US2259465A - Apparatus for consolidating metal powders - Google Patents

Description

0ct.'21, 1941. c. HARDY APPARATUS FOR CONSOLIDATING METAL POWDERS Original Filed Dec. -l5, 1937 Vazwum am a T N W M W a a -or both, and aims to angle of repose, so that it is Patented Oct. 21, 1941 Charles Hardy,

APPARATUS FOR CONSOLIDATING METAL POWDERS Pelham, N. Y., Metallurgical Corporation, New York, N.

assignor to Hardy Y., a

corporation of Delaware Original application December 15,

and t 119,898. Divided i938, Serial No. 206,146

3 Claims.

This invention relates to powder metallurgy and particularly to apparatus and processes in which coherent objects are formedby subjecting metal powders to compression or heat treatment such apparatus and processes and in such objects.

This application is a division of my co-pending application Serial No. 179,898, filed December 15, 1937.

In the heretofore customary art of powder metallurgy, metal powders have been placed in a mold under ordinary atmospheric conditions (and consequently containing entrapped air) and subjected to heating or to compression or to both to form a coherent mass. Difiiculties have been encountered in such practice. The metal powders do not fiow readily through small orifices or into confined spaces and have a relatively high difiicult to fill the mold completely, especially when it is of complex shape. This leads to the formation of objects which do not conform to the mold in all respects, and which cannot be used for their intended purpose, especially when accurate configuration is essential. Morever, expulsion of the entrained air or other gas from the powders during compression not only-requires the use of very high compressive force, but also brings about stratification and the development of planes of weakness through the object, usually substantially perpendicular to the direction in which the force is applied.

Asa result of my investigations I have discovered that the aforementioned difliculties can be avoided in large measure by conducting the operation in vacuo, i. e., at pressures substantially less than atmospheric so that the proportion of air or other gas present in the powder mass is substantially lessened. I have found that metal powders which will not flow at all or at best only slowly and have a high angle of repose under atmospheric conditions, will flow readily and have a very fiat angle of repose when the air or other gases entrained in the interstices between the powder particles are evacuated, at least in part. v Hereinafter powders in such an environment are somet es referred to as evacuated. Generally speaking, the finer the particle size of the powders, the greater is the increase in flow provide improvements in treatment operations are 1937, Serial No. his application May 5,

rate of the powder when evacuated. I have applied this discovery to mold filling operations and find that when metal powders in an evacuated condition are introduced into an evacuated mold, the speed of operation is greatly increased, the powder mass fills and conforms to the mold with substantial perfection, and the mass in the mold is more compact than when the gases entrained at atmospheric pressures are present.

There are, therefore, advantages to filling the mold under conditions of reduced gas pressure even though subsequent compression or heat not conducted under such conditions. However, further advantages accrue if the powder in the mold is kept evacuated during compression, in that the force necessary to achieve any desired degree of densityin the resulting object is only a small fraction of that necessary when gases are present in the powder mass. Moreover, stratification and consequent development of planes of weakness during compression or subsequent heat treatment is effectively prevented, probably due to the fact that the homogeneity of the powder mass undergoing com ression is not disturbed by gas which is being expelled.

My invention therefore contemplates the improvement in processes involving the filling of a mold with metal powder and its subsequent treatment to form a coherent mass which comprises filling the mold after the mold or the powder to be introduced, and preferably both; have been evacuated at least in part, whereby the rate of flow of powder into the mold is increased and the mold is filled more completely with the powder.

My invention also contemplates the compression of metal powder in the mold after gases entrained in the powder mass have been evacuated, at least in part, whereby consolidation of the powders is accomplished with a lower compressive force and the development of planes of weakness in the resultant metal object is effectively prevented. The above described process preferably is practiced in apparatus of my invention which comprises in combination a mold, a piston fitted in said mold. and slidable in the mold for compressing metal powder in the mold, a vacuum line connected .to the mold whereby the mold may be evacuated, filter means disposed in the vacuum line, a hopper, a conduit connected to said hopgree of exhaustion may to the mold, a metal powder measuring chamber in said conduit, and a'seco'nd vacuum line connected to the measuring chamber whereby 1 the measuring chamber may be evacuated. These and other aspects of my invention will be understood thoroughly in the light of the following detailed description of my presently preferred practice, taken in conjunction with the accompanying drawing in which:

Fig. 1 illustrates diagrammatically apparatus of my invention for use in such practice; and

Fig. 2 illustrates a modified form of the apparatus of Fig. 1.

Referring now to Fig. 1, it will be seen that the apparatus comprises a vertically-disposed mold III of any desired shape in which is fitted a vertically slidable piston H for compressing metal powders therein. Contrary to heretofore customary practice in which sufficient clearance was left between piston and mold to permit the escape of gas, the piston II should fit the mold with sufficient precision to insure against leakage of gas into the mold, and incidentally, to prevent the formation of fins on the object due to powder which game between the wall of the mold and the Dis- A steeply inclined conduit I 2 with an upperportion of the mold bottom of a hopper l3 for supplying to the mold. Valves l4, l5 are positioned in the communicates ment), against collapse is created in the hopper. by a tightly fitted but removable top I! and an annular gasket I8 of yieldable material.

' A vacuum line l9 communicates with a lower portion of the mold, preferably through a porous metal block 20 which conforms to the inner surface of the mold. A valve 2| is provided in the between the mold and the valve.

A second vacuum line 24 communicates with the upper portion of the hopper, preferably at a point above the level for metal powder therein. A valve 25 is provided in the second vacuum line for controlling the rate and degree of exhaustion of gases from the hopper. A manometer 26 containing a mercury column 21 may be attached to the second vacuum line, preferably at a point betweenthe valve and the hopper so that the debe determined by the operator.

A third vacuum line 28 is connected to an upper portion of the chamber l6, preferably through the porous metal block 29, so as to insure that a vacuum is maintained in the measuring chamber.

It is convenient is compressed in the clearance and with the to connect the third vacuum line to the second vacuum line at a point between the hopper and the valve 25.

A vacuum pump 30 or other apparatus for sucking gas out of the mold and out of the hopper and measuring chamber, is provided.

To facilitate movement of metal powder from the hopper through the measuring chamber into the mold and to compact the powder in the mold prior to application of compressive force by means of the piston, it is desirable to provide vibrating hammers 3|, 32, positioned respectively against the outer walls of the hopper and the mold.

The manometers should contain columns of mercury or other liquid sufliciently high to insure against breaking the seals in the manometers when the apparatus is placed under vacuum. In the case of operations conducted under sea level conditions with mercury in the manometers, the columns should be longer than 760 mm.

In the event that the metal powders to be compressed are prepared ready for use 'at a distance from the apparatus, say in another plant, it is convenient to place the powder immediately after preparation into a sealed container, from which the gas contained by the powders is then exhausted at least partially. In this way, the metal powders are ing storage. Moreover, time is saved in that the powders are already in an exhaustedcondition when introduced into the hopper, so that little or no additional removal of gas in the hopper is necessary.

' the top of the hopper,

- tering the vacuum In the case in which the metal powders are delivered ready for use in a sealed and exhausted container, the container itself is used in place of the top I! to seal the upper portion of the hopper. A container 33 can be constructed as in Fig. 2 so that when inverted, its top fits tightly against the gasket I 8. With the container in place on hopper may be ruptured by means of a spear 35,

fastened on the upper inside end of a lever 36 which projects through the wall of the hopper in a sealed joint 31', say a stufilng box. By pressing down on the outer end of the lever, the spear is forced upto rupture the top of the can or other container and the powders are then free to run into the hopper.

Assuming that the metal powder, preferably in a dry condition, is in place in the sealed hopper, with the lower valve [4 closed, the operation of the apparatus is as follows:

Both the mold and the hopper are evacuated with respect to gases. Preferably a high vacuum, say that equivalent to 750 mm. of mercury in a manometer under sea level conditions (absolute pressure of 10 mm. Hg) is permitted to develop in both hopper and mold. With the measuring chamber full of powder, the upper valve I 5 is closed. The valve I4 is then opened and the batch of powder from the measuring chamber is allowed to run into the mold. The lower valve is then closed and the piston is forced down to compress the metal powder in the mold into a mass of the required shape and density. Any gas expelled from the mold during compression is permitted to escape through the vacuum line l9, which communicates with the mold near the bottom thereof, so that the lowered piston will not shut it off. Powder is prevented from enlines I 9 and 28 by the porous blocks 20 and 29.

Following compression, the valve 2| is closed;

the compressed mass is removed from the mold;

protected from oxidation durthe inverted top of the- -weld the particles together according to any of the conventional methods, say by sin'tering. The sintering or other heating operation need not be conducted under vacuum, although it is desirable to employ a neutral or reducing atmosphere.

In the event that the powder is not supplied ready for use in an evacuated container, it is dumped into the hopper. The top I! is then placed on the hopper to seal it and with hopper, measuring chamber and mold in an evacuated condition, the operation is conducted as described hereinbefore.

The operation may also be conducted without placing the hopper under the vacuum. In such case the mold is exhausted as described hereinbefore. The valve II is closed and the valv i is open to permit powder to enter the measuring chambe after which the valve I5 is closed. The powder in the measuring chamber may then be freed from entrained gas by means of the vacuum line 28 or it may be introduced into the mold while it contains the gas which it entrapped under atmospheric pressures. If the latter procedure is followed the gas enters the mold and should be withdrawn through the vacuum line l9 prior to compression of the powder in the mold. This practice is slower and is not so satisfactory as that described previously.

The speed and thoroughness with which the mold is filled, and the degree of compactness obtainable in the powder before the piston is applied to it may be increased by vibrating the mold or the hopper or both by means of the oscillating hammers 3| and 32 disposed respectively against the hopper and the mold.

The degree of vacuum to be employed depends upon the fineness of the metal powder, the degree of porosity or of density desired in the compressed mass, and upon commercial considerations. Generally speaking, some advantage accrues to any operation in which the mold is filled or the powder is compressed while the powder mass contains less gas than it would entrain under normal atmospheric conditions.' However, it is relatively easy to obtain in the apparatus a vacuum equivalent to an absolute pressure of 10 mm. Hg (1. e. with. a manometer reading of 750 mm. Hg under sea level conditions). I prefer, therefore, to operate with the vacuum as high as this or higher.

As indicated'hereinbeiore the finer the metal powder the greater are the difflculties in getting it to flow under ordinary atmospheric conditions, and the greater is the increase in fiow rate of the metal powder and of the compactness which the powder will attain when evacuated. The increase in flow rate is illustrated by the following tests:

Dry copper powders were placed in a hopper having a valved orifice about V8" in diameter in the bottom opening into a reservoir below the hopper. Both reservoir and hopper were provided with vacuum connections so that they could be evacuated. In the first instance, the flow rate of each type of powder through the orifice was measured under atmospheric pressure,

'i. e., with atmospheric pressure (760 mm. Hg) in both hopper and reservoir. This was compared with the flow rate of the same powder in vacuo,

i. e., with both hopper and reservoir exhausted to an absolute pressure of 4 mm. Hg, as indicated by a manometer reading of 756 mm. Hg. The following results were obtained:

Flow data Type of powder 10. 5 cm. 5 es. 0

flowing through A," diameter Grams powder orifice per minute I With gas pressure corresponding to 760 mm. Hg on both sides of orifice.

-11 With gas pressure corresponding to 4 mm. Hg on both sides of orifice.

382 None Tyler scale. d Minus 200, plus 325 mesh.

As the results show, a remarkable increase in the flow rate of coarse metal powder'iType B) througha small orifice is obtained in vacuo, but the increase in flow rate over that obtainable under atmospheric conditions is greater with a finer powder (Type R), while very fine powder (Type C) which will not flow at all through an orifice under atmospheric conditions, flows rapidly through the same orifice in vacuo.

Metal powders, especially when dry, can be compacted to a greater extent in vacuum when at least a portion ot the gas which they entrain under atmospheric conditions has been removed, even though no great compressing force i applied. Moreover, vibration oi the powders in vacuo aids considerably in increasing the flow rate of the powder and also the degree of compactness obtainable. Generally speaking, the finer the powder, the more compact it becomes when entrapped gases have been removed, even partially. With. fine metal powders, say, Type C,

the volume occupied by the powder mass dee creases and the apparent density of the powder mass increases by at least 20% in 'a vacuum equivalent to an absolute pressure of 3 mm. Hg. However, a greater decrease in the volume occupied by a mass of even coarser powders, say, Type B, can be obtained if the powder is shaken or vibrated in an evacuated space. The increase in compactness of the powder, thus obtained before compression by a piston in a mold, is of assistance in making accurately formed objects. Moreover, it permits niore'metal powder to be placed in a space prior to compression, and thus permits larger articles to be manufactured in a given press. obtaining increased density with the application of a minimum of compressive force.

However, the greatest saving in compressive force is due to the elimination of the cushioning effect 01 the gas in the mold in which compression takes place. It might be thought that the onlysaving in compressive force obtainable by removing entrapped gas from the powders subjected to compression, would be at best about 15 lbs. per' square inch, since this is the maximum force exerted by the gas under atmospheric conditions. However, I have found that a much greater saving in compressive force results. For example, the density obtainable by compressing, under a force of 25 tons per square inch, metal powders (Type 0) containing the air which these It is also an important actor inpowders entrap under normal atmospheric conditions, is also obtainable with a force of only tons per square inchin an environment having an absolute pressure of 3 mm. Hg.- This represents a saving of approximately 80% in compressive force, and power.

If desired, the degree of vacuum obtaining in the mold prior to compression of the powders can be controlled to regulate the density and degree of porosity of the compressed object, the compressive forces applied by the piston being held substantially constant. Thus a press can be set to apply a maximum force of say ten tons per square inch, and a dense body of small but controlled porosity obtained by exhausting the gas from the powders with a or a more porous body can be obtained through the application of the but with a lesser degree ofvacuum and consequent increase in the amount of gas entrained in the powders subjected to compression.

Almost any metal powders or powder mixtures can be employed in the practice of my invention. Thus, mixtures of copper powder, tin powder and powdered graphite, such as is used for making bearings, may be employed. Alloy steel mixtures containing iron powder, finely-divided carbon and powdered alloy ingredients such as silicon, chromium, nickel, vanadium, tungsten, and the like, may be used.

The powders should be relatively dry in order to in the .mold prior to compression. However, binders such as powdered metallic soaps may be employed because these materials do not interfere markedly with the flow rates of powders in which they are included.

The metal powder mixture, for example, mixtures of powdered copper, tin and graphite, for use in bearing manufacture, can be prepared at a distance and shipped in evacuated containers I as described hereinbefore.

The advantages or my invention may be recapitulated as follows:

(1) Increased speed of mold filling andconsequent acceleration of operations and increased production for a press.

(2) Increased accuracy of configuration in the pressed article, especially when the mold is of complicated shape. Y

(3) Elimination of planes of weakness and high degree of vacuum.

same compressive force obtain maximum flow rate and compactness elimination of lack of homogeneity in structure and density of the finished article.

(4) Greater compactness of powders prior to compression, leading to the production of longer objects in a given press.

(5) Reduction of oxidation ders prior to and during treatment, with consequent improved welding together of the particles.

(6) Lower compressive force to obtain a product of given density, thereby increasing the size of objects obtainable in a given press and decreasing power consumption.

I claim:

1.' In apparatus for compressing metal powder the combination which comprises a mold, a piston fitted in and slidable in the mold, a vacuum line connected to the mold, filter means disposed in the vacuum line, a hopper, .means for sealing the hopper, a second vacuum line connected to said hopper, a conduit connected to the mold and to the hopper and extending downwardly from the hopper to the mold, a metal powdermeasuring chamber in said conduit, and a third vacuum line connected to the measuring chamher.

2. In apparatus for compressing metal powder the combination which comprises a mold, a piston fitted in said mold and slidable in the mold for compressing metal powder in the mold, a vacuum'line connected to the mold whereby the mold may be evacuated, filter means disposed in the vacuum line, a hopper, a conduit connected to said hopper and extending downwardly from the hopper to the mold, ametal powder measuring chamber in said conduit, and a second vacuum line connected to the measuring chamber whereby the measuring chamber may be evacuated.

3. In apparatus for compressing metal powder the combination which comprises a mold, means for compressing metal powder vin said mold, means for maintaining a vacuum in said mold prior to and during compression, a hopper, an airtight container for metal powder, means for sealing the container to the hopper, means for creating a vacuum in said hopper, a conduit connected to the hopper and to the mold to permit metal powder to pass from the hopper to the mold, and means disposed within the hopper for piercing a wall of the airtight container which is sealed to the hopper. CHARLES HARDY.

of the metal pow-

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