SYSTEM AND METHOD TO STABILIZE TRANSITION METAL PRECIPITATES IN CAST ALUMINUM ALLOYS DURING PRIMARY SOLIDIFICATION

Abstract

A system for casting an aluminum alloy includes a first chamber for containing a first melt at a first temperature, a second chamber for containing second melt at a second temperature that is lower than the first temperature, a mixing chamber in communication with the first chamber and the second chamber for simultaneously receiving and mixing the first melt from the first chamber with the second melt from the second chamber, and a mold chamber in communication with the mixing chamber and for receiving the mixed melt.

Claims

1. A system for casting an aluminum alloy comprising: a first chamber for containing a first melt at a first temperature; a second chamber for containing second melt at a second temperature that is lower than the first temperature; a mixing chamber in communication with the first chamber and the second chamber for simultaneously receiving and mixing the first melt from the first chamber with the second melt from the second chamber; and a mold chamber in communication with the mixing chamber and for receiving the mixed melt.

2. The system of claim 1, wherein the first melt comprises Aluminum and at least one peritectic transition metal element.

3. The system of claim 1, wherein the first melt comprises one of Zirconium, Scandium, Cobalt, Chromium, Niobium, Tantalum, Titanium, Vanadium, Tungsten, Molybdenum, Hafnium and Boron.

4. The system of claim 1, wherein the second melt has a composition that includes a higher percentage of Silicon than the first melt.

5. The system of claim 1, wherein the second melt has a composition that includes a higher percentage of Copper than the first melt.

6. The system of claim 1, wherein the second melt has a composition that includes a higher percentage of Magnesium than the first melt.

7. The system of claim 1, wherein the first temperature is higher than the liquidus temperature of an aluminum precipitate in the first melt and the second temperature is lower than the liquidus temperature of the aluminum precipitate in the first melt and above the liquidus temperature of the mixed melt.

8. The system of claim 1, wherein the first temperature is higher than the liquidus temperature of an aluminum precipitate in the first melt and the second temperature is lower than the liquidus temperature of the aluminum precipitate in the first melt and below the liquidus temperature of the mixed melt.

9. The system of claim 7, wherein the aluminum precipitate comprises at least one of an Aluminum-Vanadium precipitate, an Aluminum-Zirconium precipitate, an Aluminum-Titanium precipitate, an Aluminum-Scandium precipitate, an Aluminum-Cobalt precipitate, an Aluminum-Chromium precipitate, an Aluminum-Niobium precipitate, and Aluminum-Tantalum precipitate, and Aluminum-Tungsten precipitate, an Aluminum-Molybdenum precipitate, an Aluminum-Hafnium precipitate, and an Aluminum-Boron precipitate.

10. A method of casting an aluminum alloy, comprising: providing a first melt at a first temperature in a first chamber; providing a second melt at a second temperature that is lower than the first temperature in a second chamber; mixing the first melt and the second melt in a mixing chamber to form a mixed melt; flowing the mixed melt into a mold chamber; and solidifying the mixed melt in the mold chamber.

11. The method of claim 10, wherein the first melt comprises Aluminum and at least one peritectic transition metal element.

12. The method of claim 10, wherein the first melt comprises one of Zirconium, Scandium, Cobalt, Chromium, Niobium, Tantalum, Titanium, Vanadium, Tungsten, Molybdenum, Hafnium and Boron.

13. The method of claim 10, wherein the second melt has a composition that includes a higher percentage of Silicon than the first melt.

14. The method of claim 10, wherein the second melt has a composition that includes a higher percentage of Copper than the first melt.

15. The method of claim 10, wherein the second melt has a composition that includes a higher percentage of Magnesium than the first melt.

16. The method of claim 10, wherein the first temperature is higher than the liquidus temperature of an aluminum precipitate in the first melt and the second temperature is lower than the liquidus temperature of the aluminum precipitate in the first melt and above the liquidus temperature of the mixed melt.

17. The method of claim 10, wherein the first temperature is higher than the liquidus temperature of an aluminum precipitate in the first melt and the second temperature is lower than the liquidus temperature of the aluminum precipitate in the first melt and below the liquidus temperature of the mixed melt.

18. The method of claim 16, wherein the aluminum precipitate comprises at least one of an Aluminum-Vanadium precipitate, an Aluminum-Zirconium precipitate, an Aluminum-Titanium precipitate, an Aluminum-Scandium precipitate, an Aluminum-Cobalt precipitate, an Aluminum-Chromium precipitate, an Aluminum-Niobium precipitate, and Aluminum-Tantalum precipitate, and Aluminum-Tungsten precipitate, an Aluminum-Molybdenum precipitate, an Aluminum-Hafnium precipitate, and an Aluminum-Boron precipitate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0018] FIG. 1 is a schematic representation of an exemplary casting system in accordance with the present invention;

[0019] FIG. 2 is a phase diagram for Aluminum-Zirconium;

[0020] FIG. 3 is a phase diagram for Aluminum-Titanium;

[0021] FIG. 4 is a phase diagram for Aluminum-Silicon;

[0022] FIG. 5A a phase diagram for an Aluminum-Zirconium alloy with a small amount of Silicon as an impurity; and

[0023] FIG. 5B a phase diagram for an Aluminum-Zirconium alloy with about ten percent Silicon for castability, and about 1.5% Copper and 0.4% Magnesium for hardenability.

DETAILED DESCRIPTION

[0024] FIG. 1 is a schematic representation of an exemplary casting system 100 in accordance with the present invention. The casting system 100 includes a first chamber 102 and a second chamber 104. The first chamber 102 contains a first melt at a first temperature and the second chamber 104 contains a second melt at a second temperature. The first temperature is higher than the second temperature. The system 100 further includes a mixing chamber 106 in communication with each of the first chamber 102 and the second chamber 104. The mixing chamber 106 is in further communication with a mold 108.

[0025] In an exemplary method, the first melt and the second melt may be simultaneously introduced into the mixing chamber 106 whereby heat will be rapidly exchanged between the first and second melts during the mixing which initiates rapid nucleation of dispersoids of a stable aluminide precipitate from components in the first melt. In this manner, the mixing of the lower temperature second melt with the first melt performs a liquid quench in which high temperature stable precipitates are formed in the mixed melt. The mixed melt, which includes the high temperature precipitate, then flows from the mixing chamber into the mold where solidification of the mixed melt occurs. A quick transition from the mixing chamber into the mold minimizes the agglomeration of the high temperature precipitates (or dispersoids). Further, the resultant low diffusion of transition metal atoms minimizes transformation of dispersoids to more complex intermetallic phases during the mixed melt solidification in the mold and inhibits dissolution during any subsequent heat treatment or during temperatures experienced by the casting during use. This results in a casting that has stable high temperature properties.

[0026] In an exemplary embodiment, the first melt includes Aluminum and one of a peritectic transition metal element and is held within the first chamber 102 at a temperature which is above the liquidus temperature of the first melt and the second melt includes an Aluminum alloy and is held within the second chamber 104 at a temperature which is below the liquidus temperature of the first melt and above the liquidus temperature of the mixed melt.

[0027] The second melt may also include Silicon which is a known poison to the high temperature precipitate forming reaction, but may be provided within the second melt to improve castability of the mixed melt during the casting process. In this manner, the second melt, containing the higher Silicon composition is separately prepared. The second melt may also contain age-hardening elements such as, for example, Copper and Magnesium.

[0028] In this manner, the composition, temperature, and volume of the two precursor melts may be tailored to optimize aluminide volume fraction and dispersion to provide optimum properties. The lower melting point eutectic second melt being held at a lower temperature than the first melt causes a rapid reduction in the temperature of the first melt during the mixing phase which initiates nucleation of high temperature precipitate dispersoids in the mixed melt just prior to introduction into the mold and subsequent imminent solidification. Thus, the inventive process provides two mutually-exclusive solidification reactions that provide highly grain-refined primary transition metal aluminides in the resultant casting.

TABLE-US-00001 TABLE 1 Initial Melt 1 Initial Melt 2 Target Alloy weight 25 75 100 Si 0.1 12 9.025 Fe 0.1 0.5 0.4 Cu 0.1 3.5 2.65 Mg 0.1 0.5 0.4 Ti 0.5 0.2 0.275 Mn 0.3 0.3 0.3 Zn 0.1 0.3 0.25 V 0.75 0.01 0.195 Zr 1 0.01 0.2575 Temperature 1000 600 700

[0029] Table 1 illustrates an exemplary set of melt compositions and temperatures that when mixed together just before introduction into the mold cavity result in a target alloy having improved characteristics. The first melt is held at a temperature of 1000 degrees Celsius in the first chamber 102 and the second melt is held at a temperature of 600 degrees Celsius in the second chamber 104. The first melt contains transition metal elements, such as, for example Zirconium and Vanadium, that usually solidify through a peritectic reaction during the mixing with the second melt in the mixing chamber 106. Due to the nature of the peritectic reaction, only a very small amount of dispersoids may form during solidification and because of the high temperature nature of the phases that do form, the dispersoids phases cannot be subsequently formed using a solid state heat treatment process. The second melt, by contrast, contains a much higher concentration of Silicon, Copper, and Magnesium than the first melt. As explained above, those elements improve the castability of the mixed melt and provides precipitation strengthening benefits.

[0030] While FIG. 1 illustrates a mixing chamber 106 that is external to the casting mold 108, the mixing chamber 106 may be in other locations, such as, for example a gating chamber inside the mold, without limitation. In general, the mixing should occur prior to introducing the mixed melt to the casting chamber such that the melt is homogeneous within the casting mold to provide consistent qualities and characteristics throughout the entire casting. Inconsistency or variance in characteristics in the qualities of the casting is one of the problems which this invention solves.

[0031] Referring now to FIG. 2, when the high temperature first melt is mixed with the lower temperature second melt in the mixing chamber 106, the first melt is quenched from a fully liquid state into a two phase liquid and dispersoid region of the phase diagram 200. The high temperature first melt starts initially at point A in the phase diagram. Point A corresponds to a fully liquid melt. When the first melt is mixed with the second melt, the temperature is reduced which causes quenching of the first melt as indicated by the transition from point A in the phase diagram 200 to point B. Once the temperature crosses the curved line just below point A, some solid ZrAl3 will start to form. This liquid-to-liquid quench causes the immediate precipitation of the high temperature phase ZrAl3 as indicated by the line at C. The number of dispersoids is maximized by the high driving force that is generated by the temperature change, thereby minimizing the size of the dispersoids. Dispersoids are then uniformly distributed throughout the mixed melt by the mixing occurring in the mixing chamber 106 prior to introduction into the mold and prior to the onset of solidification.

[0032] The subsequent immediate solidification of the casting ensues rapidly, which precludes any appreciable growth of the dispersoids or their transformation into more complex intermetallic phases. In this manner, the resultant dispersoid phases provide excellent nuclei for the primary aluminum solidification which aids in grain refinement of the casting.

[0033] FIG. 3 provides a phase diagram 300 of another exemplary high temperature melt that may form high temperature phases in a similar peritectic reaction. In conventional casting alloys, the pouring temperature must be minimized because the Hydrogen solubility increases with temperature. A higher Hydrogen content increases gas porosity in the resultant solidified casting. To put even one percent of Titanium into an aluminum alloy, the temperature of the melt must be over about 900 degrees Celsius as indicated the phase diagram of FIG. 3. In contrast, conventional alloys are cast at temperatures which exceed no more than about 720 degrees Celsius. Thus, this invention enables the possibility to provide many other high temperature phases, in addition to those which are described herein, without limitation, which were not previously possible.

[0034] The higher temperature melt is held at a temperature which is just above the liquidus of that melt. The composition of the melt may essentially include Aluminum along with any number of other peritectic transition metal elements, such as, for example, Zirconium, Scandium, Cobalt, Chromium, Niobium, Tantalum, Titanium, Vanadium, Tungsten, Molybdenum, and the like, without limitations. The total of all the elements in the alloy may be limited by the resultant liquidus temperature. In exemplary embodiments, the melt may be held below a temperature of about one thousand degrees Celsius, but may be heated to twelve hundred degrees Celsius or higher for short periods of time such as may be experience if the melt is produced using a melt-on-demand system which may only melt the amount of material needed for each cast. In such a system, the temperature is likely only to exceed a temperature of one thousand degrees Celsius for a short period of time.

[0035] The lower temperature melt may be held at a temperature that is above the liquidus temperature of that melt. The composition of that melt may be controlled to minimize the liquidus temperature to provide a near-eutectic Silicon composition of between about 10-12% Silicon with the mixed melt having a Silicon composition of between about 6-10% Silicon. Alloying elements in the lower temperature melt may further include elements which improve the hardenability such as, for example, Copper and Magnesium, but may also include other elements such as, for example, Silver, Zinc, Manganese and the like. In an exemplary embodiment, the lower temperature melt may include a mixed alloy composition of between about 0.5-5.5% Copper, between about 0.1-0.6% Magnesium, between about 0.1-3.0% Zinc, and/or between about 0.1-0.6% Manganese.

[0036] The mixed melt results in a temperature above the liquidus temperature of the mixed melt composition. The composition, temperature and volume may be determined by rules of mixtures considering the composition of the high temperature melt and the low temperature melt. Following mixing, casting may proceed immediately after to minimize agglomeration of the high temperature dispersoids. The low diffusion of transition metal atoms minimizes transformation of the high temperature dispersoids to more complex intermetallic phases during solidification and inhibits dissolution at heat treatment temperature and at service temperatures to produce stable elevated-temperature properties for the lifetime of the casting.

[0037] FIG. 5A illustrates a phase diagram 500 for an Aluminum-Zirconium alloy with a small amount of Silicon as an impurity. FIG. 5B illustrates a phase diagram 502 for an Aluminum-Zirconium alloy with about ten percent Silicon for castability, and about 1.5% Copper and 0.4% Magnesium for hardenability. Comparing the two phase diagrams 500 and 502 illustrates that the additions of Silicon, Copper, and Magnesium may prevent high temperature ZrAl3 from forming at room temperature. The present invention obviates this problem by preventing the equilibrium phase distribution by controlling the kinetics of formation of dispersoid phases by quenching a high temperature melt alloy, which the phase diagram 500 illustrates, to form dispersoids during the mixing of the two alloys and then casting the mixed melt to solidify the mixed melt to form the remaining phases that are necessary for castability and subsequent heat treatment to produce precipitation hardening. The high temperature precipitates that are formed during the mixing of the two melts do not dissolve during subsequent solidification because the diffusion rates for Zirconium (and other transition metals) are very low at these temperature ranges. This accounts for the inability to form the dispersoids phases by conventional methods, such as by, for example, heat treatment.

[0038] FIG. 4 illustrates a phase diagram 400 for Aluminum-Silicon. Conventional casting alloys generally contain between about six to ten percent of Silicon. Thus, the liquidus temperature is about 620-640 degrees Celsius, which allows for pouring at about 720 degrees Celsius (i.e. a 100 degree Celsius superheat). In an exemplary aspect, the lower temperature melt contains a higher amount of Silicon than these conventional alloys, which means the melt has an even lower liquidus, of about 600 degrees Celsius. Mixing this lower temperature, high Silicon melt with the high temperature phase melt will raise the temperature of the high Silicon melt while lowering the amount of Silicon in the mixed melt, which raises the liquidus temperature of the mixed melt over that of the initial high Silicon melt. This maintains the temperature of the mixed melt above the required superheat at about 100 degrees Celsius above the liquidus temperature of the mixed melt. In this manner, the lower holding temperature of the low temperature, high Silicon melt reduces the Hydrogen content and results in less gas porosity in the final casting.

[0039] The composition, temperature and volume of the two precursor melts may be tailored to optimize aluminide volume faction and dispersion to obtain desired properties in the resultant casting.

[0040] Table 2 below illustrates exemplary ranges for a preferred set of melts:

TABLE-US-00002 TABLE 2 Minimum Maximum Combined Combined Broad At Least 1 of the following Range Range Dispersoid Zr, Sc, Co, Cr, Nb, Ta, 0.1 5.5 Formers Ti, V, W, Mo, B Silicon Si 4.5 12.5 Precipitation Cu, Mg, Zn, Mn 0.2 7 Strengtheners

[0041] Table 3 below illustrates one preferred embodiment of melts:

TABLE-US-00003 TABLE 3 Minimum Maximum Combined Combined Preferred 1 At Least 1 of the following Range Range Dispersoid Zr, Sc, Co, Cr, Nb, Ta, 0.3 0.6 Formers Ti, V, W, Mo, B Silicon Si 4.5 12.5 Precipitation Cu, Mg, Zn, Mn 0.2 0.7 Strengtheners

[0042] Table 4 illustrates another preferred embodiment for melts:

TABLE-US-00004 TABLE 4 Minimum Maximum Combined Combined Preferred 2 At Least 1 of the following Range Range Dispersoid Zr, Sc, Co, Cr, Nb, Ta, 0.3 1.3 Formers Ti, V, W, Mo, B Silicon Si 4.5 12.5 Precipitation Cu, Mg, Zn, Mn 3 4 Strengtheners

[0043] Table 5 illustrates one special exemplary set of melt conditions:

TABLE-US-00005 TABLE 5 Minimum Maximum Combined Combined Specialty At Least 1 of the following Range Range Dispersoid Zr, Sc, Co, Cr, Nb, Ta, 0 1.3 Formers Ti, V, W, Mo, B Silicon Si 11 22 Precipitation Cu, Mg, Zn, Mn 0.2 7 Strengtheners

[0044] Additional exemplary melt compositions and resulting mixed melt compositions are illustrated by Tables 6 through 8:

TABLE-US-00006 TABLE 6 Initial Melt 1 Initial Melt 2 Target Alloy weight 20 80 100.00 Si 0.1 12 9.62 Fe 0.1 0.5 0.42 Cu 0.1 3.5 2.82 Mg 0.1 0.5 0.42 Ti 0.3 0.1 0.14 Mn 0.3 0.1 0.14 Zn 0.1 0.5 0.42 V 0.5 0.01 0.108 Zr 0.5 0.01 0.108 Temperature 1200 620 736

TABLE-US-00007 TABLE 7 Initial Melt 1 Initial Melt 2 Target Alloy weight 50 50 100 Si 0.1 12 6.05 Fe 0.1 0.5 0.3 Cu 0.1 3.5 1.8 Mg 0.1 0.5 0.3 Ti 0.3 0.1 0.2 Mn 0.3 0.1 0.2 Zn 0.1 0.5 0.3 V 0.5 0.01 0.255 Zr 0.5 0.01 0.255 Temperature 800 620 710

TABLE-US-00008 TABLE 8 Initial Melt 1 Initial Melt 2 Target Alloy weight 10 90 100.00 Si 0.1 12 10.81 Fe 0.1 0.5 0.46 Cu 0.1 3.5 3.16 Mg 0.1 0.5 0.46 Ti 0.3 0.1 0.12 Mn 0.3 0.1 0.12 Zn 0.1 0.5 0.46 V 0.7 0.02 0.088 Zr 0.7 0.02 0.088 Temperature 1100 640 686

[0045] In yet another exemplary embodiment, primary Silicon may be refined to improve, for example, wear resistant properties of the resultant casting. Referring back now to FIG. 4, the higher temperature melt may include a high Silicon content Aluminum alloy. At about 900 degrees Celsius, an alloy with about 25% Silicon is fully molten as indicated at point A in the phase diagram 400. A lower temperature melt contains eutectic or hypoeutectic composition as indicated by point B in the phase diagram 400. The combined alloy (or mixed melt) is indicated by point C which forms an alloy in the range of conventional hypereutectic alloys or even higher Silicon than has previously been possible because of the kinetic nucleation of a profuse number of primary Silicon crystals. Casting from the combined alloy C maintains the refined structure of the Silicon crystals such that the primary mode of subsequent solidification is eutectic solidification with some growth of the primary Silicon crystals. This leaves open the possibility of using chemical modifiers in the lower temperature melt such that the assolidified eutectic structure remains modified achieving the heretofore unattainable microstructure exhibiting both primary Silicon and modified eutectic Silicon.

[0046] In an exemplary embodiment, the present invention may produce components for an internal combustion engine, such as, for example, and Aluminum alloy cylinder head that has improved high temperature properties in comparison to that which was previously achievable. In this manner, the cylinder head is able to withstand higher temperatures, which improves the efficiency of the combustion process, which may provide improved fuel economy and/or improved performance of a vehicle incorporating an internal combustion engine with an aluminum cylinder head incorporating the features which are obtainable through the use of the present invention.

[0047] This description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims.