EP0438004A1 - Method and device for making composite products having a metal matrix - Google Patents

Abstract

Process and device for producing composite products from an at least partially liquid metal bath subject to stirring, into which insoluble pulverulent materials are introduced. In this process, the said product is brought by gravity along a practically rectilinear direction within the said bath, a centrifugal force substantially perpendicular to the direction of entry is applied to it, it is placed in contact with the bath around the said direction in the form of a vein by imparting to it a rotary motion whose combination with the stirring causes the dispersion of the product in the bath. The device comprises a hollow rotor (1) provided in its lower part with a disc (2) in which a chamber (5) is drilled, communicating with the bath via channels (7). Inside the rotor there is a stator (9) which opens into the chamber and in its upper part communicates with a system for feeding (10) product (11). This invention makes it possible to manufacture composites in which the insoluble materials are uniformly distributed in the matrix. <IMAGE>

Description

The present invention relates to a method and a device for producing composite products with a metal matrix constituted in particular by aluminum and its alloys.

Those skilled in the art know that in most of their applications, metals are used in the form of alloys in order to improve in particular their mechanical characteristics.

But, given the evolution of techniques, the requirements of users in terms of these characteristics become more and more and these alloys then have shortcomings such as poor resistance at high temperature, low resistance to fatigue. or wear by friction or a too low modulus of elasticity.

This is why a person skilled in the art then has recourse to composite products, that is to say to products in which the metal matrix is reinforced, for example, by a dispersion of pulverulent materials generally insoluble in the metal.

However, in order for the pulverulent materials to effectively play their reinforcing function, it is important to resolve the problem of their regular distribution in the matrix so as to give the composites obtained the most homogeneous characteristics possible. Certainly, there are already different ways of developing these metal matrix composites.

Thus, the most mature techniques today use processes such as powder metallurgy, molding-forging, etc. But, they are complex and expensive and are therefore reserved for "high-end" products .

However, a potential market is also emerging for more common reinforced products. For these simpler techniques should be used. The most attractive is to disperse fine particles directly in the liquid metal or in the liquid-solid interval. Unfortunately, this technique proves to be extremely difficult to apply for several reasons:

First of all, the reinforcing particles, whether oxides, carbides, nitrides or other refractory compounds, have great difficulty in being wetted by the metal. It is therefore very difficult, on the one hand to introduce them into the metal and it is sometimes even impossible outside very strict operating conditions, and on the other hand to disperse once in the metal, the harmful clusters on a plane metallurgical, which formed during this difficult introduction.

• - la désorption de surface des particules introduites
• - l'air présent entre les particules, si celles-ci rentrent dans le métal par paquets
• - et éventuellement le dispositif lui-même d'introduction et de dispersion des particules.

Then, it turns out to be very difficult not to bring, at the same time as the particles are introduced, an unacceptable oxidation and gassing of the metal. Indeed, several causes of pollution are possible:

• - surface desorption of the particles introduced
• - the air present between the particles, if these enter the metal in bundles
• - And possibly the device itself for introducing and dispersing the particles.

The production methods known up to now have one thing in common: they seek to immerse particles thrown on the surface of the metal bath. Mention may be made, for example, of the methods described in United States patents No. 4,473,103 and No. 4,759,995.

• - cette peau forme une barrière et un piège pour les particules à introduire, d'où une difficulté supplémentaire venant s'ajouter aux problèmes de mouillabilité.
• - quand les particules pénètrent finalement dans le métal, elles entraînent avec elles une partie de cette peau, d'où une pollution du métal, et des difficultés accrues à disperser les amas, si des paquets de particules se sont faits littéralement "emballer" par cette peau.

However, an immersion of this kind is always disturbed by the fatal presence on the surface of the bath of an oxide skin:

• - This skin forms a barrier and a trap for the particles to be introduced, hence an additional difficulty which adds to the wettability problems.
• - when the particles finally penetrate into the metal, they carry with them part of this skin, resulting in pollution of the metal, and increased difficulties in dispersing the clusters, if bundles particles were literally "wrapped" by this skin.

Two basic methods are commonly used for this immersion. The first can be described as mechanical, since the powder is pushed under the surface by pallets animated by various movements. The second can be described as hydraulic since it is based on the suction of a vortex created in liquid metal.

But these two methods unfortunately have the same tendency to generate by themselves more dross on the surface of the bath and to accentuate the internal pollution of the metal.
They therefore present the paradox of making the conditions for introducing the particles more delicate and even worse and they naturally do not lead to very good results.

Placing under a controlled atmosphere (vacuum, argon, etc.) as well as various operating precautions (skimming, prior degassing, etc.) improve the situation but are not enough to make these methods truly effective. An oxide skin always forms on the surface, the powder is fatally submerged in packs, and the duration of the operation remains very long.

• - des porosités,
• - des oxydes,
• - des amas de particules globulaires (et souvent sans métal entre les grains) ou en chapelet (pris à la même peau d'oxyde).

The products thus produced therefore always present:

• - porosities,
• - oxides,
• - clusters of globular particles (and often without metal between the grains) or in strings (taken with the same oxide skin).

The introduction is not the only problem. Once introduced, the particles must be dispersed by stirring and / or intense shearing of the metal. Admittedly, intense mixing is easy to obtain if the introduction takes place by a mechanical mode: the 2 operations are then compatible.

On the other hand, the formation of a vortex requires a centrifugation of the metal, which is incompatible with intense mixing, and even tends to favor a settling of the powder introduced on the side walls. The dispersion of the particles can then only be effective by replacing the impossible stirring with intense shearing, obtained by no longer working with fully liquid metal, but with pasty metal, comprising a non-negligible solid fraction.

Faced with these drawbacks, the Applicant has therefore been led to develop a process and a production device.

According to the invention, this is a process in which one starts from an at least partially liquid metal bath subjected to stirring and into which insoluble pulverulent materials are introduced and which is characterized in that one brings by gravity and in a practically rectilinear direction, said product within said bath, a centrifugal force is applied to it substantially perpendicular to the direction of supply, it is brought into contact with the bath around said direction in the form of veins and by printing it a rotational movement whose combination with stirring causes the product to disperse in the bath.

Thus, in the invention, the products are introduced in the pulverulent state and the powder is brought directly under the surface of the bath, that is to say away from oxide skins.

Due to the centrifugal force, these particles acquire a high speed, they cannot therefore form bundles and arrive in the bath almost detached from each other.

In addition, due to their entrainment around the supply direction, they have a point of impact with the bath which is continuously moving, which combined with stirring, promotes their introduction and dispersion.

The introduction under the surface of the bath is preferably carried out so as to bring the product into contact with the bath in at least two veins, which promotes the dispersion of the particles.

Also preferably, these particles are desorbed from their moisture before being introduced into the bath in order to avoid any oxidation phenomenon which contributes to developing the oxide skins.

In addition, it is also possible to bring products from a distribution hopper within the bath under an inert atmosphere with respect to the metal of the bath, without the entire enclosure which contains the bath itself being under controlled atmosphere.

The introduction of the particles can be favored by exerting on the latter and in the direction of supply, a gas pressure which cooperates at least partially with the action of gravity, then of the centrifugal force.

The rotational movement imparted to the product is preferably carried out at a speed of between 300 and 1500 revolutions per minute; a speed below this limit gives rise to a too low centrifugal force, which does not allow good dispersion. A speed greater than this limit can on the one hand generate too great a centrifugal force which hinders the good flow of the veins, and on the other hand cause vortex effects, whereas one of the objects of the invention is precisely s 'to free it.

The bath can also be continuously renewed by adding a new metal bath on one side and drawing off the composite on the other; this is the case for example when the bath is placed in a treatment pocket disposed between an oven and a casting station.

In this case, the rate of introduction of the product is linked to the rate of the new metal bath added for the renewal of the bath. All pulverulent products can be used in this process, but preferably oxides, carbides, nitrides, borides, silicides, carbon and graphite are chosen.

In order to obtain a homogeneous dispersion, particles preferably having a particle size of between 1 μm and 5 mm are used, the finer particles wetting poorly and having difficulty flowing in veins, and the larger particles. dispersing so irregular.

The best results are nevertheless obtained in a narrower range between 5 µm and 100 µm. As regards the percentage of product for reinforcing the bath, it is preferably between 3 and 30% by volume, the low percentages hardly make it possible to modify the properties of the metallic matrix and the higher percentages leading to more mixed or less heterogeneous.

The invention preferably applies to aluminum alloys and their composite with alumina and silicon carbide.

The invention also relates to a device for implementing the method.

This device, in which means for mixing the bath and means for introducing particles are combined and which comprises a hollow rotor connected at its lower part to a circular disc immersed in the bath contained in an enclosure whose plane is perpendicular to the axis of the rotor, the side wall of which is provided with at least two blades, the mass of which is indented on the top and in the vicinity of its center so as to form a distribution chamber communicating with the hollow part of the rotor and provided on its side wall of substantially radial channels which open into the bath, is characterized in that inside the rotor and without being integral with the latter, a hollow stator elongates which opens into the chamber and communicates with its part upper with a product supply system.

Thus the device according to the invention differs from the prior art constituted for example by USP 4802656 by the presence inside the rotor of a stator which ensures the transfer by gravity of the particles of the product between a system of feeding and a chamber, and the transfer, under the action of the centrifugal force created in the chamber, of said particles towards the bath via channels.

According to this arrangement, the particles descend more easily by gravity without being braked by centrifugal force. Indeed, if it was the rotor itself which was used as a descent pipe, the centrifugal force would tend to press the powder on the side walls and therefore slow down the descent.

If we want to avoid the re-entry of air with the product, we use a supply system consisting of a closed hopper, maintained under an atmosphere of dry gas inert to the metal and tightly connected to the stator and the space separating the stator from the rotor is closed by placing a rotary joint there.

The rotor has blades at its periphery intended to set the bath in motion. Brewing is intensified and the tendency to vortex reduced by a judicious choice of the orientation of the blades and / or by the introduction of at least one baffle in the enclosure which contains the bath.

These blades, thanks to hydrodynamic effects of the zones on the periphery of the rotor also allow the particles to exit more easily from the rotor and thus avoid clogging of the channels.

• - soit une turbine de brassage fixée sur le rotor et dont le diamètre est suffisant pour lui conférer une vitesse périphérique convenable;
• - soit un système de brassage rotatif indépendant immergé dans le bain;
• - soit un système de brassage électromagnétique qui équipe l'enceinte et agit sur le bain.

Although the rotor-stator assembly essentially has the function of dispersing the particles in the bath, it also exerts, due to the rotational movement, a certain mixing of said bath. However, this mixing may be insufficient. Furthermore, it has been observed that when the speed of rotation becomes too high, the channels can become engorged with particles, resulting in a blockage in their distribution in the bath. It is therefore necessary to limit this speed and therefore the stirring of the bath can also become insufficient. This is why recourse is made in certain cases to stirring means different from the distribution rotor which can be:

• - Or a stirring turbine fixed on the rotor and whose diameter is sufficient to give it a suitable peripheral speed;
• - either an independent rotary stirring system immersed in the bath;
• - or an electromagnetic stirring system which equips the enclosure and acts on the bath.

To have total security against pollution by the surrounding atmosphere, it is also preferable to have a bath above the bath. atmosphere of dry gas inert to metal.

Such devices can be installed in ovens as well as in ladles or pouring heads.

The invention will be better understood with the aid of the attached drawing board, which represents in FIG. 1 an axial vertical section view of a device according to the invention, the rotor of which is immersed in a liquid metal bath contained in a crucible and in FIG. 2 a sectional view of the rotor along a horizontal plane of trace XY in FIG. 1.

In more detail, in FIG. 1 there is a hollow rotor 1 provided at its lower part with a circular disc 2 immersed in a metallic bath 3 liquid contained in a crucible 4. This disc has, in the vicinity of its center, a chamber 5 distribution which communicates with the hollow part 6 of the rotor. This chamber is provided on its side wall with substantially radial channels 7 which open into the bath on the side wall of the disc, on and / or between these blades 8 with which said wall is fitted. Inside the rotor and without connection with its internal wall, a stator 9 elongates, the lower part of which opens into the distribution chamber and the upper part of which communicates with a hopper 10 for supplying product 11 to be dissolved or dispersed in the mass of the bath.

A rotating joint 12 placed between the stator and the rotor allows the chamber to be isolated from the atmosphere. The rotor drive mechanism is not shown.

FIG. 2 shows the disc 2 provided with blades 8 having, near its center, a distribution chamber 5 provided on its side wall with channels 7 which open into the bath between the blades 8.

The invention can be illustrated using the following application examples:

EXAMPLE 1

In an oven, 3 kgs of aluminum alloy AS7G 0.3 were produced. (alloy containing by weight 7% silicon and 0.3% magnesium). To this base metal have been added, by the present method used in its simplest version (air hopper), 10% by weight of silicon carbide particles with particle size F240 (approximately 50 μm). The introduction lasted 8 min and did not cause any deterioration in the quality of the metal. The particles were properly distributed and did not show any clumps. The rotor used, of diameter 60 mm, included 4 blades as well as 4 holes of diameter 1.5 mm for the introduction of the particles. The rotation speed was 700 rpm.

EXAMPLE 2

In an oven, 3 kgs of aluminum alloy A-S7G 0.3 were produced. To this base metal were added 10% by weight of particles of silicon carbide with a particle size F600 (approximately 10 μm). Given the fineness of the particles, this method was used this time in the version where the hopper is under a neutral atmosphere (argon). The introduction lasted 25 min and did not cause any deterioration in the quality of the metal. The particles were well distributed and did not show any clumps. The rotor used, of diameter 60 mm, included 4 blades, as well as 8 channels of diameter 2.0 mm for the introduction of the particles. The rotation speed was 700 rpm.

Claims (23)

Process for the preparation of composite products from an at least partially liquid metal bath subjected to stirring and into which insoluble pulverulent materials are introduced, characterized in that said product is brought by gravity and in a practically rectilinear direction to within said bath, a centrifugal force is applied to it substantially perpendicular to the direction of supply, it is brought into contact with the bath around said direction in the form of veins by imparting to it a rotational movement whose conjugation with stirring causes the dispersion of the product in the bath. Process according to claim 1, characterized in that the product is brought into contact with the bath in at least two veins. Method according to claim 1 characterized in that the product is desorbed from its humidity before bringing it into the bath. Process according to Claim 1, characterized in that the product is brought under an inert gas atmosphere with respect to the metal. Method according to claim 1 characterized in that a gas pressure is exerted on the product in the direction of supply. A method according to claim 1 characterized in that the rotational movement takes place at a speed between 300 revolutions / min and 1500 revolutions / min. Process according to Claim 1, characterized in that the bath is continuously renewed by adding a new metal bath on one side and drawing off the composite product on the other. Process according to Claim 7, characterized in that the rate of introduction of the product is linked to the rate of new metal bath added for the renewal of the bath. Method according to claim 1 characterized in that the product belongs to the group consisting of oxides, carbides, nitrides, borides, silicides, carbon and graphite. A method according to claim 1 characterized in that the product has a particle size between 1 µm and 5 mm. Process according to Claim 9, characterized in that the particle size is between 5 µm and 100 µm. Method according to claim 1, characterized in that the proportion of product in the bath is between 3 and 30% by volume. A method according to claim 1 characterized in that the metal bath is an aluminum alloy. Method according to claim 13, characterized in that the product belongs to the group consisting of alumina and silicon carbide. Device for carrying out the method according to claim 1, in which means for mixing the bath and means for introducing particles are combined and which comprises a hollow rotor (1) connected by its lower part to a disc (2) circular immersed in the bath (3) contained in an enclosure (4) whose plane is perpendicular to the axis of the rotor, the side wall of which is provided with at least two blades and the mass of which is indented on the top and in the vicinity from its center so as to form a distribution chamber (5) communicating with the hollow part of the rotor and provided on its side wall with substantially radial channels (7) which open into the bath, characterized in that inside the rotor and without being integral with the latter, a hollow stator (9) extends which opens into the chamber and communicates at its upper part with a product supply system (10) (11). Device according to claim 15 characterized in that a rotary joint is placed between the rotor and the stator. Device according to claim 15 characterized in that the feed system is a closed hopper and maintained under a gas atmosphere dry and inert towards metal. Device according to claim 15 characterized in that the enclosure is internally equipped with at least one baffle. Device according to claim 15 characterized in that the rotor is equipped on its axis with a stirring turbine. Device according to claim 15 characterized in that the rotor is associated with at least one rotary stirring system immersed in the bath. Device according to claim 15 characterized in that the enclosure is equipped with an electromagnetic stirring system of the bath. Device according to claim 15 characterized in that the bath is placed in the enclosure under an atmosphere of dry gas inert with respect to the metal. Device according to claim 15 characterized in that the enclosure belongs to the group consisting of ovens, pockets, pouring heads.

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