US3356495A - Method for producing high density tungsten ingots - Google Patents

Description

' Dec.5,1967 ETAL 3,356,495

METHOD FOR PRODUCING HIGH DENSITY TUNGSTEN INGOTS Filed March 13, 1967 5 S t l STEP 1, MIXING SUB MICRON METAL POWDER ABOVE l Kbor.

INVENTORS JOHN F LAKNER BY GORDON E. ZIMA MQQ A TTORNE Y Dec. 5, 1967 Filed March 13, 1967 THEORETICAL DENSITY G. E. ZIMA ETAL 3,356,495

METHOD FOR PRODUCING HIGH DENSITY TUNGSTEN INGOTS 3 Sheets-Sheet 2 TEMPERATURE IOSOC 90. TIME IOMIN.

' OPISTON CYLINDER APPARATUS 9Q XBELT APPARATUS 60 y I I I 1 1 PRESSURE (K boys) INVENTORS JOHN F LAKNER BY GORDON E.Z/MA

ATTORNEY Dec. 5, 1967 G. E. ZIMA ETAL 3,356,495

DENSITY TUNGSTEN INGOTS METHOD FOR PRODUCING HIGH 3- Sheets-Sheet 5 Filed March 13, 1967 PRESSURE 25W DClrS) TIME IO MIN.

I650 zuiwmow TC v INVENTORS JOHN F LAKNER GORDON E. 2: MA

ATTORNEY TEMPERATURE (C) United States Patent ABSTRACT OF THE DISCLOSURE A process for producing tungsten ingots of high structural integrities and of above 98% theoretical density by compacting tungsten powder at temperatures above 900C. and at pressures up to about 50 kilobars, and then decompressing the powder before the temperature is low ered below about 700 C.

The present invention was evolved in the course of, or under, Contract W-7405-ENG-48 with the United States Atomic Energy Commission.

Background of the invention Field of the invention.This invention relates to the production of metallic ingots, and particularly to a powder metallurgical process for producing ingots of pure tungsten or tungsten alloys by compaction of a powder mixture under heat and pressure.

Description of the prior art.The treatment of tungsten by standard metallurgical techniques has always posed considerable problems, due to the extremely high melting point of tungsten (3370 C.) and the extraordinary react-ivity of the metal at such elevated temperatures. Accordingly, the melting step involved in conventional methods requires special apparatus, and, in spite of all precautions, is usually accompanied by the introduction of considerable quantities of impurities from the crucibles, furnace materials and atmosphere surrounding the tungsten melt.

In view of the difficulties encountered in the treatment of tungsten by ordinary methods, it has also been attempted to apply the methods of powder metallurgy to the production of pure tungsten and tungsten alloy bodies. However, the production of satisfactory ingots by this method has thus far been unsuccessful.

The principal shortcoming of tungsten ingots prepared from tungsten powder is the poor density, which falls far short of the theoretical densities for the material. The low densities are known to be due to a large fraction of void spaces. It has been attempted to increase the density by vacuum-treating the powder and compressing it at elevated pressures. However, while the void fraction is somewhat reduced thereby, the ingot surfaces craze and crack to a highly detrimental degree. The depth of these imperfections increase as higher pressures are applied. For example, at consolidation pressures of about 100,000 p.s.i., the ingot densities still fall far short of the theoretical den s-ity, yet the cracks already penetrate deep into the ingot interior, reducing most of the processed composite bodies to waste.

Accordingly, it is an object of the present invention to provide a process for producing tungsten ingots of near theoretical densities, e.g., above about 98% theoretical density.

Another object of the present invention is to provide a process for consolidating tungsten powder at pressures above 1 kb. (kilobar) and up to 50 kb. without producing cracks and crazes in the ingot.

These and other objects are achieved by the present process which generally involves the steps of preforming a green ingot from the metal powders, compressing this green ingot in a suitable die and high pressure apparatus to pressures above 1 kb. and preferably between about 2 and 30 kb., and heating the powder to at least 1000 C. during compression. At the end of the compression cycle, the temperature and pressure are controlled so that the pressure on the specimen is essentially reduced to atmospheric pressure before the temperature drops below about 700 C. The ingots produced by this procedure have proven to be invariably free from flaws and discontinuities, and have been ranging in density from about 98 to 99.6%.

The best density values are achieved by applying high pressures in the range of 15-50 kilobars. 'For generating pressures in this range of magnitude, resort must be had to special apparatus, e.g., belt type compression apparatus, which will be discussed in detail below, although the high pressure apparatus itself is not a part of this invention. Tungsten ingots of desired density and integrity are produced by raising the temperature of green ingots to about 1'000-1200 C., while compressing the specimen to 15 kb. or higher. After holding the pressure and temperature for a period of about 10 minutes, the ingots must be depressurized before the ingot temperature falls below about 700 C. as mentioned previously. With the belt type apparatus, a minimum depressurization rate of about 100,000 p.s.i./sec. was employed to satisfy the above condition.

For tungsten compaction at pressures below about 15 kb., the compaction is generally carried out with a con ventional piston apparatus, which affords processing con siderably larger ingots with a minor sacrifice in the overall density achieved. The temperature and time parameters of compaction are the same as with the high pressure belt apparatus described above. It is again essential that the pressure be reduced sufficiently rapidly to near atmospheric prior to allowing the temperature to drop below about 700 C.

The critical feature in preventing cracking of the ingots is the manner in which the ingot is depressurized. The condition that the ingot pressure be relieved before the temperature drops below the 700 C. region is a necessary condition, but evidently not suflicient as indicated by the experiments. The rate at which the pressure is dropped has a pronounced effect upon ingot integrity, also. Thus, if the specimens are compacted at pressures above 15 kilobars by means of a high pressure belt apparatus, the pressure must be decreased at an extremely rapid rate, e.g., about 1-0 p.s.i./sec., in order to obtain sound ingots. The experiments conducted in the relatively lower pressure regions of about 2-15 kb. invariably failed if the pressure was reduced at such an extremely rapid rate. On the other hand, incomplete depressurization before the temperature dropped below the 700 C. region also damaged the ingots. Sound ingots were produced only when the pressure was reduced at a comparatively moderate rate, e.g., about 1000 p.s.i./sec.

Although we have several hypotheses, e.g., that cracking is a function of the stress-strain relationships at the ingot surfaces, which may be favorably influenced by choice of a suitable depressurization rate, or that the depressurization is determined by the apparatus used in the compaction, the precise mechanisms and reasons are at present not known with certainty. It is clear, however, that the ingots must be depressurized at a rate which is in approximate conformity with the rate at which the tem perature can be decreased.

The specific features of the invention will be discussed 'in detail in the following description, in conjunction with the drawings, of which:

FIGURE 1 is a schematic fiow chart of the present process, including green ingot manufacture;

FIGURE 2 is a graph of the final density vs. the compaction pressure;

FIGURE 3 illustrates a high pressure belt apparatus, together with sample container and instrumentation in cross section;

FIGURE 4 is a graph of the final density vs. compaction temperature; and

FIGURE 5 shows a charged piston apparatus in cross section for ingot production at pressures below kb.

In general, the procedure for producing high density tungsten or tungsten alloy ingots consists of a sequence of preliminary treatment steps for producing a cohesive green ingot, from which gaseous occlusions have been largely eliminated. This green ingot is then further compressed to its final density at multikilobar pressures.

The procedure is set forth in the flow chart shown in FIGURE 1. Referring now to FIGURE 1, the tungsten material is prepared for final compression by first producing a green ingot from the tungsten and alloy additives, if any. The ingredients must be all in powder form, preferably of submicron particle size. The first step is to intimately mix and blend the powder additives, as in a conventional V-blender. Subsequently, the material is preliminarily compacted into a cohesive body as shown in step 2. The preliminary compaction is accomplished by compressing the powder at about 100-l50K p.s.i. The manner of compaction, per se, is not critical, and any suitable equipment may be used. It is preferred, however, to compress the material into an oversize slug which is similarly shaped as the desired green ingot for loading into the high pressure apparatus. Similarly, since the pressure of gases in the ingot void spaces comprises the coherence and final density of the tungsten composite, it is advan tageous to eliminate gaseous matter from the outset. Accordingly, the preferred preliminary compaction procedure for producing green ingots is to load the blended metal powders into plastic or metal containers under vacuum conditions. Typical densities for the powder mixture prior to pressing are about 5 gms./cc., and cold pressing in an oil hydrostat at about 150K p.s.i. results in densification to between 11 and 12 gm./cc. Accordingly, the container should be somewhat larger than twice the volume of the ingot size for insertion into the high pres sure apparatus. In step 3, the cold pressed material is subjected to a vacuum treatment at 200" C. to further drive off residual gases.

After the low temperature vacuum treatment, the green tungsten ingot is subjected to a minor sintering by heating it to a temperature of about 900 C. in a hydrogen atmosphere and' holding this temperature for about an hour. The final step prior to the high pressure treatment is to machine the ingots to the proper size and emplace the ingotsinto a metal container. The preferred container material is molybdenum, which can be readily removed from the tungsten by an acid leach.

The green ingot preparation is completed by baking the container and sample assemblies at about 500 C. and at pressures below 10- torr. After baking, the container is sealed under vacuum, e.g., by electron beam welding the lid to the capsule.

The final condensation is carried out in a device capable for applying pressures in the kilobar range.

The density vs. pressure profile shown in FIGURE 2 is typical of the family of curves obtained under different conditions of heating, compaction temperature, and compaction duration. The ingot density is generally above 98% for pressures exceeding 2 kilobars, and increases gradually as higher pressures are applied. In the 1 kb. region, the final density rapidly deteriorates. The exact pressure at which the steep density decline begins is dependent on the remaining variables, primarily the temperature.

Densities averaging about 99.5% theoretical density,

i.e., above 19.2 g./cc. in the case of pure tungsten, are achieved by compressing the green ingots to between 19 and 35 kilobars. For exerting such high pressures, resort must be had to equipment such as the belt type apparatus shown in FIG. 3 in cross section.

Referring now to this figure, a central sample or compression chamber 10 is defined by annular high strength steel belt 11, which laterally circumscribes the chamber, and tapered steel pistons 12, which project into the central opening of annular belt 11 and define the top and bottom bounds of the chamber 10. Green ingot 13, sealed into container 14, is disposed in the center of the chamber 10, laterally surrounded by graphite furnace 16. The container 14 is electrically insulated from the furnace 16 by insulating jacket 17, which is usually made'from a suitable ceramic such as boron nitride. This assembly is laterally enclosed in a pyrophyllite sleeve 18. Steel discs 19 provide the necessary electrical paththrough graphite furnace 16 via steel pistons 12. Annular pyrophyllite jackets 21 seal the sample chamber 10 and extrude into the void spaces between pistons 12 and belt 11 when pressure is applied to the pistons.

The temperature in the sample chamber is monitored by a thermocouple 22', the junction of which is imbedded adjacent container 14. 'The thermocouple leads are brought out to the exterior through the pyrophyllite gasket 22.

Pressurization is accomplished by placing the belt into a suitable press. For accurate pressure control, the force exerted by the press, as measured by the diagnostic equip ment associated with the press, is calibrated against ma= terials which exhibit readily discernable transitions at definite pressures. Thus, bismuth, tellurium, cesium, and barium undergo distinct electrical conductivity changes at 25.4 and 89, 37, 42 and 59 kb., respectively, which is used to obtain a graph of the effective pressures prevailing in the compression chamber interior in response to the total pressure exerted by the press.

To consolidate the tungsten sample, the pistons are compressed to the maximum desired pressure, between about 19 and 35 kb. Above 19 kb., the final density has been found to vary only slowly in the applied pressure. For example, the density of a sample compressed at 19 kb. was 19.21 g./cc., and the density of samples compressed at 23', 25 and 35 kb. were about 19.23 each. Accordingly, since the ultimate pressures determine the maximum sample size which can be processed, pressures of about 23 to 25 kb. are preferred, in view of the fact that the density has been found to increase only slowly above 25 kb. The rate at which the pressure is increased has not been found to have a critical elfcct upon the outcome of the sample; however, pressurization is preferably carried out at a moderately slow rate, e.g., up to about 5 minutes to increase the pressure to 30 kilobars, in order to protect the charge package from deleterious damage. The pressure is then held at this value and the temperature is increased to the desired consolidation temperature.

The temperature to which the ingot is raised is another parameter which has a profound influence upon the final density. The relation between temperature and density is illustrated in FIG. 4. As can be seen from this figure, densities above 98% theoretical are achieved at temperatures between about 900 and 1400, with maximum densities obtaining around 1000-1100 C. After the specimen is heated to the desired temperature, it is held at this temperature and under pressure for a period of 5-20 minutes. The final density increases somewhat with the duration of the heat treatment. For example, after holding the pressure and temperature at 30 kilobars and 1170 C. for about 1 minute, the final density is about 98.6 grams/emf, whereas the final density rises to about 98.8 grams/ cc. if the compaction is prolonged to 20 minutes.

The above discussed parameters of pressure, temperature and time all affect the final density of the specimen,

as discussed supra. The steps of terminating the compaction process, however, are critical to the integrity of the finished ingot.

The high pressure work, e.g., above about 19 kb., such as produced in the belt apparatus discussed above, calls for an extremely rapid depressurization which is especial- 1y critical to achieve sound ingots. Thus it has been found that the pressure must be relieved entirely in a matter of less than 2-3 seconds, or at a rate of much more than 10,000 p.s.i./sec., preferably about 100,000 p.s.i./sec. If the depressurization step is prolonged over greater periods of time, the finished ingots exhibit cracks and crazes of increasing severity. If the ingot is compressed in conjunction with temperatures of more than 1000 C., it is preferable to lower the temperature to about 900 C. for a short period, e.g., 1 minute, prior to decompression.

FIGURE 5 is a cross sectional view of a single piston apparatus containing a green ingot, prepared as outlined supra, and furnace for heating the ingot. The piston apparatus is useful for carrying out the final compression step under pressures below about 15 kb.

With reference to FIGURE 5, the green tungsten ingot 110, hermetically enclosed in capsule 111, is disposed in the working chamber of the piston-cylinder apparatus 112. The metallic capsule 111 is surrounded by dielectric enclosure 113 to insulate the capsule 111 from the graphite sleeve furnace 114. The furnace is in turn surrounded by pyrophyllite sleeve 116. The numeral 118 refers to a conventional thermocouple anchor and guide assembly.

To compress the tungsten ingot, the piston-cylinder assembly is placed into a press (not shown) and the ram is actuated to apply a compressive force on the piston 117 until the desired pressure is reached in the sample chambet. The pressure is increased at a moderate rate of 5-10 kb. per minute. After the ultimate pressure is attained, the sample is heated to a temperature between about 1000 and 1200 C. and held at this temperature for up to about 10 minutes. While the pressure-temperature treatment may be prolonged further, the benefits attained are marginal. Thereafter, the pressure is again lowered prior to reducing the temperatures to less than 700 C. In the case of the relatively lower pressure treatment in pistoncylinder' apparatus, the pressure is lowered at a rate of about 1000 p.s.i./ sec.

At discussed supra, lowering the pressure comprises the final densities somewhat. However, the application of lower pressures has distinct advantages in that considerably larger ingots can be processed.

EXAMPLES Green ingots were prepared by cold pressing tungsten powder of the particle size distribution and constituency given in Tables I and II, and subsequent vacuum treatment and sintering as outlined supra.

Table III summarizes results obtained by compressing in a belt type apparatus.

Table 1.Chemical analysis Table IL-Particle size analysis DISTRIBUTION BY 'PHOTELOMETER AVERAGE PARTICLE DIAMETER BY FISHER SUB-SIEVE SIZER Lab. milled Number (microns) .49 Porosity .660

Table III Percent p0 (gm./Cu P;(kbars) T, (C.) 6; (min) p theor.

density 19. 3 1,178 10 19. 20 99. 4s 23. 0 921 10 19. 03 9s. 50 23.0 1,022 10 19.09 98.91 23. 1 1, 401 10 18,97 98,28 30. 0 1,180 l 12 19. 04 9s. 05

1 Seconds.

In every instance, the ingot was inspected by die penetrant inspection methods and were found to be absolutely devoid of flaws and cracks.

While the invention has been described in terms of specific process embodiments with reference to specific apparatus, it will be apparent to those skilled in the art that the process and apparatus for carrying out the process is capable of further modifications. It is intended to cover all such modifications and variations in the present application, the scope of which should therefore be limited only by the appended claims.

We claim:

1. In a process for producing high density tungsten ingots from degassed green ingots by compaction of said green ingots in a belt type high pressure apparatus, the steps comprising:

(a) compressing said green ingot to a pressure of at least about 1 kilobar, and

(b) heating said green ingot to a temperature between about 900 and 1400 C.,

(c) thereafter reducing said pressure to atmospheric pressure and reducing said temperature to ambient temperature, said pressure being reduced at a sufficient rate to depressurize said ingot prior to reducing the temperature of said ingot to about 700 C.

2. The process of claim 1, further defined in that said green ingot is compressed to about 2 to 50 kilobars.

3. The process of claim 1, further defined in that said pressure of at least 1 kilobar and said temperature between 900 and 1400 C. are maintained for at least 1 minute before reducing said pressure and temperature.

4. The process of claim 1, further defined in that said rate at which said pressure is reduced corresponds approximately to the rate at which the temperature drops to about 700 C.

5. The process of claim 1, further defined in that said green ingot is prepared by intimately mixing tungsten and additives in the form of a powder of submicron particle size in intimate admixture; compacting said tungsten and additives powder mixture at a pressure on the order of 10 psi; heating said mixture in a hydrogen atmosphere to about 900 C. for about 1 hour; heating said mixture under vacuum conditions to remove occluded gases therefrom; and enclosing and hermetically sealing said mixture into a gas-tight container.

6. In a process for producing tungsten ingots of at least 98% theoretical density from degassed green ingots by compaction of said green ingots in a belt type high pressure apparatus, the steps comprising:

(a). compressing said green ingot to a pressure between about 15 and 50 kilobars,.and

(b) heating. said green ingot to a temperature between about 900 and 1.400 C.,.

(c) maintaining said temperature and pressure to at least about 1 minute,

( 1) reducing said pressure to atmospheric pressure at a rapid rate of more than 10 p.s.i./sec., and

(e) thereafter reducing the temperature to below 700 C. and ambient temperature.

7. The process of claim 6, further defined in that said green ingot is heatedto about 1000--1100 C.

8. The process of claim 7, further defined in that said temperature is reduced to about 900 C. before said pressure is reduced to atmospheric pressure.

9. The process of claim 6, further defined in that said temperature of 900 to 1400 C. and said pressure between 15 and 50 kilobars is maintained for about 10 minutes.

10. The process of claim 6, further defined in that said pressure is reduced from between 15-50 kilobars to atmospheric pressure at a rate of about 10 p.s.i./sec.

11. In a process for producing tungsten ingots of at least 98% theoretical density from degassed green ingots by compaction of said green ingots in a piston-cylinder high pressure apparatus, the steps comprising:

(a) compressing said green ingot to a pressure between about 2 and 15 kilobars', and

(b) heating said green ingot to a temperature between References Cited UNITED STATES PATENTS 2,916,809 12/1959 Schell 226 FOREIGN PATENTS 1,207,041 9/1958 Germany.

700,658 12/1953 Great Britain.

BENJAMIN R. PADGETT, Primary Examiner.

A. J. STEINER, Assistant Examiner.

Claims (1)

1. IN A PROCESS FOR PRODUCING HIGH DENSITY TUNGSTEN INGOTS FROM DEGASSED GREEN INGOTS BY COMPACTION OF SAID GREEN INGOTS IN A BELT TYPE HIGH PRESSURE APPAARATUS, THE STEPS COMPRISING: (A) COMPRESSING SAID GREEN INGOT TO A PRESSURE OF AT LEAST ABOUT 1 KILOBAR, AND (B) HEATING SAID GREEN INGOT TO A TEMPERATURE BETWEEN ABOUT 900 AND 1400*C., (C) THEREAFTER REDUCING SAID PRESSURE TO ATMOSPHERIC PRESSURE AND REDUCING SAID TEMPERATURE TO AMBIENT TEMPERATURE, SAID PRESSURE BEING REDUCE AT A SUFFICIENT RATE TO DEPRESSURIZE SAID INGOT PRIOR TO REDUCING THE TEMPERATURE OF SAID INGOT TO ABOUT 700*C.

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