METHOD AND APPARATUS FOR PROCESSING A LIQUID ALLOY

Abstract

A method and apparatus for producing solid alloy components from its liquid state are provided. The molten alloy is rapidly cooled using a chill to temperatures below the thermosolutal transition temperature of the alloy. Finite-amplitude acoustic vibration is applied on the chill to shake off dendrites that form on the chill surface, to stir the slurry containing the fragments of dendrites, and to shake off slurry material that sticks on the surface of the chill as the chill is separating from the slurry. The slurry is then immediately poured into a chamber of a forming machine or a mold cavity shaped into solid components.

Claims

1. A method of producing a metallic component from its liquid alloy, comprising of: preparing a liquid alloy that is free from its primary solid phase material and transferring a predetermined quantity of liquid alloy to a holding vessel or a trough; contacting the liquid alloy in the holding vessel with a vibration coupled chill to form solid crystals on the chill-liquid interfaces, and to rapidly cool the bulk of the molten alloy to below its thermosolutal transition temperature; vibrating the chill to shake off the solid crystals formed on the chill surfaces, to drive them to the bulk liquid, and to cause forced convection to mix the solid-liquid mixture containing a small fraction of non-dendritic solid crystals; separating the chill from the mixture after the solid content in the slurry has risen to 1% to 20% while vibrating the chill to shake off the slurry that may stick to the surfaces of the chill; and pouring the slurry containing a small fraction of non-dendritic solid particles into a component forming apparatus and shaping the slurry into a desired solid component.

2. The method of claim 1, wherein the liquid alloy is maintained at minimum superheat within 80° C., ideally within 30° C., above its liquidus temperature to reduce the processing time and costs.

3. The method of claim 1, wherein one of the liquid alloys is an aluminum alloy at temperatures above its liquidus temperature.

4. The method of claim 1, wherein one of the liquid alloys is a magnesium alloy at temperatures above its liquidus temperature.

5. The method of claim 1, wherein one of the liquid alloys is a zinc alloy at temperatures above its liquidus temperature.

6. The method of claim 1, wherein the molten alloy is rapidly cooled, using a solid chill of high thermal conductivity, to below the thermosolutal transition temperatures of the alloy in order to produce enough non-dendritic fragments and to maintain these fragments in the solid-liquid mixture containing a small fraction of solid in the range of about 1% to 20%.

7. The method of claim 1, wherein finite-amplitude vibration is coupled to the chill to shake off dendrites and to stir the bulk liquid while maintaining the top surface of the melt relative quiescent.

8. An apparatus for direct production of a slurry containing a small fraction of non-dendritic solid particles from a liquid alloy for subsequent forming into solid alloy components, comprising of: a vessel or a trough for containing a quantity of liquid alloy and for pouring the slurry into another fast cooling chamber of a forming apparatus or a mold cavity; a plurality of a solid chill containing at least one chill block for rapidly cooling the liquid metal in the vessel; and a plurality of a small-amplitude vibrator coupled to the chill for producing non-dendritic solid particles in the liquid and for shaking of the slurry material that sticks on the surfaces of the chill while it is separating from the slurry.

9. The apparatus of claim 8, wherein the said vessel or the said trough is made of ceramic materials or steel protected with a layer of coating to prevent the steel from reacting to the liquid alloy.

10. The apparatus of claim 8, wherein the vessel is a ladle used for HPDC process or other casting processes.

11. The apparatus of claim 8, wherein the small amplitude vibration is acoustic vibration with a frequency in the range of 500 to 500,000 Hz and power of 100 watts to 60,000 watts.

12. The apparatus of claim 8, wherein the chill is made of at least a material, such as steel, cast iron, titanium alloy or niobium alloy, having high thermal conductivity and thermal capacity for effective cooling the liquid metal.

13. The apparatus of claim 8, wherein the chill consists of a plurality of metallic sonotrode.

14. The apparatus of claim 8, wherein the chill consists of a plurality of metallic sonotrode containing at least one sonotrode surrounded by a block of metal chill.

15. The apparatus of claim 8, where in the chill has a total volume and a total surface area comparable to that of the liquid alloy in the said vessel or trough.

16. The apparatus of claim 8, wherein the chill can be cooled internally or externally to enhance its cooling capability and to maintain its desired temperatures using a fluid including, but not limited to, compressed air, water, cooling oil, ionic liquid, or liquid metallic alloy.

17. The apparatus of claim 8, wherein the chill is made of a titanium alloy.

18. The apparatus of claim 8, wherein the chill is made of steel or cast iron.

19. The apparatus of claim 8, where in the chill is made of niobium alloys.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a binary phase diagram showing the liquidus, solidus, and the thermosolutal transition temperature for an alloy at a given bulk concentration.

[0019] FIG. 2 is a schematic illustration of an apparatus in accordance with an embodiment of the present invention.

[0020] FIG. 3 is a schematic illustration of another embodiment in accordance with the present invention.

[0021] FIG. 4 is a schematic illustration of another embodiment in accordance with the present invention.

[0022] FIG. 5 is a schematic illustration of yet another embodiment in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0024] The present invention provides a method and apparatus for producing a solid-liquid mixture of an alloy containing a small fraction of discrete, non-dendritic primary solid phase particles in a ladle which can be poured into a shot chamber during HPDC or forging process for making solid components. The major solid phase that first precipitates from the molten alloy is termed the primary phase. In aluminum alloys, the primary phase is the aluminum-rich fcc phase which grows into dendrites or tree-like particles on cooling of the molten alloy below its liquidus. These dendrites can be broken up into non-dendritic fragments by vigorous stiffing, re-heating or isothermal coarsening in semi-solid temperatures [4-5]. Non-dendritic fragments are usually discrete ellipsoidal- or spherical-shaped particles.

[0025] The present invention is made based on the inventor's understanding on the rate of remelting and dissolution of a primary phase solid particle in the molten alloy at various temperatures. FIG. 1 illustrates a binary phase diagram containing elements A and B. On cooling from liquid, an A matrix alloy containing element B with a composition of C.sub.0 starts forming the primary solid phase dendrites with its composition of k.sub.0C.sub.0 at or slightly below the liquidus, T.sub.L, where k.sub.0 is the partition coefficient of the element B at the solid-liquid interface, or dendrite-liquid interface during freezing. Equilibrium solidification, i.e., solidification under extremely slow cooling rates, of the alloy completes at the solidus, T.sub.S, which is the eutectic temperature on the phase diagram. Consider the dendrites that precipitate near the liquidus temperature, T.sub.L. Their composition is k.sub.0C.sub.0 and the corresponding liquidus temperature is T.sub.T. These dendrites are relatively stable, except coarsening, at temperatures below T.sub.L but will either dissolve back into the liquid or melt when the local temperature are higher than T.sub.L.

[0026] Dissolution, melting, or isothermal coarsening of dendrites leads to smoothing out dendrites into non-dendritic fragments. However, the former two phenomena result in the disappearance of the fragments in the melt. Still, any residual of each fragment can serve as a nucleus for a new dendrite to grow from the liquid as soon as the local temperature is reduced to below T.sub.L. Enough number of such residual particles prevents new solid particles from growing into dendrites, which is effective in forming non-dendritic solid particles from the molten alloy. The issue is how long these fragments will survive before they fully totally disappear in the liquid at temperatures higher than T.sub.L. Research has suggested that T.sub.T is actually a thermosolutal transition temperature above which the particles of composition k.sub.0C.sub.0 melt and below which these particles dissolve. The melting process is controlled by heat transfer to the particle from adjacent liquid and the dissolution process is controlled by diffusion of element B between the particle and the liquid. Since the thermal diffusivity is a few orders of magnitude higher than the diffusion coefficient of a solute element, the rate of melting is much faster than the rate of dissolution [12-13]. Experimental data also suggest that at temperatures slightly below T.sub.T, the dissolution rate of a solid particle is in the order of a few micron meters per seconds. The dissolution rate decreases with decreasing temperature. Thus, it will take over 10 seconds for a particle large than 50 micron meters to dissolve into the melt at temperatures below T.sub.T [12-13]. Such a survival time is long enough for the dendrite to be broken into multiple non-dendritic fragments before cooling under the liquidus temperature of the alloy by using a proper size chill to enhance the cooling of the melt.

[0027] The process of the present invention comprises of a first step of forming a liquid alloy with a vessel at prescribed temperatures at a minimum amount of superheat to reduce the use of energy for heating up the alloy and to shorten the production cycle. The vessel is usually a melting or holding furnace. The minimum temperature in this vessel can be as low as the liquidus of the alloy but is usually higher to account for temperature fluctuation which may lead to the growth of solid in the molten alloy.

[0028] The process of the present invention comprises of a second step of transferring the molten alloy 10 prepared in the first step into a second vessel 40, shown in FIG. 2. The second vessel 40 is usually a ladle used in the HPDC process but can also be a trough or other means of holding molten metal before pouring the molten metal for making castings. In the meantime, a chill 50 coupled with vibrators for vibration 60 is prepared. The chill 50 is made of a solid material and is maintained at prescribed temperatures to keep it dry, free from moisture, using internal or external thermal control if needed. The coupling of vibration 60 to the chill 50 can take place in many ways. It can be a plurality of metal sonotrode, or a plurality of sonotrodes surrounded by a metal chill, having a total mass large enough to cool the melt 10 in the vessel 40. It can also be a single block of metal connected to a vibrator or a hollow block of metal with vibration coupled in the hollow block with a fluid serving both as the coupling liquid and as a coolant.

[0029] The process of the present invention comprises of a third step of cooling and stiffing the molten metal 10 using a vibration 60 coupled chill 50 shown in FIG. 3 to form dendrites 20 on the chill-melt interface and to detach these dendrites 20 shown in FIG. 2 to form non-dendritic fragment 30 using the vibration 60 shown in FIG. 4. The duration of this step lasts for just a few seconds to create fragments 30 in the molten alloy 10 which becomes a mixture of solids and liquids containing a small fraction of non-dendritic solid phase particles. After enough fragments 30 have been made, the chill 50 is separated from the mixture while the vibration 60 shakes off residual liquid that may adhere on the surface of the chill 50. The chill 50 coupled with vibration 60 can also be used in a trough to create fragments of dendrites for casting processes other than the HPDC process.

[0030] The process of this invention comprises of a fourth step of pouring the mixture of solid-liquid containing a small fraction of non-dendritic fragments 30 from the vessel 40 into a shot chamber 80 wherein a ram 70 is used to push the mixture 10 into the cavity 80 in dies 85 and 90 to be solidified into a solid component, shown in FIG. 5. The mixture of the solid-liquid containing a small fraction of solid can also be poured into the cavity of casting molds for making components.

[0031] The physics associated with the present invention is illustrated in FIG. 3 where the temperatures vs. distance profiles are depicted. The temperature in the molten alloy prior to contacting the chill 50 is T.sub.0, which is higher than the liquidus, T.sub.L, of the alloy. At the moment when the chill 50 contacts the molten alloy 10, the temperature of the melt 10 at the chill-melt interface is T.sub.1, which is much lower than the liquidus, T.sub.L, of the alloy. As a result, dendrites 20 form on the chill-melt interface on the wall of the chill 50. In the meantime, the bulk temperature of the molten alloy 10 decreases due to heat extraction by the chill 50. Vibration 60 applied on the chill 50 ensures that dendrites 20 formed on the chill 50 are detached off the wall of the chill 50. The detached dendrites enter the bulk molten metal 10 where they are broken up into fragments 30 due to increased local temperature and vigorous stiffing caused by the vibration 60, shown in FIG. 4. The breaking up of detached dendrites leads to a multiplication in the number of solid phase crystals because each dendrite 20 can be broken into many fragments 30 and each fragment 30 is an individual crystal. The step shown in FIGS. 3 and 4 is maintained for a few seconds. During this step, the temperature profile drops from T.sub.0 to that corresponding to time t.sub.1, or t.sub.2 as the duration increases, shown in FIG. 3. The optimal temperature profile is preferably in the shaded range defined by the curves corresponding to t.sub.1 and t.sub.2. With the temperature profile at duration of t.sub.1, majority of the molten alloy is in the temperatures below the thermosolutal transition temperature, T.sub.T, allowing for most of the dendritic fragments 30 to survive for many seconds while experiencing morphological smoothing out. At the temperature profile associated with the duration of t.sub.2, the melt 10 is at sub-liquidus temperatures so that all dendritic fragments 30 can survive while experiencing Oswald Ripening. Further vigorous stiffing of the mixture makes the non-dendritic fragments 30 more ellipsoidal or even spherical. The existence of enough non-dendritic fragments 40, shown in FIG. 5 in the shot sleeve 80, makes the local formation of new dendrites impossible so that the cooling and stiffing in the shot sleeve 80 only make the non-dendritic fragments 30 grow while coarsening.

[0032] The invention teaches that the temperature in molten alloy in the first vessel, which can be a holding furnace or a melting furnace, has to be higher than the liquidus of the alloy in the first step of the present invention. This is to ensure that no solid alloy particles are formed from the melt in the first vessel because these particles tend to coarse in the vessel holding the alloy at extended times.

[0033] The invention also teaches that the surface area of the chill that is in contact with the molten alloy in the second vessel, which is but not limited to the ladle, should be comparable in size to that of the molten metal such that enough dendrites can be produced at the chill-melt interface. The cooling capability of the chill needs to be designed such that 1) the temperature in the melt at the chill-melt interface is below the liquidus of the alloy during the chill cooling process to encourage the nucleation of dendrites on the chill wall, and 2) the bulk temperature of the melt is reduced to below the thermosolutual transition temperature, T.sub.T, towards the end of each chill cooling to ensure that majority of the fragments survives before the mixture of the solid-liquid is poured into a shot chamber, a trough to a mold, or a mold cavity for making castings. Such a cooling capability of the chill can be designed using its volume of the chill, internal cooling in the chill, or external cooling on the chill surface. Internal or external cooling may also need to restore the initial designed temperature of the chill before it is used for the next cycle for process a melt in the ladle.

[0034] The present invention further teaches that vibration needs to be coupled to the chill to shake off the dendrites on the chill surface, to stir the melt, and to shake off the residual liquid that may stick to the chill surface as it is separated from the melt. For these purposes, any kind of mechanical vibration can be used. The intensity of vibration, defined as power per unit surface area on the chill surface, does not need to be as high as those technologies using high-intensity ultrasonic vibration for grain refining or for making semi-solid materials [6-11]. Small amplitude vibration is preferred as such kind of vibration is unlikely to cause damage (rapture) to the top surface of the melt in the second vessel. For aluminum alloys, for example, the top surface of the melt is covered by a protective layer of oxides. Damage to this layer of oxides leads to pollution to the molten alloy, such as hydrogen absorption and increased oxide formation.

[0035] The invention further provides examples of producing a solid-liquid mixture of an alloy containing a small fraction of non-dendritic primary phase solid particles in a ladle which can be poured into a shot chamber during HPDC or forging process for making solid components. The examples provided below are meant merely to exemplify several embodiments, and should not be interpreted as limiting the scope of the claims, which are delimited only by the specification.

[0036] While the invention has been described in connection with specific embodiments thereof, it will be understood that the inventive methodology is capable of further modifications. This patent application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth and as follows in scope of the appended claims.

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