GB2275008A

GB2275008A - Particulate metal matrix composites

Info

Publication number: GB2275008A
Application number: GB9402087A
Authority: GB
Inventors: John Cook; Neil John Keegan; Stig Brusethaug
Original assignee: Foseco International Ltd; UK Atomic Energy Authority; Hydro Aluminium AS
Current assignee: Foseco International Ltd; UK Atomic Energy Authority; Hydro Aluminium AS
Priority date: 1993-02-13
Filing date: 1994-02-03
Publication date: 1994-08-17
Anticipated expiration: 2014-02-03
Also published as: GB9402087D0; GB9302921D0; GB2275008B

Abstract

A method of manufacturing a particulate reinforced metal matrix composite comprises the operations of producing a primary melt containing an initial volume percentage of a particulate reinforcing material in a metal matrix material, maintaining the temperature of the melt constant for a period of time sufficient to allow an optimum volume percentage of the particulate material within the settled volume to be achieved, and removing the particulate reinforcing material depleted portion of the melt. The optimum period is determined by a previous calibration experiment. The metal matrix material is preferably an aluminium/silicon alloy and the reinforcing material is preferably silicon carbide with a particle size in the range 15 - 25 mu m. The volume percentage of reinforcing material is suitably less than 20%.

Description

Particulate Metal Matrix Composites The present invention relates to the production of particulate metal matrix composite materials in which a metal matrix is reinforced by a material in the form of particles rather than fibres or whiskers.

The advantage which particulate reinforced composite materials have over fibre-reinforced materials is that they can be used as a primary feedstock material for secondary processing into shaped artifacts by means of traditional metallurgical techniques such as casting.

Particulate reinforced metal matrix composite materials are most economically produced by melt techniques in which the solid particulate reinforcing material and the metal which is to form the matrix are sheared past each other by stirring to promote the wetting of the reinforcing particles by the matrix material and ensure their complete incorporation into the melt. One example of such a technique is disclosed in the specification of a patent application filed under the Patent Co-operation Treaty in the name of the Dural Aluminium Corporation USA and bearing the reference number PCT/US87/0094O.

However, to be useful as a primary feedstock material for secondary melting and casting the particulate reinforced metal matrix composite material must remain stable during the secondary processing operations. Specifically, this means that the particulate reinforcing material must resist chemical attack by the metal matrix material while it is at elevated temperatures, particularly while it is in the molten state, and should also remain uniformly distributed, resisting gravity induced segregation, while the matrix material is molten.A uniform distribution of the particulate reinforcing material within the molten matrix material can be assured by constant stirring, but if the particulate reinforced composite material is to be processed by conventional foundry techniques, then the rate of settlement of the particulate reinforcing material after stirring has ceased must be commensurate, at least with the time taken to complete the casting or other process being carried out.

The extent of any chemical reaction between a matrix material and a particulate reinforcing material can be reduced by making the particle size as large as is practicable. This is because, by so doing, for any given volume percentage of particulate material, the total surface area of the particulate material is reduced.

However, a consequence of this is that the rate of settlement of the particulate material after stirring has ceased is increased.

Another factor which has to be considered is that the viscosity of the melt for a given set of melt conditions, is a function of the volume percentage of the particulate reinforcement material in the melt. This limits the quantity of particulate that can be successfully incorporated in the melt by a melt stirring technique.

It is an object of the present invention to provide a method of manufacturing an improved particulate reinforced metal matrix composite material for use as a feedstock material.

The invention is particularly applicable to systems in which the metal matrix material is an aluminium silicon alloy and the particulate material is silicon carbide in the size range 10 > m to 25am. The use of such relatively coarse particles minimises the rate of reaction between the aluminium in the matrix material and the silicon carbide. However, the silicon carbide particles settle out quickly from the melt when stirring is stopped, particularly when the volume percentage of the silicon carbide particles is less than or in the order of 15.As a result, the resulting composite materials usually are not compatible with conventional foundry practices and their use in casting processes such as the production of large slow cooling sand castings can be inhibited because of the gravity induced segregation of the particulate reinforcing material, which leads to inhomogeneities in the microstructure, and hence properties of the castings. On the other hand, if one tries to use volume percentages of silicon carbide > 20, then difficulties arise in effectively stirring the melt, thus making the large scale manufacture of such materials difficult.

According to the present invention there is provided a method of manufacturing a particulate reinforced metal matrix composite material, including the operations of producing a primary melt containing an initial volume percentage of a particulate reinforcing material in a metal matrix material, maintaining the melt at a particular temperature for a settlement period sufficient to allow an optimum volume percentage of the particulate material within the settled volume to be achieved, and removing that part of the melt above the settled volume, the settlement period having been determined by a previous calibration experiment.

The particulate-depleted portion of the melt may be recycled and the particulate-rich portion of the melt may be cast into artefacts directly or used as the feedstock for a secondary casting operation.

Preferably the metal matrix material is an aluminium/silicon alloy and the particulate reinforcing material is silicon carbide, but the technique is applicable generally to particulate reinforced metal matrix composite materials.

The invention will now be explained, by way of example, with reference to the accompanying drawings, in which: - Figure 1 shows the rates at which particulate reinforcing material is depleted from the upper portion of a melt when stirring of the melt is stopped, and Figure 2 shows the change in settlement rate and the establishment with time of a settlement depth.

Referring to the drawings, it can be seen that when a melt consisting of an aluminium casting alloy together with a proportion of particulate silicon carbide is allowed to stand after it has been stirred, settlement of the silicon carbide occurs at a rate which varies with the holding time, the particle size and the volume percentage of the silicon carbide.

In general, the settlement rate is not linear, but decreases with time, as shown specifically for a melt containing 15 volume percent silicon carbide in Figure 2.

Also, it has been observed that the particulate distribution within the settled volume of a particulate reinforced metal matrix composite melt which has been allowed to settle until it has reached its slower settling rate, is fairly uniform and has as a consequence of its settling, a volume percentage of particulate material which is greater than that of the initial feedstock material. For any particular holding time and set of melt conditions, the settled volume will be different for different particle sizes, being smaller for larger particles, such that for a particular particle size the melt will begin to stabilise at a particular volume percentage when the particles begin to interact with one another before coming into physical contact.

An ingot feedstock material prepared from the settled volume of the primary melt which has been allowed to settle for the appropriate period of time is, therefore, very stable with respect to the influence of gravity induced particulate settlement and has a higher volume percentage of particulate material than did the primary melt. A stable, high volume percentage coarse particulate reinforced metal matrix composite material therefore can be produced.

The appropriate settlement time for a given melt formulation and temperature can be determined as follows:1) A standard primary melt containing a known volume percentage of particulate material is made; 2) A sample (typically 3 Kg) of the primary melt is taken and placed in a holding furnace at the required melt temperature; 3) A set of free standing closed end tube moulds (typically made of mild steel 300 mm long x 36 mm internal diameter and coated internally with a mould wash) also are placed in the furnace; 4) The sample of primary melt is thoroughly stirred and then poured immediately into the tube moulds; 5) At regular intervals a tube mould is removed from the furnace, quenched and the sample of particulate reinforced metal matrix composite material removed from the mould; 6) The casting is machined to reveal the interface between the enriched and depleted regions of the sample and the distance of the interface from the surface of the sample is measured and recorded against its settlement time; 7) A plot is made of the settlement depth, that is, the thickness of the particulate enriched region of the casting, with time (typically as shown in Figure 2) and the time of the onset of the slower settlement rate is determined; 8) The settlement depth is used to calculate the settled volume; 9) The volume fraction of the enriched settled region of the casting from the mould showing the onset of the slower settlement rate is determined by any convenient method.

Other calibration methods using techniques known to those practised in the art are equally applicable.

The time taken to achieve the slower settlement rate and the corresponding settled volume of the sample, determined as above, are used to determine the settlement level in the primary melt at the appropriate time. The upper, particle depleted volume of the primary melt is removed for recycling. The lower, enriched, settled region is cast into ingot form to produce a stabilised particulate concentrated material for secondary melting and casting into artifacts. Examples of the application of the invention are: Example 1 - The appropriate settlement time and settled volume for an A357 matrix alloy containing 13 volume per cent of 14 Wm mean size silicon carbide particles was performed using steps 1 to 9 above. 19 kg of the primary ingot feedstock was melted down in a graphite crucible using a lift out furnace.The melt was heated to 7450C, stirred and allowed to settle for 1 hour. 9 Kg of the denuded upper layer of the melt was decanted off. The stable, settled fraction was then left to solidify. This was then remelted to 7450C. The melt was stirred, and four 7.5 cm sand moulded test cubes were cast. The cubes were fed by a feeder directly placed on the top of the cube face, it measured 6 cm in diameter and 7 cm in height. The cast cubes were sectioned vertically, the faces polished and examined. The particulate was well distributed throughout the crosssection.

Example 2 - A stable concentrated feedstock was produced as in Example 1 and again remelted to 7450C.

The melt was stirred and on this occasion a total of three gravity die castings each weighing approximately 2.5 Kgs were produced. The die casting mould was uncoated but had been pre-heated using a small propane burner. The castings were sectioned horizontally and vertically and again examined. All sections showed a uniform particle distribution.

Example 3 - A larger 50 Kg melt of stable concentrated feedstock was produced as in Example 1 and remelted in a bale out furnace. Again die castings were produced. There were examined and again showed a good distribution of the particulate in the cross-sections.

Attempts to produce similar castings using primary particulate reinforced metal matrix composites feedstock material which had not been stabilised by settlement resulted in very poor particulate distribution, and on occasions, very low levels of particulate within the casting, making them useless as cast particulate reinforced metal matrix components.

Claims

1. A method of manufacturing a particulate reinforced metal matrix composite material, including the operations of producing a primary melt containing an initial volume percentage of a particulate reinforcing material in a metal matrix material, maintaining the melt at a particular temperature for a settlement period sufficient to allow an optimum volume percentage of the particulate material within the settled volume to be achieved, and removing that part of the melt above the settled volume, the settlement period having been determined by a previous calibration experiment.

2. A method according to Claim 1 wherein the particulate-depleted portion of the melt is recycled.

3. A method according to Claim 1 or Claim 2 wherein the metal matrix material is an aluminium/silicon alloy.

4. A method according to any of Claims 1 to 3 wherein the particulate reinforcing material is silicon carbide.

5. A method according to any preceding claim wherein the particle size of particulate reinforcing material lies in the range 15-25 pin.

6. A method according to any preceding claim wherein the volume percentage of the particulate reinforcing material is less than 20%.

7. A method of manufacturing a particulate reinforced metal matrix composite material substantially as hereinbefore described.