A sheet metal forming limit testing apparatus and method

Abstract

The present application belongs to the technical field of sheet forming limit test, and particularly relates to a metal sheet forming limit test device and method. The device comprises a forming die system, a heating system, a temperature monitoring and feedback control system, a cooling system, a strain detection system and a general control system. The heating system integrates resistance and induction heating modules to realize heating rate adjustment of 1-100 ℃ / s. The temperature monitoring and feedback control system acquires the global temperature field of the sheet through a thermal imager in real time and dynamically adjusts the heating power. The cooling system supplies the lower die with cooling medium with adjustable temperature and flow rate to simulate the cooling effect of the die in stamping. The strain detection system collects deformation images and calculates strain in real time. The general control system coordinates the subsystems to realize automatic test of the whole process. The present application solves the problems of low heating rate and inability to simulate real non-isothermal forming conditions in the prior art, and provides accurate forming limit data for rapid hot stamping process.

Description

Technical Field

[0001] This invention belongs to the field of sheet metal forming limit testing technology, specifically relating to a sheet metal forming limit testing device and method. Background Technology

[0002] The forming limit of sheet metal is a key indicator for measuring its plastic deformation capacity under specific process conditions, directly determining the forming quality and process window design of hot-stamped parts. Traditional high-temperature forming limit testing technology originated from traditional hot forming processes, which have relatively low requirements for heating rates, generally between 3℃ / s and 5℃ / s, and the die and sheet metal are heated simultaneously. The core design of the testing equipment revolves around "stable testing under isothermal conditions." With the surge in demand for lightweight, high-strength parts in the automotive, aerospace, and other fields, rapid hot stamping forming processes have been widely used. This process requires rapid heating to bring the sheet metal to a very high stamping temperature in a short time, followed by rapid contact with a room-temperature die, obtaining high-strength, high-precision parts in an integrated "quenching + forming" process. The core advantage of this process relies on the synergistic effect of "rapid heating" and "non-isothermal forming": rapid heating avoids coarse grains in the material, ensuring uniform microstructure; non-isothermal forming achieves quenching and strengthening through rapid cooling of the die.

[0003] However, existing sheet metal forming limit testing equipment generally adopts the method of placing the sheet metal and the mold in a heating furnace for simultaneous heating, and then conducting forming limit testing under constant temperature environment after heating to a preset temperature. In this method, the sheet metal and the mold must be heated simultaneously during the testing phase, and the entire heating process is completed because the sheet metal is clamped between the upper and lower molds or stamping fixtures. The heating rate can generally only be maintained between 3℃ / s and 5℃ / s, and the temperature change of the sheet metal surface is in a "blind box" state, resulting in a large deviation between the measured forming limit data and the actual forming performance, which cannot provide a reliable basis for process optimization. Summary of the Invention

[0004] To address the problems existing in the prior art, the purpose of this invention is to provide a metal sheet forming limit testing device and method that enables wide-range adjustment of the heating rate, accurately simulates real non-isothermal forming conditions, controls the temperature uniformity across the entire surface of the sheet, facilitates the acquisition of the plastic state of the sheet after rapid heating, and provides accurate forming limit data for rapid hot stamping processes.

[0005] The technical solution of this invention is: A metal sheet forming limit testing device, comprising: The forming mold system has an upper mold and a lower mold arranged opposite each other, and the lower mold is provided with a cooling channel; The heating system includes a resistance heating module and an induction heating module disposed between the upper mold and the lower mold, which are used to adjust the heating rate of the sheet material by working alone or in concert. The temperature monitoring and feedback control system includes at least one thermal imager installed above the upper mold for real-time imaging and acquisition of the temperature field distribution across the entire plate. The cooling system is connected to the cooling channel of the lower die through a pipeline, and is used to introduce a cooling medium with adjustable temperature and flow rate into the cooling channel to simulate the cooling effect of the die on the sheet metal during the actual stamping process. The strain detection system includes at least two cameras symmetrically mounted on both sides of the lower or upper mold, and a DIC analysis module. The cameras are used to acquire surface deformation images in real time during the forming process of the sheet metal. The DIC analysis module is electrically connected to the cameras and is used to obtain the strain distribution of the sheet metal based on the acquired image data. The central control system is electrically connected to the forming mold system, heating system, thermal imager, cooling system, and DIC analysis module, respectively. It is used to dynamically control the start-up and shutdown and power of the heating system, the movement of the forming mold system, and the parameters of the cooling medium based on the temperature field distribution of the entire sheet material, so as to form a closed-loop regulation. It also collects the sheet material strain distribution obtained by the DIC analysis module and compares it with a preset threshold to control the start-up and shutdown status of each system.

[0006] Preferably, the resistance heating module and the induction heating module are integrated into a composite heating head; wherein, the resistance heating module includes a resistance wire array disposed on an insulating support and divided into multiple independent temperature control zones, and the induction heating module includes an induction coil surrounding the resistance wire array; the insulating support is mounted on the worktable via a lifting and rotating mechanism, the lifting and rotating mechanism being used to drive the insulating support to rotate the composite heating head into the gap between the upper mold and the plate during the heating stage to heat the plate, and to rotate the gap out before forming test.

[0007] Preferably, the forming mold system further includes a driver and a pressing assembly. The output end of the driver is connected to the lower mold and is used to drive the lower mold to move linearly in the direction of the upper mold. The pressing assembly is disposed on the worktable and is used to apply pressure to the edge of the sheet metal.

[0008] Preferably, the cooling system includes a low-temperature water tank, a high-temperature water tank, and a three-way proportional mixing valve. The three-way proportional mixing valve has two inlets and one outlet. The two inlets are respectively connected to the outlets of the low-temperature water tank and the high-temperature water tank, and the outlet is connected to the pipeline for providing a temperature-adjustable circulating cooling medium to the cooling channel of the lower mold.

[0009] Preferably, the central control system includes: The first PLC controller is electrically connected to the driver, heating system, and temperature monitoring and feedback control system of the forming mold system, respectively, and is used to control the power of the heating system, the motion parameters of the forming mold system, and the blank holder force; The second PLC controller is electrically connected to the cooling system and is used to control the temperature and flow rate of the cooling medium in the cooling system according to a preset temperature curve. The host computer communicates with the first PLC controller, the second PLC controller, the thermal imager, and the DIC analysis module, and is used for parameter setting, data acquisition, processing, and display.

[0010] A method for testing the forming limit of metal sheets, implemented using any of the aforementioned devices, includes the following steps: Test parameters are set in the overall control system, including heating rate, target heating temperature, lower mold motion parameters, deformation rate, and necking judgment threshold. The heating system and thermal imager are started, and the board is heated according to the set heating rate. The thermal imager collects the full-area temperature image of the board in real time and transmits it to the central control system. The central control system compares the measured temperature with the target temperature through the control module, and dynamically adjusts the output power of the heating system based on the PID algorithm until the temperature of the board reaches and stabilizes at the target temperature. The strain detection system is activated, and the lower die of the forming mold system is controlled to move towards the upper die according to the set motion parameters to apply forming force to the sheet metal. At the same time, the cooling system is controlled to introduce a cooling medium with a preset temperature and flow rate into the cooling channel of the lower die. The strain detection system acquires surface images of the sheet metal in real time through a camera and calculates the strain distribution of the sheet metal through the DIC analysis module. When the DIC analysis module detects that the local strain of the sheet material reaches the necking threshold, it controls the forming mold system, heating system and strain detection system to stop working. Based on the image data acquired by the strain detection system, the ultimate principal strain and secondary strain of the necking region are extracted to generate the forming limit value.

[0011] Preferably, the heating rate range of the heating system adopts different heating modes: When the heating rate is between 1℃ / s and 20℃ / s, the resistance heating module is used for heating, and the induction heating module is turned off. When the heating rate is 20℃ / s to 60℃ / s, the induction heating module is used as the main heat source, and the resistance heating module is used to compensate for the local low temperature area identified by the thermal imager. When the heating rate is between 60℃ / s and 100℃ / s, the induction heating module outputs a constant power for rapid heating. Within a preset temperature difference range before the plate temperature approaches the target temperature, the power of the induction heating module is reduced, and the resistance heating module performs the final temperature regulation.

[0012] Preferably, the method for determining the cooling medium at the preset temperature and flow rate includes the following steps: Step 1: Establish a simulation model of the actual hot stamping process in finite element software, calculate the temperature change of the sheet metal throughout the stamping process, and obtain the target cooling curve. ; Step 2: Establish an experimental equivalent model of the testing equipment in the finite element software, and calculate the experimental temperature change curve of the plate when cooling is performed only through the lower mold cooling channel. ; Step 3: Using the temperature of the cooling medium and traffic Using variables as variables and minimizing the error function as a constraint, the predicted temperature curve is calculated iteratively. With the target cooling curve The error between them is minimized, thus determining the optimal solution. and combination.

[0013] Preferably, the flow rate of the cooling medium Through the convective heat transfer coefficient Influence on the predicted temperature curve Determined according to the following formula: , in, , , In the formula, Pr It is a Prandtl number; D h The hydraulic diameter of the cooling channel; μ w , k w , ρ w Here are the physical properties of the cooling medium, where... μ w For dynamic viscosity, k w For thermal conductivity, ρ w For density.

[0014] Preferably, the temperature of the cooling medium and traffic During the iterative calculation process, corrections are made according to the following formula: , in, for k Temperature of the cooling medium in the next iteration and traffic , denoted as [ T water , Q water ] k ; J This is the Jacobian matrix, whose elements are the predicted temperature variables. and The partial derivatives ∂Tpre / ∂Q and ∂Tpre / ∂Tw; R For residual vectors T pre −T tar ; The damping coefficient; I It is an identity matrix.

[0015] Compared with the prior art, the metal sheet forming limit testing device and method of the present invention have the following advantages: This invention first achieves a wide range of heating rate adjustment, from 1℃ / s to 100℃ / s, through the coordinated operation of a resistance heating module and an induction heating module. This allows the test conditions to match the rapid heating requirements of the sheet metal in rapid hot stamping processes. Furthermore, by setting a cooling channel in the lower die and introducing a cooling medium with adjustable temperature and flow rate, the non-isothermal cooling effect during the contact between "high-temperature sheet metal and room-temperature die" in actual stamping can be more accurately simulated in the experimental environment. Simultaneously, real-time monitoring and closed-loop feedback control of the entire temperature field of the sheet metal using a thermal imager ensures the uniformity and stability of the heating process. Through the synergistic effect of these structures, the equipment can accurately measure the strain of the sheet metal during rapid heating and non-isothermal deformation under conditions that highly simulate real production processes. This provides accurate and reliable forming limit data that can be directly used to guide process optimization, fundamentally overcoming the data deviation problems caused by insufficient heating rate and distorted operating conditions in traditional testing methods. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the overall structure of the device in an embodiment of the present invention; Figure 2 This is a test flowchart in an embodiment of the present invention; Figure 3 This is a flowchart of the temperature feedback control in an embodiment of the present invention.

[0017] Explanation of reference numerals in the attached figures: 1. Resistance heating module; 2. Induction heating module; 3. Lifting and rotating mechanism; 4. Thermal imager; 5. Upper mold; 6. Lower mold; 7. Sheet metal; 8. Edge pressing assembly; 9. Camera; 10. Central control system; 11. Cooling system; 12. Driver. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0020] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0021] See Figures 1 to 3 As shown, in order to solve the problems of low heating rate and inability to simulate real non-isothermal forming conditions in the existing technology, and to facilitate the acquisition of the plastic state of the material after rapid heating, so as to provide accurate forming limit data for rapid hot stamping process, this embodiment provides a metal sheet forming limit testing device and method.

[0022] Specifically, the equipment includes a worktable, a forming mold system, a heating system, a temperature monitoring and feedback control system, a cooling system 11, a strain detection system, and a central control system 10.

[0023] like Figure 1 As shown, the forming mold system includes an upper mold 5, a lower mold 6, a blank holder assembly 8, and a driver 12 arranged opposite each other. The lower mold 6 is fixed on the worktable, and the upper mold 5 is fixed at the output end of the driver 12. The upper mold 5 is slidably connected to the worktable via a guide post. The driver 12 (servo motor drive mechanism) drives the lower mold 6 to move linearly towards the upper mold 5 to achieve stamping of the metal sheet 7. Specifically, the driver 12 is located below the lower mold 6 and includes: a servo motor, a precision reducer, a ball screw pair, and a lifting slide. The power output end of the servo motor is connected to the precision reducer, which drives the ball screw to rotate. The lifting slide is mounted on the ball screw and is slidably connected to the worktable via a guide post. Thus, under the drive of the servo motor, the rotational motion is converted into linear motion, that is, the lifting slide with the lower mold 6 fixed on it is driven to perform a high-precision linear stamping motion upward along the guide post to achieve forming and testing of the metal sheet 7.

[0024] The edge-pressing assembly 8, designed to balance heating avoidance and anti-warping functions, comprises several low-pressure cylinders connected to the top of the frame and several servo electric cylinders connected to the bottom of the frame. Both the low-pressure cylinders and servo electric cylinders are vertically oriented. Each low-pressure cylinder has an upper edge-pressing ring fixed to its output end, and each servo electric cylinder has a lower edge-pressing ring fixed to its output end, with each pair of upper and lower edge-pressing rings corresponding one-to-one. During the heating and anti-warping stage, the bottom servo electric cylinder drives the lower edge-pressing ring upwards, causing the metal sheet 7 to adhere to and lift the floating upper edge-pressing ring. At this time, the top low-pressure cylinder is partially compressed, applying a flexible pre-tightening force of 1kN~5kN to the sheet 7 through gas back pressure. After clamping, the sheet metal assembly 7 is suspended, with sufficient clearance between its upper part and the base of the upper mold 5 for the heating module to screw in. This pneumatic pre-tightening force effectively prevents warping of the sheet metal 7 due to rapid heating. During the formal forming stage: after the heating module is removed, the bottom servo electric cylinder increases its output to overcome the pneumatic resistance of the top cylinder (the cylinder is compressed and releases force), pushing the floating upper pressure ring along with the sheet metal 7 upwards until the back of the floating upper pressure ring is completely in contact with the rigid pressure ring of the upper mold 5 base. Subsequently, the servo electric cylinder continues to output, providing an adjustable pressure force of 10kN~50kN to firmly lock the sheet metal 7.

[0025] The upper mold 5 adopts a metal-quartz composite inlay structure, including a rigid pressure ring and a central quartz observation window. Both the rigid pressure ring and the lower mold 6 are made of Cr12MoV steel with a surface hardness of HRC58-62. The working surface of the mold is a smooth plane, suitable for biaxial stretching. The mold as a whole does not have a heating device and maintains an ambient temperature of 20℃~25℃. The lower mold 6 adopts a split design, including a high thermal conductivity head and a water-cooled base, which are connected by an interference fit. The high thermal conductivity head is printed using 3D printing technology, and the water-cooled base is made of wear-resistant tool steel. Multiple spiral cooling channels are arranged 3mm~5mm below the surface of the water-cooled base. At the same time, multiple cooling channels use independent water flow in the middle and side walls to adapt to temperature changes in different parts of the sheet. The water flow circulates in a closed loop inside the water-cooled base. Heat is conducted to the water-cooled base through the high thermal conductivity head and then carried away by the water flow.

[0026] The heating system is composed of a composite heating head, which includes a resistance heating module 1 and an induction heating module 2. The two are set at the test station between the upper mold 5 and the lower mold 6 through a lifting and rotating mechanism 3. They are used to adjust the heating rate of the plate 7 from 1℃ / s to 100℃ / s by working alone or in combination. Specifically, the lifting and rotating mechanism 3 is installed on the worktable and consists of a servo motor and a horizontal rotating arm. The horizontal rotating arm is fixed on the output shaft of the servo motor. The resistance heating module 1 includes a resistance wire array set on an insulating bracket and divided into multiple independent temperature control zones. The induction heating module 2 includes an induction coil surrounding the resistance wire array. The insulating bracket is connected to the horizontal rotating arm. The lifting and rotating mechanism 3 drives the insulating bracket to rotate the heating head into the gap between the upper mold 5 and the plate 7 during the heating stage to heat the plate 7. After forming, the heating head is smoothly rotated out by the lifting and rotating mechanism 3 within 2s to 3s to make room for photography, optical inspection and stamping.

[0027] The resistance heating module 1 employs an iron-chromium-aluminum alloy resistance wire and a porous fused silica support (i.e., an insulating support). A 50μm~100μm thick alumina ceramic coating is applied to the surface of the resistance wire. The resistance wires, with a diameter of 0.5 mm, are arranged in a "serpentine" or "grid-like" pattern at 5 mm intervals, nested in grooves on the surface of the quartz support. During the wire laying, it is ensured that the direction of most resistance wires forms a 90-degree angle with the induced current, and a low-pass filter is installed on each resistance wire. A layer of nano-sized silica is coated on the back of the quartz support to reflect the upward heat radiation from the resistance wires back to the surface of the plate 7, improving heating efficiency and protecting the camera 9. After the composite heating head is screwed in, the distance between the resistance wires and the metal plate 7 is 1mm~2mm. Heating is achieved through Joule heating radiation. The resistance wire array is divided into 16 independent control zones, each controlled by a high-frequency PWM via an independent solid-state relay.

[0028] Induction heating module 2: It consists of a medium frequency induction coil and a cooling medium circuit. The coil is wound with a rectangular copper tube (frequency 10kHz~50kHz, number of turns 15~20 turns) and is installed around the periphery of the resistance wire array. After the composite heating head is screwed in, the coil is 3mm~5mm away from the plate 7 and generates eddy current heating through electromagnetic induction.

[0029] The temperature monitoring and feedback control system includes a thermal imager 4 for real-time imaging and acquisition of the temperature field distribution across the entire surface of the sheet metal 7, and a control module. The thermal imager 4 is a FLIR A655sc model with a resolution of 640×512, a temperature measurement range of 0-1200℃, and a frame rate ≥30fps. It is installed above the testing station to capture real-time images of the temperature across the entire surface of the metal sheet metal 7. The thermal imager 4 is electrically connected to the host computer of the main control system 10 via a first PLC controller. The temperature field data acquired by the thermal imager 4 is transmitted to the host computer. The host computer calculates the temperature deviation using a PID algorithm and uses feedback to control the output power of the heating system (response time ≤0.5s) to dynamically adjust the heating rate of different areas of the sheet metal 7.

[0030] The cooling system 11 adopts a dual-tank independent water supply design, including a low-temperature water tank, a high-temperature water tank, and a three-way proportional mixing valve. The three-way proportional mixing valve has two inlets and one outlet. The two inlets are connected to the outlets of the low-temperature and high-temperature water tanks, respectively. The outlet is connected to multiple cooling channels via pipelines. A solenoid valve is installed at the connection end of each cooling channel and pipeline. Through the cooperation of the low-temperature water tank, high-temperature water tank, three-way proportional mixing valve, and solenoid valves, a cooling medium with precisely controllable temperature and flow rate is introduced into the cooling channels to simulate the cooling effect of the die on the sheet metal during actual stamping. Preferably, the water temperature of the low-temperature water tank is set at 5±0.5℃, and the water temperature of the high-temperature water tank is set at 80±0.5℃, with built-in electric heating elements. This provides immediate cold and heat source support for the system; and the three-way proportional mixing valve regulates the water temperature, enabling rapid switching of the punch temperature.

[0031] The strain detection system includes at least two cameras 9 and a DIC analysis module. The cameras 9 are 20-megapixel industrial cameras (with a shooting frame rate ≥100fps). The system also includes a supplementary lighting source and a speckle pattern tool. The supplementary lighting source uses a cold LED light source to avoid affecting the temperature of the sheet material 7. The speckle pattern tool is used to mark the sheet material 7 with matte black paint and white dots, the dots having a diameter of 0.1mm~0.2mm. The two industrial cameras 9 are symmetrically mounted on both sides of the testing station, with the shooting direction perpendicular to the surface of the sheet material 7 to ensure alignment. The two industrial cameras 9 acquire image data of the sheet material 7 in real time during the forming process and transmit it to the DIC analysis module, which calculates the strain distribution of the sheet material 7. The DIC analysis module is based on binocular stereo vision and digital image correlation algorithms. Its specific working principle is as follows: a three-dimensional coordinate system of the test space is established in advance through camera 9 calibration; the initial speckle image before deformation is used as a reference benchmark, and it is divided into multiple pixel sub-regions. In the continuously acquired deformation image sequence, the gray-scale matching algorithm is used to track the position change of each pixel sub-region frame by frame, thereby calculating the full-field displacement vector of each speckle on the surface of plate 7 in three-dimensional space; then, the module performs smoothing processing and spatial differentiation on the acquired displacement field data, and can directly calculate and output the full-field surface strain distribution of plate 7 during the deformation process.

[0032] The central control system 10 comprises a first PLC controller, a second PLC controller, a PMW control module, and a host computer. The first PLC controller, a Siemens S7-1500, is electrically connected to the drive mechanism of the forming mold system, the heating system, and the temperature monitoring and feedback control system, controlling the power of the heating system, the motion parameters of the forming mold system, and the blank holder force. The second PLC controller, a Siemens S7-1200, is electrically connected to the cooling system 11, tracking the temperature curve and controlling the temperature and flow rate of the cooling medium in the cooling system 11 according to the preset temperature curve. The PMW control module controls 16 independent solid-state relays to adjust the power of the resistance wires in each zone. The host computer is used for parameter setting, real-time display of temperature and strain data, data storage and analysis. Thus, the central control system 10 coordinates and automatically controls the start / stop and power of the heating system, the motion of the forming mold system, the medium parameters of the cooling system 11, the closed-loop adjustment of the temperature monitoring and feedback control system, and the synchronous triggering and data acquisition of the strain detection system, thereby achieving fully automated testing.

[0033] like Figure 2 and Figure 3 As shown, based on the above equipment structure, a method for testing the forming limit of metal sheet 7 under wide heating rate and non-isothermal conditions is proposed, including the following steps: Select the metal sheet 7 to be tested (such as 6-series aluminum alloy, 1.5mm thick; 22MnB5 high-strength steel, 1.2mm thick), cut it into 200mm×200mm specifications, and use a speckle making tool to make uniform speckles on the surface of sheet 7.

[0034] Based on the material properties of plate 7 and the heating rate requirements, the heating method is adjusted. For example, aluminum alloy mainly uses resistance heating module 1, while high-strength steel mainly uses induction heating module 2.

[0035] The plate 7 is fixed at the center of the lower pressing edge, and the edge of the plate 7 is pre-pressed by the pressing edge assembly 8 with a pre-pressure of 1kN~5kN to ensure that the plate 7 will not warp during the heating process.

[0036] Set the test parameters in the main control system 10, and input the test parameters in the host computer test control software: heating rate (e.g., aluminum alloy 50℃ / s, high-strength steel 100℃ / s), target heating temperature (aluminum alloy 450℃, high-strength steel 850℃), lower mold 6 upward speed (10mm / s~400mm / s), deformation rate (aluminum alloy 0.1s). -1 High-strength steel 0.5s -1 ), Necking threshold (sudden increase in local strain ≥50%).

[0037] Heating and temperature control stage: 1. Start the thermal imager 4, set the temperature measurement range and shooting frame rate, and start real-time acquisition of the temperature image of the entire board 7. The temperature data is transmitted to the host computer in real time and displayed.

[0038] 2. Start the heating system and heat the board 7 according to the set heating rate: low-rate heating (1℃~10℃ / s) uses constant power mode, and high-rate heating (50℃~100℃ / s) uses pulse power mode to avoid temperature overshoot. 3. The main control system 10 compares the measured temperature with the target temperature and dynamically adjusts the output power of the heating system based on a PID algorithm until the temperature of the board 7 reaches and stabilizes at the target temperature. Specifically, the host computer compares the measured temperature with the target temperature in real time and adjusts the heating power through a PID algorithm: when the temperature deviation is >3℃, the heating power is increased by 10%; when the temperature deviation is <1℃, the current power is maintained; when the temperature reaches the target temperature ±1℃, the heating system switches to the heat preservation mode and maintains the temperature stable for 30 seconds.

[0039] Forming and Strain Detection Stage: First, adjust the pressure of the blank holder assembly 8 on the edge of the sheet metal 7 (generally, for aluminum alloy sheet metal 7, the pressure is set to 15kN~25kN; for high-strength steel sheet metal 7, the pressure is set to 30kN~50kN) to ensure the sheet metal 7 is firmly pressed against the blank holder ring, guaranteeing the stability of the sheet metal 7's edge during the stamping process. Then, activate the strain detection system and control the lower die 6 of the forming mold system to move towards the sheet metal 7 to apply forming force. Simultaneously, control the cooling system 11 to introduce a cooling medium with a preset temperature and flow rate into the cooling channel of the lower die 6. The strain detection system acquires surface images of the sheet metal 7 in real time during the deformation process and calculates the strain distribution.

[0040] 1. After the insulation is completed, the host computer issues a command to start the strain detection system, and the two industrial cameras 9 begin to synchronously acquire images of the board 7.

[0041] 2. The driver 12 drives the lower mold 6 to move upward, contacting the high-temperature sheet 7 and applying forming force. At the same time, the second PLC controller reads the temperature setpoint. and By adjusting the temperature and flow rate of the cooling medium, plate 7 undergoes plastic deformation under the action of the mold.

[0042] 3. The strain detection system acquires image data during the deformation process in real time, transmits it to the DIC analysis module, and calculates and displays the strain distribution (principal strain ε1, secondary strain ε2) across the entire plate 7 in real time. Test termination and data processing stage: When the strain detection system detects that the local strain of plate 7 reaches the necking judgment threshold, the forming mold system, heating system and strain detection system are controlled to stop working; the necked metal plate 7 specimen is taken out, and the DIC analysis module performs subsequent processing on the acquired image sequence, extracts the ultimate principal strain and secondary strain data of the necking area, and generates the forming limit value under this working condition.

[0043] The host computer automatically stores test parameters, temperature change curves, strain change curves, forming limit values, and other data, and supports exporting reports in Excel format.

[0044] Furthermore, the heating rate range of the heating system employs different heating modes: When the heating rate is between 1℃ / s and 20℃ / s, resistance heating is mainly used, and the induction heating module 2 is turned off. The temperature uniformity of the entire plate 7 is achieved through zoned temperature control electrodes (temperature difference ≤ ±1℃). The temperature monitoring module finely adjusts the resistance power of the 16 zones in real time based on thermal imaging feedback. Due to the low heating rate, the system has sufficient time to eliminate temperature gradients.

[0045] When the heating rate is between 20℃ / s and 60℃ / s, the sample is rapidly heated by the eddy current effect of electromagnetic induction. The resistance wire array then performs directional compensation based on local cold spots (such as the heat dissipation area near the pressure ring) detected by thermal imaging. Induction heating can easily lead to overheating at the edges (skin effect). In this case, the resistance wires in the edge zones will reduce their power or even turn off, while the resistance wires in the central zone will increase their power to ensure that the entire plate 7 heats up synchronously at a constant rate.

[0046] When the heating rate is between 60℃ / s and 100℃ / s, the induction power supply acts as the main heat source, outputting constant power. As the temperature approaches the target temperature, induction heating generates significant thermal inertia. To maintain a constant heating rate without overshoot, the system reduces the induction power 50-80℃ before reaching the target temperature, with the resistance wire providing final temperature regulation.

[0047] Furthermore, the method for determining the preset temperature and flow rate of the cooling medium is calculated in reverse: A simulation model of the actual hot stamping process was established in the finite element software (Abaqus) to calculate the temperature change of the sheet metal throughout the stamping process and obtain the target cooling curve. .

[0048] In actual hot stamping, the main driving force for the temperature drop of the sheet metal is the contact heat transfer between the sheet metal and the die. Abaqus calculates heat flux density by defining the "interfacial heat transfer coefficient (IHTC)". q int :

[0049] , in, The contact heat transfer coefficient is typically determined by the contact pressure. P The function. In actual back-calculation, it is necessary to ensure that the pressure loading curve of the test bench is consistent with that of the production line, or to correct the coefficient in Abaqus. a b c These are experimental constants related to materials.

[0050] , in, T sheet , T die These are the temperatures of the sheet metal and the mold surface, respectively.

[0051] The core of Abaqus's solution is the three-dimensional transient heat conduction equation. For the interior of the mold and sheet metal:

[0052] , In the above equation, the left-hand side represents the rate of change of the sheet metal's internal energy over time (i.e., the rate of temperature change, which determines...). The slope); the first term on the right represents the heat conduction inside the sheet (heat diffusion from high-temperature areas to low-temperature areas); the second term on the right... This represents the heat generated during plastic deformation; the third item on the right. This indicates the amount of heat lost to the environment or mold through the surface.

[0053] In order to obtain specific The following formulas for the physical process specific to hot stamping must be substituted into the equation.

[0054] During the stamping deformation stage, approximately 90% of the plastic work is converted into heat, causing the sheet temperature to rise slightly. , in, This is the Taylor-Quinny coefficient, usually taken as 0.9. This refers to the flow stress of the material. This is the equivalent plastic strain rate. It consists of three parts, and the dominant component plays a different role in different stages of stamping (transfer, mold closing, and pressure holding):

[0055] , Air convection: , Thermal radiation: , Mold contact heat exchange: , To fit the algorithm, a "corrected form of Newton's law of cooling (exponential decay model)" is used to approximate the solution to the physical process: , in, Mold / cooling water temperature (final equilibrium temperature); The initial temperature of the sheet metal; The effective heat transfer coefficient varies with time (very small before mold closing, extremely large after mold closing). This is the temperature rise term caused by heat generated during deformation.

[0056] Step 2: Establish an experimental equivalent model of the testing equipment in the finite element software, and calculate the experimental temperature change curve of the plate when cooling is achieved only through the lower mold cooling channel. ; The rate of change is determined by the heat balance equation of the sheet metal, but in the experimental equivalent model, the boundary heat flow exhibits significant asymmetry: , Where V is the volume of the board material; , This refers to the contact area between the lower and upper surfaces of the sheet metal and the mold. It generates heat for the plastic deformation of the sheet material; For the heat flow of the cooling channel; This refers to the heat flow on the upper surface of the plate.

[0057] The lower mold is a steel mold with cooling channels, allowing heat to flow... great: , The upper mold is made of quartz glass. Because the thermal conductivity of quartz is much lower than that of mold steel, and the glass surface has a high degree of smoothness, heat transfer is extremely slow. In Abaqus, the heat flow on the upper surface... It is usually defined as:

[0058] , in, It is usually only 1 / 20 to 1 / 50 of the steel mold side.

[0059] If we integrate the above differential equation, the experimental temperature change curve... Mathematically, this is represented as a modified exponential decay function: , Step 3: Using the temperature of the cooling medium and traffic Using variables, through iterative calculations, the predicted temperature curve is obtained. Compared with the target cooling curve The error between them is minimized, so that the temperature change of the sheet metal in the actual hot stamping process can be simulated by controlling the temperature of the lower die alone.

[0060] In Abaqus, water flow rate It's not directly input; it's obtained through the convective heat transfer coefficient. It acts on the wall of the mold cooling channel. Therefore, the core of the back calculation is to calculate the cooling water flow rate. and temperature Transform into Based on the Dittus-Boelter empirical correlation in fluid mechanics and heat transfer (applicable to turbulent cooling water), the following was established: arrive The mapping.

[0061] Calculate the Reynolds number: , Calculate the Nusel number: , It is a Prandtl number.

[0062] Calculate the convective heat transfer coefficient: , in, D h The hydraulic diameter of the flow channel. μ w , k w , ρ w These are the physical properties of water (cooling medium). Among them, μ w The value is the dynamic viscosity, ranging from 1.00×10³Pa·s to 0.35×10³Pa·s. k w is the thermal conductivity, with values ​​ranging from 0.57 W / (m·K) to 0.67 W / (m·K). ρ w The value is the density, ranging from approximately 972 kg / m³ to 1000 kg / m³.

[0063] To predict the temperature curve As close as possible to the target cooling curve Establish a minimum error function between the two as a constraint, and find the optimal combination of variables X=[ , ]: The optimization algorithm minimizes the error function. accomplish: , in, The sampling time points during the stamping process, This represents the total number of sampling points.

[0064] because and( , The relationship between the input variables and the equations is nonlinear and implicit (it must be solved using Abaqus finite element method), and the inverse calculation formula is usually expressed as an iterative correction scheme. The input variables are corrected through an optimization algorithm until the error satisfies | T pre −T tar |≤ 3℃:

[0065] , in, for k Temperature of the cooling medium in the next iteration and traffic , denoted as [ T water , Q water ] k ; JThis is the Jacobian matrix, whose elements are the predicted temperature variables. and The partial derivatives ∂Tpre / ∂Q and ∂Tpre / ∂Tw; R For residual vectors T pre −T tar ; The damping coefficient; I It is an identity matrix.

[0066] Step 4: Calculate the optimal cooling medium temperature. and traffic The parameters are pre-stored in the main control system as control parameters.

[0067] This enables precise temperature control across the entire surface area. Through real-time monitoring and power feedback adjustment using a thermal imager, the temperature difference across the entire surface area of ​​the board is ensured to be ≤±3℃, reducing testing errors.

[0068] In summary, the present invention has the following technical advantages: 1. Comprehensive heating rate coverage: It achieves a wide range of heating rates from 1℃ / s to 100℃ / s, which is more than 20 times higher than the existing technology (3℃ / s to 5℃ / s), and fully matches the heating requirements of rapid hot stamping process.

[0069] 2. The working condition simulation is realistic and accurate: For the first time, non-isothermal forming test of "high temperature sheet material - room temperature mold" was realized. The test working condition is consistent with the actual production process, and the accuracy of forming limit value characterization is improved by 80%, providing reliable data support for process optimization.

[0070] 3. High temperature control accuracy: Through full-area monitoring by thermal imager and PID feedback control, the temperature difference of the board is ≤±3℃. Compared with the existing technology (local temperature measurement, temperature difference ≥10℃), the temperature uniformity is significantly improved, and the test error is reduced from 10% to less than 3%.

[0071] 4. Wide adaptability: The electro-magnetic co-heating module can be flexibly turned on and automatically adjusts the power of different areas. It is compatible with a variety of metal plates (thickness 0.5mm~2mm) such as aluminum alloy, high-strength steel, and magnesium alloy, covering mainstream hot stamping materials in the automotive, aerospace and other fields.

[0072] 5. High testing efficiency: The entire process is automated, with a single test cycle of ≤15min (including heating, forming, and data processing). Compared with existing equipment (single test cycle ≥30min), the testing efficiency is improved by more than 50%.

[0073] 6. Easy to operate: The host computer has a visual interface that supports parameter setting, real-time data display, and report export, allowing test operations to be completed without the need for professional technicians.

[0074] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A sheet metal forming limit testing apparatus, characterized by, include: The forming mold system has an upper mold and a lower mold arranged opposite each other, and the lower mold is provided with a cooling channel; The heating system includes a resistance heating module and an induction heating module disposed between the upper mold and the lower mold, which are used to adjust the heating rate of the sheet material by working alone or in concert. The temperature monitoring and feedback control system includes at least one thermal imager installed above the upper mold for real-time imaging and acquisition of the temperature field distribution across the entire plate. The cooling system is connected to the cooling channel of the lower die through a pipeline, and is used to introduce a cooling medium with adjustable temperature and flow rate into the cooling channel to simulate the cooling effect of the die on the sheet metal during the actual stamping process. The strain detection system includes at least two cameras symmetrically mounted on both sides of the lower or upper mold, and a DIC analysis module. The cameras are used to acquire surface deformation images in real time during the forming process of the sheet metal. The DIC analysis module is electrically connected to the cameras and is used to obtain the strain distribution of the sheet metal based on the acquired image data. The central control system is electrically connected to the forming mold system, heating system, thermal imager, cooling system, and DIC analysis module, respectively. It is used to dynamically control the start-up and shutdown and power of the heating system, the movement of the forming mold system, and the parameters of the cooling medium based on the temperature field distribution of the entire sheet material, so as to form a closed-loop regulation. It also collects the sheet material strain distribution obtained by the DIC analysis module and compares it with a preset threshold to control the start-up and shutdown status of each system.

2. The metal sheet forming limit testing device according to claim 1, characterized in that, The resistance heating module and the induction heating module are integrated into a composite heating head. The resistance heating module includes a resistance wire array mounted on an insulating support and divided into multiple independent temperature control zones. The induction heating module includes an induction coil surrounding the resistance wire array. The insulating support is mounted on the worktable via a lifting and rotating mechanism. The lifting and rotating mechanism is used to drive the insulating support to rotate the composite heating head into the gap between the upper mold and the plate during the heating stage to heat the plate, and to rotate the gap out before forming and testing.

3. The metal sheet forming limit testing device according to claim 2, characterized in that, The forming mold system also includes a driver and a pressing assembly. The output end of the driver is connected to the lower mold and is used to drive the lower mold to move linearly in the direction of the upper mold. The pressing assembly is set on the worktable and is used to apply pressure to the edge of the sheet metal.

4. The metal sheet forming limit testing device according to claim 1, characterized in that, The cooling system includes a low-temperature water tank, a high-temperature water tank, and a three-way proportional mixing valve. The three-way proportional mixing valve has two inlets and one outlet. The two inlets are respectively connected to the outlets of the low-temperature water tank and the high-temperature water tank, and the outlet is connected to the pipeline for providing a temperature-adjustable circulating cooling medium to the cooling channel of the lower mold.

5. The metal sheet forming limit testing device according to claim 3, characterized in that, The central control system includes: The first PLC controller is electrically connected to the driver, heating system, and temperature monitoring and feedback control system of the forming mold system, respectively, and is used to control the power of the heating system, the motion parameters of the forming mold system, and the blank holder force; The second PLC controller is electrically connected to the cooling system and is used to control the temperature and flow rate of the cooling medium in the cooling system according to a preset temperature curve. The host computer communicates with the first PLC controller, the second PLC controller, the thermal imager, and the DIC analysis module, and is used for parameter setting, data acquisition, processing, and display.

6. A method for testing the forming limit of metal sheets, implemented using the equipment described in any one of claims 1-5, characterized in that, Includes the following steps: Test parameters are set in the overall control system, including heating rate, target heating temperature, lower mold motion parameters, deformation rate, and necking judgment threshold. The heating system and thermal imager are started, and the board is heated according to the set heating rate. The thermal imager collects the full-area temperature image of the board in real time and transmits it to the central control system. The central control system compares the measured temperature with the target temperature through the control module, and dynamically adjusts the output power of the heating system based on the PID algorithm until the temperature of the board reaches and stabilizes at the target temperature. The strain detection system is activated, and the lower die of the forming mold system is controlled to move towards the upper die according to the set motion parameters to apply forming force to the sheet metal. At the same time, the cooling system is controlled to introduce a cooling medium with a preset temperature and flow rate into the cooling channel of the lower die. The strain detection system acquires surface images of the sheet metal in real time through a camera and calculates the strain distribution of the sheet metal through the DIC analysis module. When the DIC analysis module detects that the local strain of the sheet material reaches the necking threshold, it controls the forming mold system, heating system and strain detection system to stop working. Based on the image data acquired by the strain detection system, the ultimate principal strain and secondary strain of the necking region are extracted to generate the forming limit value.

7. The method for testing the forming limit of metal sheets according to claim 6, characterized in that, The heating rate range of the heating system is achieved using different heating modes: When the heating rate is between 1℃ / s and 20℃ / s, the resistance heating module is used for heating, and the induction heating module is turned off. When the heating rate is 20℃ / s to 60℃ / s, the induction heating module is used as the main heat source, and the resistance heating module is used to compensate for the local low temperature area identified by the thermal imager. When the heating rate is between 60℃ / s and 100℃ / s, the induction heating module outputs a constant power for rapid heating. Within a preset temperature difference range before the plate temperature approaches the target temperature, the power of the induction heating module is reduced, and the resistance heating module performs the final temperature regulation.

8. The method for testing the forming limit of metal sheets according to claim 6, characterized in that, The method for determining the cooling medium at the preset temperature and flow rate includes the following steps: Step 1: Establish a simulation model of the actual hot stamping process, calculate the temperature change of the sheet metal throughout the stamping process, and obtain the target cooling curve. ; Step 2: Establish an experimental equivalent model of the testing equipment and calculate the experimental temperature change curve of the plate material when cooling is performed only through the lower mold cooling channel. ; Step 3: Using the temperature of the cooling medium and traffic Using variables as variables and minimizing the error function as constraints, the predicted temperature curve is calculated iteratively. With the target cooling curve The error between them is minimized, thus determining the optimal solution. and combination.

9. The method for testing the forming limit of metal sheets according to claim 8, characterized in that, The flow rate of the cooling medium Through the convective heat transfer coefficient Influence on the predicted temperature curve Determined according to the following formula: ， in, , , In the formula, Pr It is a Prandtl number; D h The hydraulic diameter of the cooling channel; μ w , k w , ρ w Here are the physical properties of the cooling medium, where... μ w For dynamic viscosity, k w For thermal conductivity, ρ w For density.

10. A method for testing the forming limit of metal sheets according to claim 8, characterized in that, The temperature of the cooling medium and traffic During the iterative calculation process, corrections are made according to the following formula: ， in, for k Temperature of the cooling medium in the next iteration and traffic , denoted as [ T water , Q water ] k ; J This is the Jacobian matrix, whose elements are the predicted temperature variables. and The partial derivatives are ∂Tpre / ∂Q and ∂Tpre / ∂Tw; R For residual vectors T pre −T tar ; The damping coefficient; I It is an identity matrix.

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