WO1990015681A1

WO1990015681A1 - Metal infiltration apparatus, methods and composites obtained thereby

Info

Publication number: WO1990015681A1
Application number: PCT/GB1990/000918
Authority: WO
Inventors: Nigel Mykura; Derek Anthony Johnson; Robert Edward Emmett
Original assignee: Advanced Materials Systems Limited
Priority date: 1989-06-14
Filing date: 1990-06-14
Publication date: 1990-12-27
Also published as: AU5816190A; GB8913632D0

Abstract

Metal infiltration apparatus comprising a die assembly (1, 2) formed of two or more separable members having opposed surfaces which define between them a die chamber (5) capable of evacuation via outlet (7) and receiving molten metal via feed line (8), and at least one mould assembly (3, 4) comprising separable parts and locatable within said die chamber, the separable parts (3, 4) having surfaces which wholly or partially define between them a mould cavity (5a) which can receive a body (10) to be infiltrated and which is capable of fluid communication with said die chamber when located therein, evacuation means and molten metal supply means being provided such that the pressure within the, or each, mould cavity can be reduced and molten metal can reach the mould cavity, or each of them, to infiltrate a body when located therein.

Description

METAL INFILTRATION APPARATUS, METHODS AND COMPOSITES OBTAINED THEREBY

The invention concerns metal infiltration apparatus and composite bodies produced therefrom. More particularly the invention is directed to the production of fibre reinforced and particulate reinforced metals as composite bodies having attractive uses and properties.

One known method of manufacture of fibre-reinforced metals is by using a liquid pressure forming machine (LPF) as described in GB-A-2115327. Ceramic fibres are placed in a mould which is heated and into which molten metal is forced under pressure so as to infiltrate the fibres and then allow it to solidify. This means that the die must be at a sufficiently high temperature to allow the ceramic fibres to be infiltrated and yet must be allowed subsequently to cool to enable the metal to solidify.

The present invention seeks to provide metal impregnation apparatus and methods for impregnating fibres and/or particulate matter in an improved manner. Ceramic bodies or other shaped preforms of rigid or semi-rigid form can be metal infiltrated to form composites for subsequent use.

According to a first aspect of this invention, there is provided metal infiltration apparatus comprising a die assembly of two or more separable members having opposed surfaces which define between them a chamber capable of evacuation and receiving molten metal, and at least one mould assembly locatable within said chamber, the or each mould assembly having a surface which wholly or partly defines a mould cavity which can receive a mass of material to be infiltrated by said molten metal, the or each mould cavity being capable of fluid communication with said chamber when located therein, evacuation means and molten metal supply means being provided such that the pressure within the or each mould cavity can be reduced, and molten metal can be supplied to the die assembly to reach said mould cavity or cavities, whereby said mass of material can be infiltrated by said molten metal.

The mass of material can be a rigid body, semi¬ rigid or loosely bound material, preferably particulate or fibrous in nature, although combinations thereof are included within the scope of the present invention.

The mould assembly may be fabricated in metal or ceramic non-sacrificial form. A single unit could be used such as a sacrificial or otherwise disposable mould having a hollow region. With such an arrangement, a mould assembly of one-piece integral construction can be formed or built up around the body or mass of particulate and/or fibrous material to be infiltrated with metal in the assembly. The mould assembly, of first or second aspects of the invention, conveniently referred to as an 'internal mould' can be formed in situ around the mass to be infiltrated. Disposable moulds can be constructed of, e.g. ceramic. plaster, graphite or other suitable material capable of withstanding the temperatures and evacuation pressures likely to be encountered in use whilst maintaining its integrity during and after impregnation but permitting ready separation of the infiltrated body or other material therefrom, by destructive means if desired.

According to a second aspect of this invention there is provided metal infiltration apparatus comprising a die assembly formed of two or more separable members having opposed surfaces which define between them a die chamber capable of evacuation and receiving molten metal, and at least one mould assembly comprising separable parts and locatable within said die chamber, the separable parts having surfaces which wholly or partially define between them a mould cavity which can receive a body to be infiltrated and which is capable of fluid communication with said die chamber when located therein, evacuation means and molten metal supply means being provided such that the pressure within the, or each, mould cavity can be reduced and molten metal can reach the mould cavity, or each of them, to infiltrate a body when located therein.

In accordance with either aspect of the invention, the die assembly will preferably completely surround the mould assembly but embodiments are contemplated where such is not essential.

The apparatus may comprise a top bolster die half and lower bolster die half, generally based on existing such hardware in that it can include a die vacuum outlet, a metal feed and a chamber but modified to accommodate one or more internally locatable mould assemblies. In arrangements according to the second aspect the 'internal' mould assembly can be two-part die halves in the form of a reusable metal internal mould having a medial mating surface which substantially defines the mould cavity having a shape corresponding to that required in the finished composite body. The or each internal mould can be easily located before metal infiltration, and easily removed immediately afterwards.

Apparatus according to the invention can be in the form of a 'double container¹, one locatable substantially or wholly within a chamber defined by the other, in contradistinction to the current use of a single mould.

The fibre preform can be manufactured from any suitable ceramic fibres, or mixtures thereof. Fibres that have been effectively infiltrated include, for example,

(i) Carbon

(ii) Alumina

(iii) Boron

(iv) Silicon carbide.

The internal mould allows the fibres to be held in position preventing movement caused by the inrush of metal during infiltration by molten metal. It allows the preform to be located in a suitable container at the time of manuf cture and which can remain properly located throughout the infiltration process and subsequently during cooling. This means that less binder can be used (e.g. where a rigid preform is used as the body) so that fabrication of preforms can be easier, quicker and more economical resulting in less residue on the fibres. Preforms which utilize a minimum of binder lend themselves to better infiltration and result in a better fibre/metal bond. Because the mould assembly or assemblies can be preheated to the required temperature away from the main bolster, it is much easier to obtain the correct temperature required throughout the fibres. It is also possible to heat a comparatively large number of mould assemblies simultaneously and these can then be used either simultaneously for batch production or in comparatively rapid succession. Thus a plurality of internal moulds can be located simultaneously within the die chamber, all being in necessary fluid communication as specified above.

The use of a mould assembly which is separable from the die assembly enables the reinforcement whether of fibres and/or ceramic particles and whether rigid or loosely bound to be heated to much higher temperatures than a single die system. Higher mould temperatures and hence higher reinforcement temperatures, can reduce significantly the difficulty of infiltrating reinforcing materials. For infiltration of some ceramic particle reinforcements it may be necessary to exceed 800^*C. This is easily achieved by heating the one or more 'internal moulds' in a separate furnace, and after infiltration the internal mould is rapidly cooled by the die assembly which may be at any temperature from 20^*C to 600^*C e.g. 300 - 400 ^*C, but generally at a lower temperature than the or each internal mould. A single die system would require special heating and cooling equipment to raise the whole die up to these temperatures and then cool it down and it could only be done with extreme difficulty and a very slow production rate.

When the heated internal mould is placed in the bolster die the injection takes place before the temperature has been allowed to drop below the critical temperature for infiltration. Because the bolster is at a much lower temperature than would normally be required for infiltration, far less heating is required, and in some instances particularly high production rates, it is necessary to have cooling channels in the bolster to prevent overheating from the input of heat from the molten metal.

Although the described and illustrated embodiment of apparatus is concerned with a primary die with a single internal mould, it could be a multiple system with several internal moulds. The lower bolster is conveniently attached to a rigid fixed platen while the upper bolster is attached to a moving platen connected to a hydraulic system so that the two halves can be opened or closed. The whole system can operate with the bolsters in either the vertical or horizontal plane.

The metal can be forced into the bolster under pressure and the pressure can be applied by either of two methods .

(i) Gas pressure acting on a metal reservoir, or

(ii) Hydraulic pressure acting on a piston which forces the metal into the bolster.

The internal mould is relatively small and externally can have a simple shape. This enables the bolster to have one or more standard chambers which can accept a wide range of moulds with identical external dimensions but with different internal cavities, either on a separate or multiple basis.

The reinforcing material preform can be the same shape as the cavity in the internal mould and fill it completely resulting in a component which is reinforced through its entire structure, or, the reinforcing material preform can fill part or parts of the mould cavity resulting in a component which is selectively reinforced in some areas, with unreinforced metal in the remainder of the component.

The present process and apparatus is suitable for infiltrating material or bodies with, e.g. molten aluminium, magnesium, zinc, copper, iron, cobalt, titanium, nickel, most other metals and alloys of one or more thereof.

Particulate ceramics can be used alone or in conjunction with fibrous material in the form of a rigid preform, or as loose material suitably located within a suitable mould assembly.

Refractory ceramic particulate materials can be used as the material or body, optionally bonded together in one mass as a rigid perform. Preferred particulate material includes, for example the following compounds either alone or in any combination of two or more thereof:

Silicon carbide (SiC), Boron carbide (B₄C), Alumina (A1₂0₃) Silica (Si 0₂), Silicon Nitride (Si₃N₄), and Zirconia (Zr0₂)

In the case of using a rigid preform, prior to infiltration and also in the final composite, the particles can be bound together intimately into a rigid structure by e.g., sintering. In the case of a mass of material other than a rigid preform, a binder, preferably an evaporable binder such as wax, can be used to hold the mass in its required shape. This is particularly useful where a sacrificial mould assembly is to be formed about the mass.

The particles can vary in size and may be in the range of 0.001 mm to 0.5 mm.

Although bonded together, the particles of a rigid preform do not form a solid mass, and the preform can have a level of porosity or free space between the particles suitable for infiltration with molten metal. The proportion of porosity can be in the range from 20% to 75% in the case of bonded preforms.

The metal for infiltration into the mass of material, can be molten aluminium or alloy thereof or copper, lead, zinc, magnesium, nickel, iron, cobalt, titanium or an alloy thereof. The metal can completely surround ceramic particles in ceramic composites formed except where the particles are in intimate contact with each other.

Suitable preform structures therefore include many refractory oxides and carbides where the particles are formed into a rigid network with interconnecting pores of free space that can be infiltrated with molten metal such as aluminium. The composite has beneficial properties due to the effect of combining the two materials. It produces a material which may be much harder, stiffer and stronger than the alloy would be on its own and tougher than the ceramic would be on its own. It can have a much lower coefficient of thermal expansion than the alloy and good resistance to wear and abrasion. Embodiments of composites which can be manufactured and their potential applications are listed below:

Composite % Ceramic Application

SiC/Al 65 Electronic component support.

AI2O3/AI 70 Wear resistant internal combustion engine components.

A1₂0₃/A1 65 Ballistic shield. Si0₂ + Zr0 /Al 60 Controlled expansion component for space vehicle.

In order that the invention may be illustrated and readily carried into effect, embodiments thereof will now be described by way of non-limiting example only, with reference to the accompanying drawing wherein:

Figure 1 is an exploded isometric view of an infiltrating apparatus according to both first and second aspects of the invention, and

Figure 2 is a cross sectional view through the assembled components of Figure 1.

The fibres or particles may be placed in the internal mould 3, 4 (a split internal mould) as a preform 10. The internal mould can consist of two or more interlocking metal or ceramic pieces or otherwise have complementary mating surfaces, which, when fitted together form a generally solid or fixed block with an internal cavity 5a that corresponds to the shape of the metal- infiltrated composite required. Although a two-part component is shown, the internal mould could function in one-piece construction. Two-piece, three-piece or other multi-part form, could be deployed if such can be located together to form a block having a cavity or chamber to receive the mass of material, in solid or loose form.

An essential part of the manufacture of fibre reinforced metals using molten metal is that the fibres be heated to a high enough temperature to allow them to be infiltrated by the molten metal. This is normally done in the actual machine 1,2. By putting the fibres in an internal mould, the mould including the fibres can be heated up to the required temperature outside the primary assembly in a separate furnace. Once up to the required temperature, the loaded mould is then transferred to a die assembly in the form of a bolster die, into a preformed chamber 5. The bolster die, which includes top bolster die half 1 and lower bolster die half 2, has such an internal chamber specifically designed to accept the internal mould.

The internal mould and the main die are designed such that the internal mould effectively becomes part of the main die except that the main die normally will be at a lower temperature than the internal mould. The main die is closed about its mating surfaces and air is evacuated via die vacuum outlet 7 from the bolster die and internal mould at the same time. The internal mould has one or more openings 9 leading to an ingate 6a to allow air to be evacuated from the channel 6 and consequently the chamber and cavity, and also to allow molten metal to infiltrate into the fibres. The primary bolster has one or more metal supply channels 6 in communication with the internal mould opening cut in it to allow the molten metal to be supplied under pressure from a furnace below the bolster, via a molten metal inlet 8 to the opening 9 in the internal mould. Once the metal has fed through the bolster into the internal mould and infiltrated the fibres, the metal is allowed to solidify and the main bolster is then opened and the internal mould is then removed. The bolster is now ready to accept another mould and fibres. The mould with the completed component can then be opened and the part removed to allow the mould to be cleaned and prepared for re-use. In place of a fibre preform, a bound particulate ceramic preform can be infiltrated with metal using the same apparatus and procedure. Examples are described below.

If the internal mould assembly used is of one piece construction, e.g. a mould which has been formed of plaster, ceramic, graphite or other similar material, around the mass of material to be infiltrated, it can be removed destructively by breaking or by stripping that mould from the finished reinforced composite. An example of the use of such a sacrificial mould is presented as example 4 below. Example 1

A shaped fibre preform consisting of continuous alumina fibres was manufactured using organic binders.

The fibres have a specific orientation and packing density dictated by the requirements of the component being manufactured. The shape of the preform was identical to the shape of the final component, as the whole component was being reinforced.

The preform was then located in the shaped cavity contained in the two halves of the internal mould, the cavity having the same shape as the preform component. The internal mould is small and allows for easy handling of the preform and holds the fibres in the correct position throughout the process. The internal mould containing the fibre preform was then placed in a furnace at 600 °C to preheat the internal mould and fibres. Once at the required temperature, the mould was removed from the furnace and transferred to the main bolster die which had been preheated to a lower temperature of 400 °C. The bolster die was then closed such that it was hermetically sealed. This enabled the air to be pumped out of the internal mould and bolster, through the die vacuum outlet. When the air had been pumped out of the system to a vacuum of less than 1 mb, the molten metal was injected into the system through the metal feed. The metal rises up the metal feed sprue in the lower bolster. It is injected through a system of runners to the internal mould where it is forced into the fibres in the shaped cavity through the ingate in the mould.

The molten metal infiltrated the alumina fibres and was then allowed to solidify, resulting in a fibre reinforced metal component. As soon as the metal had solidified the bolster die was opened and the two halves of the mould were ejected complete with the component inside. The component was then removed after the mould had cooled. Example 2

A cylindrical preform 30 mm in diameter consisting of 70% fused silica and 30% Zirconia with a total porosity of 40% was placed in a cavity between heated die plates in a mould as shown in the drawing. The cavity containing the ceramic was then evacuated. When the ceramic achieved the required temperature it was infiltrated with molten aluminium at a temperature of 720 °C under pressure using compressed air to apply the pressure to the aluminium. Once the liquid aluminium had infiltrated the porosity in the ceramic preform it solidified. The die was then opened and the mould containing the composite consisting of aluminium and ceramic particles was removed. The resulting composite consisting of 60% ceramic particles and 40% aluminium alloy was harder and more wear resistant than composites with a lower volume fraction of ceramic. Example 3

A Square plate measuring 150 mm x 150 mm x 10 mm was manufactured by sintering alumina (AI2O3) particles at high temperatures. The resulting plate consisted of 70% alumina particles and 30% free space. The particles varied in size from 2 μ to 100 μ with a bi-modal distribution. The square plate was then placed in a cavity within a mould in a two part steel die and was heated to the operating temperature of 540 ^*C. The pores of the ceramic plate were then evacuated, and subsequently infiltrated by the molten aluminium using the same procedure as for example 2.

The resulting composite consisted of 70% alumina ' and 30% Aluminium. Example 4

Random alumina (Saffil) fibres 25% volume fraction were formed into the shape of a compressor blade. The shape was surrounded by a thin (0.10mm) layer of wax which was attached to a larger tapered wax ingate. This wax assembly containing the Saffil fibre preform was then invested in (surrounded by) a plaster bound ceramic material to form a one piece internal mould. When the plaster - ceramic had set the mould was heated to 700^*C to remove all traces of the wax. The internal mould still containing the Saffil fibre preform, correctly located, was reduced to a temperature of 540^*C then transferred to the bolster die which had been preheated to 400^*C. The bolster die was then closed and the preform infiltrated with aluminium as example 1.

The plaster - ceramic mould was removed from the blade by shot blasting revealing a net shape replica of the original. The 25% Saffil fibres were infiltrated with 75% aluminium and the composite surrounded by 0.10mm of unreinforced aluminium which could be polished and surface treated in the same manner as unreinforced aluminium.

The advantages the present die apparatus and method have over the known single die include the following:

The internal mould assembly or assemblies, whether or not in the form of a internal mould can be rapidly heated to the required temperature in a much shorter time.

The die assembly can be run at a much lower temperature than normal as the mould assemblies can keep the fibres or particles at the required temperature for sufficient time to allow infiltration .

A plurality of mould assemblies can be heated simultaneously in the furnace minimising delay prior to correct fibre temperature improving production rate.

The mould assemblies may be relatively small, completely or substantially surrounded by the die assembly, and easy to handle as they do not need to withstand high pressures. The die assembly acts to contain the molten metal under pressure.

The internal mould assembly assists in protecting the fibrous and/or particulate preform during handling because they can be contained within it throughout the process, especially during ejection from the main die.

It is much easier and less costly to modify a mould than the bolster die, should this be required.

A wide range of different infiltrated composites can be made using one-piece or multi-piece constructions with different cavities. The production can be changed easily from one component to another without changing the bolster die and all the ancillary equipment with it (heaters, pressure and vacuum connections, valves, cooling circuits etc. ).

The use of mould assemblies which can be separated from the die assembly enables the reinforcement whether it is made of fibres or ceramic particles to be heated to much higher temperatures than a single die system. Higher mould temperatures and hence higher reinforcement temperatures, can reduce significantly the difficulty of infiltrating reinforcing materials. For infiltration of some ceramic particle reinforcements it is necessary to have temperatures in excess of 800 ^*C. This is easily achieved by heating the internal mould in a separate furnace, and after infiltration the mould is rapidly cooled by the bolster which is at a much lower temperature e.g. 300 - 400 ^*C. A single die system would require special heating and cooling equipment to raise the whole die up to these temperature and then cool it down and it could only be done with extreme difficulty and a very slow production rate.

Claims

1. Metal infiltration apparatus comprising a die assembly of two or more separable members having opposed surfaces which define between them a chamber capable of evacuation and receiving molten metal, and at least one mould assembly locatable within said chamber, the or each mould assembly having a surface which wholly or partly defines a mould cavity which can receive a mass of material to be infiltrated by said molten metal, the or each mould cavity being capable of fluid communication with said chamber when located therein, evacuation means and molten metal supply means being provided such that the pressure within the or each mould cavity can be reduced, and molten metal can be supplied to the die assembly to reach said mould cavity or cavities, whereby said mass of material can be infiltrated by said molten metal.

2. Apparatus as claimed in claim 1 wherein the or each mould assembly is of one-piece integral construction and separate from the die assembly.

3. Apparatus as claimed in claim 1 or 2 wherein the or each mould cavity contains fibrous and/or particulate matter.

4. Apparatus as claimed in any preceding claim in which the mould assembly can be sacrifically removed from the infiltrated mass by breakage or shot-blasting.

5. Apparatus as claimed in any preceding claim wherein the mass is bound by an evaporable binder which can be removed by heating the mould assembly.

6. Apparatus as claimed in any preceding claim including a plurality of mould assemblies, each of which contains a particulate and/or fibrous mass in fluid communication with said evacuation means and molten metal supply means.

7. Metal infiltration apparatus comprising a die assembly formed of two or more separable members having opposed surfaces which define between them a die chamber capable of evacuation and receiving molten metal, and at least one mould assembly comprising separable parts and locatable within said die chamber, the separable parts having surfaces which wholly or partially define between them a mould cavity which can receive a body to be infiltrated and which is capable of fluid communication with said die chamber when located therein, evacuation means and molten metal supply means being provided such that the pressure within the, or each, mould cavity can be reduced and molten metal can reach the mould cavity, or each of them, to infiltrate a body when located therein. δ. Apparatus as claimed in claim 7 wherein the or each mould assembly comprises a generally rigid preform of fibrous or particulate matter.

9. Apparatus as claimed in claim 7 or 8 wherein at least one mould assembly comprises a two-part resuable mould.

10. Apparatus as claimed in any preceding claim wherein the mould cavity corresponds in shape to that required in the infiltrated composite body to be formed.

11. Apparatus as claimed in any preceding claim wherein at least one mould assembly contains ceramic fibres or particulate ceramic material.

12. A method of infiltrating a mass of material with metal to form a composite comprising providing a mass of material, locating said mass within a cavity of a mould assembly, placing the mould assembly within a heated die assembly, evacuating a chamber of the die assembly at least partially, supplying molten metal to said mould assembly cavity such that metal infiltrates the mass, allowing the metal-infiltrated mass to cool and removing it from said mould assembly in the form of a solidified composite. 13. A method as claimed in claim 12 using apparatus as claimed in any one of claims 1 to 6.

14. A method as claimed in claim 12 using apparatus as claimed in any one of claims 7 to 11.

15. A metal-infiltrated composite produced by a method as claimed in claim 12 or 13.

16. A metal-infiltrated composite produced by a method as claimed in claim 14.