Method for measuring gas content in die casting

Abstract

The invention discloses a method for measuring the gas content in a die casting, and belongs to the technical field of material detection. The method comprises the following steps: loading a sample into the closed desorption cavity; vacuumizing and pre-degassing the cavity by using an oil-free high-vacuum acquisition module formed by connecting a molecular pump and a dry-type screw pump in series; stepwise heating is carried out on the sample through program temperature control; the released gas is guided into a differential pressure sensing metering unit through a low-thermal-conductivity aluminum oxide ceramic isolation tube; the unit comprises a reference cavity and a measuring cavity which are arranged in the same constant temperature device, and the gas inflow rate is dynamically represented by measuring the pressure difference between the two cavities; and based on the ideal gas state equation and a calculation model containing cavity thermal expansion compensation, calculating the amount of the gas substance in unit time in real time. The problems of poor precision and repeatability caused by oil pollution, thermal interference and discontinuous metering are solved, and continuous, dynamic and high-repeatability automatic measurement of the gas content in the die casting is realized.

Description

Technical Field

[0001] This invention belongs to the field of materials testing and casting technology, specifically a method for determining the gas content inside die-cast parts. Background Technology

[0002] Die casting is widely used in high-end manufacturing industries such as automotive, aerospace, and electronics due to its advantages of high efficiency, near-net-shape forming, and excellent surface quality. However, during die casting, air or gases from the decomposition of release agents inevitably get trapped inside the molten metal, creating dispersed porosity defects. These internal gases not only significantly weaken the mechanical strength and fatigue life of the casting but also induce blistering, deformation, and even cracking during subsequent heat treatment, severely restricting the consistency and reliability of product performance.

[0003] The invention patent with announcement number CN105300834B involves heating a die-cast sample to a specific temperature within a sealed crucible, causing the release of adsorbed or dissolved gases. A multi-stage vacuum system, consisting of a mechanical pump and an oil diffusion pump, is used to extract, collect, and measure the volume of the released gases. A quantitative gas supply module is also introduced for system calibration to improve the accuracy of the measurement results. From a technological evolution perspective, this scheme is the first to combine thermal desorption under vacuum with quantitative gas measurement, effectively avoiding the problem of difficulty in capturing gas escapes at atmospheric pressure. It achieves, to a certain extent, indirect quantification of the gas content in die-cast parts, providing data support for subsequent process improvements.

[0004] However, while existing oil diffusion pumps can achieve high vacuum, their working medium is high-temperature silicone oil. Under frequent start-stop or pressure fluctuations, there is a risk of oil vapor backflow, potentially contaminating the sample surface and the inside of the measurement chamber, thus affecting gas composition identification and volume measurement. Furthermore, this system relies on the coordinated switching of dual quantitative tubes and multiple shut-off valves to complete the gas acquisition and calibration process. Under typical conditions where the gas release rate exhibits non-steady-state characteristics (e.g., rapid initial release followed by slow subsequent precipitation), its continuous response capability to the dynamic release process is limited, and the repeatability between multiple measurements is not ideal in small-batch, multi-variety production scenarios. This solution integrates the two major functions of "high vacuum acquisition" and "precise gas measurement" into the same complex flow path, without decoupling the gas release kinetics and the metering sensing mechanism, thus creating technical challenges in balancing system simplification, contamination resistance, and dynamic adaptability.

[0005] The large-scale application of existing methods in industrial settings is hampered by several factors: firstly, the high cost of equipment and relatively complex operation procedures limit their adoption in small and medium-sized die-casting enterprises; secondly, measurement results are easily affected by operator experience and environmental disturbances, falling short of the data reliability requirements of intelligent manufacturing systems. Therefore, an isolation method capable of blocking high-temperature heat conduction and a constant-temperature scheme capable of maintaining a highly stable temperature for the metering unit are needed to eliminate the impact of thermal drift on measurement accuracy. Summary of the Invention

[0006] The purpose of this invention is to provide a method for determining the gas content inside die-cast parts, which solves the technical problems of sample contamination and system complexity caused by the use of oil diffusion pumps in the prior art, as well as the inability to accurately respond to the unsteady gas release process due to the use of discontinuous metering methods.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for determining the gas content inside a die-casting part, comprising: Step S1: The die-cast sample to be tested is loaded into a sealed desorption chamber made of Inconel 718 high-temperature resistant alloy. The desorption chamber is rigidly connected to the main vacuum system through a CF35 metal sealing flange. Step S2: Start the oil-free high vacuum acquisition module consisting of a molecular pump and a dry screw pump connected in series, evacuate the desorption chamber to an absolute pressure not higher than 5×10⁻³Pa, and maintain this vacuum level for not less than 10 minutes to complete the pre-degassing of the chamber. Step S3: The desorption chamber is programmed to be heated by an embedded resistance heater in a stepwise manner at a heating rate of 5°C / min to 20°C / min. The target temperature is set to 450°C to 650°C and held at the target temperature for no less than 30 minutes. Step S4: During the heating process, the gas released from the sample is introduced into the differential pressure sensing and metering unit through an alumina ceramic isolation tube with an inner diameter of 3 mm and a wall thickness of 2 mm. The two ends of the ceramic isolation tube are respectively sealed to the outlet of the desorption chamber and the inlet of the metering unit through VCR metal connectors. Step S5: The differential pressure sensing and measuring unit includes two reference chambers and a measuring chamber made of 316L stainless steel. The volumes of the two chambers are 50.00mL ± 0.05mL respectively, and they are placed in the same constant-temperature aluminum block to maintain temperature fluctuations not exceeding ± 0.1℃. In the initial state, both chambers are connected to the main vacuum system through independent angle valves and evacuated to the same high vacuum state. Then, the angle valves are closed to completely seal the reference chamber, while the measuring chamber is connected to the desorption chamber through a ceramic isolation tube. A high-precision capacitive differential pressure sensor is installed on the connecting pipe between the two chambers through a KF16 flange and outputs a pressure difference signal in real time. Step S6: The analog voltage signal output by the differential pressure sensor is sampled by a 24-bit Σ-Δ analog-to-digital converter and transmitted to the central data processing module, based on the ideal gas law. Calculate the amount of gaseous substance entering the measuring chamber per unit time; in The measured pressure difference value (unit: Pa); For measuring the effective volume of the cavity (unit: m³); This is the universal gas constant (valued at 8.314 J / (mol·K)). The real-time temperature of the thermostat block (unit: K) is acquired by a PT100 platinum resistance sensor and used for dynamic compensation. Step S7: Numerical integration is performed on the amount of gaseous substance calculated during the entire heating cycle to obtain the total amount of gaseous substance released from the sample. The gas molar volume of 22.414 L / mol under standard conditions (0℃, 101.325 kPa) is converted into a standard volume and used as the final measured value of the gas content inside the die casting.

[0008] Preferably, in step S1, the desorption chamber is equipped with a graphite heat insulation bushing, and the sample is suspended in the center of the chamber by a molybdenum wire basket to avoid direct contact with the chamber wall; the CF35 metal sealing flange uses a copper gasket to achieve vacuum sealing, and the flange bolts are tightened in three stages in a diagonal sequence to a torque value of 25 N·m.

[0009] Preferably, in step S2, the exhaust port of the dry screw pump of the oil-free high vacuum acquisition module is connected to the exhaust gas treatment device, the molecular pump speed is stabilized at 90,000 rpm, the vacuum gauge adopts a cold cathode Penning gauge, and its signal feedback is connected to the closed-loop control system to maintain the cavity pressure fluctuation less than ±5%.

[0010] Preferably, in step S3, the stepped heating program includes three temperature platforms: the first stage is to raise the temperature from room temperature to 300°C and hold it for 10 minutes to remove adsorbed water vapor from the surface; the second stage is to raise the temperature to the target temperature range; and the third stage is to hold the temperature at the target temperature. The heating process is regulated by a PID controller, and a thermocouple is placed 5 mm directly above the sample for temperature feedback.

[0011] Preferably, in step S4, the alumina ceramic isolation tube is 300mm long, and its low thermal conductivity blocks the heat conduction from the high temperature of the desorption chamber to the metering unit, preventing temperature drift in the measuring chamber; the VCR connector uses a nickel-based alloy sealing ring to ensure that vacuum sealing is maintained at 650℃.

[0012] Preferably, in step S5, the volumes of the reference cavity and the measuring cavity are calibrated by a laser interferometer, the constant temperature aluminum block is embedded with a circulating liquid cooling channel, the cooling medium is deionized water, and the flow rate is controlled at 1.5L / min; the differential pressure sensor has a range of 0 to 10 Pa, a resolution of 0.1 mPa, and a zero drift of less than 0.5 mPa / 24h.

[0013] Preferably, in step S6, the central data processing module uses an ARM Cortex-M7 microcontroller, which collects pressure difference and temperature data every 100ms. The calculation of the amount of gaseous substance uses a fourth-order Runge-Kutta method for differential approximation. The temperature compensation model includes a correction term for the thermal expansion coefficient of the cavity, and the correction formula is as follows: Where coefficient 3 is the conversion coefficient for converting the linear expansion coefficient to the volume expansion coefficient. The volume of the measuring cavity is calibrated (unit: m³). The coefficient of linear expansion for 316L stainless steel (value 16.5 × 10⁻⁻⁴). 6 K⁻¹), The reference temperature is calibrated (unit: K, value: 293.15 K).

[0014] Preferably, in step S7, the numerical integration uses the trapezoidal rule to accumulate the discrete time series, and the integration formula is as follows: ,in This represents the total amount of gaseous substance (unit: mol). The instantaneous amount of gaseous substance (in mol) during the i-th sampling period. The instantaneous amount of gaseous substance (in mol) during the (i-1)th sampling period. The sampling time step is 0.1s. The standard volume calculation results are automatically stored on the local SD card and uploaded to the factory MES system via the RS485 interface.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention completely eliminates the risk of oil vapor backflow contamination by employing an oil-free high-vacuum acquisition module. It decouples gas release kinetics from the metering mechanism using a differential pressure sensing and metering unit, and uses a constant-temperature dual-chamber structure to counteract environmental disturbances. Combined with a low-thermal-conductivity ceramic isolation tube to block thermal interference, it achieves a continuous linear response to the unsteady-state gas release process. The entire system requires no valve group or metering tube switching, simplifying the structure and increasing automation. Through high-precision differential pressure sensing and real-time temperature compensation, the relative standard deviation of multiple measurements is less than 3%, meeting the repeatability requirements of small-batch, multi-variety production scenarios. Simultaneously, it eliminates the need for oil diffusion pumps and complex calibration processes, significantly reducing equipment maintenance costs and operational barriers, making it suitable for the online quality control needs of small and medium-sized die-casting enterprises. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is the overall flowchart of the present invention. Detailed Implementation

[0018] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the embodiments of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0019] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0020] Example 1: See Figure 1 This embodiment discloses a system for measuring the gas content inside a die-casting part. The system consists of four main parts: a desorption chamber, an oil-free high vacuum acquisition module, a differential pressure sensing and metering unit, and a central data processing module. Each part is connected to the electrical circuit through a vacuum pipeline to form a complete working system.

[0021] The desorption chamber is rigidly connected to the oil-free high vacuum acquisition module via a CF35 metal sealing flange, and is also connected to the differential pressure sensing and metering unit via an alumina ceramic isolation tube. The analog signal output by the high-precision capacitive differential pressure sensor in the differential pressure sensing and metering unit is connected to the central data processing module via a shielded cable. The latter also receives temperature signals from PT100 platinum resistance thermometers and thermocouples, and coordinates and controls the overall system operation status.

[0022] The desorption chamber consists of a shell integrally machined from Inconel 718 high-temperature alloy, with a graphite heat insulation bushing nested inside. This bushing has a cylindrical structure, a thickness of 5 mm, and a height consistent with the interior of the chamber, in order to reduce heat radiation loss to the chamber wall during sample heating.

[0023] A molybdenum wire basket is installed on the central axis of the cavity. Its upper end is fixed to the insulating support at the top of the cavity, and its lower end is suspended. It is used to support the die-cast sample to be tested, ensuring that the sample does not come into direct contact with any part of the cavity during the heating process, and avoiding distortion of gas release behavior due to local overheating or uneven heat conduction.

[0024] Embedded resistance heaters are arranged spirally and evenly around the outer shell of the cavity. Their leads pass through a ceramic insulating sleeve at the bottom of the cavity and are connected to an external temperature control power supply. A CF35 metal sealing flange is located at the bottom of the desorption cavity and is integrally formed with the cavity shell by welding. The flange face has standard CF35 bolt holes. Annealed pure copper gaskets are used as the sealing medium. During assembly, a torque wrench is used to apply tightening torques of 10 N·m, 18 N·m, and 25 N·m in three stages diagonally to ensure uniform pressure on the sealing interface and prevent leakage during high-temperature vacuuming.

[0025] In addition, the top of the desorption chamber is provided with a thermocouple mounting hole, through which a K-type thermocouple probe is inserted and positioned 5mm directly above the sample to provide real-time feedback on the temperature around the sample, serving as the input signal for the PID temperature control system.

[0026] The oil-free high vacuum acquisition module consists of a dry screw pump and a molecular pump connected in series. The air inlet of the dry screw pump is connected to the exhaust port of the molecular pump through a bellows, and the air inlet of the molecular pump is connected to the bottom of the desorption chamber through a CF35 metal sealing flange.

[0027] The dry screw pump's exhaust port is equipped with a tail gas treatment device to adsorb any trace amounts of volatile substances that may escape, preventing environmental pollution. The molecular pump's operating speed is set to 90,000 rpm, and its start-up and shutdown are automatically controlled by the central data processing module based on the vacuum level signal. Vacuum measurement uses a cold cathode Penning gauge, installed on the KF16 interface on the side wall of the desorption chamber. Its output signal is connected to the closed-loop control system. When the chamber pressure exceeds 5 × 10⁻³ Pa, the molecular pump automatically starts to maintain the set vacuum level, with pressure fluctuations controlled within ±5%.

[0028] During the pre-degassing stage of the system, the dry screw pump is first turned on to roughly pump the system to below 1 Pa, and then the molecular pump is started to further pump to 5×10⁻³ Pa. This pressure is maintained for no less than 10 minutes to fully remove the residual gas adsorbed on the inner wall of the desorption chamber and the sample surface.

[0029] The differential pressure sensing and measurement unit includes a reference chamber, a measuring chamber, a constant-temperature aluminum block, and a high-precision capacitive differential pressure sensor. Both the reference chamber and the measuring chamber are precision-machined from 316L stainless steel, each with a nominal volume of 50.00 mL and a tolerance controlled within ±0.05 mL. Their volume values ​​are calibrated using a laser interferometer at standard temperatures.

[0030] Two chambers are embedded side-by-side inside the same thermostatic aluminum block, which measures 200mm × 150mm × 100mm. The block features a serpentine circulating liquid cooling channel with deionized water as the cooling medium, driven by an external constant flow pump at a stable flow rate of 1.5L / min to maintain temperature uniformity throughout the block. A PT100 platinum resistance sensor is also embedded within the block to monitor the ambient temperature in real time, controlling temperature fluctuations within ±0.1℃. Each chamber, both the reference and measurement chambers, has an independent angle valve at its top. The other end of each valve is connected to a branch pipe of the main vacuum system via a CF16 flange. During system initialization, both valves open simultaneously, evacuating both chambers to the same high vacuum level (≤5×10⁻³Pa) as the desorption chamber. The valves are then closed, completely sealing the reference chamber, while the measurement chamber is connected to the desorption chamber via an alumina ceramic isolation tube.

[0031] The alumina ceramic isolation tube is 300mm long, 3mm in inner diameter, and 2mm thick. Its two ends are sealed to the desorption chamber outlet and the measuring chamber inlet via VCR metal connectors. The VCR metal connectors use nickel-based alloy sealing rings, maintaining reliable metal-to-metal sealing performance even at 650℃.

[0032] The high-precision capacitive differential pressure sensor is installed in the middle of the transverse connecting pipe between the reference chamber and the measuring chamber via a KF16 flange. The sensor has a range of 0 to 10 Pa, a resolution of 0.1 mPa, and a zero drift of less than 0.5 mPa / 24h. Its output is a 0 to 5V analog voltage signal, which is transmitted to the central data processing module via a shielded twisted pair cable.

[0033] The central data processing module uses a 32-bit microcontroller based on the ARM Cortex-M7 core as the core processor and is equipped with a 24-bit Σ-Δ analog-to-digital converter. It synchronously acquires the differential pressure signal output by the high-precision capacitive differential pressure sensor, the temperature signal of the isothermal aluminum block fed back by the PT100 platinum resistance thermometer, and the temperature signal of the sample area acquired by the thermocouple at a period of 100ms.

[0034] Within each sampling period, the system first reads the current differential pressure ΔP (in Pa) and the real-time temperature T (in K) of the isothermal aluminum block, and then applies the ideal gas law: ; Calculate the amount of gaseous substance entering the measuring chamber per unit time; in: The effective volume of the measuring cavity (in m³, converted from the nominal volume of 50.00 mL) is used for measurement. R is the universal gas constant (8.314 J / (mol・K)).

[0035] To improve calculation accuracy, a correction term for the thermal expansion coefficient of the 316L stainless steel cavity was introduced into the temperature compensation model, i.e., the actual volume: ; in: For calibrated volume (unit: m³); The coefficient of linear expansion for 316L stainless steel (value 16.5 × 10⁻⁻⁴). 6 K⁻¹). The reference temperature is set (unit: K, value: 293.15 K); coefficient 3 is used to convert the linear expansion coefficient to the volume expansion coefficient.

[0036] The time derivative of the gaseous substance quantity is approximated using the fourth-order Runge-Kutta method to reduce integration errors caused by discrete sampling. Throughout the heating cycle, the system accumulates the gaseous substance quantity calculated for each time step (100 ms). The numerical integration method employs the trapezoidal rule to calculate the instantaneous gaseous substance quantity for each sampling period. (The calculation method is as described above) The results are accumulated, that is: ; in: Total amount of gaseous substance (unit: mol); The instantaneous amount of gaseous substance (in mol) during the i-th sampling period; The instantaneous amount of gaseous substance (in mol) during the (i-1)th sampling period.

[0037] After integration, the total amount of substance will be... Multiply the gas molar volume (in mol) by the standard gas volume (0℃, 101.325kPa) of 22.414 L / mol to obtain the standard volume of gas released from the sample (in mL). This value is the final determination result of the gas content inside the die casting.

[0038] In practice: First, the die-cast sample to be tested (e.g., an aluminum alloy ADC12 die-cast part, weighing approximately 50g) is placed into the molybdenum wire basket, the top cover of the desorption chamber is closed, and the seal is completed through the CF35 metal sealing flange.

[0039] Start the oil-free high vacuum acquisition module, evacuate the system to 5×10⁻³Pa and maintain it for 10 minutes to complete the pre-degassing.

[0040] The embedded resistance heater is then activated and runs according to the preset stepped heating program: The first stage involves raising the temperature from room temperature to 300℃ at a rate of 10℃ / min, and then holding the temperature at 300℃ for 10 minutes to remove the water vapor physically adsorbed on the sample surface. The second stage continues to heat to the target temperature (e.g., 550°C) at a rate of 15°C / min. The third stage involves maintaining a constant temperature of 550℃ for 30 minutes to ensure that the dissolved gases inside the sample are fully released.

[0041] During the entire heating process, the gas released from the sample is introduced into the measurement chamber through the alumina ceramic isolation tube. Since the reference chamber is in a closed state, a pressure difference is generated between the two chambers. This pressure difference is detected in real time by a high-precision capacitive differential pressure sensor and converted into a voltage signal.

[0042] The central data processing module continuously collects and processes data, updating the gas release rate every 100ms and simultaneously recording temperature changes. After heating is complete, the system automatically performs numerical integration and standard volume conversion, storing the results on a local MicroSD card and simultaneously uploading them to the factory's Manufacturing Execution System (MES) via an RS485 communication interface for quality traceability and process optimization.

[0043] In practical implementation, the desorption chamber, oil-free high-vacuum acquisition module, and differential pressure sensing and metering unit are physically isolated and connected by a metal sealing flange and a ceramic isolation tube. The CF35 metal sealing flange ensures a high-vacuum seal between the desorption chamber and the vacuum pump assembly, while the VCR metal connector guarantees a reliable connection between the alumina ceramic isolation tube and the metering unit inlet at high temperatures. A constant-temperature aluminum block maintains an isothermal environment between the reference chamber and the measurement chamber through liquid cooling circulation, eliminating measurement drift caused by ambient temperature fluctuations.

[0044] All vacuum chambers are constructed using materials with low outgassing rates (Inconel 718, 316L stainless steel, graphite, and alumina ceramic) to avoid introducing additional gas interference. The electrical control system and data acquisition system are integrated into a central data processing module, enabling fully automated operation without manual intervention for quantitative tube switching or calibration. Through the precise coordination and synergistic work of these components, this device can achieve highly repeatable and linear measurements of the gas content inside die-cast parts under unsteady-state gas release conditions.

[0045] To enable those skilled in the art to fully understand and implement this invention, the following detailed explanation of the operating principle and key structural coordination mechanism of this invention is provided in conjunction with a typical application scenario.

[0046] When measuring the internal gas content of an aluminum alloy die-cast part (material ADC12, mass approximately 50g) used in an automotive engine block, the sample was first placed in a molybdenum wire basket. The basket was suspended at the geometric center of the desorption chamber by a top insulating bracket, ensuring uniform heating around the sample and preventing metal-to-metal contact. This avoided distortion of the gas release rate due to thermal short circuits or localized overheating. The desorption chamber cover was then closed, and a CF35 metal sealing flange was used for overall sealing. Annealed pure copper gaskets formed a reliable metal-sealed interface after applying torque in stages (10 N·m, 18 N·m, 25 N·m), effectively preventing leakage under high-temperature, high-vacuum conditions.

[0047] After activating the oil-free high-vacuum acquisition module, the dry screw pump operates first, reducing the system pressure from atmospheric pressure to below 1 Pa. Subsequently, the molecular pump starts and accelerates to 90,000 rpm, further reducing the pressure within the desorption chamber to 5 × 10⁻³ Pa. A cold cathode Penning gauge monitors the chamber pressure in real time and feeds the signal back to the central data processing module, which uses this information to maintain the molecular pump's operation, keeping pressure fluctuations within ±5%. Maintaining this high-vacuum environment for 10 minutes effectively desorbs water vapor and air adsorbed on the chamber's inner wall and sample surface, eliminating background gas interference with subsequent measurements.

[0048] Subsequently, the embedded resistance heater begins to heat up according to a preset program: The first stage involves raising the temperature to 300℃ at a rate of 10℃ / min and holding it at that temperature for 10 minutes. This process primarily removes physically adsorbed water. The second stage involves further heating at 15℃ / min to 550℃; The third stage involves maintaining a constant temperature of 550℃ for 30 minutes to promote the complete release of hydrogen and other gases dissolved in the aluminum matrix. A type K thermocouple is used to collect the temperature 5 mm above the sample in real time, which is then used as the input to the PID controller to ensure accurate reproduction of the heating curve.

[0049] During heating, the gas released from the sample is introduced into the differential pressure sensing and measurement unit via an alumina ceramic isolation tube. This ceramic isolation tube 9 is 300mm long, 3mm inner diameter, and 2mm thick. Its low thermal conductivity (approximately 30W / m·K) effectively blocks heat conduction from the desorption chamber (up to 650℃) to the measurement chamber, preventing volume expansion or sensor drift caused by temperature rise in the measurement chamber. Simultaneously, the VCR metal connector uses a nickel-based alloy sealing ring, maintaining a metal-to-metal seal even at high temperatures, avoiding the outgassing or failure of traditional rubber or polymer seals under high-temperature vacuum.

[0050] In the differential pressure sensing and metering unit, both the reference chamber and the measuring chamber are initially evacuated to the same high vacuum state (≤5×10⁻³Pa) as the desorption chamber through their respective angle valves. Subsequently, the angle valves close, completely sealing the reference chamber, while the measuring chamber is connected to the desorption chamber via a ceramic isolation tube 9. When gas enters the measuring chamber, its pressure rises, while the pressure in the reference chamber remains constant, creating a pressure difference ΔP. This pressure difference is detected in real time by a high-precision capacitive differential pressure sensor. Since both chambers are embedded within a constant-temperature aluminum block 12, and the aluminum block 12 is cooled by a 1.5L / min deionized water circulation system to maintain temperature fluctuations ≤±0.1℃, changes in ambient temperature have a consistent effect on both chambers. The pressure difference signal only reflects the net pressure change caused by gas injection, effectively suppressing common-mode interference.

[0051] The central data processing module synchronously acquires the voltage signal output by the differential pressure sensor 13, the temperature T of the isothermal aluminum block fed back by the PT100 platinum resistance thermometer, and the temperature of the sample area measured by the thermocouple at a period of 100ms. Within each sampling period, the system processes the data according to the ideal gas law: Calculate the instantaneous amount of gaseous substance; in; To account for the actual volume of the 316L stainless steel body after expansion; The volume is specified in m³. The coefficient of linear expansion for 316L stainless steel is 16.5 × 10⁻⁻. 6 K⁻¹; The reference temperature is set in K, with a value of 293.15 K; a coefficient of 3 is used to convert the linear expansion coefficient to the volumetric expansion coefficient. This calculation model compensates for changes in the cavity's geometric dimensions caused by temperature fluctuations, significantly improving metrological linearity.

[0052] During the entire 30-minute isothermal phase, the system calculates the temperature every 100ms. The value is integrated numerically using the trapezoidal rule, that is: ,in This represents the total amount of gaseous substance (unit: mol). The instantaneous amount of gaseous substance (in mol) during the i-th sampling period. The instantaneous amount of gaseous substance (in mol) during the (i-1)th sampling period.

[0053] After obtaining the total amount of gaseous substances released, Multiplying by 22.414 L / mol converts the gas volume (mL) to standard conditions (0℃, 101.325 kPa), providing a quantitative result for the gas content inside the die-cast part. This result is automatically stored on a MicroSD card and uploaded to the factory's MES system via an RS485 interface for closed-loop optimization of process parameters.

[0054] In the above process, the oil-free high vacuum acquisition module completely avoids the risk of silicone oil vapor backflow caused by the oil diffusion pump, preventing contamination of the sample and chamber; the differential pressure sensing and metering unit, through its dual-chamber isothermal design and high-resolution differential pressure detection, achieves a continuous linear response to unsteady, low-flow gas release; the combination of the alumina ceramic isolation tube and the VCR metal connector ensures high-temperature sealing reliability while blocking thermal interference paths; the central data processing module integrates high-precision acquisition, real-time compensation, and automatic integration functions, eliminating the need for manual switching of the quantitative tube or calibration operations, significantly improving measurement repeatability and automation. Therefore, under typical operating conditions, this device can achieve a relative standard deviation of less than 3% for multiple measurements, meeting the core requirements of rapid, stable, and automated determination of gas content in die-cast parts in industrial settings.

[0055] Traditional solutions couple the high-temperature gas release chamber, high-vacuum maintenance, and gas metering function into the same complex flow path, resulting in structural redundancy and mutual interference. This invention decouples the high-temperature desorption process from the high-precision metering process both physically and functionally by introducing an independent differential pressure sensing and metering unit. The desorption chamber focuses on efficiently and controllably releasing gas under set temperature and vacuum conditions; while the independent metering unit, in a stable environment maintained by a thermostat, is dedicated to high-fidelity, continuous, and dynamic measurement of the gas inflow. This decoupled architecture allows for the separate optimization of the two subsystems, simplifies the overall process, and completely eliminates the direct interference of the high-temperature environment on the precision sensors.

[0056] This invention abandons the traditional static collection or intermittent sampling mode and proposes a continuous measurement method based on dynamic integration of differential pressure. Traditional methods rely on switching valves to introduce gas into a quantitative tube for volume measurement, making it difficult to capture the dynamic process of unsteady gas release. This invention utilizes a symmetrical differential structure composed of a reference chamber and a measurement chamber to convert the time-varying flow signal of gas release into a continuous, high-resolution pressure difference signal between the two chambers. This signal is detected in real time by a high-precision capacitive differential pressure sensor, and numerical integration is performed by a central data processing module to completely reconstruct the kinetic curve of gas release throughout the entire heating cycle, thereby obtaining the total release amount. This method achieves "full waveform" capture of the gas release process, and the measurement results more accurately reflect the overall gas content within the die-cast part.

[0057] This invention employs an oil-free high-vacuum acquisition module consisting of a molecular pump and a dry screw pump connected in series, eliminating oil vapor backflow contamination at the source. It utilizes an alumina ceramic isolation tube with a metal sealing joint, effectively blocking heat conduction from the high temperature of the desorption chamber to the differential pressure sensing and measurement unit while maintaining gas path connectivity, thanks to the low thermal conductivity of the ceramic material. The reference chamber and measurement chamber are precisely machined and placed within the same temperature-controlled device, ensuring both chambers are in a highly uniform thermal environment, thus converting the impact of ambient temperature fluctuations on the measurement into common-mode interference and suppressing it. Furthermore, the programmed temperature control of the embedded resistance heater, the use of a thermal insulation bushing and a basket, collectively ensure uniform sample heating and accurate gas release.

[0058] This invention, through architectural decoupling, principle innovation, and component collaborative design, forms a complete and autonomous solution. It constructs a novel methodology and hardware system capable of continuous, dynamic, interference-resistant, and highly repeatable automatic measurement of the gas content inside die-cast parts, providing a reliable data foundation for precision control and quality improvement in the die-casting process.

[0059] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. It should be noted that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the gas content inside a die-casting part, characterized in that, Includes the following steps: Step S1: Place the die-cast sample to be tested into the sealed desorption chamber; Step S2: Evacuate the desorption chamber and pre-degas it; Step S3: Perform programmed temperature control heating on the desorption chamber; Step S4: The gas released from the sample during the heating process is introduced into the differential pressure sensing and measurement unit through the isolation tube; Step S5: The differential pressure sensing and metering unit includes a reference chamber and a measuring chamber. Initially, both chambers are evacuated to the same vacuum state. Then, the reference chamber is closed, and the measuring chamber is connected to the desorption chamber through the isolation tube. The gas inflow is characterized by measuring the pressure difference between the reference chamber and the measuring chamber. Step S6: Calculate the amount of gaseous substance entering the measuring chamber per unit time based on the pressure difference, the known volume and temperature of the measuring chamber; Step S7: Integrate the amount of gaseous substance during the heating cycle to obtain the total amount of gas released, and convert it into the volume under standard conditions as the measured value.

2. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S1, the desorption chamber is equipped with a heat insulation liner, and the sample is suspended in the center of the chamber by a basket.

3. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S2, an oil-free high vacuum acquisition module consisting of a molecular pump and a dry screw pump connected in series is used to evacuate the vacuum.

4. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, The programmed temperature control heating in step S3 is a stepped heating, which includes at least a first temperature plateau for removing adsorbates from the sample surface and a second temperature plateau for fully releasing the gas inside the sample.

5. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S4, the isolation tube is a low thermal conductivity ceramic tube, and its two ends are connected by metal sealing joints.

6. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S5, the reference cavity and the measuring cavity are placed in the same constant temperature device, and the volumes of the reference cavity and the measuring cavity are calibrated.

7. The method for determining the gas content inside a die-casting part according to claim 6, characterized in that, In step S5, the constant temperature device is equipped with a circulating liquid cooling channel.

8. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S6, when calculating the amount of the gaseous substance, the volume of the measuring cavity is compensated for thermal expansion according to temperature changes.

9. The method for determining the gas content inside a die-casting part according to claim 1, characterized in that, In step S1, the desorption chamber is connected to the vacuum system via a metal sealing flange.

10. The method for determining the gas content inside a die-casting part according to claim 2, characterized in that, The heat insulation bushing is a graphite bushing.

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