US 3346914 A
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0d. 17, 1967 J SANDSTRQM ET AL 3,346,914
DEVICE FOR CONSOLIDATING METAL PQWDERS 5 Sheets-Sheet 1 Original Filed June 5, 1964 'rJ/// ll INVENTOR. Donald .1 Sands/ram Charles L. Terrell W Q.GWLA.MM
0d. 17, 1967 SANDSTROM ET AL 3,346,914
DEVICE FOR CONSOLIDATING METAL POWDERS Original Filed June 5, 1964 5 Sheets-$heet 2 Charge SW. 16 ea. I gn/fran 5w.
Power pp y Coaxial Load INVENTOR.
Dana/o J Sands/ram BY Charles L. Terra/l Get. 17, 1967 D. J. SANDSTROM ET AL 3 7 DEVICE FOR CONSOLIDATING METAL POWDERS Original Filed June 5, 1964 3 Sheets-Sheet 3 INVENTOR.
Donald L/. Sandsfrom BY Char/es Lferre/l United States Patent 3,346,914 DEVICE FOR CONSOLIDATING METAL POWDERS Donald J. Sandstrom and Charles L. Terrell, Los Alamos, N. Mex, assignors to the United States of America as represented by the United States Atomic Energy Commission Continuation of application Ser. No. 372,426, June 3, 1964. This application Nov. 10, 1966, Ser. No.
Claims. (Cl. 18-5) ABSTRACT OF THE DISCLOSURE The invention described herein was made in the course of, or under, Contract lV7405ENG36 with the US. Atomic Energy Commission. This application is a continuation of SN. 372,426, filed June 3, 1964.
This invention relates to a device for compacting metal powders and in particular to a device that uses a magnetic field produced in a coaxial conductor to consolidate the metal powders into any desired shape.
This invention makes use of opposing magnetic fields in a coaxial conductor to consolidate the metal powders into tubular or rod shapes. The advantage of this device lies in the fact that extremely high pressures are attainable over extremely long lengths of compacted powdered material. The nature of the coaxial configuration is such that the magnetic field is extremely uniform over these long lengths. Comparisons of density of materials compacted using this technique and device indicate improvement over conventional hydrostatic techniques with peak pressures attainable in the range of 50,000 pounds per square inch. A decided advantage of this process over explosive compaction techniques lies in the fact that magnetic techniques are more reproducible. Another advantage of this device is that extremely intricate shapes may be produced, these shapes being dictated by the mandrel geometry.
In addition to all the aforementioned advantages, the magnetic compaction technique of this invention has additional advantages in that it is practical to perform the compaction operation in a limited shop area.
It is an object of this invention to provide a device for compaction of metal powders into rods, tubes, or any desired shape and having a green densityof at least 60 percent of theory.
Embodiment of this invention will now be described by way of example with reference to the accompanying drawings wherein:
FIGURE 1 is a sectional view of the coaxial device in a configuration used for consolidating metal particles into a tubular shape.
FIGURE 2 is a simplified electrical schematic of the capacitor bank power supply and coaxial load, and the ignitron switching header.
FIGURE 3 is an enlarged cross-sectional view of the upper end of FIGURE 1 and allows greater detail to be shown.
FIGURE 1 depicts the outer conductor 9 which has been clad with a nonmagnetic stainless steel sleeve 10 while the inner conductor is an annealed copper tube 7 which acts as the pusher and confinement for the powders to be compacted. Electrical connections are made to the copper tube by means of tapered copper plugs 2 which are driven into the inner conductor 7 to produce a good electrical contact. The top plug 2, which is held in place by brass lock nut 1, also shorts across the annular gap 8 between the inner and outer conductor thereby completing the current path. The annular gap between the inner and outer conductor must be .insulated with a dielectric 12 (also see FIG. 3) of sufficiently high breakdown voltage so as to prevent any arcing which might occur between the said conductors. Any arcing which does occur between these conductors significantly decreases the efiiciency of the process. Vinyl tubing has been found to be an effective means of insulating these conductors. This insulation must be, however, held to. a minimum thickness if the efficiency is to be kept high. The powder 6 is placed in the annular gap created between the inner conductor 7 and the mandrel 5. This mandrel is held in place in the center of the tube with micarta plugs 4 which are bored to hold the terminal ends of the mandrel. Tight fitting rubber plugs 3 are inserted over the mandrel holders to serve the dual purpose of creating leaktight seals and acting as electrical insulation between the shorting plugs 2 and the mandrel 5. The coaxial conductor is then firmly gripped to a two-way-header 13 and 16 fabricated from aluminum, and is connected to the coaxial cables 14 which are in turn connected to a capacitor bank (not shown). Upon discharging the capacitor bank, the current is carried up the inside of the outer conductor 9, and back to ground 19, through the inner conductor 7 and a copper tube 11 and ground lead 19. The magnetic fields generated by this high current are repelling to each other. Due to the fact that the inner conductor has a lower mechanical strength than the outer conductor, the inner conductor is crushed by the magnetic field.
FIGURE 2 shows the power supply, the capacitor bank, ignitron switching header, and the coaxial load. The bank which is being used to supply this coaxial device with electrical power has a capacitance of 715 microfarads at a peak voltage of 20 kv. The bank consists of 48 capacitors with switching accomplished through 16 ignitron tubes with a single trigger ignitron. The peak current produced using the coaxial load configuration is approximately 1200 ka. Using this particular system it is possible to develop instantaneous peak pressures which are in excess of 50,000 pounds per square inch.
FIGURE 3, in addition to showing all the components shown in the top terminal in FIGURE 1, shows a thin walled (0.010 inch) cellulose acetate sack 18. The cellulose acetate sack contains the metal powder and is inserted into the copper inner conductor 7 which is slipped inside a vinyl tubing 12. The mandrel is lubricated with a thin film of silicon grease with the said greased mandrel being slipped inside a polyethylene or Teflon shrink tube 17.
Operating principles The above described device uses the ability of a copper conductor carrying very high pulse currents (in the range of 10 to 10 amperes) to crush itself due to the magnetic field generated around the conductor. The time scale for the compacting. operation is of the order 10- to 1() seconds, determined by the ringing frequency of the power supply system. The energy stored in the capacitor bank is of the order of 10 joules or watts-sec 0nd. This magnetic field is confined in the annular gap between the inner and outer conductors such that the field acts as if it were a compressed gas tending to crush Materials compacted Tubular shapes have been magnetically compacted using tungsten, .molybdenum, tungsten+uranium oxide, and molybdenum+uranium oxide powders as starting to be extracted easily while the shrink tube is subsequently removed from the compact by the application of heat. A temperature of 150 C. is sufiicient to shrink the plastic tube away from the green compacted interior diameter of the tube. The green powder compact is separated from the copper pusher by means of acid etch- Once the green compacted powder tube has beenseparated from the copper pusher and the polyethylene or Teflon tubing, it is then ready to be sintered. Any well known sintering operation may be performed on the green compacted powder and the sintering operation is not considered a part of this invention. The results of density measurements performed on various compacted, sintered tubes are presented in the following table.
TABLE I.-DENSI'IY OF SINTERED REFRACTORY METAL TUBING FABRICATED BY MAGNETIC COMPACTION Sintered Properties 1 Sintered Properties 1 Particle Size Percent Material f T eo!- Green Percent Percent Percent Percent Percent Percent Density Open Closed Theor. Open Closed Theor.
Porosity Porosity Density Porosity Porosity Density Mo compacted over steel mandre 4- 06 0. 69 5. 57 93. 74 0.27 4. 52 95. 21 Mo compacted over a cerro alloy mandrpl 4. in 1.6 6. 53 91.87 0.42 4.18 95.4 Mo compacted over a steel mandrel 1. 8*; 0.52 2. 22 97. 62 Mo+ v/o U0: compacted over a steel UO2= 2 m h; 8. 95 2. 67 88.38 2. 98 5.12 91. 4
mandrel. MO =1.8*. W+40 v/o U0 compacted over a steel UOa=4- 18.39 0.12 81.43 11.56 2. 56 85.88
mandrel. W compacted over a steel mandre 3- 6).-- 64. 4 2. 49 10.31 87.2
1 Sintered at 1,700 O. in H: for 3 hours. 2 Resintered at 2,000 O. in vacuum for 2 hours.
materials. In all cases, no binders were added to thepowders. The green density and. strength of the as compacted tubes allows handling prior to sintering even Without the use of binders. In the case of molybdenum, various sizes of particles have been compacted and in all cases the green density is considerably greater than the density produced using hydrostatic techniques.
The technique used for filling the copper pusher prior to compaction consists of centering the mandrel in a thin walled cellulose acetate tube 18 (FIGURE 3). The cellulose acetate tube is used to protect powders such as molybdenum and the uranium oxided loaded cermets from acid attack during the dissolution of the copper pusher after the compaction. Packing of the loose powder into the confinement acetate sack is accomplished with an ultrasonically vibrated table. The nature of this invention is such that uniform powder packing is mandatory; however, it is not necessary to have a high packed density. If the packed density is not uniform, the compacted tube will not be uniform either. Although FIGURE 1 shows a steel mandrel 5 being used to form a powder compacted tube, fusible alloy mandrels may be used, but since the modulus of these materials is extremely low, it is difficult to keep the mandrel centered and consequently the com pacted tube will not be concentric.
After the powders have been compacted, the shorting plugs 2 are still in the inner conductor 7. These end plugs 2 and vinyl insulation 12 (FIGURE 3) are removed and the mandrel is extracted by either manual pulling or, in some cases, by application of a draw bench. The copper pusher should not be removed until the mandrel has been extracted from the compact. The copper sheath gives the compact enough mechanical strength to prevent the development of compression cracks in the green tube. In order to facilitate the removal of long mandrels from compacted tubes, the steel mandrel is first lubricated with a thin film of silicone grease. The greased mandrel is subsequently slipped inside a polyethylene or Teflon shrink tube 17 (FIGURE 3). This tubing shrinks upon heating due to plastic memory and thus prevents any lubricant from coming in contact with the compacted metal powders. The lubricant on the mandrel allows the mandrel num powder withoutthe aid of binder materials. This molybdenum compact has a theoretical density in excess of 97%. Other sintered densities for molybdenum +uraniurn oxide and tungsten+uranium oxide are listed in Table I where the uranium oxide material had an average particle size of 4.2 microns. Although the data presented in Table I was obtained without the use of binders in conjunction with the magnetic compacting method, the inventors have done a limited amount of work using well known epoxies as binders. The epoxies appear to be excellent binder materials from the standpoint of low residue upon sintering and high strength in green cured state.
In particular, although the foregoing specification only describes a device for forming tubular shapes of metal powders, the removal of the mandrel from this device allows solid rods to be formed. Thus, the foregoing illustrations of the present invention are not intended to limit its scope which is to be limited entirely by the appended claims.
What is claimed is:
1. A molding device for compacting metal powders into tubular or rod shapes comprising in combination:
(a) an inner electrical conductor which is shaped to provide a molding cavity and having the general shape of the article desired to be produced,
(b) an outer electrical conductor that completely surrounds the said inner conductor and which is electrically insulated from said inner conductor by a thin dielectric material,
(c) said outer conductor being clad with a nonmagnetic metallic sleeve,
(d) a power supply capable of supplying 10 amperes for a period of about 10 seconds, and
(e) tapered shorting plugs, which electrically connect the outer conductor to the inner conductor so as to allow current to pass from the power supply to ground by the triggering of a suitable switching means.
2. The device of claim 1 in which a mandrel is concentrically located within the inner conductor and is electrically insulated from it.
3. The device of claim 2 in which the mandrel is composed of a fusible metal alloy.
4. The device of claim 1 in which said dielectric material is a thin vinyl tube inserted over the said inner conductor.
5. The device of claim 1 in which said sleeve material is selected from the class consisting of austenitic stainless steel and aluminum.
6. The device of claim 5 in which said sleeve material is austenitic stainless steel.
7. The device of claim 5 in which said sleeve material is aluminum.
8. The device of claim 1 in which the mandrel is held in place by tight fitting rubber plugs which are inserted References Cited UNITED STATES PATENTS 2,149,546 3/1939 Gillett et a1.
3,092,165 6/ 1963 Harvey.
3,149,372 9/1964 Stinger.
3,171,014 2/1965 Ducate.
3,248,215 4/1966 Bonis et al. 1816.5
WILLIAM J. STEPHENSON, Primary Examiner.