I'm contemplating melting and pouring a silver insert into a copper base. the hole to accept the silver will be approximately 1" in diameter and about 0.025"-0.050" deep.

Has anyone here ever cast silver? I bought a 1 troy oz. piece of 0.999 pure silver to melt and pour. My first question is this: Can I use a stainless steel crucible to melt the silver in, without worrying about any of the alloying metals melting out and contaminating the silver? I want to keep it as pure as possible to ensure the highest thermal conductivity is obtained.

Second, does anyone see any problems with pouring the molten silver directly into the copper? I know copper has a higher melting point than that of silver, so I should have some headroom with the pouring temperature before I melt the copper. Maybe I'll keep a little water under the copper to help dissipate heat.

Can anyone provide suggestions to make this go smoothly? Thanks in advance!

why not cast the silver into a rod and machine the silver for a press fit?

i'd worry about the copper distorting from the heat.

I wouldn't get water near the copper cause if the molten silver hits it you'll get a shower of molten silver...

you should heat the copper up if you are going to cast the silver into it ... enough at least to get rid of any moisture...

also the silver is going to probably shink after you pour it.

just my 2 cents

the press fit would be nice if I were only looking for a mechanical connection, but I'm wanting as close as possible to a 'perfect' thermal mating between the copper and silver. A press fit will not give that, there will always be some tiny air gaps.

The reason for doing this is to increase the performance of a watercooled heat exchanger. There is only a small amount of thermal conductivity to be gained by using silver instead of copper. I don't want lose this advantage to a poor thermal interface.

Why don't you simply melt the silver into the hole in the copper, using an ordinary torch, and a little flux to clean the impurities? We call it silver soldering. Silver bonds well to copper, no mechanical fit is required.

Note that you will have to overfill the hole a bit because of shrinkage, but this buildup is easy to do with a freehand welding technique. For a plain casting method, you would have to increase the depth of the pocket to allow for shrinkage, then plane it off flush afterwards.

The thermal conductivity of Ag is 419 W/m-K, and of Cu 385 W/m-K (per www.matweb.com). That is less than 10% difference.
The junction is probably going to introduce a few percentage points of reduction in thermal conductivity. Why not bypass the Ag-Cu junction entirely and use all silver?

robotic regards,

Tom

Here are some of the problems you might encounter with this method. Shrinkage, porosity, fire scale. Also to properly melt the silver you would normally use borax for a flux so that could introduce borax glass to the mix.

I have cast silver for years and for that reason, I would recommend a machined and pressed fit or a compression fit. Cut the plug to a fairly tight press fit then tap the plug from the center out with a planishing hammer. Then machine or sand to finish the surface.

Alan T.

Thought I'd chime in here... I don't think that you are going to get any thermal conduction benefit from having a silver inlay (For lack of better term) into the copper. You are still going to have the copper layer that is limiting the heat dissapation. Perhaps a better solution is to use a silver wire of whatever diameter and go "through" the copper with it. And compress it at the ends to seal it in the hole and machine it flush to the copper. Perhaps a series of through rods in a pattern would work well. At first glance it seems like a "Through" solution is the only one that would give you the thermal advantage of the silver.

anoel, I'm highly considering this idea, too. Maybe I'll buy another chunk of silver for this purpose to compare.

Alan T., if you've cast silver for years, I'll probably heed your advice. I've personally never cast silver.

ToyMaker, the reason for not using solid silver is simply cost. To replace the entire chunk of copper with silver, I would go from a $5 piece of billet to over $100 in billet, while only gaining the above mentioned 10% in thermal conductivity. Plus, I don't know of a place to buy 0.999 pure silver in 1/4"x3" barstock. Does anyone here know?

Thanks a lot!
~Neo

http://www.riogrande.com


Should have silver in whatever size or configuration you would need.

Alan T.