Method Of Bonding Particles Into Unitary Bodies

Abstract

A method for bonding particles into an integrated mass by placing the particles, which may be in powder form, in a confining structure, and applying to the body of powder a controlled pulse, impulse or burst of electrical or electromagnetic energy, or a combination thereof, in conjunction with a controlled pressure pulse.

Description

SUMMARY OF THE INVENTION

This invention relates to a method for bonding particles into an integrated mass of controlled form.

Heretofore, it has been known to sinter metal particles of various compositions, sizes and forms into integrated masses of desired form by mechanically compacting the particles, with or without additives such as lubricants to enhance compaction or binders to retain the compacted form, to a green compact. The green compact is then heated to temperatures generally within the range of two-thirds to four-fifths of the absolute temperature melting point of the metal to bond the particles into an integrated mass.

These sintering processes can be accomplished without a liquid phase, particularly where all particles are of material having similar melting temperatures or with a liquid phase present, as where a material of the particle mix has a low melting temperature relative to other materials. Various techniques for heating the green compact have been employed including resistive heating wherein direct or alternating current is passed through the compact to raise its temperature to the sintering range.

The present invention involves bonding particles into an integrated mass by subjecting a body of the particles to a controlled pressure pulse and applying a controlled pulse of electrical or electromagnetic energy to the pressed body. Energy pulses of short duration, either single pulses or a few successive pulses, are applied only briefly, e.g. a single pulse of 1,000 microsecond duration, are employed to bond the particles to each other or to skins, membranes or filaments of weldable material.

An object of the invention is to improve the method of bonding particles to other bodies and particularly to simplify the techniques and apparatus required to bond particles which heretofore have been bonded by complex sintering techniques and apparatus.

Another object is to improve the quality of the products formed from bodies of particles bonded together.

A further object is to bond a broad class of materials which are in finely divided form without subjecting those materials to sintering temperatures.

DESCRIPTION OF THE DRAWING

FIG. 1 is a flow diagram of the method of this invention;

FIG. 2 is a schematic diagram of one form of apparatus for the practice of this invention;

FIGS. 3a and 3b are plots of typical single and double pulse programs including pressure and electrical control and applied pressure pulses vs. time; and

FIGS. 4--6 are photomicrographs of cross sections of typical integrated bodies produced according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As set forth in FIG. 1, the invention comprises the method of bonding finely divided particles of weldable material into an integrated body of a density depending upon the pressures imposed during bonding. The process involves the simultaneous application of one or more high power electrical pulses in sychronism with pressure pulses to a body 11 of finely divided particles confined between two or more pressure-platens 12 and 13 through which the pressure and in the example electrical energy is applied.

As a first step, the particles which may be produced by reduction of oxides, electrolysis, atomization or mechanical comminution may be of a wide range of sizes of from 0.5 micron diameter and may be uniformly sized, mixed in size, or stratified as to sizes. The variations in parameters in part determine the physical properties, especially density, of the ultimate integrated body. Crystal particles of angular of flake form can also be bonded together or mixed and bonded with powder forms of particles. Sheets, bars, fibers or filaments can also be bonded with the particles in the powder or crystal form both within the matrix developed from the particles and as skins on that matrix. The arrangement shown in FIG. 2 depicts an upper and lower outer skin 14 and 15 applied to the outer faces of the mass of particles.

As with welding, mixtures of metals can be bonded by this method and nonmetallic weldable materials can be bonded to each other and to metals. Where fibers or filaments are embedded in the matrix they can be prestressed, and can be arrayed to develop optimum strengths for the ultimate product. A mass of particles may be piled on a pressure-platen with no external lateral constraint other than the vector forces within the particle array of material or developed by the shape of the platen. However, the preferred technique is to provide lateral constraint of a mold or cell as the split ring 16 into which at least one electrode-platen closely fits for relative motion along the compression axis. A guide 17 maintains washer 16 in place and receives platen 13 for reciprocation.

In FIG. 2 the pressure-platens 12 and 13 are also the electrodes for passing the electrical pulses to the particle mass 11. Shunting of the pulses around the mass 11 is avoided by insulating the ring 16 as by forming it of an insulating material such as teflon.

The constraining mold or cell is advantageously vented to permit any gas developed in the bonding process to escape. Such gases can be vented laterally as through the split (not shown) of the ring 16 with little or no flow of the particles into the vent.

The upper pressure platen 12 is stationary in the illustrative embodiment and the lower pressure platen is moveable along a path toward the upper platen by a pneumatic piston and cylinder having locking means to maintain the initially compressed work piece (none of which is shown). The force mechanism 18 also includes a pressure transformer acting on the lower platen to be superimposed upon the initial force. This second applied force can be of a to compact the body of particles to be bonded prior to the application of pulsed electromagnetic energy thereto, and can be developed in equipment of the type depicted in U.S. Pat. No. 3,059,094 of Oct. 16, 1962 to A. Vang for "Pressure Transformer." When the electrical energy is pulsed through pressure platens 12 and 13 a pressure pulse is imposed by the pressure transformer of the force mechanism 18 of the Vang patent.

In practice, pneumatically developed pressures of the order of 3000 p.s.i. have been employed. These pressure levels are not considered to be critical and can be varied over a wide range. The pressures superimposed thereon by the pressure transformer can be of a peak value of the order of 2000 p.s.i. imposed with a wave form as illustrated at A in the curves of FIG. 3. However, the compacting pressure actually imposed upon the sample is presumed to be considerably less than the sum of these pressures since the pulse of electrical energy into the electrode-platens 12 and 13 develops a repulsive force therebetween of undetermined value. The particles may reorient themselves under the influence of the electric, magnetic and/or electromagnetic fields through the mass of particles whereby they interfit more closely and ultimately bond into the unitary mass, and thermal or chemical changes may occur as by the breakdown of surface films on the particles, and expulsion of the products of such breakdown. Thus, it is believed that the maximum pressure imposed on the particles are substantially less than the peaks generated by the pressure transformer since a preponderance of that pressure as developed against a relatively unyielding measuring device is utilized in the process to overcome the repulsive force and comply with the shrinkage of the processed mass.

One electrical control wave form which has been employed is shown in FIG. 3. It was developed in the secondary 19 of a welding transformer 21 having a primary 22 connected to a power source 23 through an electronic switch 24 controlled in timed relation to the pressure from the pressure transformer of the force mechanism 18 by a process control unit 25. Excellent bonds have been made with single impulses of 32,600 joules per cubic inch of material to be bonded applied over an interval of 1000 microseconds.

It will be noted from FIG. 3 that step function input is applied from the power switch 26 to the pulse transformer of force mechanism 18 approximately 90.degree. in advance of the bond impulse applied from electronic switch 24 to the bond transformer 21 and persists for about 120.degree. or about 5.5 milliseconds. This results in a pressure wave form which peaks about 135.degree. after the initiation of the pressure signal and the process control 25 times the bond power switch to pass power as a step function approximately 90.degree. or 4.1 milliseconds after pressure power is initiated. This power is sustained for about 1000 microseconds.

It is theorized that the mechanism involved in the bonding involves electrostatic or electromagnetic forces which are imposed across the particle interfaces in magnitudes sufficient to cause diffusion of the atoms to establish atomic bonds across the interfaces as a solid-state bond. That is the energy pulse is of such short duration that undesirable melting of the composite structure is avoided and no significant temperature rise in the integrated mass is observed. Concentration of the applied energy at the particle surfaces is suggested since the inner structure of the particles is of low electrical resistance as compared with that across the interfaces.

As shown in FIG. 3, the energy can be a single pulse or a plurality of pulses which can be unidirectional or of alternating sign. Further, if the pressure impulses are so close that the induced magnetic field does not decay between pulses, a build up of forces can result when plural pulsing is employed. Thus some time spacing can be employed between multiple pulses to insure uniform pressure applications.

The form at the electrode determines the distribution of energy in the body of particles to be bonded. Thus, where energy is to be applied to a circular disc having a periphery thicker than its center, the electrode structure would be more massive in the area in registry with the periphery and will be thinned in the area of the center. In such an instance the electrode is concave to conform to the ultimate shape of the part to be bonded and the thinned region is backed up with a nonconductive material which provides the requisite support for the application of compressive forces while reducing current density in that region. Variations in the resistivity of the amassed particles, as by variations in the body form of the particles of a region of the body, can be accommodated in the bonding by application of current densities at the electrode-mass interface which are inversely related to the resistivity of the underlying mass of material. A line of ridges in a pressure platen can be employed to impose greater compressive forces in the body of particles registering with the ridges whereby the region of greatest compression is bonded to the greatest density while that of less compression tends to offer porous internal passages or channels.

A bonded body having the characteristics of a thermally sintered body of pure iron has been produced from pure electrolytic iron powder. The powder was placed in a split teflon ring 16 and compressed between parallel planar surfaces of unitary electrode-platens of electrolytic copper. The pneumatic jack of force mechanism 18 was actuated to raise the lower electrode-platen and impose 3000 p.s.i. on the powder body. Bonding was completed to the form shown in FIG. 4 by the application of 32,600 joules per cubic inch in a single pulse of 1000 microseconds and the pressure switch turned on 4.2 milliseconds ahead of the initial application of electrical energy and turned off about 0.5 milliseconds ahead of the termination of the application of electrical energy.

The photomicrograph of FIG. 4 is for a polished sample and is at a magnification of four hundred. It will be noted that bonds are established uniformly throughout the body while the particles of powder retain their identity. The samples from which this photomicrograph was made were pellets one-fourth inch in diameter and one-sixteenth inch thick.

FIG. 5 is a photomicrograph of a sample of electrolytic iron powder bonded to a skin 27 of steel foil and shows the bond between the foil and the integrated mass of bonded powder 28 as an effective unitary structure. Membranes or skins such as titanium or steel foil in laminate form or as an exterior skin can be formed in the bodies produced by this method.

FIG. 6 is a photomicrograph of a sample of electrolytic iron powder in which glass fibers 29 are embedded. A mechanical lock established between the matrix 31 of bonded powder and the fibers enables the fibers to function as reinforcement. Titanium powder can be bonded in a mass with boron filaments as reenforcement with the present method and achieve a bond between the matrix and the filaments.

Other embodiments of the product of this method is a matrix of bonded weldable particles in which is embedded a mesh or cloth of wire or glass; or solid structures such as invested rods, handles, brackets or inserts; or metallized ceramic or oxide particles. Further, combinations of skins and internal filaments can be made with the bonded particles of the matrix.

Variants on the method of forming bodies from a mass of particles utilizing a correlated pressure and electrical impulse of high energy and short duration include the practice of the method in special environments as in a vacuum or an inert reducing carburizing or decarburizing atmosphere. Where extended body forms are to be produced the process can be practiced on increments of the material in a continuous operation where bonding is accomplished to previously bonded body portions while bonding the particles.

The illustrative examples of the method have not set forth the variants necessary for irregular shaped products of the method in any detail. It is to be understood that a multiplicity of pressure-platens can be employed to vary the compaction applied to the various portions of the body being bonded and that these pressures can be directed along axes which are parallel or angularly related. Further one or more of the pressure-platens can be isolated from the applied electrical energy or the electrodes for applying the energy can be separate from all platens so that the current paths need hot be parallel to the axis along which pressure is imposed. While a combination of pneumatic pressure followed by pressure from a pressure transformer has been shown, other pressure sources can be employed if their pressure pulses can be synchronized with the electrical pulses during bonding, for example, hydraulic actuators or detonating devices may be applied as the force mechanism 18. Accordingly, the above description is to be read as illustrative of the invention and not in a limiting sense.

Claims

I claim:

1. A method of forming a body of weldable material comprising the steps of amassing particles of weldable material, imposing a compressive force over an area of the amassed particles and applying electrical energy in a single pulse of a current density of at least a million amperes per square inch for an interval of the order of a millisecond to said compressed mass of particles to bond said mass of particles into a unitary body.

2. A method according to claim 1 including the step of confining said amassed particles to a predetermined form during the application of compressive force and electrical energy to establish the shape of said body.

3. A method according to claim 1 including the steps of increasing the application of compressive force and electrical energy to said particles with time and wherein said compressive force is peaked in magnitude within a millisecond of the peaked magnitude of said pulse form of electrical energy.

4. A method according to claim 1 wherein a common electrode is employed to apply said compressive force and said electrical energy to said amassed particles.

5. A method according to claim 1 including the step of mounting a sheet of weldable material in contact with said amassed particles, whereby said sheet is bonded to the bonded mass of particles.

6. A method according to claim 1 including the step of mounting a filament within the body of amassed particles, whereby said filament is embedded within said bonded mass of particles.

7. A method according to claim 6 wherein said filament is of a material weldable with said material of said particles whereby said filament is bonded in a matrix of said bonded particles.

8. A method according to claim 1 including the step of controlling the current density of said electrical energy distributed across the surface regions of said amassed particles as a function of the resistivity of the amassed particles in registry with said surface regions.

9. A method according to claim 8 wherein said function is an inverse relationship.

10. A method according to claim 1 including the step of imposing a greater compressive force on a predetermined surface region of said amassed particles subjected to compressive force than on other regions subjected to compressive force whereby the bonded mass in registry with said predetermined surface region along the axis of compressive force is of greater density than that bonded mass in registry with said other regions along the axis of compressive force.

11. A method according to claim 1 including the step of imposing a greater current density through a predetermined region of said amassed particles than through other regions of said amassed particles whereby the bonded mass in said predetermined region is of greater density than the bonded mass in said other regions.

12. A method according to claim 1 including the step of applying an additional pulse of electrical energy of a current density of at least a million amperes per square inch for an interval of the order of a millisecond to said amassed particles.

13. A method according to claim 1 including the steps of increasing the electrical energy and compressive force applied to said amassed particles with time wherein the compressive force imposed during the current pulse is sufficient to prevent sparking between particles in the mass and the peak of electrical energy is time displaced from the peak of the imposed force.

14. A method according to claim 1 including the step of mounting a continuous surface of weldable material in contact with said amassed particles whereby said mass of particles is bonded to the surface.

15. A method according to claim 1 comprising the steps of amassing additional particles of a weldable material in contact with said first bonded mass of particles, imposing a compressive force over an area of said additional particles, and applying electrical energy in a single pulse of a current density of at least a million amperes per square inch for an interval of the order of a millisecond to said additional particles to bond said additional particles into a unitary mass bonded to said first bonded mass.