Gradient exhaust sand mold for aluminum alloy thin-wall part and preparation method of gradient exhaust sand mold

Abstract

The invention relates to the technical field of sand mold casting, and particularly discloses a gradient exhaust sand mold for an aluminum alloy thin-walled workpiece and a preparation method thereof.The sand mold comprises a cavity surface layer (with the hardness ranging from 90 HB to 95 HB), a transition layer (with the hardness ranging from 80 HB to 70 HB and a back sand layer (with the hardness ranging from 60 HB to 65 HB) which are arranged from inside to outside, and microporous ceramic exhaust rods (with the diameter ranging from 0.3 mm to 0.8 mm and the distance ranging from 1 mm to 3 mm) are embedded in the cavity surface layer; the exhaust rod is composed of a mullite fiber matrix and a nanometer gamma-AlO coating, the porosity is 40-50%, and the exhaust rod penetrates through the back sand layer to form a directional exhaust network. The preparation method comprises the following steps: mixing sand layer by layer (the cavity layer contains 1.0-1.5% of phenolic resin, and the back sand layer contains 0.4-0.6% of resin and 4-6% of starch); when sand is filled to the position 1-3 mm away from the surface of the cavity, an exhaust rod is implanted (the positioning precision is + / -0.1 mm); gradient compaction is carried out (the high pressure of a cavity is 3.0-4.0 MPa, the vibration of a transition area is 20-30 Hz, and the back sand airflow is 0.1-0.2 MPa); and carrying out microwave curing (2.5 to 3.5 kW, 60 to 120 s). The defects of hot cracks and air holes of the thin-wall part (smaller than or equal to 5 mm) are overcome synchronously, and the casting yield is increased to 95% or above.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sand casting, and particularly discloses a composite exhaust sand mold structure and manufacturing process for thermal cracks and gas hole defects of aluminum alloy thin-walled parts (wall thickness <= 5 mm). BACKGROUND

[0002] In the field of sand casting of aluminum alloy thin-walled parts (wall thickness <= 5 mm), the coordinated control of thermal cracks and gas hole defects is a long-standing technical bottleneck. Such defects are caused by the double characteristics of the solidification process of aluminum alloy:

[0003] 1. The solidification shrinkage rate is as high as 6%, and the rapid cooling of the thin-walled area leads to insufficient feeding in the thick-walled area, causing shrinkage stress concentration;

[0004] 2. The hydrogen solubility sharply changes (0.65 cc / 100 g in liquid state to 0.034 cc / 100 g in solid state), and micro air traps (AirTrap) form gas holes or pinholes.

[0005] The current technical solution faces the following core contradictions:

[0006] Conflict between exhaust requirement and structural strength - although the traditional exhaust duct (diameter > 3 mm) can macroscopically exhaust, it cannot cover the micro air trap, and reducing the exhaust hole diameter will significantly reduce the sand mold resistance to metal liquid erosion;

[0007] Conflict between stress buffering and mold rigidity - although a uniformly high-hardness sand mold (> 90 HB) can resist deformation, it exacerbates shrinkage stress concentration and leads to thermal cracks; and although reducing the overall hardness can alleviate stress, it is difficult to meet the dimensional accuracy requirements of thin-walled parts.

[0008] The existing technical route has the following significant limitations:

[0009] 1. Homogeneous exhaust structure (such as uniformly arranged exhaust needles): cannot direct the air mass, and large-size exhaust holes are easy to be invaded and blocked by metal liquid;

[0010] 2. Single-hardness sand mold: high compactness of the sand mold leads to poor collapsibility and low sand cleaning efficiency (about 25 min per piece); and low-hardness sand mold causes sanding defects due to insufficient strength;

[0011] 3. Composite material modification (such as adding a collapsibility agent): although the collapsibility of the back sand layer is improved, the problem of the coordinated control of micro-exhaust on the mold cavity surface and gradient stress release is not solved.

[0012] The mainstream improvement direction in the industry also has inherent defects:

[0013] Physical vibration exhaust requires complex equipment and is easy to cause cold separation of thin-walled parts;

[0014] Chemical exhaust agents (such as CaCO3 decomposition) produce uncontrollable gas and are prone to form secondary blowholes;

[0015] Uniform microporous sand molds cannot be targeted to strengthen exhaust in hot spot areas (temperature gradient > 50℃ / mm) due to consistent air permeability throughout the entire area.

[0016] Therefore, there is an urgent need to develop a sand mold structure and process that has micro directional exhaust, gradient stress buffering, and partitioning functions to simultaneously solve the hot cracking and blowhole defects of thin-walled aluminum alloy parts, while improving sand cleaning efficiency and casting yield. SUMMARY

[0017] To solve the above-mentioned prior art problems, the present application provides a gradient exhaust sand mold for aluminum alloy thin-walled parts and a preparation method thereof.

[0018] The present application includes the following technical solutions:

[0019] A gradient exhaust sand mold includes a mold cavity surface layer with a hardness of 90-95, a transition layer with a hardness gradient of 80→70, and a back sand layer with a hardness of 60-65. The mold cavity surface layer is embedded with microporous ceramic exhaust rods, the exhaust rods have a diameter of 0.3–0.8mm, a spacing of 1–3mm, and extend through to the back sand layer. The above-mentioned scheme buffers shrinkage stress concentration through three-layer gradient hardness design, and combines with the through-type microporous exhaust rod to direct gas flow, thereby simultaneously reducing the hot cracking risk and blowhole defects of thin-walled parts (in some embodiments, hot cracking is reduced by 83%, and blowhole grade is improved by 2-3 levels).

[0020] Further, the microporous ceramic exhaust rod of the above-mentioned gradient exhaust sand mold is composed of a mullite fiber matrix and a surface nano γ-Al2O3 coating, the coating has a thickness of 20μm, a porosity of 40–50%, and a gas permeability ≥200cm 3 / (min·cm 2 ), and a compressive strength ≥15MPa. The nano coating in the above-mentioned scheme enhances surface erosion resistance, the high porosity of the mullite matrix realizes efficient escape of micro gas (in some embodiments, the gas permeability ≥200cm 3 / (min·cm 2 ), and the compressive strength ≥15MPa ensures the stability of the mold cavity (temperature resistance >1600℃)

[0021] Further, the mold cavity surface layer of the above-mentioned gradient exhaust sand mold is composed of silica sand and 1.0–1.5wt% phenolic resin; and the back sand layer is composed of silica sand, 0.4–0.6wt% resin, and 4–6wt% starch. In the above-mentioned scheme, the high resin content (1.0-1.5%) of the surface layer resists metal liquid erosion, and the addition of starch (4-6%) in the back layer promotes collapse (in some embodiments, the sand cleaning time is shortened by 68% to 8min), and reduces organic volatilization by 30%.

[0022] Further, the gradient venting sand mold described above, the distribution density of the microporous ceramic venting rod on the surface of the mold cavity is 8-10 rods / cm 2 , and the density in the area where the temperature gradient is >50℃ / mm in the CAE hot spot simulation of the casting is ≥10 rods / cm 2 . In the above scheme, the directional encryption layout based on the CAE hot spot analysis strengthens the venting in the high stress area (>50℃ / mm area density ≥10 rods / cm 2 ), and in some embodiments, the hot spot crack length is reduced by 84%. The present application also discloses a preparation method of the above-mentioned gradient venting sand mold, comprising the following steps:

[0023] S1. Sand mixing: respectively prepare the mold cavity layer sand material: silica sand + 1.0-1.5% phenolic resin and the back sand layer sand material: silica sand + 0.4-0.6% resin + 4-6% starch;

[0024] S2. After layering and filling sand, implant the microporous ceramic venting rod when filling sand to a depth of 1-3 mm from the surface of the mold cavity, with a positioning accuracy of ±0.1 mm.

[0025] In the above scheme, the layered sand mixing ensures the functionality of the components; the accurate implantation timing (1-3 mm from the surface) prevents displacement of the venting rod, and the ±0.1 mm accuracy adapts to complex cavities.

[0026] Further, in the above preparation method, the venting rod is automatically embedded by a mechanical hand in step S2, and the implantation depth penetrates to the back sand layer. In the above scheme, the mechanical hand embedding ensures that the penetrating venting channel reaches the back sand layer, avoiding gas retention, and in some embodiments, the pore density is reduced by 85%.

[0027] Further, the above preparation method further comprises a S3 gradient compaction step:

[0028] The mold cavity area is compacted at a high pressure of 3.0-4.0 MPa;

[0029] The transition area is compacted by vibration at an amplitude of 1-3 mm and a frequency of 20-30 Hz;

[0030] The back sand area is pre-compacted by airflow at a pressure of 0.1-0.2 MPa. In the above scheme, the zoned compaction process matches the functional requirements of each layer: high pressure ensures surface density, vibration optimizes stress transition, and airflow pre-compaction maintains back layer permeability.

[0031] Further, the above preparation method further comprises a S4 microwave curing step: microwave power 2.5-3.5 kW, time 60-120 s, and temperature difference during curing process ≤10℃. In the above scheme, uniform microwave curing (temperature difference ≤10℃) avoids local overburning, the strength difference is ≤2.1 MPa, and the gradient hardness is accurately realized (qualification rate 100%).

[0032] Further, the preparation method described above, the exhaust rod layout in step S2 is automatically generated based on the hot spot analysis of the three-dimensional model of the casting, and the area density of the temperature gradient > 50℃ / mm is ≥10 roots / cm 2 In the above scheme, the CAE intelligent driving layout is used, and the exhaust network in the high stress area is dynamically encrypted, and in some embodiments, the number of hot spots is reduced to ≤2 places / piece.

[0033] The application also discloses application of the gradient exhaust sand mold in casting of aluminum alloy thin-walled parts with a wall thickness ≤5mm, including but not limited to AlSi7Mg alloy castings.

[0034] Compared with the prior art, the application has the following outstanding beneficial effects:

[0035] 1. Dual defect synergistic control:

[0036] Micro-porous ceramic exhaust rod (air permeability ≥200cm 3 / (min·cm 2 )) forms a micro directional exhaust channel, eliminates the air trap generated by the hydrogen solubility mutation, and reduces the X-ray porosity level from 4 to 1-2, and the porosity density is reduced by 85%;

[0037] The three-layer gradient hardness structure buffers the shrinkage stress, the number of hot spots is reduced from 12 places / piece to ≤2 places / piece, the residual stress is reduced by 55%, and the stress concentration caused by insufficient feeding in the thin-walled area is solved.

[0038] 2. The casting efficiency is significantly improved:

[0039] The back sand layer adds 4-6% starch, and the collapse strength is reduced to 0.8MPa, and the sand cleaning time is shortened by 68%(from 25min to 8min);

[0040] Microwave curing (temperature difference ≤10℃) ensures that the hardness gradient is accurately realized, the sand mold strength range is ≤2.1MPa, and the qualified rate is 100%.

[0041] 3. Intelligent process advantages:

[0042] Based on the CAE hot spot simulation (temperature gradient > 50℃ / mm area), the exhaust rod layout (≥10 roots / cm 2 ) is automatically encrypted, and the crack length in the hot spot area is reduced by 84%;

[0043] Mechanical hand precise implantation (±0.1mm) combined with layered compaction process, adapts to complex thin-walled part cavities, and the yield is improved by 27%(68%→95%).

[0044] 4. Material and environmental protection benefits:

[0045] Mullite-based exhaust rod temperature resistance >1600℃, no softening, service life increased by 3 times;

[0046] Starch replaces part of the resin, reduces organic volatile emissions by 30%. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The cross-sectional view of the gradient exhaust sand mold according to the present application is shown in the figure;

[0048] Figure 2 The three-dimensional structure diagram of the gradient exhaust sand mold according to the present application is shown in the figure;

[0049] Figure 3 The SEM image of the microporous ceramic exhaust rod of the gradient exhaust sand mold according to the present application is shown in the figure. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0051] Embodiment 1

[0052] As shown in the figure, the gradient exhaust sand mold is prepared Figures 1-2

[0053] 1、Sand mold structure

[0054] Cavity surface layer: silica sand + 1.2wt% phenolic resin (hardness 93HB)

[0055] Transition layer: silica sand + 0.8wt% resin (hardness 80→70 gradient)

[0056] Back sand layer: silica sand + 0.5wt% resin + 5wt% starch (hardness 63HB)

[0057] Microporous ceramic exhaust rod:

[0058] Material: mullite fiber matrix + nano γ-Al2O3 coating (20μm thick)

[0059] Parameters: diameter 0.5mm, spacing 2mm, porosity 45%, air permeability 250cm 3 / (min·cm 2 )

[0060] Layout: based on CAE thermal section simulation (temperature gradient >50℃ / mm area encrypted to 10 roots / cm 2 , the rest of the area 8 roots / cm 2 )​

[0061] 2. Preparation process

[0062] S1. Mixing sand: The sand for the cavity layer / back sand layer was prepared according to the above proportions respectively;

[0063] S2. Layered sand filling and exhaust rod implantation:

[0064] When the sand filling reaches a depth of 2 mm from the surface of the cavity, the mechanical hand implants the exhaust rod (positioning accuracy ± 0.1 mm) and penetrates to the back sand layer;

[0065] S3. Gradient compaction:

[0066] Cavity area: high-pressure compaction 3.5 MPa;

[0067] Transition area: vibration compaction (amplitude 2 mm, frequency 25 Hz);

[0068] Back sand area: airflow pre-compaction 0.15 MPa;

[0069] S4. Microwave curing: power 3.0 kW, time 90 s, temperature difference ≤ 8°C.

[0070] As Figure 3 Figure 2 is a cross-sectional electron microscope SEM image of the microporous ceramic exhaust rod.

[0071] Example 2

[0072] Preparation of gradient exhaust sand mold

[0073] 1. Sand mold structure

[0074] Cavity surface layer: silica sand + 1.0 wt% phenolic resin (hardness 90 HB)

[0075] Back sand layer: silica sand + 0.4 wt% resin + 4 wt% starch (hardness 60 HB)

[0076] Microporous ceramic exhaust rod: diameter 0.3 mm, spacing 3 mm, porosity 40%

[0077] Layout: temperature gradient > 50°C / mm area density 9 rods / cm 2

[0078] 2. Preparation process

[0079] S2. Exhaust rod implantation depth: 3 mm from the surface of the cavity;

[0080] S3. Gradient compaction:

[0081] Cavity area high-pressure compaction 3.0 MPa

[0082] Back sand area airflow pre-compaction 0.1 MPa (transition layer vibration omitted);

[0083] S4 Microwave curing: power 2.5 kW, time 120 s, temperature difference < 10 °C.

[0084] Example 3

[0085] Gradient vented sand mold preparation

[0086] 1. Sand mold structure

[0087] Cavity surface layer: silica sand + 1.5 wt% phenol formaldehyde resin (hardness 95 HB)

[0088] Microporous ceramic venting rod: diameter 0.8 mm, pitch 1 mm, porosity 50% Layout: temperature gradient > 50 °C / mm Area density 12 rods / cm 2

[0089] 2. Preparation process

[0090] S2 Venting rod implantation depth: 1 mm from the cavity surface;

[0091] S3 Gradient compaction: high pressure compaction in the cavity zone 4.0 MPa;

[0092] S4 Microwave curing: power 3.5 kW, time 60 s, temperature difference < 5 °C.

[0093] Comparative Example 1

[0094] No gradient hardness layer

[0095] Sand mold structure: uniform hardness 90 HB throughout the domain (silica sand + 1.2% resin)

[0096] Venting rod: same as Example 1 (diameter 0.5 mm @ 2 mm pitch)

[0097] Preparation process: high pressure compaction throughout the domain 3.5 MPa, microwave curing same as Example 1.

[0098] Comparative Example 2

[0099] No microporous ceramic venting rod

[0100] Sand mold structure: same as Example 1 with three layers of gradient hardness

[0101] Venting structure: traditional macroscopic venting channels (diameter 3 mm, pitch 10 mm)

[0102] Preparation process: S2 venting rod implantation is omitted, the rest is the same as Example 1.

[0103] Comparative Example 3

[0104] No CAE layout optimization

[0105] Sand mold structure: same as Example 1

[0106] Venting rod layout: evenly distributed 8 rods / cm 2 (no temperature gradient)

[0107] Preparation process: same as Example 1.

[0108] Comparative Example 4

[0109] No starch added to the backing sand layer

[0110] Sand mold structure: the backing sand layer is silica sand + 0.5% resin (hardness 80 HB)

[0111] The rest of the structure and process are the same as Example 1.

[0112] Test Example 1

[0113] Gradient venting sand mold hot crack inhibition ability verification

[0114] Objective: Quantify the buffering effect of the gradient hardness layer on shrinkage stress

[0115] Method:

[0116] 1. Sample preparation:

[0117] A step block with a wall thickness of 3.5 mm was cast using AlSi7Mg alloy (the angle of the thick-thin transition zone is 60°)

[0118] Comparison sand mold:

[0119] Test group: Example 1 sand mold (three layers of gradient hardness)

[0120] Control group: Comparative Example 1 sand mold (global hardness 90 HB)

[0121] 2. Hot crack detection:

[0122] Industrial CT scanning (accuracy 0.1 mm) to locate cracks, and the number of cracks per unit length (place / cm) is counted.

[0123] Residual stress test: X-ray diffraction method (XRD, sin 2 ψ method) to measure the surface stress of the thick-thin transition zone.

[0124] The results are shown in Table 1 below

[0125] Table 1: Gradient venting sand mold hot crack inhibition ability verification

[0126] Group Hot cracking density (place / cm) Residual stress (MPa) Example 1 0.3±0.1 68±5 Comparative Example 1 2.1±0.3* 152±11*

[0127] (*P <0.01 vs Example 1)

[0128] Conclusion: The gradient hardness layer reduces the risk of hot cracking by 85%, and the residual stress decreases by 55%, effectively buffering the shrinkage stress concentration in the thick-thin transition zone.

[0129] Test Example 2

[0130] Efficiency of micro-porous venting system degassing test

[0131] Objective: To verify the ability of micro-porous ceramic rod to eliminate micro- gas traps

[0132] Method:

[0133] 1. Pore quantification:

[0134] Casting 50x50x3mm sheet test pieces (AlSi7Mg, hydrogen content 0.25cc / 100g)

[0135] Sand mold comparison:

[0136] Test group: Example 1 (micro-porous ceramic rod @ 10 rods / cm 2 )

[0137] Control group: Comparative Example 2 (traditional vent spacing)

[0138] 2. Detection technique:

[0139] X-ray pore rating (ASTM E505 Level 1-4, Level 1 optimal)

[0140] Metallographic statistics: per mm 2 Pore number (pore size > 0.1mm)

[0141] The results are shown in Table 2 below:

[0142] Table 2: Efficiency of micro-porous venting system degassing test

[0143] Group X-ray porosity grade Pore density (number / mm 2 )]> Example 1 1-2 grade 0.8±0.2 Comparative Example 2 4 grade 5.3±0.7*

[0144] (*P < 0.01 vs Example 1)

[0145] Conclusion: The micro-porous ceramic rod reduces the pore density by 85% and directs the hydrogen bubbles to escape to the back sand layer.

[0146] Test Example 3

[0147] Necessity verification of CAE hot spot optimized venting

[0148] Objective: To prove the value of vent rod encryption layout based on temperature gradient

[0149] Method:

[0150] 1. Hot spot simulation:

[0151] Design a plate-shaped casting with a hot spot boss (diameter 20mm), and ProCAST simulation confirms that the temperature gradient is >80℃ / mm

[0152] Sand contrast:

[0153] Test group: Example 1 (hot spot density 12 strands / cm 2 )

[0154] Control group: Comparative Example 3 (uniform density 8 strands / cm 2 )

[0155] 2. Defect location:

[0156] Fluorescent penetrant testing (PT) marked hot spot cracks, and the total length of cracks (mm) was counted. The results are shown in Table 3.

[0157] Table 3: CAE hot spot optimization exhaust necessity verification

[0158] Group Hot cracking length (mm) Example 1 1.2±0.3 Comparative Example 3 7.5±1.1*

[0159] (*P <0.01 vs Example 1)

[0160] Conclusion: Hot spot exhaust density increased by 50%, reducing local crack length by 84%.

[0161] Test Example 4

[0162] Quantitative evaluation of back sand layer collapsibility

[0163] Objective: To analyze the effect of starch addition on sand cleaning efficiency

[0164] Method:

[0165] 1. Sand cleaning test:

[0166] Casting motor housing (same as Example 1), after curing, using the same air blast equipment (pressure 0.6 MPa)

[0167] Sand contrast:

[0168] Test group: Example 1 (back sand layer containing 5% starch)

[0169] Control group: Comparative Example 4 (back sand layer without starch)

[0170] 2. Efficiency index:

[0171] Complete sand cleaning time (s)

[0172] Residual sand strength: The breaking strength of the back sand layer (MPa) was measured by a universal testing machine.

[0173] Results are shown in Table 4

[0174] Table 4: Quantitative evaluation of back sand layer collapsibility

[0175] Group Sand cleaning time (s) Residual sand strength (MPa) Example 1 480±15 0.8±0.1 Comparative Example 4 1080±30* 2.5±0.3*

[0176] (*P < 0.01 vs Example 1)

[0177] Conclusion: Starch addition reduces back sand strength by 68% and improves sand cleaning efficiency by 125%.

[0178] Test Example 5

[0179] Microwave curing uniformity verification

[0180] Objective: To confirm the effect of temperature difference control on the consistency of sand mold strength

[0181] Method:

[0182] Temperature difference comparison:

[0183] Test group: Example 1 (microwave curing temperature difference ≤ 8°C)

[0184] Control group: Conventional electric heating curing (temperature difference > 30°C)

[0185] Strength test:

[0186] Take 10 x 10 mm samples from different areas of the sand mold (cavity / transition / back sand layer)

[0187] Universal testing machine measures compressive strength (MPa), calculates regional strength range

[0188] Results are shown in Table 5

[0189] Table 5: Microwave curing uniformity verification

[0190] Group Strength range (MPa) Back sand layer collapse qualification rate Example 1 2.1±0.3 100％ Control group 8.7±1.2* 62％*

[0191] (*P < 0.01 vs Example 1)

[0192] Conclusion: Microwave-assisted curing reduces strength fluctuation by 76%, ensuring accurate implementation of gradient hardness.

[0193] Test example summary as shown in Table 6

[0194] Table 6: Test example summary

[0195]

[0196] It is worth noting that the above embodiment is focused on interpreting the technical solutions of the present application, rather than precisely defining its protection scope. Those skilled in the art should understand that based on the technical details disclosed in the embodiments of the present application, appropriate adjustments and optimizations can be made, or equivalent substitutions can be made to individual or even all technical elements. Such adjustment and substitution measures will not deviate from the core essence of the technical solutions of the present application, and should be included in the technical protection scope of the embodiments of the present application. In short, the protection scope of the present application should not be subject to the concrete presentation of the above embodiments, but should widely cover all equivalent changes and improvements that do not deviate from its basic idea. In summary, the protection scope of the present application should be based on the statement of the claims, and the above embodiments are only used as a reference guide to understand the present application.

Claims

1. A gradient vented sand mold, characterized by: The cavity surface layer comprises, from inside to outside, a cavity surface layer with a hardness of 90-95, a transition layer with a hardness gradient of 80→70, and a backing layer with a hardness of 60-65; the cavity surface layer is embedded with microporous ceramic exhaust rods, the exhaust rods have a diameter of 0.3-0.8 mm, a spacing of 1-3 mm, and extend through to the backing layer.

2. The gradient vented sand mold of claim 1, wherein: The microporous ceramic exhaust rods are composed of a mullite fiber matrix and a surface nano-gamma-Al2O3 coating, the coating has a thickness of 20 μm, a porosity of 40-50%, a gas permeability of ≥200 cm³ / (min·cm²), and a compressive strength of ≥15 MPa.

3. The gradient vented sand mold of claim 1, wherein: The cavity surface layer is composed of silica sand and 1.0-1.5 wt% phenolic resin; the backing layer is composed of silica sand, 0.4-0.6 wt% resin, and 4-6 wt% starch.

4. The gradient vented sand mold of claim 1, wherein: The distribution density of the microporous ceramic exhaust rods on the cavity surface is 8-10 roots / cm², and the density in the area with a temperature gradient of >50 ℃ / mm in the CAE thermal section simulation of the casting is ≥10 roots / cm².

5. The method of making a gradient vented sand mold according to any one of claims 1-4, wherein, The method comprises the following steps: S1. Mixing sand: respectively prepare cavity layer sand: silica sand + 1.0-1.5% phenolic resin and backing sand: silica sand + 0.4-0.6% resin + 4-6% starch; S2. After layering and filling sand, implant microporous ceramic exhaust rods when filling sand to a depth of 1-3 mm from the cavity surface, with a positioning accuracy of ±0.1 mm.

6. The method of claim 5, wherein: In step S2, the exhaust rods are automatically embedded by a mechanical hand, and the implantation depth penetrates through to the backing layer.

7. The method of claim 5, wherein: It further comprises a S3 gradient compaction step: The cavity area is compacted at a high pressure of 3.0-4.0 MPa; The transition area is compacted by vibration compaction at an amplitude of 1-3 mm and a frequency of 20-30 Hz; The backing area is pre-compacted by airflow at a pressure of 0.1-0.2 MPa.

8. The method of claim 5, wherein: It further comprises a S4 microwave curing step: microwave power of 2.5-3.5 kW, time of 60-120 s, and a temperature difference of ≤10 ℃ during the curing process.

9. The method of claim 5, wherein: In step S2, the exhaust rod layout is automatically generated based on the thermal section analysis of the three-dimensional model of the casting, and the density in the area with a temperature gradient of >50 ℃ / mm is ≥10 roots / cm².

10. Application of the gradient exhaust sand mold of any one of claims 1-4 in the casting of an aluminum alloy thin-walled part with a wall thickness of ≤5 mm, including but not limited to an AlSi7Mg alloy casting.

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