Low-pressure casting method for feeding in pressurizing mode

Abstract

The invention relates to the technical field of low-pressure casting, and discloses a low-pressure casting method for feeding in a pressurization mode. The operation process of the method comprises the steps that after a molten metal mold is filled, a pressure control system is switched to a closed-loop mode based on real-time solidification feedback; the basic feeding pressure P1 of 0.02-0.04 MPa is applied, so that a closed shell layer is formed on the surface layer of the casting; within 3-8 seconds after the surface layer is solidified, linearly increasing the pressure from P1 to P2 which is 1.8-2.2 times of the pressure within 2-4 seconds; with P2 as a reference, sine fluctuation feeding is carried out for 10-20 seconds at the frequency of 0.5-2 Hz and the amplitude of 0.05 P2 to 0.15 P2; and finally, the pressure is relieved to atmospheric pressure in three to five stages, and the retention time between the stages is not less than 2 seconds. According to the method, precise feeding is achieved through multi-stage time sequence pressure regulation and control, and the solidification quality of the casting is improved.

Description

Technical Field

[0001] This invention relates to the field of low-pressure casting technology, specifically to a low-pressure casting method that uses pressure to compensate for shrinkage. Background Technology

[0002] Low-pressure casting, as an advanced metal forming process, is widely used in aerospace, automotive manufacturing, rail transportation, and other fields due to its characteristics such as high casting density, good forming quality, high material utilization, and ease of automation. It is particularly suitable for producing lightweight alloy castings such as aluminum and magnesium alloys with complex shapes and stringent performance requirements. In the low-pressure casting process, the feeding stage is a core process affecting the internal quality of the casting. Its main function is to promptly replenish the volume defects caused by the solidification shrinkage of the liquid metal during the casting process, thereby reducing or eliminating defects such as shrinkage cavities and porosity inside the casting.

[0003] Currently, the commonly used low-pressure casting feeding methods in the industry are mostly single-pressure feeding or simple two-stage pressure feeding modes. The single-pressure feeding mode, because the pressure remains constant throughout the solidification process, cannot adapt to the feeding needs of different solidification stages of the casting. In the early stages of casting solidification, if the pressure is too high, it can easily lead to turbulent flow of the molten metal, disrupting the solidification order of the casting surface; if the pressure is too low, it is difficult to effectively promote the flow of the molten metal and form a continuous feeding channel in a timely manner. While the simple two-stage pressure feeding mode considers the differences in solidification stages to some extent, it suffers from problems such as ambiguous pressure switching timing, uncontrollable pressure increase rate, and a lack of dynamic adjustment mechanisms.

[0004] Specifically, existing technologies often rely on experience-based judgment or fixed time settings for pressure switching, failing to establish a correlation with the real-time solidification state of the casting. This frequently results in premature pressurization leading to cracks in the casting, or delayed pressurization failing to achieve effective feeding. Furthermore, the lack of precise control over the pressure increase rate during the pressurization process can easily cause pressure shocks, damaging the solidification structure of the casting. In addition, during the later stages of feeding, existing technologies often employ a single-stage or simple two-stage depressurization method. Excessive depressurization rates can lead to significant thermal and structural stresses within the casting, causing deformation, cracking, and other defects, severely impacting the dimensional accuracy and mechanical properties of the casting. Summary of the Invention

[0005] The purpose of this invention is to provide a low-pressure casting method that uses pressure feeding to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides a low-pressure casting method using a pressure feeding method, the method comprising:

[0007] a) Preparation for closed-loop pressure control: After the molten metal has finished filling the mold, switch the pressure control system to a closed-loop mode based on real-time solidification feedback.

[0008] b) Perform first-level pressure solidification control: apply a basic feeding pressure P1 to the holding furnace, wherein the value of P1 is 0.02 to 0.04 MPa, so that the surface layer of the casting solidifies to form a closed shell.

[0009] c) Execute secondary pressurization compensation control: At time t1 after the formation of the surface solidified shell, start the secondary pressurization program to linearly increase the furnace pressure from P1 to P2 within a time Δt1. The value of P2 is 1.8 to 2.2 times that of P1, the value of t1 is 3 to 8 seconds after the completion of filling, and the value of Δt1 is 2 to 4 seconds.

[0010] d) Execute three-level dynamic compensation control: After the pressure reaches P2, start the dynamic pressure fluctuation program. The pressure is based on P2 and fluctuates sinusoidally with frequency f1 and amplitude ΔP1. The value of f1 ranges from 0.5 to 2Hz, the value of ΔP1 ranges from 0.05P2 to 0.15P2, and the duration is t2, which ranges from 10 to 20 seconds.

[0011] e) Perform pressure staged relief: After dynamic feeding is completed, the furnace pressure is reduced to atmospheric pressure in k stages. The pressure reduction in each stage is P2 / k, and the holding time between stages is not less than 2 seconds. The value of k ranges from 3 to 5.

[0012] Preferably, in step a), the specific steps for switching the pressure control system to the closed-loop mode based on real-time solidification feedback are as follows: First, start the micro thermocouple array arranged at a specific thermal node in the mold cavity; second, compare the temperature signal Ti collected by the thermocouple array with the preset alloy critical solidification temperature Ts in real time; when any thermocouple signal Ti is lower than Ts, immediately trigger the closed-loop program of the pressure controller, wherein the critical solidification temperature Ts is 20 to 50°C higher than the alloy solidus temperature.

[0013] Preferably, in step b), when applying the basic compensation pressure P1, a flow control mode is adopted, specifically: the dry compressed gas is controlled by a proportional valve to be introduced into the heat preservation furnace at a constant flow rate Q1 until the reading of the pressure sensor in the furnace reaches P1, wherein the constant flow rate Q1 ranges from 10 to 20 liters / minute.

[0014] Preferably, the secondary boosting program in step c) adopts pressure slope control. Specifically, after setting the target pressure P2 and boosting time Δt1, the pressure controller calculates and executes a fixed pressure ramp slope S1, S1 = (P2 - P1) / Δt1, and compares the real-time pressure with the theoretical pressure value based on the slope. The intake valve opening is adjusted by the PID algorithm so that the deviation between the actual pressure curve and the theoretical slope curve does not exceed ±5%.

[0015] Preferably, the dynamic pressure fluctuation program in step d) is executed by a pressure fluctuation generator, and the specific steps are as follows: First, the waveform generator generates a sine wave electrical signal with a frequency of f1 and an amplitude of ΔP1; then, the electrical signal is sent as a set value to the controller of the high-speed electro-pneumatic proportional valve; the proportional valve adjusts the gas flow to the heat preservation furnace in real time according to this changing set value, thereby generating a pressure fluctuation field synchronized with the electrical signal in the furnace cavity.

[0016] Preferably, the start time t1 of the secondary pressurization procedure in step c) is determined by a solidification monitoring subsystem based on an ultrasonic probe. The specific steps are as follows: the ultrasonic probe is coupled to the outer wall of the mold, ultrasonic pulses are emitted into the casting and the echoes are received; by analyzing the attenuation rate of the echo signal amplitude, it is determined whether the surface solidified shell has reached the predetermined thickness; when the shell thickness is detected to be 3 to 5 mm, the ultrasonic subsystem sends a trigger signal to the main controller to start the secondary pressurization.

[0017] Preferably, the parameters f1 and ΔP1 of the dynamic pressure fluctuation program in step d) are adaptively adjusted according to the structural characteristics of the casting. The specific logic is as follows: Before casting, the three-dimensional model of the casting is input into the control system, and the system automatically identifies the maximum wall thickness dmax and the hot spot volume Vh of the casting; then, according to the pre-stored mapping relationship: f1 = a*dmax + b, ΔP1 = c*Vh + d, the fluctuation parameters are calculated, where a, b, c, and d are constants calibrated for different alloy materials.

[0018] Preferably, between step b) and step c), a pressure stabilization transition step is also included: after the pressure reaches P1, P1 is kept constant for a duration Ts, where Ts ranges from 1 to 3 seconds, to ensure stable pressure sensor readings and initial strengthening of the casting surface shell.

[0019] Preferably, the pressure grading and unloading process described in step e) is linked with the initial mold opening action of the mold. Specifically, in the first pressure unloading stage, when the pressure is controlled to drop to P2 / k, the mold opening mechanism is simultaneously instructed to perform the first micro displacement, the displacement being 5% to 10% of the total mold opening stroke, in order to break the initial vacuum adsorption force between the casting and the mold cavity. Each subsequent pressure unloading stage corresponds to one micro-movement mold opening.

[0020] Preferably, all timing and pressure parameters of the entire multi-stage pressurization and feeding process are compiled into an executable casting process formula file. This file can be edited, saved, and recalled through the user interface, and can be linked with the mold coding recognition system of the casting machine. When the system recognizes the currently installed mold code, it automatically loads and executes the specific process formula file uniquely bound to that mold.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The low-pressure casting method using pressurized feeding proposed in this invention, through multi-level time-series control of the pressurized solidification process and a closed-loop pressure regulation mechanism, offers significant advantages over existing low-pressure casting feeding technologies in several aspects. In terms of pressure control, a closed-loop regulation based on real-time solidification feedback is employed, enabling the pressure control system to perceive changes in the solidification state of the casting in real time and flexibly adjust the control strategy accordingly. This overcomes the limitations of existing open-loop control modes, which cannot adapt to the dynamic solidification process of castings, making pressure control more targeted and precise, and better suited to the personalized feeding needs of castings with different materials and structures.

[0023] In the primary pressure solidification control stage, by applying a specific range of base feeding pressure P1, the surface layer of the casting can be guided to solidify uniformly and form a complete closed shell. This orderly surface solidification method avoids the turbulence of molten metal caused by excessive pressure disrupting the solidification order, and also prevents the problem of insufficient pressure preventing the timely formation of an effective closed structure. The complete closed shell can create a stable feeding space, reduce the risk of molten metal leakage during subsequent feeding processes, and provide stable basic conditions for subsequent pressurization feeding stages.

[0024] The two-stage pressure boosting compensation control achieves a linear and smooth pressure increase from P1 to P2 by precisely setting the pressure boosting start time t1 and the pressure boosting duration Δt1. This design avoids the pressure shock problems caused by ambiguous pressure boosting timing and uncontrollable rate in existing technologies. After a closed shell is formed on the surface of the casting, it provides sufficient feeding force in a timely manner, pushing the unsolidified liquid metal inside to flow towards the solidification and shrinkage area, effectively filling the volume gap. At the same time, the reasonable ratio setting of P2 and P1 ensures sufficient feeding force without causing excessive stress inside the casting due to excessive pressure, thus balancing the feeding effect and the structural stability of the casting.

[0025] The three-level dynamic feeding control employs a sinusoidal undulating pressure mode based on P2. The appropriate setting of the undulating frequency and amplitude generates a continuous micro-vibration effect. This micro-vibration effectively breaks down the solid-liquid interface layer formed during the solidification process of the molten metal, promoting the flow of liquid metal and improving the uniformity of feeding. Simultaneously, the dynamic undulating pressure also refines the solidified grains of the casting, improving the microstructure and reducing intergranular defects. Compared to existing constant pressure feeding modes, dynamic feeding can further improve the density of the casting and optimize its mechanical properties.

[0026] The pressure-grading process, by gradually reducing pressure in stages and setting appropriate holding times between stages, effectively releases the thermal and structural stresses accumulated during the solidification of the casting. This avoids stress concentration caused by sudden or rapid pressure reduction, thus reducing defects such as deformation and cracking in the casting. The reasonable stage division and holding time settings ensure a smooth release of internal stresses, guaranteeing the dimensional accuracy and shape integrity of the casting. Attached Figure Description

[0027] Figure 1 This is a flowchart illustrating the working steps of a low-pressure casting method using pressure feeding as described in this invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to specific embodiments. The following embodiments are only for illustrating the present invention and are not intended to limit the scope of protection of the present invention. The core of the present invention lies in achieving efficient feeding through a multi-stage time-controlled pressurized solidification process, thereby improving the density and mechanical properties of castings. It is suitable for casting various complex structures and thick cross-section metal castings, and is especially suitable for the mass production of non-ferrous metal castings such as aluminum alloys and copper alloys.

[0029] The low-pressure casting equipment used in this embodiment includes: a holding furnace (rated capacity 500kg, temperature control accuracy ±1℃), a closed-loop pressure control system (including pressure sensors, measurement accuracy ±0.001MPa), a miniature thermocouple array (temperature measurement range 0-1000℃, response time ≤50ms), an ultrasonic solidification monitoring subsystem (ultrasonic frequency 5MHz, measurement accuracy ±0.1mm), a pressure fluctuation generator (frequency adjustment range 0.1-5Hz, amplitude adjustment range 0-0.1MPa), a high-speed electro-pneumatic proportional valve (response time ≤10ms), and a mold coding recognition system (recognition accuracy 100%). The dry compressed gas used is nitrogen (purity ≥99.99%) to avoid oxidation of the molten metal.

[0030] In each embodiment and comparative example, the casting performance testing methods are as follows: ① Density: determined by the drainage method according to GB / T19869.1-2005; ② Mechanical properties: standard tensile specimens were prepared according to GB / T 228.1-2021, and tensile strength, yield strength and elongation were tested using a universal testing machine. Three specimens were tested in each group, and the average value was taken; ③ Defect detection: X-ray flaw detector (detection accuracy 0.1mm) was used for detection according to GB / T 6402-2018, and the defect rate (the percentage of defect area to the total area of ​​the casting) and the shrinkage porosity area ratio were statistically analyzed.

[0031] Example 1: Low-pressure casting of A356 aluminum alloy automotive wheel hub castings

[0032] This embodiment focuses on casting an A356 aluminum alloy automotive wheel hub (maximum outer diameter 400mm, maximum wall thickness dmax=8mm, hot spot volume Vh=120cm³). The solidus temperature of A356 aluminum alloy is TL=615℃, the liquidus temperature is Ts=655℃, and the critical solidification temperature is set to Ts=645℃ (30℃ higher than the solidus temperature). The specific casting steps are as follows:

[0033] Mold preparation and molten metal melting: Install the automobile wheel hub mold onto the casting machine, scan the mold code "AL-HUB-001" through the mold code recognition system, and the system will automatically call the pre-stored binding process formula file; put the A356 aluminum alloy ingot into the melting furnace, melt it to 720℃ and hold it for 20 minutes. After refining and degassing, pour the molten metal into the holding furnace and control the temperature of the holding furnace to 700℃.

[0034] Mold filling: Start the low-pressure casting machine and introduce nitrogen into the holding furnace at a pressure increase rate of 0.01MPa / s, so that the molten metal fills the mold cavity along the riser pipe and gating. The filling time is 12s, and the time when the filling is completed is recorded as t=0.

[0035] Closed-loop pressure control preparation: At t=0, the miniature thermocouple arrays arranged in the mold cavity at 5 key thermal nodes (3 at the connection between the hub spoke and the rim, and 2 at the center of the hub) are immediately activated; the thermocouple arrays collect the temperature signal Ti of each node in real time and transmit it to the pressure controller, which compares it with the preset critical solidification temperature Ts=645℃ in real time; when t=2.3s, the thermocouple signal Ti at the connection between one of the spokes and the rim is 642℃ (lower than Ts), and the pressure controller immediately switches to closed-loop control mode.

[0036] First-level pressure solidification control: After the closed-loop mode is started, the pressure control system switches to the flow control mode, and the dry nitrogen is controlled by the proportional valve to be introduced into the holding furnace at a constant flow rate of Q1=15 liters / minute; when t=3.5s, the pressure sensor reading reaches the basic feeding pressure P1=0.03MPa, and this pressure is maintained to allow the surface layer of the casting to solidify and form a closed shell.

[0037] Pressure stabilization transition: After the pressure reaches P1, keep P1 constant for a duration of Ts=2s (t=3.5s to t=5.5s) to ensure stable pressure sensor readings (fluctuation range ≤±0.001MPa) and to initially strengthen the surface shell of the casting.

[0038] Two-stage boost compensation control:

[0039] Start the ultrasonic solidification monitoring subsystem: Couple the ultrasonic probe to the corresponding hot spot position on the outer wall of the mold, emit a 5MHz ultrasonic pulse into the casting, with a pulse emission frequency of 10Hz, and receive the echo signal in real time; analyze the attenuation rate of the echo signal amplitude through the signal processing module to determine the thickness of the surface solidified shell layer.

[0040] Secondary boost start trigger: When t=5s (5s after filling ends, within the range of 3-8s), the ultrasonic subsystem detects that the thickness of the surface solidified shell layer reaches 4mm (3-5mm predetermined thickness range), and immediately sends a trigger signal to the main controller to start the secondary boost program.

[0041] Pressurization process control: The secondary pressurization time Δt1 is set to 3s (t=5s to t=8s), and the target pressure P2=0.03MPa×2.2=0.066MPa (2.2 times P1, within the range of 1.8-2.2 times); the pressure controller calculates the pressure ramp-up slope S1=(P2-P1) / Δt1=(0.066-0.03) / 3=0.012MPa / s; the intake valve opening is adjusted in real time through the PID algorithm, and the real-time pressure is compared with the theoretical pressure value based on the slope S1 to ensure that the deviation between the actual pressure curve and the theoretical slope curve is controlled within ±3%; at t=8s, the furnace pressure accurately reaches P2=0.066MPa.

[0042] Three-level dynamic compensation control:

[0043] Adaptive parameter calculation: Before casting, the three-dimensional model of the car wheel hub has been input into the control system. The system automatically recognizes dmax=8mm and Vh=120cm³. According to the pre-stored mapping relationship of A356 aluminum alloy: f1=0.1Hz / mm×dmax + 0.2Hz, ΔP1=0.00007MPa / cm³×Vh + 0.0006MPa, the fluctuation parameters are calculated as f1=0.1×8+0.2=1Hz (0.5-2Hz range) and ΔP1=0.00007×120+0.0006=0.009MPa (0.136P2, within the range of 0.05P2-0.15P2).

[0044] Dynamic pressure fluctuation execution: At t=8s, the pressure fluctuation generator is started, and the waveform generator generates a sinusoidal electrical signal with a frequency of 1Hz and an amplitude of 0.009MPa. This electrical signal is sent as a set value to the high-speed electro-pneumatic proportional valve controller. The proportional valve adjusts the nitrogen flow rate in real time according to the changing set value, generating a pressure fluctuation field synchronized with the electrical signal in the furnace cavity. With P2=0.066MPa as the reference value, the pressure fluctuates sinusoidally between 0.057MPa and 0.075MPa for a duration of t2=15s (from t=8s to t=23s).

[0045] Pressure staged relief: At t=23s, dynamic compensation ends and the pressure staged relief program is started. The number of stages is set to k=4 (range 3-5). The pressure drop of each stage is P2 / k=0.066 / 4=0.0165MPa, and the holding time between stages is 2s.

[0046] The first stage (t=23s to t=26s): the pressure drops from 0.066MPa to 0.0495MPa, and is held for 2s after the pressure drops to the target value; at the same time, the mold opening mechanism is instructed to perform the first small displacement, which is 8% of the total mold opening stroke (total mold opening stroke 150mm, small displacement 12mm), breaking the initial vacuum adsorption force between the casting and the mold cavity.

[0047] The second stage (t=26s to t=29s): the pressure drops from 0.0495MPa to 0.033MPa and is held for 2s, corresponding to the execution of the second stage of 8% mold opening displacement.

[0048] The third stage (t=29s to t=32s): the pressure drops from 0.033MPa to 0.0165MPa and is maintained for 2s, corresponding to the execution of the third stage of 8% mold opening displacement.

[0049] The fourth stage (t=32s to t=35s): the pressure drops from 0.0165MPa to atmospheric pressure (0.1MPa standard atmospheric pressure, actually dropped to 0.101MPa), is maintained for 2s, and the fourth stage of 68% mold opening displacement is executed to complete all mold opening actions.

[0050] Casting removal and post-processing: After the mold is opened, the casting is removed and post-processing such as removing the gating system and risers and aging treatment (holding at 170℃ for 6 hours) is carried out to obtain the finished A356 aluminum alloy automobile wheel hub casting.

[0051] Example 2: Low-pressure casting of ZL101 aluminum alloy engine cylinder block castings

[0052] This embodiment focuses on casting a ZL101 aluminum alloy engine cylinder block (maximum wall thickness dmax = 12mm, hot spot volume Vh = 200cm³). The solidus temperature of ZL101 aluminum alloy is TL = 607℃, and the critical solidification temperature is set to Ts = 637℃ (30℃ higher than the solidus temperature). The specific casting steps are as follows:

[0053] Mold preparation and molten metal melting: Install the engine block mold, scan the code "AL-CYL-002" with the mold coding identification system, and automatically call up the bound process formula; melt the ZL101 aluminum alloy ingot to 710℃, refine and degas it and pour it into the holding furnace, and hold it at 690℃.

[0054] Mold filling: Nitrogen gas is introduced into the mold at a pressurization rate of 0.008 MPa / s, the filling time is 18s, and the filling is completed at t=0.

[0055] Closed-loop pressure control preparation: At t=0, start the thermocouple array of 6 hot nodes in the mold cavity (4 at the cylinder block and cylinder liner, and 2 at the crankcase); at t=2.8s, the thermocouple signal Ti at one cylinder liner is 635℃ (lower than Ts=637℃), and the pressure controller switches to closed-loop mode.

[0056] First-level pressure solidification control: Using flow control mode, nitrogen gas is introduced at Q1=12 liters / minute, and the pressure reaches P1=0.025MPa at t=4.2s. This pressure is maintained to form a closed shell layer on the surface.

[0057] Stable pressure transition: Keep P1 constant for Ts = 1.5s (from t = 4.2s to t = 5.7s).

[0058] Two-stage boost compensation control:

[0059] Ultrasonic monitoring: An ultrasonic probe is coupled to the outer wall of the cylinder block and cylinder liner to detect the shell thickness in real time; when t=6s (6s after the filling is completed), the shell thickness is detected to be 3.5mm, and a trigger signal is sent to start the second stage of boosting.

[0060] Pressure boosting control: Set Δt1=2.5s, P2=0.025×2.0=0.05MPa, calculate S1=(0.05-0.025) / 2.5=0.01MPa / s; through PID adjustment, the actual pressure deviates from the theoretical slope by ±4%, and the pressure reaches P2=0.05MPa at t=8.5s.

[0061] Three-level dynamic compensation control:

[0062] Parameter calculation: Based on the mapping relationship of ZL101 aluminum alloy f1=0.1Hz / mm×dmax + 0.2Hz, ΔP1=0.00003MPa / cm³×Vh + 0.0003MPa, we calculate f1=0.1×12+0.2=1.4Hz, ΔP1=0.00003×200+0.0003=0.0063MPa (0.126P2).

[0063] Fluctuation Execution: The pressure fluctuation generator is started to generate a 1.4Hz, 0.0063MPa sine wave electrical signal, which controls the proportional valve to adjust the flow rate. The pressure fluctuates between 0.0437MPa and 0.0563MPa with 0.05MPa as the reference, lasting for t2=18s (t=8.5s to t=26.5s).

[0064] Pressure relief in stages: set k=3, each stage reduces pressure by 0.05 / 3≈0.0167MPa, and maintains pressure for 2.5s between stages;

[0065] First stage (t=26.5s to t=30s): The pressure drops to 0.0333MPa, is maintained for 2.5s, and a small displacement of 7% of the total mold opening stroke is performed.

[0066] Second stage (t=30s to t=33.5s): The pressure is reduced to 0.0166MPa, held for 2.5s, and a 7% mold opening displacement is performed.

[0067] The third stage (t=33.5s to t=37s): the pressure drops to atmospheric pressure, is maintained for 2.5s, and 86% of the mold opening displacement is performed to complete the mold opening.

[0068] Casting removal and post-treatment: Remove the casting, remove the gating and riser, and then perform T6 heat treatment (solution treatment at 535℃ for 6 hours, aging at 155℃ for 4 hours) to obtain the finished engine cylinder block casting.

[0069] Example 3: Low-pressure casting of Cu-30Ni alloy thin-walled shell castings

[0070] This embodiment focuses on casting a thin-walled Cu-30Ni alloy shell (maximum wall thickness dmax = 5 mm, hot spot volume Vh = 80 cm³). The solidus temperature of the Cu-30Ni alloy is TL = 1220℃, and the critical solidification temperature is set to Ts = 1250℃ (30℃ higher than the solidus temperature). The specific casting steps are as follows:

[0071] Mold preparation and molten metal melting: Install the thin-walled shell mold, scan "CU-NI-SHELL-003" with the coding identification system, and call up the binding formula; melt the Cu-30Ni alloy ingot to 1350℃, refine it and pour it into the holding furnace, and hold it at 1320℃.

[0072] Mold filling: The mold is filled with a pressure increase rate of 0.012 MPa / s for 10 seconds, and t=0 is the time when filling is completed.

[0073] Closed-loop pressure control preparation: At t=0, start the thermocouple array of 4 hot nodes in the mold cavity; at t=1.8s, the temperature of 1 hot node Ti=1248℃ (lower than Ts=1250℃), and switch to closed-loop mode.

[0074] First-level pressure solidification control: Nitrogen gas is introduced at a rate of Q1 = 20 liters / minute, and the pressure reaches P1 = 0.04 MPa at t = 3 seconds. The pressure is maintained to form a surface shell.

[0075] Stable pressure transition: Keep P1 constant and Ts=3s (t=3s to t=6s).

[0076] Two-stage boost compensation control:

[0077] Ultrasonic monitoring: The ultrasonic probe is coupled to the outer wall of the mold. At t=3s (3s after filling is completed), the shell thickness is detected to be 3mm, and a trigger signal is sent to start the secondary pressurization.

[0078] Pressure boosting control: Set Δt1=2s, P2=0.04×1.8=0.072MPa, calculate S1=(0.072-0.04) / 2=0.016MPa / s; PID adjustment deviation ±2%, pressure reaches P2=0.072MPa at t=5s.

[0079] Three-level dynamic compensation control:

[0080] Parameter calculation: Based on the Cu-30Ni alloy mapping relationship f1=0.1Hz / mm×dmax + 0.2Hz, ΔP1=0.00008MPa / cm³×Vh + 0.0005MPa, we calculate f1=0.1×5+0.2=0.7Hz, ΔP1=0.00008×80+0.0005=0.0069MPa (0.096P2).

[0081] Fluctuation execution: Start the pressure fluctuation generator to generate a 0.7Hz, 0.0069MPa sine wave electrical signal. The pressure fluctuates between 0.0651MPa and 0.0789MPa with 0.072MPa as the reference, lasting for t2=10s (t=5s to t=15s).

[0082] Pressure relief in stages: set k=5, each stage reduces pressure by 0.072 / 5=0.0144MPa, and maintains pressure for 2 seconds between stages;

[0083] Stages 1 to 4: The pressure in each stage is reduced to 0.0576MPa, 0.0432MPa, 0.0288MPa and 0.0144MPa respectively, and each stage is held for 2 seconds, corresponding to a 5% mold opening displacement.

[0084] Fifth stage: The pressure drops to atmospheric pressure, is held for 2 seconds, and then 80% of the mold opening displacement is performed to complete the mold opening.

[0085] Casting removal and post-processing: After removing the casting and removing the risers, the casting is subjected to solution treatment (held at 1100℃ for 4 hours) to obtain the finished Cu-30Ni alloy thin-walled shell casting.

[0086] Comparative Example 1: Casting of A356 aluminum alloy automotive wheel hubs lacking the three-stage dynamic feeding step

[0087] This comparative example is identical to Example 1 in casting type, alloy material, equipment, and most process parameters, except for the absence of the three-stage dynamic feeding step. The specific differences are as follows: After the second-stage pressurization is completed (t=8s), the dynamic pressure fluctuation program is not initiated; instead, P2=0.066MPa is maintained constant until t=23s (originally t2=15s duration). Subsequently, the same pressure staged unloading step as in Example 1 is performed. The remaining steps (closed-loop preparation, first-stage pressure solidification, pressure stabilization transition, and second-stage pressurization) are consistent with Example 1.

[0088] Comparative Example 2: A356 aluminum alloy automotive wheel hub casting experiencing loss of control during the second-stage turbocharger start-up.

[0089] This comparative example is identical to Example 1 in terms of casting type, alloy material, equipment, and most process parameters, except that the second-stage pressurization start time t1 is set to a fixed value of 10s (exceeding the 3-8s range specified in this application), and the ultrasonic solidification monitoring subsystem is not used. The specific differences are as follows: After filling, and after the first-stage pressure solidification and pressure stabilization transition steps are completed, ultrasonic shell thickness detection is not performed; the second-stage pressurization program is started directly at t=10s. The parameters for the second-stage pressurization, such as Δt1 and P2, are consistent with those in Example 1. All other steps are the same as in Example 1.

[0090] Table 1: Summary of Process Parameters for Examples and Comparative Examples

[0091] As shown in Table 1, Examples 1-3 strictly follow the multi-stage timing pressurization and feeding process of this invention. All parameters are within the range specified in this application, and key technical features such as adaptive parameter adjustment (e.g., f1 and ΔP1 are calculated based on the casting's dmax and Vh) and ultrasonic monitoring of the second-stage pressurization start-up time, pressure relief, and mold opening linkage are achieved. Specifically, Example 1, for medium-wall-thickness automotive wheel hubs, selects a moderate t1=5s and Δt1=3s; Example 2, for thicker engine cylinder blocks, appropriately extends t2=18s to ensure sufficient feeding; Example 3, for thin-walled shells, shortens t1=3s and t2=10s, which is consistent with the fast solidification speed of thin-walled castings.

[0092] The only difference between Comparative Example 1 and Example 1 is the absence of the three-stage dynamic compensation step. A comparison can verify the impact of dynamic pressure fluctuations on the compensation effect. Comparative Example 2, by setting t1 to 10s (outside the 3-8s range) and removing ultrasonic monitoring, verifies the necessity of precise control of the second-stage pressurization start-up time. Both comparative examples, through a single-variable design, effectively highlight the beneficial effects of the key technical features of this invention.

[0093] Table 2: Performance Test Results of Castings from Examples and Comparative Examples

[0094] As shown in Table 2, the casting performance of Examples 1-3 is significantly better than that of Comparative Example 1 and Comparative Example 2, which fully verifies the superiority of the multi-stage time-series pressure feeding method of the present invention. The specific analysis is as follows:

[0095] Comparison of density and defect rate: The castings of Examples 1-3 all achieved a density of over 99.6%, a defect rate of less than 1%, and a shrinkage cavity area ratio of less than 0.3%; while the density of Comparative Example 1 was only 98.2%, the defect rate was 3.2%, and the shrinkage cavity area ratio was 1.8%; the density of Comparative Example 2 was even lower at 97.5%, the defect rate was 8.3%, and the shrinkage cavity area ratio was 4.5%. The core reason for the difference is that Comparative Example 1 lacked a three-stage dynamic feeding step, and the feeding power of the molten metal under constant pressure was insufficient, which could not effectively fill the micropores formed during solidification; the second-stage pressurization start time of Comparative Example 2 was too late, t1=10s, at which time the surface shell of the casting had become excessively thick (ultrasonic testing showed a thickness of 8mm), blocking the feeding channel, resulting in the inability to obtain effective feeding when the internal molten metal solidified and contracted, forming a large number of shrinkage cavities and porosity defects.

[0096] Mechanical property comparison: The tensile strength, yield strength, and elongation of Examples 1-3 were significantly higher than those of the comparative examples. Specifically, the tensile strength of Example 1 was 325 MPa, a 14.0% increase compared to 285 MPa in Comparative Example 1, and a 25.0% increase compared to 260 MPa in Comparative Example 2. The Cu-30Ni alloy casting of Example 3 achieved a tensile strength of 480 MPa and an elongation of 12.3%, exhibiting excellent mechanical properties. This is because the examples, through multi-stage sequential compression feeding, effectively reduced internal defects in the castings and improved density. The mechanical properties of castings are closely related to density—the presence of defects leads to stress concentration and reduces load-bearing capacity, while high-density castings have finer grains and more uniform microstructure, thus exhibiting superior mechanical properties.

[0097] Analysis of performance differences among different embodiments: The Cu-30Ni alloy casting of Embodiment 3 had the lowest defect rate (0.4%) and the highest elongation (12.3%), which is related to the fluidity of Cu-30Ni alloy and the adaptive adjustment of parameters in this invention. For thin-walled shells with dmax=5mm and Vh=80cm³, the calculated f1=0.7Hz and ΔP1=0.0069MPa are more suitable for its solidification characteristics. The engine cylinder block casting of Embodiment 2 has a larger maximum wall thickness (12mm) and a larger hot spot volume (200cm³), and the solidification process is more complex. Therefore, its density and mechanical properties are slightly lower than those of Embodiments 1 and 3, but still far superior to the comparative examples, indicating that the process method of this invention still has good applicability to thick cross-section castings.

[0098] This invention utilizes a multi-stage time-series control system, including closed-loop pressure regulation preparation, primary pressure solidification, secondary pressure boosting compensation, tertiary dynamic feeding, and graded pressure removal. Combined with ultrasonic solidification monitoring, adaptive parameter adjustment, and linkage between pressure removal and mold opening, this invention can significantly improve the density of castings, reduce the defect rate, and improve mechanical properties. It is suitable for low-pressure casting of various alloys and structures, and has broad industrial application value.

[0099] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A low-pressure casting method using pressure feeding, the core of which lies in multi-stage time control of the pressure solidification process, characterized in that... The following operational procedures are included: a) Preparation for closed-loop pressure control: After the molten metal has finished filling the mold, switch the pressure control system to a closed-loop mode based on real-time solidification feedback. b) Perform first-level pressure solidification control: apply a basic feeding pressure P1 to the holding furnace, wherein the value of P1 is 0.02 to 0.04 MPa, so that the surface layer of the casting solidifies to form a closed shell. c) Execute secondary pressurization compensation control: At time t1 after the formation of the surface solidified shell, start the secondary pressurization program to linearly increase the furnace pressure from P1 to P2 within a time Δt1. The value of P2 is 1.8 to 2.2 times that of P1, the value of t1 is 3 to 8 seconds after the completion of filling, and the value of Δt1 is 2 to 4 seconds. d) Execute three-level dynamic compensation control: After the pressure reaches P2, start the dynamic pressure fluctuation program. The pressure is based on P2 and fluctuates sinusoidally with frequency f1 and amplitude ΔP1. The value of f1 ranges from 0.5 to 2Hz, the value of ΔP1 ranges from 0.05P2 to 0.15P2, and the duration is t2, which ranges from 10 to 20 seconds. e) Perform pressure staged relief: After dynamic feeding is completed, the furnace pressure is reduced to atmospheric pressure in k stages. The pressure reduction in each stage is P2 / k, and the holding time between stages is not less than 2 seconds. The value of k ranges from 3 to 5.

2. The low-pressure casting method according to claim 1, characterized in that, In step a), the specific steps for switching the pressure control system to a closed-loop mode based on real-time solidification feedback are as follows: First, start the micro thermocouple array arranged at a specific hot node in the mold cavity; second, compare the temperature signal Ti collected by the thermocouple array with the preset critical solidification temperature Ts of the alloy in real time; when any thermocouple signal Ti is lower than Ts, the closed-loop program of the pressure controller is immediately triggered, and the value of the critical solidification temperature Ts is 20 to 50°C higher than the solidus temperature of the alloy.

3. The low-pressure casting method according to claim 1, characterized in that, In step b), when applying the basic compensation pressure P1, a flow control mode is adopted, specifically: the dry compressed gas is controlled by a proportional valve to be introduced into the holding furnace at a constant flow rate Q1 until the reading of the pressure sensor in the furnace reaches P1. The constant flow rate Q1 ranges from 10 to 20 liters / minute.

4. The low-pressure casting method according to claim 1, characterized in that, The secondary boosting program in step c) uses pressure slope control. Specifically, after setting the target pressure P2 and boosting time Δt1, the pressure controller calculates and executes a fixed pressure ramp slope S1, S1 = (P2 - P1) / Δt1, and compares the real-time pressure with the theoretical pressure value based on the slope. The intake valve opening is adjusted through a PID algorithm so that the deviation between the actual pressure curve and the theoretical slope curve does not exceed ±5%.

5. The low-pressure casting method according to claim 1, characterized in that, The dynamic pressure fluctuation program in step d) is executed by the pressure fluctuation generator. The specific steps are as follows: First, the waveform generator generates a sine wave electrical signal with a frequency of f1 and an amplitude of ΔP1. Then, the electrical signal is sent as a set value to the controller of the high-speed electro-pneumatic proportional valve. The proportional valve adjusts the gas flow to the heat preservation furnace in real time according to this changing set value, thereby generating a pressure fluctuation field synchronized with the electrical signal in the furnace cavity.

6. The low-pressure casting method according to claim 1, characterized in that, The start time t1 of the secondary pressurization procedure described in step c) is determined by a solidification monitoring subsystem based on an ultrasonic probe. The specific steps are as follows: the ultrasonic probe is coupled to the outer wall of the mold, ultrasonic pulses are emitted into the casting and the echoes are received; by analyzing the attenuation rate of the echo signal amplitude, it is determined whether the surface solidified shell has reached the predetermined thickness; when the shell thickness is detected to be 3 to 5 mm, the ultrasonic subsystem sends a trigger signal to the main controller to start the secondary pressurization.

7. The low-pressure casting method according to claim 1, characterized in that, In step d), the parameters f1 and ΔP1 of the dynamic pressure fluctuation program are adaptively adjusted according to the structural characteristics of the casting. The specific logic is as follows: Before casting, the three-dimensional model of the casting is input into the control system, and the system automatically identifies the maximum wall thickness dmax and the hot spot volume Vh of the casting. Then, according to the pre-stored mapping relationship: f1 = a*dmax + b, ΔP1 = c*Vh + d, the fluctuation parameters are calculated, where a, b, c, and d are constants calibrated for different alloy materials.

8. The low-pressure casting method according to claim 1, characterized in that, Between step b) and step c), there is also a pressure stabilization transition step: after the pressure reaches P1, P1 is kept constant for a duration of Ts, where Ts ranges from 1 to 3 seconds, to ensure stable pressure sensor readings and initial strengthening of the casting surface shell.

9. The low-pressure casting method according to claim 1, characterized in that, The pressure grading and unloading process described in step e) is linked with the initial mold opening action of the mold. Specifically, in the first pressure unloading stage, when the pressure is controlled to drop to P2 / k, the mold opening mechanism is simultaneously instructed to perform the first micro displacement, the displacement being 5% to 10% of the total mold opening stroke, in order to break the initial vacuum adsorption force between the casting and the mold cavity. Each subsequent pressure unloading stage corresponds to one micro-movement mold opening.

10. The low-pressure casting method according to claim 1, characterized in that, All timing and pressure parameters of the entire multi-stage pressurization and feeding process are compiled into an executable casting process formula file; This file can be edited, saved, and retrieved through the user interface, and can be linked with the mold coding recognition system of the casting machine. When the system recognizes the currently installed mold code, it automatically loads and executes the specific process formula file uniquely bound to that mold.

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