US2198612A - Powder metallurgy - Google Patents

Description

SR f q Swim 5 59am. OR W Y 6) April 30, 1940. c. HARDY ,198,612

POWDER METALLURGY Filed Dec. 15, 1937 I9 P J0 Vacawn INVENTOR (bar/e5 Hard Patented Apr. 30, 1940 UNITED STATES Sears PATENT OFFlCE POWDER METALLURGY Charles Hardy, Pelham Manor, N. Y., assignor to Hardy Metallurgical Corporation, New York, N. Y., a corporation of Delaware Application December 15, 1937, Serial No. 179,898

4 Claims.

This invention relates to powder metallurgy and particularly to processes in which coherent objects are formed by subjecting metal powders to compression or heat treatment or both, and aims to provide improvements in such processes and in such objects.

Zln 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. Difliculties have been encountered in such practice. The metal powders do not flow readily through small orifices or into confined spaces and have a relatively high angle of repose, so that it is 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. Moreover, 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.

As a result of my investigations I have discovered that the aforementioned diificulties 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. Hereinafter powders in such an environment are sometimes referred to as evacuated. Glenerally speaking, the finer the particle size of the powders, the greater is the increase in flow 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 im 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 treatment operations are 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 density in 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 are effectively prevented, probably due to the fact that the homogeneity of the powder mass undergoing compression 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 fiow 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 resulting metal object is effectively prevented. lIhese 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 ll] of any desired shape in which is fitted a vertically slidable piston II for compressing metal powders therein. Contrary to heretofore customary practice in which sufiicient clearance was left between piston and mold to permit the escape of gas, the piston l I should fit the mold with sufiicient 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 is compressed in the clearance space between the wall of the mold and the piston.

A steeply inclined conduit l2 communicates with an upper portion of the mold and with the bottom of a hopper I3 for supplying metal powder to the mold. Valves M, [5 are positioned in the conduit and spaced from each other to form a measuring chamber l6 for controlling the amount of powder introduced into the mold in each batch and for assuring that the batches supplied to the mold are uniform in amount. It is desirable to provide a sight glass (not shown) in the wall of the measuring chamber or to construct the chamber of glass or other transparent material so that the operator may be sure that the chamber is completely filled.

The hopper has an inverted conical bottom and its walls (like those of all the other equipment), should be sufficiently strong to insure against collapse or distortion. when a vacuum is created in the hopper. The hopper is sealed by a tightly fitted but removable top I! and an annular gasket l8 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 vacuum line for controlling the rate and degree of exhaustion of the mold. A manometer 22, preferably containing a column of mercury 23 communicates with the vacuum line at a point bev tween 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 between the valve and the hopper so that the degree of exhaustion may be determined by the operator.

A third vacuum line 28 is connected to an upper portion of the chamber 16, preferably through the porous metal block 29, so as to insure that a vacuum is maintained in the measuring chamber. It is convenient 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 and other liquid suficiently 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 protected from oxidation during storage. Moreover, time is saved in that the powders are already in an exhausted condition when introduced into the hopper, so that little or no additional removal of gas in the hopper is necessary.

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 l8. With the container in place on the top of the hopper, the inverted top of the 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 stufl'ing box. By pressing down on the outer end of the lever, the spear is forced up to 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 l4 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 mm. Hg) is permitted to develop in both hopper and mold. With the measuring chamber full of powder, the upper valve [5 is closed. The valve M 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 I9, which communicates with the mold near the bottom thereof, so that the lowered piston will not shut it off. Powder is prevented from entering the vacuum lines l9 and 28 by the porous blocks 20 and 29.

Following compression, the valve 2| isclosed; the compressed mass is removed from the mold; the mold is again evacuated, and the process is repeated. The compressed mass rernoved from the m lwe era ywismsi gme. d .399. heati vvfeldil e particles together ccording to any of the c onventiona ods b sinterin'g;"'The'slfitering"o1"""6tlier heating operation hed 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 M is closed and the valve is open to permit powder to enter the measuring chamber, after which the valve [5 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 (i. 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 hereinbefore, the finer the metal powder the greater are the difiiculties in getting it to flow under ordinary atmospheric conditions, and the greater is the increase in flow 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 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. c. 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 B C R Screen analyses of powder":

Percent plus 100 mesh 3x6 ()3 Trace Percent minus 100 mesh, plus 150 mesh 14. 07 2.0 Percent minus 150 mesh, plus 200 mesh 20. 0 33 10. 5 Percent minus 200 mesh, plus 250 mesh 7. 0 23} 21 5 Percent minus 250 mesh, plus 325 mesh 23. O 4. 83 Percent minus 325 mesh 35. 7 94. 50 65. 0 Grams powder flowing through 3 dia.

orifice per minute:

I. With gas pressure corresponding to 760 mm. Hg on both sides of orifice 382 None 110 II. With gas pressure corresponding to 4 mm. Hg on both sides of orifice 600 410 535 *Tyler scale.

iiMinus 200, plus 325 mesh.

As the results show, a remarkable increase in the flow rate of coarse metal powder (type B) through a 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 Search Room A 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 of the gas which they entrain under atmospheric conditions has been removed, even though no great compressing force is applied. Moreover, vibration of the powders in vacuo aids considerably in increasing the fiow 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 decreases 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 more metal powder to be placed in a space prior to compression, and thus permits larger particles to be manufactured in a given press. It is also an important factor in 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 of the gas in the mold in which compression takes place. It might be thought that the only saving 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 C) containing the air which these powders entrap under normal atmospheric conditions, is also obtainable with a force of only 5 tons per square inch in 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 high degree of vacuum, or a more porous body can be obtained through the application of the same compressive force but with a lesser degree of vacuum 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.

as described hereinbefore.

The advantages of my invention may be capitulated as follows:

1) Increased speed of mold filling and consequent 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.

(8) Elimination of planes of weakness and 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 of the metal powders 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:

I. In a process involving the introduction of metal powder into a mold and the subsequent molding of the powder to form a coherent mass, the improvement which comprises flowing the powder into the mold while maintaining a pressure less than atmospheric in interstices between the particles of powder and while vibrating the metal powder particles, the molding of the powder being conducted while maintaining a pressure less than atmospheric in said interstices.

2. In a process involving the introduction of metal powder into a mold and the compression of the powder in the mold to form a coherent mass, the improvement which comprises introducing the metal powder into the mold while maintaining interstices between the powder particles at a pressure substantially less than atmospheric and subjecting the powder particles to vibration while maintaining intersticesb'etween the particles at a pressure substantially less than atmospheric prior to compressing the \powders in the mold, the compression being conducted while the interstices between the powdered particles are in an evacuated condition.

3. In a process involving the introduction of a metal powder into a mold and the subsequent compression of the metal powder in the mold to form a coherent mass, the improvement which comprises storing the metal powder in an evacuated container, removing the powder from the evacuated container into an evacuated chamber while maintaining the powder in an evacuated condition, flowing the powder from the evacuated chamber to an evacuated mold while maintaining the powder in an evacuated condition, and compressing the powder in an evacuated condition in the mold.

4. In a process involving the introduction of metal powder into a mold and the subsequent compression of the metal powder in the mold to form a coherentrgmass, the improvement which comprises conducting the powder in an evacuategcondition from a hopper into a measuring chamber maintained in an evacuated "condition, sealing the measuring chamber from the hopper, and thereafter flowing the powder while subjecting it to vibration in an evacuated condition from the measuring chamber to a mold maintained in an evacuated condition and compressing the powder in said evacuated condition in the mold.

' CHARLES HARDY.

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