EP0524887A1 - Process and apparatus for the production of powders, in particular metal powders by atomisation - Google Patents

Abstract

Device for the production of metal powders by atomisation, comprising means (B) for melting the material to be atomised, an atomisation chamber (C) in which is disposed a dispersion head (9) rotating at high speed in order to diffuse the molten material in atomised form, means (9) for cooling the atomised material and the head (9) and means (17) for collecting the cooled powdered material thus obtained, the said melting means (B) comprising at least one inductive plasma tower furnace (4) producing an envelope of plasma-producing gases (12) containing the upper face of the dispersion head and the said cooling means comprising a first series of members (15, 16) for dispensing a coolant fluid arranged in the upper part of the atomisation chamber in order to create a cold zone (13) at the periphery of the envelope (12) and a second series of members (11) for circulating a coolant fluid arranged in the lower part of the chamber (C) in order to create a cold zone at the lower face of the head (9). <IMAGE>

Description

The present invention relates to a method and a device for producing powders and in particular metal powders by atomization.

Metallic powder production facilities already exist, in which atomization techniques are used. According to these known techniques, molten metal is poured onto a horizontal disc driven in rotation by a spindle rotating around a vertical axis. The metal is then projected towards the outside of the disc under the effect of centrifugal force and is divided into fine metal droplets which solidify on contact with a fluid or a cold wall.

However, whatever the current techniques, the main disadvantages are, on the one hand, the problem of powder pollution during melting, atomization, quenching and collection operations, on the other hand, the difficulties encountered to atomize a liquid of perfectly homogeneous material.

The present invention aims to overcome these technical problems and in particular to be able to disperse a sufficiently hot metallic liquid, without there being any chemical interaction between the dispersing means and the liquid, to create a quenching zone eliminating any possibility of pollution of the atomized metal, and to provide a "cold chain" allowing the powders obtained to be used without polluting them before manufacturing the final solid product, after compaction and sintering.

This object is achieved according to the invention by means of a device for the production of powders and in particular of metallic powders by atomization comprising means for melting the material to be atomized, an atomization enclosure in which is arranged a dispersion head rotating at high speed for diffusing the molten material in atomized form, means for cooling the atomized material and the head and means for collecting the cooled powder material thus obtained, characterized in that said melting means comprise at least one vertical oven inductive plasma producing a gas envelope plasmagens containing the upper face of the dispersion head and said cooling means comprise a first series of members for distributing a cooling fluid disposed in the upper part of the atomization enclosure to create a cold zone at the periphery of the envelope and a second series of bodies for circulating a cooling fluid disposed in the lower part of the enclosure to create a cold zone on the underside of the head.

According to an advantageous characteristic of the invention, said first series of members for distributing a cooling fluid consists of a ramp of nozzles producing jets of tangential fluid at the surface of said envelope and of nozzles producing a tangential washing at the compound.

According to another characteristic of the invention, said envelope of plasma gases consists of a cylindrical tube whose vertical axis is parallel to the vertical axis of the rotary head, and preferably the axis of the cylindrical tube is coincident with the 'axis of the head.

According to yet another characteristic, said vertical inductive plasma furnace is arranged above the upper face of the rotary head.

Another object of the invention is a method for manufacturing powders and in particular metallic powders by atomization, comprising the continuous melting of the material to be atomized flowing vertically and coaxially above a dispersing head rotating at great speed. speed intended to diffuse the molten material in atomized form, in an envelope of plasma-producing gases, then quenching of the atomized material and collection of the cooled powder material thus obtained, characterized in that the material is atomized by fusion by dispersing it by friction on the upper face of the rotary head and quenching by passing said atomized material through a cooling vortex situated at the periphery of the envelope of plasma gas.

Yet another object of the invention lies in the ultra pure metallic powders obtained by the above process.

Thanks to the cooled dispersion head rotating with a speed of up to 125,000 rpm, the device of the invention can absorb a large heat flow produced by a plasma torch and onto which the liquid material falls. The atomized material then enters a quenching zone at the periphery of the head formed by a cylindrical tube of plasma gas moving parallel to the vertical axis of the head and enveloped in cold fluid. Finally, the powders obtained are recovered in a collection zone comprising at least one chamber containing a neutral gas in the gaseous, liquid or solid state, before their use in formed or shaped products.

The powders obtained by the process of the invention with very rapid cooling are ultra pure and have a very fine particle size.

The invention will be better understood on reading the description which follows and relating to the accompanying drawings.

Figure 1 is a schematic representation of the atomizing device of the present invention.

FIG. 2 is an enlarged view of the central part of the device in FIG. 1.

FIG. 3 represents the quenching zone with the members for distributing the cooling fluid.

FIGS. 4a and 4b schematically represent embodiments of the means for melting and supplying molten metal to the atomization enclosure.

As shown in FIGS. 1 and 2, the material to be melted and to be atomized is introduced by supply means A into the apparatus, for example in the initial form of a cylindrical bar 1 whose diameter is related to the power of the melting means, notably consisting of a plasma oven B.

According to variants of the implementation of the process, the material to be atomized is initially in the form of pieces of various sizes, powders, shots or else may be directly brought to the molten state in the device.

The bar 1 is placed vertically in the axis of the oven B, the valve V1 then being closed, maintaining the oven B and the enclosure C under a neutral atmosphere. After vacuuming and purging the bar supply chamber several times, valve V1 is open. The bar 1 is then lowered with a hydropneumatic or electromechanical cylinder regulated at the speed which corresponds to the desired flow rate. It is preheated in the preheating furnace 3 by the induced currents with one or more inducing turns 5 at a frequency between 10 and 30 kHz, depending on its diameter.

It is also possible to carry out the melting of the material to be atomized by means of a melting device by direct induction in a cold cage with electromagnetic confinement of the molten charge as described in French patent 88 04 460.

The bar then enters the inductive plasma oven 4. The plasma is ignited by creating an electric arc between the bar brought to high voltage and a retractable mobile electrode 8 located at ground. Depending on the more or less advanced position of the bar in the flame, during the casting, the vein or the liquid drops of molten materials pass more or less long over the hottest part of the plasma to, on the one hand, reach overheating and , on the other hand, pass through the most reactive area of the oven.

Preferably, a cold cage 7 is used to protect the oven enclosure, and polished to increase the thermal efficiency of the plasma. The bar 1 is thus heated at its periphery by direct induction of the HF fields - skin effect -, and by thermal conduction and convection of the plasma gases. It melts in a cone, point directed downwards, with an angle whose opening depends on the nature of the plasma gases. There is thus a casting which is, depending on the power of the furnace and the penetration of the bar into the plasma, continuous or not and perfectly axial. As for the diameter of the liquid stream or of the drops, it depends on the flow of liquid and the opening of the cone.

Under these conditions, the material to be atomized is first received in fusion in a cold crucible (as in French patent 2 697 050) from which it flows by gravity passing through an electromagnetic and / or composite nozzle before enter the atomization enclosure as shown in Figures 4a and 4b. The electromagnetic and / or composite nozzle constitutes a means of feeding and regulating the flow of molten metal and optionally makes it possible to maintain the metal in the desired thermal state.

The device shown in FIGS. 4a and 4b comprises means B for melting the solid material (metal) M consisting for example of a plasma torch. The molten material then flows into a cold crucible 100 to form a bath of molten metal. The heat losses on the surface of the bath are possibly compensated by additional heating means B ′. The material in the molten state then flows vertically through the bottom of the crucible through an electromagnetic nozzle 101 (FIG. 4a) or composite 102 (FIG. 4b).

French Patent No. 87 00 866 already describes a composite nozzle 102 used for controlling a flow of liquid metal operating for example with a coil 102b at 450 kHz.

The electromagnetic nozzle 101 comprises a peripheral coil 101b inducing a high frequency field so as to create a constriction of the liquid stream thus causing a variation in the flow of molten material. The molten material then enters the atomization enclosure to come into contact with the dispersion head 9.

In FIGS. 1 and 2, the molten material flows into the atomization enclosure C at the center of the upper face of a dispersion or atomization head driven in rotation by the spindle 10 at a speed of up to '' at 125,000 rpm. The shape of the dispersion head 9 is determined according to the optimal thermal mapping and advantageously, it is produced in the form of a cylinder whose dimensions are determined by the nature of the constituent material and the temperature sought on the upper side coming into contact with the molten material according to the particle size sought for the powders. The upper face of the head is preferably located in a substantially horizontal plane and is crossed vertically by a heat flow generated by the plasma gases heated by induction by the inductor 6. The plasma zone consists of an envelope of plasma gas in cylindrical tube shape whose vertical axis is parallel, being close to or coincident with the vertical axis of said head 9. The underside of the cylindrical head 9 and the spindle 10 are cooled by axial circulation 11 of a fluid cooling which can be either water for the most important thermal fluxes, or a gas or a liquid gas such as argon or helium for example, in the case where a surface temperature of the head is desired more high.

The atomizing cylindrical head 9 is either made of copper, or of tungsten, or of a refractory alloy or not depending on the surface temperature which must be reached.

The underside of the cylinder constituting said head 9 is advantageously provided with a hemispherical cavity licked by the cooling fluid 11 circulating axially. The cooling of the underside of the head 9 creates a temperature gradient in the mass thereof which is included for copper between 60 and 180 ° C / cm and between 200 and 500 ° C / cm for tungsten.

The supply of heat by the plasma to the liquid metal up to the very surface of the head and the thermal resistance between the liquid material and the said head mean that the dispersed material remains liquid (despite the heat extracted through the head)

To increase the thermal resistance and, on the one hand to have the coolest possible dispersion head having regard to its mechanical properties, on the other hand, to have a liquid to be dispersed hot enough to remain homogeneous, atomization is carried out by "erosion" means "erosion" consisting in diffusing and dispersing the liquid by friction and thus avoiding its "wetting" with the upper side of the head.

• a. fondre le matériau dans des conditions géométriques et thermocinétiques optimales, pour obtenir une coulée parfaitement axiale et stable ;
• b. surchauffer la veine liquide pour obtenir un liquide homogène ;
• c. créer un flux thermique à travers la face supérieure de la tête d'atomisation 9 et assurer une cartographie thermique compatible avec la tenue mécanique de ladite tête ;
• d. maintenir la pureté des produits lors de l'atomisation jusqu'à la trempe.

The use of the plasma "torch" allows to:

• at. melt the material under optimal geometrical and thermokinetic conditions, to obtain a perfectly axial and stable casting;
• b. overheating the liquid vein to obtain a homogeneous liquid;
• vs. create a thermal flow through the upper face of the atomization head 9 and ensure thermal mapping compatible with the mechanical strength of said head;
• d. maintain the purity of the products during atomization until quenching.

After atomization, the liquid particles pass directly from the plasma zone 12 which envelops the head, to a quench zone 13 consisting of a cooling medium, two-phase or not, forming a vortex around the plasma. To this end, a series of nozzles 15 placed on a circular ramp 14 at the top of the atomization enclosure C, sends the cooling fluid tangentially to the plasma gas tube 12.

According to an advantageous embodiment as shown in FIG. 3, there is a ramp of eighteen nozzles 15 distributing a total flow of liquid argon sufficient to obtain complete cooling of the powders. The nozzle ejection axis X 15 is inclined relative to the plane of the upper face of the head 9 with a jet width determined so as to obtain rapid cooling and a rotation effect of opposite direction (counter-rotating) to that of the head 9 in order to brake the movement of powders.

The nozzle ejection orifice 15 is located above the powder ejection triangle.

The passage from the plasma zone consisting of the envelope of plasma gases 12 at high temperature, to the quenching zone 13 at low temperature, on the one hand, eliminates the chemical reactions which occur between 1500 ° C and 200 ° C and very particularly those of oxidation in the case of metals and alloys and, on the other hand, avoids the formation of phases intermediates not allowing microcrystalline and even amorphous structures to be obtained.

The refrigerant vortex 13 thus formed entrains the liquid and then solid particles in spiral trajectories, thus avoiding, on the one hand, direct impacts with the walls of the enclosure C, on the other hand, gas turbulence towards the top of the device, turbulence which may disturb the plasma and atomization.

The nozzles 16 oriented towards the walls of the enclosure, project onto them a mist of argon which flows along the walls, thus driving the powders down and thus ensuring a tangential washing to the enclosure.

The mixture of liquid and powder is deposited at the bottom of enclosure C.

The powder obtained is therefore deposited at the bottom of the enclosure C and is recovered in the container 17.

The cooling and the collection of the powder are thus carried out using a neutral gas in the gaseous, liquid or solidified state after immersion of the collected powder in the liquid phase.

The invention also provides the possibility of combining in the same unit several atomization devices arranged around the energy sources: medium frequency preheating generator (MF) and plasma torch generator (HF).

The description which follows illustrates an example of the operating mode of the method of the invention with reference to the device illustrated in FIG. 1.

Example

Elaboration in the device of the invention, of 10 kg of alloy powder with two bars of 24 mm in diameter.

The operation is semi-continuous, in sequence of 2 bars.

We will start with the loading operation of the bar n ° 1 then by that of preheating by the Medium Frequency oven of 10 kHz of 30 kW, followed by those of fusion by the plasma torch of 100 kW, of centrifugal dispersion and of cooling by liquid argon in gaseous helium, finally by that of recovery of the powder in a collector cooled by liquid nitrogen.

In the following, D denotes a flow, P a pressure, T a temperature, V a valve, B a flange.

PRELIMINARY Operations:

• Degassing at room temperature with the PV1 pump, then with the PV2 molecular pump to obtain in the enclosure the collector, the disperser or rotary head, the argon pipes and the liquid argon accumulator, a static vacuum of 10 -5 torr;
• Argon U sweep at 1 bar;
• Closing of valve V1
• Vacuum at 10-3 torr;
• Filling with helium by the V4 valve via a pressure regulation device (MKS) to maintain at 2 bars;
• Opening of valve VA9 of the disperser gas bearing, with PA9 = 2 bars;
• Rotation of the disperser at low speed, at around 5,000 rpm;
• Introduction of cooling water from the head, at a flow rate DE1 = 10 g / s;
• Cooling of the enclosure and the collector with liquid nitrogen at 3 bars;
• Cooling of the accumulator to 2 bars;
• Filling the accumulator by condensing argon U
• Introduction of gaseous argon into the cold cage of the plasma torch by the valve VA2 at a flow DA2 = 0.3 l / s;
• Pressurization, PA6 = 3 bars, of the argon accumulator (not shown) and opening of the valves VA3, VA4 and VA5 for degassing of the liquid argon pipes and priming of the pumps cryogenic;
• Filling the liquid nitrogen expansion tanks (not shown) up to the "ni" levels respectively at pressures PNi = 2 bars, for i = 1 to 6.

• 1 h 8 min en décantant entre 2 barreaux ou
• 48 min en remplissant l'accumulateur d'argon liquide avec de l'argon liquide en réserve.

To obtain 10 kg of alloy powder in a collector,

• 1 h 8 min by decanting between 2 bars or
• 48 min by filling the accumulator with liquid argon with liquid argon in reserve.

The method and the device of the invention make it possible to manufacture powders from various families of materials and in particular superalloys based on nickel, titanium and titanium alloys, aluminum, Niobium alloys, etc.

Claims (21)

Device for producing powders and in particular metal powders by atomization comprising means for melting (B) of the material to be atomized (1), an atomization enclosure (C) in which is disposed a dispersion head (9) rotating at great speed speed for diffusing the molten material in atomized form, means for cooling the atomized material and the head (9) and means for collecting (17) the cooled powder material thus obtained, characterized in that said melting means ( B) comprise at least one vertical inductive plasma oven (4) producing an envelope of plasma gas (12) containing the upper face of the dispersion head and said cooling means comprise a first series of distribution members (15,16 ) a coolant disposed in the upper part of the atomization enclosure to create a cold zone (13) at the periphery of the envelope (12) and a second series of members circulation (11) of a cooling fluid disposed in the lower part of the enclosure (C) to create a cold zone on the underside of the head (9). Device according to claim 1, characterized in that said first series of members for distributing a cooling fluid consists of a nozzle ramp (15) producing jets of fluid tangential to the surface of said envelope and of nozzles (16) producing a tangential washing to the enclosure. Device according to claim 2, characterized in that said nozzles of the first series are placed above the powder ejection triangle and have an ejection axis X inclined relative to the plane of the upper face of the dispersion head (9). Device according to one of the preceding claims, characterized in that the said envelope of plasma-producing gases consists of a cylindrical tube whose vertical axis is parallel to the vertical axis of the rotary head (9). Device according to claim 3, characterized in that the vertical axis of the cylindrical tube is close to or coincident with the vertical axis of the head. Device according to one of the preceding claims, characterized in that the said vertical inductive plasma oven (4) is arranged above the upper face of the rotary head (9). Device according to one of the preceding claims, characterized in that said dispersion head is cylindrical and its upper face is arranged in a substantially horizontal plane. Device according to one of the preceding claims, characterized in that said inductive plasma oven (4) is associated with a preheating oven (3) with induced currents. Device according to one of the preceding claims, characterized in that it further comprises a cold crucible (100) disposed under the melting means (A) for receiving the material to be atomized in the molten state and a nozzle (101,102) adjusting the flow rate of said molten material to feed the atomization enclosure (C). Process for the manufacture of powders and in particular of metallic powders by atomization, comprising the continuous melting of the material to be atomized flowing vertically and coaxially over a rotating dispersing head (9) at high speed intended for diffusing the material molten in atomized form, in a casing of plasma gas, then quenching the atomized material and collecting the cooled powder material thus obtained, characterized in that the atomization of the molten material is carried out by dispersing it by friction on the upper face of the rotary head (9) and quenching by passing said atomized material through a cooling vortex situated at the periphery of the envelope of plasma gases. Process according to claim 10, characterized in that the powder is collected in a neutral gas in the gaseous, liquid or solid state. Method according to one of claims 10 or 11, characterized in that atomization is carried out at pressures greater than atmospheric pressure. Method according to one of claims 10 to 12, characterized in that the plasma is ignited by creation of a high voltage electric arc between the material to be atomized (1) and an electrode placed in the axis of the furnace (4) . Method according to one of claims 10 to 13, characterized in that the atomized material is quenched by contact with a cold, gaseous, liquid or two-phase fluid making it possible to obtain microcrystalline or amorphous structures. Method according to one of claims 10 to 14, characterized in that the gases produced during the quenching are liquefied in a condenser and the powders are recovered with part of the liquefied gases in at least one container making it possible to maintain the mixture at liquid or solid state. Method according to one of claims 10 to 15, characterized in that the dispersion head (9) is rotated at a speed of between 30,000 and 125,000 rpm. Method according to one of claims 10 to 16, characterized in that a temperature gradient of 60 to 180 ° C / cm is obtained for the copper head and from 200 to 500 in the dispersion head (9) ° C / cm for a tungsten head. Method according to one of claims 10 to 17, characterized in that the atomized material is quenched by means of nozzles (15) distributing a total flow of liquid argon sufficient to obtain complete cooling of the powders; the axis of ejection of said nozzles being inclined relative to the plane of the upper face of said dispersion head (9) and the width of the jet being determined so as to produce a counter-rotating effect to that of said head (9) to slow down the movement of powders. Method according to one of claims 10 to 18, characterized in that the material to be atomized is initially in the form of a cylindrical bar (1). Method according to one of claims 10 to 19, characterized in that the material to be atomized is first received in the molten state in a cold crucible (100) from which it flows through a nozzle ( 101.102) for adjusting the flow rate to the atomization enclosure (C). Ultra pure metallic powders obtained by the process according to one of claims 10 to 20.

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