US2027532A - Powder metallurgy - Google Patents

Description

Patented Jan. 14, 19 36 2.0 1.532 rowers rm'rnmu'nor f Charles fiardv. Pelham, N. Y. ass s' or to a; ll slcal Copany, .New York, N.- Y a corporation of ware No Drawing. Application April a, 193% A Serial No. 719,935

. 2 Claims. This invention relates to the manufacture of metallic articles from finely" divided metals or metal powders and has for its object improvements in the methods of producing the same. 5 Metal powders, such as those used in the formation of metallic objects, are frequently produced in a wet state; .thus for example, copperpowders are electrolytically deposited from, a solution of copper salts. Metallic powders may also be precipitated from solutions through the use of a suitable reducing agent. In many instances, powders precipitated from solution have a dendritic or tree-like structure when first precipitated. Frequently such metallic powders are soft and amorphous'in character. I

. According to the heretofore customary practice such powders are washed to free them of soluble impurities; Usually the final washing step is conducted with alcohol or some other volatile liquid which tends to preserve the metal powders against oxidation. The powders are then dried at a fairly high temperature and in the dry condition they are compressed and sintered to form a coherent mass of suitable density and 2s. shape.

In the case of metal powders produced by wet methods, the drying treatment is ordinarily responsible for a marked change in the-:physical structure of the particles. Even at moderate temperatures the dendritic structureis lost to a large extent and the hardness of the particles is considerably increased. In thejcase of copper powder, this change is particularly marked. The powders which are originally soft, of low density and tree-like in their structure, become-more compact and lose their irregular configuration, even when subjected to a temperature of 300 centigrade fora period of 60 minutes. The dried product is granular as compared with the amor- 49 phous character of the wet powder.

Objects made from dry metal. powders have satisfactory characteristics if a sufficiently high compressive force is applied in their manufacture. In some instances. however, a force of 50 tons per square inch is necessary to produce a coherent non-porous mass, and the necessity of such a high degree of compressive force has preeluded-the use of the process of powder metal- 1 lurgy in the case of comparatively largeobjects. This will be apparent whenit is realized that to form an object with a cross sectional area of two square feet may; require the application of a totalforce of approximately 14,000 tons. The dimculty of manufacture is further aggravated by the fact that this pressure should be uniformly applied overthe entire cross section of the body. Even with the most modern of hydraulic presses a uniform application of a force of this magnitude is difiicult, if not impossible. In the heretofore customary art of powder metallurgy the use of substantially dry powders has been considered necessary for several reasons first, because it was believed that the presence of a substantial quantity of,liquid interfered in the production of a satisfactory metallic article;

second, because the difllculties of handling wet powder in a mold were thought to be considerable,

, and, third, in the case of a powder precipitated from solutions the amorphous or dendritic structure was considered to be less satisfactory than 15 the granular form which pertains in the case of dried powders. I As a result of my investigations! have discovered that it is possible to produce a very satis factory metallic object by employing metal powgo ders in a .wet state. Thus. for example, copper powders produced by electrolytic deposition from solution maybe washed and compressed in a mold while they still contain a considerable quantity of entrained moisture. After si-ntering, an object produced in this manner is .in no way inferior to one produced from dried copper powders. Moreover, the compressive force which is necessary in order to produce an object of a given density is but a small fraction of that required when dry powders are employed.

According to a, presently preferred practice of my invention wet metal powders are washed with water to remove entrained electrolyte or dissolved impurities. The washed powder is then placed in a centrifuge and a portion of the entrained water is thus removed. The powder while still retaining a considerable percentage of moisture is then placed in a suitable mold and compressed under a pressure of approximately 2.5 tons per square inch, and a coherent substantially nonporous mass is thus produced. The mass is subjected to heat treatment at a temperature below its melting. point and preferably in a reducing atmosphere in order to preclude oxidation of its 5 surface. The resulting object is compact. and

possessed of great tensile and compressive strength. In the event that a porous mass is desired the pressure may be considerably reduced.

objects produced by the compression of wet pow- 1 ders may be wetted with the requisite amount of a non-viscous liquid of non-oxidizing character, such as water, alcohol, acetone, or benzine, compressed at a relatively low pressure and sintered.

compressed in the wet state. with regard to metal powders which, have been The optimum amount of wetting agent will vary with the character of the powders employed and the required density of the finished object. In

some instances the amount of liquid which will no be entrained in an uncompacted mass of the powder may be desirable. In other cases a lesser amount of wetting agent produces satisfactory results. However, I have discovered that more power and a higher degree of compressive force are required when powders produced by dry methods are wetted and subjected to'compression, than when powders precipitated from solution are This is also true precipitated from solution, subjected to a mild drying-treatment and subsequently wetted and compressed. Such practice also required greater compressive stress than in the case of freshly precipitated wet powders which are compressed without a drying treatment. I attribute this phenomenon tov the fact that the softness of freshly precipitated powders enables compressionto be accomplished with less friction between particles and between the particles and the wall of the die or mold.

' According to another practice of my invention I take two or more dissimilar metal powders, at

least one ofwhich is in a wet state, and mix them to secure thorough incorporation. The mass,

while still wet is subjected to a compression and heat treatment operation in "the conventional manner. This method of operation is advantageous when it is desired to produce an object into by dry methods, are to be incorporated. It sometimes happens that the amount of liquid naturally entrained in a 'powder produced by wet methods is more than is required to obtain minimum pressure in the compression operation. Dry powders may thenbe added to reduce the average liquid content of the mass.

When two or more metal powders of's'ubstantially diiferent specific gravities are mixed, a centrifuging operation to remove excess water is undesirable, since a certain amount of segregation results. when such tendency for segregation emsts I have determined that it is preferable to remove excess liquid by means of a filter of either. the vacuum or pressure type.

It is frequently desirable to produce metallic objects from metal powders the surfaces of which have been plated witha dissimilar metal. By

greater proportion of the object. The use of plated metal powders is alsodesirable when it is necessary to combine two or more'metals in proportions in'wliich' they will not alloy with one. another. Since the plating of metal powders is usually accomplished by wet methods-either by accuses electrolytic deposition or by chemical replacement, my process is particularly applicable to the manufacture of metallic objects from plated metal powders. For example, iron powder may be coated with copper by rapidly agitatinga. mix- 5 ture of iron powder and copper sulphate solution. By controlling the respective amounts and concentration of iron powder and copper sulphate I am able to produce a uniform copper plating of the desired thickness on the iron particles; The resulting powder is then washed to remove iron "alts and excess copper salts and while still wet is compressed to the desired degree of density, and heat treated until the copper binds the iron particles .together. In the event that other ingredients are desired these may be added in dry or wet powder form to the wet plated powder.

According to another aspect of my invention I may combine two or more metallic elements by first preparing an alloy in powder form by meth- .iods of wetprecipitation; this powder being subsecarbon disulphide may also be used, but since the temperatures generated in the compression of the powders may be high, the use of inflammable liquids such as alcohol ,or benzine introduces a fire hazard. It should also benoted that in most instances the use of water as a wetting agent has resulted in a more satisfactory product or in a 40 lower compressive force. Thermoval of soluble impurities from the metal powders, by washing out entrained electrolyte etc., is usually desirable, but is not essential to the practice of my invention, since practice has demonstrated that en- 45 trained electrolyte may be almost completely squeezed out in the mold. 1

The wet powders do not present inordinate handling difficulties in the compression step. In factthe wet character of the powder actually improves the operation in that it prevents dusting. This is especially advantageous where such dust may be carried into the lungs of the operator.

The amount of liquid present with the powder, may be varied to. a considerable degree. Due to the fact that the mold must be provided with a means for egress of theliquidduringcompressionit is undesirable to employ quantities in excess of that required to obtain a minimum compressive 60 force for the degree of density which is desired. I have found that in most instances the amount of water which isretained in a powder that has been subjected to a mildcentrifuging or filtration operation is suflicient for this purpose. The 65 quantity ofthe finished object,however, is not affected by the use of liquid in excess of that required for minimum compressive force. In the case of copper powders of a fineness of approximately 300 mesh which have been produced by 7 electrolytic processes, I have found that a moisture content of approximately 15% is desirable. It will be apparent that the amount of wetting agent present will be dependent upon the nature oil thepcwders employed, that is, their composi- 16..

tion, fineness and physical structure, and also upon the characteristics which are desired in the finished object.

The advantages of my invention may be enumerated as follows:

1. In the case of powders which have been prepared by wet methods the drying step is eliminated, thus saving time, labor and reagents, such as alcohol, which are employed to prevent the oxidation of the powder during the drying operation.

2. There is a great saving in power, since when wet powders are employed the force necessary to produce a mass of predetermined density is much less than whendry powders are employed.

3. Less compressive force need be applied in the case of wet powders, making it possible to produce much larger articles without encountering the difliculties which arise from an unequal dis-'- tribution of compressive stress.

4. Improved operating conditions.

My methods are applicable to mixtures of two or more metal powders. I have found that powders of iron, copper, zinc, and other metals'may be satisfactorily employed either alone'or mixed with one or more other metals. In certain instances it is also desirable to mix non-metallic powders, such as porcelain dust, or other materials of high dielectricstrength with wet metal powders in order to produce structures of high electrical resistance. As has been noted, the process is also applicable to plated powders. In

' many instances, it may be desirable to mix powders which have been prepared by dry methaman skilled in the art.

- I claim:

1. In a process for making a metallic object in which metal powders that have been platedwith metal in an aqueous'solution' are employed, the 15 improvement which comprises compressing the plated powders into a coherent mass while said plated powders are still wet with water and before the original soft amorphous character of the plating has been altered substantially by heating, 20 thereby reducing the compressive force necessary to form a coherent mass of predetermined density. v

2. Ina process for making a metallic object in which metal powders deposited from an aqueous 25 v solution are employed, the improvement which comprises "compressing said powders into a coherent mass while the powders are still wet with water and before the original soft amorphous character of 'the powder has been altered sub 30 stantially-by heating, thereby reducing the compressive force necessary to form a coherent mass of predetermined density.

CHARLES HARDY.