Alloy of high density

Patented Apr. 19, 1949

2,467,675

UNITED STATES PATENT OFFICE

2,467,675

ALLOY OF HIGH DENSITY

Jacob Kurtz, Teaneck, and Harold G. Williams, Ramsey, N. J., assignors to Callite Tungsten Corporation, Union City, N. J., a corporation of Delaware

No Drawing. Original application September 30, 1942, Serial No. 460,226. Divided and this application March 10,

Claims.

The present invention relates to alloys of refractory metals, such as tungsten, molybdenum, rhenium, and tantalum. More particularly, it relates to alloys of such metals that are of high density and free from the usual porosity and intergranular voids that are characteristic of alloys made by the methods of powder metallurgy heretofore practised.

In our copending application Serial No. 460,226, of which this application is a division, a method of making an alloy of refractory metals such as tungsten, molybdenum, rhenium and tantalum is disclosed. This alloy has a density approximating the theoretical density of the refractory metals referred to when they have been hot swaged, and contains grain refining additions which control the grain size and structure of the alloy and give certain other characteristics thereto. The alloy of the present invention contains no grain refining additions but nevertheless it has a density approximating the theoretical density of the swaged refractory metals contained therein.

According to methods commonly in use, tungsten or molybdenum and similar refractory metals having high melting points are usually worked from their powders. For instance, tungsten or molybdenum metal powder is placed in a die and compressed under high pressure Until it is formed into the shape desired, such as a rod or ingot. It is then presintered, that is, sintered at a temperature and for such a period of time that the article is strong enough to be handled for further treatment. Further treating and sintering at temperatures equal to approximately 90% of the fusing current for the particular rod or ingot, further shrinks and strengthens the rod or ingot. The treated ingot, however, is still quite porous and has many intergranular voids, the density at this stage being approximately 17-18 grams per cubic centimeter in the case of tungsten.

The rod or ingot must then be hot swaged until it is further compacted whereby the density increases rapidly from the values of 17-18 to a value approximately 19.3 for fully swaged rods, having a reduction in area of 80-90%. This density is generally referred to as the theoretical density.

According to the method of the present invention, purified refractory metal powder, such as tungsten or molybdenum, having a particle size of approximately from 1 to 25 microns, is combined with a small but effective amount of powdered metals that result in the alloying, bonding and densifying of the rod or ingot without the necessity of swaging. These alloying metals are formed from an appropriate selection

1943, Serial No. 478,610 (Cl. 75—22) 2 that are more properly described as alloying metals, and a second group consisting of the noble metals. In the first group are manganese, iron, cobalt and nickel, and in the second group, ³ platinum, palladium, gold, osmium, iridium, ruthenium and rhodium.

This alloying, bonding and densifying metal consists of a mixture of finely divided metal povzders approximately one-half by weight of base ¹⁰ metal and the balance noble metal. After these two constituents are thoroughly mixed they are added to the refractory metal powder in the proportion of approximately .25%-4% by weight, thoroughly ball-milled and mixed to assure ¹⁸ uniform distribution of the densifying metal throughout the refractory metal. The mixture is then ready for pressing into formed pieces, discs, and the like, and subsequent heat treatment at a temperature of 1500-2000° C. in a ²⁰ hydrogen or neutral atmosphere or even in a high frequency vacuum furnace, using a molybdenum or tungsten tube or elongated crucible.

The following example illustrates the method of forming this alloy:

Example

First, the alloying, bonding and densifying metal mixture is prepared. Approximately equal proportions by weight of pure finely divided ⁸⁰ nickel powder, and pure finely divided platinum powder are thoroughly mixed and ball-milled. Platinized nickel powder (containing approximately 50% by weight of platinum) made by shaking nickel powder in a solution of chlor35 platinic acid may be used. Then, approximately 7-2% by weight of this prepared mixture is added to finely divided tungsten powder of a particle size of 1-25 microns and is thoroughly mixed. It may be ball-milled for a period of several hours, 40 or at least long enough to assure the uniform distribution of the small amount of alloying powders throughout the mass of tungsten powder.

This mixture is then ready for pressing in a ⁴³ suitable mold either in a tablet machine, or in a conventional hydraulic press, depending on the size and shape of the desired finished piece. After pressing, the formed pieces are then heat treated in an electric furnace at a temperature ⁵⁰ of 1500-1850° C. in a dry hydrogen atmosphere for about one hour, the time depending, of course, on the size, shape and number of pieces inserted ' into the hot zone, and the characteristics of the furnace.

⁵⁵ After this heat treatment, the pieces have been fully sintered and shrunk in volume in a uniform manner maintaining in a characteristic way all ' the sharp contours of the pieces. The volume

3,487,676 shrinkage will vary from about 15-25% of the original volume depending on pressures and sintering temperatures used. With ¹A% Ni and %% Pt, 99.5% tungsten, densities are obtained without swaging as high as 19.35 grams per cubic centimeter, comparable to the density of a piece of hot swaged tungsten rod of 19.3.

This high density is an indication of a fully sintered body exceptionally free from porosity and intergranular voids. The micro-structure confirms this, and shows a well ordered, well developed, fairly large grained crystal structure.

An alternate method of adding the nickel and platinum in the above example may be employed with equally satisfactory results. Standard stock solutions of the soluble salts or compounds, or the metals to be added, are made up. The salts chosen belong to the class which when completely dehydrated are easily decomposible to their metallic states when heated in a reducing atmosphere. Examples of such are the acetates, chlorides, and nitrates of iron, cobalt, nickel and manganese; chlorplatinic acid, ammonium chlorplatinate, gold chloride, and similar compounds of the other metals used. A measured amount of the solution of the base metal and the noble metal is added to the oxide of the refractory metal, thoroughly dried in a manner to assure uniform distribution of the salts, and subsequently this mixture is reduced in the usual manner to metal powder. Or the solutions may be added to the refractory metal powder forming a slurry, care being taken to mix and stir the powder thoroughly to get uniform distribution of the metallic salts added. The slurry is carefully dried with constant stirring, and, when dry, it is sieved to break up any lumpy formation, put into a nickel boat and run through a reduction furnace to insure complete reduction, to their metallic states. The powders are then sieved through 200 mesh and are now ready for pressing into any desired form or shape.

While the example given above illustrates the method of making the dense alloy of the invention with tungsten as the refractory metal, it will be understood that a similar method of alloying molybdenum, rhenium and tantalum also comes within the scope of the invention. In the case of molybdenum, however, about 1% of the alloying, bonding and densifying metals should be added.

The alloys of the invention are extraordinarily suitable for making the face plates of contact points and may be used for making large shaped objects such as crucibles and cylinders, both solid and hollow. They may, in fact, be used wherever hard, strong, dense metal capable of withstanding high temperatures is desired.

Subsequent heating at approximately 600°1000° C. for periods of about 1-10 hours will cause precipitation hardening whenever desired.