US2481232A - Powder metallurgy - Google Patents

Description

$ EWWM Filed June 2'7, 1945 N. C. MOORE POWDER METALLURGY 2 Sheets-Sheet l Filed June 27, 1945 2 Slwets-Sheec; 2

Patented Sept. 6, 1949 POWDER METALLURGY Norman Clement Moore, Plumpton, mear Towcester, England, assignor to The Ples'sey Company Limited, Ilford, England, a British com- Application June 27, 1945, Serial No. 601,736 In Great Britain June 23, 1944 1 Claim. 1

This invention relates to improvements in powder metallurgy, and has for one of its objects the provision of an efiicient method of pressing powder compacts.

Another object of this invention is to overcome certain objections and/or disadvantages which exist in connection with the manufacture of powder compacts.

A further object of this invention is to provide means for the manufacture of powder compacts.

It is known in the art, that when powder is compacted in a moulding tool by subjecting the tool to a high pressure which is released immediately after the maximum pressure has been applied, deformation and slip cracks frequently occur. After th compact is removed from the press tool, these cracks are visible, or will reveal themselves after the compact has been standing for some time in the atmosphere. Deformation sometimes occurs when the compact is sintered, for example, a straight bar will curve in the form of an arc.

To overcome these defects, it has been a general practice to add a lubricant, such as camphor or paraffin Wax to the powder, so that the individual particles are coated by a thin film of the lubricating material, thus permitting the said particles to move independently of each other during the application of the pressure. The disadvantage of adding a lubricant is that inter alia it increases the porosity of the material during sintering.

When the powder is placed in a die and pressure applied, the distribution of the pressure through the powder does not follow in any respect the theories which apply to liquids or homogeneous solids. It is considered that the most important period, affecting the conditions of pressing, arises when the pressure is applied at its earliest stages when exceedingly low pressures are exerted on the surface of the material.

Supposing the powder is held within a square section die and the pressure applied by movement of the top tool, the build up of particles will immediately take place close to the surface of the top tool and as the tool enters the powder this build-up of particles, close to the surface of the moving tool, will increase considerably as the maximum pressure is applied, This means that the compact will have a high density surface where the tool is entered and low density at the base where conditions were static throughout. When this condition occurs and the compact is sintered it will shrink to a greater extent on the low density side thus causing deformation.

In view of these facts the ideal condition for 2 applying pressure to the compact would be to apply pressure at the same rate and magnitude on all faces of the compact.

When pressure is applied to powder in a die under atmospheric conditions, the powder contains air and other gases, depending upon the previous treatment to which the powder has been subjected, This absorbed air and gas is compressed at the same time as the powder, the pressures to which the absorbed air and gas is subjected may be exceedingly high and give rise to a very complex condition, so that when pressure is released the air and gas entrapped will start to diffuse in various ways.

If the pressure is released rapidly, the conditions of gas and air dispersion may be such that large bubbles or pockets are formed, which may give rise to uneven return pressures within the structure of the compact. These air and gas pockets would follow the weakest of the low density strata and assist in causing cracks.

The condition can be avoided by releasing pressure in stages, particularly during the low pressure release period, thus permitting gradual gas and air dispersion.

When pressure is applied to the powder confined in the tool, the surface of the powder compacted follows exactly the surface contour of the die and tools, with the result that the surfaces of the compact will adhere exceedingly well to the surfaces of the die and tools. During the extraction period, the strong adhesion causes the surface of the compact to be pulled out in the form of a lid, a condition commonly known as capping.

When pressure is released rapidly on all or one surface of the compact, two conditions arise. Firstly, the gas expands rapidly at the weakest points within the compact and forces movement outwardly. Secondly, the sides or side of the die and tools which are free to move are forced out with expansion of the compact. This means that the surface of the compact adhering to the movable part will be pulled away from the rest of the material. Should the tool be extracted mechanically, this condition is exaggerated. By releasing pressure progressively in all directions, these conditions are avoided.

If the compact is extracted from the die in one direction and the adhesion of the compact to the sides of the die is great, a bending moment will be given to the compact, particularly if there is no support to the compact on the side where it is being extracted. This will give rise to a deformation which causes a redistribution of parpowder which consists in subjecting the compact to a very high pressure and simultaneously with the relief of this pressure the compact is allowed to expand substantially in all directions.

In United States Patent No. 1,321,125 a method is described for compressing powder tungsten, tantalum and the like which are also subjected to a very high pressure to compress the compact in all directions and the pressure is then withdrawn in such a manner as to be relieved on all sides of the compact simultaneously.

In United States Patent No. 2,259,094 a further method is described for making bodies of compacted powder material which consists in the gradual application of pressure to compress the material into a compact form which pressure is subsequently reduced progressively in one direction.

According to this invention the method consists in the production of a compact by the application of pressure which is reduced first in one plane and then at right angles to said plane after the maximum pressure has been applied. This said reduction is preferably applied to the compact in two diametrically opposed directions, and the pressure is first reduced to a small amount and finally to a minute portion of the original pressure.

Although a primary feature of this invention is to use a powder free of lubricant, it is to be understood that lubricant can of course be used when desired.

The process is carried out by means of a mould in which independent pressures and reduction thereof can be applied at the top and bottom or on allsides either separately or in combination. For example, the mould may be divided into two identical halves, fitted with a top and bottom tool and subjected to pressure from some source, such as a fast action hydraulic press. The hydraulic pressure is applied to the top and/or bottom tool and may be applied to the two parts of the mould.

In the drawings:

Fig. 1 is a sectional elevation of a double acting hydraulic press.

Fig. 2 is a diagrammatic sketch showing the different stages in the progressive reduction of the pressure applied to the compact, and

Fig. 3 isan enlarged scale of A and B of Fig. 2, showing the taper exaggerated.

It is to be clearly understood that the apparatus illustrated is given by way of example only; the invention is not limited to any particular device.

Referring to the drawing:

An upper hydraulic cylinder I and a lower hy draulic cylinder 2 are supported a predetermined distance apart by means of spacing bolts 3 and nuts 4.

Mounted in the upper cylinder I is a piston 5,

with an upper press tool 6 depending therefrom.

The lower cylinder 2 also contains a piston I from which extends a lower press tool 8 which is in axial alignment with the upper press tool 6 as shown in Fig. 1. Said upper and lower cylinders I and 2 each havea recess 9 for receiving cup-shaped packings III which are held in position around the respective press tools 6 and 8 by means of a cover plate II secured by bolts I2 to the upper and lower cylinders respectively.

A bracket I3 with a central threaded aperture is secured to the spacingbolts 3 by means of locking screws "I4 in such a manner that said bracket I3 can be fixed at any position between the upper and lower hydraulic cylinders I and 2.

A die I6 consisting of an externally threaded hub I? and a downwardly tapered aperture I8 is fixed, at its upper end, to a wheel I9; said hub is screwed into thebracket I3, whereby it can be moved relatively to same by rotation of the wheel I9 in a clockwise of anti-clockwise direction for the purpose of reducing the side or peripheral pressures as hereinafter set forth.

Both the upper and lower hydraulic cylinders I and 2 are provided with conduits 20, 2| for the admission and discharge of high pressure fluid for operating the press tools 6 and 8.

The process of manufacture will now be described with reference to Figs. 1 and 2.

The lower press tool 8 is first elevated to a short distance from the top of the die I 6, when it is filled with a metal powder until flush with the upper surface of said die I6; said lower press tool 8 together with the powder pact is lowered into the mould I6; the upper press tool 6 is then brought in contact with the powder compact 22 and a pressure of about 5 tons per square inch is applied to both the upper and lower press tools 6 and B for approximately one second. This pressure on both tools is then reduced to about .5 ton per square inch as indicated by the diagrammatic sketch A in Fig. 2. The die I6 is then move-d downwardly relatively to the two press tools 6 and 8 as indicated at B in Fig. 2. This movement is carried out manually by rotation of the wheel I9 a predetermined amount; thus pressure is relieved on all sides of the compact 22. The next step is to reduce the fluid pressure acting upon both press tools 6 and 8 to a very small degree, say lbs. per square inch as indicated at C in Fig. 2. The die I6 is again moved relatively to the press tools 6 and 8 to further relieve the side pressure as indicated at D in Fig. 2 when the upper press tool 6 is removed entirely as shown at E, Fig. 2. Finally the powder compact 22 can be removed by elevating the lower press tool 6 flush with the top of the die I6 as indicated at F in Fig. 2.

The amount of reduction of the side pressures may vary with different powders, and in all cases it is very small.- With the kind of press herein described, this result can'be achieved by varying the taper of the die I6.

The cycle of reducing the pressure may be repeated several times before final release.

I claim:

Apparatus for the production of powder compacts comprising in combination an upper fluid operated press tool, a lower fluid operated press tool, stationary spacing members adapted to support the respective upper and lower press tools in co-axial alignment, a transverse bracket provided with a central threaded aperture adjustably connected to said spacing members, a cylindrical mould provided with a downwardly tapering aperture and an external thread, said mould being located in the central aperture in said transverse bracket and the upper and lower press tools co-operate with opposite ends of the aperture in said mould, and a wheel attached to said mould for rotating same relatively to said transverse bracket.

NORMAN CLEMENT MOORE.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Ronay Jan. 3, 1905 Number 6 Name Date Pfanstiehl June 3, 1919 Pfanstiehl Nov. 11, 1919 Stacy June 24, 1930 Smith June 24, 1930 Whipple Aug. 19, 1941 Wellman Oct. 14, 1941

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