EP0304167A2

EP0304167A2 - Production of fibre reinforced metal sections

Info

Publication number: EP0304167A2
Application number: EP88306655A
Authority: EP
Inventors: Andrew Winsloe Clifford; William Joseph Cook
Original assignee: CRAY ADVANCED MATERIALS Ltd
Current assignee: CRAY ADVANCED MATERIALS Ltd
Priority date: 1987-07-20
Filing date: 1988-07-20
Publication date: 1989-02-22
Also published as: GB8717067D0; JPS6448658A; EP0304167A3

Abstract

Fibre reinforced metal sections are produced by forcing molten metal into a die chamber 6 through which fibres are fed at a controlled speed from a fibre inlet orifice in ingate 5 to an exit die 7 having an exit orifice corresponding to the cross-section of the desired reinforced cast section. The molten metal is held in a pressurisable second chamber 11 from which it is forced by the pressure of an inert gas into the die chamber 6 so as to infiltrate the fibres. Important parameters in operation of the process are infiltration pressure, ingate die temperature and exit die temperature, molten metal temperature, and fibre feed rate.

Description

This invention relates to the production of fibre reinforced metal sections. In particular it relates to a process and apparatus for continuous production of fibre reinforced metal sections.
The production of fibre reinforced metal components has been described, using a casting process in which the fibre reinforcement is placed in a die which is then evacuated, as is also a holding vessel containing a charge of molten metal, and in which pressure is then applied to the vessel by means of a compressed gas so as to force molten metal up a connecting tube into the die and to surround substantially all the fibres in the die cavity. Such a process is described in GB-B-2115327 and in WO-A-83/02782.
In GB-A-2168032 there is described a process for making reinforced rings by stacking modules in a die which are formed by co-winding of a filamentary reinforcement and of a metal strip, with interspersed foils of a matrix material, such as aluminium, titanium, magnesium or copper. This assembly can then be consolidated by application of heat and pressure.
Impregnation of a porous body with a filler metal is described in US-A-3867177. The body is first contacted with an activator and then immersed in the filler metal.
US-A-3913657 describes a method for preparing a composite structure which comprises melting a metal in a first chamber, pressurising the chamber, lowering a second evacuated chamber containing a filamentary material into the molten metal, and then piercing the second chamber to allow molten metal to flow under the pressure differential from the first chamber into the second and to infiltrate the filamentary material.
Melting of a slug of aluminium so as to infiltrate a fibrous reinforcement is disclosed in US-A- 3547180. Gas pressure is used to improve infiltration.
Production of fibres and their pretreatment in readiness for use in reinforcing metals is disclosed in a number of patent specifications, including US-A-3754112, US-A-4045597, US-A-4068037, US-A-4123583, US-A-4127659, US-A-4142008, US-A-4315968, US-A-4340636, US-A-4415609 and US-A-4481257.
Particular combinations of fibre reinforcement and matrix metal are disclosed in GB-A-1185349 and GB-A-1331728.
The present invention seeks to extend the availability of fibre reinforced metal articles by providing a novel method for continuous casting of fibre reinforced metal sections and a novel apparatus for carrying out the method .
According to the present invention there is provided a process for continuous casting of a fibre reinforced metal section which comprises:
providing a die chamber having (i) an exit orifice, through which a cast section can exit the die chamber in an exit direction, and (ii) at least one fibre inlet orifice opposite the exit orifice through which fibres can be fed into and through the die chamber in the exit direction;
passing fibres at a controlled speed into and through the die chamber;
providing a pressurisable second chamber containing a charge of molten metal and in flow connection with the die chamber whereby molten metal can pass into the die chamber upon pressurisation of the second chamber;
applying a controlled pressure to the contents of the second chamber so as to cause molten metal to enter the die chamber and infiltrate the fibres therein;
controlling the temperature of the charge of molten metal in the second chamber to a selected value higher than the melting point of the charge of molten metal and sufficient to avoid premature solidification of the molten metal prior to infiltration of the fibres;
withdrawing consolidated cast fibre reinforced metal section from the exit orifice in the exit direction at a controlled rate; and
controlling the temperature of said at least one inlet orifice and of said exit orifice each to a respective predetermined value sufficient to prevent total freezing of molten metal in the die chamber yet not so high as to permit escape of molten metal therefrom.
The invention further provides apparatus for continuous casting of a fibre reinforced metal section comprising:
a die chamber having (i) an exit orifice through which a consolidated cast section can be withdrawn in an exit direction, and (ii) at least one fibre inlet orifice opposite the exit orifice through which fibres can be passed into and through the die chamber in the exit direction;
a pressurisable second chamber for containing a charge of molten metal, said second chamber having an inlet port for connection to a source of a pressurised gas and having a flow connection to the die chamber so that, upon applying a gas pressure to the contents of the second chamber via the inlet port, molten metal can be forced into the die chamber;
heating means for heating the charge of metal in the second chamber to a predetermined temperature higher than the melting point of the charge;
means for passing fibres into and through the die chamber via said at least one fibre inlet orifice; and
withdrawal means for withdrawing consolidated fibre reinforced metal section in the exit direction from the exit orifice.
The fibres may be used in the form of individual filaments, in the form of tows or bundles, or in woven form (e.g. in tubular woven form), or in any other form suitable for metal reinforcement. In some cases, when the fibres are used in the form of tows or bundles, the fibres may for ease of handling be coated with a size or bonded together with the aid of a binder. Typical binders include fugitive binders, such as an acrylic resin. The fibres may be as thin as about 3 µm in diameter or less or as large as 200 µm or more.
Examples of suitable fibres include steel wire, and ceramic fibres (e.g. alumina fibres, carbon fibres and silicon carbide fibres). Any of the types of fibre which are suitable for use in production of fibre reinforced metal composites, and preforms made therefrom, can be used. Unlike other systems available, no coating of the fibres is required to facilitate infiltration. Indeed it is preferred that all sizes and binders should be burnt off to present clean, uncontaminated fibres to the molten metal in the die chamber.
Similarly the metal can be any metal suitable for forming the matrix metal of a fibre reinforced metal composite. Typical metals include aluminium, copper, magnesium, zinc and lead, as well as alloys of each of these metals.
Although many variations are possible in the practice of the invention, the fibres typically constitute from about 20% to about 50% by volume of the consolidated cast fibre reinforced metal section, corresponding to a metal:fibre volume ratio of from about 4:1 to about 1:1. However, in some cases metal:fibre volume ratios outside this range may be used so that the volume percentage of fibres in the fibre reinforced metal section is less than about 20% or is higher than 50%.
In order that the invention may be clearly understood and readily carried into effect a preferred process for the continuous casting of a fibre reinforced metal section, and an apparatus for use in connection therewith, will now be described, by way of example only, with reference to the accompanying drawing, which is a vertical section through the apparatus.
Referring to the drawing, one or more bundles of fibres 1 in the form of tows are drawn off creels 2 and are passed through a guide 3 into a heating chamber 4 in which organic binder is burnt off. They then pass into an extrance die or ingate 5 which is provided with a number of small orifices, each a little larger than the diameter of the fibre tow, which are positioned so that the fibre tow or tows will be appropriately aligned in the consolidated cast metal section. Typically the clearance between the tow and the walls of the orifice is about 25 µm. The fibres pass from ingate 5 through a die chamber 6 and out through an exit die 7 which is formed with an exit orifice corresponding to the cross-section of the desired reinforced cast section. Finally, after passing through cooling chamber 8 through which air is blown, they pass between a set of pinch rollers 9 which are driven by a motor 10 or alternatively onto a tensioned collecting drum of sufficient diameter to allow for collection of the reinforced material.
Positioned below die chamber 6 is a pressure vessel 11 which has an insulated lining 12 within which is a crucible 13. Crucible 13 and its contents can be heated by means of an electrical heating element 14. An inlet port 15 can be connected to a source of compressed inert gas (not shown), e.g. a cylinder of argon.
Crucible 13, in use, holds a charge 16 of molten metal, e.g. aluminium or an aluminium alloy. A venturi tube 17 connects die chamber 6 and the bottom of crucible 13. Electrical heating elements 18, 19 can be used to heat venturi tube 17 and prevent freezing of the molten metal as it passes from crucible 13 into die chamber 6.
Die chamber 6 is lagged, as are ingate 5 and exit die 7. Thermocouples (not shown) are provided for monitoring and controlling the temperature of the various critical parts of the apparatus. Further electrical heating elements (not shown) are provided to maintain ingate 5, die chamber 6, and exit die 7 each at the respective desired temperature.
To manufacture a fibre reinforced metal section crucible 13 is charged with metal and heated to melt the metal to a predetermined temperature above the melting point of the metal charge. Then, while feeding fibres at an appropriate speed through die chamber 6, pressure is applied via inlet port 15 to cause molten metal to pass up venturi tube 17 into die chamber 6. By controlling the gas pressure, the temperature of the die chamber 6, of the ingate 5 and of the exit die 7, the temperature of the molten metal, the rate of feed of the fibres, and the cooling effect of cooling chamber 8, consolidated cast fibre reinforced metal section is produced.
The illustrated process and apparatus are suitable for continuous casting of sheet, strip, tube, bar or shaped section, using an appropriate exit die 7. In some cases another type of preform may be used in place of a tow or tows. For example, a woven tubular preform may be used when casting a tubular section.
For the production of sound material it is important to control solidification at the ingate die 5 and at the exit die 7, to control the solidification front in the die chamber 6, and also to control the gas pressure so as to effect satisfactory infiltration of the fibres by molten metal within die chamber 6.
Four operational parameters, in particular, are mutually dependent in order for sound material to be produced. These are:

1. Infiltration pressure;
2. Ingate die and exit die temperatures;
3. Molten metal temperature; and
4. Fibre feed rate.

The minimum infiltration pressure applied via inlet port 15 that is necessary to achieve infiltration is desirably used. This varies according to fibre type and metal:fibre volume ratio but is typically in the range 11.35 bar (150 psig) to 104.35 bar (1500 psig). However those experienced in the art will appreciate that the use of pressures lower than 11.35 bar (150 psig) may be feasible for some compatible metal/fibre systems.
The molten metal in crucible 13 is also held at the minimum temperature necessary to maintain sufficient superheat to avoid premature solidification in die chamber 6. For aluminium and aluminium alloys this is typically in the range of from about 600°C to about 750°C, depending upon the composition of the metal of the charge 16.
The die temperatures are desirably maintained at values such that a thermal gradient exists across the entrance and exit areas to the die chamber 6. Typical operating temperatures are from about 500°C to about 750°C at the ingate 5 and from about 400°C to about 550°C at the exit die 7.
Fibre feed rate is dependent upon the precise operating conditions used and upon the fibre type and the section being produced. Generally speaking the rate of draw off of cast fibre reinforced metal section is in the range of from about 1 mm/sec to about 500 mm/sec.

Claims

1. A process for continuous casting of a fibre reinforced metal section which comprises:
providing a die chamber having (i) an exit orifice, through which a cast section can exit the die chamber in an exit direction, and (ii) at least one fibre inlet orifice opposite the exit orifice through which fibres can be fed into and through the die chamber in the exit direction;
passing fibres at a controlled speed into and through the die chamber;
providing a pressurisable second chamber containing a charge of molten metal and in flow connection with the die chamber whereby molten metal can pass into the die chamber upon pressurisation of the second chamber;
applying a controlled pressure to the contents of the second chamber so as to cause molten metal to enter the die chamber and infiltrate the fibres therein;
controlling the temperature of the charge of molten metal in the second chamber to a selected value higher than the melting point of the charge of molten metal and sufficient to avoid premature solidification of the molten metal prior to infiltration of the fibres;
withdrawing consolidated cast fibre reinforced metal section from the exit orifice in the exit direction at a controlled rate; and
controlling the temperature of said at least one inlet orifice and of said exit orifice each to a respective predetermined value sufficient to prevent total freezing of molten metal in the die chamber yet not so high as to permit escape of molten metal therefrom.

2. A process according to claim 1, in which the fibres are in the form of individual filaments.

3. A process according to claim 1, in which the fibres are in the form of tows or bundles.

4. A process according to claim 1, in which the fibres are in woven form.

5. A process according to any one of claims 1 to 4, in which the diameter of the fibres ranges from about 3 µm to about 200 µm.

6. A process according to any one of claims 1 to 5, in which the fibres are selected from steel wire, alumina fibres, carbon fibres and silicon carbide fibres.

7. A process according to any one of claims 1 to 6, in which the metal is selected from aluminium, copper, magnesium, zinc, lead, and alloys of one of these metals.

8. A process according to any one of claims 1 to 7, in which the metal:fibre volume ratio is in the range of from about 4:1 to about 1:1.

9. A process according to any one of claims 1 to 8, in which the controlled pressure applied to the contents of the second chamber so as to cause molten metal to enter the die chamber and infiltrate the fibres therein is in the range of from about 11.35 bar (150 psig) to 104.35 bar (1500 psig).

10. A process according to any one of claims 1 to 9, in which the metal is selected from aluminium and aluminium alloys and in which the temperature of the charge of molten metal in the pressurisable second chamber is in the range of from about 600°C to about 750°C.

11. A process according to claim 10, in which the temperature at the ingate is in the range of from about 500°C to about 750°C and in which the temperature at the exit orifice is in the range of from about 400°C to about 550°C.

12. A process according to any one of claims 1 to 11, in which the fibre feed rate is In the range of from about 1 mm sec to about 500 mm/sec.

13. Apparatus for continuous casting of a fibre reinforced metal section comprising:
a die chamber having (i) an exit orifice through which a consolidated cast section can be withdrawn in an exit direction, and (ii) at least one fibre inlet orifice opposite the exit orifice through which fibres can be passed into and through the die chamber in the exit direction;
a pressurisable second chamber for containing a charge of molten metal, said second chamber having an inlet port for connection to a source of a pressurised gas and having a flow connection to the die chamber so that, upon applying a gas pressure to the contents of the second chamber via the inlet port, molten metal can be forced into the die chamber;
heating means for heating the charge of metal in the second chamber to a predetermined temperature higher than the melting point of the charge;
means for passing fibres into and through the die chamber via said at least one fibre inlet orifice; and
withdrawal means for withdrawing consolidated fibre reinforced metal section in the exit direction from the exit orifice.