GB2176804A

GB2176804A - Fibre-reinforced aluminium castings

Info

Publication number: GB2176804A
Application number: GB08615160A
Authority: GB
Inventors: Eggert Tank
Original assignee: Daimler Benz AG
Current assignee: Daimler Benz AG
Priority date: 1985-06-21
Filing date: 1986-06-20
Publication date: 1987-01-07
Also published as: JPS61295344A; GB8615160D0; DE3522166C1

Abstract

The use of an aluminium alloy with an effective quantity of 0.1 to 5% by weight in total of one or more of the metals lead, bismuth, cadmium or thallium for the preparation of fibre-reinforced aluminium castings is described. The alloy is distinguished by very good infiltration behaviour, so that it is possible to apply lower casting pressures than hitherto.

Description

SPECIFICATION Use of an aluminium alloy for the preparation of fibre-reinforced aluminium castings The preparation of fibre-reinforced light metals by impregnation of fibre packs, fibre mouldings and the like with liquid light metal has been known for a long time (compare German Patent Specification 2644272). A considerable number of high-strength and high-rigidity ceramic or metallic fibres in the most diverse processing and application forms are used. Such fibres consist, for example, of alumina, silicon carbide, glass or also Al203-SiO2-mullite ceramics, if appropriate with a proportion of B203. Metal whiskers of, for example, SiC are also used. In order to obtain an improvement in the mechanical properties of the aluminium castings, the fibres must be ordered in a clearly defined quantity and arrangement within the volume of the component or semi-finished article to be produced.This arrangement must be capable of being impregnated with molten metal without substantial geometrical changes.

However, any accumulation of fibres, which may be of fibre felt, a fibre moulding with or without binder, fabric packs, fibre plies and the like, exerts a resistance to the penetration of molten metal. This resistance is composed of several components: The surface tension of a metal tends to prevent an enlargement of the surface of the metal.

However, this necessarily takes place due to the continuous breaking-up of a large curved metal surface with large radii of curvature into many small curved surfaces as a result of the action of the fibres which obstruct the flow of metal.

If surface skins (refractory oxides, nitrides, sulphides or fluorides with very high melting points) form on the molten light metal, the fibres are additionally supported on these skins during the penetration of the metal. These skins acts as though the surface tension of the metal were substantially increased. The skins consist of materials of a strength and hardness similar to those of the fibres themselves and can therefore exert a considerable resistance. This applies in particular when their thicknesses come into the 1 um range, that is to say into the range of the fibre diameters (0.1 ym in the case of whiskers and 20 jtm in the case of alumina fibres).

The narrowness of the interspaces exerts a flow resistance to the continued flowing of a metal which has already penetrated. This resistance depends on the geometry of the interspaces, the viscosity of the metal, the flow velocity and the type of flow (turbulent-laminar).

The gases present must be displaced from the moulding by the metal. Their outflow is subject to a flow resistance which depends on the geometry of the channels, the viscosity of the gases, the flow velocity and the type of flow. This resistance can be eliminated by evacuating the fibre pack.

These resistances, some of which are dealt with in German Patent Specification 2644272, result in the following disadvantages: All fibre mouldings are reduced in size during the infiltration by the metal (the metal front).

This happens to a particularly great extent with mouldings of low filling density, that is to say mouldings of low fibre content or low density. Such bodies can very easily be compressed, whereas their resistance to infiltration can already be relatively high.

With progressive reduction in size, the bodies are increasingly compacted, so that finally metal pressures of up to 2000 bar must be applied at the end of the filling phase as, for example, in German Patent Specification 2644272, in order to achieve complete impregnation. For fibre mouldings of SiC whiskers, it is known that even higher pressures of 3000 to 4000 bar must be applied ("squeeze-cast" process).

Such pressures are, however, no longer attainable by the conventional low-pressure and highpressure casting devices. The casting machines required for this purpose already belong to the class of extremely expensive so-called "squeeze-cast" machines.

Due to the deformation of the fibre mouldings by the penetrating molten aluminium, complete and uniform filling of a component with fibres is very difficult, and even impossible in the case of some fibres. Moreover, there is a risk of channels forming in the fibre moulding, the metal flowing around fibre regions which have not been impregnated. This fibre region, by-passed by the flow and sometimes still filled with gases, can then be filled with metal only with very great difficulty, and frequently not at all.

It is the object of the invention to provide a possibility for enabling fibre mouldings to be impregnated with an aluminium melt even without the application of extremely high pressures and thus for producing fibre-reinforced aluminium castings in a relatively simple manner.

According to the present invention, the preparation of a fibre-reinforced aluminium casting includes the use of an aluminium alloy effectively containing in total between 01 and 5% by weight of at least one of the metals lead, bismuth, cadmium or thallium. The invention also provides a fibre-reinforced aluminium casting comprising an aluminium alloy containing, in total, between 0.1 and 5% by weight of at least one of the metals lead, bismuth, cadmium or thallium.

It has been found that the impregnation of the said fibres is substantially facilitated and improved by the addition of an effective quantity of 0.1 to 5% by weight in total of one or more of the metals lead, bismuth, cadmium or thallium to aluminium or one of its alloys, - Even glass fibres, which are very difficult to impregnate by aluminium, as is known, can be wetted very readily and without destruction.

The common feature of the alloy constituents lead, bismuth, cadmium and thallium, when admixed to aluminium, is the limited miscibility (solubility) in the liquid state (so-called miscibility gap in the liquid state) and the complete immiscibility (insolubility) in the solid state.

The so-called "monotectic points", that is to say the points of maximum miscibility in the liquid state at the lowest temperature, are at the following values: Al-Pb 1.5% by weight and 658.5"C; Al-Bi at 3.4% by weight and 657"C; Al-Cd at 6.5% by weight and 549"C, and Al-TI at about 1.5% by weight and 659"C. The surface tension of the aluminium is greatly reduced by the addition of these alloying metals, so that impregnation of the fibre moulding can be carried out in a substantially simpler manner. It is also possible that the mode of oxide formation on the liquid aluminium and the viscosity of the latter are modified.

To achieve this result, an effective content of 0.1 to 5% by weight in total of one or more of the metals lead, bismuth, cadmium or thallium suffices. An effective content is understood as an actually present free quantity of this metal. Thus, for example in the case of magnesiumcontaining alloys, the compound Mg3Bi2 would form on additional alloying with bismuth. The bismuth would thus be bound and hence largely ineffective. If such alloy constituents are present, which form compounds with the metals to be added and hence prevent effectiveness, either a change-over to another effective metal is necessary or an alloy must be used which does not contain this harmful alloy constituent.

Either pure aluminium or aluminium alloys are correspondingly improved by an addition of the metals lead, bismuth, cadmium or thallium. Particularly good results are obtained when the effective quantity of these metal is 0.8 to 3% by weight in total. If a quantity of 5% by weight is exceeded an improved effect no longer results; on the contrary, the properties of the alloy may in some cases already be impaired.

The following aluminium alloys were used for the impregnation of the fibre felts and fabrics: 1.

an alloy of the composition 0.6 to 1.2% of Mg, 0.6 to 1.4% of Si, 0.4 to 1.0% of Mn, 0.1% of Cu and 1.0 to 2.5% of (Bi + Cd + Pb + Sn), the remainder being Al (commercially available wrought alloy DIN 1725 AlMgSiPb, 3.0615).

2. A commercially available wrought alloy Al 99 with 1% in total of admixtures of iron, silicon, copper manganese and magnesium was additionally alloyed with 1.5% by weight of lead.

3. A commercially available casting alloy DIN 1725 G-AISi 12, 3.2581, having a composition of 11 to 13.5% of Si, O-Q.4% of Mn, O to 1% of Fe in small quantities (0.1%) of Ti, Zn, Cu and Mg, the remainder being Al, was additionally alloyed with 1.5% by weight of lead.

Felts and fabrics of Alp03, SiC, glass crystalline aluminium silicate (mullite) or boron-containing crystalline aluminium silicate were impregnated with these alloys. In particular in the cases of the sensitive glass fibres, there was no destruction of the fibres. In spite of the low infiltration pressures of only a few tenths of a bar (0.1 to 0.3 bar), the wetting and filling of the felts and fabrics was extremely satisfactory.

As a result of the use of the alloy according to the invention, the preparation of fibrereinforced aluminium castings is considerably facilitated, because of the reduced casting pressures. Even in processes for the production of composite materials, wherein fibres or particles are stirred into an aluminium melt, advantages are obtained when the alloy according to the invention is used, since the introduction of the fibres into the melt or the acceptance of the fibres by the melt is substantially facilitated by the low surface tension and the reduced viscosity. The composite materials thus prepared are usually further processed by massive forming (forging, pressing, extrusion) into semi-finished articles or components.

Claims

1. In the preparation of a fibre-reinforced aluminium casting, the use of an aluminium alloy with an effective quantity of 0.1 to 5% by weight in total of one or more of the metals lead, bismuth, cadmium or thallium.

2. An aluminium alloy according to Claim 1 with an effective quantity of 0.8 to 3% by weight in total of at least one of the metals lead, bismuth, cadmium or thallium.

3. A fibre-reinforced aluminium casting comprising an aluminium alloy containing, in total, between 0.1 and 5% by weight of at least one of the metals lead, bismuth cadmium or thallium.

4. A method of preparing a fibre-reinforced aluminium casting substantially as hereinbefore described.

5. A fibre-reinforced aluminium casting substantially as hereinbefore described.