WO1995022424A2

WO1995022424A2 - Metal composite casting method

Info

Publication number: WO1995022424A2
Application number: PCT/GB1995/000339
Authority: WO
Inventors: Patrick Joseph Helly
Original assignee: New Pro Foundries Limited
Priority date: 1994-02-18
Filing date: 1995-02-17
Publication date: 1995-08-24
Also published as: AU1670095A; GB9403146D0; GB2286786A; WO1995022424A3

Abstract

The casting of metal matrix composites (MMC's) is associated with numerous precautions necessary to prevent the formation of inclusions and imperfections in the molten metal during solidification. The disclosure relates to a method of casing metal matrix composites in which molten metal or alloy is poured under gravity into an enclosed mould (2), the method including the steps of allowing the molten metal or alloy to solidify in the said cavity, thereby to form the required casting; and removing the casting from the mould, characterised in that there is used a bottom fill mould wherein the molten metal or alloy is fed to the mould substantially exclusively via a sprue (3) inclined at an angle of between 45° and 75° to the horizontal, the sprue opening substantially directly into the mould cavity (2) at its lowermost point. An advantage of the method is that casting of metal matrix composites can be achieved more reliably.

Description

Metal Composite Casting Method

This invention relates to a method of casting metals and Metal Matrix

Composites (MMC) , i.e. metals and alloys containing a finely divided ceramic material, such as silicon carbide, and especially ceramic reinforced aluminium and aluminium alloys such as those sold under the Trade Mark

DURALCAN, and which give rise to high viscosity melts.

Castings of reinforced aluminium and aluminium alloys containing upto 20 volume percent or more of ceramic particles, such as silicon carbide, are much in demand as lightweight, high strength castings for a variety of different applications ranging from golf club heads, to bicycle frames, to rotor discs for motor vehicle disc brakes and brake shoes for aircraft brakes in which latter applications, at least, the ability of the alloy to function at high temperature without loss of strength is of great value. However, because of the high melt viscosity of such composite materials casting becomes a problem, and conventional casting methods, such as gravity feed to a sand mould via a vertical sprue, lead to a high reject rate. In particular, because of the high viscosity of the molten composite, air and gas bubbles trapped in the melt cannot escape leading to air or gas inclusions in the cast, or to surface defects and surface blistering. Slag and oxide inclusions are also a problem, again due to the viscosity of the melt inhibiting the movement of the slag and oxide particles through the melt to the surface. Manufacturers' recommendations are, therefore, that the molten composite be poured into the mould with a minimum of turbulence, and preferably into an inclined mould which ensures an even filling of the mould. Also according to manufacturers' recommendations, the molten composite material is fed to the mould, which may be either a sand mould or a permanent mould, via a runner which incorporates a skimmer core/riser combination which serves to trap and remove any particles of surface slag or oxide which may become entrained in the melt during pouring, and/or to trap air or gas bubbles likewise entrained during pouring of the molten composite into the runner via a vertical sprue, and prior to feeding to the mould.

In accordance with the present invention, we have found that these artifices, i.e. feeding the molten composite to the mould through a runner which incorporates a skimmer core and a riser, can be eliminated by feeding the molten composite material, under gravity, substantially exclusively through an inclined sprue, inclined at an angle of from 45° to 75° to the horizontal, preferably from 55° to 65° , most preferably at about 60°, and which communicates substantially directly with the mould cavity at or adjacent its lowest point so that the mould is filled from the bottom upwards at a preferred mass flow rate of from about 0.25 to 0.75 kg/sec, preferably from 0.4 to 0.6 kg/sec and at a linear flow velocity of from 5 to 50 cms/sec, preferably 10 to 35 cm/sec, both rates applying at the point of entry into the mould. Preferably also, at the point of entry of the inclined sprue into the mould cavity the acute angle between the inclined sprue and the side wall of the mould cavity is radiussed, the radius of curvature being anything from 2 to 30 mm, preferably from 10 - 15 ram, depending on the diameter of the sprue, and the dimensions of the cavity, thus providing a smoothly curved surface at the point of entry into the mould. Usually but not necessarily a filter, preferably a ceramic filter will be incorporated either into the sprue itself or else substantially at the point of entry into the mould cavity.

Whilst it is generally preferred that the inclined sprue should communicate substantially directly with the mould cavity, this may not always be possible for geometrical reasons. In such circumstances, therefore, it may be necessary to incorporate into the mould a short runner extending between the end of the inclined sprue and the mould cavity, but preferably this should be kept as short as possible, no more than a few centimetres. For the larger casting it may be necessary to cast the metal composite through two or more inclined sprues.

The method of the invention has been particularly developed for the gravity fed, sand mould casting of ceramo-metallic composites, but may equally be applied to other methods of casting, and to other castable metals and alloys, whether or not containing a particulate ceramic as a reinforcement material. Casting temperatures will be in accordance with manufacturers' instructions . For silicon carbide reinforced aluminium , and silicon carbide reinforced aluminium alloys, e.g. the precipitation hardening alloys of aluminium, which contain small amounts of silicon , copper and nickel as well as aluminium, casting temperatures will be in the range 680° to 750°C, preferably from 700° to 730°C.

Since, in the case of these reinforced metals and metal alloys, they contain a particulate ceramic material, e.g. silicon carbide, which usually will have a higher density than the matrix, it is important that the melt should be thoroughly mixed before casting to ensure uniform dispersion of the particles throughout the melt. Again, in accordance with manufacturers' recommendations, this is usually achieved using a rotary impeller, preferably with inclined blades or thread to impart a vertical lift to the melt, and to the particles, which tend to sink to the bottom of the melt, if left too long without stirring. Also recommended is the use of an internally fluted crucible, the flutes of which are inclined to assist the vertical lifting and dispersion of the particles throughout the melt. Depending on the quantities involved, uniform homogenisatLon of the melt using a rotary impeller can take as long as 20-30 minutes, which adds quite considerably to the length of the casting process. In a separate and independent aspect of the present invention , it has been found that the homogenisation time can be substantially reduced by reciprocal agitation of the melt in a vertical direction rather than by rotational stirring and using an agitator tool or plunger comprising a lift member mounted on the end of a rod and extending radially therefrom and by means of which the lift member can be reciprocated in the melt along a vertical axis , either manually or mechanically . Preferably the length of the stroke and the speed of reciprocation will be such as to cause minimum - k - disruption or turbulence at the surface of the melt. Homogenisation of the melt will be carried out under an inert atmosphere, e.g. under nitrogen, or more preferably argon. Degassing of the melt can be performed either subsequently or simultaneously by bubbling argon (or nitrogen) through the melt, either using a separate wand, or through the shaft of the agitator. Using these two techniques in combination. Metal Matrix Composite (MMC) castings have been made substantially free from inclusions and surface imperfections, so much so that casting yields (i.e. weight of casting of acceptable quality against total amount of metal cast) have risen from 40% , using a conventional sand mould with a vertical sprue , to 62% or more using a sand mould with an inclined sprue.

The invention is further described with reference to the accompanying drawings of preferred, exemplary embodiments of the invention, in which: Figure 1 is an imaginary section through a sand mould as used in accordance with the invention, and

Figure 2 is an imaginary section through a crucible containing molten metal to be cast, and showing the tool used to homogenise the melt prior to casting. Referring to the drawings. Figure 1 shows an imaginary section through a sand mould (1) defining a U-shaped mould cavity (2) . The molten metal to be cast, which, as indicated, is preferably a ceramo- metallic composite such as a silicon carbide reinforced aluminium or aluminium alloy, is gravity fed to the bottom of the mould cavity via a sprue (3) which, in accordance with this invention is inclined at an angle of from 45° to 75° to the horizontal, preferably about 60° as shown, and which communicates substantially directly with the mould cavity, i.e. with only a minimum length , if any , of horizontal runner connecting the bottom of the sprue to the mould cavity. A ceramic filter block (4) is preferably located towards the bottom of the sprue to filter the molten composite passing into the mould cavity.

As indicated the sand mould is radiussed at a radius "r" , preferably about 10 mm, at the point of entry of the sprue into the mould cavity. Also, preferably, the mouth of the sprue is radiussed at a radius R, the value of which is not at all critical, and is there merely to provide a smoothly curved surface over which the metal flows at the point of entry to the sprue, as well as at the point of entry into the mould cavity.

As in all foundry practice, steps down in the flow path of the metal are to be avoided wherever possible. Steps up can be tolerated, but generally the flow path is as smooth as possible.

In the casting of ceramo-metallic composites, especially silicon carbide reinforced aluminium and aluminium alloy, the molten composite is poured into the sprue (3) at a mass flow rate of from 0.25 to 0.75 kg/sec. Preferably from 0.4 to 0.6 kg/sec, and at a linear velocity of from 5 to 50 cm/sec, preferably 10 to 35 cms/sec.

Before pouring, the molten composite must be thoroughly mixed to ensure homogenous dispersion of the ceramic particles throughout the melt . Preferably this is done in the manner shown in Figure 2.

In Figure 2, the molten composite (5) is shown in a pouring crucible (6) . Prior to pouring, the molten composite is homogenised by the reciprocal action of a plunger (7) comprising a plate (8) or other similar lift member located on the distal end of a rod (9) which is reciprocated vertically, or substantially so, within the crucible, either by hand, or mechanically. As indicated by the arrows, this reciprocal movement of the plate rapidly disperses the ceramic particles, which tend to settle at the bottom of the crucible, upwardly into the melt. Homogenisation will be done under an inert atmosphere, e.g. under nitrogen or argon, and with or without the sparging of argon or nitrogen through the melt. This may be done either using a separate wand, or as shown using a hollow tubular plunger (7) as a conduit for passing the gas into the melt, usually through a ceramic gas diffuser (10) located in the distal end of central bore in the plunger (7) . Using this method, molten ceramo-metallic composites can be homogenised very rapidly in from 5 - 10 minutes, depending on the volume of the melt, thus considerably reducing the time required to produce the composite metal casting.

As will be apparent numerous modifications and improvements may be made in the above methods without departing from the scope of the invention as hereinafter claimed.

Claims

CLAIMS:

1. A method of sand or die casting a molten metal or metal alloy, which comprises pouring the molten metal or alloy under gravity into an enclosed mould having a suitably vented mould cavity formed therein and defining the shape of the required casting, allowing the molten metal or alloy to solidify in the said cavity, thereby to form the required casting, and removing the casting from the mould, characterised in that there is used a bottom fill mould wherein the molten metal or alloy is fed to the mould substantially exclusively through a sprue inclined at an angle of from 45 to 75° to the horizontal, said sprue opening substantially directly into the mould cavity at its lowermost point, the mould cavity thus being filled from the bottom up .

2. A method according to claim 1 , wherein the angle of inclination of the sprue is in the range 55° to 65°.

3. A method according to claim 2 , wherein the angle of inclination of the sprue is about 60°.

4. A method according to any one of claims 1 to 3 , where at the point of entry of the sprue into the mould cavity and in the acute angle formed between the inclined sprue and the well defining the mould cavity, the surface of the mould is radiussed and provides a curved surface over which the molten material flows from the sprue into the mould cavity.

5. A method according to any one of claims 1 to 4, wherein the mould is a sand mould.

6. A method according to any one of claims 1 to 5, wherein the molten metal or alloy is cast at a mass flow rate in the range 0.25 to 0.75 kg/sec and at a linear flow velocity in the range 5 to 50 cms/sec, that mass flow rate and linear flow velocity being measured at the point of entry of the molten metal or alloy into the mould cavity.

7. A method according to claim 6, wherein the mass flow rate is in the range 0.4 to 0.6 kg/sec and the linear velocity is in the range 10 to 35 cms/sec.

8. A method according to any one of claims 1 to 7, as applied to the casting of a reinforced metal composite comprising particles of ceramic material dispersed in a metal or alloy matrix .

9. A method according to claim 8, as applied to the casting of silicon carbide reinforced aluminium or aluminium alloy.

10. A method according to claim 8 or 9, which includes the step of homogenising the melt before pouring.

11. A method according to claim 10 whereby the homogenisation being effected by vertical agitation of the melt under an inert atmosphere using a reciprocating plunger.

12. A method according to claim 10 or 11, wherein the melt is simultaneously degassed by bubbling nitrogen or argon into the melt during the homogenisation process.