Large size aluminum alloy shell metal outer mold composite sand core low pressure casting device and method

Abstract

This invention belongs to the field of low-pressure casting technology, specifically a low-pressure casting device and method for large-size aluminum alloy shell metal outer mold composite sand core. The large-size aluminum alloy shell casting has a height of not less than 600mm, an inner diameter of not less than 700mm, and an average wall thickness of 14~20mm. The low-pressure casting device includes a mold system, a sand core system, a gating system, a chill system, and risers. The gating system adopts a bottom pouring + stepped pouring method. The structure of the gating system is designed according to the height, lateral characteristic dimensions, and average wall thickness of the casting. The gating system is deeply optimized, and a design of multiple sprues and circumferentially distributed horizontal runners is adopted to ensure that the feeding channel solidifies later than the casting, the holding pressure remains effective, and casting defects such as cold shuts, porosity, shrinkage cavities, and shrinkage porosity are avoided. This significantly improves the mechanical properties and airtightness of large-size castings and increases cost efficiency.

Description

Technical Field

[0001] This invention belongs to the field of low-pressure casting technology for large-size aluminum alloy shell structural components, specifically relating to a low-pressure casting device and method for large-size aluminum alloy shell metal outer mold composite sand core. Background Technology

[0002] Large-sized, complex, thin-walled aluminum alloy shell structures are widely used in aerospace, shipbuilding, and power equipment industries due to their lightweight, high strength and toughness, and excellent corrosion resistance. For example, key components such as large aluminum alloy hulls, sections, hydraulic valve bodies, and large tanks for power transmission and transformation not only need to meet the requirements of complex internal cavity structures but also must possess excellent mechanical properties and airtightness to ensure long-term reliability. Traditionally, these large-sized, complex, thin-walled aluminum alloy castings are produced using sand casting low-pressure or sand casting gravity casting methods. Sand casting allows for flexible design of complex internal cavities, gating systems, and venting channels through sand cores; however, the poor thermal conductivity of sand molds leads to slow solidification, coarse grains, and defects such as shrinkage porosity and gas bubbles, affecting the density and mechanical properties of the casting. In addition, for large-sized castings, the production cycle of integral sand molds is long and the material consumption is large. Especially when using 3D printing to make the entire sand mold of large-sized castings, the cost increases significantly, the printing time is long, and the overall strength of the sand mold is low, making it easy to deform or crack during the pouring process, which makes it difficult to meet the needs of large-scale production.

[0003] While metal mold casting (hard mold casting) accelerates cooling, refines grain structure, and improves the strength and density of castings through the high thermal conductivity of the metal outer mold, it suffers from high mold manufacturing costs, long production cycles, and difficulty in adapting to design changes related to complex internal cavity structures. Particularly for large castings, the manufacturing and adjustment of the integral metal mold is extremely inconvenient, and the optimization space for the gating and riser system is limited, easily leading to problems such as incomplete filling or localized overheating, thus affecting casting quality.

[0004] Large-sized aluminum alloy hollow shells are typical high-difficulty parts in casting production due to their thin walls and large cavities. They have large height and lateral distances, and the flow of molten metal is extremely long. If the gating system is not designed properly, it is very easy for it to solidify prematurely due to excessive temperature drop, resulting in incomplete filling or cold shuts. This can lead to a series of defects such as porosity, shrinkage, deformation, and inclusions, with scrap rates as high as 30-50% in severe cases. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, the present invention aims to provide a low-pressure casting device and method for large-size aluminum alloy shell parts using a metal outer mold and a composite sand core. Specifically, for large-size aluminum alloy shell machine parts, a composite casting process combining a metal outer mold and an inner cavity sand core is designed. This process utilizes the high thermal conductivity of the metal outer mold to accelerate casting solidification, while simultaneously employing a sand core to form a complex internal cavity structure, and optimizing the design of the gating and venting system. This method leverages the flexibility of the sand core in structural design and the rapid cooling advantage of the metal mold, improving the density and mechanical properties of the casting while reducing production costs and increasing production efficiency.

[0006] The complete technical solution of this invention includes:

[0007] A low-pressure casting device for a large-size aluminum alloy shell metal outer mold composite sand core, wherein the large-size aluminum alloy shell includes a base located below, a boss located above, and reinforcing ribs located on the inner wall of the large-size aluminum alloy shell. The height of the large-size aluminum alloy shell is not less than 600mm, the inner diameter is not less than 700mm, and the average wall thickness is 14-20mm.

[0008] The large-size aluminum alloy shell metal outer mold composite sand core low-pressure casting device includes a mold, a gating system, a cooling system, and risers. The mold includes a metal outer mold and an internal sand core. The gating system adopts a bottom pouring + stepped pouring method. The aluminum liquid rises under the action of gas pressure, enters the gate through the liquid riser channel, and enters the sprue from the gate. The sprue includes multiple steps in the longitudinal height direction. Each step is designed with a sprue module including a certain number of horizontal runners. The sprue enters the casting cavity through the horizontal runner modules located on the steps at different longitudinal heights to complete the filling.

[0009] Furthermore, the gating system is designed based on the height, lateral feature dimensions, and average wall thickness of the casting.

[0010] Furthermore, the specific design method of the gating system is as follows:

[0011] (1)

[0012] (2)

[0013] (3)

[0014] In the formula, For the number of direct pouring channels, This is the lateral characteristic dimension. For castings with a circular cross-section, its value is the maximum diameter D (in mm). For castings with a non-circular cross-section, ... S is the maximum cross-sectional area; The average wall thickness of the casting. This is the direct casting coefficient, with a value ranging from 0.35 to 0.5. This refers to the number of steps in a stepped pouring system, i.e., the number of horizontal runner modules at different heights. The height of the casting is in mm. This is the step coefficient, with a value ranging from 0.05 to 0.11. The number of horizontal runners on a single step. The variable value for the horizontal runner is 0.024-0.045. During the calculation process, it is based on the shape parameters of the casting and the corresponding coefficients. The calculation result can be rounded up to ensure accuracy. , , All values ​​are positive integers, which gives the corresponding number of straight runners, stepped runners, and horizontal runners.

[0015] Furthermore, the reinforcing ribs and bosses are areas where the cross-section changes abruptly during the filling process. The wall thickness of the reinforcing ribs is 30-35mm, and the wall thickness of the bosses is 50-55mm.

[0016] Furthermore, the metal outer mold includes a bottom mold and side molds. The bottom mold is connected to the lower platform through a connector and is used to support the sand core. The side molds have a four-opening structure.

[0017] Furthermore, the top of the side mold mates with the top of the sand core to seal the molten aluminum.

[0018] Furthermore, the side mold is provided with a side mold heating rod, which is parallel to the mold cavity.

[0019] Furthermore, the internal sand core is located inside the space enclosed by the side mold. The shape of the internal sand core is designed according to the internal cavity structure of the product and is provided with weight reduction holes. The gating system and risers are both set in the sand core. The internal sand core is formed by 3D printing.

[0020] Furthermore, the cooling system includes reinforcing rib chills and boss chills, which are respectively disposed at the reinforcing ribs and bosses in the inner cavity of the housing, with a thickness of 20mm-30mm. The reinforcing rib chills and boss chills are made of aluminum alloy.

[0021] Furthermore, in the large-size aluminum alloy shell metal outer mold composite sand core low-pressure casting device, during the filling stage, the control system dynamically adjusts the filling pressure rise rate based on the real-time cross-sectional area change during the aluminum liquid rise process.

[0022] Furthermore, the design method for a low-pressure casting gating system for large-size aluminum alloy shells involves designing the structure of the gating system based on the height, lateral characteristic dimensions, and average wall thickness of the casting.

[0023] Furthermore, the method for low-pressure casting of large-size aluminum alloy shell metal outer mold composite sand core using the system includes the following steps:

[0024] 1) Liquid rising stage: Driven by compressed air, the aluminum liquid in the holding furnace rises through the riser pipe until it completely fills the gate area;

[0025] 2) Filling stage: The compressed air pressure is continuously increased to push the molten aluminum through the mold's gating system in sequence. The filling pressure is dynamically adjusted according to the liquid level to complete the cavity filling. First, it passes through each sprue, then enters each gating module from low to high, filling the cavity of the large-size aluminum alloy shell casting, and finally reaches the top riser and venting position.

[0026] 3) Crystallization pressurization stage: After the cavity is completely filled, the control system further pressurizes the casting based on the existing compressed air pressure to form a solidified shell.

[0027] 4) Crystallization and pressure holding stage: Maintain the compressed air pressure of the crystallization and pressure increasing stage. Under the continuous compressed air pressure, the aluminum liquid in each part of the cavity completes the sequential solidification until the entire casting is completely solidified.

[0028] 5) Decompression stage: The compressed gas in the holding furnace is gradually released through the exhaust valve. The aluminum alloy, which is still in a liquid state in the gating channel and riser pipe, falls back into the holding furnace under the action of gravity, completing the low-pressure casting cycle of a large-size aluminum alloy shell.

[0029] The beneficial effects of this invention are as follows:

[0030] 1. Firstly, by employing a composite structure of a metal outer mold and a sand core cavity, the high thermal conductivity of the metal mold (refining grains and improving density) and the flexibility of the sand core structure (forming complex internal cavities), along with optimization of the gating system, significantly improve the mechanical properties and airtightness of large-size hollow castings, achieving a breakthrough in quality performance, enhanced process adaptability, and providing advantages in production efficiency and cost. Specifically, this includes:

[0031] 2. The gating and riser system is optimized for thin-walled, large-cavity cylindrical parts with diameters >700mm. A bottom-pouring + stepped pouring method is adopted, combining the advantages of low-pressure casting's bottom-pouring and stepped pouring. Each step is designed with a certain number of horizontal runners to ensure that the feeding channels solidify later than the casting, maintaining effective pressure. Based on actual production processes, a general gating system design method for thin-walled, large-cavity castings is proposed. For the first time, the transverse dimensional characteristics of the casting are incorporated into the design to solve the problem of uneven filling in large-span cavities. A multi-sprue + circumferentially distributed horizontal runner design reduces casting defects such as porosity, shrinkage, deformation, and inclusions, improving the casting yield. Furthermore, risers with venting holes are installed at the ends of independent bosses to simultaneously address the challenges of feeding and deep cavity venting.

[0032] 3. Mold Structure Innovation: A side mold top sealing design, combined with a sand core, replaces the traditional top mold, eliminating the need for upper templates / main hydraulic cylinders and other equipment modules. The modular side mold design reduces material consumption and is suitable for controlling radial deformation of large-size shells. Furthermore, the embedded heating rod in the side mold (25-30mm from the cavity) achieves precise temperature control, solving the problems of uneven heating and high gas consumption in the baking mold holder, and reducing inter-mold heating time by more than 50%.

[0033] 4. Innovative design for sand core 3D printing: While meeting strength requirements, the sand core is designed with weight reduction holes (avoiding the gating and riser system), which shortens the printing time by more than 30% compared to conventional processes and reduces the transfer load; and chilled iron positioning grooves and venting channels can be flexibly configured inside to reduce subsequent assembly processes.

[0034] 5. Cold iron synergistic sequential solidification control: conformal cold irons are pre-placed at the reinforcing ribs / boobs to form a bidirectional chilling zone with the outer metal mold, forcing directional solidification from the cold iron zone to the ingate, eliminating shrinkage defects. Furthermore, aluminum alloy cast cold irons that match the thermal expansion coefficient of the casting are used to avoid the interface gap affecting the chilling effect.

[0035] 6. During the filling stage, a dynamic filling method is adopted, which adjusts the filling pressure and speed in real time based on the real-time cross-sectional area change during the rise of the aluminum liquid. This avoids the problems of drastic changes in the flow velocity of aluminum liquid during the filling process of large-sized construction castings with complex internal cavity shapes and abrupt changes in cross-sectional area, which can lead to air entrapment, porosity, oxide inclusions, and cold shuts. Attached Figure Description

[0036] Figure 1 This is an overall schematic diagram of the low-pressure casting device provided by the present invention.

[0037] Figure 2 for Figure 1 A sectional view.

[0038] Figure 3 This is a front view of the gating system.

[0039] Figure 4 This is a top view of the gating system.

[0040] Figure 5 Diagram of chiller and exhaust system.

[0041] In the figure, 1-shell casting, 2-base, 3-reinforcing rib, 4-boss, 5-bottom mold, 6-side mold, 7-side mold heating rod, 8-sand core positioning plate, 9-sand core, 10-sand core weight reduction hole, 11-sprue system, 12-gland system, 13-first sprue, 14-second sprue, 15-third sprue, 16-fourth sprue, 17-first gland module, 18-second gland module, 19-third gland module, 20-fourth gland module, 21-bottom riser, 22-top riser, 23-vent hole, 24-reinforcing rib chill, 25-boss chill. Detailed Implementation

[0042] The present invention will now be described in detail with reference to embodiments and accompanying drawings. However, it should be understood that the embodiments and drawings are for illustrative purposes only and do not constitute any limitation on the scope of protection of the present invention. All reasonable modifications and combinations included within the inventive spirit of the present invention fall within the scope of protection of the present invention.

[0043] like Figures 1-5 As shown, in the structure of the large-size aluminum alloy shell metal outer mold composite sand core low-pressure casting device disclosed in this invention, the aluminum alloy shell casting 1 includes a base 2 located below, a reinforcing rib 3 located on the inner wall of the shell, and a boss 4 located on the top of the shell. The overall height of the shell casting is 600-750mm, the diameter is 700-850mm, and the shell wall thickness is 12-20mm. The reinforcing rib and the boss are the parts where the cross-section changes abruptly during the filling process. The wall thickness at the reinforcing rib is 30-35mm, and the maximum wall thickness at the boss is 50-55mm.

[0044] The castings involved in this invention are large hollow castings with large internal spaces and relatively thin walls. The aluminum liquid has a long flow path, which makes it very easy for problems such as excessively rapid cooling and delayed filling to occur. In addition, aluminum alloys are prone to oxidation, and their fluidity is significantly affected by the wall thickness. The inventor's prior patent application CN111673072B discloses a wheel forming device and method based on central pressurization of multiple liquid risers. The device uses multiple liquid risers and gates for filling to solve the problems of long filling distance and difficulty in forming and feeding of aluminum alloy wheel castings. However, the aluminum alloy shell casting involved in this invention has a larger overall size and a more complex internal structure, and cannot be formed using an all-metal mold structure. It requires the formation of the internal cavity composite structure through a sand core. Therefore, the previously applied-for casting method cannot be used. Thus, this invention adopts a bottom-pouring + stepped casting method. The molten aluminum enters the sprue from the bottom gate. The sprue includes multiple steps in the longitudinal height direction. Each step is designed with a sprue module including a certain number of horizontal runners. The molten aluminum enters the casting cavity through the horizontal runner modules on the steps at different heights to complete the filling. This casting system is set inside a 3D-printed sand core and is integrally formed during the sand core printing process.

[0045] The gating system for the casting process is designed based on the shape characteristics of the casting. A reasonable number and connection method of straight / horizontal runners are selected to achieve proper mold filling. For the first time, the transverse dimensional characteristics of the casting are incorporated into the design to address the problem of uneven filling in large-span cavities. Specifically:

[0046] (1)

[0047] (2)

[0048] (3)

[0049] In the formula, For the number of direct pouring channels, This is the lateral characteristic dimension. For castings with a circular cross-section, its value is the maximum diameter D (in mm). For castings with a non-circular cross-section, ... S is the maximum cross-sectional area; The average wall thickness of the casting is taken as 12-20 mm in this invention. The direct sprue coefficient ranges from 0.35 to 0.5, and is preferably 0.47 in the aluminum alloy castings used in this embodiment. This refers to the number of steps in a stepped pouring system, i.e., the number of horizontal runner modules at different heights. The height of the casting (mm) is taken as 600-750 in this invention. The step coefficient ranges from 0.05 to 0.11, and is preferably 0.098 in the aluminum alloy castings used in this embodiment. The number of horizontal runners on a single step. The variable value is the horizontal runner coefficient, ranging from 0.024 to 0.045. In the aluminum alloy casting used in this embodiment, a value of 0.042 is preferred. During the calculation process, calculations are performed based on the shape parameters of the casting and the corresponding coefficients. The calculation results can be rounded up to ensure accuracy. , , All values ​​are positive integers, which gives the corresponding number of straight runners, stepped runners, and horizontal runners.

[0050] In the design process, the above parameters, considering the casting height, lateral feature dimensions, and wall thickness, were used to rationally design the number of sprues, steps, and single-step runners. In particular, the influence of lateral dimensional features was considered for the first time. The design concept is that the larger the lateral dimension, the more sprues are needed to shorten the lateral flow radius of the molten aluminum. Simultaneously, increasing the number of runners increases the number of ingates, preventing insufficient filling and cold shuts that can easily occur during the pouring of large hollow castings due to excessively long filling paths causing a drop in molten aluminum temperature. Furthermore, the higher the casting height, the more critical layered filling becomes to avoid lag in filling the upper cavity; the thinner the casting wall thickness, the greater the flow resistance and the faster the cooling rate, requiring dispersed supply channels and increased step density to shorten the time difference between upper and lower filling layers, as well as increasing runners to shorten local filling distances.

[0051] When designing a casting system using the above principles, it is also necessary to impose certain boundary condition constraints based on the actual spatial layout and the strength requirements of the sand core to avoid extreme situations. Specifically:

[0052]

[0053] The maximum distance between the centers (or central axes) of adjacent sprues is limited to prevent the molten aluminum in some sprues from cooling too quickly.

[0054]

[0055] This represents the height difference between adjacent steps.

[0056]

[0057] The maximum coverage of a single horizontal runner is limited to ensure that the filling of each part of the cavity is synchronized, so as to avoid material shortage at the far end.

[0058] Based on the above design principles and constraints, and based on the measured structural parameters of the shell casting of this invention, ( =621mm, =812mm, =18.6mm), the low-pressure casting mold and gating system were designed. The resulting sprue system includes four roughly evenly distributed sprues, with four steps in the height direction, that is, four sets of horizontal sprue modules of different heights. Each horizontal sprue module has eight horizontal sprues. Because the casting of the present invention is a relatively regular circle in the horizontal direction, each sprue is designed to connect two horizontal sprues on each step. Based on the feeding effect, the diameter of the sprues and horizontal sprues is designed to be 80mm-100mm.

[0059] Based on the above casting system, the mold structure of the present invention is obtained. The mold structure includes a bottom mold 5, a side mold 6, and a sand core 9. The bottom mold 5 is connected to the lower platform of the low-pressure casting device through bolts and other connecting parts, and is used to support the sand core.

[0060] The number of side molds 6 depends on the product shape design. In most cases, they can be designed as 2-opening or 4-opening side molds. In this embodiment, since the radial dimension of the casting shell is large, it is designed as a 4-opening mold, which is beneficial to control the mold temperature and save the outer mold material. The top of the side mold matches the top of the sand core to seal the molten metal. In this case, there is no need for a top mold, and the corresponding casting device does not need an upper template, main oil cylinder and related support structure.

[0061] The side mold 6 is equipped with a side mold heating rod 7, which is positioned parallel to the mold cavity. A through hole, 25mm-30mm from the mold cavity, is machined to hold the heating rod 7, with the hole diameter matching the heating rod. After the mold and low-pressure casting device are installed, the heating rod is activated to heat the mold, overcoming the shortcomings of traditional mold heating racks, such as long heating time, high gas consumption, and uneven heating. Simultaneously, the mold can be heated from the time it opens until the next mold closing, avoiding situations where prolonged intermediate operation time causes mold cooling, affecting the quality of the next casting or requiring reheating of the mold, thus delaying production.

[0062] It also includes a sand core positioning plate 8, which is connected to the bottom mold 5 by bolts and is designed to cooperate with the sand core 9 for sand core positioning.

[0063] The sand core 9 is located inside the space enclosed by the side mold 6. The shape of the sand core is designed according to the internal cavity structure of the product, and the gating system, venting system and riser are also set in the sand core.

[0064] The sand core 9 is designed with sand core weight reduction holes 10. The present invention uses 3D printing technology to prepare sand cores. Under the premise of meeting the strength of sand cores, the casting, venting and venting systems can be avoided. The sand core weight reduction holes are designed to shorten the 3D printing cycle and reduce the weight of sand cores, which facilitates transportation and subsequent process operations.

[0065] The gating system structure specifically includes a sprue system 11 and a runner system 12. The sprue system includes a first sprue 13, a second sprue 14, a third sprue 15, and a fourth sprue 16 arranged in a ring. The runner system includes a first runner module 17, a second runner module 18, a third runner module 19, and a fourth runner module 20 arranged sequentially from bottom to top. Each runner module has the following horizontal structure: every two runners connect to one sprue, and each runner module includes a total of eight runners.

[0066] Furthermore, since the outer wall structure of the shell parts in this invention is relatively simple and is formed by a metal mold, risers are not designed on the metal mold. Moreover, most of the bosses inside the shell are connected to the runner, allowing for good feeding during solidification. Therefore, in this invention, only for independent structures that protrude a significant distance from the inner surface of the shell, namely the shell base 2 and the boss 4 at the top of the shell, are bottom risers 21 and top risers 22 respectively. The top riser 22 has vent holes 23, which can provide feeding for the solidification of the metal at the far end of the structure and avoid the difficulty of venting during filling of deep cavity structures.

[0067] The chill system of this invention includes reinforcing chills 24 and boss chills 25. Chills with a thickness of 20mm-30mm are arranged at the reinforcing ribs and bosses located between the ingates in the shell cavity. Before casting, these chills are fixed to the sand core with an adhesive. Chill grooves are designed at corresponding positions on the sand core, ensuring that the placement of the chills does not affect the formation of the shell cavity structure. When molten aluminum fills to this position, the chills and the opposite metal outer mold form the first solidified area, which then solidifies sequentially towards each ingate, providing good feeding for the product body. The system determines whether the shell part, under the aforementioned gating and riser system, metal outer mold, and sand core mold cavity forming conditions, solidifies first at a distance from the intersection of the runner and the shell, or whether there is an isolated liquid phase area during solidification. If not, a chill is placed at this position. The contact surface between the chills and molten aluminum is designed to conform to the casting structure, with a thickness of approximately 20mm, and is made of aluminum alloy, obtained through a pouring method. During sand core preparation, the chills are placed in the pre-reserved grooves on the sand core using an adhesive. Ultimately, the casting solidifies sequentially from the chill or a position far from the sprue towards the sprue, thus obtaining a casting with the required internal quality.

[0068] The low-pressure casting process for the shell casting of this invention is as follows:

[0069] 1) Liquid rising stage: Driven by compressed air, the liquid aluminum alloy in the holding furnace rises slowly through the riser pipe until it completely fills the gate of the mold; specifically, the pressure is increased to 120mbar-150mbar in 10s-15s, so that the liquid aluminum material rises through the riser pipe to the bottom gate of the mold.

[0070] 2) Filling Stage: The system continuously increases the air pressure, pushing the molten aluminum through the mold's gating system in sequence. Based on the direct proportional relationship between pressure and liquid level, the molten aluminum fills the cavity in a predetermined order: first through each sprue, then from low to high into the horizontal runner branches, filling the shell and its internal structure, and finally reaching the top riser and venting position; the filling pressure increases to 350mbar-400mbar during the filling process to ensure the molten aluminum fills the mold cavity completely.

[0071] In the low-pressure casting process, a fixed pressure increase rate is usually used for filling. However, the internal cavity shape of the large-sized component casting of this invention is complex, with multiple bosses, concave sections, and other places where the cross-sectional area changes abruptly. Moreover, the wall thickness is relatively thin. During the rise of the aluminum liquid, when it encounters the part where the cross-section changes abruptly, the flow velocity of the aluminum liquid will change drastically. According to the fluid continuity equation and Bernoulli effect, when the liquid flow enters the cavity where it suddenly narrows, its velocity will increase significantly due to the constant flow rate. This impacts the end of the cavity or the opposite side wall. The high-speed liquid flow can easily entrain air in the cavity into the molten metal, forming pores. Furthermore, the violent turbulence will destroy the oxide film on the liquid surface, causing it to break and be entrained into the casting. In addition, the impact of the high-speed liquid flow will scour the surface of the sand core, resulting in sand holes.

[0072] When the liquid enters the cavity where the cross-section suddenly increases, the flow rate will drop rapidly and the liquid will spread in all directions, forming eddies in corners or dead zones, and even producing reverse flow in some areas. If the flow rate of the molten metal converging from different directions is too slow and the temperature drops, cold shut defects may be formed. Furthermore, the molten metal in the slow-flowing area dissipates heat faster, which may cause the solidification sequence to be disordered.

[0073] For castings with complex cross-sections, dynamically adjusting the pressurization rate to improve flow stability has been studied in low-pressure casting processes in recent years. Currently, the mainstream solution is to evolve the filling method from the traditional segmented pressurization structure to a more dynamic nonlinear pressurization method. For example, CN108723338A uses a segmented liquid level matching and multi-stage air pressure control system to solve the problem of unstable liquid level in castings. However, existing methods still suffer from insufficient precision in adjustment and a lack of specificity.

[0074] Based on the above problems, the device of the present invention adopts a dynamic control filling method through the control system.

[0075] First, based on the casting process conditions, a reasonable baseline filling pressure rise rate is determined. Then, based on the real-time changes in the cross-sectional area of ​​the molten aluminum during the actual filling process, the filling pressure rise rate is dynamically adjusted. Specifically, this includes:

[0076] (1) Determination of reference filling pressure and rate of increase

[0077] First, based on process conditions such as pouring temperature, mold preheating temperature, and thermal properties of molten aluminum, simulation and experimental verification were conducted. Considering the minimum wall thickness of the casting, the minimum flow rate of molten aluminum to avoid cold shuts and the maximum flow rate to prevent air entrapment were determined. Based on this, a baseline flow rate that ensures smooth mold filling was obtained. The preferred pressure is 0.032–0.038 m / s. Subsequently, based on the pressure balance of low-pressure casting filling, the pressure inside the crucible must overcome the static pressure of the molten metal and the flow resistance. The reference pressure increase rate is obtained in the following way:

[0078]

[0079] The reference boost rate is expressed in mbar / s. This is a temperature correction factor, with a value ranging from 0.95 to 1.35 under fixed pouring and mold temperatures. The density of molten aluminum is taken as 2380 kg / m³. The acceleration due to gravity is taken as 9.81 m / s². The drag coefficient is denoted as 3.5–4.0, and H is the total height of the casting. Based on the above model, the baseline pressurization rate is obtained as 8 mbar / s–12 mbar / s.

[0080] (2) Real-time dynamic adjustment during the filling process

[0081] This process is the core step in dealing with sudden changes in the filling cross section and achieving flow stability. It identifies sudden changes in the cross section by the real-time height position of the molten aluminum and dynamically adjusts the pressurization rate.

[0082] Using a 3D CAD model of the casting, the casting is sliced ​​along its height and discretized into n height steps, preferably n = 50-120. The real-time equivalent cross-sectional area is obtained by dividing the volume within each step by its height, and a lookup table corresponding to the filling height h and the cavity cross-sectional area A is established. During the actual filling process, a reference filling pressure is initially used for increasing the filling speed. Simultaneously, the control system uses real-time h(t) (which can be obtained through multiple pre-installed non-contact liquid level sensors in the mold or through pressure feedback calculation) to interpolate the real-time equivalent cross-sectional area A(h) from this table. Table 1 shows a partial example of the lookup table data.

[0083] Table 1. Example of a lookup table for filling height h and cavity cross-sectional area A.

[0084] Inflation height (mm) Equivalent cross-sectional area (×10³ mm²) Area change α Feature region 0 53.9 - Base area 60 35.3 -34.5% Standard wall thickness of the casing 200 40.0 +13.3% Inner wall reinforcing ribs 210 35.4 -11.5% Reinforcing ribs finished 590 58.8 +66.0% Reaching the boss 610 58.5 -0.5% Boss body

[0085] Define the area change rate threshold =20%. The rate of change of the equivalent cross-sectional area between two adjacent step sizes (height intervals). , For the next step, the equivalent cross-sectional area, This is the equivalent cross-sectional area of ​​the current step size. When this threshold is exceeded, dynamic adjustments are made.

[0086]

[0087] To accelerate the pressure in the next long period, The pressure increase rate is set for the current step size. This is an adjustment coefficient, with a value range of 0.5–0.7.

[0088] The filling method of this invention, compared to the prior art, first predicts the shape of the cavity ahead through a pre-stored lookup table, and then combines this with real-time liquid level for feedforward-feedback composite control. This achieves a shift from passive execution to active adaptation, allowing pressure to be adjusted in advance before the molten aluminum reaches the abrupt change region, rather than afterward, greatly improving control accuracy.

[0089] Secondly, a balance between rapid and stable filling was achieved. By setting a reasonable baseline flow rate, safety was ensured at the thinnest wall thickness. In areas with wide cross-sections, the allowable pressure rise rate was appropriately increased through dynamic adjustment to compensate for the filling time, thereby optimizing the overall filling speed and efficiency while ensuring no air entrapment or scouring.

[0090] Furthermore, all parameters of the filling method described above in this invention have clear physical meanings and are directly related to the actual physical processes (solidification, flow resistance, cross-sectional changes), exhibiting strong predictability. They can be pre-calibrated and optimized through numerical simulation, reducing the cost and risk of on-site trial and error. Moreover, by combining lookup tables and model adjustments, the filling process scheme is fully digitized. Once successfully debugged for a certain type of casting, the method can be quickly replicated in the development of similar new products, significantly improving the level of process standardization.

[0091] 3) Crystallization and Pressurization Stage: After the cavity is completely filled, the control system implements precise pressurization based on the existing pressure. The purpose of this pressure increase is to enhance the tight contact between the casting and the cavity wall, and promote the rapid formation of a uniform and dense solidified shell on the surface of the casting; specifically, the pressure is increased to 420mbar-460mbar in 5s-10s, with a pressurization rate of 5mbar / s-7mbar / s, so that the area where the shell solidifies first is fully fed back.

[0092] 4) Crystallization and holding pressure stage: Maintain a constant pressure state after pressurization to ensure that the molten aluminum in each part of the cavity solidifies sequentially under continuous pressure until the entire casting is completely solidified. Specifically, after solidification and pressurization, continue to hold the pressure for 80s-120s so that the shell is completely solidified under continuous pressure feeding.

[0093] 5) Depressurization Stage: Compressed gas in the holding furnace is released in an orderly manner through the equipment's exhaust valve, and the system pressure gradually decreases. Under the action of gravity, the molten metal that remains liquid in the gating channel and riser pipe naturally falls back into the holding furnace, completing a complete casting cycle. Specifically, after the solidification and holding time ends, the compressed gas in the holding furnace is discharged through equipment control to achieve depressurization, allowing the molten aluminum that has not yet solidified in the gating and riser pipe to flow back into the holding furnace for subsequent use.

[0094] The foregoing detailed description only illustrates preferred embodiments of the present invention and is not intended to limit the invention. Those skilled in the art, upon considering the disclosure in the specification and embodiments, will readily conceive of other embodiments of the invention. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and embodiments are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

Claims

1. A low-pressure casting device with a large-size aluminum alloy shell, metal outer mold, and composite sand core, characterized in that: The large-size aluminum alloy shell includes a base located below, a shell boss located above, and reinforcing ribs located on the inner wall of the large-size aluminum alloy shell. The height of the large-size aluminum alloy shell is not less than 600mm, the inner diameter is not less than 700mm, and the average wall thickness is 14~20mm. The large-size aluminum alloy shell metal outer mold composite sand core low-pressure casting device includes a mold, a gating system, a cooling system, and risers. The mold includes a metal outer mold and an internal sand core. The gating system adopts a bottom pouring + stepped pouring method. The aluminum liquid rises under gas pressure, enters the gate at the bottom of the casting through the liquid riser channel, and then enters the sprue from the gate. The sprue includes multiple steps in the longitudinal height direction. Each step is designed with a sprue module including a certain number of horizontal runners. The sprue enters the casting cavity through the horizontal runner modules on the steps at different heights to complete the filling. The gating system is designed based on the height, lateral feature dimensions, and average wall thickness of the casting. The specific design method of the gating system is as follows: （1） （2） （3） In the formula, This refers to the number of direct pouring channels. This is the lateral characteristic dimension. For castings with a circular cross-section, its value is the maximum diameter D; for castings with a non-circular cross-section, it is... S is the maximum cross-sectional area; The average wall thickness of the casting. This is the direct casting coefficient, with a value ranging from 0.35 to 0.

5. This refers to the number of steps in a stepped pouring system, i.e., the number of horizontal runner modules at different heights. For the height of the casting, This is the step coefficient, with a value ranging from 0.05 to 0.

11. The number of horizontal runners on a single step. The variable value for the horizontal runner is 0.024-0.

045. During the calculation process, it is calculated based on the shape parameters of the casting and the corresponding coefficients. The calculation result is then rounded up to ensure accuracy. , , All values ​​are positive integers, which yield the corresponding number of straight runners, stepped runners, and horizontal runners; The resulting gating system structure is as follows: it includes a sprue system and a gating system. The sprue system includes a first sprue, a second sprue, a third sprue, and a fourth sprue arranged in a ring. The gating system includes a first gating module, a second gating module, a third gating module, and a fourth gating module arranged from bottom to top. Each gating module has the following horizontal structure: every two gating modules are connected to one sprue. Each gating module includes a total of 8 gating modules.

2. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 1, characterized in that, The reinforcing ribs and bosses are the parts where the cross-section changes abruptly during the filling process. The wall thickness of the reinforcing ribs is 30~35mm, and the wall thickness of the bosses is 50~55mm.

3. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 2, characterized in that, The metal outer mold includes a bottom mold and a side mold. The bottom mold is connected to the lower platform through a connector and is used to support the sand core. The side mold has a four-opening structure and is equipped with a side mold heating rod. The side mold heating rod is parallel to the mold cavity and is used to heat the mold after the low-pressure casting device is installed.

4. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 3, characterized in that, The top of the side mold fits into the top of the sand core and seals the molten aluminum.

5. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 4, characterized in that, The side mold is provided with a side mold heating rod, which is located in the side mold at a position parallel to the mold cavity.

6. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 5, characterized in that, The sand core is located inside the space enclosed by the side mold. The shape of the sand core is designed according to the internal structure of the product and is equipped with weight reduction holes. The gating system and risers are both located in the sand core. The sand core is formed by 3D printing.

7. The apparatus for low pressure casting of large size aluminum alloy shell metal outer mold composite sand core according to claim 6, characterized in that, The cooling system includes reinforcing chills and boss chills, which are respectively disposed in the reinforcing ribs and bosses in the inner cavity of the housing, with a thickness of 20mm~30mm. The chills are made of aluminum alloy.

8. A method of performing low pressure casting of large size aluminum alloy housing metal outer mold composite sand core using the low pressure casting apparatus according to claim 7, characterized in that, Includes the following steps: 1) Liquid rising stage: Driven by compressed air, the liquid aluminum alloy in the holding furnace rises through the riser pipe until it completely fills the gate of the mold. 2) Filling stage: The air pressure is continuously increased to push the molten aluminum through the mold's gating system. The filling pressure is dynamically adjusted according to the liquid level to complete the cavity filling: First, it passes through each sprue, then enters the horizontal sprue branch from low to high, filling the shell and its internal structure, and finally reaches the top riser and venting position. 3) Crystallization pressurization stage: After the cavity is completely filled, the control system pressurizes the cavity based on the existing pressure to form a solidified shell. 4) Crystallization and pressure holding stage: Maintain the constant pressure state of the crystallization and pressure increasing stage. The aluminum liquid in each part of the cavity completes sequential solidification under continuous constant pressure until the entire casting is completely solidified. 5) Decompression stage: The compressed gas in the holding furnace is released through the exhaust valve, and the pressure gradually decreases; under the action of gravity, the aluminum alloy that is still in liquid state in the gating channel and riser pipe naturally falls back into the holding furnace, completing a complete casting cycle.

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