FR2533150A2 - Method for producing composite materials based on a preform produced in a first reinforcement component in association with a second component consisting of a light metal alloy, as well as the products obtained by this method - Google Patents

Abstract

The method is characterised in that the composite material is formed starting with a preform comprising silicon carbide in association with a light metal alloy. The preform is produced in silicon carbide in a first stage and pretreated with a fluorinated agent containing at least a double fluoride of a transition element or a metallic element or an element belonging to the rare earths and the pretreated preform is impregnated with light metal alloys. Application to the production of composite pieces with fibre reinforcements.

Description

PROCESS FOR THE PRODUCTION OF COMPOSITE MATERIALS BASED
OF A PREFORMEREALIZED IN A FIRST COMPONENT OF
REINFORCEMENT IN ASSOCIATION WITH A SECOND COMPONENT (:: 0NSTITUE D't N ALLIAGE LECL: R, AS WELL AS THE PRODUCTS- GET-
NUTED BY THIS PROCESS
The present invention relates to processes for the preparation of composite materials, and more particularly to composites comprising a preform produced in a first component to form the main framework of the composite, and a second wetting component, in the liquid state, this first component, in order to obtain this composite material with all the qualities which are inherent in this kind of material.

The strengthening of light alloys by fibers with high mechanical characteristics has not hitherto known a development comparable to that of plastics, mainly due to the difficulties encountered on the technical level during its implementation.

However, the metal matrix fibrous composites have certain interesting specific characteristics:. possibility of use in temperature, absence of aging, stability with regard to many media, high electrical and thermal conductivities, etc.

Among these materials, matrix composites consisting of an aluminum-based alloy reinforced with fibers based on silicon carbide such as those resulting from the pyrolysis of a polycarbosilane precursor are of particular interest because of: (1) characteristics mechanical ex of the availability of silicon carbide-based fibers, (2) the low density, low melting point and low cost of light alloys and (3) the low chemical reactivity of silicon carbide fibers - compared to that of carbon fibers - with aluminum on the one hand, - and with oxygen in the air on the other hand, at the temperatures of preparation and use of these composites. of composites, in the form of complex parts, is currently considerably hampered by the fact that their simplest production process, liquid impregnation, is complicated by the poor wetting of the fibers by most of the alloys of luminium in the liquid state.

A silicon carbide surface is, in fact, like a graphite surface, poorly wetted by liquid aluminum (and most of its usual foundry alloys) at atmospheric pressure and at usual temperatures (y 750 C) the foundry of light alloys.

As a result, the liquid aluminum is not likely to penetrate easily by capillary action at the pore surface - some of which are very small in size - present (in the wicks of silicon carbide fibers or in the made-up porous preforms from them, especially if the volume fraction of fibers is high.

French patent application g2106448 describes a process for the preparation, by liquid means, of composites consisting of a fibrous carbon reinforcement coated in a matrix of aluminum base alloys
It has now been established that this process could be extended to homologous composites comprising a fibrous reinforcement based on silicon carbide.

More specifically, the subject of the present invention is a process for preparing composite materials formed on the basis of a preform produced in a first reinforcement component, with a second component playing the role of matrix according to claim 1 of the main patent, characterized by the fact that said composite material is formed on the basis of a preform comprising silicon carbide in association with a light alloy, in particular based on aluminum, said preform being produced in a first step in said silicon carbide, said preform being pretreated with a fluorinated agent containing at least a double fluoride of a transition element such as titanium, zirconium, hafnium, vanadium, niobium or tantalum or a metallold element such as boron or silicon.

The invention will now be described with the aid of Figures I and 2 showing the various stages of two embodiments according to the invention, depending on whether the fibrous preform is produced outside the mold (Fig. 1) or within the mold, from fibrous semi-finished products (Fig. 2).

Referring to FIG. 1, a fibrous preform based on silicon carbide is produced 1 in order to reinforce all or part of the part to be produced, using the fibrous processing techniques, starting from the fibers themselves, or better from half
fibrous products (felts, mats, fabrics, multidirectional fibrous architectures of fibers or rods consolidated or not by silicon carbide) or even of porous C / SiC or SiC / SiC composites. Depending on the application envisaged, the fibrous preform may correspond to the entire volume of the mold (integral reinforcement) or occupy only a part of the latter (local reinforcement). To be easily impregnated with the liquid alloy, the fibrous preform must have a relatively large open porosity.

The preform thus produced is then pretreated 2 using fluorinated species 3 before being introduced 4 into a prepared mold 6. This operation which constitutes, in terms of technical implementation,
The essential step of the process according to the invention can be carried out in various ways depending on whether the pretreatment is carried out in the solid state or using a solution (or a suspension), as has been described. in patent application 82/06448. Although the two routes are also usable, the pretreatment using a solution (such as for example aqueous solutions of K2ZrS6 alone or mixed with other fluorinated species), which must necessarily wet the preform well, has the advantage of distributing the active fluorinated species more uniformly within the preform itself and of limiting the quantity of reagents to the value just necessary to subsequently obtain good wetting for the metal alloy liquid. The pre-treatment of the preform can be carried out in an inert atmosphere or even in air.

After treatment, the fiber preform placed in the mold 4 of the type used in the foundry of light alloys (metal shell possibly covered, on its internal face, with a protective coating, for example) is impregnated 7 with the liquid metal 8 (AS7G alloys, for example).

Referring now to FIG. 2, the preform can be produced, using fibrous semi-products (fabrics, mats, etc.), directly in the mold. In this case, the fibrous semi-products are first pretreated 1 with the fluorinated reagent 3 outside the mold (by immersion in a solution (or suspension) of the fluorinated reagent, then wringing). They are then draped against the walls of the mold and a new addition, of fluorinated reagent is possibly made. The mold is then closed and preheated 4, this preheating eliminating, moreover, the excess of solvent.

Referring to the two figures i and 2, the casting operation is carried out according to the rules of the art. It can advantageously be carried out in a mold previously placed under vacuum, the metal being then introduced under a slight overpressure to promote its progression within the fibrous preform. In the same way, the impregnation is favored if the mold / preform assembly has been previously - preheated to a sufficient temperature (the higher the less porous the preform) so as not to cause too rapid cooling of the alloy in contact with the preform.

The invention as described in application 82/06448 and in the present one, therefore differs from the process for the preparation of composite fibers of silicon carbide - aluminum base alloy, described by S.

 YAZIMA.

Indeed, this process, which does not require any specific pretreatment of the fibers, is based on an impregnation in liquid phase under high pressure (use of a press with heating plates) inside a vacuum sealed bag. . As a result, it does not make it possible to obtain composite parts, of massive or complex shape, by the techniques commonly used in the casting of light alloys.

The invention will now be illustrated with the aid of a few examples without the latter, however, being able to be considered as a limitation of its scope, as will be clear to those skilled in the art.

Example 1:
A fibrous preform in the form of a parallelepiped (26x50x50mm) was produced from fabrics of fibers based on silicon carbide (NIKALON type fibers of iOvV m of diameter). The fiber volume fraction was about 20%. An aqueous solution of unsaturated K2ZrF6 was prepared by dissolving this salt in water (at a rate of 20 grams of K2ZrF6 per 100 ml of solvent) at a temperature sufficient for there to be no solid residue. The fibrous preform has been immersed, in air, in this solution which perfectly wets the fibers based on silicon carbide and rises by capillary action in all the interstices of the preform. After drying in the oven (110-1200C), it was observed that a thin and uniform layer of fluorinated product covered the fibers.The preform thus pretreated was placed in a steel shell (including a protective coating) of the type of those commonly used in light alloy foundries. Shell and preform were preheated to around 5500C, then casting was carried out in air with the alloy AS7G06 at 7500C. After cooling, the part was cut off and examined under a metallographic microscope and a scanning electron microscope. fi appeared that, under the conditions of the test, the aluminum alloy wetted the fibers perfectly and penetrated by capillary rise into all the interstices of the preform, leading to a composite with approximately 20% of fibers and Approximately 80% of AS7G06 matrix. In addition, the study of the distribution of the various chemical elements involved in the pretreatment, carried out using an electronic microanatyster with X-ray spectrometry, has not made it possible to detect significant accumulations of chemical species which could have been detrimental to the properties of the composite.

Example?
The test described in Example 1 was repeated on an identical fibrous preform which had not received the pretreatment with the fluorinated agent
K2ZrF6 before impregnation. This test resulted in failure. After cooling and analysis, it appeared in fact that the liquid AS7CE06 alloy had not wetted, at 750 "C, the fibers based on silicon carbide and had, therefore, practically not penetrated within the fibrous preform.

These two examples therefore show that a pretreatment, by a process such as that recommended in the present application, is essential for developing, by foundry techniques and from a porous fibrous preform, a composite consisting of a reinforcement fibrous based on silicon carbide and an aluminum-based matrix, at least when the fibers are of very small diameter and when their volume fraction in the material is relatively high Example 3
The test described in Example 1 was repeated from a woven multidirectional preform made using fibers based on silicon carbide (NIKALON fibers 10 mm in diameter) in which the volume fraction of fibers was around 32%. After pretreatment with the K2ZrF6 tltiorê agent in aqueous solution, drying, preheating of the preform in the mold cast in air (alloy AS7G06 at 750 "C) and cooling, it appeared that said preform had been suitably impregnated, leading to a composite has about 32 by volume by fiber.

However, and as has been pointed out above, in order to obtain good impregnation, in the case of this preform less porous than those used in Examples 1 and 2, preheating before casting had to be carried out at a higher temperature (of around 650 "C) so that the liquid alloy does not solidify too quickly in the shell in contact with the preform and has time to penetrate all the pores. Due to the excellent resistance to oxidation of silicon carbide, this operation has not resulted in any significant degradation of the fiber reinforcement properties, whereas the same operation should have been carried out in a neutral atmosphere in the case of a preform of the same type consisting of carbon fibers.

Example 4
Contrary to the previous examples where the fibrous preform was produced outside the mold, pretreated with the fluorinated agent and then introduced into the pout before casting, in this example, the fibrous reinforcement used in the form of fiber fabrics based on silicon carbide (NIKALON fibers) pretreated with the fluorinated agent K2ZrF6 in aqueous solution and then wrung out, was draped while still wet on the walls of the mold, then layer upon layer so as to fill practically the entire volume of the latter. After closing the shell, mold and fibrous reinforcement were brought to 1200C in air, to remove excess water, then preheat to around 4500C. Impregnation by alloy AS7G06 at 7500C was carried out, under satisfactory conditions, in air.

Example 5
The procedure described in Example 1 was repeated, the preform consisting of a local reinforcement (4 × 20 × 10 mm) of ceramic-ceramic type composites based on fibers and on silicon carbide matrix (of the type of those which are marketed by SEP under the CERASEPJ brand.

To allow good attachment, at the time of casting, of the insert in the metal, the cornposite reinforcement had a significant residual porosity on its internal face and, on the contrary, practically zero residual porosity on its external face. The composite insert, pretreated with the fluorinated agent K2ZrF6 as indicated in the example so that the pores which it presented on its internal face have received a. thin layer of fluorinated product, was placed at the bottom of the mold and the casting carried out using alloy AS7C06 at 750 "C (after preheating to 500-6006 approximately) and under partial vacuum. The analyzes carried out after cooling showed that the liquid alloy had penetrated into the pores of the insert, to a depth of approximately 2 to 3 mm, thus ensuring good anchoring of the composite insert in the metal.

Claims (10)

K E V E N D I C A T I O N S 1 / Process for the preparation of composite materials formed on the basis of a preform produced in a first reinforcement component with a second component acting as a matrix according to claim 1 of the main patent, characterized in that said composite material is formed based on a preform comprising silicon carbide in association with a light alloy, in particular based on aluminum, said preform being produced in a first step in said silicon carbide, said preform being pretreated with a fluorinated agent containing at least a double fluoride of a transition element such as titanium, zirconium, hafnium, vanadium, niobium or tantalum or a metallold element such as boron or silicon, associated with a cation like alkali cations, ammonium NH4 +, calcalinoterrous or rare earths, and to impregnate said preform pre-prepared with light alloys, in particular aluminum. 2 / A method according to claim 1, characterized in that the double fluoride is taken in one of the following elements: potassium fluotitanate K2TiF6, potassium fluozirconate K2ZrF6, a mixture of potassium fluotitanate K2TiF6 and potassium fluoborate KBF4. 3 / Method according to one of the preceding claims, characterized in that to said fluorinated species is added at least one alkali, alkaline earth or rare earth halide. 4 / Method according to one of the preceding claims, characterized in that the fluorinated agent is in glassy form. 5 / Method according to one of claims 2 to 4, characterized in that the fluorinated agent is used in the solid state, in the form of a powder with fine particle size dispersed on the surface of the preform, the temperature of the preform being brought to a temperature slightly lower than that of the melting of the fluorinated agent so as to form a uniform and sufficiently resistant fluorinated crust around said preform. 6 / Method according to one of claims 2 to 4, characTérisé in that the fluorinated agent is introduced into the pores of the preform in the form of a suspension or a solution in a solvent capable of wetting the preform , the solvent being subsequently removed physically. 7 / A method according to claim 6, characterized in that said solution is a saturated aqueous solution, or almost saturated, in potassium fluozir conate K22rF6 taken at a temperature ensuring the dissolution of a sufficient quantity of fluozirconate. 8 / A method according to claims 2 to 7, characterized in that said pre-treatment of the preform with the fluorine agent is carried out in the evening in an inert gas atmosphere, either under vacuum, or simply simply in air. 9 / A method according to one of claims 2 to 8, characterized in that the preform is a local composite insert of the ceramic-ceramic type comprising fibers and a matrix based on silicon carbide and having on the faces in contact with the metal a large open porosity which cannot be filtered, after pretreatment with the fluorinated agent, by light liquid alloys. 10 / A method according to claim 9, characterized in that said fibrous preform is produced and treated with the fluorinated agent outside a mold, placed in the mold, the assembly being preheated to a sufficient temperature depending in particular porosity of the preform and which is between 3500C and the melting temperature of the alloy, said preform then being impregnated within said mold using a light alloy according to foundry techniques.