ULTRASOUND ASSISTED VERTICAL SHOT CHAMBER FOR DIE CASTING APPLICATIONS

Abstract

A method and apparatus for producing semi-solid material castings from its liquid state in a shot chamber of a vertical die casting machine where the liquid material is poured into a shot chamber and rapidly cooled from its liquid state to temperatures below its liquidus. High-intensity ultrasonic vibration is applied to the cast material. The combined action of rapid cooling from the shot chamber, vigorous pushing by the plunger, and radiation of ultrasonic vibration on the cast material in the shot chamber directly turns the initial liquid material directly into a semi-solid slurry. The slurry is then injected into a die cavity to form a casting.

Claims

1. A method for producing a semi-solid material directly from its liquid material in a shot chamber in a vertical cold chamber die casting machine for making castings, comprising the steps of: embedding at least one sonotrode in a plunger with one tip of each sonotrode in direct contact with the liquid material to be introduced in the shot chamber; preparing the liquid material and pouring the liquid material into the shot chamber at a temperature slightly higher than its liquidus temperature; transmitting high-intensity ultrasonic vibration through said at least one sonotrode to the liquid material immediately as the liquid material is introduced into the shot chamber wherein the tip of each sonotrode is vibrating with an intensity of ultrasonic vibration capable of creating cavitation in the liquid material nearby; and forcing slurry formed from the liquid material to fill die cavity using the plunger while high-intensity ultrasonic vibration is applied to the slurry in the shot chamber.

2. The method of claim 1, wherein the liquid material is poured into the shot chamber at superheat within 150 C. above its liquidus temperature.

3. The method of claim 1, wherein the tip of said at least one sonotrode is smaller than a tip of the plunger.

4. The method of claim 1, wherein the at least one sonotrode is made of a ferrous alloy, a Ti alloy, a ceramic material, or a refractory metal alloy including Nb alloy, W alloy, and Mo alloy.

5. The method of claim 1, wherein the tip of the at least one sonotrode is made of refractory materials or coated with a layer of refractory materials which are defined as ceramics, refractory metals and their alloys including Nb, W, Mo, Ta, and Ha based alloys.

6. The method of claim 1, wherein the intensity of ultrasonic vibration at the tip of each sonotrode is greater than 100 W/cm.sup.2 and vibrational frequency is higher than 15,000 Hz.

7. The method of claim 1, wherein the liquid material is a metallic alloy including aluminum alloys.

8. A method for producing a semi-solid material directly from its liquid material in a shot chamber in a vertical cold chamber die casting machine for making castings, comprising the steps of: preparing the liquid material and pouring the liquid material into the shot chamber at a temperature slightly higher than its liquidus temperature; submerging a sonotrode into the liquid material and then withdrawing the sonotrode out of the shot chamber within a predetermined duration; transmitting high-intensity ultrasonic vibrations through the sonotrode into the liquid material immediately as the tip of the sonotrode is in contact with the liquid material; and forcing slurry formed from the liquid material to fill the die cavity using the plunger.

9. The method of claim 8, wherein the liquid material is poured in the shot chamber at a superheat within 150 C. above its liquidus temperature.

10. The method of claim 8, wherein the sonotrode is made of a ferrous alloy, a refractory metal alloy such as niobium alloy, or a ceramic material.

11. The method of claim 8, wherein the tip of the sonotrode is made of refractory materials or coated with a layer of refractory materials which are defined as ceramics, refractory metals and their alloys including Nb, W, Mo, Ta, and Ha based alloys.

12. The method of claim 8, wherein the intensity of ultrasonic vibration at the tip of the sonotrode is greater than 100 W/cm.sup.2 and vibrational frequency is higher than 15,000 Hz.

13. The method of claim 8, wherein the liquid material is a metallic alloy including aluminum alloys.

14. The method of claim 8, wherein the sonotrode is submerged into the liquid material, lowered to bottom of the shot chamber, and then withdrawn out of the shot chamber within a predetermined duration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 schematically represents a cold chamber die casting process and a plunger coupled with high-intensity ultrasonic vibration in accordance with an embodiment of the present invention.

[0016] FIG. 2 schematically represents a cold chamber die casting process and a shot plate coupled with high-intensity ultrasonic vibration in accordance with another embodiment of the present invention.

[0017] FIG. 3A schematically represents a vertical die casting process in accordance with another embodiment of the present invention.

[0018] FIG. 3B schematically represents a vertical die casting process in accordance with another embodiment of the present invention.

[0019] FIG. 3C schematically represents a vertical die casting process in accordance with another embodiment of the present invention.

[0020] FIG. 4 represents a cold chamber die casting process with plunger and shot plate coupled with high-intensity ultrasonic vibration in accordance with the present invention.

[0021] FIG. 5 represents a cold chamber die casting process with a plurality of sonotrode embedded in the tubular wall of a shot chamber in accordance with the present invention.

[0022] FIG. 6 represents a hot chamber die casting process and a sprue-spreader coupled with high-intensity ultrasonic vibration 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] In a preferred embodiment, the present invention relates to a method and apparatus for producing a slurry containing discrete non-dendritic primary phase solid particles in a shot chamber by using the high cooling capacity of the shot chamber to the molten metal and high-intensity ultrasonic vibration applied directly on to the molten metal during its early stage of solidification in the shot chamber. Here high-intensity ultrasonic vibration is defined as a type of vibration having its intensity of vibration greater than 100 W/cm.sup.2 and its vibrational frequency higher than 15,000 Hz. The molten metal includes an aluminum alloy, magnesium alloy and zinc alloy.

[0025] A non-dendritic, semi-solid material is a material containing liquid material and discrete solid non-dendritic particles dispersed in the liquid material. Non-dendritic particles generally have a spherical or ellipsoidal shape. This type of particles is formed as a result of forced convection in a solidifying liquid during its nucleation and early stage of dendritic growth below the liquidus temperature of the material. The general understanding is that the forced vigorous convection breaks up dendrite arms from dendritic crystals and enhances the subsequent ripening of these fragments, turning them into spherical or ellipsoidal particles. This convective effect on the morphology of the solidifying material is pronounced during the early stage of dendritic solidification at high cooling rate when the precipitated dendrites are thin and small.

[0026] Under die casting conditions, the massive shot chamber provides rapid cooling on the liquid material to initiate its early stage of solidification. Vigorous convection occurs when pouring the liquid material into the die chamber, pushing the solidifying material by a plunger to fully fill the shot chamber, and finally injecting the solidifying material from the shot chamber to the die cavity [4]. Such a combination of rapid cooling and vigorous stirring causes certain fragmentation of dendrites formed in the shot chamber but is not sufficient to produce fully non-dendritic solid particles in the semi-solid slurry.

[0027] In a preferred embodiment, the present invention relates to a method and apparatus for producing a slurry containing discrete non-dendritic primary phase solid particles in a shot chamber. High intensity ultrasonic vibration is coupled to the plunger to assist in forming discrete non-dendritic primary phase solid particles from the molten alloy. High-intensity ultrasonic vibration can affect both the nucleation and the growth stages of dendritic solidification. With ultrasonic vibration applied to the melt, cavitations occur which give rise to the formation of a large number of tiny discontinuities or cavities. These cavities expand and collapse instantaneously, causing undercooling which leads to copious nucleation and eventual formation of the globular structures desired for semi-solid processing [5-9]. Such an acoustically induced nucleation effect is enhanced by the rapid cooling of the massive shot chamber on the molten alloy. High-intensity ultrasonic vibration is also effective in breaking up dendrites adjacent to the acoustic radiator [10]. This effect, however, decays with increasing distance from the radiator owing to acoustic attenuation in the viscous semi-solid slurry. To overcome the acoustic attenuation issue, the present invention teaches that the acoustic radiator or acoustic vibration is coupled to the tip of the plunger. As the plunger travels throughout the shot chamber and pushes the slurry towards the die cavity, the tip of the plunger encounters a large number of dendrites in the slurry. In the meantime, forced convection in the melt brings dendrites to the tip of the acoustically active plunger as well. As a result, the acoustic attenuation issue is, to a large extent, avoided, and the acoustically activated plunger can be used to process a large volume of slurry effectively.

[0028] In another preferred embodiment, the present invention relates to a method and apparatus for producing a slurry containing discrete non-dendritic primary phase solid particles in a shot chamber. High-intensity ultrasonic vibration is applied on the shot plate near the entrance to the die cavity so that the molten metal prior to entering the die cavity is processed with the acoustic radiator or the acoustically activated shot plate. High-intensity ultrasonic vibration is effective in breaking up dendrites adjacent to the acoustic radiator [10]. The dendritic fragments formed near the radiator will smooth out rapidly to form globular particles under the combined influence of acoustic streaming and vigorous turbulence caused by the plunger when the slurry is entering the die cavity.

[0029] FIG. 1 shows an ultrasound assisted cold chamber die casting process in accordance with an embodiment of this invention. An ultrasonic vibrator 12 or sonotrode 12 is coupled to a plunger 22. The sonotrode 12 is made of a ferrous alloy, a Ti alloy, a ceramic material, or a refractory metal alloy including Nb alloy, W alloy, and Mo alloy. The tip of the sonotrode 12 is made of refractory materials or coated with a layer of refractory materials which are defined as ceramics, refractory metals and their alloys including Nb, W, Mo, Ta, and Ha based alloys. Molten alloy 10 is transferred from a holding furnace or a melting furnace using a ladle 18 to a cavity 20 of shot chamber 24. As the molten metal 10 is poured into the cavity 20, ultrasonic vibration is applied to the molten metal 10 during its solidification resulted from the rapid cooling of the massive shot chamber 24. Immediately after the pouring is completed, the ultrasonic vibrator 12 and the plunger 22 push the slurry (molten metal plus dendrites that have formed) towards the end of the shot chamber 24 before entering the die cavity 28 defined by the movable die 30 and the fixed die 32. During this pushing process, high-intensity ultrasonic vibration is transmitted into the slurry to break up dendrites that form in the shot chamber and smooth them out into globular particles that are not likely to choke the mold filling at the in-gates. As a result, the molten metal 10 can be poured into a shot chamber at much lower temperatures than that using a conventional die casting process. Having a low pouring temperature is beneficial in extending die tooling life and in improving the internal integrity of the castings. Although FIG. 1 illustrates a horizontal shot chamber, the sonotrode 12 embedded plunger 22 can also be used in a vertical shot chamber in a cold chamber vertical die casting machine.

[0030] In the embodiment of this invention shown in FIG. 1, the ultrasonic vibrator 12 is coupled with the plunger 22 in a various way. For instance, the tip of the ultrasonic vibrator 12 can be protruding the tip of the plunger 22, flush with it, or lagging behind it. In the case that water cooling is applied near the tip of the plunger 22, ultrasonic vibration can be coupled through the cooling water to the tip of the plunger 22. Ultrasonic vibration can also be directly coupled to the plunger tip so that the entire tip of the plunger 22 becomes the tip of the ultrasonic vibrator 12. This means that the ultrasonic vibrator 12 and the plunger tip can be combined into one-piece.

[0031] FIG. 2 illustrates another ultrasound assisted cold chamber die casting process in accordance with an embodiment of this invention. An ultrasonic vibrator 14 is coupled to the shot plate 24, which is designed to rapidly solidify the biscuit, i.e., the metal remaining at the end of the shot chamber cavity 20 after the die cavity 28 is totally filled. Molten alloy 10 is transferred from a holding furnace or a melting furnace using a ladle 18 to the cavity 20 of shot chamber 24. As the molten metal 10 is pouring into the cavity 20, ultrasonic vibration is applied to the molten metal 10 during its solidification that occurs due to the rapid cooling of the massive shot chamber 24. Immediately after the pouring is completed, the molten metal 10 becomes a slurry containing liquid and dendrites that form from the liquid. The plunger 22 pushes the slurry 10 towards the shot plate 26 before entering the die cavity. During this pushing process, high-intensity ultrasonic vibration is transmitted into the slurry 10 to break up dendrites that form in the shot chamber and smooth them out into globular particles that are not likely to choke the mold filling at the in-gates [2]. Because the ultrasonic radiator 14 is placed very close to the entrance of the die cavity 28, all dendrites in the molten metal 10 in the shot chamber 24 are forced to pass by the ultrasonic radiator 14 and experience high-intensity ultrasonic vibration, resulting in the formation of non-dendritic globular solid particles in the slurry. As a result, the molten metal 10 can be poured into a shot chamber at much lower temperatures than that using a conventional die casting process. Having a low pouring temperature is beneficial in extending die tooling life and in improving the internal integrity of the castings.

[0032] In the embodiment of this invention shown in FIG. 2, an ultrasonic vibrator 14 is coupled with the shot plate 26 in a various way. For instance, the tip of the ultrasonic vibrator 12 can be protruding the tip of the shot plate 26, flush with it, or lagging behind it. In the case where water cooling is applied near the surface of the shot plate 26, ultrasonic vibration can be coupled through the cooling water to the shot plate 26. Ultrasonic vibration can also be directly coupled to the shot plate 26 so that the entire shot plate 26 becomes the ultrasonic vibrator 14.

[0033] For the vertical cold chamber die casting process, or a vertical indirect squeeze casting process, the shot chamber 24 shown in FIG. 2 is vertical and the plunger 22 is placed at the bottom of the shot chamber 24 shown in FIG. 3. The other end of the shot chamber 24 is open to receive molten metal 10 from the ladle 18 shown in FIG. 3A. After the cavity 20 in the shot chamber 24 has finished receiving the molten metal 10 shown in FIG. 3A, the sonotrode or the ultrasonic vibrator 14 is then directly submerged into the molten metal 10 within a predetermined duration and withdrawn from the molten metal. The sonotrode can also be directly submerged into the molten metal 10, lowered to the bottom of the shot chamber 24 and then withdrawn out of the shot chamber 24 within a predetermined duration to form a slurry containing a small fraction of non-dendritic fragments of dendrite shown in FIG. 3B. The shot chamber 24 is then swung back to under the dies 30 and 32 shown in FIG. 3C to allow the plunger 22 to inject the slurry 10 into the mold cavity 28 in molds 30 and 32. High-intensity ultrasonic vibration is applied through the sonotrode to the molten metal 10 while the sonotrode is in contact with the molten metal 10 in the shot chamber 24. The motion of the ultrasound-activated sonotrode in the molten metal 10 from top of the shot chamber 24 to the bottom of the shot chamber 24 as shown in FIG. 3B breaks up dendrites when the sonotrode encounters them, forming non-dendritic fragments of dendrite in the slurry 10.

[0034] FIG. 4 illustrates yet another ultrasound assisted cold chamber die casting process in accordance with an embodiment of this invention. The approach shown in FIG. 4 is actually a combination of the previous two approaches shown in FIG. 1 and FIG. 2. High-intensity ultrasonic vibrators, 12 and 14, are used at both of the ends of the shot chamber 24: one in the plunger 22 and the other in the shot plate 26. Molten metal 10 poured into the cavity 20 of the shot chamber 24 is subjected to high-intensity ultrasonic vibration from both ends in the shot chamber 24 as well as severe stirring due to the pushing of the plunger 22. Such a combination of ultrasound induced effects and plunger induced stirring enhances the completeness of the formation of a slurry containing liquid and spherical solid particles formed in the shot chamber, leading to an extended die tooling life and improved the internal integrity and mechanical properties of the castings made using this invention.

[0035] FIG. 5 shows an ultrasound assisted cold chamber die casting process in accordance with yet an embodiment of this invention. A plurality of an ultrasonic vibrator 15 is embedded into the tubular wall of the shot chamber 24. Molten alloy 10 is transferred from a holding furnace or a melting furnace using a ladle 18 to the cavity of shot chamber 24. As the molten metal 10 is poured into the cavity 20, ultrasonic vibration is applied directly to the molten metal 10 through the sonotrode 15 embedded in the shot chamber 24. The ultrasonic vibration energy to each sonotrode is stopped immediately before the plunger 22 passes over the sonotrode 15. The sonotrode 15 is so placed that the tip of the sonotrode contacts the molten metal 10 so that the ultrasonic vibration energy is fully transmitted to the molten metal to produce non-dendritic fragments of solid particles.

[0036] FIG. 6 illustrates an ultrasound assisted hot chamber die casting process in accordance with an embodiment of this invention wherein an ultrasonic radiator is used as the sprue-spreader 16. As shown, molten metal 10 is held in a holding furnace 36 wherein the gooseneck 38 is partially submerged into the molten metal 10. A reciprocating plunger 22 is used to draw molten metal 10 into the cavity 39 in the gooseneck 38 and to inject the molten metal 10 from the cavity 39 through the nozzle 40 into the die cavity 28 defined by the movable die 30 and the fixed die 32. A sprue-spreader 16 is conventionally used to guide the mold filling and to freeze the metal locally so that molten metal in the mold cavity is not sucked back into the gooseneck cavity 39. This invention teaches to use an ultrasonic vibrator to replace the sprue-spreader 16 or to couple ultrasonic vibration to the sprue-spreader 16. The tip of the ultrasonic radiator faces the flow of the mold filling so that the molten metal flows pass it. Since high-intensity ultrasonic vibration is capable of breaking up dendrites in semi-solid slurry, mold filling is not likely to be choked by the formation of dendrites in the molten metal 10 in the channel 42.

[0037] The benefit of using an ultrasonic vibrator to replace the sprue-spreader 16 in FIG. 4 is that the hot chamber die casting process can be used for operation at temperatures in the molten metal that are slightly higher than the liquidus temperature of the alloy. Dendrite formation in the nozzle 40 and in the channel 42 will be less of an issue. Lower temperatures in the molten is beneficial in extending die tooling life and in improving the internal integrity of the castings. Furthermore, thermal control such as forced cooling can be used on the nozzle and the die materials near the channel 42 so that a certain small fraction of solid in the form of dendrites can be allowed to form from the molten metal. The combination of thermal management on the molten metal, the nozzle, and the die materials near the flow channel 42 may open opportunities of using the hot chamber die casting process for semi-solid material processing.

[0038] 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.

REFERENCES

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