Patent Application
20220048106
None
The present invention relates to industrial metal forming, more specifically, an apparatus and process for forming metal components from non-dendritic, semi-solid metal slurries in a shot chamber used in the die casting process.
Die casting, also termed as high pressure die casting (HPDC), is a widely-used metal casting process that is characterized by forcing molten metal under high pressure into a die cavity. The metal, commonly aluminum, magnesium, zinc, and their alloys, and sometimes copper, titanium, and their alloys, is transported into a chamber containing a cylindrical channel connected to the mold cavity and then is injected with a reciprocating cylinder referred to as a plunger into the die cavity where it solidifies and forms a solid component. Die casting is generally considered to be a cost-effective process capable of producing precision (net-shaped) products at high production rates. Currently, die casting processes are used to produce over 70% of the annual tonnage of all aluminum castings in the United States.
There are two kinds of die casting processes: hot-chamber and cold-chamber die casting. The hot-chamber die casting process uses “gooseneck” as the chamber containing a cylindrical channel connected to the die cavity. Part of the gooseneck is submerged into the molten metal in a pot or a holding furnace so the chamber is hot. A reciprocating plunger in the cylindrical channel draws the molten metal in and injects it into the mold cavity through a nozzle. A sprue spreader, located near the exit of the nozzle, prevents molten metal in the die cavity from being sucked back into the gooseneck. The injection system for hot-chamber die casting consists of a gooseneck, plunger, nozzle, and arguably a sprue spreader.
The cold-chamber process involves pouring hot metal into a cold shot chamber or shot sleeve containing a cylindrical channel and injecting it using a reciprocating plunger into the die cavity. Sometime, a water-cooled shot plate is placed at the exit of the shot sleeve to cool down the hot shot biscuit. The injection system for cold-chamber die casting consists of a plunger or ram, shot sleeve, and shot plate.
The casting equipment and the metal dies represent large capital costs. The tooling for the HPDC process is fairly expensive, thus, increasing tooling life leads to reduced costs for this process. Tooling damage is usually associated with die soldering and heat checking. The tendencies of die soldering and heat checking increase with increasing temperatures so that tooling life is strongly affected by the pouring temperature of the molten alloy [1]. The lower the pouring temperature, the longer the tooling life. Unfortunately, the pouring temperature has to be significantly higher than the liquidus of the alloy. This is because after being poured into the steel shot sleeve, the molten cools quickly to below its liquidus to form primary tree-like crystals called dendrites from the liquid within the massive shot sleeve. Recently, the inventor of this present invention has found that slurry containing these tree-like dendrites can choke the mold filling near the in-gate in the runner/gating system before the dies are completely filled [2]. In adequate fluidity of the alloy leads to the formation of casting defects such as misruns, cold shuts, folds, flow marks, and etc. The only way to lower the pouring temperature of the molten metal is to produce a slurry containing non-dendritic crystals. Semi-solid materials having non-dendritic or spherical primary particles are known to be castable at temperatures much lower than the liquidus using the HPDC process [3]. The fraction solid in the semi-solid material during mold filling is in the range of about 0.3 to about 0.5 with the remainder being the liquid phase.
Methods for producing semisolid materials are described in U.S. Pat. No. 3,948,650 to Flemings et al. and U.S. Pat. No. 3,954,455 to Flemings et al. As disclosed by these patents, a metal alloy in the semi-solid state can be vigorously agitated to break up dendrites into spherical particles. Slurries made in such a way can then feed into the shot sleeve of a die casting machine for making a casting. The benefits of semisolid materials having non-dendritic or spherical primary particles include improved mold filling, lower mold erosion, no die soldering, and thus increased die life and shot tooling life. Other advantages of the semisolid process include less shrinkage during solidification, less porosity in the casting, and more uniform mechanical property. Because of these advantages, several techniques have been developed to produce semisolid materials by applying agitation during solidification, including mechanical stirring, electromagnetic stiffing, and ultrasonic vibrations. These techniques utilize different media or means to achieve agitation at the semi-solid state of an alloy before transferring the semi-solid slurry into the shot chamber of a die casting machine for making components [U.S. Pat Nos. 5,114,998, 5,865,240, 5,901,78, 6,645,323, and European Pat. Appl. No. 96,108,499]. Methods capable of a direct formation of non-dendritic crystals in the shot chamber of a die casting machine are certainly more cost-effective and easier to utilize.
When molten metal is poured into the shot chamber, the molten metal cools down rapidly due to the massive shot chamber so dendritic solidification of the molten metal starts immediately. U.S. Pat. No. 5,579,825 to Shibata et al. and U.S. Pat. No. 10,448,103 by Hong et al. disclose a method of utilizing coils outside the shot chamber to produce electromagnetic stiffing in the molten metal to break up dendrites into fragments and to enhance ripening of the fragments into non-dendritic particles. However, one issue is that the external electromagnetic field produces eddy currents both in the molten metal as well as in the steel of the massive shot sleeve. The eddy currents heat up the molten metal and the shot tool steel, leading to decreased tooling life.
Therefore, there is a need to develop an improved method that is capable of using the rapid cooling capability of the shot chamber to produce solid non-dendritic crystals from the molten metal that is poured into the shot chamber. The semi-solid slurry produced in-situ in the shot chamber is then injected into the mold cavity for producing castings of high internal integrity at high production rates and low costs.
The present invention relates to a method and apparatus for producing a slurry of an alloy containing non-dendritic primary phase solid particles in a shot chamber during die casting for making solid components. In this method, a molten metallic alloy is prepared in a melt holding vessel. The molten alloy is then transferred into the shot chamber of a die casting machine. High-intensity ultrasonic vibration is coupled directly into the molten metal in the shot chamber, breaking up dendrites that form in the shot chamber into non-dendritic solid particles and forming a semi-solid slurry to be injected into the mold cavity for the production of solid components.
In another embodiment, the present invention relates to a method and apparatus for producing a slurry of an alloy containing non-dendritic primary phase solid particles in a shot chamber, wherein the first vessel containing the molten metal is a holding furnace and the second vessel is the shot chamber used for the die casting process. The method involves preparing molten metal using the first vessel, cooling the molten metal rapidly in the second vessel, and forcing the molten metal to flow using the plunger. High-intensity ultrasonic vibration is applied directly to the molten metal through a plurality of sonotrodes embedded in the shot sleeve. The combined action of rapid cooling by the second vessel and vigorous stiffing by the plunger and sonotrodes on the molten metal turns dendrites into non-dendritic fragments. As a result, the molten metal can be poured into the second vessel at much lower temperatures than those during a conventional HPDC process which is beneficial in improving the castability of the alloy, extending die and shot tooling life, and reducing porosity in the final casting components.
In yet another embodiment, the invention relates to a method and apparatus for producing a slurry of an alloy containing non-dendritic primary phase solid particles in a shot chamber wherein the water-cooled plunger and/or the shot plate are coupled with high-intensity ultrasonic vibration. High-intensity ultrasonic vibration is transmitted from the tip of a sonotrode embedded in the plunger and/or the shot plate to the semi-solid slurry formed in the shot chamber as the plunger pushes the slurry towards the mold cavity. Vigorous convection in the slurry pushed by the plunger coupled with ultrasonic vibration breaks up dendrites and smoothes out the dendritic fragments into spherical solid particles, which is beneficial in improving the castability of the alloy, extending die and shot tooling life, and reducing porosity in the final casting components.
In still another embodiment, the invention relates to a method and apparatus for producing a slurry of an alloy containing non-dendritic primary phase solid particles in the gooseneck and nozzle of a hot chamber die casting process where the sprue-spreader is coupled with high-intensity ultrasonic vibration. A partially solidified material is forced in the gooseneck to pass the ultrasound coupled sprue spreader which breaks up dendrites in the partially solidified material and forms a slurry containing non-dendritic fragments of dendrites.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings, specification, and claims.
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.
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.
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.
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 stiffing 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.
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.
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.
In the embodiment of this invention shown in
In the embodiment of this invention shown in
For the vertical cold chamber die casting process, or a vertical indirect squeeze casting process, the shot chamber 24 shown in
The benefit of using an ultrasonic vibrator to replace the sprue-spreader 16 in
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.
