GB1562270A - Process for manufactruing a composite structure having an aluminium matrix - Google Patents

Description

(54) PROCESS FOR MANUFACTURING A COMPOSITE STRUCTURE HAVING AN ALUMINIUM MATRIX (71) We, UNITED TECHNOLO GIES CORPORATION, a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 1, Financial Plaza, Hartford, Connecticut, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for manufacturing a composite structure having an aluminium matrix with a plurality of layers of filaments therein, and a composite manufactured by the process.

The quick-bond process described in Patent No. 1,426,476 was developed to make possible the manufacture in air of an aluminium matrix composite reinforced by a plurality of parallel layers of unidirectional filaments.

This "air bonding" process comprises positioning a plurality of filament-reinforced aluminium matrix monolayer tapes in a stack and then pressing the stack between heated platens or dies at high pressure in air to densify the matrix, the platen temperature causing bonding of the matrix to the filaments.

It was believed, in the prior Patent, that the monolayer tapes were best made by bonding the layer of filaments to an aluminium backing foil by a plasma-sprayed metal coating, such as aluminium, on the surface of the filaments opposite to the foil. It was hoped that other less expensive forms of tapes could be developed that would produce equally acceptable matrix composites.

According to this invention there is provided a process for manufacturing a composite having an aluminium matrix with at least one layer of filaments therein, comprising the steps of forming a single layer of collimated substantially parallel filaments of boron, silicon carbide, or silicon-coated boron, securing the layer of filaments into a mat by interweaving metallic crosswires with the filaments, assembling the mat or a plurality of the mats with aluminium sheets to form a stack such that the or each mat is disposed between a pair of sheets, and densifying and bonding the stack in air to form a composite.

Since the compacting and bonding is done in air, as in the process of the prior Patent, use of a vacuum chamber for the compaction and bonding is avoided as well as the long time cycle required for vacuum pump-down and subsequent heating in a vacuum or inert atmosphere.

The invention will now be described by way of example, with reference to the drawing, in which: Fig. 1 is a perspective view of the tape; Fig. 2 is an end view of a stack of tapes; and Fig. 3 is an end view of a completed composite.

A particular method for assembly and bonding a plurality of stacked tapes is de- scribed in the prior Patent, where the assembly, densification, and bonding are all carried out in air thereby being a low-cost, quick bond with fewer limitations on the finished size of the composite. By this process the finished size is limited only by the size of the press available and not by the size of available vacuum chambers which were necessary previously as the densification and bonding were done in a vacuum. The process of this invention uses a simple form of tape that is particularly suitable in this process and which itself is simple to produce and inexpensive. This tape can be used in the air bonding process to produce an acceptable composite structure of adequate strength.

To produce this tape, a plurality of filaments or fibres 2, of boron fibres, silicon carbide fibres, or silicon-coated boron fibres, are collimated in a single layer, as shown in Fig. 1. These filaments in the layer are not in contact with one another and are held in the desired spaced relation and in the single layer configuration by crosswires 4 woven with the fibres, preferably at 900 to the fibres, and in spaced relation to one another. Only enough of these crosswires are used as are needed to hold the fibres in position while the tapes are being handled in making the composite. These crosswires are metallic and may be of aluminium to bond with the aluminium of the interleaved foils in forming the composite. This tape differs from the usual tape in that it has no backing as such. The tape is solely the fibres and the interwoven support wires. The usual aluminium foil backing sheet is not needed.

In use the aluminium for the matrix of the composite is provided by interleaving sheets 6 of aluminium foil between adjacent tape layers 8, Fig. 2. This arrangement permits faster assembly of the several tapes or sheets for the multilayer composite and permits better control of the percentage of matrix in the completed composite structure, since thicker or thinner aluminium roil sheets may be interleaved with the tapes or the number of itnerleaved foil sheets may be varied to produce the proper percentage of aluminium matrix.

The completed tapes, that is the fibres and the interwoven crosswires, are cut to appropriate sized strips for stacking with the interleaved foil sheets to build up the composite structure or stack 14, Fig. 2, and the assembled stack is then heated and densified by compression between heated platens or dies in a press, as described in the prior Patent, to form the completed composite 12, Fig. 3. The deformation of the aluminium foil sheets during compaction, and the heating of the composite to a temperature nearly the liquidus temperature of the foils assures a bonding of the aluminium foils to one another and to the fibres of the several tapes. The aluminium crosswires become an integral part of the matrix during the compaction and bonding under pressure.

It was expected that the fibres would oxidize during the assembly and densification because there was no coating on the fibres to prevent oxidation as in the plasma sprayed tapes.

High strength composites were achieved, however, even without the protection against oxidation. It was found desirable to minimize the assembly time for the composite and also to minimize the time prior to the application of pressure on the composite after positioning the composite between the heated platens.

It was found that adequate strength in the composite could be obtained with a tape of this character and even with the densification and bonding occurring in air. It is believed that the minimum of oxidation that occurs in the process is not detrimental and the loss of fibre strength due to oxidation may be reversible and may in fact produce a stronger fibre if the extent of the oxidation is limited by the time involved in the densification and bonding.

These tapes and the resultant composite are significantly lower in cost than the plasma sprayed tapes described in the prior Patent.

It was originally expected that this woven mat type of tape would prove unusable in air bonding because 1) of the oxidation of the fibres since there was no coating on the fibres that would be impervious to air and 2) the matrix would not bond because of the oxide on the fibres.

Neither of these problems appears to be serious. As above stated, by minimizing the time prior to the application of pressure in the press any oxidation has been minimised.

Further, the amount of oxidation that takes place does not appear to reduce significantly the fibre strength. The ability to handle the woven mats of fibres independently of the aluminium foil sheets making up the matrix has facilitated the stacking of the mats and foils in making the composite and has also permitted a control of the percentage of matrix in the completed composite.

The results of tests showed these mats to be comparable to the plasma sprayed tapes of the prior Patent when in the completed composite structure, as shown in the following comparison:-

Fibre Strength Three Point Bend Strength Tape (kg/cm) (kg/cm) Panel Size Torsional Fibre Binder ATM (cm) Tape Composite Test Measured 50% Fibre 142.24 B Plasma Air 5.0 x 12.7 - 41880 # 2250 Excellent 26200 25060 24920 203.2 B 5056 wire Air 1.27 x 7.62 33700 # 4500 34680 # 3940 Excellent 9504 19000 203.2 B 5056 wire Air 1.27 x 7.62 33700 # 4500 34620 # 5550 Excellent 11065 19700 The 5056 wire in an aluminium wire 0.05 mm in diameter and these wires were spaced apart about 6.35 mm in the tape or mat. The "tape" fibre strength is the strength of the tape fibre before being made into the composite, the "composite" fibre strength is that of a fibre extracted from the composite. The "measured" bending strength is determined by experiment and the 50% bend strength is determined by calculation, based on the assumption of 50% fibres by volume in the composite and utilization of the measured strength.

The data was obtained from composites made as above described and bonded in air.

The fibre strengths and torsional performances all indicated excellent material strength in the finished product as compared with the plasma sprayed composites. The composite structures made with the woven mat fibres were shown to be adequate for any of the presently known uses for this type of composite.

When the stack of tapes is completed, the top of the stack is preferably covered by a layer of foil 14 to form a covering for the exposed fibres of the top tape or tapes in the stack. This top layer of foil serves to protect these fibres from oxidation during heating and compaction; and when the compaction is completed, this top foil is bonded to the adjacent foil and to the enclosed fibres in the same manner as the interleaved foils between the tapes.

It will be understood that a stack may in certain instances consist of a single tape with foil on each side where the thickness of the completed compacted composite does not permit more than the single layer of tape.

Whether the composite is made from a single tape stack or a multitape stack, the crosswires of the tape, if aluminium, are incorporated into and become an integral part of the matrix.

In this way the composite, when densified and bonded, consists of the filaments and matrix; the wires, having served their purpose, have disappeared into the matrix.

Claims (7)

1. WHAT WE CLAIM IS:1. A process for manufacturing a composite having an aluminium matrix with at least one layer of filaments therein, comprising the steps of forming a single layer of collimated substantially parallel filaments of boron, silicon carbide, or silicon-coated boron, securing the layer of filaments into a mat by interweaving metallic crosswires with the filaments, assembling the mat or a plurality of the mats with aluminium sheets to form a stack such that the or each mat is disposed between a pair of sheets, and densifying and bonding the stack in air to form a composite.
2. 2. A process according to claim 1 wherein the densification and bonding is done by heating and pressing the stack between heated platens.
3. 3. A composite manufactured by the process according to claim 1 or claim 2 and comprising a plurality of tapes each comprising a collimated single layer of parallel filaments of boron, silicon carbide, or silicon-coated boron and metallic crosswires spaced apart and interwoven with the filaments to hold the filaments in a collimated single layer mat form, a plurality of layers of aluminium foil interleaved with the plurality of tapes to form a stack comprising alternate layers of aluminium foil with layers of filaments there-between, the stack being heated and compacted in air to press the foil around the filaments of the several layers and to cause bonding of the adjacent foil layers to one another between the filaments and to the filaments to produce the densified composite.
4. 4. A composite according to claim 3 wherein the crosswires are of aluminium.
5. 5. A composite according to claim 3 or claim 4 wherein the crosswires become an integral part of the matrix in the densified composite.
6. 6. A process according to claim 1 for manufacturing a composite structure having an aluminium matrix with a plurality of layers of filaments substantially as herein described with reference to the drawing.
7. 7. A composite according to claim 3 substantially as herein described with reference to the drawing.