EP0217835A1

EP0217835A1 - Process for the manufacture of metallic powders

Info

Publication number: EP0217835A1
Application number: EP86901772A
Authority: EP
Inventors: Peter Boswell; Dag Richter; Georges Haour
Original assignee: Battelle Memorial Institute Inc
Current assignee: Battelle Memorial Institute Inc
Priority date: 1985-04-16
Filing date: 1986-04-11
Publication date: 1987-04-15
Also published as: WO1986006013A1; JPS62502478A; CH666639A5; US4915729A

Abstract

On met en contact un métal fondu avec un lit de perles en mouvement; ce métal fondu se fragmente en fines particules lesquelles se refroidissent brusquement au contact de ces perles et acquièrent, de ce fait, une structure typique d'un tel refroidissement rapide.A molten metal is brought into contact with a bed of moving beads; this molten metal breaks up into fine particles which suddenly cool on contact with these beads and thereby acquire a structure typical of such rapid cooling.

Description

METHOD FOR MANUFACTURING METAL POWDERS

The present invention relates to a method for manufacturing metal powders. It also relates to a device for implementing this method as well as variants of this device.

We know the great interest currently aroused by metallic powders which result from the rapid cooling of particles of a molten metal or alloy. Indeed, during such an almost instantaneous solidification (quenching), the metal retains or acquires a typical structure which depends on the cooling rate. As examples of such structures, mention may be made of "amorphous", "metallic glass" or "m.c." structures. whose physical properties are often very different from those of the same metal after slower cooling. It is not uncommon for "metallic glasses" to be endowed with interesting magnetic, mechanical and chemical properties which make them suitable for many applications in the electrical fields, machine building and chemical engineering.

Many metals and alloys can thus be fragmented, in particular ferrous metals, steel, nickel, chromium, copper, aluminum, zinc, etc. The powders of these metals are then compacted by the techniques of powder metallurgy in various forms of commercial use, in particular of ingots, bars, filaments, ribbons and others as well as products molded directly by sintering.

There are already many processes for converting metals and alloys into powders. Thus, document US-A-3,598,567 describes a process according to which a jet of molten metal is sprayed by a stream of air and the droplets thus produced are rapidly cooled by a refrigerating fluid, in particular a gas or a liquid at low temperature or a metal surface with high thermal conductivity, such as copper, silver, steel, etc. To obtain the desired effect (ie conservation of the amorphous structure or "mc" of the molten metal), cooling rates of the order of a hundred degrees C / sec may be suitable in certain cases ; however, higher speeds, on the order of 10 ⁴ to 10 ⁶ ° C / sec, may be advantageous in other cases.

Other similar methods are described in the following documents: US-A-3,325,277; 3,046,177; 3,764,255; 3,313,196 and 3,356,513: MH Kim et al, Proc. 4th Int. Conf. on Rapidly Quenched Metals (Sendai 1981), pp. 85-88; using different techniques, we can also cause fragmentation of a jet of molten metal by directing it onto a fast-moving solid surface; for example a rotating disc or cylinder, the liquid metal thereby undergoing a shearing effect leading to the formation of fine particles. Such a process is described in document US-A-2, 555.131 and in document US-A-4,386,896 which combines both the atomization of the molten metal and the projection of the atomized particles onto a rotating disc (see also G. THURSFIELD et al., J. Phys. E: Sci. Instrum. 4 (1971), pp. 675-676, ARE SINGER et al., Powder Metallurgy 2, (1980) pp. 81-85; M. Lebo et al., Metallurgical Transactions, 5 (1974), pp. 1547-1554.

More recently, a process has been described (see USA-4,355,057) for manufacturing metallic powders whose particles have a calibrated dimension. To do this, one knocks against each other, on the one hand, molten metal droplets and, on the other hand, solid metallic particles, by projecting, approximately horizontally, a jet of molten metal sprayed onto a flow. shot falling vertically. On contact with a solid grain of the shot, a droplet of molten metal suddenly solidifies and, adhering to it, increases its size. The grains are then recycled in the process until, by their successive contacts with other liquid metal droplets, they have acquired a determined size from which they can be harvested by means of a sorting device.

Despite their various advantages, the processes of the state of the art are not intended to, on the one hand rapidly cool a molten metal while simultaneously dividing it into fine particles with sharp angles and / or, on the other hand, ensure simultaneous grinding of such particles, in particular in the form of flakes in the case where the solidification of the molten metal occurs by crushing drops of this metal against a cold surface with consecutive formation of a film of solid metal on this surface.

Now, we have now reached such results thanks to the process of the invention defined in claim 1. In fact, according to a first embodiment, we begin by dividing the molten metal into fine droplets, for example by atom -ization or spraying according to known techniques, and these droplets are projected onto the beads of the moving bed so that these droplets, by crushing and cooling in contact with the beads which they meet, form there a film which is then ground into flakes due to the mechanical effect that exert hundred on this film, the pearls of the moving bed which collide against each other. When the straws have reached, after a certain period of residence in the moving bed, the desired size, it is easy to harvest them, continuously or not, thanks to a sorting device (grid, sieve or the like).

It is quite obvious, and this is one of the particularly advantageous elements of the invention, that the characteristics of these flakes can be controlled by modification of the various operating parameters of the present process, parameters such as speed and type of movement of the pearls, nature and dimensions of these, temperature and number of beads per unit of working volume. Other parameters such as the nature and temperature of the molten metal or alloy, as well as its flow rate and the size of the droplets resulting from the spraying, are also important with regard to the size, shape and thickness of the flakes resulting from this. process. Thus, the larger the balls and the drops of molten metal, the more extensive will be the films of solidified metal resulting from the crushing and the sudden cooling of said drops on the balls; similarly, the more the balls suddenly collide, the more effective the spraying of the films of solidified metal and the finer the flakes.

The flakes obtained according to the present process can be used after dispersion in an appropriate binder, to produce metallized coating compositions. Depending on their composition, such coatings can have decorative purposes; or they can be used as a coating reflecting certain electromagnetic wavelengths or as electroconductive coatings (Faraday cages), in particular in the field of electronic devices.

As binders, it is possible to use resins known in the field of pastes, paints, coatings, varnishes, etc., such materials being well known to those skilled in the art, either in the form of polymers or in the form of polymerizable monomers (for example thermally or photochemically example). Furthermore, the present compositions for metallized coatings can comprise any additives and solvents generally used in this field. Such materials are well known in the art of coatings and there is no need to detail them further.

According to another embodiment of the present method, the molten metal is directed, previously divided into droplets or not, against the animated beads with a sufficient speed so that after having struck those Ci, it fragments into particles which, by cooling (this intervening practically simultaneously), acquire, if desired, sharp angles. It is in fact known that, during their subsequent transformation by powder metallurgy techniques, powders formed from metal particles at acute angles have notable advantages over powders consisting of particles with rounded angles.

Among the documents identified by the search for novelty in this area, we can cite the following texts:

US-A-4,508,666 (HOECHST); 4,435,342 (WENTZELL), 4,396,420 (DORNIER); 3,963,811 (TAMURA); 3,897,016 (LINDE); 3,624,796 (ENGLISH CLAYS); 2,380,253 (McCOY) 1,938,876 (TAKATA); an extract from the "XEROX DISCLOSURE JOURNAL" from May-June 1977; US-A-4, 104,342 and DE-A-2,144,220 (MANNESMANN); US-A-3,665,837 (RED ET al); 4,374,633 (HART) and 3,726,621 (CICHY).

The documents US-A-4,104,342 and DE-A-2,144,220 relate to a process for the manufacture of metallic powder by generally known means, in which a molten metal is atomized into particles and these are cooled by a bed of sand. silica, the purpose of such a bed being to prevent agglomeration of the particles. A sand bed has only a very distant analogy with the beads or beads of the present invention whose mechanical action during the shaping of the particles of the present powder is fundamental.

Document US-A-3,655,837 discloses the atomization into particles of a molten metal followed by their agglomeration with solid particles of the same metal for the production of particles of irregular shapes. None of the typical characteristics of the process of the present invention relating to the implementation of a bed of moving pearls is included in this reference.

Document US-A-4, 374,633 teaches periodic stripping of a nozzle for spraying molten metal using an abrasive powder. This reference in no way suggests the process of the present invention.

Document US-A-3, 726,621 relates to the manufacture of refractory oxides with abrasive properties. According to this process, a molten mineral mixture is added to a bed of metal spheres. This bed is however not agitated with movements such that a dynamic effect (shock) occurs during contact between the liquid and the spheres and, consequently, the specific effects and advantages of the present invention do not emerge from this document. . Consequently, teaching it would not encourage the skilled person to apply a bed of rapidly moving beads to the manufacture of metallic powders with grains of non-spherical shape as is done in the present invention.

The pearls used in the present process can be made of very diverse materials and adopt a wide variety of shapes. Preferably, the materials of which the pearls are made are hard and resistant to impact and abrasion (unless, of course, in special cases, only the material resulting from a certain degree of wear of the materials is desired. pearls do not mix with the metallic powder obtained). As such materials, mention may be made of metals and alloys and mineral materials, for example certain ceramics and cermets. As metals, mention is made more particularly of steel, nickel, cobalt, copper, bronze, chromium, precious metals, etc. As mineral materials, mention is made of metal carbides, nitrides and borides (also as coating of surfaces on pearls with a metallic core), alumina, corundum, zirconia, etc. All these materials forming the pearls must of course be good conductors of heat in order to allow rapid cooling of the molten metal coming into contact with them. It is however obvious that, according to the needs (that is to say according to the degree of fineness of the powder which results from the fragmentation of the liquid metal under the impact of the pearls), it may be advantageous to limit this speed of cooling and delaying the hardening of the particles during fragmentation by a judicious choice of the heat absorption coefficient of the pearl material. Generally, the thermal conductivity of the pearl material will be in the range 1-50 calºC m / sec, values higher or lower than this range may however be suitable in certain special cases.

We can almost say that the shape of the pearls can be arbitrary with the reservation, however, that if their shape does not correspond to that of a volume of revolution, their angles must be sufficiently rounded so as not to lead to breakage of the pearls during of the shocks to which they are subjected. Preferably, ovoid or spherical balls of very variable sizes are used, that is to say of the order of a fraction of mm to about 15-20 mm in diameter. The diameter of the pearls is, of course, in relation to the nature of the powder which it is desired to obtain, large balls producing more violent but less frequent impacts than small balls. Generally speaking, beads with a diameter of about 0.5 to 10 mm are advantageously used, but these values can be exceeded in special cases. In the case of ellipsoidal beads, the ratio of large to small diameter will preferably be between 1.2 and 4; the non-spherical balls have, in the present process, an action perhaps less regular and homogeneous on the fragmentation and the grinding of the metal of the desired powder than the round balls; however, the transfer of kinetic energy from the stirring device to the bead bed is more efficient in the case of ovoid beads than in the case of spherical beads.

The types of agitation movements to which the bed of pearls is subjected according to the present process are also very varied and depend as much on the desired effects as on the nature of the materials used. It is indeed possible to subject the pearls to translational movements, for example displacement, oscillation or vibration, or rotations. These movements can also be combined. These movements can be jerky or continuous. Depending on their nature, they will print on the balls, taken themselves individually, more or less random movements of translation and rotation depending on the type of pulse applied to the bed and the density of the bed of balls, that is to say tell the mean free path of each of them. Preferably, a general gyratory movement is imparted to the bed of beads in the horizontal or vertical plane, the centrifugal force resulting from this movement providing for the bouncing of the beads against the walls of the container containing the bed and, consequently, mutual shocks between the balls. One can also act on the balls by projecting them vertically upwards, their fall against the bottom of the container and their rebound following this shock produce comparable effects.

The device for implementing the present method is defined in claim 10. For a more exhaustive understanding of this device as well as of the method, reference will be made to the attached drawing.

Figure 1 schematically shows a device for manufacturing metal powders from a molten metal.

FIG. 2 represents another embodiment of such a device, and

Figure 3 shows yet another embodiment.

Figure 4 shows another embodiment of this device where the beads are driven in a vertical reciprocating movement.

Figure 5 shows another embodiment of the present device where the beads are circulated from top to bottom of a vertical enclosure, harvested at the bottom of the enclosure and recycled from the top of this one.

The device shown diagrammatically in FIG. 1 comprises an enclosure 1 containing a bed of pearls 2, these pearls being made of a hard and impact-resistant material - for example steel - and set in motion by an axial agitator 3 with paddles 3a actuated by a motor 4. This enclosure 1 is double-walled so that between them there remains a sleeve 5 allowing the circulation of a cooling fluid coming from a supply and outlet piping 6 for liquids.

The present device is provided, in its upper part, with an element 8 allowing the molten metal to be maintained in the liquid state and to distribute it, according to a chosen mode, inside the spraying enclosure. 1. This element 8 consists of a tank of molten metal 9 provided with a heating device, for example an inductive winding 10, and a flow nozzle 11 whose flow rate is controlled by a needle rod 12 The temperature of the molten metal can be measured using a thermocouple 13 and its protection against oxidation is ensured by an inert gas, for example Ar, coming from a pipe 14a.

The enclosure 1 also comprises a gas supply 14b, a gas outlet 15 and a safety device against overpressure represented in the drawing by a valve 16. Furthermore, the gas supply 14b leads to gas injection nozzles 17 arranged concentrically with the flow of molten metal from the nozzle 11 and used to spray (or atomize) this liquid metal into fine droplets.

The enclosure 1 also comprises a sorting element, consisting of a sieve 18 whose meshes are calibrated to allow the metallic powder formed in the enclosure to pass through while retaining the pearls therein, as well as a funnel 19 making it possible to harvest metal powder from this device.

The operation of the latter is summarized as follows: the metal to be fragmented is introduced into the reservoir 9 and is kept molten at a temperature sufficient to ensure its free flow through the nozzle 11 under the control of the needle 12. The temperature of this metal, generally of the order of 10 to 50 ° C above the liquefaction point, is kept constant by the heating element 10.

The agitator 3 is started, which, thanks to the pallets 3b, drives the pearls of the bed 2 in a rapid rotary movement; this movement causes, due to the centrifugal force to which the bed of pearls is subjected and the existence (preferably) of asperities on the internal surface of the girdle 1, a rotation on themselves of the pearls and a violent mixing of the bed in the enclosure. The balls are thrown in all directions and, as a result, collide.

We then regulate the entry into the enclosure 1 of the liquid metal through the nozzle 11 as well as the pressure of the atomizing gas coming from the nozzles 17 so that the molten metal is transformed into droplets 20 of chosen desired dimensions which are projected at the meeting of the bed of balls 2 in movement.

Under the effect of the shock between the molten metal and the balls, the latter still fragments once or more and, in so doing, the metal fragments solidify into compact particles with sharp angles. These particles finally fall to the bottom of the enclosure where they pass through the sorting grid 18 and are collected, thanks to the hopper 19, in a tank 21.

The device shown diagrammatically in FIG. 2 comprises a rotary drum 31 containing a bed of balls 32 and provided on one of its axial faces with a central circular opening 33 and, on the other face, with an annular opening 35. The drum 31 is rotated by a motor 34 via a pinion 36 of the axis thereof and a ring gear 37 secured to the external face of the drum. The latter rests in rotation, in an oblique position, on rollers 39. The device of FIG. 2 also comprises an element 38 for distributing molten metal represented in the drawing by a block. This element is, in. done, almost identical to the corresponding block 8 of Figure 1 and, for simplicity, it has not been shown in detail in Figure 2. The present device also includes a nozzle

40 for injecting gas (for example Ar or another inert gas), this gas serving to atomize, if desired, the metal flowing from a nozzle 38a of the member 38. Finally, the device also comprises means cooling

41 shown schematically in the form of a water jet 41 projected onto the external surface of the drum 31.

The operation of the present variant of the device of the invention can be summarized as follows:

In a first mode of operation, the atomization of the molten metal is carried out as in the case of the device in FIG. 1 under the influence of a gas jet coming from the nozzle 39. The drops of molten metal resulting from this atomization are then projected onto the bed of pearls 32 set in motion by rotation of the drum 31. The drops disperse in the pearls and, due to the repeated shocks they undergo feels while cooling, they become particles with sharp angles; the speed of rotation of the drum, the volume and the weight of the beads are all parameters that are adjusted to avoid excessive crushing of particles which would result in excessively rounding them before they leave the drum through the opening annular 35 (too narrow to allow the pearls to pass through) and are not collected in the tray 43.

According to another mode of operation, the pressure of the gas in the nozzle 39 is adjusted so that the metal jet coming from the nozzle 38a undergoes only a very coarse fragmentation (relatively large drops) instead of atoraisation. These drops, arriving against the beads, spread over their surface forming a thin film which solidifies into a solid film; due to the movement of the pearl bed, this solid film comes off and undergoes crushing which converts it into flakes during its movement in the drum, the latter playing the role of a ball mill.

We can therefore act on the shape and structure of the particles by playing not only on the parameters related to the movement of the beads in the drum (and the cooling rate), but also on the preliminary fragmentation (and non-fragmentation) of the jet of molten metal from nozzle 38a.

The modification shown diagrammatically in FIG. 3 somehow combines the advantages of the variants of FIGS. 1 and 2. It comprises, in succession, a vertical enclosure 51 and an inclined rotary drum 52. The enclosure 51 is swept by an agitator 53 with paddles 53a driven by a motor 54 and receives at its upper part a jet of divided molten metal generated by an element 58 (identical in all points to the corresponding element 8 in FIG. 1) provided with a nozzle 55, this jet being divided by a stream of gas coming from an intake pipe 56 and nozzles 57.

The enclosure 51 also comprises an inlet 59 through which beads constituting a bed of beads 60 are added. These beads move progressively from the enclosure 51 to the drum 52 (they enter there through a first annular opening 61 of the latter) and come out through a second opening 62 of the opposite axial face; the pearls are then temporarily stored in a reservoir 63 after having passed over a sorting member 64 (sieve) from where they are again routed into the enclosure 51 by the inlet 59.

The drum 52 is rotated by the motor 54 by means of a pinion 65 and a crown with a toothing 66.

The fresh metal from nozzle 55 is sprayed in a first time in the enclosure 51 by the effect of the balls driven in horizontal rotation by the agitator 53, then it is further ground when, after leaving the enclosure 51, it has entered the drum 52 through the opening 61 in company of the pearls in circulation, this grinding effect resulting from the cascading movement to which these pearls are subjected in the drum 52 in rotation. Finally, the pearl / powdered metal mixture passes over the screen 64 where it is separated, the metal powder passing through the meshes to be collected in the tank 67.

Note that, as in the previous variants, this device is designed to work in the presence of a protective gas (for example a noble gas), the intake and exhaust pipes not having been shown in the drawing for simplicity . It will also be noted that, instead of exerting a protective effect, the gas admitted into the spraying enclosure can be a reactive gas, for example a gas causing the oxidation, nitriding or surface carburization of the particles of the metal powder. we want to get. As such reactive gases, mention may be made of the following gases: methane and other votalil hydrocarbons, carbon monoxide, cracked ammonia, etc.

The modification of the device shown diagrammatically in FIG. 4 comprises, like the previous variants, a spraying and grinding chamber 71 containing a bed of balls 72 and a member 73 for agitating these balls. Furthermore, this device also comprises a sorting member (sieve) 74 placed at the bottom of the enclosure 71, a gas inlet 75 provided, at its end with nozzles 76 for spraying gas and with a member 78 intended (as the corresponding members 8, 38, 58 of the preceding variants) to supply a descending jet of molten metal through a nozzle 79, this jet being able to be divided by the action of the pressurized gas coming from the nozzles 76.

The lower part of the agitator 73 is articulated on a connecting rod 80 whose foot is articulated on a crank 81 secured to a motor 32. Thus, unlike the cases of the previous variants, the agitator 73 is driven by a movement of vertical back and forth. The balls are therefore projected unilaterally upward at each oscillation of the blades 73a of the agitator and fall by gravity. This movement (with a certain degree of organization) results in a particular effect on the shape and structure of the metal particles formed by interaction between the beads and the droplets of atomized molten metal. In fact, when they cool, these particles tend to acquire a rather elongated, almost filamentary shape; manufactured items by compaction or fried Tage of such powders have properties distinct from those obtained from powders (of identical composition) but the particles of which are either compact (shapes approaching the parallelipiped), or flakes (micro-chips) obtained by grinding thin films. It should be noted that in the embodiments of figs 1 to 4, one could use a stream of molten metal not previously sprayed; a specific example of powder formation from such a stream of non-pulverized metal is described with reference to FIG. 5.

The device shown in fig. 5 comprises, as in the embodiments of the preceding figures, a fragmentation enclosure 91 containing a bed of pearls 92. This enclosure can be provided with a double heating wall as in FIG. 1 although this is not shown in the drawing.

The device also comprises, like the previous forms (see FIG. 1) an element 88 for distributing molten metal in the form of a continuous jet of liquid metal 89. This jet is not sprayed in micro-droplets as in the models previous but simply flows into the bed of moving beads so that the impact of the beads causes its fragmentation.

The device further comprises two turbines 90 for accelerating the beads supplied by hoppers 94. These hoppers extend towards the axial zone of the turbines and the balls penetrating therein are expelled (as shown in 95) by centrifugal force resulting from the rotation of the turbine rotors. By varying the speed of the turbines, the output speed of the beads, their direction and the impact force between them and the molten metal are varied.

The present device also comprises, at the bottom of the enclosure, a perforated end wall 93 used for sieving the metal powder formed by the impact of the liquid metal and the pearls and the sudden cooling of the latter after its fragmentation under the impact of this impact. The device also includes an outlet channel for the beads, an accumulation tank 99 and a recycling channel 97 for the latter so as to return them to the hoppers 94. The propulsion of these beads in the recycling line 97 is ensured by usual drive means not shown in the drawing. The metal powder formed accumulates in a collecting tank 98 from which it can be taken as required.

It will be noted that the present device may also comprise, placed at the bottom of the enclosure, and before separation of the powder from the balls, a mill for grinding, to the desired particle size, the fragmented and solidified particles resulting from the present process. Such a grinding operation can also be carried out after the fact, independently.

It can therefore be seen that the invention, in its various embodiments, leads to metal powders whose particles have (in addition to their properties depending on the choice of metal or alloy) an adjustable shape and geometry to be chosen according to the configuration of the fragmentation device, the nature and the size of the beads of the bed, the mode and the speed of stirring thereof as well as the temperatures of molten metal, the type and pressure of the atomizing gas and the degree of effectiveness of the cooling of the liquid metal, that is to say the rate of hardening of the particles during their interaction with the bed of moving pearls.

In general, it has been found that, among the above-mentioned parameters, the values below provide advantageous results:

In the case of a particulate distribution of the molten metal, size of the metal drops (with and without atomization) at the outlet of the dispensing spout: 20 μ-1 mm. Continuous metal distribution: flow 50-100 g / min.

Average speed of the pearls in the stirring bed: 1-1000 m / sec.

Apparent density of the bed of beads relative to the apparent density of the atomized material: 3 / 1-10 / 1.

Ratio between the volume of molten metal sprayed per unit of time and the speed of the pearls multiplied by their cross section: 10 ^-5 to 10 ^-1 .

The following example illustrates the invention.

Example 1

A spraying device was used in accordance with that shown in FIG. 1. The spray enclosure had an approximate diameter of 200 mm, a height of 250 mm and contained 5 kg of steel balls 3 mm in diameter. The rotating agitator gave these balls an average translational speed of 5 m / sec with an angular speed of 500 rpm. A Fe-C-Si-B alloy melted at the temperature of 1200 ° C. was distributed over these beads with a flow rate of approximately 120 g / min. For atomization, argon was used at a pressure of 4-6 bar and at a flow rate of 12 l / min, the approximate average size of the liquid metal droplets thus formed being 30-200 μm. At the bottom of the column, powder of the alloy in question was collected, through the sieve (18), consisting of stocky particles with sharp angles, the approximate average size of which was 20-50 μm. These particles have been used to manufacture, by compacting and sintering by the usual means, mechanical parts of particularly high strength.

Example 2

A spraying device was used in accordance with that shown in FIG. 5 comprising a cylindrical enclosure 0.5 m in diameter and 1 m long. The oblique sieving area included a mesh screen of about 0.5 mm.

The enclosure was equipped with 0.25 m diameter turbines rotating at speeds of the order of 2000 to 5000 rpm. 0.2 cm steel balls were used and circulated with a flow rate of 0.25 to 1 kg / sec. The molten metal dispensing spout provided a 0.5 mm diameter jet.

Molten aluminum was heated heated to 860 ° C and delivered at the rate of 2.5 g / sec and a powder formed from flakes with sharp edges of a particle size between 50 and 400 μm was obtained.

We also operated on molten tin at 630 ° C pulverized at a flow rate of 5 g / sec with 1 kg / sec of steel balls with approximately similar results.

The above aluminum powder was pasted by grinding part of it by weight with 0.9 parts of isobutyl methyl ketone and 0.1 parts of methanol.

Then a coating base was formed from a 3: 4: 3 mixture (by weight) of trimethylol propane triacrylate, diethylene glycol diacrylate and EBECRYL-600 (acrylic prepolymer of Belgian Chemical Union) and methyl amyl ketone, due to 4 parts of monomeric resins per L part of solvent.

One part of the aluminum paste was then mixed with 6 parts of the base and 0.1 parts of polymerization catalyst.

A sheet metal plate was coated with an approximately 800 µm layer of the coating mixture and then after 1 h. in air, it was heated for 2 hours at 60-80 ° C in the oven. This gave a metallized coating with a decorative appearance.

Claims

RE VE ND I CA TIONS

1. Process for the manufacture of metallic powders by fragmentation of a liquid metal into particles followed by bringing them into contact with a cooling element sufficiently effective to cause a quenching effect so as to give them a specific structure typical of 'rapid cooling, characterized in that this cooling and solidifying element consists of a bed of beads of solid material resistant to impact and subjected to rapid movement.

2. Method according to claim 1, characterized in that the metal is first divided into droplets and that they are directed on said moving beads, said droplets forming, by s' crushing on the surface of the beads and by solidifying there, a metallic film which is then ground into fine flakes by the pearls which collide against each other.

3. Method according to claim 1, characterized in that the molten metal is directed against the moving beads with sufficient kinetic energy so that, under impact, it divides into particles which then solidify, the bed of pearls acting, as well as an element of fragmentation as well as cooling.

4. Method according to claim 3, characterized in that the fragments thus obtained are particles with sharp angles.

5. Method according to claim 1, characterized in that the pearls are of rounded, ovoid or spherical shape.

6. Method according to claim 5, characterized in that the beads are metal balls or ceramic impact resistant.

7. Method according to claim 1, characterized in that the beads have a translational speed of 1 to 100 m / sec.

8. Method according to claim 1, characterized in that the pearls are in rotation at a speed of 10 ² -10 ⁴ rpm.

9. Method according to claim 1, characterized in that after solidification, the metal powder thus formed is separated from the beads of the bed.

10. Device for manufacturing metal powders by implementing the method according to claim 1, characterized in that it comprises: a) a fragmentation chamber comprising a bed of moving pearls; b) means for setting these beads in motion; c) means for melting the metal to be reduced to powder, for forming a jet of this molten metal and directing this jet of molten metal in or against these moving beads, the operating conditions being adapted so that said metal in fusion is fragmented into particles, which then solidify on contact with these pearls; d) means for collecting the fragments of solidified divided metal constituting the desired metal powder.

11. Device according to claim 10, characterized in that the means for setting the bed of pearls in motion comprise a rotary or oscillating agitator.

12. Device according to claim 10, characterized in that the stirring means of the pearl bed comprise a rotary drum whose axis is oriented obliquely, so that the metal particles gradually accumulate at the bottom of this drum from which they are discharged and separated from the beads by passing through a sieve.

13. Device according to claim 12, characterized by. the fact that the beads accumulating at the bottom of the drum are removed and reintroduced at the top of said spray enclosure.

14. Device according to claim 10, characterized in that the ratio of the spraying rate in volume of molten metal per minute to the speed of the beads multiplied by the area of their cross section is of the order of 10 ^-5 to 10 ^-2 .

15. Device according to claim 10, characterized in that the means for forming a jet of molten metal consist either of an orifice delivering a continuous jet of metal in the liquid state, or a nozzle for spraying this metal into droplets.

16. Device according to claim 10, characterized in that the means for setting the pearls in motion circulate them generally through the enclosure, from top to bottom thereof, the fragmentation of the molten metal taking place during the meeting between the jet of molten metal and the pearls.

17. Device according to claim 16, characterized in that the means for circulating the pearls comprise the following elements: at least one upper hopper by which the balls are brought into the enclosure at its top at a predetermined rate; at least one dynamiσue element for projecting the pearls into the enclosure at a speed and in a predetermined direction against the jet of molten metal; a sieve for separating the metal powder formed; an outlet channel at the bottom of the enclosure through which the beads leave the latter and a recycling channel through which, after separation of the metal powder, the beads are returned to the top of the enclosure to the inlet hopper.

18. Metallic powder obtained according to the method of claim 2, characterized in that the particles of this powder are in the form of flakes.

19. Use of the flake powder according to claim 18, for producing a coating composition making it possible to cover substrates with a metallized layer, characterized in that these flakes are dispersed in a coating medium comprising resins and other coating additives.