CA2232177A1

CA2232177A1 - Aluminium matrix composite material and process of producing same

Info

Publication number: CA2232177A1
Application number: CA002232177A
Authority: CA
Inventors: Franz Josef Feikus; Achim Sach
Original assignee: VAW Motor GmbH
Current assignee: VAW Motor GmbH
Priority date: 1997-03-26
Filing date: 1998-03-16
Publication date: 1998-09-26
Also published as: HUP9800652A1; HU216623B; EP0867517A1; DE59800182D1; ATE194170T1; JPH10265870A; BR9806333A; EP0867517B1; MX9802324A; DE19712624C2; HU9800652D0; DE19712624A1

Abstract

The invention relates to an aluminium matrix composite material consisting of a porous fibre preform which is embedded in an aluminium alloy and which has a higher strength and an improved wear behaviour as compared to the aluminium matrix, wherein the fibre preform consists of a shaped fibre member which comprises a metallic and/or intermetallic structure and into which there is infiltrated a silicon-containing aluminium alloy melt, with the Si-content of the melt amounting to 5 - 14 % by weight. Furthermore, the invention relates to a process of producing an aluminium matrix composite material which consists of a porous fibre preform infiltrated with an aluminium alloy.

Description

VAN motor GmbH 28th January 1998 Friedrich-Wohler-Str. 2 MW/scb (allO771) 53117 Bonn P96906EO10 Aluminium matrix composite material and process of producing same Description The invention relates to an aluminium matrix composite material consisting of a porous fibre preform which is embedded in an aluminium alloy and which has a higher strength and an improved wear behaviour as compared to the aluminium matrix, and to a process of producing such composite materials.

Such aluminium matrix composite materials are produced for example in the form of single cylinder blocks or multi-cylinder blocks from an aluminium alloy, and into the cylinder there is cast a hollow-cylindrical member consisting of ceramic fibres with silicon members inserted into same, which hollow-cylindrical member forms the cylinder barrel and is penetrated by a hypoeutectic aluminium alloy. The purpose of this measure is to improve the tribological properties of the cylinder barrel face.

One problem which occurs when subjecting such composite materials to mechanical loads consists in that the high silicon content results in an increase in tool wear when machining the cylinder block and causes piston wear under operational conditions. Said disadvantage can be reduced in that the proportion of fibres and the percentage of sicilon grains are calculated to be such that a large area of contact between the aluminium alloy matrix of the cylinder and the tool and piston respectively is avoided. Nevertheless, the piston shank has to be provided with a layer of iron in order to keep the piston shank wear within acceptable limits.

A further problem of the prior art fibre composite materials consists in that the raw materials can only be re-used if they are subjected to expensive recycling processes which include additional separating stages for the fibres. Such processes are not able to separate metallic and non-metallic materials completely which, therefore, subsequently, have to be processed separately.

Furthermore, the wetting behaviour of ceramic fibres when coming into contact with metal melts is known to be very disadvantageous. Therefore, it has so far been regarded as necessary to allow the aluminium alloy melt to act under high pressures of up to 3000 bar up to the point of solidification. This greatly increases the costs of producing aluminium matrix composite materials.

The same applies to reinforcing elements which are produced from spray-compacted materials and which, in accordance with EP O 271 222 A2, may consist of several layers of fireproof materials. However, in this case there is a risk of a reaction between the fibre material and the melt, so that infiltration times have to be kept short to prevent the formation of metal carbides or metal nitrides.

Furthermore, following a large number of tests it has been found that, as a result of the different temperature behaviour of ceramics and metal materials, there may occur very disadvantageous tolerance ranges under the different operating conditions of internal combustion engines. Under extreme conditions, these differences lead to the formation of grooves between the piston shank and the cylinder barrel or, in another extreme case, to increased oil losses due to through-blowing effects.

It is the object of the present invention to improve the tribological properties of prior art aluminium matrix composite materials, especially when used for cylinder sleeves for engine blocks and to narrow the tolerance range by adapting the temperature-dependent material properties to one another, with the disadvantages of prior art aluminium matrix composite materials being avoided by improving the wetting behaviour during infiltration.

This objective is achieved by the characteristics listed in the claims.

The wear behaviour of an aluminium matrix composite material produced in accordance with the invention can be influenced as follows:

1. For the first time, it has become possible to produce pairs of the same type of (metallic) material for the friction partners of fibre-reinforced materials, with the temperature-dependent strength behaviour of same being improved considerably.

2. By controlling the pore size it is possible to influence the distribution of volume of the matrix and of the fibre content, so that the stength values and friction pairs can be set locally.

3. In an extreme case, the friction faces can be produced entirely from fibre materials and the visual faces can be produced entirely from a metal matrix.

The extreme case referred to above can be explained with the help of the following example:

When used for the cylinder barrels of a single- or multi-cylinder engine, the inventive aluminium matrix composite material can be provided with particularly advantageous properties when tolerances and clearances have to be compensated for. By using the same type of material (infiltration material) for cylinder sleeve faces and outer piston faces, no dimensional deviations between the used materials have been identified, neither when cold-starting an engine nor under continuous operating conditions, neither during acceleration cycles nor full throttle cycles. This means that also the emission behaviour of the inventive running faces of single- or multi-cylinder engines has been improved considerably over the entire operational range, as compared to conventionally produced composite materials reinforced by fully ceramic fibres.

A further advantage refers to the advantageous wetting behaviour during infiltration because the metallic and/or intermetallic fibres can be wetted particularly easily by an aluminium casting alloy, i.e. also in a pressure-less way. This is particularly important in connection with automated processes, for example in the case of pressure die casting processes, because the individual process stages can take place in a relatively short time while nevertheless ensuring complete wetting of the fibres of the fibre preform. Satisfactory wetting means that the die is well filled and that the fibre preform is well anchored in the matrix, so that the dimensional stability of the composite material in accordance with the invention is at least twice as high as that of a conventionally die-cast aluminium matrix material with oxide-ceramic fibres.

As a result of the improved dimensional stability of the fibre preform it is also possible to achieve high production speeds when using modern production processs.
For instance, it is possible to speed up the injecting sequence while simultaneously increasing the injection pressure during the pressure die casting process, so that, while the number of rejects is reduced, the production output is more than three times as high as that of comparable composite materials based on ceramics.

The proportion of fibres of the inventive aluminium matrix composite material can be adapted to the loads applied in the respective application. It is thus possible to produce new composite materials with a gradient structure and specific strength/elongation properties. Production costs are thus reduced, especially in the case of components with complicated shapes.

Comprehensive machining operations on the working surface with the improved rheological properties can be avoided if the part is designed accordingly. The process in accordance with the invention makes it possible to provide a component with both highly loaded sliding faces and temperature-loaded sealing faces, e.g. in the cylinder head region. Even tensioned parts with an improved elongation behaviour can be produced by a cost-effective overall process.

Decisive factors in this context are the structure, design and composition of the fibre preform which may comprise anopen-pore metallic structure in the porosity range of 30 - 97% with pore sizes of 1 ~ to 3 mm.
Furthermore, the surface roughness of the porous structure can be varied, which means that the ability of the melt to infiltrate the porous structure can be influenced. It is thus possible to achieve very good mechanical anchoring of the fibres with the help of a rough structure, whereas with a smooth fibre surface, the porous structure can be infiltrated more easily.

As the inventive aluminium matrix composite material consists of constituents of the same type, it has a homogeneous strength structure which results in a very good thermal shock behaviour, and since, furthermore, because of its metallic basis, its thermal conductivity reaches very high values, it is particularly suitable for being used in internal combustion engines. In addition, it is very suitable for chip-forming and non-chip-forming machining operations, especially in the transition zone between purely metallic and aluminium matrix composite materials, which so far led to problems due to different material structures.

To be able to use cylinder barrels made of composite materials it is necessary for the expansion coefficient of the shaped fibre members to be largely adapted to the matrix material and the base alloy. This has been achieved for the first time with the aluminium matrix composite material in accordance with the invention because the shaped fibre member consisting of a metallic and/or intermetallic fibre material can be largely adapted to its metallic environment by changing its composition. Porous ceramic structures, on the other hand, have a very limited spectrum of properties.

For optimising the wear properties, there are available the base materials of iron and nickel or the intermetallics such as iron-, nickel-, titanium-aluminides, tungsten, copper, cobalt, magnesium. Depending on requirements, it is possible, by selecting a suitable combination, advantageously to combine specific properties such as low density, good thermal conductivity, good corrosion resistance and good machining properties.

In addition to selecting the material, it is possible in the case of the aluminium matrix composite material, to achieve a gradient structure in the fibre preform.
For example, said member may comprise a rising or falling porosity or a rising or falling pore size across its cross-section. In this way it is possible to ensure that the properties of the metal matrix fibre composite meet the load requirements.

Below, there will be described an example of producing the inventive aluminium matrix composite material and some specific preferred applications which demonstrate the inventive idea and reveal further advantageous properties. In particular, reference is made to using the above material as a bearing material according to Figure 2, for a cylinder barrel according to Figure 3, and for a valve seat according to Figure 4.

As can be seen in Figure 1, process stage I, first a shaped fibre member 9 is connected, and provided with a firm structure, by being sintered in a furnace 10.
After the shaped fibre member has cooled down, it can be converted into a fibre preform 11.

During process stage II, an aluminium alloy forming the matrix material 12 is cast around the fibre preform 11.
There is then produced an aluminium matrix composite material 13 which can be used as a bearing material, for a cyllnder barrell or a valve guide.

When the aluminium matrix composite material 13 is used as a bearing material as shown in Figure 2, it is proposed that the shaft 1 and the bearing bush 2 consist of aluminium alloys, for example that the shaft consists of a wrought aluminium alloy and the bearing bush of a cast aluminium alloy in the form of an infiltrated fibre preform, which means that materials of the same type are being paired. The outer casing 3 can consist of a pure matrix metal, for example of an easily weldable AlMgSi or AlZnMg alloy.

When the inventive material is used for a cylinder barrel, the paired materials as used for the piston 4 and the inliner 5 according to Figure 3 lead to improved sliding properties. For example, the piston 4 consists of aluminium and the cylinder barrel (inliner 5) of an infiltrated gradient material based on aluminium. There is no need for any clearances to be compensated for, because the outer layer of the infiltrated fibre preform features an almost identical thermal expansion behaviour to the outer piston face.

In the third application as illustrated in Figure 4, the valve seat and the valve 6 are produced from different materials. Whereas the valve material, for instance steel, must have very good strength properties and a high thermal resistance, the valve guide 7 is produced from an infiltrated gradient material with an aluminium matrix alloy. The wear behaviour of the latter can be adapted by means of the gradient structure to the wear behaviour of the valve shank 8.

As can be seen from the above examples, the engine-specific running properties and the engine-specific wear behaviour can be greatly improved by using the paired materials as described. It is to be expected that the running performance is considerably improved by the constant wear behaviour and the constant tolerance values.

Furthermore, oil consumption and thus emission behaviour are improved in those areas where the inventive composite materials are used. These expectations result from the knowledge that the tribological properties of the materials used and the emission behaviour are closely related to one another. Oil adhesion is improved as a result of the specific surface of the inventive composite materials.

The abrasion behaviour of the inventive composite materials is comparable to that of the nickel calcium silicate layers which comprise a very advantageous wear behaviour. The expansion behaviour is very similar to that of ceramic composites. By using oxidation-resistant alloys, infiltration of the fibre preform is so advantageous that it is correct to speak of complete wetting of the composite materials.

To achieve a specific structure of the inventive composite materials it is also possible to use directed fibre structures; they are directed magnetically into a predetermined shape. In this way it is possible to produce any shape of asymmetric fibre preforms whose individual fibres are orientated into the direction where the load is applied.

In spite of the large number of applications and the different possible alloy compositions in the infiltrated fibre preform and in the matrix, the inventive composite materials can be very easily recycled. The previously required complicated cleansing operations and procedures of separating non-ceramic fibres no longer have to be carried out.

Claims

1. An aluminium matrix composite material consisting of a porous fibre preform which is embedded in an aluminium alloy and which has a higher strength and an improved wear behaviour as compared to the aluminium matrix, characterised in that the fibre preform consists of a shaped fibre member which comprises a metallic and/or intermetallic structure and into which there is infiltrated a silicon-containing aluminium alloy melt, with the Si-content of the melt amounting to 5 - 14 % by weight.

2. An aluminium matrix composite material according to claim 1, characterised in that the fibres of the shaped fibre member are connected by being sintered at the points of intersection.

3. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the composite material comprises at least one working surface with an improved wear behaviour, with the embedded fibre preform, on its side pointing towards the working surface, having lower porosity values than on the side of the composite material facing away from the working surface.

4. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the working surface is formed by the preform structure of the composite material.

5. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the preform structure, if viewed across its cross-section, comprises a gradient, with the porosity values ranging between 20 and 98 %.

6. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the preform contains homogeneous regions with a porosity ranging between 20 and 98%.

7. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the preform is layered, having a supporting layer with a coarse-pore preform structure and an outer layer with a fine-pore preform structure, which layers are sintered together at the points of intersection.

8. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the layers consist of fibres with different chemical and physical properties.

9. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the fibre preform consists of fibres of an iron chromium aluminium alloy which is insensitive to oxidation, with Fe = 50 - 85 % by weight, Cr = 10 - 30 %
by weight, A1 = 5 - 20 % by weight, from which alloy there were obtained, by melt extraction, individual threads of a length L = 0.5 - 5 mm with a fibre diameter of 1 - 50 µm.

10. An aluminium matrix composite material according to any one of the preceding claims, characterised in that for producing melt-extracted fibres, use is made of an aluminium nickel alloy with aluminium contents of 7 up to 40%.

11. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the fibres are produced from an iron nickel aluminium alloy.

12. An aluminium matrix composite material according to any one of the preceding claims, characterised in that the fibres of the shaped fibre member consist of intermetallic aluminides of type AlFe, AlTi, AlNi.

13. A process of producing aluminium matrix composite materials consisting of a porous fibre preform into which an aluminium alloy was infiltrated, characterised in that the fibres of the fibre preform are produced by a melt extraction process, are processed to obtain a homogenous or gradient structure and are connected by sintering to form a firm, porous preform;
that the fibre preform is pre-heated to a temperature of > 200 °C; and that subsequently, a silicon-containing aluminium casting alloy melt is infiltrated into the fibre preform.

14. A. process according to any one of the preceding claims, characterised in that the fibre preform is provided in the form of a shaped fibre member with a specific fibre orientation of the metallic fibres, with the points of intersection being sintered.

15. A process according to the preceding claim, characterised in that infiltration takes place in accordance with the gravity casting process.

16. A process according to any one of the preceding claims, characterised in that infiltration takes place in accordance with the pressure die casting process at a gating speed of >
5m/sec.

17. A process according to any one of the preceding claims, characterised in that a pressure die casting alloy is infiltrated into the fibre preform in a pressure die casting die at a minimum pressure of 80 bar.

18. A process according to any one of the preceding claims, characterised in that the fibre preform comprises an open porosity of 20 - 98 %, with porosity being controlled by the fibre geometry and the fibre orientation during the layering operation.

19. A process according to any one of the preceding claims, characterised in that the pore size of the fibre preform is determined by the fibre content per unit of volume of the preform, ranging from 20 µm to 1000 µm.

20. A process according to any one of the preceding claims, characterised in that infiltration takes place during a pressure-assisted casting process, with the pressure being maintained until a diffusion zone has formed between the fibre material and the matrix.

21. A process according to any one of the preceding claims, characterised in that while aluminium melt is being infiltrated, or after aluminium melt has been infiltrated, a matrix material is cast around the fibre preform, which matrix material is selected from one one or several metals of the following group:
aluminium silicon magnesium zinc (casting materials) in combination with the intermetallics of iron-, nickel-, titanium-aluminides tungsten, copper, cobalt and/or magnesium.