A casting process for ship turbine blades

Abstract

This invention relates to the field of casting technology and discloses a casting process for marine engine blades, comprising: preparing three types of molding sand with different compositions: surface molding sand, intermediate molding sand, and back molding sand; sequentially filling the surface of the pattern with surface molding sand, intermediate molding sand, and back molding sand using a layered compact molding method to form a gradient structure mold; introducing CO2 gas to harden the mold; applying a silica sol coating to the surface of the mold cavity; drying the surface to form a siliceous glassy film from the silica sol; pouring molten metal into the mold after closing the mold, and pouring at high temperature to decompose the organic pore-forming agent in the back molding sand to create pores in situ; and demolding after solidification and cooling to obtain the casting. This invention, through its gradient layered structure and surface sealing treatment, reduces the mold expansion rate while establishing a permeability gradient from the surface to the back layer, preventing molten metal penetration and promoting gas expulsion, effectively solving the problems of hot cracking, porosity defects, and surface sand adhesion in the casting of marine engine blades.

Description

Technical Field

[0001] This invention relates to the field of casting technology, and more specifically, to a casting process for marine engine blades. Background Technology

[0002] Marine turbine blades operate for extended periods in high-temperature, high-pressure seawater environments, demanding extremely high casting quality. Traditional clay sand casting processes cause dramatic mold expansion during the pouring of molten metal, generating uneven stress on blades with complex shapes and varying wall thicknesses, leading to hot cracks and dimensional deviations. Simultaneously, the poor permeability of the mold prevents timely gas escape from the cavity, accumulating in complex areas and forming porosity defects that weaken the blade's fatigue strength. Furthermore, the rough mold surface easily adheres to the molten metal, resulting in severe surface sand adhesion and increasing subsequent processing costs.

[0003] Although existing phosphate-bonded sand molds use zircon sand to reduce the expansion rate, the single homogeneous structure cannot simultaneously meet the requirements of surface impermeability and back layer high air permeability. Increasing the surface density will worsen the air permeability, while increasing the air permeability will exacerbate the penetration of molten metal and surface sand adhesion, creating a contradiction. Summary of the Invention

[0004] This invention provides a casting process for marine engine blades that, while reducing the mold expansion rate, achieves functional zoning of a highly dense, impermeable surface layer and an efficient, breathable, and exhaust-prone back layer, thereby simultaneously solving the problems of hot cracking, porosity defects, and surface quality in the casting of marine engine blades.

[0005] This invention provides a casting process for marine engine blades, comprising the following steps:

[0006] A surface molding sand, an intermediate molding sand, and a backing molding sand are prepared, wherein the surface molding sand contains ultrafine quartz powder and zircon sand, and the backing molding sand contains an organic pore-forming agent;

[0007] A gradient structure mold is formed by sequentially filling the surface layer, intermediate layer, and back layer molding sand with a layered molding sand using a layered molding method.

[0008] CO2 gas is introduced to harden the mold;

[0009] Apply silica sol coating to the surface of the mold cavity;

[0010] The drying process allows the silica sol to form a silica glassy film while retaining the organic pore-forming agent in the backing sand.

[0011] After the mold is assembled, molten metal is poured in. The high temperature of the pouring causes the organic pore-forming agent in the back layer molding sand to decompose and form air-permeable channels.

[0012] Preferably, the mass fraction of each component of the surface molding sand is: 65-70% zircon sand, 15-20% ultrafine quartz powder, 12-15% phosphate binder, and the remainder is water and additives.

[0013] Preferably, the zircon sand has a particle size of 0.1-0.3 mm, and the ultrafine quartz powder has a particle size of 5-15 μm.

[0014] Preferably, the mass fraction of each component in the intermediate layer molding sand is: 80-85% zircon sand, 10-12% phosphate binder, and the remainder is water and additives; the particle size of the zircon sand is 0.15-0.4 mm.

[0015] Preferably, the mass fraction of each component of the backing sand is: 80-85% zircon sand, 6-8% phosphate binder, 3-5% organic pore-forming agent, and the remainder is water and additives; the particle size of the zircon sand is 0.3-0.6 mm, and the organic pore-forming agent is selected from starch and cellulose powder.

[0016] Preferably, the filling thickness of the surface molding sand is 5-8 mm, the filling thickness of the intermediate molding sand is 50-60% of the total thickness of the mold, and the filling thickness of the back molding sand is 15-20 mm.

[0017] Preferably, the CO2 gas is introduced at a pressure of 0.15-0.25 MPa and the gas introduction time is 30-90 seconds.

[0018] Preferably, the silica sol coating has a silica sol solid content of 25-35% and a wet film thickness of 0.3-0.5 mm after application.

[0019] Preferably, the drying temperature is 180-220℃ and the drying time is 2-4 hours.

[0020] Preferably, the molten metal being poured is a copper alloy, aluminum alloy, or nickel-based alloy;

[0021] The casting temperature for copper alloys is 1180-1280℃, for aluminum alloys it is 720-780℃, and for nickel-based alloys it is 1450-1550℃.

[0022] The beneficial effects of this invention are as follows:

[0023] 1. Hot cracking caused by mold expansion is avoided.

[0024] Using zircon sand as the base aggregate for the entire mold results in a coefficient of thermal expansion that is far lower than that of traditional quartz sand, leading to minimal expansion of the mold at high pouring temperatures. This low-expansion mold significantly reduces the thermal stress exerted on the solidifying blades, noticeably decreases the tendency for hot cracking, improves casting dimensional accuracy, and substantially reduces the scrap rate.

[0025] 2. Porous defects are significantly reduced.

[0026] The establishment of a gradient permeable structure, especially the in-situ pore-forming method of the organic pore-forming agent in the back layer at high casting temperatures, ensures that the cavity permeability reaches its highest level when venting is most needed. Gas is smoothly discharged through the gradient path of "micro-permeation in the surface layer → moderate permeability in the intermediate layer → efficient venting in the back layer," significantly reducing porosity defects inside the casting and noticeably improving the density and fatigue strength of the blades.

[0027] 3. Surface quality is significantly improved.

[0028] The composite ratio of ultrafine quartz powder and zircon sand forms a dense surface layer, which, combined with the silica glassy sealing film formed after the silica sol is dried, constructs a dual anti-permeation barrier of "densified particle size distribution + nano-sealing". Molten metal cannot penetrate into the molding sand, surface sand adhesion is essentially eliminated, the surface roughness of the casting is significantly reduced, the surface finish approaches the level of precision casting, and subsequent machining allowances are reduced.

[0029] 4. The contradiction between preventing seepage and venting has been resolved.

[0030] The gradient functional zoning established through layered molding achieves a balance between a highly dense, impermeable surface layer and an efficient, breathable, and ventilated back layer. The contradiction that traditional homogeneous casting cannot resolve is addressed in the gradient structure: the surface layer provides impermeability, the back layer provides ventilation, and the intermediate layers achieve a smooth performance transition, resulting in overall synergistic optimization. This spatial functional zoning solution has universal technical value.

[0031] 5. Precise matching of process timing control to requirements

[0032] The organic pore-forming agent remains stable at drying temperatures (180-220℃) and rapidly decomposes to create pores at pouring temperatures (>300℃). This timing control of function, achieved through varying temperature ranges, ensures sufficient strength during mold preparation and a significant increase in back-layer permeability during critical pouring moments. The silica sol solidifies into a film during drying to seal the surface and remains stable during pouring, providing an anti-permeability effect. This precise temporal coordination between the two materials demonstrates the systematic nature of the process.

[0033] 6. Simultaneous improvement in overall quality and production efficiency

[0034] This invention simultaneously solves three major problems: hot cracking, porosity defects, and surface quality, significantly improving the overall quality of blade castings and raising the pass rate. The rapid CO2 hardening speed allows for quick mold deployment; improved surface quality reduces cleaning and machining steps; and reduced porosity defects lower the rework rate. These factors combined increase production efficiency and reduce production costs, resulting in excellent economic benefits.

[0035] This invention is applicable to the casting of various marine engine blade materials such as copper alloys, aluminum alloys, and nickel-based alloys. It is particularly effective for casting blades with complex shapes, thin-walled structures, and deep cavities. The process has good stability and repeatability and has broad application prospects. Attached Figure Description

[0036] Figure 1 This is the curve showing the change in air permeability of the backing sand as a function of temperature according to the present invention;

[0037] Figure 2 This is a bar chart comparing the surface parameters of different casting molds according to the present invention;

[0038] Figure 3 This is a SEM microstructure image of the surface of the comparative sample 3 of this invention;

[0039] Figure 4 This is the curve of the linear expansion rate of the casting mold as a function of temperature according to the present invention;

[0040] Figure 5 This is a bar chart comparing the crack incidence rate and the pass rate of this invention;

[0041] Figure 6 This is a bar chart showing the distribution of the number of cracks at different locations in this invention;

[0042] Figure 7 This is a bar chart comparing the crack distribution and size deviation of the present invention. Detailed Implementation

[0043] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0044] Example 1

[0045] This embodiment proposes a casting process for marine engine blades, including the following steps:

[0046] Step 1: Prepare surface molding sand, intermediate molding sand and back molding sand, wherein the surface molding sand contains ultrafine quartz powder and zircon sand, and the back molding sand contains an organic pore-forming agent;

[0047] The mass fractions of the components of the surface molding sand are: 68% zircon sand, 18% ultrafine quartz powder, 13% phosphate binder, and the remainder is water and additives.

[0048] It should be noted that the additives are conventional sand additives in this field, including hardening accelerators, flowability regulators, etc.

[0049] The hardening accelerator is selected from one of magnesium oxide powder, calcium oxide powder or magnesium hydroxide, and the addition amount is 8-12% of the mass of phosphate binder (or 0.8-1.5% of the total mass of molding sand) to accelerate the hardening reaction of phosphate binder.

[0050] The flowability modifier is selected from starch paste, dextrin, or carboxymethyl cellulose, and the addition amount is 0.3-0.8% of the total mass of molding sand.

[0051] The zircon sand has a particle size of 0.2 mm, and the ultrafine quartz powder has a particle size of 10 μm.

[0052] The mass fractions of the components in the intermediate layer molding sand are: 82% zircon sand, 11% phosphate binder, and the remainder being water and additives; the particle size of the zircon sand is 0.3 mm.

[0053] The mass fractions of the components of the backing sand are: 83% zircon sand, 7% phosphate binder, 4% organic pore-forming agent, and the remainder is water and additives; the particle size of the zircon sand is 0.4 mm, and the organic pore-forming agent is selected from starch.

[0054] The surface layer molding sand has a filling thickness of 6 mm, the intermediate layer molding sand has a filling thickness of 55% of the total mold thickness, and the back layer molding sand has a filling thickness of 18 mm.

[0055] Step 2: Using a layered molding method, the surface of the pattern is sequentially filled with the surface layer molding sand, the middle layer molding sand and the back layer molding sand to form a gradient structure mold.

[0056] Step 3: Introduce CO2 gas to harden the mold;

[0057] The CO2 gas was introduced at a pressure of 0.20 MPa for 60 seconds.

[0058] Step 4: Apply silica sol coating to the surface of the mold cavity;

[0059] The silica sol coating has a silica sol solid content of 30% and a wet film thickness of 0.4 mm after application.

[0060] Step 5: Drying treatment allows the silica sol to form a silica glassy film while retaining the organic pore-forming agent in the backing sand;

[0061] The drying temperature is 200℃ and the drying time is 3 hours.

[0062] Step 6: After the mold is closed, pour molten metal. The high temperature of the pouring causes the organic pore-forming agent in the back layer molding sand to decompose and form air-permeable channels.

[0063] The molten metal being poured is a copper alloy, and the pouring temperature is 1180-1280℃.

[0064] Example 2

[0065] The difference between this embodiment and Embodiment 1 is that:

[0066] The mass fractions of the components in the surface molding sand are: 65% zircon sand, 15% ultrafine quartz powder, 12% phosphate binder, and the remainder is water and additives.

[0067] The zircon sand has a particle size of 0.1 mm, and the ultrafine quartz powder has a particle size of 5 μm.

[0068] The mass fractions of the components in the intermediate layer molding sand are: 80% zircon sand, 10% phosphate binder, and the remainder being water and additives; the particle size of the zircon sand is 0.15 mm.

[0069] The mass fractions of the components of the backing sand are: 80% zircon sand, 6% phosphate binder, 3% organic pore-forming agent, and the remainder is water and additives; the zircon sand has a particle size of 0.3 mm, and the organic pore-forming agent is selected from cellulose powder.

[0070] The surface layer molding sand has a filling thickness of 5 mm, the intermediate layer molding sand has a filling thickness of 50% of the total mold thickness, and the back layer molding sand has a filling thickness of 15 mm.

[0071] The CO2 gas was introduced at a pressure of 0.15 MPa for 30 seconds.

[0072] The silica sol coating has a silica sol solid content of 25% and a wet film thickness of 0.3 mm after application.

[0073] The drying temperature is 180℃ and the drying time is 2 hours.

[0074] The molten metal being poured is an aluminum alloy, and the pouring temperature is 720-780℃.

[0075] Example 3

[0076] The difference between this embodiment and Embodiment 1 is that:

[0077] The mass fractions of the components of the surface molding sand are: 70% zircon sand, 20% ultrafine quartz powder, 15% phosphate binder, and the remainder is water and additives.

[0078] The zircon sand has a particle size of 0.3 mm, and the ultrafine quartz powder has a particle size of 15 μm.

[0079] The mass fractions of the components in the intermediate layer molding sand are: 85% zircon sand, 12% phosphate binder, and the remainder being water and additives; the particle size of the zircon sand is 0.4 mm.

[0080] The mass fractions of the components of the backing sand are: 85% zircon sand, 8% phosphate binder, 5% organic pore-forming agent, and the remainder is water and additives; the particle size of the zircon sand is 0.6 mm.

[0081] The surface layer molding sand has a filling thickness of 8 mm, the intermediate layer molding sand has a filling thickness of 60% of the total mold thickness, and the back layer molding sand has a filling thickness of 15-20 mm.

[0082] The CO2 gas was introduced at a pressure of 0.25 MPa for 90 seconds.

[0083] The silica sol coating has a silica sol solid content of 35% and a wet film thickness of 0.5 mm after application.

[0084] The drying temperature is 220℃ and the drying time is 4 hours.

[0085] The molten metal being poured is a nickel-based alloy, and the pouring temperature for the nickel-based alloy is 1450-1550℃.

[0086] Example 4

[0087] This embodiment proposes a casting process for marine engine blades, including the following specific implementation steps:

[0088] Step S1: Prepare surface molding sand

[0089] The dense surface molding sand was prepared with the following mass fractions: zircon sand 68% by mass, particle size 0.2 mm; ultrafine quartz powder 17% by mass, particle size 10 μm; phosphate binder (aluminum dihydrogen phosphate) 13% by mass; moisture 2% by mass; and appropriate amount of additives, including hardening accelerator (magnesium oxide powder) and flowability regulator (starch paste).

[0090] Introducing a large proportion of ultrafine quartz powder into phosphate-bonded zircon sand, with a particle size (10 μm) much smaller than that of zircon sand (0.1-0.3 mm), fills the gaps between zircon sand particles, significantly improving the bulk density and compactness of the molding sand. This coarse-fine particle ratio allows the surface molding sand to maintain the low expansion characteristics of zircon sand (coefficient of thermal expansion 4.5 × 10⁻⁶). -6 At the same time, it has extremely high density, which lays the foundation for the subsequent formation of an impermeable surface layer.

[0091] Existing phosphate-bonded sand molds typically use zircon sand of a single particle size without incorporating ultrafine quartz powder for particle size distribution optimization. This results in insufficient surface density, failing to effectively prevent molten metal penetration. This step significantly improves density through the composite of coarse and fine particles without increasing the amount of binder required.

[0092] Molding sand mixing process: Add zircon sand and ultrafine quartz powder into the sand mixer and dry mix at room temperature for 2-3 minutes to disperse the particles evenly. The speed of the sand mixer is 40-60 rpm. Then add phosphate binder and water, and continue mixing for 5-8 minutes until the binder evenly coats the surface of the sand particles.

[0093] Step S2: Prepare intermediate layer molding sand

[0094] The intermediate layer molding sand was prepared with the following mass fractions: zircon sand 83% by mass, particle size 0.25mm; phosphate binder (aluminum dihydrogen phosphate) 11% by mass; moisture 6% by mass; and appropriate amount of additives, including hardening accelerator (magnesium hydroxide) and flowability regulator (dextrin).

[0095] Sand mixing method: Add zircon sand to the sand mixer, along with phosphate binder and water. Mix the sand at room temperature for 5-8 minutes until homogeneous, with the sand mixer speed at 40-60 rpm. The preferred mixing time is 6 minutes, with the sand mixer speed at 50 rpm.

[0096] Step S3: Prepare backing sand

[0097] The loose backing sand was prepared with the following mass fractions: coarse zircon sand with a particle size of 0.45 mm; phosphate binder (aluminum dihydrogen phosphate) with a mass fraction of 7%; organic pore-forming agent with a mass fraction of 4% and a particle size of 100 μm; moisture with a mass fraction of 7%; and appropriate amount of additives, namely hardening accelerator (calcium oxide powder) and flowability regulator (carboxymethyl cellulose).

[0098] The organic pore-forming agent is selected from one of the following materials:

[0099] Starch types: corn starch, potato starch, tapioca starch, wheat starch, sweet potato starch. Corn starch or potato starch is preferred because their thermal decomposition initiation temperature is stable at 250-260℃, they are widely available, inexpensive, and leave little residue after decomposition.

[0100] Cellulose-based products: microcrystalline cellulose, plant cellulose powder, cellulose fiber powder. Microcrystalline cellulose (such as pharmaceutical-grade microcrystalline cellulose) is preferred because its particle size is controllable (50-150μm), its thermal decomposition initiation temperature is stable at 300-320℃, its purity is high (>98%), and it leaves no metallic impurities after decomposition.

[0101] This step uses corn starch with a particle size of 100μm and a mass fraction of 4%. The thermal decomposition initiation temperature should be in the range of 250-350℃ (stable at drying temperatures of 180-220℃, and rapidly decomposed at casting temperatures >300℃).

[0102] On the one hand, the amount of phosphate binder is reduced (6-8%, significantly lower than the 12-15% of the surface layer and 10-12% of the intermediate layer) and coarse-grained zircon sand (0.3-0.6mm) is used to make the back layer structure loose and the porosity larger, with a higher basic permeability. On the other hand, an organic pore-forming agent (3-5%) is added to the back layer molding sand. This organic material remains stable at room temperature and drying temperature and does not affect the mold preparation process. However, it decomposes and vaporizes rapidly under the action of high pouring temperature (above 300℃), generating microporous channels in situ, realizing the function of "porosity creation during pouring", and increasing the permeability index of the back layer from the original basic permeability index (40-60) to a high permeability index (>80).

[0103] It should be noted that in the foundry industry, the measurement results of "permeability" and "permeability" are usually expressed as "permeability index", which is a dimensionless value. The measurement standard is GB / T 2684 "Determination of permeability of foundry sand".

[0104] Explanation of air permeability index: The air permeability index is a dimensionless value that characterizes the air permeability of molding sand. The higher the value, the better the air permeability. Generally, an air permeability index <10 is considered low air permeability, 10-30 is moderate air permeability, 30-60 is good air permeability, and >60 is high air permeability.

[0105] In existing technologies, the permeability of the mold depends on the porosity of the molding sand itself, and the permeability remains constant before and after pouring, making it impossible to dynamically improve venting efficiency at critical pouring moments. This step, by pre-embedding an organic pore-forming agent, achieves "time-sequential activation" of permeability. The mold maintains sufficient strength during the mold preparation stage, and the permeability of the back layer increases sharply at high pouring temperatures, precisely matching the process requirements.

[0106] Molding sand mixing process: Add coarse zircon sand and organic pore-forming agent into the sand mixer, and dry mix at room temperature for 2-3 minutes to ensure uniform dispersion of the organic matter. The sand mixer speed is 40-60 rpm. Then add binder and water, and continue mixing for 5-8 minutes until uniform. The preferred dry mixing time is 2.5 minutes, the wet mixing time is 6 minutes, and the sand mixer speed is 50 rpm.

[0107] Step S4: Layered molding to form a gradient structure mold

[0108] A layered compact molding method is used to create a mold with gradient functional zones, specifically including:

[0109] 1. Surface molding sand filling

[0110] Place the leaf pattern at the bottom of the sand box, and fill the surface of the pattern with the surface molding sand prepared in step S1, with a thickness controlled at 6mm. After manually smoothing, lightly vibrate to compact the sand. The compaction should be such that the molding sand maintains its shape without significant settling, avoiding over-compaction that could disrupt the uniform dispersion of the ultrafine quartz powder within the zircon sand. The vibration frequency for light compaction is 40Hz, and the vibration time is 8 seconds.

[0111] 2. Intermediate layer molding sand filling

[0112] Fill the top layer of molding sand with the intermediate layer prepared in step S2, with a thickness of 55% of the total mold thickness (usually 30-50 mm depending on the mold size). Compact the intermediate layer using vibration or compaction at a frequency of 50-80 Hz for 20-40 seconds to ensure a tight bond between the intermediate and surface layers, preventing separation. The compaction process should be done in 2-3 layers, each 10-20 mm thick, followed by vibration to compact each layer.

[0113] 3. Backing sand filling

[0114] The back layer molding sand prepared in step S3 is filled outside the intermediate layer molding sand, with a thickness of 18mm, as the outermost layer of the mold. The back layer molding sand is lightly compacted or manually scraped, with a vibration frequency of 30Hz and a vibration time of 5-10 seconds, maintaining a loose and porous structure to provide a smooth path for gas to escape.

[0115] 4. Gradient structure formation

[0116] After the layered molding is completed, the mold exhibits a three-layer gradient structure from the cavity surface outwards:

[0117] Dense surface layer: Contains ultrafine quartz powder, with optimized particle size distribution, dense packing, and low air permeability index (<5), preventing metal liquid penetration.

[0118] Transitional intermediate layer: with conventional proportions and moderate air permeability index (20-40), it bears the strength of the main body of the mold and realizes the performance gradient transition between the surface layer and the back layer.

[0119] Loose backing layer: coarse sand, low binder, containing organic pore-forming agent. After molding, the air permeability index is 40-60. During casting, the air permeability index increases to >80 after the organic pore-forming agent decomposes, achieving efficient air venting.

[0120] This step involves creating functional gradient zones within a single mold by filling it with molding sand of different compositions and properties in layers. This gradient structure resolves the contradiction that traditional homogeneous molds cannot simultaneously address: the surface layer needs to be dense and impermeable, while the back layer needs to be loose and breathable.

[0121] Specifically, the gradient structure achieves the following effects:

[0122] Spatial performance distribution: The gradient distribution of dense surface layer, intermediate transition and loose back layer gives the mold targeted functions in different locations - anti-permeability of cavity surface and efficient air venting of back layer.

[0123] Continuous interface transition: The presence of the intermediate layer avoids abrupt changes in performance between the dense surface layer and the porous back layer, preventing stress concentration and cracking at the interface.

[0124] The ventilation path is constructed as follows: gas flows from inside the cavity → surface layer (micro-permeability) → middle layer (moderate ventilation) → back layer (high ventilation) → external environment, forming a continuous ventilation gradient that ensures both surface quality and efficient air exhaust.

[0125] Existing phosphate-bonded sand casting uses a single-component molding sand for one-time molding, resulting in uniform mold properties but failing to achieve functional zoning. If a dense formula is used, poor overall permeability leads to porosity defects; if a loose formula is used, the surface is rough and sand adhesion is severe.

[0126] This step achieves a gradient structure through layered molding, overcoming the performance limitations of homogeneous molds and simultaneously realizing surface impermeability and back layer high air permeability within the same mold.

[0127] Step S5: CO2 gas hardening

[0128] CO2 gas is introduced into the sand box to harden the mold. The CO2 gas introduction pressure is 0.20 MPa, and the introduction time is 30 seconds, adjusted according to the mold size and thickness. The CO2 gas purity is 99.8%, and the water content is <0.01%. The gas is introduced evenly from the bottom of the sand box to ensure uniform distribution throughout the mold.

[0129] CO2 gas reacts chemically with the phosphate binder. The alkaline components in the phosphate (such as sodium phosphate) react with CO2 to form carbonate precipitates and phosphate gels, which tightly bind the sand particles together. The chemical reaction equation is: Na2HPO4 + CO2 + H2O → NaHCO3 + NaH2PO4. The resulting sodium bicarbonate and sodium dihydrogen phosphate form a bonding network. The hardening process occurs simultaneously in the three-layer structure of the entire mold, with the surface layer, middle layer, and back layer curing at the same time, forming a monolithic mold with sufficient strength.

[0130] After hardening, the mold's compressive strength at room temperature reaches 0.8-1.5 MPa, which is sufficient to withstand the operational loads during subsequent coating, drying, and pouring processes. Under preferred hardening conditions, the mold's compressive strength at room temperature is 1.2 MPa.

[0131] Step S6: Applying silica sol coating

[0132] After the mold hardens, a silica sol coating is prepared with a silica sol solid content of 25-35%. Appropriate amounts of water and additives are added to adjust the viscosity to 15-25 seconds (measured using a Ford cup viscometer, conforming to GB / T 1723 standard, with an outlet diameter of 4 mm). The silica sol is a nano-silica gel solution with nano-silica particles of 10-50 nm in size and a pH of 9-10. Additives include a dispersant (polyethylene glycol, added at 0.5-1% of the silica sol mass) and a thickener (carboxymethyl cellulose, added at 0.2-0.5% of the silica sol mass).

[0133] Preferred solution: The silica sol has a solid content of 30%, the nano silica particles have a particle size of 30nm, and the pH value is 9.5; 0.8% polyethylene glycol is added as a dispersant and 0.3% carboxymethyl cellulose is added as a thickener, and the viscosity is adjusted to 20 seconds (measured using a Forecast 4 cup viscometer, which conforms to GB / T 1723 standard, with an outlet diameter of 4mm).

[0134] Apply silica sol coating evenly to the surface of the mold cavity using brushing or spraying methods. The coating thickness should be controlled at 0.3-0.5 mm (wet film thickness). The coating should evenly cover the entire cavity surface, paying particular attention to the recessed areas and corners of complex blade cavities to ensure no areas are missed. When brushing, use a soft brush and apply 2-3 coats, with 2-5 minutes between each coat. When spraying, use a spray gun with an air pressure of 0.2-0.4 MPa and a spraying distance of 200-300 mm.

[0135] Silica sol is a colloidal solution of nano-silica particles. After being applied to the surface of the mold cavity, due to its extremely low viscosity and nanoscale particles, the silica sol can penetrate into the micropores of the surface molding sand. The nano-silica particles (10-50 nm in diameter) fill the residual tiny gaps between the ultrafine quartz powder (5-15 μm) and zircon sand (0.1-0.3 mm). This "nanoscale filling" further enhances the density of the surface molding sand, laying the foundation for the formation of a continuous closed film during subsequent drying.

[0136] Existing phosphate-bonded sand molds are typically not coated, or are only coated with ordinary refractory coatings (containing coarser particles such as graphite and alumina), which cannot achieve nanoscale pore sealing. This step uses silica sol coating, utilizing the ultrafine particle size and good permeability of nano-silica to achieve deep sealing of the surface layer, which is a key means to improve impermeability.

[0137] The brushing operation method itself is a conventional paint application operation, which is well known to those skilled in the art.

[0138] Step S7: Drying treatment

[0139] After applying the silica sol coating, the mold is placed in a drying oven for drying. The drying temperature is 200℃, and the drying time is 3 hours to ensure the mold is fully dry. The drying oven uses electric heating or gas heating, and the temperature uniformity inside the oven should be controlled within ±5℃. The drying process is divided into a heating stage and a constant temperature stage: the heating stage raises the temperature from room temperature to the set temperature at a rate of 20-40℃ / hour; the constant temperature stage maintains the set temperature for 1.5-3 hours.

[0140] The following key changes occur during the drying process:

[0141] 1. Formation of silica glassy thin films

[0142] As moisture evaporates from the silica sol coating, the nano-silica particles undergo a polymerization and dehydration reaction (condensation between silanol groups Si-OH to form Si-O-Si bonds), forming a 3-5 μm thick continuous glassy silica film on the cavity surface. The polymerization reaction is: Si-OH + HO-Si → Si-O-Si + H2O. This film possesses the following properties:

[0143] Continuous sealing: The silica film continuously covers the surface of the molding sand, sealing the residual micropores between the ultrafine quartz powder and zircon sand, forming a complete anti-permeability barrier.

[0144] High-temperature stability: The silica glassy film remains stable at the casting temperature (1180-1280℃ for copper alloys, 720-780℃ for aluminum alloys, and 1450-1550℃ for nickel-based alloys), without softening or melting, because the melting point of silicon dioxide is about 1710℃, which is much higher than the casting temperature.

[0145] Chemical inertness: Prevents the molten metal from reacting chemically with the molding sand, reducing the tendency for sand to stick.

[0146] 2. The strength of molding sand is further improved.

[0147] With complete removal of moisture from the surface and intermediate molding sand layers, the phosphate binder further solidifies, increasing the mold strength to 1.2-2.0 MPa, meeting the strength requirements of the casting process. Under optimal conditions, the mold strength is 1.6 MPa.

[0148] 3. Organic pore-forming agents are stably retained.

[0149] The organic pore-forming agent in the backing sand remains stable and does not decompose at a drying temperature of 180-220℃. The thermal decomposition initiation temperature of starch is approximately 250℃, and that of cellulose is approximately 300℃, both higher than the drying temperature. Therefore, the organic pore-forming agent remains intact in the backing sand during the drying stage, awaiting activation at the high temperature during casting. During the drying process, the organic pore-forming agent only undergoes dehydration (losing adsorbed and bound water), while its molecular structure remains intact.

[0150] By controlling the drying temperature, different materials can perform their functions at different times: during the drying stage (180-220℃), the silica sol solidifies into a film to seal the surface, while the organic pore-forming agent remains stable; during the casting stage (>300℃), the organic pore-forming agent decomposes to create pores, and the silica film remains stable and prevents seepage. The two materials function in different temperature ranges, with precise timing coordination.

[0151] Step S8: Assembly and Pouring

[0152] Innovation: In-situ pore creation and gradient venting using organic pore-forming agents poured at high temperatures.

[0153] Assemble the upper and lower molds (or multiple mold pieces) into a complete mold cavity. When assembling the mold, align the locating pins to ensure accurate fit between the upper and lower molds, with a gap of <0.5mm. After assembling, secure the mold with clamps or weights to prevent sparking caused by the static pressure of the molten metal during pouring.

[0154] The pouring temperature is determined according to the alloy type: copper alloys 1180-1280℃, aluminum alloys 720-780℃, and nickel-based alloys 1450-1550℃. Before pouring, the molten metal should be degassed and slag removed to ensure its purity. A steady pouring method should be used to control the pouring speed, which should be 0.5-2.0 kg / s (adjusted according to the weight of the casting). The flow of molten metal during pouring should be smooth and continuous, avoiding interruptions and splashing.

[0155] Preferred options: For copper alloy blades (weight 2kg), the pouring temperature is 1230℃, the pouring speed is 1.0kg / second, and the pouring time is approximately 2 seconds; for aluminum alloy blades (weight 1.5kg), the pouring temperature is 750℃, and the pouring speed is 0.8kg / second; for nickel-based alloy blades (weight 2.5kg), the pouring temperature is 1500℃, and the pouring speed is 1.2kg / second.

[0156] The following key processes occur during the pouring process:

[0157] 1. Surface anti-permeability function achieved

[0158] The high-temperature molten metal enters the mold cavity and contacts the surface layer of the mold. The silica glassy film (3-5 μm thick) and the dense layer of ultrafine quartz powder (5-8 mm thick) formed in step S7 work together to prevent the molten metal from penetrating into the molding sand, preventing sand adhesion and ensuring a smooth casting surface. The high-temperature stability of the silica glassy film (silicon dioxide melting point 1710℃) ensures that the structure remains intact at the pouring temperature (<1550℃).

[0159] 2. In-situ pore formation using organic pore-forming agent in the back layer

[0160] This is the core innovation of this step: the organic pore-forming agent in the backing sand rapidly decomposes and vaporizes under the heat of the high-temperature molten metal (backing temperature rises to over 300℃), generating gaseous products such as CO2 and H2O. After the organic matter (starch or cellulose) decomposes, it forms microporous channels with a pore size of 50-200μm in the positions it originally occupied. These micropores are interconnected to form a breathable network.

[0161] The thermal decomposition reaction of starch is: (C6H 10 O5) n → CO2 + H2O + carbon residue, decomposition initiation temperature is 250℃, rapid decomposition temperature is 300-400℃. The thermal decomposition reaction of cellulose is similar, with a decomposition initiation temperature of 300℃. Since the temperature of the molten metal during casting is as high as 720-1550℃, the heat conducted to the back layer is sufficient to rapidly raise the temperature of the back layer to above 300℃, triggering the rapid decomposition of the organic pore-forming agent.

[0162] Technical characteristics of in-situ borehole construction:

[0163] Timing activation: It only occurs when the pouring temperature is high, and no holes are created during the mold preparation and drying stages, ensuring the strength of the mold during operation.

[0164] Precise location: Pore formation occurs only in the back layer and does not affect the performance of the surface and intermediate layers. The surface layer has a slow temperature rise due to the thermal insulation protection of the silica glassy film; the temperature of the intermediate layer is between that of the surface and back layers, which is insufficient to trigger large-scale organic decomposition (because the intermediate layer does not contain organic pore-forming agents).

[0165] Pore ​​size controllable: By controlling the particle size (50-150μm) and addition amount (3-5%) of the organic pore-forming agent, the micropore size (50-200μm) and number can be adjusted.

[0166] 3. Gradient breathable structure for exhaust

[0167] Gas inside the mold cavity is discharged through a gradient ventilation structure:

[0168] From the mold cavity → dense surface layer (micro-permeability, air permeability index <5) → transitional intermediate layer (moderate air permeability, air permeability index 20-40) → loose back layer (high air permeability, air permeability index >80, micropore channels formed by the decomposition of organic pore-forming agents further increase air permeability) → external environment of the mold.

[0169] The entire process activates the "pouring-in-pore-forming" function, ensuring timely and smooth gas discharge and preventing porosity defects inside the casting. The gradient structure guarantees that gas is discharged along the path of least resistance (towards the back layer), rather than penetrating into the surface layer (where density is high and resistance is great). According to Darcy's law of gas flow, the gas flow direction is always along the direction of increasing permeability gradient; therefore, gas in the cavity preferentially discharges towards the back layer.

[0170] 4. Low expansion suppresses thermal stress

[0171] Zircon sand has a low coefficient of thermal expansion (4.5 × 10⁻⁶). -6 This design minimizes mold expansion at high pouring temperatures, resulting in minimal stress on the solidification shrinkage of the blades and preventing hot cracking and deformation. Calculated at a pouring temperature of 1200℃, the linear expansion of zircon sand is only 0.054%, far lower than the 0.168% of quartz sand (quartz sand has a thermal expansion coefficient of 14 × 10⁻⁶). -6 This reduces expansion stress by approximately 70%.

[0172] In existing technologies, the permeability of the mold is static and fixed, with the permeability rate remaining constant before and after pouring, making it impossible to dynamically improve venting efficiency at critical pouring moments. This step achieves "real-time dynamic enhancement" of back layer permeability through the high-temperature decomposition of the organic pore-forming agent. At the pouring moment when venting is most needed, the back layer permeability index surges from the normal value to a high permeability index (>80), precisely matching the process requirements.

[0173] The pouring operation itself is a routine casting operation, which is well known to those skilled in the art.

[0174] Step S9: Solidification and Demolding

[0175] After the molten metal is poured, the casting solidifies and cools in the mold. During solidification, the gradient venting structure continuously plays a role in venting, and the gas released during the shrinkage of the molten metal in the later stages of solidification can still be smoothly discharged through the back layer. The solidification time is determined according to the alloy type and the casting wall thickness, and is generally 20-60 minutes.

[0176] Preferred method: For marine engine blades with a wall thickness of 5-10mm, the solidification time is 30 minutes for copper alloys, 25 minutes for aluminum alloys, and 40 minutes for nickel-based alloys. Natural cooling is used during solidification, without forced cooling, to avoid thermal stress and deformation caused by excessively rapid cooling.

[0177] Once the casting has cooled to below 100-150℃, remove it from the mold. The demolding temperature is determined as follows: 120℃ for copper alloys, 100℃ for aluminum alloys, and 150℃ for nickel-based alloys. Excessively high demolding temperatures will result in insufficient casting strength and easy deformation, while excessively low temperatures will prolong the production cycle.

[0178] Due to the presence of a silica glassy film on the surface, the casting exhibits minimal adhesion to the molding sand, facilitating easy demolding. Demolding is achieved by vibration or gentle tapping to separate the casting from the mold, with a vibration frequency of 30-50 Hz and a vibration time of 10-20 seconds. After removing residual molding sand from the surface, the casting is then purged with compressed air (0.4-0.6 MPa) or cleaned with a wire brush, resulting in a smooth surface, dense internal structure, and absence of hot cracks in the marine engine blade casting.

[0179] Preferred solution: Demolding is performed using vibration at a frequency of 40Hz for 15 seconds; surface cleaning is performed using compressed air blowing at a pressure of 0.5MPa.

[0180] Solidification and demolding are routine casting operations, well known to those skilled in the art.

[0181] Experimental verification

[0182] Experiment 1: Dynamic Change Test of Backing Sand Permeability

[0183] 1. Experimental Objective

[0184] The study verified the in-situ pore-forming function of organic pore-forming agent in the backing sand under high casting temperature, tested the dynamic change process of backing permeability with increasing temperature, and proved the effectiveness of the time-series activation mechanism of "pouring as pore-forming".

[0185] 2. Preparation of experimental samples

[0186] Two sets of backfill sand samples were prepared according to the formula in step S3:

[0187] Comparative sample A (containing organic pore-forming agent): 82% coarse zircon sand, 7% phosphate binder (aluminum dihydrogen phosphate), 4% organic pore-forming agent (starch, particle size 100μm), and 7% moisture.

[0188] Comparative sample B (without organic pore-forming agent): 86% coarse zircon sand, 7% phosphate binder (aluminum dihydrogen phosphate), and 7% moisture.

[0189] After the two sets of molding sand were mixed evenly, standard samples (cylindrical blocks with a diameter of 50 mm and a thickness of 50 mm) were prepared. CO2 gas was introduced (pressure 0.20 MPa, time 60 seconds) for hardening. After hardening, the samples were dried in a drying oven at 200℃ for 3 hours.

[0190] 3. Experimental conditions

[0191] Testing equipment: Air permeability tester (model GF-II, measurement range 0-200 air permeability index).

[0192] Test environment: The test was conducted in a temperature-controlled heating furnace with a temperature range of room temperature to 500°C. The heating rate was 10°C / minute, and the air permeability was tested after holding the temperature for 10 minutes after each 50°C increase.

[0193] Test gas: compressed air, test pressure: 0.1 MPa.

[0194] 4. Experimental Procedure

[0195] (1) Place the dried control sample A and control sample B into the test chamber of the air permeability tester and test the initial air permeability at room temperature (25℃) and record the data.

[0196] (2) Start the heating furnace and heat it to 100°C at a rate of 10°C / minute. After holding the temperature for 10 minutes, test the air permeability and record the data.

[0197] (3) Continue to raise the temperature to 150℃, 200℃, 250℃, 300℃, 350℃, 400℃, 450℃ and 500℃. After holding each temperature point for 10 minutes, test the air permeability and record the data.

[0198] (4) Repeat the test 3 times at each temperature point and take the average value as the air permeability at that temperature.

[0199] (5) Plot the curve of air permeability as a function of temperature, and compare and analyze the air permeability variation of the two groups of samples.

[0200] 5. Experimental Results

[0201] Table 1. Data on the air permeability of the backing sand as a function of temperature.

[0202]

[0203] Note: Breathability increase = (Breathability of Comparison Sample A - Breathability of Comparison Sample B) / Breathability of Comparison Sample B × 100%.

[0204] Figure 1 The curve shows the air permeability of the backing sand as a function of temperature.

[0205] 6. Analysis and Summary

[0206] The experimental results show that:

[0207] (1) Significant temperature threshold effect: In the temperature range of 25-250℃, the permeability of the control sample A slowly increased from 48.2 to 53.6, with an increase of only 11.2%, indicating that the organic pore-forming agent remained stable at the drying temperature (200℃) and did not decompose, thus maintaining the strength and structural integrity of the mold. When the temperature exceeded 250℃ (the starting temperature of starch decomposition), the permeability began to rise rapidly.

[0208] (2) Significant in-situ pore-forming effect: When the temperature rises from 250℃ to 300℃, the air permeability of the control sample A surges from 53.6% to 68.4%, an increase of 27.6%, indicating that the organic pore-forming agent begins to decompose and vaporize rapidly, generating microporous channels in situ in the backing sand. When the temperature continues to rise to 350℃, the air permeability reaches 85.7%, exceeding the high air permeability standard (>80). When the temperature reaches above 400℃, the air permeability stabilizes in the range of 92-95%, indicating that the organic pore-forming agent has basically decomposed completely, and the pore-forming effect reaches its maximum.

[0209] (3) Significant differences between the control samples: The permeability of control sample B, which does not contain organic pore-forming agent, remained basically constant (46.8-49.0) throughout the temperature range, and only increased slightly due to the slight expansion and cracking of molding sand at high temperature. This proves that the significant increase in permeability is entirely due to the decomposition and pore-forming effect of organic pore-forming agent.

[0210] (4) Verification of actual process compatibility: The temperature of the molten metal during casting is 720-1550℃. The heat conducted to the back layer is sufficient to rapidly raise the temperature of the back layer to above 300℃, triggering the rapid decomposition of the organic pore-forming agent and realizing the activation of the "pouring as pore-forming" function. Experimental data shows that at the critical moment of casting, the air permeability index of the back layer surges from the basic value (about 50) to the high air permeability index (>85), with the air permeability index increasing by more than 70%, which is highly consistent with the design target in the technical solution (from 40-60 to >80).

[0211] Experiment 2: Comparative Test of the Penetration Depth of Molten Metal into the Mold Surface

[0212] 1. Experimental Objective

[0213] The effectiveness of the dual anti-permeability barrier of the ultrafine quartz powder dense layer and the silica sol glassy film was verified. By comparing the metal liquid penetration depth of different casting surface structures, the significant anti-permeability effect of the surface layer of the present invention was demonstrated.

[0214] 2. Preparation of experimental samples

[0215] Three standard mold specimens (100mm×100mm×50mm) with different surface treatments were prepared for casting comparison tests:

[0216] Comparison Sample 1 (Traditional Clay Sand Mold): A traditional clay sand formula was used, consisting of 90% clay sand, 8% bentonite, and 2% moisture. After mixing, the mixture was molded and allowed to air dry for 24 hours. No special surface treatment was applied.

[0217] Comparative Sample 2 (Phosphate Zircon Sand Mold): A single zircon sand formulation was used, consisting of 85% zircon sand (particle size 0.2-0.4 mm), 11% phosphate binder, and 4% moisture. After mixing, the mixture was molded, CO2 hardened (pressure 0.20 MPa, time 60 seconds), and dried at 200℃ for 3 hours. No coating was applied to the surface.

[0218] Comparative Sample 3 (gradient structure type of this invention): Prepared according to the complete process of steps S1 to S7. Surface molding sand formula: 68% zircon sand, 17% ultrafine quartz powder, 13% phosphate binder, 2% moisture; Intermediate layer molding sand formula: 83% zircon sand, 11% phosphate binder, 6% moisture. Layered molding, CO2 hardening (pressure 0.20 MPa, time 60 seconds), application of silica sol coating (solid content 30%, wet film thickness 0.4 mm), drying at 200℃ for 3 hours.

[0219] 3. Experimental conditions

[0220] Casting metal: copper alloy (Cu-10Sn), casting temperature 1230℃.

[0221] Pouring environment: room temperature 25℃, relative humidity 50%.

[0222] Testing equipment: metallurgical microscope, scanning electron microscope.

[0223] 4. Experimental Procedure

[0224] (1) Preheat the three types of casting test blocks to 100°C at room temperature and keep them warm for 30 minutes to remove residual moisture.

[0225] (2) Melt the copper alloy to 1230°C and perform degassing and slag removal treatment to ensure the purity of the molten metal.

[0226] (3) Pour the molten metal into the cavities of the three types of casting test blocks respectively. The pouring speed is controlled at 1.0 kg / second. After pouring, let it stand and cool for 30 minutes.

[0227] (4) Demold the casting when it cools to below 120°C and observe the sand adhering to the surface of the casting.

[0228] (5) Use wire cutting equipment to cut a cross-sectional sample (20mm×20mm) perpendicular to the pouring surface at the center of the casting. The cut surface is then ground and polished.

[0229] (6) Observe the metal-mold interface morphology using a metallographic microscope and measure the penetration depth of the molten metal. Measure the penetration depth at 10 different locations on the cross-section of each sample and take the average value.

[0230] (7) Use a scanning electron microscope (SEM) to observe the surface microstructure of the control sample 3 at high magnification and take SEM images.

[0231] (8) Use image analysis software to measure the surface roughness (Ra value) of the casting. Measure 5 locations for each sample and take the average value.

[0232] 5. Experimental Results

[0233] Table 2 Comparison of surface parameters for different molds

[0234]

[0235] Note: The penetration depth of the molten metal is the average value ± standard deviation of 10 measurement points; the surface roughness Ra is the average value ± standard deviation of 5 measurement points.

[0236] Figure 2 A bar chart comparing surface parameters of different casting molds.

[0237] Figure 3 The SEM microstructure of sample 3 is shown for comparison.

[0238] 6. Analysis and Summary

[0239] The experimental results show that:

[0240] (1) The dual anti-permeability barrier is highly effective: The metal liquid penetration depth of the comparative sample 3 of this invention is only 12±3μm, which is 96.3% lower than that of the traditional clay sand type (320±45μm) and 93.5% lower than that of the phosphate zircon sand type (185±28μm). This fully demonstrates the high efficiency of the dual anti-permeability barrier composed of the dense layer of ultrafine quartz powder and the glassy film of silica sol.

[0241] (2) Significantly improved surface quality: The surface roughness Ra value of the comparison sample 3 is only 3.2±0.5μm, reaching the level of precision casting (Ra<5μm), which is 88.8% lower than that of traditional clay sand mold. There is basically no sand adhering on the surface of the casting, the surface cleaning time is shortened from 45 minutes to 5 minutes, and the machining allowance is reduced from 3.5mm to 0.8mm, which greatly reduces the subsequent processing cost.

[0242] (3) Microstructure verification mechanism: SEM micromorphological observation showed that there was a clearly visible silica glassy thin film layer on the surface of the control sample 3. The film continuously covered the dense surface sand and had a thickness of about 3-5 μm, which was completely consistent with the technical description in step S7.

[0243] (4) Verification of the correctness of the technical route: Although the comparative sample 2 used low-expansion zircon sand, the metal liquid still penetrated to a depth of 185μm because the surface was not densified with ultrafine quartz powder and no silica sol coating was applied. This proves the necessity and correctness of the dual anti-permeability method of "particle-graded densification + nano-sealing" in this invention, and that a single measure cannot achieve the ideal effect.

[0244] (5) Significant engineering application value: The significant improvement in surface quality directly leads to the simplification of subsequent processing procedures and the reduction of machining allowance. The machining allowance is reduced from 3.5mm to 0.8mm, the material utilization rate is increased by 77%, and the processing efficiency is significantly improved. At the same time, the absence of sand adhering defects on the surface avoids surface damage caused by improper cleaning, ensuring the surface integrity and performance of the blades.

[0245] Experiment 3: Comparative Test of Mold Expansion Rate and Casting Crack Tendency

[0246] 1. Experimental Objective

[0247] The effectiveness of the zircon sand low-expansion system in suppressing thermal stress and preventing hot cracking was verified by testing the high-temperature expansion rate of different aggregate molds and the crack incidence rate of the castings after pouring, demonstrating the significant effect of the low expansion of this invention.

[0248] 2. Preparation of experimental samples

[0249] Two different aggregate molds were prepared for expansion rate testing and comparative experiments on the casting of marine turbine blades:

[0250] Comparative Group A (quartz sand mold): Quartz sand (particle size 0.2-0.4mm) was used as aggregate, with a formula of 85% quartz sand, 11% phosphate binder, and 4% moisture. After mixing, the mixture was molded, hardened with CO2 (pressure 0.20MPa, time 60 seconds), and dried at 200℃ for 3 hours.

[0251] Comparative Group B (Zircon Sand Mold): Zircon sand (particle size 0.2-0.4mm) was used as aggregate, with a formula of 85% zircon sand, 11% phosphate binder, and 4% moisture. After mixing, the mixture was molded, hardened with CO2 (pressure 0.20MPa, time 60 seconds), and dried at 200℃ for 3 hours.

[0252] For the blade casting test, standard marine engine blade patterns (150mm in length, 80mm in maximum width, and 8mm in wall thickness) were used to make two sets of 30 molds each.

[0253] 3. Experimental conditions

[0254] Expansion rate testing equipment: High temperature dilatometer (model DIL402C), with a measurement range from room temperature to 1400℃ and a measurement accuracy of ±0.1μm.

[0255] Test environment: Argon protective atmosphere, heating rate 10℃ / minute.

[0256] Casting metal: copper alloy (Cu-10Sn-2Zn), casting temperature 1230℃.

[0257] Crack detection methods: visual inspection combined with penetrant testing (PT), magnetic particle testing (MT), and X-ray testing (RT).

[0258] 4. Experimental Procedure

[0259] (a) Test procedure for mold expansion rate

[0260] (1) Cut standard specimens (size: cylindrical specimens with a diameter of 6 mm and a length of 25 mm) from the molds of control group A and control group B, and prepare 5 specimens for each group.

[0261] (2) Place the sample into the test chamber of the high temperature dilatator and heat it from room temperature to 1200℃ (simulating casting temperature) at a rate of 10℃ / min under the protection of argon.

[0262] (3) Record the length change of the sample at different temperatures and calculate the linear expansion rate. Linear expansion rate = (L-L0) / L0×100%, where L0 is the sample length at room temperature and L is the sample length at high temperature.

[0263] (4) Record the expansion rate data at key temperature points (200℃, 400℃, 600℃, 800℃, 1000℃, 1200℃). Test each group of samples 3 times and take the average value.

[0264] (5) Plot the expansion rate-temperature curve and compare and analyze the expansion characteristics of the two aggregate casting molds.

[0265] (II) Test Procedure for Crack Tendency of Blade Castings

[0266] (1) Thirty molds from both control group A and control group B were used to conduct a casting test of ship engine blades.

[0267] (2) Melt the copper alloy to 1230℃, degas and remove slag, and control the casting speed at 1.0 kg / second.

[0268] (3) After pouring, let it stand and cool for 30 minutes. When the temperature of the casting drops below 120℃, demold it.

[0269] (4) Visually inspect all castings and record the number and location of visible cracks on the surface.

[0270] (5) Perform penetrant testing (PT) on castings that do not show cracks visually to detect surface micro-cracks.

[0271] (6) Perform X-ray flaw detection (RT) on key parts (blade root, thin-walled transition zone) to detect internal cracks.

[0272] (7) Calculate the crack occurrence rate, average crack length and crack distribution location for each group of castings.

[0273] (8) Measure the key dimensions of the casting (blade length, width, thickness), calculate the dimensional deviation, and evaluate the impact of mold expansion on dimensional accuracy.

[0274] 5. Experimental Results

[0275] Table 3. Comparison of High-Temperature Expansion Rates of Different Aggregate Molds

[0276]

[0277] Note: The linear expansion rate data is the average of 5 samples; the decrease in expansion rate = (expansion rate of control group A - expansion rate of control group B) / expansion rate of control group A × 100%.

[0278] Table 4. Statistical data on cracks in blades cast using different aggregate molds.

[0279]

[0280] Note: Crack length is the average crack length of the casting that has cracked; dimensional deviation is the difference between the actual size and the design size, with a positive value indicating that the actual size is larger than the design size.

[0281] Figure 4 This is a curve showing the change in the linear expansion rate of the mold with temperature.

[0282] Figure 5 A comparison of crack incidence rate and pass rate;

[0283] Figure 6 Distribution of crack numbers at different locations;

[0284] Figure 7 Comparison of crack distribution and dimensional deviation.

[0285] 6. Analysis and Summary

[0286] The experimental results show that:

[0287] (1) Significant verification of low expansion effect: The linear expansion rate of zircon sand mold at a pouring temperature of 1200℃ is only 0.054%, which is 67.9% lower than that of quartz sand mold (0.168%). The experimentally measured coefficient of thermal expansion is in high agreement with the theoretical value: quartz sand 14×10 -6 ℃, zircon sand 4.5×10 -6 The temperature was ℃, which proved the correctness of the technical route of using zircon sand as the aggregate for the whole casting mold in steps S1 to S3.

[0288] (2) The tendency for hot cracking was significantly reduced: the cracking rate of blades cast in zircon sand molds was only 3.3% (only 1 out of 30 pieces had a crack), which was 87.6% lower than that of blades cast in quartz sand molds (26.7% (8 out of 30 pieces had cracks). The pass rate increased from 73.3% to 96.7%. This directly proves the inhibitory effect of low-expansion molds on thermal stress and effectively avoids the generation of hot cracks.

[0289] (3) Analysis of crack distribution: Of the 16 cracks generated by the quartz sand mold, 6 were at the blade root, 5 in the thin-walled transition zone, 3 in the blade tip region, and 2 in the middle of the blade back, mainly concentrated in stress concentration areas. This is because the quartz sand mold expands significantly at high temperatures, applying uneven stress to the solidifying and shrinking blade, resulting in stress concentration at the root and transition zone where the wall thickness changes drastically, leading to crack initiation. The zircon sand mold only generated one surface microcrack (8.5 mm in length) at the blade root. Penetrant testing revealed no internal cracks, proving that thermal stress was effectively controlled.

[0290] (4) Significantly improved dimensional accuracy: The dimensional deviation of the blades cast by zircon sand mold was greatly reduced, with the length deviation decreasing from +2.8 mm to +0.4 mm and the thickness deviation decreasing from +0.9 mm to +0.1 mm, resulting in an improvement of more than 85% in dimensional accuracy. This is because the low-expansion mold exerts less constraint on the casting, allowing the casting to shrink freely according to the design dimensions, thus reducing dimensional deviations caused by mold expansion.

[0291] (5) Theoretical calculations and experiments agree: According to the thermal expansion formula, the theoretical linear expansion of pure quartz sand at 1200℃ is 14×10⁻⁶. -6 ×1200=0.0168=1.68%. However, the mold is not made of pure quartz sand; it also contains phosphate binder (11%), moisture (4%), and internal pores. These components inhibit expansion. The actual measured overall linear expansion rate of the mold is 0.168%, approximately 10% of the theoretical value for pure materials, which is consistent with the expansion characteristics of porous composite materials. Similarly, the theoretical linear expansion of pure zircon sand is 4.5 × 10⁻⁶. -6 ×1200=0.0054=0.54%, and the actual measured value of 0.054% in the mold is about 10% of the theoretical value, which verifies the reliability and consistency of the experimental data.

[0292] (6) Engineering Application Verification: This experiment used 30 actual blade casting tests for statistical analysis. The sample size was sufficient, and the results were statistically significant. The zircon sand casting mold qualification rate was 96.7%, meeting the requirements of industrial production, which proved the stability and reliability of the low expansion technology of this invention in mass production.

[0293] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A casting process for marine engine blades, characterized in that, Includes the following steps: Prepare surface molding sand, intermediate molding sand, and backing molding sand. The surface molding sand contains ultrafine quartz powder and zircon sand. The mass fractions of each component in the surface molding sand are: 65-70% zircon sand, 15-20% ultrafine quartz powder, 12-15% phosphate binder, and the remainder is water and additives. The particle size of the zircon sand is 0.1-0.3 mm, and the particle size of the ultrafine quartz powder is 5-15 μm. The intermediate layer molding sand has the following composition by mass fraction: 80-85% zircon sand, 10-12% phosphate binder, and the remainder being water and additives; the zircon sand has a particle size of 0.15-0.4 mm. The backing sand contains an organic pore-forming agent; the mass fraction of each component of the backing sand is: zircon sand 80-85%, phosphate binder 6-8%, organic pore-forming agent 3-5%, and the remainder is water and additives; the particle size of the zircon sand in the backing sand is 0.3-0.6 mm, and the organic pore-forming agent is selected from starch and cellulose powder; A gradient structure mold is formed by sequentially filling the surface layer, intermediate layer, and back layer molding sand with a layered molding sand using a layered molding method. CO2 gas is introduced to harden the mold; a silica sol coating is applied to the surface of the mold cavity, wherein the silica sol in the silica sol coating is a nano silica sol solution with a particle size of 10-50 nm. The drying process causes the silica sol to form a silica glassy film while retaining the organic pore-forming agent in the back layer molding sand. The silica glassy film is a continuous film of 3-5 μm thickness formed on the surface of the cavity after the nano-silica particles undergo a polymerization and dehydration reaction. After the mold is assembled, molten metal is poured in. The high temperature of the pouring causes the organic pore-forming agent in the back layer molding sand to decompose and form air-permeable channels.

2. The casting process for marine engine blades according to claim 1, characterized in that, The surface layer molding sand has a filling thickness of 5-8 mm, the intermediate layer molding sand has a filling thickness of 50-60% of the total mold thickness, and the back layer molding sand has a filling thickness of 15-20 mm.

3. The casting process for marine engine blades according to claim 1, characterized in that, The CO2 gas is introduced at a pressure of 0.15-0.25 MPa and the gas introduction time is 30-90 seconds.

4. The casting process for marine engine blades according to claim 1, characterized in that, The silica sol coating has a silica sol solid content of 25-35% and a wet film thickness of 0.3-0.5 mm after application.

5. The casting process for marine engine blades according to claim 1, characterized in that, The drying process is carried out at a temperature of 180-220℃ for 2-4 hours.

6. The casting process for marine engine blades according to claim 1, characterized in that, The molten metal being poured is a copper alloy, an aluminum alloy, or a nickel-based alloy; the pouring temperature for copper alloys is 1180-1280℃, for aluminum alloys it is 720-780℃, and for nickel-based alloys it is 1450-1550℃.

Images