EP3325681B1 - Process for manufacturing an al/al3b48c3 composite - Google Patents

Description

TECHNICAL AREA

The present invention relates to the field of synthesis of metal matrix composite materials and ceramic particulate reinforcements.

In particular, the invention relates to a process for producing a composite material having an aluminum matrix in which Al ₃ B ₄₈ C ₂ reinforcements are dispersed.

This process can notably be applied in the fields of aeronautics and the automobile.

STATE OF THE PRIOR ART

In the automotive and aerospace industries, manufacturers are seeking lightweight and durable materials. However, most light industrializable materials are very weak.

This is the reason why metal matrix materials (CMM), which comprise a metal matrix (metal or metal alloy) in which are incorporated reinforcements (particles, fibers or other) metal or ceramic, are particularly preferred. Indeed, the advantage of CMM compared to light alloys (based on aluminum, magnesium or titanium) is that they have ratios E / ρ (elastic modulus / density) and σ / ρ (limit of elasticity / density) very high.

One of the main problems to solve in the development of a metal matrix material is that of the chemical reactivity between the matrix and the reinforcement. This reactivity is indeed necessary so that the interface between the matrix and the reinforcement is mechanically resistant, but it can also lead to effects. catastrophic side effects for the composite material. This reactivity must therefore be strictly controlled.

Take the example of the composite material Al / B ₄ C, which is a particularly interesting composite material because boron carbide is one of the hardest known materials, it is lightweight and has a higher melting temperature at 2400 ° C.

The chemical reactivity between the Al phase and the B ₄ C phase of the Al / B ₄ C composite is necessary because the liquid aluminum does not wet the B ₄ C particles, which implies a difficulty in ensuring within the composite material an intimate interface between the two phases. As a result, the adhesion work, ie the chemical force of the interface, will be weak within the composite material, leading to a mechanically weak interface. However, in most metal matrix composites and ceramic reinforcement, there is an objective of transfer of the mechanical load from the matrix to the reinforcement through the interface: it must therefore be mechanically resistant.

However, the chemical reactivity must also be controlled. Indeed, the equilibrium between the phases in the Al-BC system indicates that Al and B ₄ C are not in equilibrium below a temperature which is not precisely known, but which is estimated in the literature to be superior at 1400 ° C.

Thus, any synthesis based on the mixture of Al and B ₄ C precursor powders and carried out at a temperature below 1400 ° C. thus leads to the decomposition of the B ₄ C reinforcement by Al and to the formation of Al ₃ BC ₃ carbide. . The latter, although less sensitive to hydrolysis in the presence of moisture than Al ₄ C ₃ carbide, still remains subject to this phenomenon which leads to the release of large amounts of CH ₄ gas. The gas produced in the core of the composite material then generates stresses that lead to the ruin of the composite material (return to the powder state). Moreover, other phases such as AlB ₂ and AlB ₁₂ borides can also be formed during the interaction between Al and B ₄ C. The fragility of these phases then induces embrittlement of the composite material.

EP0322336 discloses a process wherein Al / Al ₃ B ₄₈ C ₂ is formed from the high temperature reaction of molten aluminum with sintered B ₄ C powder.

JC Viala et al., Journal of Material Science 32, 4559 (1997) ) discloses the decomposition of AlB ₂ into Al ₃ B ₄₈ C ₂ in the presence of B ₄ C.

US5017217A discloses methods for obtaining Ti-Al-B-based composite via the peritectic decomposition reaction of AlB ₂ with TiH ₂ .

A synthesis carried out at high temperature (at least greater than 1400 ° C) in the area where Al and B ₄ C are in equilibrium would be confronted with the same difficulty, but this time during the cooling then leading to the same consequences.

The solutions envisaged in the literature to produce a composite Al / B ₄ C are aimed at solving the reactivity problem by acting on the kinetics of reaction, either by placing at very low temperature (cryogenic) in order to slow down as much as possible. reaction kinetics between Al and B ₄ C, either by limiting the interaction time at high temperature during the consolidation / shaping step. But these solutions are not suitable for industrial production.

The first solution, which will be called the cryogenic method, was developed by Julie Schoenung at the University of California, Davis. This method encounters the difficulty of implementing high energy grinding in liquid nitrogen for large quantities of material. The change of scale from laboratory to industrial production seems difficult. Moreover, this method can not ignore a consolidation step that must be carried out hot.

The second solution is to minimize the duration of the hot consolidation step to limit as far as possible the progress of the reaction between Al and B ₄ C. The main difficulty lies again in the amount of usable material. Indeed, the hot forming requires that the matrix Al is brought to a temperature sufficient to be subject to plastic deformation by creep. However, in the case of a large volume of material, the standardization of the temperature in the entire volume requires a significant temperature maintenance time as well.

A third solution combining these two approaches was proposed by Julie Schoenung's team and was the subject of a patent application (document [1] ). It consists of mixing and grinding Al and B ₄ C precursor powders in liquid nitrogen (freeze-grinding), cold-compacting the mixture and then sintering the compacted mixture by the SPS technique (for "Spark Plasma Sintering"). In English), so-called flash sintering, which allows to wear the compacted mixture at high temperature for a shorter time than by conventional heating techniques. This third However, this solution does not solve the problem of scaling for the cryomilling step.

Thus, because of the reactivity between Al and B ₄ C, the conventional methods of mixing, compacting and densifying the powders are unsatisfactory, except to implement them in the context of cryogenic techniques. However, such cryogenic techniques are cumbersome to implement, expensive and are not suitable for large volume production of material.

The inventors have therefore set themselves the goal of designing a process for producing an alternative composite material to the Al / B ₄ C composite, which has properties similar to those of the Al / B ₄ C composite, while being able to be produced industrially. .

STATEMENT OF THE INVENTION

1. a) la mise en place d'une poudre de formule chimique AlB₂ dans la cavité d'un creuset en graphite ;
2. b) la fermeture de la cavité par un élément en graphite ;
3. c) le chauffage du creuset à une température au moins égale à 960°C et inférieure ou égale à 1400°C pour obtenir la formation de précipités du carbure mixte de formule chimique Al₃B₄₈C₂ dans de l'aluminium liquide ;
4. d) le refroidissement du creuset pour solidifier l'aluminium liquide ;
5. e) l'élimination du creuset ;

moyennant quoi on obtient la pièce en matériau composite Al/Al₃B₄₈C₂.This object is achieved by a method of manufacturing a part made of a composite material Al / Al ₃ B ₄₈ C ₂ comprising an aluminum matrix in which particles of a mixed carbide of chemical formula Al ₃ B ₄₈ C are dispersed. ₂ , said method comprising the following steps:

1. a) the establishment of a powder of chemical formula AlB ₂ in the cavity of a graphite crucible;
2. b) closing the cavity by a graphite element;
3. c) heating the crucible at a temperature of at least 960 ° C and less than or equal to 1400 ° C to obtain the formation of mixed carbide precipitates of chemical formula Al ₃ B ₄₈ C ₂ in liquid aluminum;
4. d) cooling the crucible to solidify the liquid aluminum;
5. e) elimination of the crucible;

whereby the composite material part Al / Al ₃ B ₄₈ C ₂ is obtained.

Preferably, the graphite element for closing the cavity is a graphite piston.

When placed in the graphite crucible in step a), the AlB ₂ powder can be in various forms. According to a first variant, the powder is placed in the crucible in a compressed form, for example in the form of one or more pellets. According to a second variant, the powder is placed in the crucible in a powder form and step b) further comprises a compression of the powder. It is preferred to use the powder in powder form and compress it in the cavity of the crucible, since the AlB ₂ diborure is weakly ductile, obtaining a compact is difficult.

According to a preferred variant of the invention, when the powder is placed in the crucible in pulverulent form, step b) further comprises the compression of the powder. Preferably, the compression of the powder and the closing of the cavity of the crucible are obtained by the use of a graphite piston. The piston is dimensioned so as to slide in the opening of the crucible in order to compress the powder and to close this opening.

Preferably, in step c), the crucible is heated at a temperature ranging from 1000 ° C. to 1400 ° C. for a period ranging from 5 minutes to 30 minutes.

Preferably, the descent in temperature in step d) is fast. This makes it possible to limit the decomposition reactions of the phases formed at high temperature. Preferably, the cooling in step d) comprises a descent in temperature with a speed greater than or equal to 10 ° C / s until reaching 600 ° C.

The elimination of the crucible in step e) can be obtained separating the ingot of composite material obtained at the end of step d) (and which forms the composite material part to be obtained) of the crucible or else by proceeding to a turning operation that will destroy the crucible.

The Al / Al ₃ B ₄₈ C ₂ composite material produced according to the process according to the invention is a good alternative to the Al / B ₄ C composite material. Indeed, the ternary compound τ ₃ -Al ₃ B ₄₈ C ₂ , which forms the reinforcement, is in equilibrium with the matrix Al according to the literature. In addition, it has properties similar to those of B ₄ C, as can be seen from the table below, and is therefore a credible alternative to B ₄ C for the production of a ceramic matrix composite and reinforcement of carbide type rich in boron. Compound Density (g.cm ^-3 ) Module (GPa) Knoop (GPa) Vickers (GPa) Toughness (MPa.m ^1/2 ) Thermal conductivity (Wm ^-1 .K ^-1 ) Al ₃ B ₄₈ C ₂ 2.62 - 23 - 37 25 - 30 4 - 5.3 19.6 (310K) B ₄ C 2.52 450 - 470 39 - 37 38 3 - 4 30 - 42

According to the method of the invention, the matrix and the reinforcement (and therefore the interface) are formed at high temperature and in-situ, which has several advantages.

First of all, it makes it possible to overcome the difficulty of eliminating the oxide films present on the AlB ₂ particles. The reactivity between AlB ₂ and the graphite crucible carbon removes this oxide barrier which limits the wetting, adhesion and mechanical strength of the interface.

The reinforcements of the composite are obtained during the decomposition of the AlB ₂ particles by germination / growth in the liquid phase. The matrix / reinforcement interface is therefore chemically clean (no impurities, oxides or other) and thus leads to optimum resistance of the interface.

The reinforcements are formed in situ and have not had to undergo a grinding cycle, grinding which is often likely to induce defects which are then starting points for the cracking of the composite material.

Furthermore, the method of the invention also has the advantage of simplicity of its implementation. In particular, it makes it possible to obtain a dense ingot directly from the internal geometry of the graphite crucible, since the ingot is shaped in the liquid state in the graphite crucible.

The potential applications of the method which is the subject of the invention are numerous. These include areas requiring the production of lighter parts (parts for aeronautics (aircraft, helicopter, etc.), for the automobile, etc.). We can also mention the areas that require the production of parts with high thermal conductivity by the presence of the aluminum matrix, but low coefficient of thermal expansion by the presence of a rate of particles of reinforcements high. Such parts can evacuate heat and have good dimensional stability and are therefore of interest for applications in the space sector or in power electronics. Finally, the production of lightweight parts comprising a high rate of a ceramic reinforcement having a high hardness can also find application in the ballistic sector for personal protection purposes.

Other features and advantages of the invention will appear better on reading the additional description which follows and which refers to the appended figure.

Of course, this additional description is given by way of illustration of the invention and does not constitute in any way a limitation thereof.

BRIEF DESCRIPTION OF THE SINGLE FIGURE

The single figure is an image obtained by scanning electron microscopy of an ingot obtained according to a first embodiment according to the method which is the subject of the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The method which is the subject of the invention is based on a so-called reactive synthesis method. Indeed, the matrix and the reinforcement of the composite material are obtained in situ by a reaction between two precursors. The precursors chosen are aluminum diboride (AlB ₂ ) and graphite (C). AlB ₂ is in the form of a powder and is placed in a crucible which is made of graphite. Preferably, the same graphite element, preferably a graphite piston, is used to compact the powder and to seal the cavity of the crucible. The whole is then heated to high temperature. The heating is carried out at a temperature above the decomposition temperature of AlB ₂ , that is to say the temperature from which one begins to have a liquid phase. In fact, at the decomposition temperature of AlB ₂ , i.e. at 960 ° C, two phases are obtained, a liquid phase and a solid phase.

Preferably, the heating is carried out at a temperature of between 1000 ° C. and 1400 ° C., preferably between 1200 ° C. and 1400 ° C., for a period of time. which can be variable, but will generally be between 5 and 30 minutes. In fact, the duration of heating at a given temperature is adjusted according to the desired microstructure: the longer the heating time, the larger the size of the reinforcing particles.

Since the two phases AlB ₂ and C are not in equilibrium, they react with each other to form Al and the mixed carbide Al ₃ B ₄₈ C ₂ .

Preferably, the climbs and descents in temperature are fast, in order to limit both the size of the reinforcing particles and their decomposition during cooling.

At the end of the high temperature synthesis, the graphite crucible can be removed by simple machining, releasing the ingot of CMM composite material contained therein. Since this was obtained at a temperature higher than the Al fusion, the presence of the matrix in the liquid state makes it possible to directly obtain a composite with a relative density greater than 99.5%.

We will now make a composite material Al / Al ₃ B ₄₈ C ₂ according to the method object of the invention.

In a graphite crucible 8 mm in diameter, 5 mm high and whose walls have a thickness of 2 mm, 750 mg of aluminum diboride powder (AlB ₂ ) are placed. The whole is heated at 1400 ° C for 15 minutes. The heating ramp is about 340 ° C / min, while cooling is obtained by dipping the crucible directly in an oil bath cooled to 0 ° C.

The microstructure of the Al / Al ₃ B ₄₈ C ₂ composite thus obtained is observed under SEM (single figure). The white phase corresponds to the aluminum matrix and the black particles correspond to the reinforcement phase Al ₃ B ₄₈ C ₂ . It is found that the reinforcements are homogeneously dispersed in the matrix and have a size of between 200 nm and 5 μm (average size of about 700 nm).

The method of the invention makes it possible to create an interface between a matrix and a reinforcement which is mechanically strong, but without leading to the decomposition of the reinforcement and the creation of deleterious secondary phases for the properties of the composite. Indeed, during the reactive synthesis between AlB ₂ and graphite (C), there are very few minor phases that are created and the composite thus essentially comprises a phase of AI (forming the matrix) and a phase of Al ₃ B ₄₈ C ₂ (reinforcement), the minor phases being present in minimal quantities.

Finally, the process according to the invention provides a new synthetic route for producing, in a simple manner and in quantity, Al-matrix composite materials reinforced with particles of a mixed boron (B) and aluminum (Al) carbide. whose properties are close to those of a B ₄ C reinforcement.

REFERENCE CITEE

1. [1] US 11/033,099

Claims (7)

1. Method for manufacturing a part made from an Al/Al₃B₄₈C₂ composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al₃B₄₈C₂ are dispersed, said method comprising the following steps:

   a) placing a powder of chemical formula AlB₂ in the cavity of a graphite crucible;

   b) closing the cavity by means of a graphite element;

   c) heating the crucible to a temperature of at least 960°C and less than or equal to 1400°C in order to obtain the formation of precipitates of the mixed carbide of chemical formula Al₃B₄₈C₂ in liquid aluminium;

   d) cooling the crucible in order to solidify the liquid aluminium;

   e) removing the crucible;

   thereby the part made from Al/Al₃B₄₈C₂ composite material is obtained.
2. Method according to claim 1, wherein the graphite element used to close the cavity is a graphite piston.
3. Method according to claim 1 or claim 2, wherein the powder is placed in the crucible in a compressed form.
4. Method according to claim 1 or claim 2, wherein the powder is placed in the crucible in a powdery form and step b) further comprises a compression of the powder.
5. Method according to claim 4 when it is dependent on claim 2, wherein the compression of the powder and the closure of the cavity of the crucible are obtained by the use of a graphite piston.
6. Method according to any of claims 1 to 5, wherein, at step c), the crucible is heated to a temperature ranging from 1000°C to 1400°C for a period ranging from 5 minutes to 30 minutes.
7. Method according to any of claims 1 to 6, wherein the cooling at step d) comprises a temperature drop at a rate greater than or equal to 10°C/second until it reaches 600°C.

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