DE102008054561A1 - Process for producing preforms for metal-matrix composites - Google Patents

Abstract

For the production of composite materials, the preforming technique is preferred. The preform must have a high strength and required porosity for infiltration with the liquid metal. The invention consists in that the properties of the preform are improved by an appropriate selection of the size and type of the fibers and / or of the particles of the secondary phase and / or the formation of a special pore structure. In particular, by using an additional secondary phase in the nanoscale range, the strength of the preform is increased or the temperature for solidifying the preform can be lowered.

Description

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

When Metal-matrix composite, metal matrix composite (MMC), is a composite of a metal matrix with one or more embedded secondary phases designated. Count in the strict sense For the MMCs only composite materials with a metal content ≥ 50 Vol%. For example, composites with lower metal content as a ceramic-matrix composite, Ceramic matrix composite (CMC), designated. This distinction but is not always carried out consistently.

The Preform should be for infiltration with the liquid Metal have a high strength. As a secondary phase in the following all the particles or fibers denoted, the one have different composition than the metal matrix and embedded in this are. Accordingly, the term storage phase is accordingly used. Not meant here are the elimination phases at Alloys.

When Metal matrix are in principle all metals in question, in particular but those from the group of so-called light metals such as Aluminum, magnesium or titanium. With these light metals can be compared to the usual Construction materials such as cast iron or steel clear Weight advantages are achieved, with the possibly occurring Disadvantages regarding mechanical, tribological or thermal properties homogeneous incorporation of further phases into the metal matrix partially or completely can be compensated.

When Storage phases are elements and compounds that are possible due to their material characteristics are suitable for improving individual properties of the metal matrix. This can be an improvement of the mechanical properties such as strength, tribology, wear resistance or the modulus of elasticity. In other cases can improve the thermal properties such as an increase the thermal conductivity or lowering the thermal expansion can be achieved. Farther Composite materials have positive characteristics due to their composition acoustic properties regarding the reduction of sound production and the conduction of sound.

The homogeneous distribution of the secondary phases in the metal matrix can either be formed by in situ formation during the Manufacturing process done by mixing the secondary phase to molten metal or by the infiltration of a porous shaped body, a so-called preform, those from the secondary phase exists with the molten metal. The storage phases can be described as Particles, as fibers or as a mixture of both in the preform and / or later present in the network.

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 Forming of the preform is carried out according to common powder technology Methods such as axial pressing, isostatic pressing, Slip casting, filtration casting, extrusion.

It it is possible to shape the composite material close to the contours.

A local limitation of the composite to the functional areas of the workpiece is possible.

It creates a monolithic connection of a local composite material with the rest Component.

The Combinations of different composite materials in a component is possible.

at Need exists the possibility the anisotropic orientation of the secondary phase in the composite, whereby especially in the use of fibers, the component properties can be specifically influenced.

The Production of gradient materials is possible.

The Composition of a preform material is reproducible.

To preform porosity temporary fillers can be used, that is, substances that be removed from the preform again 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.

at The shaping of the preforms can be done by demixing processes, agglomerations or sedimentation, to an inhomogeneous distribution of the particles come in the preform. Such segregation occurs in particular for particles in the nanometer range. Furthermore, the Shaping process frequently to locally different compression and thus to density gradients to lead.

The Processing of nanoscale powders is in the aqueous Phase as dispersion only in high dilution and with dispersing aids possible, otherwise agglomeration of the particles will occur, causing the desired Properties are lost.

at The processing of nanoparticles as a powder comes next to problems during handling also to agglomerations, and as a result For example, property deterioration due to structural defects networked.

To the Forming process heard a solidification of the preform, which in ceramic hard materials as a secondary phase in usually done by a sintering process. One for the handling and the infiltration to achieve sufficient preform strength, have to depending on the used hard material the temperatures are chosen so high, that it is already becoming a coarsening the hard particles come. This, for example, the reinforcing effect reduced. Furthermore creates a rigid ceramic framework, causing the ductility of the composite material is reduced.

It The object of the invention, by an appropriate choice of Size and style the fibers and / or the particles of the secondary phases and the porosity agent to improve the properties of the preforms.

The Solidification of the preform after the molding process by a Temperature treatment occurs in many hard materials by training of bridges between the particles. This happens first where there are particles already touch.

The Particle size of the particles, as the secondary phase used for the production of a composite, practice in an advantageous manner Influence the composite properties. By adding Particles of orders of magnitude in the nanometer range from 1 to 500 nanometers, also as nanoparticles or nanoparticles, the number of points of contact the particles are significantly increased. In addition, the nanoparticles more reactive because of their small size, so that the training of the bridges already at lower temperatures. Due to the high number the contact points are thinner Bridges. This increases the ductility and the elasticity of the composite material. The nanoparticles act as so-called inorganic Binder between the coarser ones Particles.

requirement is that the particles as separate, not agglomerated particles available. The influence of the nanoparticles starts from one Proportion of about 1 weight percent in the sintered preform. With rising Proportion of nanoparticles also increases the preform strength and the more lower may be the required temperature for solidification. Depending on the proportion of nanoscale particles is the sintering temperature between 600 ° C and 1500 ° C.

One optimal proportion of a secondary phase in the nano-scale range in the sintered preform is in the range between about 1 weight percent and 20 weight percent. In this Range is the risk that the nanoscale particles during the Mixture of the starting materials agglomerate, not as big as at higher Shares or even preforms made exclusively from secondary phase powders be manufactured in the nanoscale range.

Around with a high proportion of nanoscale particles their positive To obtain effects and to prevent agglomeration is a defined deposition of nanoparticles on other carriers Particles, further secondary phases or porosifying agent required. Then the processing even as a powder, without causing unwanted agglomeration.

To avoid segregation processes within the secondary phase and between seconds därphase and porosity, 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.

Farther Pre-treated nanoparticles can be deposited on other secondary particles as well as fix on or in the porosity agent. This is for example, the spray granulation over a Atomizer suitable or a method for dry mixing and / or Build-up granulation, for example by means of granulating, fluidized bed and the like. By appropriate choice of the sequence of component addition and auxiliaries as well as the thermal and mechanical boundary conditions becomes one Get granules that are familiar with usual process dry molding process.

The Pore structure arises on the one hand from temporary organic porosity agents, during a special process step can be removed from the preform and with respect to Quantity, size and shape leave corresponding pores. On the other hand, by the shaping process over the processed granules pores introduced into the preform.

Corresponding The invention relates to the porosity and the inorganic Base materials in the production of the granules so coordinated, that the desired multimodal pore structure arises. It is beneficial as a porosity agent a mixture of different substances with different size distribution, To use shape and structure. Suitable are natural organic Substances such as starches, Cellulose and other vegetable substances with particle sizes between 1 and 500 microns. But also synthetic substances such as Waxes and other polymers of suitable 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 μm 0.4 wt% colloidal SiO ₂ , 5 to 50 nm, as inorganic binder 3.8 wt% cellulose powder 1.9 wt% lignin 0.7 wt% dispersing aid 0.8 wt% organic binder 50.0 wt% water

The Water was in a container submitted and the rest Components with a high-speed stirrer dissolved homogeneously therein or dispersed. Subsequently the slip was over spray-dried 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 .

Then took place annealing the binder and the porosifying agent and solidifying up to a temperature of 1400 ° C at a holding time of 0.5 to 1 hour.

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

The following table summarizes the essential properties of the preform according to the invention from example 1. Inorganic base Al ₂ O ₃ property proportion of unit grain size 1-3 microns ceramic content 40-50 v% density 1.6-1.7 g / cm ³ Total porosity 50-60 % Porosity <1 μm 2.10 Proportion of total porosity Porosity 1-10 μm 6.10 Proportion of total porosity Porosity 10-100 μm 2.10 Proportion of total porosity 3P bending strength 8.3 MPa Coefficient of thermal expansion 7.5 10 ^-6 K ^-1 spec. Heat capacity at 600 ° C 0.9 kJkg ^-1 K ^-1 Modulus of static 280 GPa

at the present embodiment 1, the proportion of the inorganic binder can also by a proportion up to 10% by weight of other colloidal oxides, for example Alumina, with a particle size of 10 to 200 nanometers replaced or supplemented become.

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 μm 2.2 wt% Al ₂ O ₃ , nanoscale powder 10 to 100 nm as inorganic binder 3.8 wt% cellulose powder 1.9 wt% lignin 0.7 wt% dispersing aid 0.8 wt% organic binder 50.0 wt% water

The Water was in a container submitted and the rest Components with a high-speed stirrer dissolved homogeneously therein or dispersed. Subsequently the slip was over spray-dried 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 1.89 g / cm ³ .

The plate was heated at about 50 K / h in air to 1250 ° C, held for 1 h at 1250 ° 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 0.2 wt% dispersing aid 0.8 wt% organic binder 50.0 wt% water

The Water was in a container submitted and the rest Components with a high-speed stirrer dissolved homogeneously therein or dispersed. Subsequently the slip was over spray-dried 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 1.91 g / cm ³ .

The plate was heated at about 50 K / h in air to 1100 ° C, held for 1 h at 1100 ° C and then cooled. The 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 based on these embodiments is clearly visible, decreases with increasing proportion of nanoscale secondary phase the sintering temperature to a preform with a for the infiltration process to produce required strength.

The Temperature treatment may be stationary as a batch process or continuously in a roller oven.

A possibly necessary processing of the sintered preforms can, comparable to green bodies, through mill and drilling done.

Claims (12)

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, silicides and combinations thereof have a particle size in the range of 1 to 50 microns and at least one of the secondary phases from the materials mentioned 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. Method according to claim 1, characterized in that that the secondary phase from particles in the nanoscale size range consists of aluminum oxide and / or silicon dioxide. Method according to claim 1, characterized in that that the secondary phase from particles in the nanoscale size range made of titanium dioxide. Method according to one of claims 1 to 3, characterized that is a pretreatment of one or more secondary phases with the particle size in the range from 1 to 50 μm by coating the particles with the nanoscale particles by means of a chemical or physical process takes place, preferably a sol-gel process. Method according to one of claims 1 to 4, characterized that for support the granule preparation and shaping coordinated organic binder be used with coordinated different solubility such as polymers, preferably polyvinyl alcohol. Method according to one of claims 1 to 5, characterized that organic temporary Porosifying agents are used such as starches, cellulose, polymers. Method according to Claim 6, characterized that porosity agent having a particle size of 1 micron up to 500 microns added become. Method according to one of claims 6 or 7, characterized that at least two in size and morphology different porosity agents are combined. Method according to one of claims 1 to 8, characterized that a self-similar Pore structure is generated by the use of a multimodal, Mixture of the porosity and / or by combination of different Granulation processes, preferably by build-up granulation or Spray granulation. Method according to one of claims 1 to 9, characterized that the precipitation or formation of the secondary phase with particles in the nanometer range directly in the production of the granules on a further, coarser secondary phase or takes place on the Porosierungsmittel. Method according to one of claims 1 to 10, characterized that deposition of the secondary phase as particles in the nanometer range on a coarser secondary phase and / or the porosity agent is carried out by a granulation process. Method according to one of claims 1 to 11, characterized that the annealing of the temporary Porosifying agent and solidifying at temperatures between 600 ° C and 1500 ° C, preferably between 1000 ° C and 1500 ° C, more preferably between 1000 ° C and 1400 ° C he follows