Conference on Production and Design Limitation and Possibilities for Powder Metallurgy (Metal Technology, January 1945) - Certain Characteristics of Silver-base Powder Metallurgical Products - Discussion

P. R. Kalischer.*—I should like to amplify a little one of the points made by Dr. Hensel, and rather violently disagree with him at the same time. He brought out the point that when the higher forming pressures are used on the green compacts there is a marked decrease in the density as sintered. We found that to be true and, of course, many of you notice that that is true with copper compacts also. But, in my opinion at least, it is not true that the expansion is due to entrapped air. We have vacuum-pressed both silver and copper compacts and obtained almost exactly the same numerical expansion as we secured by pressing in air. The powder, however, that is vacuum-annealed does show a slight decrease in expansion on sintering, but almost the entire increase on expansion can be overcome by very slow rates of heating. And it is our opinion that a majority of the expansion is due to dissolved oxygen in the discrete metal particles themselves rather than to entrapped gases. Another interesting observation that we have made is that often with silver or copper compacts it is desirable to silver solder or braze the compact into some part of a piece of equipment after completing powder metallurgical operations on it. If such a compact is sintered and coined, say a silver compact, given an initial forming pressure of 10 to 15 tons per square inch, and sintered, there is an adequate amount of shrinkage and it can be coined at about 50 or 60 tons per square inch and bring up the density nearly to theoretical. If, then, that compact is silver-soldered into some part of a fixture or a machine, it will expand very beautifully, and the only way that this can be overcome. as far as we know, has been to reanneal after the coining operation at a temperature higher than is to be used in any subsequent operation. In that way, a density very nearly theoretical can be achieved, but the coining operation cannot be carried to the excessively high pressures or expansion and bubbling of the compact will occur again.