Oxygen-free copper base alloy contain-
US 3172762 A
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United States Patent 3,172,762 OXYGEN-FREE CGPPER BASE ALLOY CONTAIN- ING CI-ROMIUM AND CADMIUM Bernard H. Wilson, Staten Island, N.Y., assignor to Phelps Dodge Copper Products Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Apr. 2, 1962, Ser. No. 184,563 4 Claims. (Cl. 75-153) This invention relates to alloys of copper with chromium and cadmium. Its object is to provide such a cop per alloy having ductility adapting it for drastic forging, extrusion, drawing, rolling or other working to form parts even of small dimension or intricate configuration which may be heat treated to exceptionally high mechanical strength and electrical conductivity, even though brazing, which usually deleteriously affects mechanical properties in other copper alloys, may have been employed in fabricating such parts with others in a larger assembly. The alloy is substantially free from copperoxide, so that it is immune to hydrogen embrittlement.
Alloys of copper, chromium and cadmium have been proposed in the literature as, for example, in United States Patent No. 2,033,710 describing such an alloy for use in making castings. Although several such prior proposals purport to deal with the factors of workability, strength and electrical conductivity, none, so far as I am aware, provides a copper alloy which successfully meets the problems inherent in the manufacture, by drastic working, of parts which must have residual high mechanical strength and high electrical conductivity. Typical of the many different articles of that nature which may be manufactured in accordance with the present invention are large and small screw-threaded cold formed parts of the type used for assembly by brazing to form semi-conductor bases for use in diodes, rectifiers, transistors, and the like and small conductor wire, a few thousandths of an inch in diameter, having high elongation characterstics as well as high tensile strength and, by suitable modification, a degree of relative selectivity in those properties.
In accordance with my invention, the copper, of which the alloy is principally composed, is substantially oxygenfree, by which I mean that it will pass the ASTM hydrogen embrittlement tests. It contains less than of the order of .001 oxygen. For that purpose, the copper is alloyed with lithium, leaving a residual lithium content in the alloy in the range of from about .005 to .015% preferably around 010%. Other deoxidizers such as boron which, like lithium, do not depress conductivity can also be used for this purpose. The alloy contains cadmium in the range of from about 20% to about 1.00% and preferably about .40%, chromium in the range of from about .20% to about .80% and preferably about 35%, and the balance substantially oxygen-free copper.
In manufacturing the alloy, it is desirable to use copper that has a minimum of copper oxide, although the exact amount is not critical as I remove substantially all the oxygen present during the manufacture. It is desirable that cathode copper, formed by electrodeposition, be used as the supply because it is oxygen-free. Copper scrap for re-melt may, however, also be used even though it contains small quantities of copper oxide. The copper supply is melted in a conventional furnace using a charcoal cover on the melt, partly for its effect in excluding oxygen from the melt, either by shielding the melt or by consuming the oxygen as the charcoal burns. I prefer to add the lithium first as a means of purging the cop per, as it were, of oxygen just prior to adding the cadmium and chromium to the furnace. The chromium and cadmium are then added to the melt by using suitable ad ditive alloys of those metals with copper to contribute the right proportions to the mix. The lithium addition insures deoxidation of the melt so that chromium is not oxidized, allowing it to form a homogeneous alloy with the copper. Lithium forms an oxide with the oxygen to be removed and is hence added in quantity suflicient for the purpose, depending upon the quantity of oxygen, with excess to leave the residual lithium above referred to. The lithium oxide forms a liquid slag which floats to the surface without impairing the pouring to leave the alloy with its desired freedom from oxygen.
Because of its freedom from oxygen, the alloy has exceptionally high ductility so that it is capable of undergoing drastic plastic deformation without developing shear cracks or failing by rupture in tension. Thereby the alloy is, I have found, well adapted for use in forming intricate parts by drastic hot or cold working.
he exceptionally high mechanical strength and electrical conductivity which my alloy is capable of developing is attained by heat treatment in two stages, first by solution annealing at a metal temperature in the range of from about 1650 F. to about 1800 F. for from about 15 minutes to 1 hour, preferably in a reducing or inert atmosphere, followed desirably, but not necessarily, by quenching, and thereafter by precipitation heat treatment (age hardening) by heating to a temperature in the range of from about 850 F. to about 1150 F. for from 15 minutes to 1 hour, also preferably in a reducing or inert atmosphere.
The characteristics of my alloy are well represented by the comparative values of certain physical characteristics after solution annealing and after age hardening, as follows:
1 At 0.5% extension under load.
The tensile strength, yield strength and Rockwell hardness can be further augmented by cold working such as by rolling, drawing, stamping, extruding, heading, etc., without appreciable loss of conductivity, with characteristics as follows depending upon size and amount of cold working:
Tensile strength p.s.i 60-95,000 Yield strength p.s.1 5085,000 Rockwell hardness F l05 1 At 0.5% extension under load.
I attribute these characteristics not simply to the constituents of the alloy but to their relative proportions, which I have found are critical withn the narrow ranges given. Outside those ranges either the alloy would not be sufficiently ductile for manufacture even by hot extrusion, forging, or rolling, as the metal would resist flow under pressure and would crack under the tremendous forces employed so that the metal could be used, if at all, only for casting or other uses not within the scope of the present invention, or the age-hardening would not be effective to produce either an electrical conductivity or a yield strength approaching the values given.
The alloy of my invention is well adapted for any use where its high ductility, combined with its characteristics of high mechanical strength and high electrical conductivity developed by heat treatment are desired, as well as its freedom from copper-oxides, whether or not brazing all is involved in the utilization of the part formed from the alloy. For example, my alloy has been found well adapted for use in the production of wire having tensile strength as high as 60,000 p.s.i. or above with an elongation of the order of 10% in 10 inches and an electrical conductivity greater than 85% of that of pure copper, in wire of any cross section and of any desired size even as fine as .0056 inch in diameter. Such wire obviously is well adapted because of its flexibility and strength for use in coil windings or for other similar purposes and the finer wire is particularly well adapted for miniaturization. In the production of wire of any cross section having the combined properties of tensile strength as high as 60,000 psi. or above, with an elongation as high as about 10% in 10 inches, I would proceed as follows:
(1) By hot extrusion of the billet followed by cold rolling or cold drawing or both, the billet is reduced to a diameter of the order of 7 6 to /2 inch diameter, depending upon desired finished size.
(2) The wire is then solution annealed and age hardened.
(3) Thereafter the wire is cold reduced to desired finished size.
(4) Finally the wire is temper heat treated by exposure to a temperature which may be, and usually is, below the age hardening temperature, preferably in the range of about 500 F. to 900 F. of surrounding atmosphere from about 15 minutes to 2 hours depending upon the size of the package of wire being treated, or until desired properties are obtained.
My alloy, furthermore, lends itself to a degree of selectivity of physical characteristics. For example, I can, by eliminating the temper heat treatment, produce a wire which, at some sacrifice to ductility, has a tensile strength in excess of 90,000 p.s.i. without appreciable loss of conductivity.
A further advantageous characteristic of this unique alloy is its ability to retain high mechanical strength and electrical conductivity after exposure to high temperatures during use of the part, as, for example, in the operation of brazing such parts with others in a larger assembly. If such subsequent exposure to elevated temperature does not raise the temperature of the part as high as about 1200 F., the part will retain its strength and conductivity without further treatment. Thus a sample of my alloy was found by test to possess a Rockwell hardness of the order of F-91 on cold-worked material even after exposure to a temperature of 1100 F. for 15 minutes. If such subsequent high temperature exposure raises the temperature of the part to above the solution annealing temperature, its strength and conductivity may readily be restored by the simple expedient of repeating the agehardening treatfreedom from oxygen, which, I have found, greatly enhances its ductility and also contributes to the use of chromium, which would come out of the alloy as a chromium oxide slag if appreciable oxygen were present. Furthermore, my alloy is, as stated, exceptionally well adapted for the formation, by drastic working, of small parts to be assembled with others by brazing because it avoids any problem of embrittlement as would attend the brazing in a reducing atmosphere of an alloy containing oxygen.
1. An alloy of high ductility for wrought metal parts, heat treatable to high mechanical strength and high electrical conductivity, consisting essentially of chromium in the range of from about 20% to about .80%, cadmium in the range of from about 20% to about 1.00%, and the balance essentially oxygen-free copper.
2. An alloy of high ductility for wrought metal parts, heat treatable to high mechanical strength and high electrical conductivity, consisting essentially of chromium in the range of from about 20% to about 80%, cadmium in the range of from about .20% to about 1.00%, a deoxidizer, and the balance copper.
3. An alloy of high ductility for wrought metal parts, heat treatable to high mechanical strength and high electrical conductivity, consisting essentially of chromium in the range of from about 20% to about .80%, cadmium in the range of from about 20% to about 1.00%, a deoxidizer in the range of from about 005% to about .015 and selected from the group consisting of lithium and boron, and the balance copper.
4. An alloy of high ductility for wrought metal parts, heat treatable to high mechanical strength and high electrical conductivity, consisting essentially of about 35% chromium, about .40% cadmium, about 010% lithium and the balance copper.
References Cited in the file of this patent UNITED STATES PATENTS 2,025,662 Hensel et al. Dec. 24, 1935 2,033,710 Hensel et al. Mar. 10, 1936 2,135,254 Hensel et al. Nov. 1, 1938 2,172,639 Hessenbruch Sept. 12, 1939 2,202,150 Hensel et al. May 28, 1941 2,249,136 Hensel et al. July 15, 1941