GB2029881A

GB2029881A - Metallic multi-layer composite material

Info

Publication number: GB2029881A
Application number: GB7914459A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1978-04-29
Filing date: 1979-04-25
Publication date: 1980-03-26
Also published as: GB2029881B; DE2819076C2; DE2819076A1

Abstract

A metallic multi-layer composite material suitable for components which are highly stressed both thermally and mechanically e.g. aircraft and spacecraft skin panels or turbine blades comprises a substrate 2 of titanium or titanium alloy to which is brazed a first layer 3 of fibre-reinforced aluminium to which is brazed a second layer 4 of wear and abrasion-resistant metal. The first layer can be of aluminium reinforced with boron carbon, alumina or silicon carbide fibres and the second layer can be of steel or titanium. When the second layer is steel it is plated with aluminium or aluminium alloy vapor deposited to act as brazing material. When the second layer is titanium its oxide film has been removed and replaced by copper, magnesium, zinc, silver or silicon in vacuum to similarly act as brazing material. <IMAGE>

Description

SPECIFICATION A metallic multi-layer composite material The invention relates to a metallic multi-layer composite material having a substrate of titanium or titanium alloy particularly for lightweight components which are highly stressed both mechanically and thermally.

At temperature of 300"C to 500"C a composite material of titanium with fibre-reinforced aluminium has a very favourable stiffness-to-weight ratio. The brazing technique previously used to join titanium and aluminium yields, in the required temperature range, results having good shear values. As used throughout this specification the term "brazing" means joining two metallic layers together by sandwiching a layer of metal (pure metal or alloy) between them and heating to cause the metal to fuse and bond the layers together. The metal will be hereafter referred to as "solder" and can be first coated onto one of the layers before brazing. The solder can however, also be introduced between the layers to be joined by other methods.Difficulties are, however, caused in these brazing methods by the stable titanium oxide layer which adheres to the titanium and which, during brazing, makes the complete wetting of the surface difficult. Therefore, in the past satisfactory brazing has been achieved only in a high vacuum.

Some components, particularly exterior parts of air and space vehicles such as rockets, are subject, in addition to thermal and mechanical stress, to erosion by rain or sand, cavitation by flow processes and impact by foreign bodies. The surface of such components have to be able to withstand such influences, expecially when a coating of fibrereinforced aluminium is present, because in this material the matrix is relatively sensitive to erosion and vacitation and the fibres are sensitive to impact. Because of the complex problems posed by the above-listed stresses only partially successful solutions have become known, such as coatings of pure aluminium or of glass-fibre-reinforced plastics material (GRP), in which respect, however, the desired properties were not totally achieved.Pure aluminium is indeed ductile and corrosion-resistant, but does not have the requisite strength properties. GRP has, like all fibre-reinforced materials, its optimum strength only in the direction of the fibres.

Using cross laminates gives only limitedly better properties.

An object of the invention is therefore to provide a metallic multi-layer composite material which withstands extreme and comprehensive thermal and mechanical stresses.

Accordingly the invention provides a metallic multi-layer composite material more especially for the production of components which are highly stressed both thermally and mechanically, comprising a substrate having a first layer of fibre-reinforced aluminium brazed thereto and a second layer of a wear-resistant and abrasion-resistant metal sheet brazed to overlie the first layer.

A multi-layer composite material of this kind combines the advantages of high-strength metallic materials with those of fibre composite materials and fulfils, to a high degree, the demands which are made on thermally and mechanically highly-stressed lightweight components. The titanium substrate and the second coating of highly wear-resistant and abrasion-resistant metal sheet impart strength to the material, especially shear and bending strength. The first coating of fibre-reinforced aluminium imparts stiffness to the material and additional strength in the direction of the fibres.

In accordance with a further development, the aluminium is reinforced by unidirectional boron fibres. This material combination is commercially available and has proven itself preferentially for lightweight components subject to tensile stress. However, it is possible to use, instead of boron fibres, other commercial fibres, such as carbon aluminium oxide or silicon carbide fibres.

The second coating can be of steel sheet plated with aluminium or of titanium sheet advantageously alloyed with aluminium. The main function of the second coating is to impart to the material a high-strength outer skin which withstands extremely high external wearing stresses such as rain, erosion, cavitation and foreign-body impacts. These stresses occur in high-speed turbine blades and in exterior components of aircraft, missiles and space vehicles travelling at high speed. It has in the past not been possible to develop a lightweight material which, with justifiable costs, is a match for all these stresses. The sole use of special steels, which are alloyed for example with chromium or vanadium, yield, as a result of their high specific weights, component weights which are too high.If, however, as in the material of the invention, a steel sheet is used for the second coating, then this can be kept very thin, to compensate for the associated weight increase. Titanium alone does not fulfil all the strength requirements and is very expensive.

The use of titanium with other materials, such as boron-fibre-reinforced alumimium, is indeed known, but such a composite material is not ideal, because aluminium does not withstand erosion or cavitation and the boron fibres are destroyed by impact. Moreover, in the past the brazing of titanium has been very difficult and technically complicated, because a very durable titanium oxide layer, which has to be removed prior to brazing, forms on titanium in the atmosphere.

In order also to be able to braze titanium without a protective gas atmosphere or a vacuum, the oxide layer is removed prior to brazing and a metallic protective layer is applied, which layer serves as solder. The brazing of the two layers can be effected at a temperature in the region of the eutectic of the coatings or of the applied metal layers.

Copper can be applied as the metallic protective layer. Removal of the titanium oxide layer can be effected by pickling and the application of the copper layer is effected, for example, in a high-vacuum chamber. Residues of the titanium oxide layer can be removed by ion etching and the copper layer deposited by evaporation or atomisation.

The brazing of the two coatings can then be effected subsequently without special protective measures at a temperature in the region of the respective eutectic or aluminium and the applied metal layers. The metallic protective layer, applied to the titanium, together with the aluminium serves as soldering material. The protective layer, to be applied only in a thickness of about 1 ym, diffuses upon the soldering process completely into the aluminium. If the protective layer on the titanium is of copper, the connection of the titanium substrate to the first coating of fibre-reinforced aluminium is effected at 548"C, which is the temperature of aluminium-copper eutectic.

Apart from copper, still further protective layers are possible, such as magnesium, zinc, silver or silicon, which have their eutectic with aluminium in conventional temperature regions. The connection between the fibre-reinforced aluminium and the second coating can be effected in the same way when the latter is of titanium sheet.

Conditions are different when steel sheet plated with aluminium is used as the second coating. In order to achieve, here too, a fluxfree soldering below the melting point of aluminium, the Al-Si eutectic, which lies at 585"C, is utilised, by an appropriate Al-Si alloy being used for the plating. On account of the different temperatures on the liquids phases of Ti-Al and Al-Si, the two coatings are first brazed together and these are brazed jointly onto the titanium substrate. A particularly high-strength second coating can be achieved upon brazing if aluminium diffuses into the second coating and in so doing forms, with the material of the second coating, an erosion-resistant intermetallic phase.

This is made possible by the second coating being in the form of a particularly thin sheet, whereby aluminium from the first coating can diffuse into the second coating. The intermetallic phase which forms in this respect is particularly hard and abrasion-resistant. This process is assisted if there is used, as titanium sheet, a Ti-Al compound which contains about 6% aluminium because titanium becomes brittle with an aluminium content of over 6%.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a metallic multi-layer composite material conforming to the invention; Figure 2 shows a turbine compressor blade; and Figure 3 shows the design of a wing profile for a flying body.

The thicknesses of the layers shows in the Figures of the metal are, for the sake of better representation, shown thicker and do not necessarily correspond to the layer thicknesses actually used. The basic construction of a metallic multi-layer composite material 1 is evident from Fig. 1. Applied to a substrate 2 of titanium or titanium alloy is a first layer 3 of fibre-reinforced aluminium and a second layer 4 of sheet metal. A titanium alloy used for the substrate 2 is, for example, TiAI6V4, in which there are contained proportions of 6% aluminium and 4% vanadium. In the first layer 3 of fibre-reinforced aluminium, the fibres can be of boron. The second layer 4 is of sturdy steel sheet or titanium sheet. It is, however, equally possible to use different high-strength metal sheets for the second layer.

The production of the depicted multi-layer composite material 1 is effected by brazing (as hereinbefore defined). For this the titanium sheet 2 is coated, after the removal of the titanium oxide layer, with, for example, a copper coating. The second layer 4 is similarly provided with a very thin metal coating. In the event of the second layer consisting of steel sheet, the latter is plated with aluminium, in the case of the choice of titanium this is provided with a copper coating. First the two layers 3 and 4 are soldered together in the temperature range of the respective eutectic of aluminium and of the metal coating applied to the second layer 4. After that, the connected layers are brazed jointly onto the titanium substrate 2 at the eutectic of aluminium and copper. The respective brazing process is effected without use of a flux.

Shown in Fig. 2 is a double blade 6 whose foot is let into a turbine disc 7. The blade 6 has a substrate 8 of titanium or titanium alloy, to which a first layer 9 of boron-fibre-reinforced aluminium and a second layer 10 of steel sheet are applied. The layers 9 and 10 are, as described with reference to Fig. 1, connected by brazing to one another and then to substrate 8. The layers 9 and 10 absorb bending and torsion forces as well as the loads applied in the blade foot. The second layer 10 of steel effectively resists abrasion and erosion which, in the case of turbine blades, is caused by cavitation or foreign-body impact.

The profile 1 2 of Fig. 3, which represents, for example the cross-section of a wing of a flying body flying at multiple supersonic speed, is constructed in the same way as the double blade 6 in accordance with Fig. 2. It thus consists of a substrate 13, a first layer 14 of boron-fibre-reinforced aluminium and a second layer 15, which advantageously is a thin sheet of the alloy TiA16V4. Upon brazing, aluminium diffuses out of the first layer 1 4 into the titanium alloy of the second layer 1 5 and produces in this an embrittled intermetallic phase, which imparts an exceptionally hard, abrasion-resistant surface to the titanium alloy layer, which surface effectively protects the wing against erosion and foreign-body impact at high speed. The boron fibres of the first coating 14 lie in the main load direction and absorb a large part of the tractive forces which occur.

Claims

1. A metallic multi-layer composite material more especially for the production of components which are highly stressed both thermally and mechanically, comprising a substrate having a first layer of fibre-reinforced aluminium brazed thereto, and a second layer of a wear-resistant and abrasion-resistant metal sheet brazed to overlie the first layer.

2. A metallic multi-layer composite material as claimed in claim 1, wherein the aluminium is reinforced with unidirectional boron fibres.

3. A metallic multi-layer composite material as claimed in claim 1, or 2 wherein the second layer is of steel sheet plated with aluminium.

4. A metallic multi-layer composite material as claimed in claim 1, wherein the second layer is of titanium sheet.

5. A metallic multi-layer composite material as claimed in claim 4, wherein the titanium sheet is alloyed with aluminium.

6. A metallic multi-layer composite material as claimed in claim 1, 4 or 5, wherein prior to brazing on the titanium substrate and the titanium sheet, the oxide layer is removed and a metallic protective coating applied, which protective coating serves as solder upon brazing.

7. A metallic muiti-layer composite material as claimed in any of claims 1 to 6, wherein the brazing of the two layers is effected at a temperature in the region of the eutectic of the layers or of the applied metal coatings.

8. A metallic multi-layer composite material as claimed in any of claims 1 to 7, wherein the two layers are brazed together and then jointly brazed onto the substrate.

9. A metallic multi-layer composite material as claimed in claim 1 or 7, wherein aluminium has been allowed to diffuse into the second layer upon brazing and in so doing form with the material of the second layer, an erosion-resistant intermetallic phase.

1 0. A turbine blade made from a metallic multi-layer composite material as claimed in any of claims 1 to 9.

11. An outer component of a flying body, such as an aircraft, rocket or spacecraft made from a metallic multi-layer composite material as claimed in any of claims 1 to 9.

1 2. A metallic multi-layer composite material substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.