PROCESS FOR MANUFACTURING A PART MADE OF AN AI/AI3B48C2 COMPOSITE MATERIAL

Abstract

A method for manufacturing a part made from an Al/Al.sub.3B.sub.48C.sub.2 composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al.sub.3B.sub.48C.sub.2 are dispersed. The method comprises the following steps: a) placing a powder of chemical formula AlB.sub.2 in the cavity of a graphite crucible; b) closing the cavity by use 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 Al.sub.3B.sub.48C.sub.2 in liquid aluminium; d) cooling the crucible in order to solidify the liquid aluminium; e) removing the crucible; thereby the part made from Al/Al.sub.3B.sub.48C.sub.2 composite material is obtained.

Claims

1. Method for manufacturing a part made from an Al/Al.sub.3B.sub.48C.sub.2 composite material comprising an aluminium matrix in which particles of a mixed carbide of chemical formula Al.sub.3B.sub.48C.sub.2 are dispersed, said method comprising the following steps: a) placing a powder of chemical formula AlB.sub.2 in a 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 formation of precipitates of mixed carbide of chemical formula Al.sub.3B.sub.48C.sub.2 in liquid aluminium; d) cooling the crucible in order to solidify the liquid aluminium; e) removing the crucible; thereby the part made from Al/Al.sub.3B.sub.48C.sub.2 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, wherein the powder is placed in the crucible in a compressed form.

4. Method according to claim 1, 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, 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 claim 1, 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 claim 1, 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.

Description

BRIEF DESCRIPTION OF THE SINGLE FIGURE

[0036] The single FIGURE is an image obtained by scanning electron microscopy of an ingot obtained according to a first embodiment according to the method that is the subject matter of the invention.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0037] The method that is the subject matter of the invention is based on a so-called reactive synthesis method. This is because the matrix and the reinforcement of the composite material are obtained in situ by a reaction between two precursors. The precursors chosen are aluminium diboride (AlB.sub.2) and graphite (C). AlB.sub.2 is in the form of a powder and is placed in a crucible that is made from graphite. Preferably, the same graphite element, preferably a graphite piston, is used for compacting the powder and for hermetically closing the cavity of the crucible. The whole is then raised to high temperature. The heating is carried out at a temperature higher than the decomposition temperature of AlB.sub.2, that is to say the temperature as from which there begins to be a liquid phase. In fact, at the decomposition temperature of AlB.sub.2, that is to say 960 C., two phases are obtained, a liquid phase and a solid phase.

[0038] Preferably, the heating is carried out at a temperature of between 1000 C. and 1400 C., preferentially between 1200 C. and 1400 C., for a period which may be variable but which will generally be between 5 and 30 minutes. In fact, the duration of the heating at a given temperature is adjusted according to the microstructure that it is wished to obtain: the longer the heating period, the larger the size of the reinforcement particles.

[0039] Since the two AlB.sub.2 and C phases are not in equilibrium, they react with each other in order to form Al and the mixed carbide Al.sub.3B.sub.48C.sub.2.

[0040] Preferably, the temperature rises and falls are rapid, for the purpose of limiting both the size of the reinforcement particles and decomposition thereof during cooling.

[0041] At the end of the high-temperature synthesis, the graphite crucible can be eliminated by simple machining, then releasing the ingot of CMM composite material contained inside. Since the latter was obtained at a temperature higher than the melting point of Al, 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%.

[0042] We shall now produce a composite material Al/Al.sub.3B.sub.48C.sub.2 according to the method that is the subject matter of the invention.

[0043] In a graphite crucible 8 mm in diameter, with a height of 5 mm and the walls of which have a thickness of 2 mm, 750 mg of aluminium diboride (AlB.sub.2) powder is placed. The whole is heated to 1400 C. for 15 minutes. The heating ramp is approximately 340 C./minute, while cooling is obtained by soaking the crucible directly in an oil bath cooled to 0 C.

[0044] The microstructure of the Al/Al.sub.3B.sub.48C.sub.2 composite thus obtained is observed under SEM (single figure). The white phase corresponds to the aluminium matrix and the black particles correspond to the reinforcement phase Al.sub.3B.sub.48C.sub.2. It can be seen that the reinforcements are dispersed in the matrix homogeneously and have a size of between 200 nm and 5 m (mean size approximately 700 nm).

[0045] The method that is the subject matter of the invention makes it possible to create an interface between a matrix and a reinforcement that is mechanically strong, but without leading to the decomposition of the reinforcement and to the creation of secondary phases that are detrimental to the properties of the composite. This is because, during the reactive synthesis between AlB.sub.2 and the graphite (C), there are very few minor phases that are created and the composite therefore behaves essentially as a phase of Al (forming the matrix) and the phase of Al.sub.3B.sub.48C.sub.2 (reinforcement), the minor phases being present in minimal quantities.

[0046] Finally, the method according to the invention provides a novel synthesis method for producing, in a simple manner and in quantity, composite materials with an Al matrix reinforced by particles of a mixed carbide of boron (B) and aluminium (Al), the properties of which are similar to those of a B.sub.4C reinforcement.

REFERENCE CITED

[0047] [1] US Ser. No. 11/033,099