Process for preparing molten metals for casting at a low to zero superheat temperature

Abstract

A process for preparing molten metals for casting at a low to zero superheat temperature involves the steps of placing a heat extracting probe into the melt and at the same time vigorous convection is applied to assure nearly uniform cooling of the melt. Then, the heat extraction probe is rapidly removed when a low or zero superheat temperature is reached. Finally, the rapidly cooled melt is quickly transferred to a mold for casting into parts or a shot sleeve for injection into a die cavity. The process may be carried out so as that small amounts of solid form in part of the melt. In this case, a key aspect of the invention is to carry out the process rapidly in order to maintain the particles in a fine, dispersed state that will not impede flow and will improve the quality of the metal parts produced. Cost of the metal parts produced is lowered due to longer die life and shorter cycle time.

Claims

1. A method for preparing molten metals for casting at low superheat temperatures, said method comprising: (a) having a melt of a metal or alloy that is initially at a superheat temperature above the liquidus temperature of the metal or alloy in a container; (b) decreasing the temperature of the metal or alloy at a rate of more than 10 C. per minute by placing at least one heat extracting probe into the melt to remove a controlled amount of heat and applying vigorous convection to the melt to assure nearly uniform cooling of the melt, wherein the vigorous convection is applied via bubbling an inert gas through the heat extracting probe through a multiplicity of gas outlets, thereby forming a semi-solid slurry of the metals, wherein the inert gas is provided at a flow rate of 2 to 10 liters per minute for less than 30 seconds; (c) removing the heat extracting probe from the cooled melt when the temperature of the metal or alloy is at a lower superheat temperature above the liquidus temperature and the fraction of solid nuclei formed in the slurry is about 3-5% by weight, in order to substantially stop further cooling, wherein the lower superheat temperature is maximally 5 C. above the liquidus temperature; and (d) transferring the cooled melt at a temperature above the liquidus temperature into a secondary container for casting in a time-regulated manner wherein the time period from the entry of the heat extracting probe into the melt to entry of the cooled melt into the secondary container is less than 60 seconds to ensure that the solid nuclei has a size of less than 70 microns in diameter for desired flow behavior.

2. The method of claim 1, wherein the container is in the form of a crucible or ladle, which is made of a material and is at a temperature such that the container extracts heat from the melt at a significantly lower rate than does the heat extracting probe.

3. The method of claim 1, wherein said metal or alloy includes a component selected from the group consisting of aluminum, silicon, and magnesium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of an apparatus in accordance with an embodiment of the invention.

(2) FIG. 2 is an optical micrograph of the rapidly cooled melt with near zero superheat temperature showing a small fraction of finely distributed solid nuclei in the matrix of the rapidly solidified melt.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(3) This present invention provides a process for preparing molten metals for casting at low to zero superheat temperature.

(4) By the phrase low to zero superheat temperature as used herein are meant that there is at least a part in the melt with the superheat temperature of less than about 5-10 degree Celsius, preferably less than 5 degree Celsius. In some metals and alloys, the superheat temperature may be essentially zero, so that the temperature of the melt in at least one part is at or slightly below the liquidus.

(5) The process of this invention comprises of four steps illustrated in FIG. 1.

(6) Step 1 starts by placing a heat extracting probe 1 into the melt 2 held inside a container 3 from which heat extraction is low. The melt is initially at a temperature higher than the liquidus temperature, preferably not more than 80 degree Celsius above the liquidus temperature.

(7) In step 2, vigorous convection is applied to the melt to assure nearly uniform cooling of the melt to a low superheat temperature. The convection may be done by various techniques such as injecting inert gas dispensed through the heat extracting probe and creating gas bubbles inside the melt, by vibration, by stirring, by rotation or by a combination thereof. Solid nuclei 4 are progressively formed in the melt.

(8) In Step 3, the heat extraction probe is rapidly removed from the rapidly cooled melt 5 when the desired melt temperature is reached, in order to substantially stop further cooling. The cooling rate of the melt during the probe immersion should be more than 10 degree Celsius per minute. In Step 4, the rapidly cooled melt 5 that has some parts with low to zero superheat temperature is then quickly transferred to a secondary container 6 such as a shot sleeve designed to inject the rapidly cooled melt into a die in die casting process 7 or a mold in gravity casting (not shown). The secondary container 6 or the die or mold for casting purpose needs to be at a lower temperature than that of the melt to stabilize and allow growth of the created solid nuclei.

(9) The time period from entry of the heat extracting probe into the melt to entry of the metal into the mold should be less than about 60 seconds to ensure that the solid nuclei are fine in size for the desired flow behavior into the die cavity. A cleaning process may be added to ensure no solid sticking on the heat extracting probe after each process cycle.

(10) Shown in FIG. 2 is the microstructure of a rapidly cooled aluminum melt at a low superheat temperature. The optical micrograph shows a small fraction of bright particles uniformly dispersed in the matrix. These bright particles are the solid nuclei 4 created during the heat extracting probe immersion (Step 2 of FIG. 1). These solid nuclei 4 are very fine in size, in the order of less than 100 micron in diameter. To create a large number of these fine solid nuclei, it is necessary to create it in a short time. Therefore, the heat extracting probe immersion time should be less than 30 seconds, preferably less than 15 seconds.

(11) The following two examples illustrate two embodiments of the present invention. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein.

Example 1

High-Pressure Die Casting of an Aluminum Alloy

(12) The following is a description and the benefits of casting molten metals at a low superheat temperature and with a small fraction of fine solid nuclei in the melt in a high-pressure die casting process of an AlMg alloy part.

(13) In this example, the AlMg alloy has the liquidus temperature of about 640 C. In the current commercial liquid casting process, the pouring temperature of the alloy into the shot sleeve of a high-pressure die casting machine is about 740 C. (the superheat temperature of about 100 C.).

(14) By applying the present invention to the current commercial production process, the key motivations are to improve the productivity, reduce production cost, and extend the die life. In this example, the AlMg alloy is treated with a heat extraction probe in the ladle at the temperature of about 660 C. for 2 seconds. The vigorous convection is achieved by flowing fine inert gas bubbles through a heat extracting probe such as a porous probe at the flow rate of 2-10 liter/minute. For each cycle of the probe immersion into the molten metal, the temperature of the probe is controlled to be nearly the same in the range of 50 C. to 150 C. After the treatment, the melt temperature is reduced to about 645 C., which is about 5 C. above the liquidus temperature (the superheat temperature of about 5 C.) with a fraction of solid estimated to be under about 3-5% by weight. The melt is then quickly transferred into the shot sleeve in less than 10 seconds and then injected into the mold in less than 3 seconds. The total time from entry of the probe into the melt to entry of the metal into the mold is about 15 seconds. Results of the mass production process with the present invention show several expected benefits, including reduction in usage of natural gas for melting aluminum by about 25%, reduction in die holding time by 40%, reduction in die spray time by 40%, and die life extension by more than 2 times, and reduction of casting reject from 30% to 5%.

Example 2

Gravity Die Casting of an Aluminum Alloy

(15) The following is a description and the benefits of casting molten metals at a low superheat temperature and with a small fraction of fine solid nuclei in the melt in a gravity die casting process of an AlSiMg alloy component.

(16) In this example, an AlSiMg alloy is cast into a metal die. This alloy has the liquidus temperature of about 613 C. The die is preheated to about 400 C. before each casting cycle. The conventional liquid casting process pours the molten metal alloy at about 680 C. (the superheat temperature of about 67 C.). With the present invention, the casting temperature is lowered to about 614 C., about 1 C. above the liquidus temperature (the superheat temperature of about 1 C.). In this example, the melt is treated with a heat extraction probe in the ladle at the temperature of about 630 C. for about 5 seconds. The vigorous convection is achieved by flowing fine inert gas bubbles through a heat extracting probe such as a porous probe at the flow rate of 2-10 liter/minute. For each cycle of the probe immersion into the molten metal, the temperature of the probe is controlled to be nearly the same in the range of 50 C. to 150 C. The melt is then quickly transferred and poured into the mold in less than 12 seconds. The total time from entry of the probe into the melt to entry of the metal into the mold is about 17 seconds. Results show that the present invention yields better mechanical properties. The liquid casting process with the superheat temperature of 67 C. gives the ultimate tensile strength of 287 MPa and the elongation of 10.5%. The casting process with the present invention gives the ultimate tensile strength of 289 MPa and the elongation of 11.2%. The productivity of the casting process using the present invention is also higher. This is because the freezing time of the melt in the mold is reduced from 133 seconds for the conventional liquid casting with the high superheat temperature of 67 C. to 46 seconds for this invention with near zero superheat temperature. This shows that the die opening time in the production process can be reduced by about 65%.

(17) Another key benefit of this present invention is the saving of the melting energy. With the present invention, the holding temperature of the furnace can be reduced by about 100 C. This reduction can significantly save the energy and extend the furnace life.

(18) The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those stilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.