GB2042385A - Casting thixotropic metals - Google Patents

Abstract

Apparatus 10 includes a duplex mould 11 for postponing solidification within the mould until the molten metal is within a magnetic field for providing magnetohydrodynamic stirring of a thixotropic slurry. The mould 11 includes a liner 42 of low thermal conductivity, eg ceramic, at the top and a main part 21 of high conductivity. <IMAGE>

Description

SPECIFICATION Process and apparatus for casting metals and alloys This invention relates to an apparatus for forming semi-solid thixotropic alloy slurries for use in applications such as rheocasting, thixocasting, or thixoforging.

The known methods for producing semisolid thixotropic alloy slurries include mechanical stirring and inductive electromagnetic stirring. The processes for producing such a slurry with a proper structure require a balance between the shear rate imposed by the stirring and the solidification rate of the material being cast.

In the mechanical stirring process, the molten metal flows downwardly into an annular space in a cooling and mixing chamber. Here the metal is partially solidified while it is agitated by the rotation of a central mixing rotor to form the desired thixotropic metal slurry for rheocasting. The mechanical stirring approaches suffer from several inherent problems. The annulus formed between the rotor and the mixing chamber walls provides a low volumetric flow rate of thixotropic slurry.

There are material problems due to the erosion of the rotor. It is difficult to couple mechanical agitation to a continuous casting system.

In the continuous rheocasting processes described in the art the mixing chamber is arranged above a direct chill casting mould.

The transfer of the metal from the mixing chamber to the mould can result in oxide entrainment. This is a particularly acute problem when dealing with reactive alloys such as aluminium, which are susceptible to oxidation.

The volumetric flow rates achievable by this approach are inadequate for commercial application.

The slurry is thixotropic, thus requiring high shear rates to effect flow into the continuous casting mould. Using the mechanical approach, one is likely to get flow lines due to interrupted flow and/or discontinuous solidification. The mechanical approach is also limited to producing semi-solid slurries, containing from about to 30 to 60% solids. Lower fractions of solids improve fluidity but enhance undesired coarsening and dendritic growth during completion of solidification. It is not possible to get significantly higher fractions of solids because the agitator is immersed in the slurry.

In order to overcome the aforenoted problems inductive electromagnetic stirring has been proposed. While the indirect nature of this electromagnetic stirring is an improve- ment over the mechanical process, there are still limitations imposed by the nature of the stirring technique.

With AC inductive stirring, the maximum electromagnetic forces and associated shear are limited to the penetration depth of the induced currents. Accordingly, the section size that can be effectively stirred is limited due to the decay of the induced forces from the periphery to the interior of the melt. This is particularly aggravated when a solidifying shell is present. The inductive electromagnetic stirring process also requires high power consumption and the resistance heating of the stirred metal is significant. The resistance heating in turn increases the required amount of heat extraction for solidification.

The pulsed DC magnetic field technique is also effective, however, it is not as effective as desired because the fore field rapidly diverges as the distance from the DC electrode increases. Accordingly, a complex geometry is required to produce the required high shear rates and fluid flow patterns to ensure production of slurry with a proper structure. Large magnetic fields are required for this process and, therefore, the equipment is costly and very bulky.

Our co-pending Application No. 79 05621 (Serial No. 2014614) (J. Winter et al 1-1-1) describes apparatus and processes in which magnetohydromagnetic motion associated with a rotating magnetic field generated by a two pole multiphase motor stator is used to achieve the required high shear rates for producing thixotropic semi-solid alloy slurries. The magnetohydromagnetic process therein disclosed provides high volumetric flow rates which make the process particularly adaptable to continuous or semi-continuous rheocasting.

According to the present invention there is provided an apparatus for forming a semisolid thioxtropic alloy slurry of degenerate dendrite primary solid particles in a surrounding matrix of molten metal, said apparatus comprising: means for containing molten metal; means for controllably cooling said molten metal in said containing means; and means for generating a magnetic field for mixing said molten metal for shearing dendrites formed in a solidification zone as said metal is cooled for forming said slurry; wherein said containing means includes means for postponing solidification within said containing means until said molten metal is within said magnetic field.

The use of a duplex mould having an upper portion of low thermal conductivity and a lower portion of higher thermal conductivity ensures that the molten metal can solidify under the influence of the rotating magnetic field. This helps the resultant rheocast casting to have a degenerated dendritic structure throughout its cross-section even up to its outer surface.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic representation in partial cross-section of an apparatus for continuously or semi-continuously casting a thixotropic semi-solid metal slurry; Figure 2 is a schematic representation in partial cross-section of the apparatus of Fig. 1 during a casting operation; Figure 3 is a partial cross-sectional view along the line 3-3 in Fig. 1; Figure 4 is a schematic bottom view of a non-circular mould and linear induction motor stator arrangement; Figure 5 is a schematic representation of the lines of force at a given instant generated by a four pole induction motor stator; Figure 6 is a schematic representation of the lines of force at a given instant generated by a two pole motor stator.

In the background of this application there have been described a number of techniques for forming semi-solid thixotropic metal slurries for use in rheocasting, thixocasting, thixoforging, etc. Rheocasting as the term is used herein refers to the formation of a semi-solid thixotropic metal slurry, directly into a desired structure, such as a billet for later processing, or a die casting formed from the slurry. Thixocasting or thixoforging respectively as the terms are used herein refer to processing which begins with a rheocast material which is then reheated for further processing such as die casting or forging.

The metal composition of a thixotropic slurry comprises primary solid discrete particles and a surrounding matrix. The surrounding matrix is solid when the metal composition is fully solidified and is liquid when the metal composition is a partially solid and partially liquid slurry. The primary solid particles comprise degenerate dendrites or nodules which are generally spheroidal in shape. The primary solid particles are made up of a single phase or a plurality of phases having an average composition different from the average composition of the surrounding matrix in the fully solidified alloy. The matrix itself can comprise one or more phases upon further solidification.

Conventionally solidified alloys have branched dendrites which develop interconnected networks as the temperature is reduced and the weight fraction of solid increases. In contrast thixotropic metal slurries consist of discrete primary degenerate dendrite particles separated from each other by a liquid metal matrix, potentially even up to solid fractions of 80 weight percent. The primary solid particles are degenerate dendrites in that they are characterized by smoother surfaces and a less branched structure which approaches a spheroidal configuration. The surrounding solid matrix is formed during solidification of the liquid matrix subsequent to the formation of the primary solids and contains one or more phases of the type which would be obtained during solidification of the liquid alloy in a more conventional process.The surrounding solid matrix comprises dendrites, single or multiphased compounds, solid solution, or mixtures of dendrites, and/or compounds, and/or solid solutions.

Referring to Figs. 1 and 2, an apparatus 10 for continuously or semi-continuously rheocasting thixotropic metal slurries is shown.

The cylindrical mould 11 is adapted for such continuous or semi-continuous rheocasting.

The mould 11 may be formed of any desired non-magnetic material such as stainless steel, copper, copper alloy or the like.

Referring to Fig. 3, it can be seen that the mould wall 1 3 is cylindrical. The technique described herein is adapted for making cylindrical ingots utilizing a conventional two pole polyphase induction motor stator for stirring.

However, it is not limited to the formation of a cylindrical ingot cross-section since it is possible to achieve a transversely or circumferentially moving magnetic field with a non-cylindrical mould 11 as in Fig. 4. In the embodiment of Fig. 4 the mould 11 has a rectangular cross-section surrounded by a polyphase rectangular induction motor stator 1 2. The magnetic field moves or rotates around the mould 11 in a direction normal to the longitudinal axis of the casting which is being made.

The bottom block 1 3 of the mould 11 is arranged for movement away from the mould as the casting forms a solidifying shell. The movable bottom block 1 3 comprises a standard direct chill casting type bottom block. It is formed of metal and is arranged for movement between the position shown in Fig. 1 wherein it sits up within the confines of the mould cavity 14 and a position away from the mould 11 as shown in Fig. 2. This movement is achieved by supporting the bottom block 1 3 on a suitable carriage 1 5. Lead screws 1 6 and 1 7 or hydraulic means are used to raise and lower the bottom block 1 3 at a desired casting rate in accordance with conventional practice. The bottom block 1 3 is arranged to move axially along the mould axis 18. It includes a cavity 1 9 into which the molten metal is initially poured and which provides a stabilising influence on the resulting casting as it is withdrawn from the mould 11.

A cooling manifold 20 is arranged circumferentially around the mould wall 21. The particular manifold shown includes a first input chamber 22, a second chamber 23 connected to the first input chamber by a narrow slot 24. A discharge slot 25 is defined by the gap between the manifold 20 and the mould 11. A uniform curtain of water is provided about the outer surface 26 of the mould 11.

A suitable valving arrangement 27 is provided to control the flow rate of the water or other coolant discharged in order to control the rate at which the slurry S solidifies. In the appa ratus 10 a manually operated valve 27 is shown, however if desired this could be an electrically operated valve.

The molten metal which is poured into the mould 11 is cooled under controlled conditions by the water sprayed upon the outer surface 26 of the mould 11 from the encompassing manifold 20. By controlling the rate of water flow against the mould surface 26 the rate of heat extraction from the molten metal within the mould 11 is controlled.

In order to provide a means for stirring the molten metal within the mould 11 to form the desired thixotropic slurry a two pole multiphase induction motor stator 28 is arranged surrounding the mould 11. The stator 28 is comprised of iron laminations 29 about which the desired windings 30 are arranged in a conventional manner to provide a three-phase induction motor stator. The motor stator 28 is mounted within a motor housing M. The manifold 20 and the motor stator 28 are arranged concentrically about the axis 1 8 of the mould 11 and casting 31 formed within it.

It is preferred to use a two pole three-phase induction motor stator 28. One advantage of the two pole motor stator 28 is that there is a non-zero field across the entire cross-section of the mould 11. It is, therefore, possible to solidify a casting having the desired rheocast structure over its full cross-section.

Fig. 5 shows the instantaneous lines of force for a four pole induction motor stator at a given instant in time. It is apparent that the centre of the mould does not have a desired magnetic field associated with it. Therefore, the stirring action is concentrated near the wall 21 of the mould 11. In comparison thereto, a two pole induction motor stator as shown in Fig. 6 generates instantaneous lines of force at a given instant which provide a non-zero field across the entire cross-section of the mould 11. The two pole induction motor stator 28 also provides a higher frequency of rotation or rate of stirring of the slurry S for a given current frequency than the four pole approach of Fig. 5.

A partially enclosing cover 32 is utilised to prevent spill out of the molten metal and slurry S due to the stirring action imparted by the magnetic field of the motor stator 28. The cover 32 comprises a metal plate arranged above the manifold 20 and separated therefrom by a suitable ceramic liner 33. The cover 32 includes an opening 34 through which the molten metal flows into the mould cavity 14.

Communicating with the opening 34 in the cover is a funnel 35 for directing the molten metal into the opening 34. A ceramic liner 36 is used to protect the metal funnel 35 and the opening 34. As the thixotropic metal slurry S rotates within the mould 11, cavity centrifugal forces cause the metal to try to advance up the mould wall 21. The cover 32 with its ceramic lining 33 prevents the metal slurry S from advancing or spilling out of the mould 11 cavity and causing damage to the apparatus 1 0. The funnel portion 35 of the cover 32 also serves as a reservoir of molten metal to keep the mould 11 filled in order to avoid the formation of a U-shaped cavity in the end of the casting due to centrifugal forces.

Situated directly above the funnel 35 is a downspout 37 through which the molten metals flows from a suitable furnace 38. A valve member 39 associated in a coaxial arrangement with the downspout 37 is used in accordance with conventional practice to regulate the flow of molten metal into the mould 11.

The furnace 38 may be of any conventional design, it is not essential that the furnace be located directly above the mould 11. In accordance with convention direct chill casting processing the furnace may be located laterally displaced therefrom and be connected to the mould 11 by a series of troughs or launders.

Referring again to Fig. 3, a further advantage of the rotary magnetic field stirring approach in accordance with this invention is illustrated. In accordance with the Fleming's right-hand rule for a given current J in a direction normal to the plane of the drawing the magnetic flux vector B extends radially inwardly of the mould 11 and the magnetic stirring force vector F extends generally tangentially of the mould wall 21. This sets up within the mould cavity a rotation of the molten metal in the direction of arrow R which generates the desired shear for producing the thixotropic slurry S. The force vector F is also tangential to the heat extraction direction and is normal to the direction of dendrite growth. This maximises the shearing of the dendrites as they grow.

It is preferred that the stirring force field generated by the stator 28 extend over the full solidification zone of molten metal and thixotropic metal slurry S. Otherwise the structure of the casting will comprise regions within the field of the stator 28 having a rheocast structure and regions outside the stator field tending to have a non-rheocast structure. In the embodiment of Figs. 1 and 2 the solidification zone preferably comprises the sump of molten metal and slurry S within the mould 11 which extends from the top surface 40 to the solidification front 41 which divides the solidified casting 31 from the slurry S. The solidification zone extends at least from the region of the initial onset of solidification and slurry formation in the mould cavity 14 to the solidification front 41.

Under normal solidification conditions, the periphery of the ingot 31 will exhibit a columnar dendritic grain structure. Such a structure is undesirable and detracts from the overall advantages of the rheocast structure which occupies most of the ingot cross-section. In order to eliminate or substantially reduce the thickness of this outer dendritic layer the thermal conductivity of the upper region of the mould 11 is reduced by means of a partial mould liner 42 formed from an insulator such as a ceramic. The ceramic mould liner 42 extends from the ceramic liner 33 of the mould cover 32 down into the mould cavity 14 for a distance sufficient so that the magnetic stirring force field of the two pole motor stator 28 is intercepted at least in part by the partial ceramic mould liner 42.The ceramic mould liner 42 is a shell which conforms to the internal shape of the mould 11 and is held to the mould wall 21. The mould 11 comprises a duplex structure inciuding a low heat conductivity upper portion defined by the ceramic liner 42 and a high heat conductivity portion defined by the exposed portion of the mould wall 21.

The liner 42 postpones solidification until the molten metal is in the region of the strong magnetic stirring force. The low heat extraction rate associated with the liner 42 generally prevents solidification in that portion of the mould 11. Generally solidification does not occur except towards the downstream end of the liner 42 or just thereafter. The shearing process resulting from the applied rotating magnetic field will further override the tendency to form a solid shell in the region of the liner 42. This region 42 or zone of low thermal conductivity thereby helops the resultant rheocast casting 31 to have a degenerate dendritic structure throughout its cross-section even up to its outer surface.

Below the region of controlled thermal conductivity defined by the liner 42, the normal type of water cooled metal casting mould wall 21 is present. The high heat transfer rates associated with this portion of the mould 11 promote ingot shell formation. However, because of the zone 42 of low heat extraction rate even the peripheral shell of the casting 31 should consist of degenerate dendrites in a surrounding matrix.

It is preferred in order to form the desired rheocast structure at the surface of the casting to effectively shear any initial solidified growth from the mould liner 42. This can be accomplished by ensuring that the field associated with the motor stator 28 extends over at least that portion of the liner 42 at which solidification is first initiated.

The dendrites which initially form normal to the periphery of the casting mould 11 are readily sheared off due to the metal flow resulting from the rotating magnetic field of the induction motor stator 28. The dendrites which are sheared off continue to be stirred to form degenerate dendrites until they are trapped by the solidifying interface 41. Degenerate dendrites can also form directly within the slurry because the rotating stirring action of the melt does not permit preferential growth of dendrites. To ensure this the stator 28 length should preferably extend over the full length of the solidification zone. In particular, the stirring force field associated with the stator 28 should preferably extend over the full length and cross-section of the solidification zone with a sufficient magnitude to generate the desired shear rates.

To form a rheocasting 31 utilising the apparatus 10 of Figs. 1 and 2, molten metal is poured into the mould cavity 14 while the motor stator 28 is energized by a suitable three-phase AC current of a desired magnitude and frequency. After the molten metal is poured into the mould cavity it is stirred continuously by the rotating magnetic field produced by the motor stator 28. Solidification begins from the mould wall 21. The highest shear rates are generated at the stationary mould wall 21 or at the advancing solidification front 41. By properly controlling the rate of solidification by any desired means as are known in the prior art the desired thixotropic slurry S is formed in the mould cavity 14. As a solidifying shell is formed on the casting 31, the bottom block 1 3 is withdrawn downwardly at a desired casting rate.

The shear rates which are obtainable with the process and apparatus 10 are much higher than those reported for the mechanical stirring process and can be achieved over much larger cross-sectional areas. These high shear rates can be extended to the centre of the casting cross-section even when the solid shell of the solidifying slurry S is already present.

The induction motor stator 28 which provides the stirring force needed to produce the degenerate dendrite rheocast structure can be readily placed either above or below the primary cooling manifold 20 as desired. Preferably, however, the induction motor stator 28 and mould 11 are located below the cooling manifold 20.

It is preferred that the entire casting solidify in the stator 28 field in order to produce castings with proper rheocast structure through their entire cross-section. Therefore, the casting apparatus 10 should preferably be designed to ensure that the entire solidification zone is within the stator 28 field. This may require extra long stators 28 to be provided to handle some types of casting.

We have found that two competing processes shearing and solidification control the casting process. The shearing produced by the electromagnetic process and apparatus of this invention can be made equivalent to or greater than that obtainable by mechanical stirring. The interaction between shear rates and cooling rates causes higher stator currents to be required for continuous type casting than are required for static casting.

It has been found in accordance with this invention that the effects of the experimental variables in the process can be predicted from a consideration of two dimensionless groups, namely ss and N as follows:

Z = angular line frequency a = melt electrical conductivity sso = magnetic permeability R = melt radius B^, = magnetic induction at the mould wall 710 = melt viscosity.

The first group, ss, is a measure of the field geometry effects, while the second group, N, appears as a coupling co-efficient between the magnetomotor body forces and the associated velocity field. The computed velocity and shearing fields for a single value of ss as a function of the parameter N can be determined.

From these determinations it has been found that the shear rate increases sharply toward the outside of the mould where it reaches its maximum. This maximum shear rate increases with increasing N. It has been concluded that the shearing is produced in the melt because the peripheral boundary or mould wall is rigid. Therefore, even when a solidifying shell is present, there should still be shear stresses in the melt and they should be maximal at the liquid solid interface 41.

Further, because there are always shear stresses at the advancing interface 41 it is possible to make a full section ingot 31 with the appropriate degenerate dendritic rheocast structure.

The following example illustrates the invention: Ingots 2.5 inches in diameter of alloy 6061 were cast using an apparatus 10 similar to that shown in Figs. 1 and 2. The bottom block 1 3 was lowered and the casting was drawn from the mould 11 at speeds of from about 8 to 14 inches per minute. The two pole three-phase induction motor stator 28 current was varied between 5 and 35 amps. It was found that at the low current end of this range, a fine dendritic grain structure was produced but not the characteristic structure of a rheocast thixotropic slurry. At the high current end of the range particularly in and around 1 5 amps fully non-dendritic structures were generated having a typical rheocast structure comprising generally spheroidal primary solids surrounded by a solid matrix of different composition.

The mould cover 32 by enclosing the mould cavity 1 4 except for the small centrally located opening 34 serves not only to prevent spillage of molten metal but also to prevent the formation of a U-shaped cavity in the end of the rheocasting. By adding sufficient molten metal to the mould to at least partially fill the funnel 35, it is possible to ensure that the mould cavity 1 4 is completely filled with molten metal and slurry. The cover 32 offsets the centrifugal forces and prevents the formation of the U-shaped cavity on solidification.

By completely filling the mould oxide entrainment in the resulting casting is substantially reduced.

While it is preferred that the stirring force due to the magnetic field extend over the entire solidification zone it is recognised that the shearing action on the dendrites results from the rotating movement of the melt. This metal stirring movement can cause shearing of dendrites outside the field if the moving molten metal pool extends outside the field.

Dendrites will initially attempt to grow from the sides or wall 21 of the mould 1 1,. The solidifying metal at the bottom of the mould may not be dendritic because of the comparatively low heat extraction rate which promotes the formation of more equiaxed grains. Suitable stator currents will vary depending on the stator which is used. The currents must be sufficiently high to provide the desired magnetic field for generating the desired shear rates.

Suitable shear rates for carrying out the process comprise from at least about 100 sec.-1 to about 1500 sex.~' and preferably from at least about 500 sec. -' to about 1200 sex.~'. For aluminium and its alloys a shear rate of from about 700 sec. - 1 to about 1100 sec. -' has been found desirable. The average cooling rates through the solidification temperature range of the molten metal in the mould should be from about 0.1 C per minute to about 1000"C per minute and preferably from about 10"C per minute to about 500"C per minute.For aluminium and its alloys an average cooling rate of from about 40"C per minute to about 500"C per minute has been found to be suitable. The efficiency of the magnetohydrodynamic stirring allows the use of higher cooling rates than with prior art stirring processes. Higher cooling rates yield highly desirable finer grain s.tructures in the resulting rheocasting. Further, for continuous rheocasting higher throughput follows from the use of higher cooling rates.

The parameter lss21 (P defined by equation (1) should comprise from about 1 to about 10 and preferably from about 3 to about 7.

The parameter in N (defined by equation (2)) should comprise from about 1 to about 1000 and preferably from about 5 to about 200.

The angular line frequency X for a casting having a radius of from about 1" to about 10" should be from about 3 to about 3000 hertz and preferably from about 9 to about 2000 hertz.

The magnetic field strength which is a function of the angular line frequency and the melt radius should comprise from about 50 to 1 500 gauss and preferably from about 100 to about 600 gauss.

The particular parameters employed can vary from metal system to metal system in order to achieve the desired shear rates for providing the thixotrophic slurry. The appropriate parameters for alloy systems other than aluminium can be determined by routine experimentation.

Solidification zone as the term is used in this application refers to the zone of molten metal or slurry in the mould wherein solidification is taking place. Magnetohydrodynamic as the term is used herein refers to the process of stirring molten metal or slurry using a moving or rotating magnetic field. The magnetic stirring force may be more appropriately referred to as a magneto motive stirring force which is provided by the moving or rotating magnetic field of this invention.

The process and apparatus described herein are applicable to the full range of materials as set forth in the prior art including but not limited to aluminium and its alloys, copper and its alloys and steel and its alloys.

Claims (11)

1. An apparatus for forming a semi-solid thioxtropic alloy slurry of degenerate dendrite primary solid particles in a surrounding matrix of molten metal, said apparatus comprising; means for containing molten metal; means for controllably cooling said molten metal in said containing means; and means for generating a magnetic field for fixing said molten metal for shearing dendrites formed in a solidification zone as said metal is cooled for forming said slurry; wherein said containing means includes means for postponing solidification within said containing means until said molten metal is within said magnetic field.

2. An apparatus as claimed in claim 1, wherein said containing means comprises a mould having at least a first portion and a second portion wherein said means for postponing solidification comprises said first portion of said mould and wherein said first portion of said mould has a low thermal conductivity and said second portion of said mould has a high thermal conductivity.

3. An apparatus as claimed in claim 2, wherein said first portion of said mould is formed of said mould is formed of a nonmagnetic metal or alloy.

4. An apparatus as claimed in claim 2 or 3, wherein said cooling means is arranged about said first portion of mould.

5. An apparatus as claimed in claim 2, wherein said mould comprises a metal wall member for surrounding said molten metal and slurry, said wall member defining a top and bottom thereof and wherein a partial mould liner is provided internally of said mould wall extending from said top of said mould wall to a position intermediate said top and bottom of said mould wall to define said first portion of said mould, said liner leaving a portion of said metal wall member exposed which defines said second portion of said mould.

6. An apparatus as claimed in claim 5, wherein said liner is formed from an insulating material.

7. An apparatus as claimed in claim 5 or 6, wherein said magnetic field overlaps said liner.

8. An apparatus as claimed in any one of claims 2 to 7, wherein said mould wall has a cylindrical shape.

9. An apparatus as claimed in any one of claims 2 to 8, wherein said mould comprises a mould for continuously or semi-continuously forming a rheocasting.

10. A casting apparatus substantially asdescribed herein with reference to Figs. 1, 2, 3 and 6 or to Figs. 4 and 6 of the accompanying drawings.

11. A casting process substantially as described herein with reference to the accompanying drawings.

1 2. A casting prepared by a process as claimed in claim 11.