WO2009080560A2

WO2009080560A2 - Method for producing preforms for metal-matrix composites

Info

Publication number: WO2009080560A2
Application number: PCT/EP2008/067410
Authority: WO
Inventors: Dirk Rogowski; Ilka Lenke; Michael Theil
Original assignee: Ceramtec Ag
Priority date: 2007-12-21
Filing date: 2008-12-12
Publication date: 2009-07-02
Also published as: WO2009080560A3; DE102008054561A1; EP2225060A2

Abstract

The preform technology is preferred for the production of composite materials. The preform must have a high strength and the necessary porosity for the infiltration with the liquid metal. The invention comprises the properties of the preform being improved by an appropriate selection of the size and type of the fibers and/or the particles of the secondary phase and/or the implementation of a special pore structure. In particular, the strength of the preform is increased and/or the temperature for solidifying the preform can be reduced by the use of an additional secondary phase in the nanoscale range.

Description

Process for producing preforms for metal-matrix composites

The invention relates to a process for the production of preforms for metal matrix composites.

Metal-matrix composite, metal matrix composite (MMC), is a composite of a metal matrix with one or more embedded secondary phases. In a strict sense, only composite materials with a metal content> 50% by volume belong to the MMCs. Composite materials with lower metal content are referred to, for example, as a ceramic matrix composite material, ceramic matrix composite (CMC). However, this distinction is generally not always carried out consistently.

The preform should have high strength for infiltration with the liquid metal. In the following, all particles or fibers which have a different composition than the metal matrix and are embedded in it are referred to as the secondary phase. Accordingly, the term storage phase is therefore used. Not meant here are the precipitation phases in alloys.

In principle, all metals are suitable as metal matrix, but in particular those from the group of so-called light metals such as aluminum, magnesium or titanium. Considerable weight advantages can be achieved with these light metals compared with conventional construction materials such as cast iron or steel, for example, and the possible disadvantages with respect to mechanical, tribological or thermal properties can be partially or completely compensated for by homogenous incorporation of further phases into the metal matrix.

As storage phases elements and compounds are possible, which are suitable due to their material characteristics to improve individual properties of the metal matrix. This can be an improvement mechanical properties such as strength, tribology, wear resistance or modulus of elasticity. In other cases, an improvement in the thermal properties, such as an increase in the thermal conductivity or lowering the thermal expansion can be achieved. Furthermore, composites have, by virtue of their composition, positive acoustic properties with respect to the reduction of sound generation and the conduction of the sound.

The homogeneous distribution of the secondary phases in the metal matrix can be done either by in-situ formation during the manufacturing process, by mixing the secondary phase to the molten metal or by infiltration of a porous shaped body, a so-called preform, which consists of the secondary phase, with the molten metal. The intercalation phases can be present as particles, as fibers or as a mixture of both in the preform and / or later in the composite.

For the production of composite materials, the preforming technique is preferred due to the following advantages:

The molding process of the preform is independent of the shape of the component.

The shaping of the preform is carried out by conventional powder technology methods such as axial pressing, isostatic pressing, slip casting, filtration casting, extrusion.

It is possible a contour near shaping of the composite material.

A local limitation of the composite to the functional areas of the workpiece is possible. The result is a monolithic connection of a local composite material with the rest of the component.

The combinations of different composite materials in one component is possible. If necessary, there is the possibility of anisotropic orientation of the secondary phase in the composite, which can be specifically influenced, especially in the use of fibers, the component properties.

The production of gradient materials is possible. The composition of a preform material is reproducible.

Temporary fillers may be used to pore the preform, that is to say substances which are removed from the preform by a thermal, chemical or physical process step. This results in a pore structure which corresponds in terms of proportion, size distribution and shape of the porosity agent. Porosizing agents used are, for example, natural or synthetic organic powders. It can be used for certain applications and fibers.

During the shaping of the preforms, segregation processes, agglomeration or sedimentation can lead to an inhomogeneous distribution of the particles in the preform. Such segregation occurs especially with particles in the nanometer range. Furthermore, the shaping processes can often lead to locally different densification and thus to density gradients.

The processing of nanoscale powders is possible in the aqueous phase as a dispersion only in high dilution and with dispersing aids, otherwise agglomeration of the particles occurs, as a result of which the desired properties are lost.

In the processing of nanoparticles as a powder it comes next to problems in handling also agglomerations, and as a result, for example, to property deterioration due to structural defects in the composite.

The shaping process involves solidification of the preform, which in ceramic hard materials as a secondary phase usually by a - A -

Sintering process takes place. In order to achieve sufficient preforming strength for handling and infiltration, the temperatures must be selected to be so high, depending on the hard material used, that coarsening of the hard material particles already occurs. As a result, for example, the reinforcing effect is reduced. In addition, a rigid ceramic framework, whereby the ductility of the composite material is reduced.

It is the object of the invention to improve the properties of the preforms by an appropriate choice of the size and type of fibers and / or the particles of the secondary phases and the porosity.

The hardening of the preform after the shaping process by a temperature treatment occurs in many hard materials by forming bridges between the particles. This happens first where particles already touch.

The particle size of the particles used as the secondary phase for producing a composite advantageously exerts an influence on the composite properties. By addition of particles with

Magnitudes in the nanometer range from 1 to 500 nanometers, also as

Designated nanoparticles or nanoparticles, the number of contact points of the particles can be significantly increased. In addition, the nanoparticles are more reactive because of their small size, so that the formation of the bridges already takes place at lower temperatures. Due to the high number of

Contact points form thinner bridges. This increases the

Ductility and ductility of the composite material. The nanoparticles act as so-called inorganic binder between the coarser particles.

The prerequisite is that the particles are present as separate, non-agglomerated particles. The influence of the nanoparticles already starts from a proportion of about 1 percent by weight in the sintered preform. As the proportion of nanoparticles increases, so too does the preform strength may be the required temperature for solidification. Depending on the proportion of nanoscale particles, the sintering temperature is between 600 ⁰ C and 1500 ⁰ C.

An optimal fraction of a nanoscale secondary phase in the sintered preform is in the range of about 1% to 20% by weight. In this area, the danger that the nanoscale particles agglomerate during the mixing of the starting materials is not yet as great as with higher fractions or even preforms, which are produced exclusively from nanoscale secondary-phase powders.

In order to maintain their positive effects and to prevent agglomeration in the case of a high proportion of nanoscale particles, a defined deposition of nanoparticles onto other carrier particles, further secondary phases or porosity agents is required. Then the processing is even possible as a powder, without causing unwanted agglomeration.

To avoid segregation processes within the secondary phase and between the secondary phase and the porosity agent, the nanoparticles of the secondary phase can either be deposited directly on the porosity agent or formed in the preform, for example via a sol-gel process. This leads directly to a preform containing nanoparticles.

Furthermore, pretreated nanoparticles can be fixed on other secondary particles as well as on or in the porosity agent. For this purpose, for example, the spray granulation is suitable via an atomizer or a method for dry mixing and / or build-up granulation, for example by means of granulating, fluidized bed and the like. By suitable choice of the sequence of component addition and auxiliaries as well as the thermal and mechanical boundary conditions, a granulate is obtained which can be processed by conventional dry shaping methods. On the one hand, the pore structure consists of temporary organic porosity agents, which are removed from the preform during a special process step, leaving corresponding pores with regard to quantity, size and shape. On the other hand, pores are introduced into the preform via the processed granules by the shaping process.

According to the invention, the porosity agents and the inorganic base materials in the preparation of the granules are coordinated so that the desired multimodal pore structure is formed. It is advantageous to use as a porosity agent a mixture of different substances with different size distribution, shape and structure. Suitable natural organic substances such as starches, cellulose and other botanicals with particle sizes between 1 and 500 microns. But synthetic materials such as waxes and other polymers with matching structure are suitable.

Reference to exemplary embodiments, the invention is explained in more detail:

An embodiment with a low proportion of nanoscale particles is presented. Colloidal SiO ₂ in the nanoscale area acts as a so-called inorganic binder. According to the following composition, the inorganic and organic base materials are mixed together.

Example 1 :

42.4 wt% Al 2 O 3, microscale powder 1 to 3 microns

0.4 wt% colloidal SiO 2, 5 to 50 nm, as an inorganic binder

3.8 wt% cellulose powder

1, 9 wt% lignin powder

0.7 wt% dispersing agent

0.8 wt% organic binder

50.0 wt% water

The water was placed in a container and the other components were dissolved or dispersed homogeneously in a high-speed stirrer. Subsequently, the slurry was spray dried over an atomizer to a granulate.

For shaping, 6000 g of the granules were pressed axially in a mold measuring 400 mm × 250 mm at 100 MPa. The result was a plate of the above dimension with a height of about 30 mm and a density of 1.88 g / cm ³ .

Subsequently, the heating of the binder and the porosity agent and the solidification was carried out up to a temperature of 1400 ⁰ C with a holding time of 0.5 to 1 hour.

The proportion of the so-called inorganic binder, based on the inorganic components of the preform, was about 1 wt% after sintering.

The following table summarizes the essential properties of the preform according to the invention from example 1.

In the present embodiment 1, the content of the inorganic binder may also be replaced or supplemented by a proportion of up to 10% by weight of other colloidal oxides, for example, alumina, having a particle size of 10 to 200 nanometers.

In the following embodiments, the proportion of nanoscale particles is greater. In the exemplary embodiment 2, it is about 5 wt%, based on the inorganic components of the sintered preform. As already stated, the required sintering temperature also drops correspondingly. Example 2:

40.6 wt% Al ₂ O ₃ , microscale powder, 1 to 3 microns

2.2 wt% Al ₂ O ₃ , nanoscale powder 10 to 100 nm as an inorganic binder 3.8 wt% cellulose powder

1, 9 wt% lignin powder

0.7 wt% dispersing agent

0.8 wt% organic binder

50.0 wt% water

For shaping, 6000 g of the granules were pressed axially in a mold measuring 400 mm × 250 mm at 100 MPa. The result was a plate of the above dimension with a height of about 30 mm and a density of 1.89 g / cm ³ .

The plate was heated at about 50 K / h in air at 1250 ⁰ C, held for 1 h at 1250 ⁰ C and then cooled. The resulting plate had a density of 1.71 g / cm ³ , which corresponds to a porosity of about 57%. The plate was solid, sawed into small pieces.

Example 3:

35.6 wt% TiO ₂ , microscale powder, 0.5 to 2 μm

8.9 wt% TiO ₂ , nanoscale powder, 5 to 50 nm, as inorganic

Binder 3.0 wt% cellulose powder

1, 5 wt% lignin powder

0.2 wt% dispersing aid

0.8 wt% organic binder

50.0 wt% water

For shaping, 6000 g of the granules were pressed axially in a mold measuring 400 mm × 250 mm at 100 MPa. The result was a plate of the above dimension with a height of about 30 mm and a density of 1, 91 g / cm ³ .

The plate was heated at about 50 K / h in air to 1100 ⁰ C, held for 1 h at 1100 ⁰ C and then cooled. The resulting plate had a density of 1.83 g / cm ³ , which corresponds to a porosity of about 57%. The plate was solid, sawed into small pieces.

As can be clearly seen from these exemplary embodiments, the sintering temperature decreases as the proportion of the nanoscale secondary phase increases in order to be able to produce a preform with a strength required for the infiltration process.

The temperature treatment can be stationary as a batch process or continuously in a roller furnace. Any required machining of the sintered preforms can be done by milling and drilling, as in the case of green bodies.

Claims

claims

1. A process for the production of preforms for metal matrix composites, characterized in that one or more secondary phases of metals, oxides, carbides, borides, nitrides, suicides and combinations thereof have a particle size in the range of 1 to 50 microns and at least one the secondary phases of said materials has a particle size of 1 to 500 nanometers, that the proportion of the secondary phase with the nanoscale particle size has a proportion of 1 to 20 weight percent of the sintered preforms and that these nanoscale particles are used as so-called inorganic binder.

2. The method according to claim 1, characterized in that the secondary phase consists of particles in the nanoscale size range of alumina and / or silica.

3. The method according to claim 1, characterized in that the secondary phase consists of particles in the nanoscale size range of titanium dioxide.

4. The method according to any one of claims 1 to 3, characterized in that a pretreatment of one or more secondary phases with the particle size in the range of 1 to 50 microns by coating the

Particles with the nanoscale particles by means of a chemical or physical process, preferably a sol-gel process.

5. The method according to any one of claims 1 to 4, characterized in that to support the granule production and shaping coordinated organic binder with coordinated different

Solubility can be used as polymers, preferably polyvinyl alcohol.

6. The method according to any one of claims 1 to 5, characterized in that organic temporary porosity agents are used such as starches, cellulose, polymers.

7. The method according to claim 6, characterized in that porosity agents are added with a particle size of 1 micron up to 500 microns.

8. The method according to any one of claims 6 or 7, characterized in that at least two different in size and morphology Porosierungsmittel be combined.

9. The method according to any one of claims 1 to 8, characterized in that a self-similar pore structure is produced by the use of a multimodal, mixing the porosity and / or by combining different granulation processes, preferably by build-up granulation or spray granulation.

10. The method according to any one of claims 1 to 9, characterized in that the precipitation or formation of the secondary phase is carried out with particles in the nanometer range directly in the production of the granules on a further, coarser secondary phase or on the Porosierungsmittel.

11. The method according to any one of claims 1 to 10, characterized in that deposition of the secondary phase takes place as a particle in the nanometer range on a coarser secondary phase and / or the porosity by a granulation process.

12. The method according to any one of claims 1 to 11, characterized in that the annealing of the temporary Porosierungsmittel and the

Solidification at temperatures between 600 ⁰ C and 1500 ⁰ C, preferably between 1000 ⁰ C and 1500 ⁰ C, more preferably between 1000 ⁰ C and 1400 ⁰ C takes place