Unstable in-situ high-temperature forming performance testing device and method

Abstract

The invention relates to the technical field of mechanical property testing of metal materials, and provides an unsteady-state in-situ high-temperature forming property testing device and method. A punch constant strain rate loading model is arranged in the device, the punch moving speed is adjusted in real time to realize punch constant strain rate loading, and the rheological behavior of a constant strain rate sensitive material in a complex stress state is accurately tested; a rapid cooling system is arranged, so that accurate simulation of complex unsteady-state forming processes such as actual hot stamping and warm forming is realized, and the unsteady-state in-situ high-temperature forming performance of the material is obtained; in combination with a DIC dynamic strain measurement technology, a strain field and deformation force are collected in real time in a plate high-temperature plane stress bulging test, and local necking strain and main strain and secondary strain change paths are determined. According to the method, the reliability of researching macroscopic rheological behaviors such as strain field distribution, flow behavior, hardening mechanism, forming limit and fracture limit of the plate in a complex stress state can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material mechanical property testing, and in particular to a non-steady-state in-situ high-temperature forming performance testing device and method. Background Art

[0002] In recent years, thin-walled metal components with large dimensions, long lifespans, high performance, and high reliability have been increasingly used in cutting-edge equipment such as next-generation aerospace vehicles, strategic missiles, and ships. The ability to manufacture precise, lightweight, thin-walled components is a core technology for the development of high-end aerospace and other transportation equipment. Forming complex sheet metal can present complex deformation scenarios. Different ratios of the two principal stresses within the sheet surface result in different allowable values for the two corresponding principal strains. These values essentially reflect material properties and should therefore be determined experimentally. The allowable values of the two principal strains measured under different stress states are plotted in a coordinate system with the smaller principal strain on the abscissa and the larger principal strain on the ordinate. A curve connecting these points is called the forming limit diagram (FLD) of the sheet metal. Any combination of the two principal strains within the sheet plane that falls above the FLD indicates rupture when the sheet deforms; otherwise, the sheet is considered safe. For sheet metal forming, FLDs are an effective tool for simulation and process analysis.

[0003] Each material has a forming limit curve, typically obtained experimentally. The main experimental method for determining the forming limit is the hemispherical punch method. In traditional rigid punch bulging experiments, the punch is typically loaded at a constant speed, which cannot guarantee that the sheet deforms at a constant strain rate. The strain rate decreases with increasing strain, resulting in a low measured flow stress. This method is not suitable for alloys that are sensitive to strain rate during high-temperature deformation.

[0004] For low-plasticity materials, such as aluminum alloys, which have low elongation, hot forming is often used. Hot stamping is a material forming method in which the blank is first heated to a certain temperature, and then stamped and quenched in the corresponding mold with a punching machine to obtain the desired shape and simultaneously achieve phase change of the metal material. It is a non-isothermal forming method. Traditional devices usually only support constant temperature testing, and the temperature environment needs to be preset before the experiment. It is impossible to dynamically adjust the temperature during the bulging process, and it is impossible to achieve rapid temperature rise and fall and temperature gradient control. Therefore, the resulting forming limit diagrams are under isothermal conditions and are not suitable for this type of forming.

[0005] Furthermore, deformation during unsteady-state high-temperature forming is complex, and traditional methods cannot accurately capture the moment of fracture. Furthermore, due to severe softening at high temperatures, the material undergoes significant local deformation before fracture, at which point the material has already failed. This requires full-scale capture to determine the local necking strain and the paths of change in the primary and secondary strains, thereby establishing a uniform strain window. While digital image correlation (DIC) technology has been proposed in academia for strain analysis, existing devices have not integrated it with bulging experiments, lacking real-time data feedback and closed-loop control capabilities. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides a non-steady-state in-situ high-temperature forming performance testing device and method.

[0007] The specific technical solutions are as follows:

[0008] A non-steady-state in-situ high-temperature forming performance testing device, comprising an experimental specimen (1), a deformation loading and pressure acquisition system, a temperature control system, and a full-field strain acquisition system;

[0009] The deformation loading and pressure acquisition system includes a bulging die, a servo loading electric cylinder (2) and a blank holding cylinder (3); the bulging die includes an upper die (4), a blank holding ring (5) and a punch (6); a pressure sensor is installed below the punch (6) to collect the bulging force of the experimental sample (1) during the bulging process in real time; a heat insulation plate (7) and a water cooling plate (8) are provided between the blank holding ring (5) and the platform (15); the servo loading electric cylinder (2) is loaded according to a program to realize the constant strain rate deformation of the experimental sample; an annular raised blank holding rib is installed on the lower surface of the upper die (4), and the blank holding cylinder (3) drives the blank holding ring (5) to move, so that the blank holding rib is pressed into the experimental sample (1), and the material is controlled not to flow radially when the experimental sample (1) is deformed;

[0010] The temperature control system comprises a heating furnace (9), a thermocouple (10), a blower (11) and a blast pipe (12); the heating furnace (9) is used to heat the experimental sample (1), and is filled with asbestos (13) to reduce the heat exchange between the mold and the external environment during the experiment; the top of the heating furnace (9) is open and is provided with high-temperature glass (14); the thermocouple (10) is used to detect the real-time temperature of the mold, and a temperature measuring hole is opened in the upper mold (4), and one end of the thermocouple (10) is inserted into the temperature measuring hole; the blower (11) and the blast pipe (12) are used to exchange hot air in the furnace with the outside world to achieve rapid cooling;

[0011] The full-field strain acquisition system includes a CCD camera (16) and a DIC system (17). The DIC optical strain measurement system is used to perform real-time acquisition and calculation of the full-field strain during the bulging process of the experimental sample. During the experiment, the speckle pre-sprayed on the sample surface changes as the sample deforms. This process is collected by the CCD camera (16) and used to calculate the displacement and strain during the deformation process.

[0012] The bulging die is made of low carbon steel, stainless steel or Ni7N.

[0013] The experimental sample (1) is a circular plate with a diameter of 180 mm and a thickness of 1 to 4 mm.

[0014] The cooling channel in the water-cooling plate (8) is a flowing normal-temperature water-cooling channel.

[0015] The blank holding pressure of the blank holding ring (5) is 5-7 MPa.

[0016] The heating furnace (9) adopts a PID system closed-loop temperature control, outputs a control signal according to the difference between the target temperature and the actual temperature, and performs temperature feedback and temperature control.

[0017] A non-steady-state in-situ high-temperature forming performance testing method specifically comprises the following steps:

[0018] Step 1: Adjust the temperature control system to control the temperature of the bulging mold to T1, and keep it warm for a period of time to ensure that the temperature at each point inside the mold is uniform;

[0019] Step 2: Prepare the experimental sample (1). First, polish the cut of the experimental sample (1). Then, scrub the surface of the experimental sample (1) to remove debris and oil generated by laser cutting. Finally, spray lubricant on the part of the experimental sample (1) that contacts the punch (6), and spray speckle on the other side.

[0020] Step 3: After the experimental sample (1) is prepared, it is placed in the mold. After pressurization, the edge holding cylinder (3) drives the edge holding ring (5) to move, so that the edge holding rib is pressed into the experimental sample (1). The edge holding pressure is controlled to P so that the material does not flow radially when the experimental sample (1) is deformed;

[0021] Step 4: heat preservation, using a heating furnace to perform solid solution treatment on the experimental sample (1) for a time of t1. After the solid solution is completed, the fan (11) is turned on to quickly cool the temperature of the experimental sample (1) to T2;

[0022] Step 5: According to the established punch constant strain rate loading model, the punch movement speed is set to perform constant strain rate bulging, and the full-field strain acquisition system collects strain information in real time;

[0023] Step 6: Unload, open the furnace, take samples and export experimental data;

[0024] Step 7: Uniform strain processing and uniform strain window establishment.

[0025] In step 2, the polishing of the experimental sample (1) was performed using an electric grinder equipped with a 400-grit sandpaper grinding head.

[0026] In step 2, the surface of the experimental sample (1) is scrubbed with anhydrous ethanol; the contact portion of the sample with the punch is sprayed with a layer of boron nitride coating and two layers of 2800 mesh graphite coating for lubrication.

[0027] The constant strain rate loading model described in step 5 is established by the following method:

[0028] The length L of the deformation zone is expressed as follows:

[0029] L=L H +2L C (1)

[0030] Where L H , L c The length of the spherical contour line and the generatrix of the punch (mm);

[0031] The length L of the spherical contour line that contacts the punch H and busbar length L C They are:

[0032]

[0033] Where D0 is the lower die aperture (mm); H i is the bulging height (mm); R' is the punch radius (mm); Half of the die taper angle (°);

[0034] Assuming uniform plastic deformation occurs in the deformation zone, the length L of the deformation zone at time t is:

[0035]

[0036] Where L0 is the initial length of the deformation zone (mm); V is the deformation speed of the deformation zone (mm / s);

[0037] strain rate is a constant value, and its relationship with the deformation velocity of the deformation zone can be expressed as:

[0038]

[0039] Integrate formula (5) and rearrange it to get:

[0040]

[0041] When t=0, V=V0, from which the deformation velocity V of the plastic deformation zone is calculated:

[0042]

[0043] Strain rate in the deformation zone It can be expressed as:

[0044]

[0045] By combining equations (7) and (8), we can get the deformation velocity of the deformation zone:

[0046]

[0047] In order to ensure a constant strain rate at the vertex, the loading speed v of the servo loading cylinder needs to be continuously adjusted. i Assuming that the time interval for the servo loading cylinder to adjust the loading speed is Δt, the end times of each loading stage are:

[0048] t1=Δt, t2=2Δt,…,t i =iΔt (10)

[0049] The average velocity of the deformation zone in each punch loading stage can be calculated by formula (9): And use this as the deformation speed V of the deformation zone each time the punch loads i :

[0050]

[0051] The length L of the deformation zone at each stage of the bulging deformation zone is calculated from this i :

[0052]

[0053] Where V1, V2..., V i A set of common ratios is The geometric progression of , we can get:

[0054]

[0055] Combined equations (10) and (13)

[0056]

[0057] Substituting equations (13) and (14) into equation (11), we can calculate the value of the “spherical cap cut cone ring” at any time t i The contour length L i It can be expressed as:

[0058]

[0059] The bulge height at any time can be obtained by combining:

[0060]

[0061] During the experiment, the deformation velocity V i Converted into the moving speed v of the punch i , adjust the punch speed v i , then the bulging height at the top of the specimen is:

[0062]

[0063] The moving speed v of the punch in each loading stage is determined from this i :

[0064]

[0065] Compared with the prior art, the present invention has the following beneficial technical effects:

[0066] The present invention proposes a non-steady-state in-situ high-temperature forming performance testing device and method. The device incorporates a punch constant strain rate loading model, establishes a punch constant strain rate loading model, and adjusts the punch movement speed in real time to achieve punch constant strain rate loading. This can accurately test the rheological behavior of strain rate-sensitive materials under complex stress states.

[0067] The present invention is equipped with a rapid cooling system, which can achieve rapid cooling during the experiment, realize accurate simulation of complex non-steady-state forming processes such as actual hot stamping and warm forming, and obtain the non-steady-state in-situ high-temperature forming performance of the material;

[0068] The present invention combines digital image correlation (DIC) dynamic strain measurement technology to collect the strain field and deformation force in real time during high-temperature sheet metal bulging experiments, obtain real-time full-field strain information during sheet metal deformation, and determine the local necking strain and the change paths of the primary and secondary strains.

[0069] The outstanding advantages of this device of the present invention are:

[0070] (1) The present invention has precise strain rate control, establishes a punch constant strain rate loading model, and adjusts the punch movement speed in real time to achieve punch constant strain rate loading. It can accurately test the macroscopic rheological behaviors such as strain field distribution, flow behavior, hardening mechanism, forming limit and fracture limit of strain rate sensitive materials under complex stress states, significantly improving the reliability of research;

[0071] (2) The test temperature of the present invention is precisely controlled and equipped with a temperature control system. The heating furnace and the fan are used in conjunction to achieve rapid temperature rise and fall and temperature gradient control, thereby achieving accurate simulation of the actual forming process and obtaining the material's non-steady-state in-situ high-temperature forming performance;

[0072] (3) The strain information collected by the present invention is accurate and comprehensive. Combined with the DIC technology, it can collect the strain field and deformation force in real time during the high-temperature bulging experiment of the plate, obtain real-time full-field strain information during the deformation process of the plate, determine the local necking strain and the change path of the main strain and secondary strain, and establish a uniform strain window. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 This is a schematic structural diagram of the non-steady-state in-situ high-temperature forming performance testing device of the present invention;

[0074] Figure 2 This is a flow chart of the non-steady-state in-situ high-temperature forming performance testing method of the present invention;

[0075] Figure 3 It is the geometric model of rigid die bulging process;

[0076] Figure 4 This is a diagram of the experimental process of non-steady-state in-situ high-temperature forming performance test;

[0077] Figure 5 Schematic diagram of the uniform strain treatment method, where (a) shows the selection of BZ and RZ regions, and (b) shows the measurement of necking and fracture regions;

[0078] Figure 6 Figure 1 shows the results of bulging experiments using 7B04 aluminum alloy as an example. Figure (a) shows the variation of the full-thickness strain in the gauge zone with bulging time. Figure (b) shows the variation curve of the through-thickness strain in the base zone BZ and the reference zone RZ. Figure (c) shows the strain path in the reference zone.

[0079] In the figure, 1. Experimental specimen; 2. Servo loading electric cylinder; 3. Blank holding cylinder; 4. Upper die; 5. Blank holding ring; 6. Punch; 7. Heat insulation board; 8. Water cooling plate; 9. Heating furnace; 10. Thermocouple; 11. Fan; 12. Blower tube; 13. Asbestos; 14. High-temperature glass; 15. Platform; 16. CCD camera; 17. DIC system. DETAILED DESCRIPTION

[0080] The present invention will be described in detail below with reference to specific embodiments and the accompanying drawings, but the protection scope of the present invention is not limited by the specific embodiments and the accompanying drawings.

[0081] Figure 1 This is a schematic diagram of the structure of the non-steady-state in-situ high-temperature forming performance testing device of the present invention, as shown in the figure:

[0082] Example 1

[0083] The non-steady-state in-situ high-temperature forming performance testing device of the present invention comprises an experimental specimen 1, a deformation loading and pressure acquisition system, a temperature control system and a full-field strain acquisition system;

[0084] The experimental specimen 1 is a circular plate with a diameter of 180 mm and a thickness of 1 mm; the deformation loading and pressure acquisition system includes a bulging die, a servo loading electric cylinder 2 and a blank holding cylinder 3; the bulging die is made of low carbon steel, and includes an upper die 4, a blank holding ring 5 and a punch 6; a pressure sensor is installed under the punch 6 to collect the bulging force of the experimental specimen 1 during the bulging process in real time; the blank holding pressure of the blank holding ring 5 is 5 MP, and an insulation plate 7 and a water-cooling plate 8 are provided between the blank holding ring 5 and the platform 15, and the cooling channel in the water-cooling plate 8 is a flowing normal temperature water cooling channel; the servo loading electric cylinder 2 is loaded according to the program to realize the constant strain rate deformation of the experimental specimen; an annular raised blank holding rib is installed on the lower surface of the upper die 4, and the blank holding cylinder 3 drives the blank holding ring 5 to move, so that the blank holding rib is pressed into the experimental specimen 1, and the material is controlled not to flow radially when the experimental specimen 1 is deformed;

[0085] The temperature control system includes a heating furnace 9, a thermocouple 10, a fan 11, and a blast pipe 12. The heating furnace 9 is used to heat the experimental sample 1. The heating furnace 9 adopts a PID system closed-loop temperature control, outputs a control signal based on the difference between the target temperature and the actual temperature, and performs temperature feedback and temperature control. The interior of the heating furnace 9 is filled with asbestos 13 to reduce heat exchange between the mold and the external environment during the experiment. The top of the heating furnace 9 is open and equipped with high-temperature glass 14. The thermocouple 10 is used to detect the real-time temperature of the mold. A temperature measuring hole is opened in the upper mold 4, and one end of the thermocouple 10 is inserted into the temperature measuring hole. The fan 11 and blast pipe 12 are used to exchange hot air in the furnace with the outside world to achieve rapid cooling.

[0086] The full-field strain acquisition system includes a CCD camera 16 and a DIC system 17. The DIC optical strain measurement system is used to collect and calculate the full-field strain of the experimental specimen during the bulging process in real time. During the experiment, the speckle pre-sprayed on the specimen surface changes as the specimen deforms. This process is captured by the CCD camera 16 and used to calculate the displacement and strain during the deformation process.

[0087] Example 2:

[0088] The difference from Example 1 is that the bulging die is made of stainless steel, the thickness of the circular plate of the experimental sample 1 is 2 mm, and the blank holding pressure of the blank holding ring 5 is 6 MPa.

[0089] Example 3:

[0090] The difference from Example 1 is that the bulging die is Ni7N; the thickness of the circular plate of the experimental sample 1 is 4 mm; and the blank holding pressure of the blank holding ring 5 is 7 MPa.

[0091] Figure 2 This is a flow chart of the non-steady-state in-situ high-temperature forming performance testing method of the present invention. Figure 4 FIG1 is a diagram of the experimental process of the non-steady-state in-situ high-temperature forming performance test; as shown in the figure, the non-steady-state in-situ high-temperature forming performance test method of the present invention specifically includes the following steps:

[0092] Step 1: Adjust the temperature control system to control the temperature of the bulging mold to T1, and keep it warm for a period of time to ensure that the temperature at each point inside the mold is uniform;

[0093] Step 2: Prepare the experimental sample 1. First, polish the cut of the experimental sample 1 to remove surface defects left by the wire cutting. Polishing is done using an electric grinder equipped with a 400-grit sandpaper grinding head. Then, scrub the surface of the experimental sample 1 with anhydrous ethanol to remove debris and oil stains generated by laser cutting. Finally, spray a layer of boron nitride coating and two layers of 2800-grit graphite coating on the part of the experimental sample 1 that contacts the punch 6 for lubrication. Speckling is sprayed on the other side.

[0094] Step 3: After the experimental sample 1 is prepared, it is placed in the mold. After pressurization, the blank holder cylinder 3 drives the blank holder ring 5 to move, so that the blank holder rib is pressed into the experimental sample 1. The blank holder pressure is controlled to P so that the material does not flow radially when the experimental sample 1 is deformed;

[0095] Step 4: Heat preservation: Use a heating furnace to perform solid solution treatment on the experimental sample 1 under conditions of T1 and t1 to eliminate the coarse precipitate phase to obtain a uniform and well-plastic microstructure. After the solid solution is completed, turn on the fan 11 to quickly cool the temperature of the experimental sample 1 to T2;

[0096] Step 5: According to the established punch constant strain rate loading model, the punch movement speed is set to perform constant strain rate bulging, and the full-field strain acquisition system collects strain information in real time;

[0097] Figure 3 The geometric model of the rigid die bulging process is shown in the figure. The constant strain rate loading model is established by the following method:

[0098] The length L of the deformation zone is expressed as follows:

[0099] L=L H +2L C (1)

[0100] Where L H , L c The length of the spherical contour line and the generatrix of the punch (mm);

[0101] The length L of the spherical contour line that contacts the punch H and busbar length L C They are:

[0102]

[0103] Where D0 is the lower die aperture (mm); H i is the bulging height (mm); R' is the punch radius (mm); Half of the die taper angle (°);

[0104] Assuming uniform plastic deformation occurs in the deformation zone, the length L of the deformation zone at time t is:

[0105]

[0106] Where L0 is the initial length of the deformation zone (mm); V is the deformation speed of the deformation zone (mm / s);

[0107] strain rate is a constant value, and its relationship with the deformation velocity of the deformation zone can be expressed as:

[0108]

[0109] Integrate formula (5) and rearrange it to get:

[0110]

[0111] When t=0, V=V0, from which the deformation velocity V of the plastic deformation zone is calculated:

[0112]

[0113] Strain rate in the deformation zone It can be expressed as:

[0114]

[0115] By combining equations (7) and (8), we can get the deformation velocity of the deformation zone:

[0116]

[0117] In order to ensure a constant strain rate at the vertex, the loading speed v of the servo loading cylinder needs to be continuously adjusted. i Assuming that the time interval for the servo loading cylinder to adjust the loading speed is Δt, the end times of each loading stage are:

[0118] t1=Δt, t2=2Δt,…,t i =iΔt (10)

[0119] The average velocity of the deformation zone in each punch loading stage can be calculated by formula (9): And use this as the deformation speed V of the deformation zone each time the punch loads i :

[0120]

[0121] The length L of the deformation zone at each stage of the bulging deformation zone is calculated from this i :

[0122]

[0123] Where V1, V2..., V i A set of common ratios is The geometric progression of , we can get:

[0124]

[0125] Combined equations (10) and (13)

[0126]

[0127] Substituting equations (13) and (14) into equation (11), we can calculate the value of the “spherical cap cut cone ring” at any time t i The contour length L i It can be expressed as:

[0128]

[0129] The bulge height at any time can be obtained by combining:

[0130]

[0131] During the experiment, the deformation velocity V i Converted into the moving speed v of the punch i , adjust the punch speed v i , then the bulging height at the top of the specimen is:

[0132]

[0133] The moving speed v of the punch in each loading stage is determined from this i :

[0134]

[0135] Step 6: Unload, open the furnace, take samples and export experimental data;

[0136] Step 7: Uniform strain processing and uniform strain window establishment.

[0137] Figure 5 The schematic diagram of the uniform strain treatment method is shown in Figure 1. Figure 1 shows the selection of the BZ and RZ regions, and Figure 1 shows the measurement of the necking and fracture regions. As shown in the figure, the uniform strain treatment method for the bulging test plate is as follows:

[0138] (1) Use the DIC test device to obtain the global strain field during the sheet bulging process. In principle, the higher the CCD camera capture frequency, the more accurate the measured strain and the local necking strain calculated later. In actual operation, the computer memory limitation is considered. The CCD camera acquisition frequency is reasonably set according to the strain rate of the specimen deformation, and it is necessary to ensure that t AN and t F There must be at least 5 photos between them. AN is the time after necking occurs, t F is the time when the fracture occurs, such as Figure 5 (b) shown.

[0139] (2) Determination of the fracture starting position. Find the full-domain strain field photo before the fracture in the collected series of photos. The position with the largest principal strain is the starting position of the crack, which is recorded as the reference zone BZ, that is, Figure 5 Point 1 in (a).

[0140] (3) Determination of the reference zone RZ. Draw a circle with a diameter of 16 mm with point 1 as the center. Take a point every 90° along the circumference and record it as point 2, point 3, point 4, and point 5 respectively. The average strain of these four points is the average strain in the reference zone RZ.

[0141] (4) Determination of the starting point of local necking. Calculation of the thickness strain during the bulging process of the reference point of the base area - point 1 |ε3 BZ | and the average through-thickness strain in the reference zone (RZ), expressed as |ε3 BZ | is the x-axis, |ε3 RZ | is the y-axis, and is plotted as Figure 5 (b) shows the curve. i' ≤t BZ and t AN ≤t i' ≤t F The least square method is used to fit straight lines 1 and 2 within the range of t. The intersection of these two curves is the beginning of local necking, and the strain before this intersection is uniform. BN and t AN are the time before and after necking, t BN The value range is 0.3t F ≤t BN ≤0.6tF , t AN The value range is 0.99t F ≤t BN ≤0.998t F ,t F The time when the fracture occurs.

[0142] Taking the non-steady-state in-situ high temperature forming performance test of 7B04 aluminum alloy as an example, Figure 6 Figure 1 shows the results of bulging experiments using 7B04 aluminum alloy as an example. Figure (a) shows the variation of the full-thickness strain in the gauge zone with bulging time. Figure (b) shows the variation curve of the through-thickness strain in the base zone BZ and the reference zone RZ. Figure (c) shows the strain path in the reference zone.

[0143] Step 1: Adjust the temperature control system to control the mold temperature to 470℃ and keep it warm for 2 hours to ensure uniform temperature at all points inside the mold.

[0144] Step 2: Prepare 7B04 aluminum alloy specimens. First, polish the specimen cuts with an electric grinder equipped with a 400-grit sandpaper grinding head to remove surface defects left by wire cutting. Then, scrub the specimen surface with anhydrous ethanol to remove debris and oil stains generated by laser cutting. Finally, spray one layer of boron nitride coating and two layers of 2800-grit graphite coating on the part in contact with the punch, and spray the other side with speckle.

[0145] Step 3: After the sample is prepared, put it into the mold. After pressurization, the blank holder cylinder drives the blank holder ring to move, so that the blank holder rib is pressed into the specimen. The blank holder pressure is controlled to 5MPa so that the material does not flow radially when the specimen is deformed.

[0146] Step 4: Solution treatment of the slab was performed in a heating furnace at 470°C for 20 minutes to eliminate coarse precipitates and obtain a uniform and well-ductile microstructure. After solution treatment, the fan was turned on to rapidly cool the sample to 115°C.

[0147] Step 5: According to the established punch constant strain rate loading model, set the punch moving speed to achieve a constant punch speed of 0.01s -1 Strain rate loading was performed to perform bulging at a constant strain rate, and the shooting frequency of the CCD camera in the strain acquisition system was set to 6 Hz to collect strain information in real time.

[0148] Step 6: Unload, open the furnace, take samples and export experimental data.

[0149] Step 7: Uniform strain processing and uniform strain window establishment.

[0150] Figure 6 (a) shows the variation of the full-thickness strain ε3 in the gauge area with the bulging time. The specimen deforms uniformly after the bulging begins. When t i' / tF = 0.6, the specimen deforms concentratedly in the suspended area and the contact area, and a light green concentrated deformation area can be observed on the thickness strain distribution cloud diagram; as the punch rises further, when t i' / t F = 0.9, local necking occurs in the concentrated deformation zone; when t i' / t F =1, the aluminum-gold plate breaks under the action of biaxial tensile stress, and the Mises equivalent strain at the crack initiation position is the largest, about 0.414.

[0151] Figure 6 (b) is the variation curve of the thickness strain ε3 of the base zone BZ and the reference zone RZ, and the |ε3 BZ |-|ε3 RZ |The curve is at 0≤t i' ≤0.5t F and 0.95≤t i' ≤t F Using the least squares method, lines 1 and 2 were fitted within the two ranges of . Their intersection marks the onset of local necking, and the strain at this point is the local necking strain of the plate, which is 0.215. The through-thickness strains of the base zone BZ and the reference zone RZ are nearly equal before necking. With the onset of necking, concentrated deformation occurs in the base zone, and the through-thickness strain of the base zone BZ gradually increases beyond that of the reference zone RZ, with the slope of the curve changing from 1.09 to 0.83. Figure 6 (c) shows the strain path of the reference zone RZ. Through calculation, the principal strain when local necking occurs is 0.116, the secondary strain is 0.099, and the principal strain when fracture occurs is 0.235, and the secondary strain is 0.177.

Claims

1. A non-steady-state in-situ high-temperature forming performance testing device, characterized by: The testing device comprises an experimental specimen (1), a deformation loading and pressure acquisition system, a temperature control system and a full-field strain acquisition system; The deformation loading and pressure acquisition system includes a bulging die, a servo loading electric cylinder (2) and a blank holding cylinder (3); the bulging die includes an upper die (4), a blank holding ring (5) and a punch (6); a pressure sensor is installed below the punch (6) to collect the bulging force of the experimental sample (1) during the bulging process in real time; a heat insulation plate (7) and a water cooling plate (8) are provided between the blank holding ring (5) and the platform (15); the servo loading electric cylinder (2) is loaded according to a program to realize the constant strain rate deformation of the experimental sample; an annular raised blank holding rib is installed on the lower surface of the upper die (4), and the blank holding cylinder (3) drives the blank holding ring (5) to move, so that the blank holding rib is pressed into the experimental sample (1), and the material is controlled not to flow radially when the experimental sample (1) is deformed; The temperature control system comprises a heating furnace (9), a thermocouple (10), a blower (11) and a blast pipe (12); the heating furnace (9) is used to heat the experimental sample (1), and is filled with asbestos (13) to reduce the heat exchange between the mold and the external environment during the experiment; the top of the heating furnace (9) is open and is provided with high-temperature glass (14); the thermocouple (10) is used to detect the real-time temperature of the mold, and a temperature measuring hole is opened in the upper mold (4), and one end of the thermocouple (10) is inserted into the temperature measuring hole; the blower (11) and the blast pipe (12) are used to exchange hot air in the furnace with the outside world to achieve rapid cooling; The full-field strain acquisition system includes a CCD camera (16) and a DIC system (17). The DIC optical strain measurement system is used to perform real-time acquisition and calculation of the full-field strain during the bulging process of the experimental sample. During the experiment, the speckle pre-sprayed on the sample surface changes as the sample deforms. This process is collected by the CCD camera (16) and used to calculate the displacement and strain during the deformation process.

2. The unsteady-state in-situ high-temperature forming performance testing device according to claim 1, characterized in that: The bulging die is made of low carbon steel, stainless steel or Ni7N.

3. The unsteady-state in-situ high-temperature forming performance testing device according to claim 1, characterized in that: The experimental sample (1) is a circular plate with a diameter of 180 mm and a thickness of 1 to 4 mm.

4. The unsteady-state in-situ high-temperature forming performance testing device according to claim 1, characterized in that: The cooling channel in the water-cooling plate (8) is a flowing normal-temperature water-cooling channel.

5. The unsteady-state in-situ high-temperature forming performance testing device according to claim 1, characterized in that: The blank holding pressure of the blank holding ring (5) is 5-7 MPa.

6. The unsteady-state in-situ high-temperature forming performance testing device according to claim 1, characterized in that: The heating furnace (9) adopts a PID system closed-loop temperature control, outputs a control signal according to the difference between the target temperature and the actual temperature, and performs temperature feedback and temperature control.

7. A non-steady-state in-situ high-temperature forming performance testing method, characterized in that: The specific steps include: Step 1: Adjust the temperature control system to control the temperature of the bulging mold to T1, and keep it warm for a period of time to ensure that the temperature at each point inside the mold is uniform; Step 2: Prepare the experimental sample (1). First, polish the cut of the experimental sample (1). Then, scrub the surface of the experimental sample (1) to remove debris and oil generated by laser cutting. Finally, spray lubricant on the part of the experimental sample (1) that contacts the punch (6), and spray speckle on the other side. Step 3: After the experimental sample (1) is prepared, it is placed in the mold. After pressurization, the edge holding cylinder (3) drives the edge holding ring (5) to move, so that the edge holding rib is pressed into the experimental sample (1). The edge holding pressure is controlled to P so that the material does not flow radially when the experimental sample (1) is deformed; Step 4: heat preservation, using a heating furnace to perform solid solution treatment on the experimental sample (1) for a time of t1. After the solid solution is completed, the fan (11) is turned on to quickly cool the temperature of the experimental sample (1) to T2; Step 5: According to the established punch constant strain rate loading model, the punch movement speed is set to perform constant strain rate bulging, and the full-field strain acquisition system collects strain information in real time; Step 6: Unload, open the furnace, take samples and export experimental data; Step 7: Uniform strain processing and uniform strain window establishment.

8. The method for testing the non-steady-state in-situ high-temperature forming performance according to claim 7, characterized in that: In step 2, the polishing of the experimental sample (1) was performed using an electric grinder equipped with a 400-grit sandpaper grinding head.

9. The method for testing the non-steady-state in-situ high-temperature forming performance according to claim 7, wherein: In step 2, the surface of the experimental sample (1) is scrubbed with anhydrous ethanol; the contact portion of the sample with the punch is sprayed with a layer of boron nitride coating and two layers of 2800 mesh graphite coating for lubrication.

10. The unsteady-state in-situ high-temperature forming performance testing method according to claim 7, characterized in that: The constant strain rate loading model described in step 5 is established by the following method: The length L of the deformation zone is expressed as follows: L=L H +2L C (1) Where L H , L c The length of the spherical contour line and the generatrix of the punch (mm); The length L of the spherical contour line that contacts the punch H and busbar length L C They are: Where D0 is the lower die aperture (mm); H i is the bulging height (mm); R' is the punch radius (mm); Half of the die taper angle (°); Assuming uniform plastic deformation occurs in the deformation zone, the length L of the deformation zone at time t is: Where L0 is the initial length of the deformation zone (mm); V is the deformation speed of the deformation zone (mm / s); strain rate is a constant value, and its relationship with the deformation velocity of the deformation zone can be expressed as: Integrate formula (5) and rearrange it to get: When t=0, V=V0, from which the deformation velocity V of the plastic deformation zone is calculated: Strain rate in the deformation zone It can be expressed as: By combining equations (7) and (8), we can get the deformation velocity of the deformation zone: In order to ensure a constant strain rate at the vertex, the loading speed v of the servo loading cylinder needs to be continuously adjusted. i Assuming that the time interval for the servo loading cylinder to adjust the loading speed is Δt, the end times of each loading stage are: t1=Δt,t2=2Δt,……,t i =iΔt (10) The average velocity of the deformation zone in each punch loading stage can be calculated by formula (9): And use this as the deformation speed V of the deformation zone each time the punch loads i : The length L of the deformation zone at each stage of the bulging deformation zone is calculated from this i : Where V1, V2..., V i A set of common ratios is The geometric progression of , we can get: Combined equations (10) and (13) Substituting equations (13) and (14) into equation (11), we can calculate the value of the "spherical cap cut cone ring" at any time t i The contour length L i It can be expressed as: The bulge height at any time can be obtained by combining: During the experiment, the deformation velocity V i Converted into the moving speed v of the punch i , adjust the speed v of the punch i , then the bulging height at the top of the specimen is: The moving speed v of the punch in each loading stage is determined from this i :

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