Device and method for measuring pure electroplastic effect based on contact cooling in electrically assisted forming

Abstract

A device and method for measuring the pure electroplasticity effect of electrically assisted forming based on contact cooling includes a loading unit, an energizing unit, a contact cooling unit, a measuring unit, and a control unit. The core of the method lies in: firstly, conducting a friction calibration experiment to quantify systematic errors; then, under closed-loop temperature control, simultaneously performing energizing, deformation, and dynamic cooling to stabilize the sample temperature at the target temperature to completely offset the Joule heating effect; finally, correcting the measurement results based on the calibration data, and comparing the corrected energized deformation data with the thermal baseline data; the difference is the precise quantification value of the pure electroplasticity effect. This invention uses a high thermal conductivity insulating ceramic sheet for contact cooling, fundamentally avoiding the material chemical contamination problem caused by liquid nitrogen spraying, and through an original friction calibration and correction process, eliminating systematic errors introduced by contact, thus achieving in-situ, accurate, and non-destructive measurement of the pure electroplasticity effect of materials.

Description

Technical Field

[0001] This invention relates to a technology in the field of electro-assisted forming, specifically a device and method for measuring the pure electroplastic effect of electro-assisted forming based on contact cooling. Background Technology

[0002] Electro-assisted forming (EAM) technology significantly reduces material flow stress and improves forming limits by applying high-density current during material deformation. Its mechanism is generally considered to stem from the coupling of non-thermal pure electroplasticity and Joule heating. To separate the pure electroplasticity effect, the influence of Joule heating must be eliminated. One mainstream method is active cooling, which involves cooling the sample simultaneously with deformation to offset the heat generated by the current and maintain the sample temperature at its initial state (e.g., room temperature). Currently, most publicly available solutions employ liquid nitrogen spraying as a cooling method. This involves directly spraying liquid nitrogen onto the sample surface, allowing the intense vaporization of the liquid nitrogen to absorb heat and remove Joule heat. However, this method has a fatal flaw: the low temperature of liquid nitrogen alters the sample surface and surrounding atmosphere. For many metallic materials (such as titanium alloys, magnesium alloys, and certain high-strength steels), liquid nitrogen spraying introduces extremely high nitrogen partial pressure and an extremely low temperature environment, resulting in a significantly higher nitrogen content on the material surface and even in the internal regions compared to the original material. During testing, the material may undergo chemical reactions or physical adsorption with nitrogen. Therefore, the "electroplastic effect" measured using such methods is actually a mixture of "electroplastic effect + material composition change effect," which deviates significantly from the true properties of the material. Furthermore, liquid nitrogen spraying also presents problems such as high cost and high safety risks. Therefore, developing a measuring device capable of precise temperature control near room temperature and completely avoiding any physicochemical reaction between the sample and the cooling medium has become a pressing technical challenge in this field. Summary of the Invention

[0003] To address the aforementioned shortcomings of existing technologies, this invention proposes a device and method for measuring the pure electroplastic effect of electric-assisted forming based on contact cooling. This device can rapidly dissipate the Joule heat generated by the current while the material is deformed by electricity, through efficient thermal management, thereby stabilizing the temperature of the sample deformation zone at room temperature and achieving direct, in-situ, and accurate measurement of the pure electroplastic effect.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a device for measuring the pure electroplasticity effect of electric-assisted forming based on contact cooling, comprising: a universal testing machine, an electrical system, a cooling system, a temperature monitoring system, and a data acquisition system.

[0006] The universal testing machine is used to apply and control loads on specimens to achieve material deformation (such as tension and compression).

[0007] The power supply system includes a DC / pulse power supply, insulating clamps, and wires. The sample is clamped onto the testing machine by two insulating clamps, and the positive and negative terminals of the power supply are connected to the upper and lower clamps respectively to apply a high-density current to the sample.

[0008] The cooling system comprises a pair of highly thermally conductive insulating ceramic sheets, a refrigeration unit, and a heat-conducting medium. The ceramic sheets possess excellent thermal conductivity (e.g., aluminum nitride, AlN; beryllium oxide, BeO) and electrical insulation, enabling efficient heat conduction while ensuring complete current flow through the sample without short circuits. The ceramic sheets are lightly pressed against both sides of the sample by a low-stiffness, flexible clamping mechanism, with the interfaces coated with high-temperature thermally conductive, insulating grease to achieve efficient heat conduction, electrical insulation, and minimized friction. The refrigeration unit is a thermoelectric cooler (TEC) or a circulating liquid cooler, with its cold end tightly connected to the outer surface of the ceramic sheets via thermally conductive silicone grease or adhesive to continuously remove the heat absorbed by the ceramic sheets.

[0009] The temperature monitoring system includes at least one high-response infrared thermometer for real-time monitoring and feedback of the sample temperature.

[0010] The data acquisition system is connected to the sensors, power supply, and temperature monitoring system of the universal testing machine to synchronously acquire signals such as force, displacement, current, voltage, and temperature.

[0011] This invention relates to a method for measuring the pure electroplasticity effect of electro-assisted forming based on contact cooling using the aforementioned device, comprising:

[0012] S1. Mount the test sample onto the universal testing machine and clamp it securely using the upper and lower insulating clamps, ensuring a firm clamping and a circuit with the power supply. Gently press and tightly adhere a pair of high thermal conductivity insulating ceramic sheets to both sides of the test area of ​​the sample using a low-rigidity flexible clamping mechanism. Simultaneously, apply a thin layer of high-temperature thermally conductive insulating grease to the contact interface to minimize friction while ensuring thermal contact. Start the data acquisition system and temperature monitoring system, and zero and initialize the various sensors.

[0013] S2. Friction calibration is used to quantify and eliminate systematic errors introduced by the ceramic plate. Under conditions of no power supply and no active cooling, the universal testing machine is started to load and deform the specimen. The load-displacement curve obtained in this test is compared with the baseline curve obtained under the same conditions for a specimen without any ceramic plate installed. The difference between the two curves is the additional friction-displacement curve introduced by the ceramic plate contact system, which will be used for subsequent data correction.

[0014] S3. Replace the sample. Without power or deformation, pre-start the cooling unit of the cooling system to begin operation. Confirm through the temperature monitoring system that the sample temperature can be stably controlled at the preset target temperature (e.g., room temperature 25℃). Simultaneously, perform a blank test without power on the universal testing machine and collect a load-displacement curve at the target temperature as baseline data for subsequent comparison.

[0015] S4. Replace the sample and start the universal testing machine to load the sample; at the same time, start the power supply system, and the power supply applies current to the sample according to preset parameters (such as current density, frequency, duty cycle); simultaneously, the cooling system runs, and the refrigeration unit performs contact forced heat conduction on the sample through a high thermal conductivity insulating ceramic sheet.

[0016] S5. The temperature signal of the test area of ​​the sample is collected in real time and continuously through a temperature monitoring system (such as an infrared thermometer). This temperature signal is compared with the preset target temperature value, and the power of the cooling unit is dynamically adjusted through a closed-loop control algorithm (such as a PID controller). The control objective is to instantly cancel out the Joule heating effect generated by power-on through forced cooling, so that the temperature of the deformation zone of the sample remains dynamically stable near the preset target temperature (fluctuation range ±1~2℃).

[0017] S6. Through the data acquisition system, the following data are simultaneously acquired and recorded throughout the entire process (S4-S5): time, load, displacement / strain, current, voltage, and sample temperature.

[0018] S7. After the experiment, the data collected in S6 is corrected based on the additional friction force collected in S2, and processed into a stress-strain curve under constant target temperature and energized condition, eliminating the influence of friction. Simultaneously, the data collected in S3 is corrected based on the additional friction force collected in S2, obtaining a stress-strain curve under constant target temperature and non-energized condition, eliminating the influence of friction. The stress difference between the two curves at the same strain represents the stress drop caused by the pure electroplastic effect after removing all thermal effects, thus achieving a quantitative measurement of the pure electroplastic effect.

[0019] Technical effect

[0020] This invention completely avoids cooling medium contamination: Compared with existing liquid nitrogen jet cooling technology, this invention uses solid, inert, high thermal conductivity ceramic sheets for contact heat conduction cooling. The sample is always in a dry air environment, completely isolating it from contact with active cooling media (such as liquid nitrogen). This fundamentally avoids the risk of sample surface oxidation, nitriding, or reactions with other elements, ensuring the consistency of material composition before and after testing, thus measuring the true pure electroplastic effect of the material. This invention provides more precise and uniform temperature control: Compared with jet convection heat transfer, contact heat conduction is more efficient and uniform in heat transfer, avoiding temperature gradients and large internal stresses caused by localized overcooling of the sample, which helps improve the accuracy and repeatability of data. This invention has wide applicability: The device has a simple structure and can be integrated into a conventional universal testing machine. It is suitable for testing various electro-assisted forming processes (such as tension and compression) and various metallic materials (such as steel, aluminum alloys, magnesium alloys, and titanium alloys). This invention employs closed-loop temperature control, which features fast response speed, high temperature control accuracy, and adaptability to experimental requirements under different current parameters. It is also safer and more economical, avoiding the safety hazards and costs associated with using and storing cryogenic liquid nitrogen. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the present invention;

[0022] Figure 2 This is a partially enlarged schematic diagram of the sample, ceramic cooling plate, and temperature measuring point locations in this example.

[0023] In the diagram: 1 Universal testing machine, 2 Upper insulating clamp, 3 Lower insulating clamp, 4 Sample, 5 High current pulse DC power supply, 6a First high thermal conductivity insulating ceramic sheet, 6b Second high thermal conductivity insulating ceramic sheet, 7 Semiconductor cooling chip, 8 Infrared thermometer, 9 Data acquisition card, 10 Flexible clamping mechanism, 11 Thermally conductive insulating grease. Detailed Implementation

[0024] like Figure 1As shown, this embodiment relates to a device for measuring the pure electroplastic effect of electrically assisted forming based on contact cooling. S1: A dog-bone shaped metal tensile specimen 4 is mounted on a universal testing machine 1, with its upper and lower ends clamped by an upper insulating clamp 2 and a lower insulating clamp 3, respectively, ensuring a secure clamping. Ceramic sheets 6a and 6b are lightly pressed onto the specimen surface by a low-stiffness, finely adjustable flexible clamping mechanism 10. This pressure is only sufficient to ensure that the ceramic sheets 6a and 6b move with the surface of the specimen 4 and maintain good thermal contact. A thin layer of thermally conductive insulating grease 11 is applied to the contact interface between the ceramic sheets 6a and 6b and the specimen, and sufficient thermally conductive silicone grease is applied between the ceramic sheets 6a and 6b and the cold end of the semiconductor cooling chip 7. The infrared thermometer 8 is mounted on a bracket, and its position and focus are adjusted so that its laser temperature measurement point is precisely aligned with the center area of ​​the gauge length of the specimen 4. Finally, all electrical circuits are connected, the data acquisition system 9 and the temperature monitoring system 8 are turned on, and each sensor is zeroed and initialized.

[0025] Furthermore, the contact surfaces of the high thermal conductivity insulating ceramic sheets 6a and 6b should be processed to an extremely high degree of smoothness;

[0026] Furthermore, ensure that the two ceramic plates 6a and 6b are precisely aligned, with their planes completely parallel to the tensile axis of the specimen, to avoid generating any component force perpendicular to the tensile direction.

[0027] S2. Under conditions of no power supply and no active cooling, start the universal testing machine 1 to load and deform the specimen 4. Compare the load-displacement curve obtained this time with the baseline curve obtained by the specimen 4 without any ceramic sheet installed under the same conditions. The difference between the two curves is the additional friction force-displacement curve introduced by the ceramic sheet contact system. This curve will be stored for subsequent data correction.

[0028] Furthermore, the strain rate is taken as 10⁻³ s⁻¹. -1 .

[0029] S3. Activate the semiconductor cooling chip 7 to lower the temperature of ceramic plates 6a and 6b. Observe the reading of the infrared thermometer 8 to confirm that the temperature of sample 4 can be stabilized at the preset target temperature (25±0.5)℃. After the temperature stabilizes, activate the universal testing machine 1 and perform a tensile test on sample 4 without power at the set strain rate. The data acquisition system 9 synchronously records the load-displacement data of this experiment, processes it, and obtains a stress-strain curve at room temperature, which is saved for later use. After the experiment is completed, reset the crossbeam of the testing machine.

[0030] S4. Replace with a new sample and repeat step S1 to complete the clamping. Set all experimental parameters: strain rate, target current density, pulse frequency, duty cycle, and target temperature (25°C). Simultaneously perform the following operations:

[0031] Start the universal testing machine 1 and begin tensile testing on specimen 4.

[0032] Start the high-current pulsed DC power supply 5 and apply a high-density pulsed current to the sample 4 according to the preset parameters.

[0033] The semiconductor cooling chip 7 is activated to force-cool the electrically heated sample.

[0034] S5. Throughout the deformation process, the infrared thermometer 8 acquires the temperature signal of the gauge length section of sample 4 at a high frequency (e.g., 100Hz) in real time and transmits the data to the PID temperature controller of the semiconductor cooler 7 in real time. This controller compares the measured temperature with the target temperature (25℃) and dynamically and automatically adjusts the current output to the semiconductor cooler 7 based on the difference, thereby precisely controlling its cooling power. Through this closed-loop feedback control, it is ensured that the temperature of sample 4 remains stable within the range of 25±1℃ under the strong Joule heating effect.

[0035] S6. Throughout steps S3 and S4, the data acquisition system 9 continuously acquires and records the following time-series data in a synchronously triggered manner: time, load and displacement signals from the testing machine, current and voltage signals from the power supply, and temperature signals from the infrared thermometer 8.

[0036] S7. After the experiment, the data collected in S6 is corrected based on the additional friction force collected in S2, and processed into a stress-strain curve under constant target temperature, in an energized state, eliminating the influence of friction. The data collected in S3 is also corrected based on the additional friction force collected in S2, and processed into a stress-strain curve under constant target temperature, in a non-energized state, eliminating the influence of friction. These two curves are then superimposed and compared. The stress difference between the two curves at the same strain point (e.g., 0.05, 0.10, 0.15) is calculated. This stress difference can be quantitatively characterized as the magnitude of the decrease in flow stress caused by the pure electroplastic effect under the current electrical parameters.

[0037] In summary, through the above steps, particularly the synchronous start-up in S4 and the closed-loop temperature control in S5, this invention successfully maintained the isothermal conditions of the sample during the electro-deformation process, thereby achieving precise exfoliation and measurement of the pure electroplastic effect. Throughout the process, the sample was kept in a room-temperature air environment, completely avoiding contamination problems caused by active cooling media such as liquid nitrogen, ensuring the authenticity and reliability of the data.

[0038] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. An application of a device based on contact cooling separation and measurement of pure electroplasticity effect, i.e., measuring pure electroplasticity effect, characterized in that, The device includes: a loading unit, a power supply unit, a contact cooling unit, a measuring unit, and a control unit; The contact cooling unit includes at least one pair of high thermal conductivity insulating ceramic sheets and a cooling module. The ceramic sheets are in contact with the surface of the test area of ​​the sample through a flexible clamping mechanism. The contact interface is provided with a thermally conductive insulating medium for actively cooling the energized sample. The device operation method includes a step of performing a friction calibration experiment to eliminate system errors introduced by the cooling unit; The flexible clamping mechanism is a spring or elastic sheet that can provide a small and constant positive pressure; The high thermal conductivity insulating ceramic sheet is made of aluminum nitride, beryllium oxide, or high-purity alumina ceramic. The thermally conductive and insulating medium is a high-temperature thermally conductive and insulating grease, such as boron nitride-based or aluminum oxide-based thermally conductive silicone grease; The cooling module is a semiconductor cooling chip or a circulating liquid cooling system; The control unit uses a PID algorithm to dynamically adjust the power of the cooling module based on the temperature signal fed back by the measurement unit, thereby achieving closed-loop temperature control. The measurement of pure electroplasticity includes: Clamp the sample by gently pressing and tightly adhering the high thermal conductivity insulating ceramic sheet to both sides of the sample's test area using the clamping mechanism. A friction force calibration experiment was conducted to obtain the relationship between the additional friction force and displacement of the system. At the target temperature, a deformation experiment without electricity was conducted to obtain thermodynamic baseline data; Simultaneously initiate power-on, deformation, and dynamic cooling, and stabilize the sample temperature at the target temperature through closed-loop control; Collect total load, displacement, electrical parameters, and temperature data during the experiment; The total load is corrected based on the aforementioned friction-displacement relationship to obtain the true deformation force; the true deformation force data is compared with the baseline data at the target temperature, and the difference is the quantification value of the pure electroplastic effect.

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