Congruent melting salt alloys for use as salt cores in high pressure die casting

Abstract

Congruent melting salt alloys for use as salt cores in high pressure die casting of metallic alloys for the production of complex metallic parts. Congruent melting salt alloys provide mechanical advantages in the high pressure die casting of both aluminum and magnesium alloys. Salt cores may be used to make complex high pressure die casting parts such as internal passages in a closed deck engine block. The congruently melting salt alloy is cast into a shape of a desired salt core. The cast salt core of the congruently melting slat alloy is placed into a high pressure die casting mold for a complex object. Molten metal is introduced into the high pressure die casting mold to form the complex part. The congruently melting salt alloys may be readily removed from the final cast part through flushing with a solvent, such as water, or through other processes known in the art.

Claims

1. A high pressure die casting salt core for use in a high pressure die casting process, the salt core comprising a congruently melting salt alloy system selected from one of the following congruently melting salt alloys: KBrKCl at 0.30 to 0.40 mole fraction KCl; KBrKI at 0.62 to 0.72 mole fraction KI; KBrNaBr at 0.44 to 0.54 mole fraction NaBr; KBrRbBr at 0.70 to 0.80 mole fraction RbBr, CaCl.sub.2SrCl.sub.2 at 0.37 to 0.47 mole fraction SrCl.sub.2; CsBrCsI at 0.42 to 0.52 mole fraction CsI; CsClKCl at 0.32 to 0.42 mole fraction KCl; CsNO.sub.3RbNO.sub.3 at 0.77 to 0.87 mole fraction CsNO.sub.3; K.sub.2CO.sub.3Na.sub.2CO.sub.3 at 0.54 to 0.64 mole fraction Na.sub.2CO.sub.3; K.sub.2SO.sub.4Na.sub.2SO.sub.4 at 0.69 to 0.79 mole fraction Na.sub.2SO.sub.4; KINaI at 0.53 to 0.63 mole fraction NaI; KINaCl at 0.45 to 0.55 mole fraction NaCl; KIRbI at 0.70 to 0.80 mole fraction RbI; LiBrLiCl at 0.32 to 0.42 mole fraction LiCl; LiClNaCl at 0.23 to 0.33 mole fraction NaCl; Na.sub.2CO.sub.3Na.sub.2SO.sub.4 at 0.31 to 0.41 mole fraction Na.sub.2SO.sub.4; KClNaCl at 0.45 to 0.55 mole fraction NaCl; NaBrNaCl at 0.20 to 0.30 mole fraction NaCl; KClKI at 0.55 to 0.65 mole fraction KI; KClNaBr at 0.45 to 0.55 mole fraction KCl or KBrNaCl at 0.45 to 0.55 mole fraction KBr.

2. The salt core of claim 1, wherein the high pressure die casting process is an aluminum alloy high pressure die casting process and the congruently melting salt alloy system is one of the following alloys KBrKCl at 0.30 to 0.40 mole fraction KCl; KBrKI at 0.62 to 0.72 mole fraction KI; KBrNaBr at 0.44 to 0.54 mole fraction NaBr; KBrRbBr at 0.70 to 0.80 mole fraction RbBr; CaCl.sub.2SrCl.sub.2 at 0.37 to 0.47 mole fraction SrCl.sub.2; CsBrCsI at 0.42 to 0.52 mole fraction CsI; CsClKCl at 0.32 to 0.42 mole fraction KCl; K.sub.2CO.sub.3Na.sub.2CO.sub.3 at 0.54 to 0.64 mole fraction Na.sub.2CO.sub.3; K.sub.2SO.sub.4Na.sub.2SO.sub.4 at 0.69 to 0.79 mole fraction Na.sub.2SO.sub.4; LiBrLiCl at 0.32 to 0.42 mole fraction LiCl; LiClNaCl at 0.23 to 0.33 mole fraction NaCl; KINaI at 0.53 to 0.63 mole fraction NaI; KIRbI at 0.70 to 0.80 mole fraction RbI; Na.sub.2CO.sub.3Na.sub.2SO.sub.4 at 0.31 to 0.41 mole fraction Na.sub.2SO.sub.4; KClNaCl at 0.45 to 0.55 mole fraction NaCl; NaBrNaCl at 0.20 to 0.30 mole fraction NaCl; KClKI at 0.55 to 0.65 mole fraction KI; KClNaBr at 0.45 to 0.55 mole fraction KCl or KBrNaCl at 0.45 to 0.55 mole fraction KBr.

3. The salt core of claim 2, wherein the high pressure die casting process is an aluminum alloy high pressure die casting process and the congruently melting salt alloy system is one of the following alloys: KBrKCl at 0.30 to 0.40 mole fraction KCl; KBrKI at 0.62 to 0.72 mole fraction KI; KBrNaBr at 0.44 to 0.54 mole fraction NaBr; NaBrNaCl at 0.20 to 0.30 mole fraction NaCl; K.sub.2CO.sub.3-Na.sub.2CO.sub.3 at 0.54 to 0.64 mole fraction Na.sub.2CO.sub.3; KClNaCl at 0.45 to 0.55 mole fraction NaCl; KClNaBr at 0.45 to 0.55 mole fraction KCl; or KBrNaCl at 0.45 to 0.55 mole fraction KBr.

4. The salt core of claim 3, wherein the salt core is a fused salt core and forms internal passages in a cast part.

5. The salt core of claim 4, wherein the internal passages are in a closed deck engine block.

6. The salt core of claim 2, wherein the salt core alloy has a shrinkage value less than a shrinkage value for a eutectic salt mixture.

7. The salt core of claim 2, wherein the salt core alloy has a shrinkage value less than either of the two pure salts that comprise the alloy.

8. The salt core of claim 1, wherein the high pressure die casting process is a magnesium alloy high pressure die casting process and the congruently melting salt alloy system is one of the following alloys KBrKCl at 0.30 to 0.40 mole fraction KCl; KBrKI at 0.62 to 0.72 mole fraction KI; KBrNaBr at 0.44 to 0.54 mole fraction NaBr; KBrRbBr at 0.70 to 0.80 mole fraction RbBr; CaCl.sub.2SrCl.sub.2 at 0.37 to 0.47 mole fraction SrCl.sub.2; CsBrCsI at 0.42 to 0.52 mole fraction CsI; CsClKCl at 0.32 to 0.42 mole fraction KCl; K.sub.2CO.sub.3Na.sub.2CO.sub.3 at 0.54 to 0.64 mole fraction Na.sub.2CO.sub.3; K.sub.2SO.sub.4Na.sub.2SO.sub.4 at 0.69 to 0.79 mole fraction Na.sub.2SO.sub.4; LiBrLiCl at 0.32 to 0.42 mole fraction LiCl; LiClNaCl at 0.23 to 0.33 mole fraction NaCl; KINaI at 0.53 to 0.63 mole fraction NaI; KIRbI at 0.70 to 0.80 mole fraction RbI; Na.sub.2CO.sub.3Na.sub.2SO.sub.4 at 0.31 to 0.41 mole fraction Na.sub.2SO.sub.4; KClNaCl at 0.45 to 0.55 mole fraction NaCl; NaBrNaCl at 0.20 to 0.30 mole fraction NaCl; KClKI at 0.55 to 0.65 mole fraction KI; KClNaBr at 0.45 to 0.55 mole fraction KCl or KBrNaCl at 0.45 to 0.55 mole fraction KBr.

9. The salt core of claim 8, wherein the salt core is a fused salt core and forms internal passages in a cast part.

10. The salt core of claim 9, wherein the internal passages are in a closed deck engine block.

11. The salt core of claim 8, wherein parts cast from the salt core exhibit less cracking than parts cast from traditional salt cores.

12. The salt core of claim 8, wherein the salt core alloy has a shrinkage value less than a shrinkage value for a eutectic salt mixture.

13. The salt core of claim 8, wherein the salt core alloy has a shrinkage value less than either of the two pure salts that comprise the alloy.

14. The salt core of claim 1, wherein the salt core alloy has a shrinkage value less than a shrinkage value for a eutectic salt mixture.

15. The salt core of claim 1, wherein the salt core alloy has a shrinkage value less than either of the two pure salts that comprise the alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is the NaClNa.sub.2CO.sub.3 eutectic phase diagram with a eutectic temperature of 635 C. and demonstrating that the eutectic is a mechanical mixture of the pure salt compounds because there is no solubility of either pure salt into the other.

(2) FIG. 2 is a graph of temperature versus mole concentration for the KBrKCl congruently melting salt alloy system.

(3) FIG. 3 is a graph of temperature versus mole concentration for the KBrKI congruently melting salt alloy system.

(4) FIG. 4 is a graph of temperature versus mole concentration for the KBrNaBr congruently melting salt alloy system.

(5) FIG. 5 is a graph of temperature versus mole concentration for the KBrRbBr congruently melting salt alloy system.

(6) FIG. 6 is a graph of temperature versus mole concentration for the CaCl.sub.2SrCl.sub.2 congruently melting salt alloy system.

(7) FIG. 7 is a graph of temperature versus mole concentration for the CsBrCsI congruently melting salt alloy system.

(8) FIG. 8 is a graph of temperature versus mole concentration for the CsClKCl congruently melting salt alloy system.

(9) FIG. 9 is a graph of temperature versus mole concentration for the CsNO.sub.3RbNO.sub.3 congruently melting salt alloy system.

(10) FIG. 10 is a graph of temperature versus mole concentration for the K.sub.2CO.sub.3Na.sub.2CO.sub.3 congruently melting salt alloy system.

(11) FIG. 1I is a graph of temperature versus mole concentration for the K.sub.2SO.sub.4Na.sub.2SO.sub.4 congruently melting salt alloy system.

(12) FIG. 12 is a graph of temperature versus mole concentration for the KINaI congruently melting salt alloy system.

(13) FIG. 13 is a graph of temperature versus mole concentration for the KIRbI congruently melting salt alloy system.

(14) FIG. 14 is a graph of temperature versus mole concentration for the LiBrLiCl congruently melting salt alloy system.

(15) FIG. 15 is a graph of temperature versus mole concentration for the LiClNaCl congruently melting salt alloy system.

(16) FIG. 16 is a graph of temperature versus mole concentration for the Na.sub.2CO.sub.3Na.sub.2SO.sub.4 congruently melting salt alloy system.

(17) FIG. 17 is a graph of temperature versus mole concentration for the NaBrNaCl congruently melting salt alloy system.

(18) FIG. 18 is a graph of temperature versus mole concentration for the KClKI congruently melting salt alloy system.

(19) FIG. 19 is a photograph demonstrating the permanent metal mold used to create the samples for Example 1.

(20) FIG. 20 is a photograph demonstrating solidified salt cores provided with a congruently melting alloy in accordance with the present invention.

(21) FIG. 21 is a photograph demonstrating solidified salt cores poured with a traditional eutectic salt alloy NaClNa.sub.2CO.sub.3 and demonstrating the cracking problem prevalent due to shrinkage with traditional salt cores.

(22) FIG. 22 is a graph of temperature versus mole concentration for the NaClKCl congruently melting salt alloy system.

DETAILED DESCRIPTION

(23) The following table summarizes the identified congruent melting salt alloy systems, the melting point of systems' constituents and the congruent melting temperature and composition of the systems:

(24) TABLE-US-00001 TABLE 1 Mole Fraction of the congruent melting Melting Point alloy [congruent Melting Point System [1st member] melting temperature] [2nd member] KBrKCl 734 C. KBr 0.35 KCl [717 C.] 771 C. KCl KBrKI 734 C. KBr 0.67 KI [663 C.] 681 C. KI KBrNaBr 734 C. KBr 0.487 NaBr [643 C.] 747 C. NaBr KBrRbBr 734 C. RbBr 0.75 RbBr [689 C.] 694 C. RbBr CaCl.sub.2SrCl.sub.2 772 C. CaCl.sub.2 0.416 SrCl.sub.2 [662 C.] 874 C. SrCl.sub.2 CsBrCsI 635 C. CsBr 0.472 CsI [578 C.] 640 C. CsI CsClKCl 645 C. CsCl 0.375 KCl [616 C.] 771 C. KCl CsNO.sub.3RbNO.sub.3 410 C. 0.825 CsNO.sub.3 312 C. CsNO.sub.3 [290 C.] RbNO.sub.3 K.sub.2CO.sub.3Na.sub.2CO.sub.3 901 C. 0.59 Na.sub.2CO.sub.3 [709 C.] 858 C. K.sub.2CO.sub.3 Na.sub.2CO.sub.3 K.sub.2SO.sub.4Na.sub.2SO.sub.4 1069 C. 0.742 Na.sub.2SO.sub.4 884 C. K.sub.2SO.sub.4 [834 C.] Na.sub.2SO.sub.4 KINaI 681 C. KI 0.588 NaI [580 C.] 660 C. NaI KIRbI 681 C. KI 0.75 RbI [640 C.] 647 C. RbI LiBrLiCl 550 C. LiBr 0.375 LiCl [525 C.] 610 C. LiCl LiClNaCl 610 C. LiCl 0.28 NaCI [554 C.] 801 C. NaCl Na.sub.2CO.sub.3Na.sub.2SO.sub.4 858 C. 0.36 Na.sub.2SO.sub.4 884 C. Na.sub.2CO.sub.3 [826 C.] Na.sub.2SO.sub.4 KClNaCl 771 C. KCl 0.50 NaCl [660 C.] 801 C. NaCl NaBrNaCl 747 C. NaBr 0.25 NaCl [740 C.] 801 C. NaCl KClKI 771 C. KCl 0.601 KI [599 C.] 681 C. KI

(25) This application identifies congruent melting alloy salts that are more ductile than the pure salts because the crystal structure of congruent melting alloys can be considered as face centered cubic (FCC) closest packing of the larger chlorine anions, with the smaller sodium and potassium cations distributed only among the octahedral voids (with a coordination number of 6) in a random fashion that minimizes the strain energy created by the different sizes of the sodium and potassium cations, and not among the tetrahedral voids (with a coordination number of 4) which are too small for either sodium or potassium cations. The coordination number of an ionic crystal is the number of nearest ions of opposite charge. A coordination number of 8 is not compatible with a congruent melting alloy system. Thus, the ionic radius ratio of the cation and anion (i.e., r/R) for a congruent melting alloy is quite restricted in the range of about 0.414 to 0.732.

(26) The above-identified congruently melting salt alloys of Table 1 do not have the properties of the component salts, but different alloy mechanical properties, and different from the properties that follow law of mixtures. Congruently melting salt alloys at 50% have cations randomly distributed on a face centered cubic (FCC) lattice. The randomly distributed cations in the 50% alloy salt have lower bond strength due to the lower melting point. This permits the congruently melting alloys to be more ductile or less brittle than either of the constituent pure salts. Further, because the cations randomly occupy the FCC lattice sites somewhat like an alloy metal, its coefficient of thermal expansion should be larger than the coefficient of thermal expansion of the component pure salts. At the congruent melting alloy point the 50% alloy should also have a lower solid density. Thus, the shrinkage at the lower liquid to solid transformation temperature, where there is no composition change, should produce a lower shrinkage than a eutectic alloy salt system because the solid density is lower and the liquid density is larger than the corresponding pure salt densities.

(27) For example, the present application recognizes that sodium and potassium atoms must occupy octahedral voids (as opposed to tetrahedral voids with a coordination number [CN] of 4) because their coordination number is calculated as 6. That is, for Na and Cl, r[Na]/R[Cl]=0.95 A/1.81 A=0.53; likewise for K and Cl, r[K]/R[Cl]=1.33 A/1.81 A=0.73. Since the radius ratio is too small for 8:8 [0.732], the coordination number of Na & Cl in NaCl and K & Cl in KCl are 6:6. Accordingly, and as an example, at the congruent melting point of the NaClKCl system at 50% NaCl, the crystal structure is FCC with the closest packing of the larger Cl anions and the smaller Na and K cations distributed among the octahedral voids in a random fashion that minimizes the strain energy created by the different sizes of the Na and K cations. Thus, all the octahedral sites [CN=6] created with the FCC packing of the larger Cl anions are occupied while none of the smaller tetrahedral voids [CN=4] are occupied. As a result, the congruently melting alloy is predicted to be more ductile, and because of the lower bond strength (due to the lower melting point) should also have a higher coefficient of thermal expansion, lower solid density, higher liquid density and shrink less than either component salt, e.g. NaCl or KCl.

(28) To select the optimal congruently melting salt alloys for use as salt cores in the casting of aluminum-silicon alloys, it is first recognized that the melting point of aluminum is 660 C. and the AlSi eutectic temperature is 577 C. The melting point of aluminum and the eutectic temperature of the AlSi alloy is then compared to the congruent melting point of the salt system to ascertain whether the salt system can withstand the heat of the molten alloy. Several congruently melting alloys have congruent melting points too low for use with aluminum high pressure die castings because the congruent melting points are either lower than the AlSi eutectic temperature, i.e. CsNO.sub.3RbNO.sub.3 at 290 C.; LiBrLiCl at 525 C.; and LiClNaCl at 554 C. Others, with a congruent melting temperature lower than the melting point of aluminum, i.e. CsBrCsI at 578 C.; CsClKCl at 616 C.; KINaCl at 580 C.; and KClKI at 599 C. may still be used, but are less optimal in aluminum silicon HPDC. Several of these lower congruent melting point salts may be used in other casting systems, particularly magnesium alloys having a eutectic temperature of 450 C. Further, while the other listed congruently melting salts meet the melting point requirements, they are less optimal for more practical reasons. For example, sulfates and carbonates of sodium and phosphorus produce congruent melting alloys having substantially higher congruent melting points than the melting point of aluminum and therefore require more energy investment. Likewise, the strontium in the CaCl2-SrCl2 system and rubidium in the RbBr and RbI systems are more expensive than necessary. Accordingly, the Na & K chlorides, bromides, and iodides are the most cost effective solutions, i.e. KBrKCl, KBrKI, KBrNaBr, K.sub.2CO.sub.3Na.sub.2CO.sub.3, KClNaCl, and NaBrNaCl. Further, coordination number analysis indicates that the KINaCl system, KClNaBr system as well as the KBrNaCl system will operate effectively as congruently melting salt alloys that may be used in creating salt cores in the casting of aluminum-silicon alloys.

(29) Unlike the eutectic composition where the eutectic is a 50/50 mechanical mixture of the pure salts, the congruent melting alloy composition is a true alloy composition and the mechanical properties of this alloy composition is unrelated to the mechanical properties of the pure salts through the law of mixtures. As a result, solid density at the congruent alloy melting point, is less than the solid density at eutectic melting point temperature or at either pure salt melting points, because the bond strengths holding together the atoms of the pure salts are much larger than the bond strengths holding together congruent melting alloy of much lower melting point. This translates into a lower shrinkage for the congruent melting alloy than either of the two pure salts used to make the alloy salt, or a corresponding eutectic composition consisting of a mechanical mixture of the pure salts. Thus, congruent melting alloy shrinkage is lower than the shrinkage of a eutectic alloy of the same composition made up of the same pure salt constituents because its solid density is smaller and its molten density is higher, making the difference between these values smaller than its eutectic alloy counterpart.

(30) In the available literature, shrinkage values of pure salts are somewhat limited, and the shrinkage values for eutectics in binary salt systems are almost non-existent. However, the densities of molten salt and molten binary salt systems are readily available. The density of most molten salts varies almost linearly with temperature and is represented by the equation: d=abT/1000 where d is the density [g/cc], a and b are constants, and T is the temperature in degrees Celsius over significant ranges of temperature, which may not include the melting point. The densities of the solid pure salts or solid alloy salts are available at room temperature but almost never available at the melting point in the solid state. Thus, the shrinkage value at the melting point is not available, except in rare instances. As a result, shrinkage values in this application were calculated for pure salts and eutectic salts using FIGS. 2-18 and 22, along with the available information from the literature. These calculations are listed in Table 2.

(31) TABLE-US-00002 TABLE 2 Salts, CN, calculated molten MP d, RT d, shrinkage = [0.91 RT d MP d]/MP d Molten T Calculated CN = a = b = Range = MP = Molten d = (g/cm3) Shrinkage = CaCl2* 6 2.4108 0.4225 787 to 950 C. 782 C. 2.08 g/cm3 MP d = 2.15 g/cm3 3.4% CaF2 6/8 3.072 0.391 1367 to 2027 C. 1360 C. 2.54 g/cm3 RT d = 3.18 g/cm3 13.9% CBr 8 3.911 1.22 MP to 860 C. 636 C. 3.14 g/cm3 RT d = 4.43 g/cm3 28.4% CsCl* 8 3.479 1.065 667 to 907 C. 646 C. 2.79 g/cm3 RT d = 3.99 g/cm3 30.1% CsF 12 4.549 1.2806 712 to 912 C. 682 C. 3.68 g/cm3 RT d = 4.12 g/cm3 1.9% CsI* 8/6 3.918 1.18 645 to 855 C. 621 C. 3.19 g/cm3 RT d = 4.51 g/cm3 28.7% KBr 6 2.733 0.825 747 to 927 C. 730 C. 2.13 g/cm3 RT d = 2.75 g/cm3 17.5% KCl* 6 1.977 0.583 777 to 947 C. 776 C. 1.52 g/cm3 MP d = 1.79 g/cm3 17.8% K2CO3* 2.293 0.442 907 to 1007 C. 891 C. 1.90 g/cm3 RT d = 2.43 g/cm3 16.4% KF 8/12 2.469 0.652 881 to 1037 C. 846 C. 1.92 g/cm3 MP d = 2.24 g/cm3 16.6% KI* 6 3.099 0.956 682 to 904 C. 686 C. 2.4 g/cm3 RT d = 3.13 g/cm3 16.7% LiBr* 6 2.888 0.652 MP to 740 C. 547 C. 2.53 g/cm3 RT d = 3.46 g/cm3 24.5% LiCl* 6 1.766 0.433 627 to 777 C. 614 C. 1.50 g/cm3 MP d = 1.90 g/cm3 26.4% Li2CO3 2.10 0.373 737 to 847 C. 723 C. 1.83 g/cm3 RT d = 2.11 g/cm3 4.9% LiF 4 2.224 0.490 876 to 1047 C. 842 C. 1.81 g/cm3 MP d = 2.34 g/cm3 29.4% LiI 4 3.540 0.918 MP to 670 C. 460 C. 3.13 g/cm3 RT d = 3.49 g/cm3 11.5% MgCl2 4 1.894 0.302 727 to 967 C. 708 C. 1.68 g/cm3 RT d = 2.32 g/cm3 25.7% MgF2 6 3.092 0.524 1377 to 1827 C. 1266 C. 2.43 g/cm3 RT d = 3.0 g/cm3 12.3% NaBr* 6 2.952 0.817 MP to 945 C. 755 C. 2.34 g/cm3 RT d = 3.20 g/cm3 24.4% NaCl* 6 1.991 0.543 803 to 1030 C. 801 C. 1.56 g/cm3 MP d = 1.95 g/cm3 24.7% NaCO3* 2.357 0.449 867 to 1007 C. 851 C. 1.97 g/cm3 RT d = 2.53 g/cm3 16.9% NaF 6 2.502 0.560 997 to 1057 C. 988 C. 1.95 g/cm3 RT d = 2.56 g/cm3 19.5% NaI* 6 3.368 0.949 MP to 915 C. 651 C. 2.75 g/cm3 RT d = 3.67 g/cm3 21.4% RbBr* 8/6 3.446 1.072 700 to 910 C. 682 C. 2.71 g/cm3 RT d = 3.35 g/cm3 12.5% RbCl 8 2.880 0.883 MP to 925 C. 715 C. 2.25 g/cm3 RT d = 2.80 g/cm3 13.2% RbF 12 3.707 1.011 820 to 1005 C. 775 C. 2.92 g/cm3 RT d = 3.56 g/cm3 10.9% RbI* 6 3.638 1.14 655 to 905 C. 642 C. 2.91 g/cm3 RT d = 3.55 g/cm3 11.0% SrCl2* 8 3.232 0.578 893 to 1037 C. 873 C. 2.73 g/cm3 RT d = 3.05 g/cm3 1.7% SrF2 8 4.579 0.751 1477 to 1927 C. 1450 C.+ 3.49 g/cm3 RT d = 4.24 g/cm3 10.6%

(32) Applicants discovered that at the congruent melting point in the congruently melting salt alloy systems, the bond strength and density in the solid state is less than the bond strength and densities of either pure component salt in the solid state. This is because applicants realized that the congruent melting point is going to be less than the melting point of either component salt melting point. Further, the density at the congruent melting point in the liquid state is higher than the densities at the melting points of the salt components in the liquid state because the congruent melting point is lower than the melting points of either of the component salts. Further, ionic solids with high melting points will have strong bonds as reflected in a relatively higher density in the solid state, while ionic solids with low melting points will have weak bonds reflected in a relatively lower density in the solid state. Because of low melting points compared with the component pure salts, congruently melting alloy salts should have low strength bounds and lower densities in the solid state. Accordingly, the congruently melting alloy has a lower shrinkage than the pure salts that were used to make the congruent melting alloy.

(33) The identified congruently melting salt alloys can optimally be used in for an expendable salt core in high pressure die casting (HPDC). Using congruently melting salt alloys provides a benefit of the salt core undergoing no change in composition on freezing, melting and freezing isothermally, exhibiting no coring in its cast structure, and having a relatively low shrinkage compared to conventional pure salt cores. As explained, the identified binary isomorphous salt systems where the liquidus and solidus interfaces of the respective constituents do not descend continuously from the melting temperature of one salt to that of the other but, instead, passes through a minimum temperature, that lies below the melting point of both salt components are advantageous. This is because the liquidus and solidus interfaces of the respective constituencies meet at the congruent melting point. Such congruent melting salt alloys do not have law of mixtures properties of the component pure salts, but different alloy mechanical properties, unrelated to the component pure salts, as detailed above. Particularly, because the liquid density is higher than the pure salts and the solid density is less than the pure salts, the shrinkage is less than the shrinkage of a eutectic alloy of the same composition and temperature. This provides a real advantage in that this reduction in shrinkage prevents cracking of the core that negatively affects the rejection rate of the HPDC cored parts. In other words, salt cores having reduced shrinkage values will result in HPDC that have significantly less flaws in the cavities firmed by the salt cores.

(34) In one embodiment, the congruently melting salt alloy is used for salt cores in the HPDC process for aluminum alloys. The salt cores may be fused salt cores used to make closed deck engine blocks or other complicated die cast parts. While each of the congruently melting salt alloys identified above may be advantageously used, the following systems provide both practical advantages in having reduced shrinkage and also lower cost production: the KBrKCl system at 0.35 mole fraction KCl with a congruent melting temperature of 717 C.; the KBrKI system at 0.67 mole fraction KI with a congruent melting temperature of 663 C.; the KBrNaBr system at 0.487 mole fraction NaBr with a congruent melting temperature of 643 C.; the NaBrNaCl system at 0.25 mole fraction NaCl with a congruent melting temperature of 780 C.; the K.sub.2CO.sub.3Na.sub.2CO.sub.3 system at 0.59 mole fraction Na.sub.2CO.sub.3 and with a congruent melting temperature of 709 C.; the KClNaCl system at 0.50 mole fraction KCl and with a congruent melting temperature of 660 C.; the KClNaBr system at mole fraction 0.50 KCl with a congruent melting temperature of 750 C. or lower; or the KBrNaCl system at mole fraction 0.50 KBr with a congruent melting temperature of 730 C. or lower. If cost advantages are not a consideration, the KBrRbBr system at 0.75 mole fraction RbBr with a congruent melting temperature of 689 C.; the CaCl.sub.2SrCl.sub.2 system at 0.416 mole fraction SrCl.sub.2 and with a congruent melting temperature of 662 C.; the K.sub.2SO.sub.4Na.sub.2SO.sub.4 at 0.74 mole fraction Na.sub.2SO.sub.4 and with a congruent melting point of 834 C.; the Na.sub.2CO.sub.3Na.sub.2SO.sub.4 at 0.36 mole fraction Na.sub.2SO.sub.4 with a congruent melting point at 826 C.; or the KIRbI system at 0.75 mole fraction RbI and with a congruent melting temperature of 834 C. may also be used with the same advantages in increased strength and ductility on the final product. Further, the following systems may also be used, but are not as optimal as those listed above because of the proximity of the melting point of aluminum to the congruent melting point: the CsClKCl system at 0.375 mole fraction KCl and with a congruent melting temperature of 616 C. and the KClKI system at 0.601 mole fraction KI and with a congruent melting temperature of 599 C.

(35) In another embodiment, the congruently melting salt alloy is used for salt cores in the HPDC process for magnesium alloys. Like aluminum alloys, the magnesium alloys may be used to make closed deck engine blocks or other complicated die cast parts with the identified salt cores. While each of the congruently melting salt alloys identified above for aluminum alloys may be advantageously used, the following systems may also be used a will also provide both practical advantages in having reduced shrinkage and also lower cost production for magnesium alloys: the KINaI system at 0.588 mole fraction NaI and with a congruent melting temperature of 580 C.; the KINaCl system at 0.5 mole fraction NaCl with a congruent melting temperature of 560 C.; the LiBrLiCl system at 0.375 mole fraction LiCl with a congruent melting temperature of 525 C.; and the LiClNaCl system at 0.28 mole fraction NaCl with a congruent melting temperature of 554 C. If cost advantages are not a consideration, the CsBrCsI system at 0.472 mole fraction CsI with a congruent melting temperature of 578 C.; the CsClKCl system at 0.375 mole fraction KCl with a congruent melting temperature of 580 C. may also be used for magnesium alloy HPDC castings.

(36) In the embodiments noted above, one of ordinary skill in the art will recognize that because the liquidus and solidus are relatively flat at the congruent melting point, the congruent alloy composition listed may be plus or minus 0.05 mole fraction of the mole fraction identified. Further, the identified melting points may be plus or minus 5 C. because the identified melting points were calculated and determined both theoretically and experimentally. Additionally, salt cores for either aluminum or magnesium castings can provide additional utility when they are not allowed to cool below 200 C. after casting, and are transferred to an oven at 200 C. to improve the congruent melting alloy's resistance to exhibiting cracks in the salt core.

EXAMPLE

(37) The congruent melting alloy system of NaClKCl with a congruent melting alloy of 0.50 mole function NaCl at 685 C. or 56.1% by weight KCl was used to experimentally create salt core tensile specimens for comparative purposes with traditional salt cores. Twenty specimens of each of the congruent melting alloy cores and the traditional salt cores were made in the metal mold of FIG. 19. The salt cores constructed were tensile feeder bars, larger diameter feeder bars, and downward space extensions with rectangular cross sections as shown in FIGS. 20 and 21. The mold was preheated to 260 C. before the molten salt alloy was poured into the mold. The conventional salt cores used were the standard of the industry NaClNa.sub.2CO.sub.3 system (FIG. 1) at the eutectic composition, while the congruent melting alloy cores were constructed from the NaClKCl system (FIG. 22) at the congruent melting point composition.

(38) Because of the large shrinkage contraction at the eutectic composition, the tensile bar salt core, the larger diameter feeder bar salt cores and the downward sprue extension salt cores with a rectangular cross section, when poured with the traditional NaClNa.sub.2CO.sub.3 salt, all cracked in the mold after pouring and solidification, as shown by example in FIG. 21. This cracking of the traditional eutectic alloy has constantly been obtained over the years with the NaClNa.sub.2CO.sub.3 system whether the pouring temperature was high at 1500 F. or low in the semi-solid state. Further, as FIG. 21 indicates, a significant gap between the cracked surfaces of each of the twenty specimens for the traditional eutectic alloy was observed.

(39) By contrast, and independent of the casting temperature, the tensile bar, larger diameter feeder bars and sprue segment salt cores powered with the congruently melting alloy salt did not exhibit cracking in the solidified structures as shown in FIG. 20. These visual differences between the two compositions clearly indicate that the congruently melting alloy has a lower shrinkage value. In the above description, certain terms have been used for brevity, clarity and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes only and are intended to be broadly construed. The different systems and methods described herein may be used alone or in combination with other systems and methods. Various equivalents, alternatives and modifications are possible within the scope of the appended claims. While each of the method claims includes a specific series of steps for accomplishing certain processes, the scope of this disclosure is not intended to be bound by the literal order or literal content of the steps described herein, and non-substantial differences or changes still fall within the scope of the disclosure.