WO2006000003A1

WO2006000003A1 - Carbon fibre-reinforced light-weight metal component and method for the production thereof

Info

Publication number: WO2006000003A1
Application number: PCT/AT2005/000201
Authority: WO
Inventors: Maria Papakyriacou; Peter Schulz; Mark Russel-Stevens
Original assignee: Arc Leichtmetallkompetenz- Zentrum Ranshofen Gmbh; Reiter, Josef
Priority date: 2004-06-23
Filing date: 2005-06-07
Publication date: 2006-01-05
Also published as: AT413704B; EP1759030A1; ATA10672004A

Abstract

The invention concerns a method for producing carbon fibre-reinforced light-weight metal components by infiltrating a fibre preform made of carbon fibres with a light-weight molten metal. In order to produce carbon fibre-reinforced light-weight metal components that have a large portion by volume of carbon fibres and are free from holes and with a low degree of mechanical strain being exerted on the fibre preform, the invention proposes that the fibre preform and solid light-weight metal material be subjected to a vacuum and be heated to a temperature at which the light-weight metal material is melted, whereafter a pressure is exerted on the light-weight molten metal by means of a gas and the fibre preform is infiltrated with light-weight molten metal, whereupon the infiltrated fibre preform is left to cool whilst the pressure is maintained. The invention also concerns a carbon fibre-reinforced light-weight metal component which contains more than 50 volume percent carbon fibres and which is produced according to a claimed method. The claimed light-weight metal component has a high modulus of elasticity for a small density as well as a small coefficient of thermal expansion and is suitable for mirrors for space travel.

Description

Carbon fiber reinforced light metal part and method of making the same

The invention relates to a method for the production of carbon fiber-reinforced light metal parts by infiltration of a fiber preform made of carbon fibers with a light metal melt.

Further, the invention includes a carbon fiber reinforced light metal part and uses thereof.

In recent decades, composites have replaced traditional materials such as metals, ceramics or plastics in many areas in the development of new design and functional elements. Above all, this applies to functional elements that have to have a specifically oriented property profile for a specific application, which can not be achieved as desired or only in an uneconomical manner when using conventional materials.

A combination of materials which is particularly promising for many applications represents the pairing of light metals with carbon fibers. Light metals, on the one hand, have a low density and are easy to process in themselves, so that lightweight components can be produced easily and inexpensively; However, the mechanical properties of light metals are limited mainly in terms of a modulus of elasticity and a tensile strength value. On the other hand, carbon fibers also have low tensile strength, high tensile strength and high modulus of elasticity. Now, if a light metal combined with carbon fibers, so this can lead to lightweight composite parts, which have compared to a light metal alone noticeably better mechanical properties. One speaks in this context of a gain of a light metal by carbon fibers.

For special applications of carbon fiber reinforced light metal parts, it is necessary that their property profiles are targeted and several criteria, such as minimum values of mechanical characteristics, are achieved simultaneously. For example, for mirrors or other components used in the Space technology to be used, required that they should have the lowest possible density at the highest possible elastic modulus and the lowest possible thermal expansion coefficient with good thermal conductivity.

In order to be able to realize such property profiles of carbon fiber-reinforced light metal parts, several factors have to be considered: On the one hand, it is necessary to set a weight proportion or volume fraction of carbon fibers in the composite part, because the mechanical properties can be significantly influenced by this proportion. On the other hand, it is known to those skilled in the art that it is possible to achieve extremely different property profiles by varying the production methods, the processing and after-treatment and the shape of the reinforcing components, although macroscopically a same composition and an equal proportion of the components involved (KU Kainer, Foundations of metal matrix composites, page 1 et seq. in "Metallic composites", Wiley-VCH, 2003, ISBN 3-527-30532-7).

A production of kohJenstofffaserverstärkten light metal parts, which are understood with long fibers, which here are understood fibers having a length of more than 1000 microns, or to be reinforced with two-dimensional fabric, is made by making a Faservorform consisting of fibers and intervening open spaces from the fibers and the fiber preform is then infiltrated with a light metal melt. In order to completely fill the free spaces in a Faservorfomi with a light metal melt, it is necessary to support the penetration of the melt. Thus, it is known in the art to press a light metal melt into a fiber preform by means of a punch having a pressure of about 1000 bar in a mold, which process is referred to as squeeze casting. However, the high pressures required in this preparation may result in damage to the solid fiber preform, such as breakage of individual fibers.

In a method alternative to squeeze casting, a light metal melt is provided and then a fiber preform is dipped into the molten metal melt. Subsequently, a pressure is applied to the surface of the light metal melt by means of a gas, which causes an infiltration of the fiber preform. The at pressures used in such processes are lower than those used in squeeze casting, and therefore fiber preform integrity after infiltration seems attainable. The comparatively lower pressures for pressure casting are intended to make it possible to protect the fiber preform during infiltration, but infiltration can often be incomplete at the same time, so that pores occur in the reinforced light metal part.

Not only do pore inclusions degrade the mechanical properties and provide weaknesses in the use of a fiber reinforced light metal part, but also cause problems when heat treatment of the fiber reinforced light metal part is additionally desired, as may be the case for high performance materials. Gas in the pores expands upon heat treatment such as solution heat treatment, causing undesirable internal stresses during the heat treatment. These can lead to material damage and thus adversely affect a property profile.

This problem is especially pronounced when lightweight metals are to be reinforced with a high proportion of carbon fibers, because with increasing fiber content, a wettability of the fibers decreases in an infiltration and a tendency to pore formation or the occurrence of voids increases.

In particular, with a view to developing mirrors for aerospace engineering, other elaborate and complicated processes for producing carbon fiber reinforced light metal parts have been developed (R. Wendt, M. Misra, Fabrication of near-net shape graphite / magnesium composites for large mirrors, SPIE Vol. 1303 Advances in Optical Structure Systems (1990) p. 554 et seq.), however, only a fiber content of about 38 percent by volume was achieved with these methods. These composite parts do not reach a property profile desired from today's perspective for mirrors in aerospace applications.

According to the aforementioned prior art, no satisfactory answer has yet been given to the question of how carbon fiber-reinforced light metal parts with a high proportion of carbon fibers can be produced without pore-free or lunker-free under low loading of the fiber preform during the infiltration, so that composite parts with good results mechanical properties are obtained. The invention now sets itself the goal of finding a solution for this.

Another object of the invention is to provide a carbon fiber reinforced light metal part, which is due to its property profile suitable for use in space technology, in particular as a mirror.

The procedural object of the invention is achieved by exposing the fiber preform and solid light metal material to a vacuum and heating them to a temperature at which the light metal material has melted, after which pressure is applied to the light metal melt by means of a gas and the fiber preform infiltrated with molten metal, whereupon the infiltrated fiber preform is allowed to cool while maintaining the pressure.

The advantage achieved by the invention can be seen in particular in that, through the intended combination of method steps, carbon fiber-reinforced light metal parts with a high proportion by weight of carbon fibers can be produced without pores or lunker-free with low mechanical loading of the fiber preform during the infiltration.

By providing evacuation gas is removed from the free spaces of the fiber preform, whereby a subsequent infiltration of the preform with light metal melt is greatly facilitated. Since the vacuum is maintained until the light metal is present as a melt, the light metal melt can be degassed by the vacuum at the same time. A heating of the fiber preform associated with a melting of a light metal, in addition to an increased wettability of the fibers, that the light metal melt can invest in the infiltration completely to the surface of the fibers without immediately solidify. Since solidification of the melt upon contact with fiber surfaces is avoided, the fiber preform can be completely filled with light metal melt under pressure. A pressure is maintained even when solidifying the molten metal; This allows a re-pressing of melt in any existing pores or holes caused by shrinkage, thus contributing to a void-free or pore-free formation of a carbon fiber reinforced light metal part. With regard to a pressurization of the molten metal melt, it is advantageous if a pressure of 50 to 220 bar, in particular 70 to 170 bar, is applied to this. A minimum pressure of 50 bar, in particular 70 bar, proves to be favorable for a high proportion by volume of carbon fibers in order to introduce liquid light metal into the spaces of the fiber preform which are not filled by carbon fibers. In order to best protect the fiber preform used in this case, it is expedient to limit a pressure further to 220 bar, better 170 bar.

If the gas is an inert gas, in particular helium or argon, then a pressurization of the light metal melt can be carried out without there being a reaction of the gas used with the light metal melt.

In a particularly advantageous variant of the invention, the carbon fiber-reinforced light metal part is additionally subjected to a heat treatment. By a heat treatment caused by cyclic thermal load between -100 ⁰ C and 100 ⁰ C caused change in length of the reinforced light metal part can be minimized.

The above effect is particularly effective when the heat treatment is from solution heat treating the carbon fiber reinforced light metal part for at least two hours, then quenching it in air, then hot aging the light metal part for at least 2 hours at at least 100 ° C and cooling the light metal part to ambient temperature consists.

Temperature-induced changes in length of a composite part can also be counteracted if the carbon fiber reinforced light metal part is maintained after production and cooling to ambient temperature (about 25 ⁰ C) for at least one more minute in liquid nitrogen. However, it is even more effective if the carbon fiber reinforced light metal part is held in liquid nitrogen for at least one minute after a heat treatment. This allows an increase in the effects already achieved with a heat treatment.

With regard to the lowest possible density of a carbon fiber reinforced light metal part, it is advantageous if the light metal magnesium or a Magnesium alloy is. Moreover, liquid magnesium has a very low tendency to react with carbon of the fibers to metal carbides.

Depending on the intended use of a light metal part produced according to the invention, different fiber preforms can be used. If unidirectionally reinforced light metal parts are desired, then the fiber preform expediently has parallel aligned carbon fibers. Alternatively, it may be required to form a light metal in two spatial directions with high tensile strength and high elasticity modulus. In this case, it is expedient if the fiber preform has one or more carbon fiber fabrics.

The further object of the invention is achieved by a carbon fiber-reinforced light metal part according to claim 11.

The advantages of a light metal part according to the invention are to be seen in particular in that it has high mechanical properties, above all high tensile strength and high elasticity modulus (s) at low density. At the same time, a thermal conductivity is good and a thermal expansion coefficient is extremely low.

Another advantage of inventive light metal parts is that they are thermally stable between room or ambient temperature (about 25 ⁰ C) and 100 ⁰ C. In other words, between approx. 25 ^° C. and 100 ^° C., ie in a temperature range important in particular for mirrors used in aerospace engineering, the dimensions of a heat-treated light metal part do not change significantly with repeated heating and cooling.

Such carbon fiber reinforced light metal parts have a wide range of applications. Due to their property profile, they are suitable as optical components, in particular mirrors for space technology. Because of their high mechanical characteristics and low density, light metal parts according to the invention are also particularly suitable for use in aerospace components.

In the following the invention is further illustrated by means of examples and figures. FIG. 1a shows a schematic representation of an undirectionally fiber-reinforced light metal part; FIG. 1b: a photograph of the cross section of a unidirectionally carbon fiber-reinforced light metal part transverse to the fiber direction; FIG. 1c: an enlarged detail from FIG. 1b; Figure 2a: A photograph of a woven fabric of carbon fibers; FIG. 2b: a photograph of the cross section of a light metal part reinforced with carbon fiber fabric; FIG. 2c shows an enlarged section from region R of FIG. 2b; FIG. 3: A graph of the fractions of density versus modulus of elasticity plotted against the fractions of thermal expansion coefficient to thermal conductivity for various materials; Figure 4: change in the length of a non-heat treated workpiece with repeated heating and cooling; FIG. 5: change in the length of a workpiece treated after a heat treatment A with repeated heating and cooling; FIG. 6 shows a change in the length of a workpiece treated after a heat treatment B during repeated heating and cooling; FIG. 7: Change in the length of a workpiece treated after a heat treatment C with repeated heating and cooling.

example 1

Long fibers of carbon (trade name: P 100, Thornel carbon fibers, available from BP Amoco Chemicls) were wrapped over a frame. In the case of the winding, it was considered that the fibers in the composite part to be produced should have a volume fraction of 60% by volume, or that the spaces between the fibers should allow room for 40% by volume of light metal, based on the total mass. After wrapping the fibers, the frame was split in half to obtain a fiber preform having unidirectionally oriented fibers.

The thus prepared fiber preform was placed in an evacuable container or autoclave. Thereafter, a light alloy AZ91 was on the Fiber preform filed; the composition of this alloy is given in Table I. The weight of the alloy was dimensioned with respect to the free spaces of the fiber preform, so that a complete filling of these spaces was made possible.

Table I: Chemical composition of alloy AZ91

Subsequently, the autoclave was evacuated and the fiber preform and the alloy were heated under vacuum to a temperature of 638 ⁰ C, so that the alloy was present as a melt. Thereafter, the melt was pressurized by helium with a pressure of 83 bar, infiltrated the fiber preform and finally allowed to cool the autoclave or the infiltrated fiber preform while maintaining the pressure.

The carbon fiber reinforced composite part thus prepared was examined more closely. The composite part has a structure as shown schematically in FIG. 1 a: parallel aligned carbon fibers 1 are embedded in a light metal matrix 2. As can be seen from FIG. 1b on the basis of a cross-sectional image, the fibers are distributed macroscopically over wide areas; Cracks in the composite part are not recognizable. From FIG. 1c, which shows an enlarged detail of region K of FIG. 1b, it can clearly be seen that carbon fibers 1 and light metal 2 together form a dense body which is free of pores or voids.

The carbon fiber reinforced light metal part had proportionally 60% by volume carbon and 40% by volume AZ91. The density was 2:02 like ^"3. An elastic modulus of the composite was in the direction of the fibers 450 GPa. For the tensile strength of a value of 1200 MPa was measured in the fiber direction. The average coefficient of thermal expansion (CTE) was in the fiber direction for the temperature range of 2O ⁰ C to 100 ⁰ C on average 0.4 ppm / K. Transverse to the fiber direction, a thermal expansion coefficient (CTE) in the same temperature range was about 30 ppm / K. The thermal conductivity K was determined to be 340 W / mK.

To determine a dimensional stability with multiple cyclic thermal load, the composite part produced was a five-fold heating / cooling between -100 ⁰ C and 100 ⁰ C subjected. It turned out that a hysteresis occurs and a sample continues to expand (FIG. 4). In order to reduce such sample expansion, carbon fiber-reinforced light metal parts produced as described above were subjected to various heat treatments A, B or C. The heat treatments were carried out as follows:

Heat treatment A: solution annealing at 410 ^° C. for 12 hours, quenching in air at ambient temperature, then heat aging at 200 ^° C. for 15 hours, finally cooling in air; Heat treatment B: After producing the light metal part, cool in air to ambient temperature and then hold the light metal part for 5 minutes in liquid nitrogen (temperature -196 ⁰ C); Heat treatment C: solution heat treatment of the light metal part at 410 ^° C. for 12 hours, then quenching in air at ambient temperature, then heat aging at 200 ^° C. for 15 hours and finally cooling to ambient temperature in air for five minutes in liquid nitrogen.

As the individual figures for the heat treatments A (Figure 5), B (Figure 6) and C show (Figure 7) in comparison with figure 4, all heat treatments cause a substantial reduction of expansion of carbon fiber reinforced composite parts at cyclic thermal load between -100 ⁰ C and 100 ^° C. This effect is most pronounced in heat treatment C.

Figures 4 to 7 also show that an inventive light metal part between ambient temperature and 100 ⁰ C does not significantly expand.

The microstructure of the heat-treated light metal parts corresponded to that of the untreated light metal parts (FIG. 1b or FIG. 1c).

Example 2

A carbon fiber reinforced light metal part was produced analogously to Example 1, wherein the fiber preform was formed by a plurality of superimposed fabrics, as shown in Figure 2b. The fabric shown is commercially available under the designation K13C2U from Mitsubishi Chemical America. The single ones Tissues were each rotated by 90 ^° C. (so-called 0.90 architecture). Again, a carbon fiber reinforced light metal part with a fiber content of 60% by volume, balance AZ91 was created. Under otherwise identical conditions of manufacture, the infiltration was carried out at a temperature of 670 ⁰ C and a pressure of 83 bar, with argon was used as the gas.

For the thus-produced light-metal part was determined to be in the temperature range of 2O ⁰ C to 100 ⁰ C, an average thermal expansion coefficient (in the tissue plane) of 0.4 ppm / K. At a density of 2.0% ^"3 , a modulus of elasticity in a plane of the fabric was 140 GPa. The composites produced are free from macroscopic cracks and microscopic pores, as shown in Figures 2b and 2c.

Due to their property profiles, as set forth above with reference to Examples 1 and 2, materials according to the invention are excellent for components used in aerospace applications. In particular, composite parts according to the invention are particularly suitable for the production of mirrors used in space technology because of a balanced property profile and high thermal stability. Such mirrors require that a quotient of density (p) to modulus of elasticity (E) as well as a quotient of thermal expansion coefficient (CTE) to thermal conductivity (K) are as low as possible. FIG. 3 shows that composite parts according to Examples 1 and 2 outstandingly fulfill these requirements in comparison to known materials such as silicon or beryllium. It is also evident that composite parts according to the invention exceed known carbon fiber-reinforced light metal parts in terms of these properties (the values 3, 3 'correspond to a carbon fiber-reinforced light metal part and are from R. Wendt, M. Misra, Fabrication of near-net shape graphite / magnesium composites for large mirrors SPIE Vol. 1303 Advances in Optical Structure Systems (1990) p. 554 et seq.).

Claims

claims

A process for the production of carbon fiber reinforced light metal parts by infiltration of a carbon fiber fiber preform with a light metal melt, characterized in that the fiber preform and solid light metal material are subjected to a vacuum and heated to a temperature at which the light metal material has melted, after which the light metal melt by means of a Gases is applied pressure and the fiber preform is infiltrated with molten metal, whereupon the infiltrated fiber preform is allowed to cool while maintaining the pressure.

2. The method according to claim 1, characterized in that on the light metal melt, a pressure of 50 to 220 bar, in particular 70 to 170 bar, is exercised.

3. The method according to any one of claims 1 or 2, characterized in that the gas is an inert gas, in particular helium or argon.

4. The method according to any one of claims 1 to 3, characterized in that the carbon fiber reinforced light metal part is additionally subjected to a heat treatment.

5. The method according to claim 4, characterized in that the heat treatment from a solution annealing of the carbon fiber reinforced light metal part for at least two hours, then quenching the same in air, subsequent heat aging of the light metal part for at least 2 hours at least 100 ⁰ C and cooling the Alloy part at ambient temperature.

6. The method according to any one of claims 1 to 5, characterized in that the carbon fiber reinforced light metal part is held after the preparation and cooling or heat treatment for at least one minute in liquid nitrogen.

7. The method according to any one of claims 1 to 6, characterized in that the light metal is magnesium or a magnesium alloy.

8. The method according to any one of claims 1 to 7, characterized in that the fiber preform has parallel aligned Kσhlenstofffasern.

9. The method according to any one of claims 1 to 8, characterized in that the fiber preform comprises one or more fabrics of Kohlenstoϊffasem.

A carbon fiber reinforced light metal part obtainable according to any one of claims 1 to 9, wherein a content of carbon is more than 50% by volume.

11. Use of a carbon fiber-reinforced light metal part according to claim 10 as an optical component, in particular mirror.

12. Use of a carbon fiber reinforced confection metal part according to claim 10 in components for aerospace