Water-soluble sand core for low-pressure casting and method for producing the same

Abstract

The application discloses a water-soluble sand core for low-pressure casting and a preparation method thereof. The water-soluble sand core comprises 20-70% of refractory aggregate and 30-80% of binary potassium salt according to mass percentage. The binary potassium salt is composed of potassium chloride and potassium carbonate, and the molar ratio of the two is 1:4-4:1. In the preparation, the potassium chloride and the potassium carbonate are heated to form a uniform melt, then the refractory aggregate is added and stirred to form a slurry, the slurry is poured into a mold and cooled to be demolded. The solidified binary potassium salt phase forms a bridge between the refractory aggregate particles, so that the water-soluble sand core has both the erosion resistance and the complex shape formability. After casting, the potassium salt bridge phase is dissolved in water, so that the refractory aggregate is scattered and discharged. The sand core does not use organic binder and is suitable for low-pressure casting of aluminum alloy and magnesium alloy complex inner cavity castings.

Description

Technical Field

[0001] This invention relates to the field of casting technology, specifically to a water-soluble sand core for low-pressure casting and its preparation method. Background Technology

[0002] Low-pressure casting typically involves molten metal filling the mold cavity smoothly from bottom to top under relatively low gas pressure and then solidifying under pressure. This process helps reduce gas entrapment and slag inclusions, improves feeding capacity and material utilization, and is therefore used for aluminum and magnesium alloy castings with complex internal cavities. For such castings, the core must maintain its shape and strength during assembly, filling, and solidification, and must also be able to be quickly removed from tortuous, closed, or elongated internal cavities after casting.

[0003] Traditional resin-bonded sand cores have good core-making adaptability, but they may generate a lot of gas under the action of molten metal, leading to defects such as porosity in the casting, and causing fumes and cleaning pollution. Although sand cores using water-soluble inorganic binders can improve cleanability, some systems have problems such as insufficient heat resistance, decreased strength after moisture absorption, or unstable collapsing rate after casting.

[0004] Water-soluble core technology emerged with the development of modern industrial technology, especially the aerospace industry. Water-soluble sand cores are simple to manufacture, do not produce harmful gases during casting, are easy and quick to clean, and produce no vibration or noise. After casting, the core can be dissolved or highly dispersed by hydraulic cleaning, making it easy to remove and resulting in a clean, smooth inner surface and good dimensional accuracy in the casting. It also saves labor and improves the hygiene of the foundry, making it an effective method for producing high-performance low-pressure castings. Existing water-soluble salt cores mainly include fused alumina soluble cores, pure salt cores made from molten sodium chloride or sodium carbonate, water-soluble cores made from alumina and potassium carbonate aqueous solutions, and ceramic water-soluble cores sintered with magnesium oxide or calcium oxide as the base material. The main problems with these water-soluble cores are: pure salt cores require a large amount of salt, resulting in high cost, high pressure to control solidification shrinkage and brittle fracture; and current water-soluble binder sand cores have poor heat resistance and low strength.

[0005] Therefore, there is an urgent need to provide a new type of water-soluble sand core, which enables refractory aggregates to undertake the main functions of heat resistance and erosion resistance, and enables water-soluble salt phases to undertake the functions of particle bridging and post-cast dissolution and core removal, thereby achieving a balance between strength, formability and water-soluble cleanability without the use of organic binders. Summary of the Invention

[0006] The present invention aims to solve the problems of large gas generation of existing resin sand cores, insufficient heat resistance of some water-soluble inorganic bonded sand cores, high salt consumption, solidification defects and high manufacturing cost of pure salt cores, and provides a water-soluble sand core for low-pressure casting and its preparation method.

[0007] To better solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] In a first aspect, a water-soluble sand core for low-pressure casting comprises 20% to 70% refractory aggregate and 30% to 80% binary potassium salt; the binary potassium salt comprises potassium chloride and potassium carbonate, wherein the molar ratio of potassium chloride to potassium carbonate is 1:4 to 4:1.

[0009] Preferably, the refractory aggregate has a particle size of 50-250 mesh; the refractory aggregate is selected from at least one of quartz sand, zircon sand, olivine sand, and corundum sand.

[0010] Preferably, when the refractory aggregate is quartz sand, the mass content of SiO2 in the quartz sand is not less than 90%; when the refractory aggregate is corundum sand, the mass content of α-Al2O3 is not less than 98.5%; when the refractory aggregate is zircon sand, the mass content of ZrO2 is not less than 66.0%.

[0011] In the water-soluble sand core, refractory aggregate particles are dispersed in a solidified binary potassium salt binder phase. The binary potassium salt binder phase at least partially covers the surface of the refractory aggregate particles and forms solidification bridging between adjacent refractory aggregate particles, so that the refractory aggregate constitutes a load-bearing skeleton.

[0012] Apart from unavoidable impurities in the raw materials, the water-soluble sand core does not contain sodium salts or organic binders.

[0013] Preferably, the molar ratio of potassium chloride to potassium carbonate is 2:3 to 3:2.

[0014] Secondly, a method for preparing water-soluble sand cores for low-pressure casting includes the following steps:

[0015] (1) Mix potassium chloride and potassium carbonate to obtain a binary potassium salt mixture;

[0016] (2) The binary potassium salt mixture is placed in a crucible and heated to melt, resulting in a mixed melt;

[0017] (3) Add refractory aggregate to the mixed melt and stir evenly to obtain a mixed slurry;

[0018] (4) Pour the mixed slurry into the mold, cool it until solidified, and then open the mold to obtain water-soluble sand core.

[0019] Preferably, in step (1), the heating and melting temperature is 650-750℃.

[0020] Preferably, in step (4), the mold is selected from metal molds, ceramic molds or plastic molds.

[0021] Compared with the prior art, the present invention has at least the following advantages:

[0022] 1. This invention uses refractory aggregate as the main load-bearing skeleton of the sand core and a binary potassium salt composed of potassium chloride and potassium carbonate as the fusion bonding phase. In its molten state, the binary potassium salt can penetrate the gaps between refractory aggregate particles and wet the particle surfaces. After cooling and solidification, the salt phase forms a continuous or semi-continuous solidification bridging structure between adjacent refractory aggregate particles, thereby binding the loose aggregate into a sand core with overall strength. Through the synergistic effect of these two components, the problem of brittle fracture easily occurring in pure salt cores that rely mainly on the salt body for support can be avoided, and the problem of insufficient bonding strength in sand cores using traditional water-soluble inorganic binders can also be overcome. Test results show that the tensile strength of the sand cores obtained in this embodiment can reach 22.8–26.0 MPa, which can meet the core strength requirements during sand core demolding, handling, assembly, and low-pressure casting filling.

[0023] 2. This invention incorporates 20%–70% refractory aggregate into the core, forming a stable granular skeleton, and controls the content of binary potassium salts at 30%–80%. The refractory aggregate itself has high thermal stability, which can constrain the solidification shrinkage of the salt phase, reducing the overall dimensional change of the core. Compared with pure salt cores, this invention can reduce the total amount of salt phase while ensuring effective particle bonding, thereby reducing solidification shrinkage, lowering cooling stress, and improving the dimensional stability and molding yield of the sand core. Test results show that the linear shrinkage rate of the sand core obtained in this embodiment is 0.54%–0.75%.

[0024] 3. The refractory aggregates used in this invention, such as quartz sand, zircon sand, olivine sand, or corundum sand, possess high refractoriness and good high-temperature stability. During low-pressure casting, the refractory aggregates can bear the main heat load of the sand core under the action of high-temperature molten metal and maintain the basic shape of the core. Compared with pure salt cores dominated by salt phases, this invention reduces the proportion of low-melting-point salt phases directly exposed to high-temperature molten metal, lowering the possibility of instability of the sand core surface due to heat softening, localized melting, or erosion. Simultaneously, the particle skeleton formed by the refractory aggregates can improve the core's resistance to molten metal impact and flow shear, helping to reduce problems such as sand core breakage, localized sand loss, internal cavity deformation, and inclusions formed by aggregate entering the casting. This invention is suitable for low-pressure casting processes of aluminum alloys, magnesium alloys, etc.

[0025] 4. This invention employs a binary potassium salt system composed of potassium chloride and potassium carbonate, rather than using potassium chloride or potassium carbonate alone. By limiting the molar ratio of potassium chloride to potassium carbonate to 1:4 to 4:1, the fluidity, wettability, solidification behavior, water solubility, and hygroscopic properties of the salt melt can be adjusted. Potassium chloride has relatively low hygroscopicity, which is beneficial for improving the moisture resistance stability of the sand core during storage and use; potassium carbonate has good water solubility, which is beneficial for improving the post-casting cleaning speed. By combining the two in a specific molar ratio, this invention utilizes potassium chloride to improve moisture resistance while utilizing potassium carbonate to improve water solubility and salt phase bridging, achieving a better balance between the sand core's normal strength, water solubility cleaning speed, and strength retention rate after moisture absorption. Test results show that the 24-hour moisture absorption weight gain rate of the sand core obtained in this embodiment is 1.4% to 2.2%, and the strength retention rate after moisture absorption is 88% to 93%.

[0026] 5. In this invention, the water-soluble binary potassium salt phase is distributed on the surface and between the refractory aggregate particles, forming salt bridges between them. After the casting is poured, solidified, and cooled, clean water can enter the core from the exposed part of the sand core, process holes, or internal channels of the casting. Upon contact with the binary potassium salt phase, the salt phase gradually dissolves, and the solidification bridging structure between the particles is destroyed. When the salt bridges lose their binding effect, the refractory aggregate particles transform from an integral skeleton into loose particles and can be discharged from the casting cavity under the action of water flow. Core removal can be achieved without relying on resin pyrolysis, mechanical vibration, or strong sand flushing, making it particularly suitable for complex cavities that are tortuous, slender, enclosed, or difficult for mechanical tools to access. Test results show that the sand core obtained in this embodiment of the invention dissolves and disintegrates in water at 25°C in 4.0–5.4 min, achieving relatively fast post-casting core removal.

[0027] 6. Except for unavoidable impurities in the raw materials, this invention does not contain organic binders. During the low-pressure casting process of aluminum or magnesium alloys, the thermal decomposition reaction of organic binders such as phenolic resin, furan resin, and cold box resin will not occur, thus significantly reducing the gases, fumes, and volatile organic compounds generated by the heating of the sand core. Detailed Implementation

[0028] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0029] Example 1

[0030] A method for preparing water-soluble sand cores for low-pressure casting includes the following steps:

[0031] (1) Weigh 400g of quartz sand, 210.2g of potassium chloride, and 389.8g of potassium carbonate by mass percentage; wherein the SiO2 content of the quartz sand is not less than 90% and the particle size is 200 mesh;

[0032] (2) Potassium chloride and potassium carbonate are premixed, placed in a crucible and heated to 700°C. After they are completely melted, quartz sand is added and stirred evenly to obtain a mixed slurry.

[0033] (3) The mixed slurry is poured into the metal core mold using a quantitative tool. After cooling and solidification, the mold is opened and the gating system is removed to obtain water-soluble sand core.

[0034] Example 2

[0035] A method for preparing water-soluble sand cores for low-pressure casting includes the following steps:

[0036] (1) Weigh 400g of quartz sand, 158.7g of potassium chloride, and 441.3g of potassium carbonate by mass percentage; wherein the SiO2 content of the quartz sand is not less than 90% and the particle size is 200 mesh;

[0037] (2) Potassium chloride and potassium carbonate are premixed, placed in a crucible and heated to 700°C. After they are completely melted, quartz sand is added and stirred evenly to obtain a mixed slurry.

[0038] (3) The mixed slurry is poured into the metal core mold using a quantitative tool. After cooling and solidification, the mold is opened and the gating system is removed to obtain water-soluble sand core.

[0039] Example 3

[0040] A method for preparing water-soluble sand cores for low-pressure casting includes the following steps:

[0041] (1) Weigh 400g of quartz sand, 268.4g of potassium chloride, and 331.6g of potassium carbonate by mass percentage; wherein the SiO2 content of the quartz sand is not less than 90% and the particle size is 200 mesh;

[0042] (2) Potassium chloride and potassium carbonate are premixed, placed in a crucible and heated to 700°C. After they are completely melted, quartz sand is added and stirred evenly to obtain a mixed slurry.

[0043] (3) The mixed slurry is poured into the metal core mold using a quantitative tool. After cooling and solidification, the mold is opened and the gating system is removed to obtain water-soluble sand core.

[0044] Example 4

[0045] A method for preparing water-soluble sand cores for low-pressure casting includes the following steps:

[0046] (1) Weigh 400g of quartz sand, 157.7g of potassium chloride, and 292.3g of potassium carbonate by mass percentage; wherein the SiO2 content of the quartz sand is not less than 90% and the particle size is 200 mesh;

[0047] (2) Potassium chloride and potassium carbonate are premixed, placed in a crucible and heated to 700°C. After they are completely melted, quartz sand is added and stirred evenly to obtain a mixed slurry.

[0048] (3) The mixed slurry is poured into the metal core mold using a quantitative tool. After cooling and solidification, the mold is opened and the gating system is removed to obtain water-soluble sand core.

[0049] Comparative Example 1

[0050] The difference between this example and Example 1 is that an equal amount of potassium chloride is used instead of potassium carbonate, while the other operations are the same as in Example 1.

[0051] Comparative Example 2

[0052] The difference between this example and Example 1 is that an equal amount of potassium carbonate is used instead of potassium chloride, while the other operations are the same as in Example 1.

[0053] Comparative Example 3

[0054] The difference between this example and Example 1 is that the molar ratio of potassium chloride to potassium carbonate is 1:5, while the other operations are the same as in Example 1.

[0055] Comparative Example 4

[0056] The difference between this example and Example 1 is that the molar ratio of potassium chloride to potassium carbonate is 5:1, while the other operations are the same as in Example 1.

[0057] Comparative Example 5

[0058] The difference between this example and Example 1 is that the mass ratio of refractory aggregate to salt phase is 75:25, while the other operations are the same as in Example 1.

[0059] Comparative Example 6

[0060] The difference between this example and Example 1 is that the mass ratio of refractory aggregate to salt phase is 15:85, while the other operations are the same as in Example 1.

[0061] Comparative Example 7

[0062] The difference between this example and Example 1 is that all raw materials are mixed together and added to the crucible, while other operations are the same as in Example 1.

[0063] The performance of the sand cores prepared in the above embodiments and comparative examples was tested. Sampling, conditioning, and testing were conducted according to GB / T 2684-2025 "Test Methods for Foundry Sand and Mixtures". The test results are shown in Table 1.

[0064] Table 1

[0065]

[0066] As can be seen from the test results in Table 1, the water-soluble sand cores prepared by mixing quartz sand and a binary potassium salt composed of potassium chloride and potassium carbonate in Examples 1-3 of this invention exhibit high tensile strength, significantly higher than that of Comparative Examples 1 and 2, thanks to the reasonable adjustment of the dosage of each component and the preparation process conditions. This result indicates that after the appropriate proportion of potassium chloride and potassium carbonate forms a binary salt phase in this invention, the Kc of the binary potassium salt melt increases during the cooling process. + Cl - and CO3 2- The formation of a continuous salt bridge phase through ion rearrangement has a better interfacial bonding force between the salt bridge phase and the surface of refractory aggregate particles than that of a single potassium salt. This can improve the wetting, filling and solidification bridging state of the aggregate particles by the melt, and enable the salt phase to form a more complete load-bearing connection between adjacent aggregate particles.

[0067] Comparative Example 1 showed the lowest moisture absorption weight gain and a strength retention rate of 95%, but its water dissolution time was 8.9 min, and its tensile strength was only 14.2 MPa. Comparative Example 2 had a water dissolution time of only 2.6 min, but its moisture absorption weight gain reached 4.9%, and its strength retention rate decreased to 68% after moisture absorption. Comparative Examples 3 and 4 also show that when the amount of potassium chloride is too small, the sand core tends to exhibit a high moisture absorption weight gain and a low moisture absorption strength retention rate; when the amount of potassium chloride is too large, the water dissolution and cleaning speed of the sand core decreases significantly. When the molar ratio of potassium chloride to potassium carbonate is between 2:3 and 3:2, the obtained sand core achieves a better balance between tensile strength, water dissolution rate, and moisture resistance.

[0068] In Comparative Example 5, the refractory aggregate content was increased to 75%. Although the linear shrinkage rate decreased to 0.28%, the tensile strength was only 10.8 MPa, and the water dissolution time exceeded 15 minutes. This was mainly due to insufficient salt phase content, which failed to continuously coat and bridge all refractory aggregate particles, leading to the formation of unbonded areas within the sand core. Simultaneously, the small amount of salt phase dispersed by the refractory aggregate pores and interparticle gaps made it difficult for water to quickly establish continuous dissolution channels. In Comparative Example 6, the refractory aggregate content was only 15%, while the salt phase reached 85%. The resulting sand core exhibited a faster water dissolution rate, but the linear shrinkage rate increased significantly, and the strength decreased after moisture absorption. These results indicate that when the refractory aggregate content is insufficient, the sand core mainly relies on the salt phase for load bearing and dimensional stability, failing to fully utilize the refractory aggregate skeleton's functions of reducing shrinkage, inhibiting brittle fracture, and stabilizing dimensions.

[0069] In summary, controlling the refractory aggregate content to 20%–70% and the binary potassium salt content to 30%–80% is beneficial for balancing the particle bridging, dimensional stability, tensile strength, and water solubility of the sand core.

[0070] In Comparative Example 7, all raw materials were added to the crucible simultaneously, resulting in a sand core with a tensile strength of 16.7 MPa; linear shrinkage increased to 0.92%, water dissolution time extended to 6.2 min; and strength retention after moisture absorption decreased to 84%. This is because when all raw materials are heated simultaneously, the refractory aggregate hinders the contact, melting, and convection mixing between salt particles. Some salt may adhere to the aggregate surface, forming localized salt-rich areas, while some refractory aggregate areas form weakly connected areas due to insufficient salt content. Furthermore, the addition of refractory aggregate absorbs heat, causing a decrease in the temperature and an increase in viscosity of the local salt phase, which is detrimental to the melt's full penetration into the interparticle spaces. In this invention, potassium chloride and potassium carbonate are first melted to form a uniform binary salt melt, and then the refractory aggregate is added. This allows the melt to more fully wet the aggregate surface and fill the interparticle spaces, forming a more uniformly distributed solidified salt bridge after cooling.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A water-soluble sand core for low-pressure casting, characterized in that, It includes 20% to 70% refractory aggregate and 30% to 80% binary potassium salt; the binary potassium salt is composed of potassium chloride and potassium carbonate, and the molar ratio of potassium chloride to potassium carbonate is 1:4 to 4:

1.

2. The water-soluble sand core for low-pressure casting according to claim 1, characterized in that, The refractory aggregate has a particle size of 50-250 mesh; the refractory aggregate is selected from at least one of quartz sand, zircon sand, olivine sand, and corundum sand.

3. The water-soluble sand core for low-pressure casting according to claim 1, characterized in that, The quartz sand contains no less than 90% SiO2 by mass; the corundum sand contains no less than 98.5% α-Al2O3 by mass; and the zircon sand contains no less than 66.0% ZrO2 by mass.

4. The water-soluble sand core for low-pressure casting according to claim 1, characterized in that, The molar ratio of potassium chloride to potassium carbonate is 2:3 to 3:

2.

5. A method for preparing water-soluble sand cores for low-pressure casting according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Mix potassium chloride and potassium carbonate to obtain a binary potassium salt mixture; (2) The binary potassium salt mixture is placed in a crucible and heated to melt, resulting in a mixed melt; (3) Add refractory aggregate to the mixed melt and stir evenly to obtain a mixed slurry; (4) Pour the mixed slurry into the mold, cool it until solidified, and then open the mold to obtain water-soluble sand core.

6. A method for preparing water-soluble sand cores for low-pressure casting according to claim 5, characterized in that, In step (1), the heating and melting temperature is 650-750℃.

7. A method for preparing water-soluble sand cores for low-pressure casting according to claim 5, characterized in that, In step (4), the mold is selected from metal molds, ceramic molds or plastic molds.

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