Fibre-Reinforced Metal Panels and Production Thereof

This invention is concerned with panels comprising a metal support sheet to which is adhered a layer of fibre-reinforced metal matrix, and with a process for their production.

Because of their light weight and very good mechanical properties, there is a considerable demand for such panels, particularly in the aeronautics industry. One of the main obstacles to the large-scale development of such fibre-reinforced metal panels is their high price which, in turn, is due to the complexity and low output of the processes by which they are manufactured. It is also difficult to carry out the non-destructive quality control of these products, which makes any improvement in the existing processes, in the sense of improving the reliability of the products formed, very desirable.

A process for the production of such panels is described in U.S. Pat. No. 3,615,277 and consists of spraying fine particles of molten metal, by means of an arc plasma torch, on to a thin metal foil (support foil) which is wound round a drum and which itself supports a winding of high modulus fibres. The metal particles sprayed at a high temperature, and with considerable force, fill the intestices between the fibres to form a metal matrix which coats and bonds the fibres, thus giving a panel of high quality when the support foil has been separated from the drum.

One of the main disadvantages of this process is that the adhesion of the metal particles to the support foil is generally low, which can cause cracks in the direction of the fibres when the panel is cut out on a template.

In order to improve the adhesion of the matrix metal to the support foil, various solutions have been proposed, which however only have a limited scope and add to the final price of the panels because of their complexity and/or because of the care which they require.

For example, the spraying conditions can be modified so that the metal particles arrive on the support foil at a higher temperature. Thus, the temperature of the plasma torch can be increased or the distance of the torch from the target can be reduced. These modifications can only be applied for a limited period otherwise the thermal stress is such that the panel obtained is distorted and/or some or all of the fibres undergo deterioration.

Another possibility consists of preheating the drum/support foil/fibres assembly. This is an operation which is complex and difficult to extend to mass production. Furthermore, it is found that the difference between the coefficients of expansion of the fibres and of the metal of the drum causes a high proportion of the fibres to break above a certain temperature.

It has also been proposed to subject the support foil to surface-stripping, for example by etching, before spraying the matrix metal. While it is found that this slightly improves adhesion, it requires an additional process step which has to be carefully controlled and is, therefore, not well suited to mass production. Such pre-treatment can also damage the support foil if the latter is very thin.

A further proposal comprises compacting the panel as initially obtained in a press (300-500 kg/mm.sup.2) at high temperatures (500.degree.-550.degree. C.) so as to cause the metal sprayed between the fibres to flow and to bring about solid phase diffusion. Disadvantages of this procedure are that it is discontinuous and requires heavy equipment.

We have now developed an improved fibre-reinforced metal panel and a method for its production.

According to the present invention, we provide a panel comprising a metal support sheet having on a surface thereof a thin coating of a bonding metal or alloy and adhered to the bonding metal coating, a layer of a fibre-reinforced metal matrix.

The present invention also comprises a process for the production of a panel according to the invention, which comprises depositing a thin layer of the bonding metal or alloy on the support sheet, positioning a layer of reinforcing fibres on the bonding metal or alloy layer and spraying the matrix metal by means of an arc plasma torch on to the reinforcing fibres to form a metal matrix reinforced with the fibres.

The thin layer of bonding metal or alloy can be deposited by any suitable method, for example by electrolysis, chemical vapour phase deposition, or physical vapour phase deposition. The deposition is, however, preferably carried out by pad electrolysis of a suitable solution in accordance with the technique described in French Pat. No. 1,585,605, and preferably after electrolytic surface-stripping of the support foil.

The bonding metal or alloy is preferably one which is capable of diffusing rapidly into the metal or alloy which constitutes the support sheet, on the one hand, and into the matrix metal on the other, and of forming a solid solution or an alloy in the diffusion zone. The bonding metal or alloy is preferably also one that does not affect the mechanical properties of the matrix metal.

It is particularly preferred that the bonding metal or alloy should be less sensitive to oxidation than the metal or alloy of the support sheet.

Although it is theoretically possible to improve the adhesion of the sprayed metal to a support sheet merely by surface-stripping the latter in order to remove the oxide coating thereon, it is found in practice that this improvement can be difficult to achieve because of the high speed at which the oxide layer is re-formed.

Preferred bonding metals and alloys for use in accordance with the invention are, for example, copper, which exhibits particularly good compatibility with the metals or alloys of which the support sheets and sprayed powders are generally formed, zinc, gold, silver, tin, indium, iron, brass, gold-indium alloys, and low-melting alloys based on lead, tin, antimony and/or bismuth.

Suitable thicknesses of the bonding metal coating depend on the nature of the bonding metal, the nature of the metal support sheet and that of the matrix metal, and the spraying conditions (temperature and torch-target distance). Thus, if the rate of diffusion of the bonding metal into the metal of the support sheet is low, it is desirable to employ a relatively low thickness. Equally, if the bonding metal (or alloy) has a melting point which is not very high, but is, nevertheless, preferably above 200.degree. C., it is appropriate to deposit a relatively thick layer of the bonding metal in order to take into account the volatilisation of the bonding metal which takes place during spraying of the matrix metal.

However, it is preferred that the thickness of the bonding layer should not exceed 5 microns in the case of a bonding metal of low volatility, such as copper, (if it is more than 5 microns, bond failure and cracks at the interface between the support foil and the matrix layer tend to occur) and should not be less than 0.05 micron (since otherwise the beneficial effect of the bonding metal layer is insignificant). Equally, with a bonding metal or alloy of low melting point and high vapour pressure, the thickness of the bonding layer is preferably from 0.1 to 10 microns, that is relatively greater, other conditions being equal, than the thickness preferred for low volatility bonding metals.

Any suitable support sheet can be used in the product and process according to the invention. The presence of the bonding metal layer does, in fact, broaden the range of support sheets that can be used because a sprayed powder that does not adhere to certain types of supports can, by the interposition of the bonding metal layer, be made to adhere thereto indirectly.

The support sheet is preferably a metal foil, but can be a relatively thicker plate. The support sheet is preferably formed of a light metal or alloy based on aluminium and containing greater or lesser amounts of other elements (such as, magnesium, silicon, iron, copper or titanium) and alloys intended for equivalent uses (particularly the alloys based on titanium).

The powder to be sprayed may be any of those conventionally used in the production of fibre-reinforced metal matrices. The melting point of the powder must be above that of the support sheet and the high modulus reinforcing fibres must be resistant to deterioration caused by the powder at the spraying temperature. Aluminium- and titanium-based powders are preferred.

The reinforcing fibres may be any of those conventionally used in the production of fibre-reinforced metal matrices. It is preferred to use boron and silicon carbide fibres and boron fibres coated with silicon carbide. Boron fibres coated with boron carbide, as described in French Pat. No. 2,136,978 are also particularly suitable.

Panels in accordance with the invention have mechanical properties which are superior to those of conventional panels, primarily because they are exposed to less severe conditions during their manufacture and, in particular, strains of thermal origin which are markedly less than arise in conventional processes.

The principal improvements are in the tensile strength in the direction of the fibres (fewer of the fibres are broken or rendered fragile because of the gentler spraying conditions) and in the direction at right angles to the fibres, where the adhesion of the powder to the support sheet matters most.

Panels of sandwich structure comprising a central support sheet with a fibre-reinforced metal matrix on both surfaces are also within the scope of the invention. The manufacture of such panels is readily carried out by using a support sheet which has been coated on both surfaces with a bonding metal, making a panel comprising one layer of fibre-reinforces metal matrix, putting this panel on a drum, with the metal matrix layer down (against the drum) and then repeating the fibre application and metal spraying steps to form a second fibre-reinforced metal matrix layer.

It is also possible, though this is of little value, to deposit several layers of bonding metal or alloy of the same nature or of different natures, but compatible with one another, on the support sheet.

In order that the invention may be more fully understood, the following examples and comparative experiments are given by way of illustration:

A support foil of aluminium alloy 4343 having a thickness of 25 microns, was wrapped around an aluminium drum having a diameter of 30 cm, the curved surface of which was polished, by means of a double-sided adhesive tape. The following operations were then carried out on the support foil: brushing, degreasing, and electrolytic surface-stripping by the pad electrolysis technique to remove the surface layer of alumina.

As soon as possible thereafter, an 0.5 micron thick copper layer was deposited on the support foil using the pad electrolysis technique described in French Pat. No. 1,585,605, the pad being a flexible graphite-containing mass.

Boron filament having a coating of boron carbide and a diameter of 105 microns, a mean tensile strength of 380 kg/mm.sup.2 and a modulus of elasticity of 40000 kg/mm.sup.2, was wound on to the copper coating in non-contiguous turns under a slight tension (2 newtons) at a speed of 25 turns per minute. The winding pitch was 125 microns.

Aluminium alloy 6061 having a mean particle size between 15 and 120 microns was then plasma sprayed on to the drum. The powder was sprayed by means of a Plasmaton-Jet arc plasma torch (made by Baystape) using a PS751 generator, a PG.100-3 gun, a CP 120 control panel, a PF 500 powder dispenser and a "high velocity" anode providing sub-sonic ejection speeds. The electricity supply to the anode was 40 volts and 300 amperes. The plasma gas was a mixture of argon and hydrogen containing 5% of hydrogen. The flow rate of the plasma gas was between 50 and 60 liters/minute under a pressure of 2.8 bars.

The powder gas was argon supplied at a flow rate of 3 to 4 liters/minute and the powder feed rate was 17 grams/minute.

The gun was mounted on an automatic reciprocating device with the axis of travel parallel to the axis of rotation of the cylinder. The speed of travel of the gun was 1 meter/minute. The speed of rotation of the drum during spraying was 120 meters/minute.

The distance from the spray nozzle to the drum was either 9 or 12 cm. After 7 passes, the amount of powder sprayed was sufficient for the panel to comprise, by volume, an equal amount of fibres and of sprayed matrix metal.

The panel was then cut along a generatrix of the drum and detached therefrom. A plane, homogeneous sheet was obtained which could be readily handled and on which adhesion tests (peel test, triangle-cutting test, and detachment test using adhesive paper) were carried out. These tests gave the results shown in the following Table, in which the adhesion of the sprayed metal to the same kind of foil as a function of the two nozzle-cylinder distances is given.

In addition to two runs with the process described above, two otherwise identical pairs of runs were also carried out, one in which the support foil was subjected to degreasing only before winding the filament and the second in which the support foil was subjected to degreasing and electrolytic surface-stripping before winding the filament. The table gives the results obtained with these treatments for the purpose of comparison.

Example 1 was repeated using the same aluminium alloy 6061 for the support foil and for the sprayed powder.

In the panel obtained, the adhesion was as good as in Example 1.

The procedure of Example 1 was repeated with the following differences. The support foil was aluminium alloy 2024 having a thickness of 38 microns. The filament was formed of boron having an anti-diffusion coating of boron carbide and a diameter of 140 microns. The winding pitch was 185 microns. The sprayed powder was aluminium alloy 4343 having a particle size between 15 and 120 microns. The number of passes was reduced to 5 so as to give a volume ratio of fibres to matrix of 50%.

The adhesion obtained was in all respects similar to that obtained in the preceding examples.