GB2187762A - Metal powder by atomization process - Google Patents

Abstract

Process for atomizing metal comprising quenching molten particulates (19) atomized in a gas medium at the surface (21) of a liquid quench medium, the position of which is adjusted to give a significant proportion of irregularly shaped particles in the powder product. <IMAGE>

Description

SPECIFICATION Atomization process The present invention is concerned with metal atomization and more particularly with atomization of metal to produce metal powder having irregularly shaped particles of relatively low oxygen content.

BACKGROUND OF THE INVENTION Atomization of metal is a process whereby a stream of molten metal (including alloys) is broken up into particulates by means of an intercepting, fast moving stream or mass of an atomizing agent usually a gas or a liquid.

Gas atomization generally employs as the atomizing agent a gas which is inert to the metal being atomized e.g., argon when atomizing a nickel-chromium alloy. On occasion, a gas reactive with the molten metal can be used provided that the product of gas-metal interaction forms a protective film around the small particulates which are produced when the gas and molten metal streams intersect.

An example of use of a reactive gas is the use of air in atomizing aluminum. By and large however, an inert gas, e.g., argon, is employed in atomizing reactive metals. A problem with gas atomization is that the -heat capacity of a gas is very low and relatively little heat is extracted from the molten metal stream when that stream is intersected by an argon gas stream. The result of this is that the particulates produced by the atomization process are molten particulates which solidify while falling under the influence of gravity in the atomization chamber. The ultimate product is a powder having mainly spherical particles or streamlined quasi-spherical particles which, if fall time is not great enough, sometime tend to bond together in the mass of powder product at the bottom of the atomizing chamber.

On occasion, persons in the prior art have suggested the use of a water bath at the bottom of a gas atomizing chamber to serve as a quench means so as to prevent bonding in the powdered mass.

For powder metallurgical purposes, spherically or quasi-spherically shaped powder particles are not particularly advantageous. Compaction of powder made up of such particles under pressure does not generally produce a coherent mass having any measurable green strength unless the compaction pressure substantially exceeds the yield strength of the particular metal throughout the compacted mass. It is well known that metal powder being compacted in the first step of producing an article by powder metallurgical techniques does not act as a Newtonian liquid would under pressure. Inter-particle effects such as bridging, complexities of compacted shapes and the like tend to dissipate applied compacting pressure.If one must rely upon deformation of particles from a spherical shape to produce a compact having reasonable green strength, enormous compacting pressures must be used to effect solid state welding especially with strong alloys such as nickelchromium alloys. Consequently, when employing metal powders made up of spherical or quasi-spherical particles, the art has resorted to canning the powders before compaction.

While this procedure is operacle and practical, it normally is expensive since the can must be provided, a protective atmosphere (including vacuum) generally is provided in the can and, ultimatelv, the can must be removed.

Another common atomization process is liquid atomization. In this process a liquid, usually water, is employed as a fast moving stream to intersect the molten metal stream.

Water, in contrast to gas, has a high heat capacity. Thus particulates formed by interaction of the molten metal stream and the water stream are instantaneously solidified into irregularly shaped particles. The difficulty with such a process for producing a powder for metallurgical use is that the product powder often contains a high level of impurity resulting from chemical reaction with the atomizing liquid. For example, powder produced by water atomization generally contains high oxygen levels even when the water contains oxidation inhibitors such as alcohol. For many purposes the high oxygen content of the powder makes it useless for the intended purpose unless an expensive, subsequent reduction operation is carried out.If, as has been previously suggested, hydrocarbon liquid is used as the atomizing medium, the resultant powder can have high carbon or carbide content generated by hydrocarbon cracking when hydrocarbon liquid intersects a molten metal stream having a temperature above about 700"C.

Metal fragmentation processes other than gas or liquid atomization which act on or produce molten metal fragments in a gas phase are known. Among these processes are included fragmentation of metal thrown off a rotating electrode of an electrical arc and fragmentation of metal occurring by pouring molten metal onto a rapidly moving (e.g., rotating) surface from which fragmentable films or droplets are projected. The present invention, may, under specific circumstances be applicable to these other fragmentation processes.

OBJECT AND PURPOSE OF THE INVENTION The present invention has for its object and purpose the provision of a process of gas atomization whereby particles similar to irregularly shaped powder particles of the liquid atomization process are produced with relatively low levels of contaminants approaching or equalling the contaminant level of normally gas atomized metal powder.

DRAWINGS Figure 1 of the drawing is a schematic cross-sectional view of a gas-liquid atomizer such as can be used for carrying out the process of the present invention.

Figure 2 of the drawing is a 5 X photograph of metal powder. produced by the process of the present invention GENERAL DESCRIPTION OF THE INVENTION The present invention contemplates a process of atomization of metal wherein molten metal, after disintegration into molten particles in an inert or reducing gas phase, is quenched in a liquid quench medium containing an oxida-tion inhibitor to provide irregularly shaped particles. The position of the gas-liquid interface of the quench medium onto which particulates impact is adjusted with respect to the locus of metal disintegration such that a significant portion of particulates traversing the gas phase and recovered from the quench medium have a shape other than spherical.

The principles -of the present invention have been elucidated by experimentation with gasatomized nickel-iron alloy using argon as the atomizing fluid and a bath of alcohol-water. It has been found that if the at-rest level of the quench medium bath surface containing about 2% to about 10% by volume isopropanol as the liquid quench medium is less than about 25 centimeters (cm;) below the atomization zone, i.e., the spot where a metal stream and high velocity streams of argon intersect, the powder-produced has a significant proportion of irregularly shaped particles and, as a whole, has a low oxygen content. If the at-rest quench medium bath surface is substantially lower than 30 cm. below the atomization zone the resultant powder particles are spherical.

The quench medium can be not only a bath below the atomization zone but also it can be a curtain of liquid sprayed so as to approximate the position of the surface of a quench bath under gas atomization conditions.

The present invention is particularly adapted for the atomization of metals and- alloys which in the main have oxides which are reducible by hydrogen at temperatures below about 1000"C. Alloys especially operable in the process of the present invention are those which have as a- major or principal constituent a metal from the group consisting of copper, iron, nickel and- cobalt and which may include minor amounts of metals such as chromium, aluminum, titanium, molybdenum, tungsten, etc.

While applicant is nowt fully aware of all the factors which are involved in the process of the present invention, it is believed that by limiting the time of flight of a molten particle between the atomization zone 19 of a conventional gas atomizer as schematically depicted in Figure 1 of the drawing and the locus of quenching 21 a particle as it hits the exposed surface of the quench medium can be in a molten, mushy or highly plastic state. In such a condition, the forces involved in impact can cause deformation of the particulate from the spherical shape. With respect to Figure 1, those skilled in the art will recognize in schematic the various parts of a conventional gas atomizer 11 including tundish 1-3, body of molten metal 15, pouring nozzle 16, gas nozzles 17 and gas vent port 23.In the practice of the present invention the level of quench medium 20 is adjusted so that the gas-liquid interface 21 (or locus of quenching 21) at rest is usually less than 25 cm below atomization zone 19. At the end of or periodically or continuously during atomization, metal powder is recovered from the liquid quench medium 20 using recovery means 12 and conventional liquid-solid separation process.

The dynamic conditions existing during atomization are quite different from those schematically depicted in Figure 1 of the drawing. For one thing the interface 21 between quench medium 20 and the gas phase is far from flat. It is highly deformed by rapid flow of gas emerging at supersonic speed from jets 17 under pressures of 8 or more atmospheres gauge.

Further, in atomization zone 19 a range of size of particulates is produced. Small particulates can be levitated by gas and held in gas suspension for longer periods of time than large particulates. It is known that convection and radiation cooling of small particulates is much faster than cooling large particulates.

This results in the phenomenon that often the process of the present invention produces powder in which the smaller sized powder fractions tend to be spherical and the larger sized powder fractions tend to be irregular in shape. This phenomenon is shown in Figure 2 of the drawing which is a 5 power photographic view of a powder of nickel-iron alloy produced by the process of the present invention. For use in powder metallurgy such a product is perfectly satisfactory without using canning because upon-compaction the larger irregular particles interlock holding the smaller spherical particles within an interlocked skeleton.

Another phenomenon that can occur during actual operation of a gas atomizer under con ditions specified in the present application is the collision of molten metal particulates with spray droplets of the quench medium. Under the applied pressure of 8 or more atmospheres, gas issues from jets 17 at high velocity and can pick up spray at interface 21.

Occasionally a droplet of quench medium can collide with a molten metal particulate result ing in either freezing the shape of the particulate prior to spheroidizing or more likely, distorting a spherically shaped particulate by exceedingly rapid almost explosive local generation of gas from the droplet of quench medium. In view of the foregoing, the product powder resulting from the process of the present invention can have a complex combination of particles of clearly non-spherical irregular shape.

Carrying out the process of the present invention is not limited to the apparatus depicted in Figure 1 of the drawing. Gas atomization chamber 11 can include deflection plates either above or below interface 21; quench liquid 20 can be circulated to enhance or oppose vortexing induced by gas from jets 17; sonic or ultra-sonic vibration can be used along with or in place of the disintegrating gas and other means of disintegration such as centrifugal shotting in association with a peripheral curtain of quench liquid can be used in place of gas atomization.

When carrying out gas atomization in accordance with the invention it is advantageous to use as the atomization medium substantially pure argon gas introduced as a plurality of high velocity gas streams which intersect a downwardly moving stream of molten metal at one or more points in space. Argon gas is employed at a rate of about 0.033 to about 1.3 standard cubic meter (sm3) per kilogram of metal atomized. Prior to the start of atomization the chamber in which atomization is to take place should be filled with the atomization gas, e.g., argon and, during atomization, steps should be taken to assure that a slight positive internal pressure exists in the atomization chamber to prevent influx of ambient air.If the metal or alloy being atomized is carburization resistant or can tolerate small amounts of carbon, the argon can be diluted with a hydrocarbon gas such as butane to thereby reduce the cost of atomization gas and provide a reducing atmosphere at the instant of atomization. In the case of atomization of an alloy such as aluminum bronze, the hydrocarbon gas, e.g., butane, can comprise a major part or all of the atomizing gas. Other gases which may be used for atomization, depending upon the metal being atomized in clude nitrogen, helium, methane, propane and carbon monoxide.

Except in the case of aluminum it is generally important that the atmosphere in the atomization chamber be substantially devoid of free oxygen, e.g., from the air. This is to prevent rapid oxidation of metal at the instant of atomization and during passage of atomized particles through the gaseous medium in the atomizing chamber. If oxide forms, it is usually difficult to reduce the oxide to metal under conditions prevailing in the atomizer chamber because (1) the metal temperature is always lower after atomization than at the time of atomization and (2) the time interval between atomization and quench is very short.

The liquid quench medium used in the gas atomization process of the present invention is advantageously a water solution of about 10% or less by volume of isopropanol e.g., 2 or 3 to 10% by volume of isopropanol held at a temperature below about 66"C. This quench medium is advantageous in that its total vapour pressure (water plus isopropanol) is less than about 0.5 atmosphere, water is cheap and isopropanol is relatively inexpensive, readily available and effective to inhibit oxidation of many conmon metals e.g., nickel, iron, copper and the like. Other readily oxidizable, water-soluble organics can be substituted in part or in whole for isopropanol but are generally not preferred because of cost, toxicity, volatility or odor considerations.Such other water-soluble organic compounds include but are not limited to methanol, ethanol, propanol, acetone, formaldehyde, acetaldehyde, glucose, invert sugar, hexatols, sorbitols, mannitol, dulcitol, other reducing carbohydrates, benzaldehyde, hydroquinone, ascorbic acid and its salts, phenol, gallic acid and its alkali metal salts, resorcinol and salicylic acid and its alkali metal salts. If desired an aqueous quench medium can contain a water-insoluble oxidation inhibitor as a dispersed phase. For metal which must be rigorously free of oxide but which can- tolerate carbon, it is possible to use a liquid hydrocarbon such as purified mineral oil as a quench medium. Care should be taken however that undesirable impurities, notably sulfur, e.g., in the form of sulfur-containing compounds, should be at a very low level in mineral oil used for this purpose.

The liquid quench medium employed in the atomizing process of the present invention should have a reasonably high heat capacity e.g., a specific heat above about 0.5 cal. per mol. deg. and be substantially unreactive with respect to the metal being atomized. The temperature of the liquid quench medium should be maintained such that the total vapor pressure of the medium is below about 0.5 at atmosphere EXAMPLE 1 A 13.6 kg heat of 42% nickel, 58% iron, 0.05% carbon alloy was melted in air and fed to a tundish above an atomizing chamber at a temperature of about 1650"C. Prior to this the atomizing chamber was filled to a point about 20 to 25 cm below the atomization zone with water containing 7.5 volume percent isopropanol.The metal was atomized by allowing a molten metal stream about 0.76 cm in diameter to run from the bottom of the tundish into the atomizing chamber where it was intersected at the atomization zone by 8 jets of argon gas emanating from a gas stream under a pressure of about 11 atmospheres absolute.

The powder produced from this heat contained 0.08% oxygen and was irregular in shape.

In a parallel experiment using a quench bath of water containing 8.6 volume percent isopropanol established 27.5 to 32.5 cm below the atomization zone and using a gas pressure of 9 atmospheres absolute, resultant powder contained 018% oxygen and was very rounded and spherical in shape.

EXAMPLE II A 14 kilogram air melted heat of an alloy nominally in weight percent containing 3035% nickel, 19-23% chromium, 0.1% maximum carbon, 1.5% maximum manganese, 1% maximum silicon, 0.75%-maximum copper, 0.15%-0.6% aluminum, 0.15-0.6% titanium, balance essentially iron was- atomized using argon at about 15 atmosphere gauge using a water-isopropanol (9%) quench liquid about 12-13 cm below the atomization zone when at rest. The resultant powder was very irregular and, after anneaiing in hydrogen at 980"C was compactable under a pressure of about 3945 atmospheres to form a disc having good green strength.

EXAMPLE III A 14 kilogram air-melted heat of alloy containing in weight percent about 35- /0 nickel, 20% chromium, 4% aluminum, 5%-cobalt, 0.4% titanium, 0.1% yttrium, 0.33% silicon, balance essentially iron was atomized in the same manner as was the alloy of Example II.

The resultant powder contained pancake-like particles, was compactable as atomized and contained about 0.07% oxygen.

While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention, those skilled- in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without-a corresponding use of the other features.

Claims (8)

1. A process for atomization of metal comprising impacting particulates of gas atomized molten metal traversing a inert gaseous phase into a liquid quench medium, which is prefera- bly an aqueous liquid quench medium containing an oxidation inhibitor the position of the gas-liquid interface of said quench medium having been adjusted such- that, a significant proportion of said particulates, deformed from spherical shape, are recoverable from said liquid quench medium.

2. A process as in claim 1- wherein gas atomization is camed out using primarily argon.

3. A- process as in claim 1 wherein the liquid quench medium-is positioned at rest less than about 25 cm below the zone of atomization.

4. A process as in claim 1 wherein said aqueous-liquid quench medium contains an effective amount of an organic oxidation inhibitor.

5. A process as in claim 4 wherein the organic oxidation inhibitor is selected from the group of lower alcohols and water-soluble carbohydrates.

6. A process as in claim 5 wherein the lower alcohol is selected from the group of methanol, ethanol, propanol and isopropanol.

7. A process as claimed in claim 1 substantially as hereinbefore described in any one of Examples I to Ill.

8. Metal powder produced by a process as claimed in any one of claims 1 to 7.