GB2257720A

GB2257720A - Fibre preform and process for its manufacture,for use in aluminium castings

Info

Publication number: GB2257720A
Application number: GB9213767A
Authority: GB
Inventors: Josef Penkava
Original assignee: Vereinigte Aluminium Werke AG; Vaw Aluminium AG
Current assignee: Vereinigte Aluminium Werke AG; Vaw Aluminium AG
Priority date: 1991-07-17
Filing date: 1992-06-29
Publication date: 1993-01-20
Also published as: FR2679226A1; DE4123677A1; GB9213767D0

Description

22 3 -7 7 2,j 1 Fibre preform and process for its manufacture, and the use

of the preform for manufacturing fibre-reinforced aluminium casting-s The invention relates to a fibre preform and to a process for its manufacture, and to the use of such a preform for manufacturing fibre- reinforced aluminium castings.

Metal fibre composite materials are those materials in which metal or nonmetal, continuous or discontinuous fibres are embedded in a metal matrix. The properties of such metal fibre composite materials are a result of the special mechanical, physical or chemical properties of the matrix and fibres. Therefore, the fibre geometry, the fibre quantity, and the fibre arrangement inside the composite material are particularly important. In addition, obviously questions of adhesion and wetting between the fibres and matrix material are of paramount importance.

The commonest method of manufacturing a composite material of the type mentioned in the introduction is to embed a separately manufactured reinforcement phase in the matrix, e.g. by melt infiltration, fibre coating and pressing, the pressing of fibre- and matrix-foil stacks, and in special cases, powder metallurgical embedding (see Ullmann, Vol. 23, pages 545 ff.). In this case, the reinforcement phase is formed either only of short fibres or whiskers or only of long fibres. In the pressing of fibre- and matrix-foil stacks, the individual fibres are held together mechanically and then the prefabricated fibres are inserted into the matrix.

Pure short-fibre preforms are very suitable for the manufacture of fibrereinforced metal castings, e.g. by pressure infiltration. The local fixing of the short fibres by the binder needs to be sufficiently strong that the fibre preforms can withstand the in-flowing melts without changing their shape. The high heat storage capacity and the homogeneous porosity permit problem-free, complete infiltration by the metal melt.

2 The strength values obtainable with such short-fibre-reinforced castings are generally higher than the strength values of unreinforced components, but the gain in strength is only limited, as the short fibres cannot counteract in a controlled manner stress of the reinforced casting in a specified direction, due to their random distribution.

In the case of pure long-fibre preforms, the usually very strong fibres absorb some of the external stress, and the maximum strength under load is found in the direction of the fibres. The transfer of force from the matrix to the fibres is effected by shearing forces at the fibrelmatrix border face, so that the properties of the composite material are predominantly affected by the fibre-matrix consistency. The long fibres are arranged predominantly parallel in the direction corresponding to the main stress direction of the fibre preform or of the fibre-reinforced casting. The long fibres must not in this case be in contact with one another over their whole length or over extended portions, but must only touch here and there, so that they can be surrounded as completely as possible by the infiltrating metal melt.

In many cases, the prefabricated fibres are introduced into the matrix uncoated or optionally provided with a layer of adhesive, but fibres are also partly coated with the matrix material even before being added to the composite material. A substantial advantage of this technique is the good adjustability of the volume ratio of matrix to fibres. The coating of individual fibres can be carried out with great accuracy, so that a definite volume ratio is also present in the finished composite material.

If a binder is used, this must be metered and distributed in such a manner that on the one hand cohesion of the preform is ensured, and on the other hand a slight gap between individual fibres is retained, in order that the preform can be infiltrated with the metal melt.

1 3 Obviously, in this fibre compound, the strength of the preform is comparatively low, and there is a consequent risk in the case of infiltration that the preform might lose its shape and the reinforcement of the casting might not be completely successful.

A further disadvantage of long-fibre preforms is the low heat storage capacity, with the result that they cool off rapidly after preheating to prepare them for infiltration. The metal melt also cools relatively rapidly after infiltration, so that the preform is not always completely filled. This leads to irregularities in the external stress of long-fibrereinforced castings.

Due to the different manufacturing processes, the mechanical properties of individual composite materials vary widely. In whisker-reinforced materials, usually lower values are found than in those reinforced with fibre, which is ascribable to the high dispersion of the whisker properties and damage of the whiskers in the manufacture of composite materials. Furthermore, the whiskers are very expensive compared to other fibre reinforcement materials due to their complex manufacture, and being respirable are damaging to health.

The object of the present invention is to indicate a fibre preform and a process for its manufacture which leads to homogeneous porosity, high strength and shape stability, high heat storage capacity, and good infiltration properties for metal melts. If the fibre preform is used for the manufacture of fibre-reinforced castings, in addition to a generally higher strength in all dimensions, in particular heat-resistance in the main stress direction of the casting is to be substantially improved.

4 This object is achieved according to the invention by the features indicated in the patent claims. In particular, there is provided according to this invention a fibre preform containing oxide ceramic short fibres fixed with a binder, which is characterised in that the fibre preform consists of a plurality of layers of a short fibre matrix formed of homogeneously distributed short fibres sintered together at their points of intersection, and in that long fibres with point-contact are located between the layers of the short fibre matrix, the individual long fibres being surrounded on all sides by support fibres. Preferably, long fibres are embedded in the short fibre matrix of the preform.

It has been found that the particular structure of the porous preform from a plurality of layers and the arrangement of short, long, and support fibres permit use for pressure infiltration for the manufacture of fibre-reinforced castings from aluminium or aluminium alloys. The metal-ceramic composite parts thus produced are characterised by a high density combined with low porosity and by a very good heat resistance, in particular in the longitudinal direction of the long fibres, and improved strength in the direction transverse to the longitudinal direction of the fibres.

The fibre preform is advantageously constructed from a plurality of layers of a short-fibre matrix, which is formed of homogeneously distributed short fibres sintered together at their points of intersection, and long fibres contacting one another locally and surrounded on all sides by support fibres are located between the layers of the short-fibre matrix.

1 The average length of the short fibres is preferably 1 - 5mm, whereas the average length of the support fibres is 50pm - 1 00pm. In this case, it has been found advantageous if the average diameter of the short and support fibres is 5 - 1 5lim, whereas the average diameter of the long fibres is 30 - 80pm. Under these conditions - together with a proportion by volume of short fibres in the preform of 25 - 60 % by volume - a porous fibre preform is obtained with pore sizes particularly suitable for infiltration of the melt.

For highly stressed machine parts, such as connecting rods, piston pins, piston heads, piston ring carriers, or cylinder heads for internal combustion engines, a proportion by volume of long fibres in the preform which has been found advantageous is between 5 and 35 % by volume. In this case, the average spacing of the long fibres in the preform should be between 10 and 50pm.

In the manufacture of the porous fibre preform according to the invention, it is important that the base materials and additives consist of the suitable materials. As a result of numerous investigations, the short fibres and support fibres consist of A1203 or mullite and the long fibres of A1203, SiC or carbon, whereas the binder is constructed from an organic component, e.g. starch, and an inorganic component with an Si02 or A1203 base. The process according to the invention provides that a mould is filled with a suspension of short fibres, water and a binder mixture, and long fibres, evenly spaced, together with support fibres, are applied to this first layer, and water is simultaneously extracted from the mould, then the layers are pressed until a fibre body having a stable shape is obtained. This process can be continued until a multi- layered structure leads to the desired fibre preform. Then drying is carried out at increased temperature and after removal of the body, firing at 800-10000C is carried out.

6 It is also possible to embed long fibres direct in the short-fibre matrix of the preform. This is achieved by incorporating individual long fibres into the first suspension of short fibres, homogeneous distribution being promoted by the simultaneous application of vibration.

But in this case, the proportion of short fibres should not be lower than a limit of 5 % by volume.

The invention is explained in more detail below with the aid of an example illustrated in the accompanying drawing.

A mould 1 is filled with a first layer 2 of a suspension of short fibres 3, water and a binder mixture. Then long fibres 4 together with support fibres 5 are applied, and water is simultaneously extracted from the mould 1 via the line 6.

To promote the homogeneous distribution of the long fibres, a vibrator 7 can be applied to the mould 1. A further layer 8 of a suspension of short fibres 9 is applied to the layer of long fibres 4. This is repeated until the mould is completely filled. To improve water discharge, the interior of the mould 1 is lined with a porous material 10.

The preform composed of the various layers is pressed into the mould with a pressure plate 11, in particular with simultaneous further extraction of residual water via the line 6. Drying of the mechanically dehydrated fibre preform is carried out at approximately 110 1 C in a time of 12-24 hours. The necessary green strength is achieved by the organic binder component, which further facilitates handling of the preform until the end of the firing operation.

7 After removal of the green body, sintering of the different fibres to form a homogeneously porous fibre preform is carried out in a separate firing process at approx. 1000 'C, and due to the sintering processes between the inorganic binder and the fibres, firm adhesion of the fibres, and hence a body with a stable form and high final strength, is achieved.

Due to the specific pore distribution, the fibre preforms according to the invention are particularly suitable for manufacturing fibre- reinforced metal pressure-diecast parts, e. g. by melt infiltration. The final form of the product can be specified as early as during manufacture of the fibre preform by appropriate shaping of the mould. But a preform can also be manufactured from which, after infiltration, the finished component is produced by machining. It is also possible to carry out shaping by machining after drying or after firing, e.g. by sawing, grinding or the like, but not until after infiltration with the metal melt, e.g. by known joining methods such as melt diffusion welding, gluing etc..

The type of bond between the fibres is a decisive factor for the strength and shape stability of the fibre preform, and is therefore necessary for subsequent processing. The binder according to the invention composed of an inorganic and an organic component collects preferably at the points of contact between the fibres. At these points, it forms strong bridge connections, which are strengthened in the subsequent thermal processes.

The cohesion of the preform is greater the higher the number of points of contact between the fibres. However, it must be ensured that the fibres only have point contact and not linear contact, for example. In the case of extended contact over part of or even the whole length of the fibres, virtually solid areas form in the preform into which no metal melt can penetrate during infiltration. This leads to a distinct decrease in strength and hence to a local weak spot, which has a strength distinctly below that of a non-reinforced casting.

8 To avoid this problem, which arises particularly if a large proportion of long fibres are used, it is proposed according to the invention to envelop the long fibres with support fibres and embed them evenly in the short fibre matrix. The support fibres in this case ensure the necessary distance between the long fibres and a maximum possible number of pointlike contact places both with the short fibres and with the long fibres. The result of the structure according to the invention is manifested in a long fibre firmly integrated into the preform, the necessary porosity for melt infiltration between the fibres involved being retained.

Obviously, the long fibres are preferably so arranged in the fibre preform that they are oriented parallel to the main direction of stress in the fibre-reinforced metal casting. In the example chosen, the mould 1 and the fibre content are shown in cross-section, so that the main direction of stress lies in a plane perpendicular to the plane of the drawing. The long fibres should if possible extend over the whole length of the preform in order to guarantee maximum strength in the main direction of stress.

For the manufacture of fibre-reinforced castings from aluminium or aluminium alloys, short fibres and support fibres of A1203 or mullite and long fibres of A1203, SiC or carbon have been found to be advantageous. These fibres are sufficiently temperature-resistant and have good wettability with respect to the aluminium melt. Furthermore, they are sufficiently chemically stable with respect to the aluminium melt and are eminently suitable for sintering with the mixture of inorganic and organic binders used.

9 The average lengths and diameters of the short and support fibres, as well as of the long fibres, are such according to the invention that, after embedding in the binder mixture and sintering, a porous preform is obtained, which has a particularly suitable micro-macro-porosity for melt infiltration. It has been discovered that the microporosity arising after sintering due to contraction fissures has a substantial effect on the suitability for pressure infiltration. Adhesion between the fibres, sinter material and metal melt is substantially improved thereby.

After drying of the preform at increased temperature and pressure, at first a green strength is achieved which must be sufficient to remove the preform from the mould 1 and optionally after initial shaping - to subject the preform to the actual sintering process. It was discovered in the case of test bodies that a proportion by volume of short fibres in the preform of at least 5 % by volume must be achieved in order to establish adequate green strength for subsequent handling of the preform. However, the shape stability in subsequent pressure infiltration requires a higher proportion by volume of short fibres, which should be at least 15 % by volume, since otherwise an increase in the strength in the fibre- reinforced casting of less than 10 % is to be expected in comparison to a non-reinforced casting.

Above 60 % by volume, the optimum packing density of the short fibres between the long fibres is exceeded, so that even in the case of a low long fibre content of less than 15 % by volume, the fibres may break up during manufacture if the preform is compressed. Due to the inhomogeneity and weakness then arising in the long fibre components, adequate strength of the casting is not ensured after infiltration of the fibre preform with the metal melt.

Similarly, a proportion of long fibres of less than 5 % by volume in the fibre-reinforced casting is ineffective, since no strength increase can be achieved.

Above 35 % by volume, the gap between long fibres is so small that a sufficient proportion of short fibres or support fibres between the long fibres is not ensured. Investigations have shown that the long fibres in this case have extensive contact with one another, giving rise to the disadvantageous consequences mentioned in the introduction. The optimum long fibre proportion in the fibre preform is therefore between 15 and 30 % by volume in order to permit a sufficient safety margin from the abovementioned limit values.

According to a particular embodiment of the invention, the proportion by volume of long fibres as a function of the long fibre diameter should be so chosen that the spacing between the long fibres - measured between the opposing fibre surfaces - is between 8 and 70pm. In the case of a long fibre spacing of more than 70pm, the possible strength increase of the fibre-reinforced casting in the direction of orientation of the long fibres is relatively slight. At spacings of less than 8pm, the same disadvantageous consequences are to be expected as in the case of long fibre contents of above 35 % by volume.

According to the invention, the binder content is 2 - 10 by weight relative to the total fibre content. A binder content of at least 2 % is necessary in order to ensure high strength by strong bonding of the long fibres into the short fibre matrix. If the binder upper limit of more than 10 % by weight is exceeded, there is a risk that homogeneous total porosity will be unachievable so that the melt infiltration will be irregular and, in parts, incomplete. At a binder content of 5 - 8 % by weight, a very high mechanical stability is achieved relative to the infiltrated melt, which is under very high pressure. Furthermore, a porous preform with the preferred proportion of binder has a very high heat storage capacity compared to a body containing a lower proportion of binder.

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Claims

1. Fibre preform, containing oxide ceramic short fibres fixed with a binder, characterised in that the fibre preform consists of a plurality of layers of a short fibre matrix formed of homogeneously distributed short fibres sintered together at their points of intersection, and in that long fibres with point-contact are located between the layers of the short fibre matrix, the individual long fibres being surrounded on all sides by support fibres.

2. Fibre preform according to claim 1, characterised in that long fibres are embedded in the short fibre matrix of the preform.

3. Fibre preform according to one of the preceding claims, characterised in that the embedded long fibres are oriented in at least one direction of the preform and extend over the whole length of the preform in this direction.

4. Fibre preform according to one of the preceding claims, characterised in that the short fibres and support fibres consist of A1203 or mullite.

5. Fibre preform according to one of the preceding claims, characterised in that the binder consists of a mixture of starch, Si02 and/or A1203.

6. Fibre preform according to one of the preceding claims, characterised in that the long fibres consist of A1203, SiC or carbon.

7. Fibre preform according to one of the preceding claims, characterised in that the average length of the short fibres is preferably 1 - 5mm, whereas the average length of the support fibres is 50tm - 10Oym.

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8. Fibre preform according to one of the preceding claims, characterised in that the average diameter of the short fibres and support fibres is 5 - 15gm, whereas the average diameter of the long fibres is 30 - 8Ogm.

9. Fibre preform according to one of the preceding claims, characterised in that the proportion of short fibres in the preform is 5 - 60 % by volume, preferably 25 - 50 % by volume.

10. Fibre preform according to one of the preceding claims, characterised in that the proportion of long fibres in the preform is between 5 and 35 % by volume, preferably between 15 and 30 % by volume.

11. Fibre preform according to one of the preceding claims, characterised in that the average spacing of the long fibres in the preform is 8 - 70gm, preferably 10 - 50gm.

12. Fibre preform according to one of the preceding claims, characterised in that the binder content is 2 - 10 % by weight relative to the total fibre content.

13. Process for manufacturing a fibre preform according to one of the preceding claims, characterised in that a mould is filled with a first layer of a suspension of short fibres, water and a binder mixture composed of organic and inorganic binders, then long fibres, evenly spaced, together with support fibres, are applied to this first layer, and water is simultaneously extracted from the mould, in that a second layer of a suspension consisting of short fibres, water and a binder mixture composed of organic and inorganic binders is then applied to the long fibres, and in that the layers are pressed until a fibre preform having a stable shape is obtained, and in that drying is then carried out at increased temperature and the fibre preform is subjected to firing at 800 - 1000 0 C after removal f rom the mould.

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14. Process according to the preceding claim, characterised in that during application of the first and second layers respectively of a suspension of short fibres, water and a binder mixture, long fibres together with support fibres are introduced into the mould.

15. Process according to one of the preceding claims, characterised in that during application of the layers, the mould or contents of the mould are subjected to vibration.

16. Process according to one of the preceding claims, characterised in that the preform is dehydrated under pressure before the drying process until a minimum content ol 5 % by volume short fibres is obtained.

17. Apparatus for carrying out the process according to one of the preceding claims, characterised in that a drainage sieve (10) with a drainage line (6) is mounted in a metal mould (1), in that the filling aperture of the mould is closable with a pressure plate (11), and in that the mould (1) and/or the pressure plate (11) communicates with a vibrator (7) in order to subject the mould contents to an oscillating load.

18. Use of a porous fibre preform composed of oxide ceramic short fibres, wherein individual long fibres encased in support fibres are embedded in the short fibre matrix of the preform, in order to manufacture fibrereinforced aluminium or aluminium alloy castings by melt infiltration.

19. Use of a porous fibre preform according to the preceding claim for manufacturing press ure-d i ecast parts with a heat resistance increased by 10 %.