Method of making porous metallic bodies
United States Patent 2464517

The present invention relates to porous metallic bodies and methods of making the same. More particularly, it relates to porous metallic bodies suitable for use as filters and while the method may be used with a wide variety of metals, in its preferred form it is applicable to the formation...

Jacob, Kurtz
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Other Classes:
210/510.1, 419/6, 419/33, 419/35, 419/36, 428/547, 428/548, 428/566
International Classes:
B22F3/11; B22F9/04
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The present invention relates to porous metallic bodies and methods of making the same. More particularly, it relates to porous metallic bodies suitable for use as filters and while the method may be used with a wide variety of metals, in its preferred form it is applicable to the formation of porous bodies of refractory metals such as tungsten and molybdenum. It has, however, a special adaptation to alloys such as iron molybdenum manganese alloy, and generally to metals that can be cast and suddenly chilled in a manner to produce a brittle formation of large crystals.

The method may generally be described as, first, forming an ingot or body of other shape having large crystals and a consequent brittleness of structure which permits the breaking up of the body into its component crystals. The individual crystals are of considerable strength and hardness although the body as a whole formed from these crystals is quite brittle. The body is first formed into large crystals either by heating the ingot in the case of the more refractory metals to the germinating temperature, or by casting the ingot and immediately chilling it. The ingot is then broken up into-its component crystals and these crystals are again broken down by mechanical means such as ball milling, crushing or hammering.

The crystal fragments so produced vary considerably in size and it is, therefore, desirable to screen the fragments into different batches in which the particles are of approximately equal sizes. This may be done by passing the powdered crystalline material through a series of screens varying from say 40 mesh to about 350 mesh.

There may thus be screens of the following sizes which are given by way of example merely: mesh, 60 mesh, 100 mesh, 150 mesh, 200 mesh, 250 mesh, 300 mesh, and 350 mesh. By combining the material retained on certain screens with that retained on others, a considerable degree of control of the porosity of the finished body is permitted. Thus, if a given proportion of larger size fragments is mingled with a given proportion of fragments of smaller sizes, a certain degree of porosity of the finished body may be secured, and this porosity may be increased by increasing the proportion of larger size fragments or it may be diminished by increasing the proportion of the smaller size fragments.

When a batch is prepared having a predetermined proportion of large and small size fragments, it is thoroughly wetted with a solution of a soluble salt of a metal such as silver, copper or nickel. The salt used should be one -which will break down to the oxide under temperature For instance, an aqueous solution of silver nitrate may be used and the powdered crystalline metal is thoroughly wetted with this solution and dried so as to form a coating of silver nitrate on the individual particles. It is then heated to a temperature sufficient to break down the nitrate tc the oxide.

A binding material which is decomposable at temperatures below sintering temperature, of the metal with which the powdered base metal is coated and which will volatilize at that temperature without leaving a residue, should be used, Among such binding materials are paraffin dissolved in carbon tetrachloride, salicyclic acid, naphthalene, benzyldichloride, and ammonium chloride, and many others. The coated powdered metal is thoroughly mixed with this binding material and the mass is shaped in a die under hydraulic pressure.

After pressing and shaping in this manner, the mass is then heated in an atmosphere of hydrogen to a temperature sufficient to volatilize the binder out of the mass without leaving a carbon residue, and when volatilization is complete the temperature is held sufficiently high to reduce the oxide coating to a metallic coating. This temperature should be further increased to a point at which the coating metal on the particles will sinter but it should not be sufficiently high to melt the coating metal to a fluid condition at which it would flow off the particles. There is very little shrinkage of the body under this treatment owing to the fact that at the temperatures used the crystalline fragments show little tendency to coalesce and the voids remaining after the volatilization of the binding material show practically no diminution of size or number. Nowithstanding this lack of shrinkage, the final product has considerable mechanical strength.

The method may be illustrated by describing the formation of such a body from tungsten.

Tungsten powder is first formed under hydraulic pressure in a die to the shape of a bar or ingot. This bar is then sintered and heat treated .in a hydrogen atmosphere at the temperature of rapid germination or about 2800* C. and held at that temperature for an adequate time to sinter the ingot and to form the whole body into large crystals. About fifteen minutes is sufficient for this purpose. The ingot so formed is quite brittle and can be broken up into its component crystals and these in turn are further broken down to smaller size in a ball mill or jaw crusher. The material is then screened in the manner hereinfore described, and the particles thus graded cording to size. Batches selected according to e are then thoroughly mixed together in a pretermined manner to produce the desired degree porosity in the finished body and the mass so :med is thoroughly wetted with an aqueous lution of silver nitrate and then dried. Into e same so formed a solution of paraffin dis[ved in carbon teterachloride is thoroughly xed and dried, and the whole is then formed to e desired shape in a die under hydraulic presre. The body so formed is then heated to a nperature of about 4000 C. until the paraffin iding material is completely volatilized without iving any residue. The temperature is regu;ed so that the silver nitrate coating has broken wn to silver oxide and the temperature is then adually raised to about 8000 C., the oxide coat; being thus reduced to metallic silver and the 3tallic silver coated particles are sintered tother. The resulting body is highly porous and considerable mechanical strength.

The invention will be more clearly understood )m the drawings in which Fig. 1 represents an paratus for carrying out the method of the inntion and the formation of a porous metallic dy having zones of different degrees of porosity, d Fig. 2 represents a finished porous metallic dy having such zones.

Referring now to Fig. 1, a mold or die I having erein a movable plunger 2 and a stationary mger or stop 3, is provided. An ingot of base Atal is heat treated as hereinabove described as to form large crystals and then broken down to individual crystals. The crystals are ished, the broken particles screened to form tches graded as to size, and then thoroughly Itted with an aqueous solution of silver nitrate d dried. Each batch is then mixed thoroughly th a binding material, such as paraffin disved in carbon tetrachloride. A batch of very .e screenings consisting, for instance, of parles that pass through a 300 mesh screen and E retained on a 350 mesh screen, mixed with is binder, is then introduced into the die I to :m a layer as indicated at 4. The plunger 2 then forced down under hydraulic pressure the layer 4 with sufficient pressure to reduce is material to about two-thirds of its original lume. The plunger is then removed and a :ond batch of screenings 5 consisting, for ample, of particles that pass through a screen 150 mesh and are retained on a 200 mesh :een, is then placed in the die 1 on top of the rtially pressed layer 4. This material is then draulically pressed in about the same degree the bottom layer 4. The plunger 2 is again re)ved and a third batch 6 consisting of particles ssing a 40 mesh screen and being retained on mesh screen, is then filled into the die I top of the partially compressed layer 5 and bjected to very heavy pressure of the order about 30,000 lbs. to the square inch, thus closely mpacting the entire mass.

The block thus formed is then removed from s die 1. It is heat treated, as hereinabove de:ibed, to volatilize out the binder, to reduce the ver nitrate to oxide and ultimately to a metallic ver coating on the particles and to sinter the ated particles into a mechanically strong, corent but porous mass.

The resultant body is illustrated in Fig. 2 in lich it will be seen that the original layers 4, 5 d 6 are still roughly preserved but that where e layers contact each other, part of each layer is forced into the other by reason of the heavy pressure ultimately applied. Marginal layers of this sort are indicated in Fig. 2 at 7 and 8. It will thus be obvious that in this manner the porosity of the finished body may be regulated from a high porosity at 6 through zones of decreasing porosity to 4.

The first step in the process, namely, that of forming the metal into large crystals, may be accomplished in a variety of ways, and it will be noted that the method used will depend upon the nature of the metal dealt with. If the metal concerned is one having a high melting point, such as tungsten, molybdenum and similar refractory metals, the method given in the example of tungsten will be found most convenient under ordinary circumstances. They will be formed into an ingot by pressing and sintering and then heat treated to the germinating temperature of the metal. On the other hand, with lower melting point metals and many alloys, such as the alloy of iron, molybdenum and manganese hereinabove suggested, the metal will be melted and cast and then suddenly chilled by quenching it in cold water. The ingot so formed will be found to have exceedingly large crystals. In fact, any method which is effective to produce large crystals in a given particular metal may be used.

Metallic bodies formed according to the present invention are characterized by a porous continuity permeating the entire structure and permit the passage of fluids therethrough while obstructing the passage of contaminating solid matter that may be contained in said fluids. Such bodies may be constructed with very small pores intercommunicating with each other so as to retain lubricants within the pores that constantly feed to the surface and function very efficiently as self-lubricating bearings. Metals such as an iron molybdenum manganese alloy or tungsten having the particles coated with lead reduced from lead nitrate in the manner hereinabove described, are especially suitable for use as bearings.

Having thus described my invention, what I claim is: 1. Method of making a porous metallic body that comprises making an ingot of metal and treating said metal so as to cause it to form into large crystals; breaking said ingot into its individual crystals, crushing the crystals into crystalline particles; then wetting said particles with a solution of a metallic salt reducible to an oxide under heat, drying said solution on said particles and heat treating said particles until said salt is reduced to an oxide coating on said particles; then mixing said oxide coated particles with a binding medium, pressing said material in a die to a desired shape and heating at temperatures regulated to volatilize said binding material completely, reduce said oxide coating on said particles to metal and sinter said particles into a strong, coherent, metallic body permeated by intercommunicating pores.

2. Method of making a porous body of tungsten that comprises making an ingot of tungsten and heat treating said ingot at a temperature of about 28000 C. until said ingot is formed into large crystals; breaking said ingot into its component crystals and crushing said crystals into crystalline particles of irregular sizes and shapes; grading said crystals into batches according to size by screening and mixing the material from said batches in predetermined proportions according to the degree of porosity desired in the finished 2,464,517 body; then thoroighly wetting said mixed par- powders ii tides with an aqueous solution of silver nitrate degrees o0 and heating until said nitrate breaks down into ing to th( a silver oxide coating; then thoroughly mixing ments con said silver oxide coated particles with a solution 5 7. Meth of paraffin in carbon tetrachloride and shaping meated bi the mass so formed in a die under hydraulic prises coal pressure; and finally heating said formed mass crystals w at temperatures regulated, first, to volatilize said powder w paraffin without leaving a residue, then, to reduce 10 pressing s said silver oxide coating to metallic silver and heating sE then to sinter said silver coated tungsten par- ture suffic tides into a strong, coherent body permeated by leaving an intercommunicating pores. cient to r 3. Method of making a porous metallic body 15 and then t according to claim 1, in which a plurality of coherent, batches of crystalline particles from separate 8. Meth screenings through screens of different mesh after meated b3 being coated and mixed with binding material prises coa are each separately partially pressed in a die in 20 broken tu layers one above the other and then further mixing sai pressed and heat treated to volatilize out said fin, pressi binding material, reduce said coating to metal heating sa and finally to sinter the same into a strong, co- tures to v herent, porous body having zones of different de- 25 any residi grees of intercommunicating porosity corre- metallic si spending to said layers. coated pa 4. Method of making a porous metallic body body. according to claim 1 in which an ingot is made of a refractory metal and is formed into large crys- 30 tals by heating at the temperature of rapid germination until said ingot is formed into large The foll crystals. ie f thi 5. Method of making a porous metallic body file of thi according to claim 1 in which an ingot is formed 35 of a metal that is cast and suddenly chilled, thereby forming said ingot into large crystals. Number 6. The method of forming a porous metallic 912,246 body having zones of different degrees of porosity 1,205,080 that consists in forming a metallic body and heat 40 1,226,470 treating it to form large crystals therein; then 1,566,793 breaking said body down to its component crys- 1,739,068 tals and crushing said crystals; then screening 1,823,869 said crushed crystals to separate the crushed 2,110,967 particles into batches of different sizes; then wet- 45 2,133,292 ting said batches with metallic salts and heating 2,199,620 to reduce said salts to oxides, thereby forming an 2,206,616 oxide coating on the crystalline fragments; then 2,267,918 mixing batches of different sized particles with a 2,298,908 binding material and filling said batches into a 50 2,319,240 die in layers of different sized particles and press- 2,332,737 ing said layers under hydraulic pressure; then 2,370,242 heating to volatilize said binding material without leaving any residue and to reduce the oxide coating on said particles to metal; and finally 55 Number further heating so as to sinter said coated metal 555,983 ito a body comprising layers of different f intercommunicating porosity accorde particle sizes of the crystalline fragiposing each layer.

od of making a metallic body per7 intercommunicating pores that comting a metal powder consisting of broken ith a metallic salt, mixing said coated 'ith a volatilizable binding material, said mixture to a desired shape and lid body so formed first to a temperaient to volatilize said binder without Yy residue, then to a temperature suffieduce said metal salt coating to metal ;o sinter said coated powder to a strong, porous body.

od of making a tungsten body perintercommunicating pores that comting a tungsten powder consisting of ngsten crystals with silver nitrate, Id coated tungsten powder with parafig said mixture to a desired shape and id pressed body at increasing temperaolatilize said paraffin without leaving ie, to reduce said silver nitrate to a ilver coating and to sinter said silver rticles to a strong, coherent, porous JACOB KURTZ.

REFERENCES CITED owing references are of record in the s patent: JNITED STATES PATENTS Name Date Kuzel -------------- Feb. 9, 1909 Baumann ---------- Nov. 14, 1916 Coolidge ----------- May 15, 1917 Gero ----- __------. Dec. 22, 1925 Harris ------------ Dec. 10, 1929 Baur ------------- Sept. 15, 1931 Andrews ---------- Mar. 15, 1938 Gordon ------------ Oct. 18, 1938 Davis ------------- May. 7, 1940 Devereux ----------- July 2, 1940 Hildabolt ---------- Dec. 30, 1941 Wentworth --------- Oct. 13, 1942 Larsen ----.. __--- May 18, 1943 Marvin et al. ---__ Oct. 26, 1943 Hensel ------------ Feb. 27, 1945 FOREIGN PATENTS Country Date Great Britain ------ Sept. 15. 1943