final surface clean.

Our own brazing does not move the components but we achieve regular vacuum tight joints.

Your methods are basically correct all though i am not sure the acid is a good idea unless you are washing it off 100%.

By silver shim do you mean Silver or silver solder? I would use 50% silver solder shim with borax flux it would spread into the joint easier than silver.

Surfaces buffed with a brass buff and degreased with acetone would be a cleaner approach than acid.

Also consider high frequency heating if you are making large quantities of these contacts This is much cleaner and you have greater consistency

By sliding as you call it you are mechnically forcing the bubbles out. The bubbles are from air gaps between foil and the two parts being soldered.

You could be right, in our brazing operations we heavily weight the assemblies to compress the joints. This would also tend to expel any gas as the braze material melts.

Nigh- We are following proper practice of cleaning the contatcs before brazing. First we dip the contacts in a solution of (H2SO4 + HNO3 with 3:2 proportion). Then it is washed in flowing distilled water, further it is dried & then brazing surface is cleaned with Wire Brush.

After going thru some books I can say that usually a gap of around 0.05mm is maintained between the brazing surfaces to impart proper strength to the joint & I think by weighing the brazing assy with heavy load u r trying to achieve the same thing.

garth- I don't know the exact composition of the silver alloy foil we are using but it contains Cd or Sb depending on the current carrying capacity of contact. The flux is Borax with some % of other elements.

erssk-Do u mean that if the there is air gap between the preplaced foil & contacts while making the brazing assy, the joint will result with more gas bubbles.

We are in the process of switching over from flame brazing to Induction brazing. As Induction brazing has the advantage of quick heating, better control & above all uniform melting of foil. What I observe is, when flame brazing is used the foil usually starts melting form the flame side & as I can perceive this creats a wall of molten filler (due to capillary action)all around the solid filler (which melts later but immediately after some time) & this wall might be resticting the gases from escaping the joint & by slight sliding u r actually helping to escape the gases as well as breaking & dispersing the big bubbles into smaller one. But with induction brazing the heat is generated inside the component so melting may be uniform & this might reduce the chances of gas trapping inside the joint.

The induction brazing should work ok, I've seen this used in air to braze carbide tips to cutters at a high throughput rate. Our own brazing is done in an oven which is ramped up to brazing temperature & held there before being allowed to cool naturally so the heat is very even.