WO2001023124A1

WO2001023124A1 - Process and apparatus for manufacturing castings from immiscible metallic liquids

Info

Publication number: WO2001023124A1
Application number: PCT/GB2000/003551
Authority: WO
Inventors: Zhongyun Fan; Michael John Bevis; Shouxun Ji
Original assignee: Brunel University
Priority date: 1999-09-24
Filing date: 2000-09-15
Publication date: 2001-04-05
Also published as: GB9922696D0; GB2354472A; AU7432200A

Abstract

A method for forming a semisolid slurry from a metallic alloy having at least two immiscible components, comprising the steps of a) providing the alloy at a temperature of at least about its demixing temperature, b) cooling the alloy to a temperature at which said components become immiscible, c) applying shear to the alloy in order to convert the alloy into a liquid suspension in which the minor component is dispersed in the major component in a liquid phase, and d) cooling the liquid suspension to its monotectic temperature or below and continuing to apply shear in order to form a semisolid slurry. Castings are formed from the semisolid slurry by transferring the slurry to a mould or to a pre-heated metal band.

Description

Process and Apparatus for Manufacturing Castings from Immiscible Metallic Liquids

This invention relates to a process and apparatus for manufacturing castings from immiscible metallic alloys. At room temperature, one of the immiscible phases is finely and uniformly dispersed in the other immiscible phase throughout the casting section. The immiscible systems can be either binary or multi-component systems.

A large number of liquid alloys, such as Al-Pb, Al-In and Pb-Ga, exhibit a limited miscibility, i.e. , the binary diagrams show a miscibility gap which represents the equilibrium between two liquids of different compositions. Those systems, often referred as the immiscibles, have great potential applications in advanced self-lubricating bearing systems, electrical contacts and 'super-conducting devices.

Here, Al-Pb binary system is taken as an example to illustrate the microstructural requirement for bearings. Usually, a material used for sliding bearings is of a composite nature with equiaxed inclusions of both hard and soft phases distributed in a mechanically strong matrix. The soft phase provides good embeddability, whereas the hard phase is responsible for the increased wear resistance. The tribological behaviours of such alloys are not only determined by the volume fraction of those phases but also by their particle size and distribution in the matrix. In the case of Al-Pb system for bearings, it is desirable to have a microstructure in which the fine equiaxed soft Pb particles and a hard reinforcing phase (e.g., alumina) are uniformly distributed throughout the Al-alloy matrix.

Mixing the immiscible metallic systems has been a long-standing challenge to metallurgists and engineers. The efforts along this direction can go back as far as last century. Unfortunately, it has not yet been possible technically to produce those alloys, because early in the cooling phase of the homogeneous liquid, the alloys separate quickly into two different liquids (one being Al-rich, the other being Pb-rich), owing to the large difference in density between the various phases. As described by Stoke's Law, the sedimentation velocity of the droplet increases as the square of the droplet radius, therefore large droplets settle much more rapidly than the small ones. When droplets of different sizes and thus different sedimentation velocities collide, they coagulate to form even larger droplets which settle much more rapidly. Owing to this phenomenon, no casting process under terrestrial conditions has yet been able to produce the desired solidified dispersion of Pb particles in the Al matrix after solidification, even if extremely high solidification rates were achieved. These systems have gained renewed interests as a result of materials research in the outer space. Many tests have been performed during space experiments to achieve an appropriate phase distribution under microgravity conditions in the 1980s. The results, however, were rather disappointing, because even under microgravity conditions a coarse phase separation occurred. The origin of this coarse demixing was found to be Marogoni motion of the droplets, which is on the Earth superimposed on the gravity-driven sedimentation (Stoke's motion) and often hidden behind its action. However, having studied the results of space experiments, scientists started to utilise those unexpected findings in the early 1990s. A new strip casting process and a planar flow casting technique were developed recently in German Aerospace Research Establishment, DLR. In both processes, an artificial temperature gradient was created to introduce a controlled Marogoni motion, which was intend to counter-balance partially the gravity-driven Stoke's motion. Unfortunately, the experimental results were unsatisfactory: Pb was found to concentrate in the middle of the strip.

The properties of interface boundaries between immiscible components are discussed in High Temp. Mater. Processes, Vol 14, pages 255-261, 1995, A Zhnkov et al. and in Zh.Fiz.Khim, Vol. 49, pages 2570-2574, 1975, V I Kononenko et al., but no attempt is made in these articles to mix or to prepare castings from the alloys. It is a primary objective of this invention is to provide a process and apparatus for manufacturing castings from the immiscible metallic alloys, in which, at room temperature, one of the immiscible phases is finely and uniformly dispersed in the other immiscible phase throughout the casting section.

Another objective of the invention is to provide an apparatus and process which is specially adapted for mixing immiscible metallic alloys which are highly corrosive and erosive in their molten or semisolid state.

According to a first aspect of the invention, there is provided a method for forming a semisolid slurry from a metallic alloy having at least two immiscible components, comprising the steps of a) providing the alloy at a temperature of at least about its demixing temperature, b) cooling the alloy to a temperature at which said components become immiscible, c) applying shear to the alloy in order to convert the alloy into a liquid suspension in which the minor component is dispersed in the major component in a liquid phase, and d) cooling the liquid suspension to its monotectic temperature or below and continuing to apply shear in order to form a semisolid slurry.

Shear is preferably applied to the alloy by means of an extruder.

Extruders are well known in the art of polymer processing. They comprise a barrel having at least one extruding screw disposed therein. Each screw generally has a shaft which is aligned with the barrel of the extruder, and a series of flights or vanes disposed along the shaft. These flights or vanes may be connected in a spiral or helical manner to form a continuous thread down the shaft. The form may be varied depending on the desired effect. Preferably, the extruder has at least two screws which are at least partially intermeshed, and most preferably substantially fully intermeshed. By this it is meant that the flights or vanes on one screw should at least partially overlap with the flights or vanes on the other screw with respect to the longitudinal axis of movement of the alloy through the extruder. Thus, in a preferred embodiment, two screws each having a continuous spiralled vane down the screw shaft are disposed such that the vanes overlap along the "line of sight" of the longitudinal axis of the shafts, which are aligned with the longitudinal axis of the extruder barrel.

In a second aspect of the invention, there is provided a method for forming a casting from a metallic alloy having at least two immiscible components, comprising forming a semisolid slurry from the alloy by means of a method as defined above, and then transferring the slurry to a mould to form the casting or to a pre-heated metal band to form a strip casting.

In a third aspect of the invention, there is provided a casting formed from a metal alloy comprising at least two immiscible components, wherein the microstructure of the casting comprises a fine and uniform dispersion of the minor component in a matrix of the major component.

More particularly, the preferred embodiment of the present invention relates to a method and apparatus for converting immiscible metallic liquid into a semisolid slurry and injecting subsequently the semisolid slurry into a die cavity for production of high integrity castings. The said method can offer semisolid slurries with high enough viscosity to prevent coarse segregation in the immiscible system. The said apparatus and method can also offer net-shaped metallic castings with a fine and uniform dispersion of the minor phase in a matrix of the major phase.

In order to facilitate the description of the present invention, a schematic binary phase diagram of an immiscible A-B system, as shown in Fig 1, is used to introduce the relevant terminology. The miscibility gap is represented by the curve MCF. For an alloy of a given composition, it is miscible above MCF and immiscible below MCF. The liquid separation occurs through the following reaction:

L <- Li + L2

The maximum temperature on the miscibility gap is called the critical temperature, denoted as Tc. A monotectic reaction occurs at the monotectic temperature, TM, to produce the solid phase A_(S) from Li phase:

Ll <r> L2 + A(_S)

and an eutectic reaction occurs at the eutectic temperature, TE, to complete the solidification process:

L2 <-> A(s) + B(s)

For a given alloy with X% of B, the liquid separation temperature will be Tχ>

Briefly described, the above mentioned objectives are accomplished according to the present invention by providing an apparatus and process, rheomixer and rheomixing process, to produce castings from the immiscible liquid alloys. This is achieved by the following two-step strategy, as depicted in Fig 2:

(1) Creation of a fine liquid dispersion by high shear mixing. An artificial shear stress- strain field is applied continuously to the immiscible alloy during the cooling process from a temperature above Tx . This shear mixing action is so extensive that it can override the demixing actions resulted from both Stoke's and Marogoni motions. Consequently, a fine homogeneous liquid dispersion is created at a temperature above T_M (Fig 2a). (2) Stabilisation of the fine liquid dispersion. Although the fine liquid dispersion created by the high shear mixing action will slow down substantially the demixing process by both Stoke's and Marogoni motions due to the very much reduced droplet size, it is still unstable. With prolonged time, it will demix with increasing speed. However, the fine liquid dispersion can be further stabilised by shearing it at a temperature below TM to create a slurry containing both liquid phase and solid phase (See Fig 1), the viscosity of which should be high enough that both Stoke's and Marogoni motions can no longer produce coarse separation (Fig 2b). In multi-component systems, the solid phase may be created at a temperature above TM- In addition, the solid phase may be fine ceramic particles introduced externally to the alloy system at a temperature above TM-

As stated above, it is preferred to employ a twin screw extruder to apply shear to the alloy(s). In the extruder, the alloy is subjected to shearing. The shear rate is such that it is sufficient to prevent the complete formation of dendritic shaped solid particles in the semisolid state. The shearing action is induced by a pair of co-rotating screws located within the barrel and is further invigorated by helical screw flights formed on the body of the screws. Enhanced shearing is generated in the annular space between the barrel and the screw flights and between the flights of two screws.

The fluid flow of the liquid alloy or semisolid slurry in the twin screw extruder is characterised by figure "8" motions around the periphery of the screws, which moves from one pitch to the next one, forming a figure "8" shaped helix and pushing the fluid along the axial direction of the screws. This is referred as the positive displacement pumping action. In this continuous flow field, the fluid undergoes cyclic stretching, folding and reorienting processes with respect to the streamlines during the take-over of the materials from one screw to the other one. Meanwhile, fluid flow in the closely intermeshing twin-screw extruder is the circular flow pattern on the axial section, which could create high intensity of turbulence for low viscosity liquid metals and/or semisolid metals. In addition, the fluid in the extruder is subjected to a cyclic variation of shear rate due to the continuous change in the gap between the screw and the barrel, which causes the material in the extruder to undergo a shear deformation with cyclic variation of shear rate. Therefore, the fluid flow in a closely intermeshing, self-wiping and co-rotating twin-screw extruder is characterised by high shear rate, high intensity of turbulence and cyclic variation of shear rate.

Unlike the viscous drag-induced type flow of materials transported in a single screw extruder, such as employed in prior art processes, the transport behaviour in a closely intermeshing twin-screw extruder is to a large extent a positive displacement type of transport, being more or less independent of the viscosity of the materials. The velocity profiles of materials in a twin-screw extruder are quite complex and more difficult to describe. There are basically four groups of forces. The first group relates to the scales of inertia forces and centrifugal forces; the second group concerns the scale of gravity force; the third comprises the scale of internal friction and the fourth group refers to the scales of elastic and plastic deformation behaviour of the materials being processed. The principal forces acting on the liquid or semi-solid alloys during the rheomoulding process between two screws and between screw and barrel are compression, rupture, shear and elasticity.

It has been found that shear rates of 5000- 10,000s^"1 can be achieved with a twin screw extruder, which results in greatly improved results. However, if the intensity of turbulence is sufficiently high, these improved results can be achieved with shear rates of perhaps 400s^"1.

The inventive rheomixer preferably consists of an extruder and a caster. The rheomixing process starts from feeding liquid metal at a temperature above T_x into one end of the extruder. The liquid metal is either rapidly cooled into the miscibility gap or maintained at a temperature about the miscibility gap in the extruder while being mechanically sheared by, preferably, at least two at least partially intermeshing screws, converting the liquid alloy into a liquid suspension. Further simultaneously cooling and shearing the liquid suspension to a temperature below T_m will allow the formation of a slurry containing solid and liquid phases with a pre-determined volume fraction of the solid phase dictated by accurate temperature control. The slurry is then discharged and injected at a high velocity into a mould cavity or discharged on a pre-heated metal band to form a strip casting.

Generally, the feeder is used to supply liquid alloy at the pre-set temperature to the twin screw extruder. The feeder can be a melting furnace or just a ladle.

Generally, the extruder, consisting of a barrel, at least a pair of screws and a driving system, is adapted to receive molten alloy through an inlet located generally toward one end of the extruder. Once in the passageway of the extruder, molten alloy is either cooled rapidly to or maintained at a predetermined processing temperature. The processing temperature can be either above or below TM depending on the alloy systems. Also, in the extruder, the alloy is subjected to shearing. The shear rate is such that it is sufficient to create fine liquid droplets in a liquid suspension in the first stage of the cooling and to prevent the complete formation of dendritic shaped solid particles in a later stage of the cooling. The shearing action is induced by screws located within the barrel and is further invigorated by helical screw flights formed on the body of the screws. Enhanced shearing is generated in the annular space between the barrel and the screw flights and between the flights of two screws. The positive displacement pumping action of a twin-screw extruder also causes the semisolid alloy to travel from the inlet of the extruder toward the outlet of the extruder, where it is discharged. Preferably, the screws of the extruder are at least partially intermeshed.

The interior environment of the extruder is characterised by high wear, high temperature and complex stresses. The high wear is a result of the close fit between the barrel and the twin-screw as well as between the screws themselves. So a suitable material for the barrel and screws or any other components inside the extruder must exhibit good resistance to wear, high temperature creep and thermal fatigue. Meanwhile, the interior environment of the extruder may also be highly corrosive and erosive if highly reactive alloys, such Al-alloys, are processed. After intensive tests and evaluation, the present invention has developed a novel machine construction which allows highly corrosive and erosive materials to be processed without any significant degradation of the machine itself.

The barrel of the extruder is constructed with an outer layer of creep resistant first material which is lined with an inner layer of corrosion and erosion resistant second material. Preferably, the outer layer material is Hl l, H13, or H21 steel or another material with excellent elasticity and the inner layer material is sialon (a modified ceramic material with excellent corrosive and erosive resistant characteristics and good strength). Bonding of the inner and outer layers is achieved by either shrink fitting or with a buffer layer between the two. For small extruders, the barrel may be constructed from monolithic ceramics or tool steel with ceramic coatings, such as boron nitride.

The screw is positioned within the passageway of the extruder. The rotation of the screw subjects the molten alloy to shear and translates the material through the barrel of the extruder. The screw is constructed with sialon components that are mechanically or physically bonded together to get its maximum resistance to creep, wear, thermal fatigue, corrosion and erosion. Additional components of the extruder, including the outlet pipe, outlet valve body and valve core, are also constructed from sialon. The extruder is driven by either an electric motor or a hydraulic motor through a gearbox to maintain the desired rotation speed. For small extruders, the screws may be constructed from monolithic ceramics or tool steel with coatings, such as boron nitride.

The caster can be connected directly to the outlet of the extruder. The caster can include a mould clamp unit connected with a cylinder-piston assembly. The slurry discharged from the extruder into the cylinder-piston assembly can be injected into the mould cavity. Alternatively the caster can be a continuous device. The slurry may be discharged from the extruder onto a preheated metal band. Continuous moving and cooling with a simultaneously applied force on the slurry and metal band makes the slurry stick to the metal band.

A number of embodiments of the invention are described in detail herein with reference to the drawings in which:

Fig 1 is a schematic binary phase diagram of the immiscible A-B system for introduction of the terminology used to describe the rheomixing process.

Fig 2 is a schematic illustration of the microstructural evolution during the rheomixing process. (1) initial stabilisation by creation of a fine L₂ dispersion in Li; (2) further stabilisation by formation of a primary solid phase in Li: (3) monotectic solidification of Li and eutectic solidification of L₂.

Fig 3 is a schematic illustration of an embodiment of an apparatus for mixing the immiscible alloys and for producing castings according to the principles of the present invention.

Fig 4 is a schematic illustration of another embodiment of an apparatus for mixing the immiscible alloys and for producing castings according to the principles of the present invention.

Fig 5 shows the microstructure of the rheomixed Ga-10wt%Pb alloy. Pb particles are uniformly distributed in the Ga matrix.

In the description of the preferred embodiment which follows, a casting is produced by twin-screw rheomixer from a lead-gallium (Pb-Ga) binary system. However, the invention is not limited to Pb-Ga system and is equally applicable to any other types of immiscible systems, such as Al-Pb, Al-In and Al-Bi. Furthermore, specific temperatures and temperature ranges cited in the description of the preferred embodiment are applicable only to Pb-Ga system, but could be readily modified in accordance with the principles of the invention by those skilled in the art in order to accommodate other alloy systems.

Fig 3 illustrates schematically a twin-screw rheomixer according to an embodiment of this invention. The rheomixing system has three sections: a liquid metal feeder, a twin- screw extruder and a cylinder-piston assembly.

The feeder 12 is provided to receive liquid alloy with the predetermined temperature from an external source, such as a melting furnace or a ladle.

The extruder has a plurality of heating elements 10, 14 and cooling channels 11, 13 dispersed along the length of the extruder. The matched heating elements 10, 14 and cooling channels 11, 13 form a series of heating and cooling zones respectively. The heating and cooling zones maintain the extruder at the desired temperature for processing immiscible alloys. For a rheomixing system designed for the Pb-Ga alloys, heating elements and cooling channels would maintain the extruder at a temperature around 280°C. The heating and cooling zones also make it possible to maintain a complex temperature profile along the extruder axis, which may be necessary to achieve certain microstructural effects during rheomixing. The temperature control of each individual zone is achieved by balancing the heating and cooling power inputs by a central control system. The methods of heating can be resistance heating, induction heating or any other means of heating. The cooling media may be water or gas depending on the process requirement. While only two heating/cooling zones are shown in Fig 3, the extruder can be equipped with between 1 to 10 separately controllable heating/cooling zones.

The extruder is also provided with two closely intermeshing, self-wiping and co-rotating screws 15, 16 which are driven by an electric motor or hydraulic motor 8 through a gear box 9. The twin-screw 15, 16 is designed to provide high shear rate which is necessary to achieve fine and uniform liquid suspension and fine and uniform solid particles. Different types of screw profiles may of course be used. In addition, any device which offers high shear mixing and positive displacement pumping actions may also be used to replace the twin-screw.

The slurry exits the extruder into a caster through a valve 20. The valve 20 operates in response to a signal from the central control system. The optional opening of valve 20 should match the process requirements.

Injection of the slurry is made by a piston 26 through the cylinder 25 into a mould cavity 24. The position and velocity of piston 26 are adjustable to suit the requirement by different processes, materials and final castings. Generally, the shot speed should be high enough to provide enough fluidity for complete mould filling, but not too high to cause air entrapment.

As shown in Fig 3, heating and cooling elements are also provided along the length of the cylinder 25 and the mould 24. In the preferred embodiment, the cylinder is preferably maintained at a temperature close to the extruder temperature to maintain the alloy in its predetermined state. The cooling rate of the solidifying alloy in the mould is controlled by the heating element 21 and cooling element 23.

In Fig 3, the barrel is made of tool steel with a sialon liner and the screws are monolithic sialon construction. However, the barrel and screws can be tool steels or any high temperature materials coated with any suitable ceramic materials, such as boron nitride.

Fig. 4 shows an alternative caster. The metal band 21 is plated by roll 22 and heated by heater 23. When the metal band 21 moves through the supporter 24, the sheared slurry is discharged on it and subsequently moved and cooled on the metal band 21. When the metal band 21 and slurry is subsequently passed through the roll 25, the at least partially solidified slurry and the preheated metal band 21 is mechanically forced and bonded together to form a metal strip 25. Fig. 5 illustrates the microstructure of the rheomixed Ga-10wt%Pb alloy. Ga particles with an average size of 5 mm are uniformly distributed in the Pb matrix.

The grey patches in the micrograph is due to the smearing effect during polishing. It indicated that the invented rheomixing process is capable of producing a fine and uniform dispersion of one immiscible phase in the other immiscible phase.

The embodiment may also have a device attached to the extruder to supply protective gas in order to minimise oxidation. Such a gas may be argon, nitrogen, carbon dioxide or any other suitable gas or gas mixture which can effectively prevent the oxidation of metal during processing.

The embodiment may also have a device attached to the extruder to supply protective gas in order to minimise oxidation. Such a gas may be argon or nitrogen.

Generally, the rheomixing system has a central control system to realise all the functions. Ideally, the control system is programmable so that the desired microstructure may be achieved easily. The control system (not shown in Fig 3) may, for example, comprise a microprocessor which may be easily and quickly reprogrammed to change the processing parameters.

While this particular embodiment according to the present invention have been illustrated and described above, it will be clear that the invention can take a variety of forms and embodiments within the scope of the appended claims.

Claims

1. A method for forming a semisolid slurry from a metallic alloy having at least two immiscible components, comprising the steps of a) providing the alloy at a temperature of at least about its demixing temperature, b) cooling the alloy to a temperature at which said components become immiscible, c) applying shear to the alloy in order to convert the alloy into a liquid suspension in which the minor component is dispersed in the major component in a liquid phase, and d) cooling the liquid suspension to its monotectic temperature or below and continuing to apply shear in order to form a semisolid slurry.

2. A method as claimed in claim 1, wherein shear is applied at a sufficiently high shear rate and intensity of turbulence to form semisolid slurry, the viscosity of which is high enough to prevent coarse segregation of the immiscible system.

3. A method as claimed in any preceding claim, wherein shear is applied to the alloy by means of an extruder.

4. A method as claimed in any preceding claim, wherein shear is applied to the alloy by means of an extruder having at least two screws which are at least partially intermeshed.

5. A method as claimed in claim 4, wherein the screws are substantially fully intermeshed.

6. A method as claimed in any of claims 3 to 5, wherein the extruder is temperature controlled with a plurality of heating and cooling zones disposed along its longitudinal axis.

7. A method for forming a casting from a metallic alloy having at least two immiscible components, comprising forming a semisolid slurry from the alloy by means of a method as claimed in any of claims 1 to 6, and transferring the slurry to a mould to form the casting.

8. A method for forming a casting from a metallic alloy having at least two immiscible components, comprising forming a semisolid slurry from the alloy by means of a method as claimed in any of claims 1 to 6 and transferring the slurry to a pre-heated metal band to form a strip casting.

9. A casting formed from a metal alloy comprising at least two immiscible components, wherein the microstructure of the casting comprises a fine and uniform dispersion of the minor component in a matrix of the major component.

10. A casting of a metal alloy comprising at least two immiscible components obtainable by means of a method as claimed in claim 7 or 8.