Device and method for hot air bulging forming control of special-shaped light alloy pipes based on thermal deformation

Abstract

This invention discloses a control device and method for hot gas expansion forming of irregularly shaped light alloy pipes based on thermal deformation, comprising: a forming mold; an induction heating system; a temperature measurement and control system; a mechanical control system; a gas expansion forming system; a displacement measurement system; and a data processing system. The data processing system is used to establish a material model and a forming limit model that can predict the stress-strain curves of the material at different temperatures and strain rates, determine whether the strain rate at a given location meets the requirements for material forming, and then feed back to the gas expansion forming system for adjustment. This invention utilizes the aforementioned control device and method for hot gas expansion forming of irregularly shaped light alloy pipes based on thermal deformation, employing a material model and a forming limit model that can predict the stress-strain curves of the material at different temperatures and strain rates. This improves production efficiency while simultaneously obtaining pipes with uniform wall thickness and ideal forming characteristics. It also reduces the range of experimental parameters, avoids unnecessary experiments, and reduces experimental costs.

Description

Technical Field

[0001] The invention relates to a metal forming manufacturing technology, and in particular to a hot air bulging forming control device and method for special-shaped light alloy pipe fittings based on thermal deformation. Background Art

[0002] Complex and special-shaped thin-walled hollow pipes are key components in high-end equipment fields such as aerospace and automobiles. For different application fields, commonly used raw materials are mostly titanium alloys, high-strength aluminum, high-strength steel and high-temperature alloys.

[0003] The existing forming methods of complex and special-shaped thin-walled pipe fittings are mainly: 1) Plate stamping + welding. The idea of ​​this method is to use stamping to form the complex curved surface of the plate into blocks, and then weld them into a whole. The disadvantages of this method are: due to the existence of the weld joint, it is impossible to guarantee the strength of the formed part, the accuracy of the welding forming, and some materials are difficult to weld; 2) Internal high pressure forming. The idea of ​​this method is to use liquid as the forming medium to achieve the purpose of forming hollow parts by controlling the internal pressure and material flow. The disadvantage of this method is that the material has poor plasticity at low temperatures and it is difficult to form parts with particularly complex shapes. 3) Superplastic forming. The idea of ​​this method is to use superplasticity, which refers to the characteristics of certain metals or alloys showing low strength and large elongation under specific conditions, that is, under low strain rate, certain deformation temperature and stable and fine grain size. The disadvantages of this method are high requirements on the organization of the material, uneven forming wall thickness and poor performance of the formed component.

[0004] In summary, the existing forming methods cannot meet the requirements of forming complex thin-walled pipes. To address this, the existing technology has also developed a hot air bulging forming process. Hot air bulging is formed under medium temperature and rapid loading conditions. It uses the double hardening phenomenon of aluminum alloy to control the material deformation and microstructure performance evolution by adjusting process parameters, and finally obtains a formed part with uniform deformation and good comprehensive performance.

[0005] However, complex aluminum alloy parts have many local small features, large outline dimensions and complex variable cross-sections, making it difficult to achieve uniform wall thickness and overall precise forming. At the same time, there are many process parameters that affect the forming process, and the deformation is complex, making it difficult to accurately obtain process parameters that meet production requirements. Summary of the invention

[0006] The purpose of the present invention is to provide a hot air bulging forming control device and method for special-shaped light alloy pipe fittings based on thermal deformation, using a material model and a forming limit model that can predict the stress-strain curve of the material at different temperatures and strain rates, and a displacement sensor to measure the bulging process of the cross-section tube in real time, and adopt a segmented forming process to adjust the process parameters. Under the premise of forming the main contour of the pipe fitting and local small features, the optimal process parameters are obtained, thereby improving the production efficiency and obtaining pipe fittings with uniform wall thickness and ideal forming characteristics. At the same time, the range of experimental parameters is reduced, useless experiments are avoided, and the experimental costs are reduced.

[0007] To achieve the above-mentioned purpose, the present invention provides a hot air bulging forming control device for special-shaped light alloy pipes based on thermal deformation, comprising:

[0008] Forming dies, used to form the pipe and transfer temperature to the pipe;

[0009] an induction heating system for heating the pipe through the forming die;

[0010] A temperature measurement and control system to ensure uniform temperature of the forming die and pipe fittings;

[0011] Mechanical control system for expanding and sealing both ends of the pipe;

[0012] The gas expansion forming system is used to introduce compressed gas into the interior of the pipe;

[0013] Displacement measurement system, used to monitor the displacement changes of the pipe section;

[0014] The data processing system is used to establish a material model and a forming limit model that can predict the stress-strain curve of the material under different temperatures and strain rates, and calculate the strain rate at each cross-sectional position of the pipe fitting in each time period based on the displacement signal collected by the displacement measurement system, and compare it with the forming limit derived from the material model, thereby judging whether the strain rate at this position meets the material forming requirements, and then feeding back to the inflation forming system for adjustment.

[0015] Preferably, the induction heating system comprises an induction heating wire spirally arranged in the forming mold around the mold cavity of the forming mold, and an induction heater electrically connected to the induction heating wire;

[0016] The induction heating wire is isolated from the forming mold by a high-temperature ceramic ring.

[0017] Preferably, the displacement measurement system includes a quartz rod whose one side contacts the outer side of the large convex part of the pipe, and the other side of the quartz rod contacts a plurality of displacement sensors arranged along the axial direction of the pipe, and the plurality of displacement sensors are electrically connected to a multi-channel recorder.

[0018] Preferably, the small characteristic deformation areas of the forming die are provided with displacement measurement holes;

[0019] The quartz rod and the displacement sensor are fitted in the clearance of the displacement measuring hole.

[0020] Preferably, the temperature measurement and control system includes a temperature sensor, a multi-channel recorder and a temperature control box;

[0021] The temperature sensor is arranged on the outside of the pipe symmetrically with respect to the displacement sensor. The temperature sensor is respectively connected to the multi-channel recorder and the temperature control box, and the temperature control box is connected to the induction heater.

[0022] Preferably, the mechanical control system includes a press connected to the outside of the forming mold and punches coaxially arranged at both ends of the mold cavity of the forming mold, the punches are connected to a horizontal servo cylinder, and the press and the horizontal servo cylinder are both electrically connected to a mechanical control cabinet.

[0023] Preferably, the inflatable forming system includes a compressed air source and an air source control cabinet. The compressed air source is connected to the interior of the forming mold via the air source control cabinet using a connecting air pipe. A pressure sensor and a one-way valve are provided in the connecting air pipe. The pressure sensor and the one-way valve are electrically connected to the air source control cabinet.

[0024] The method of the hot air bulging forming control device of the special-shaped light alloy pipe based on thermal deformation comprises the following steps:

[0025] S1. Load materials and enter initial conditions

[0026] Input the material, geometry, forming temperature and forming requirements of the special-shaped hollow pipe into the data processing system;

[0027] S2. According to the geometric shape of the special-shaped hollow pipe, the main contour area and small feature deformation rate algorithm are established, and the plastic strain ε and forming pressure P of the material at each typical cross section of the pipe are calculated according to the material shaping;

[0028] S3. First, based on the uniaxial tensile test data and forming limit data of the given pipe material, a material model and a forming limit model are established to predict the stress-strain curve of the material at different temperatures and strain rates; then, the forming process window is preliminarily determined based on the cross-sectional strain calculated according to the geometric shape of the pipe; finally, the minimum strain rate in the forming process window is selected, and the average pressurization rate P corresponding to the minimum strain rate is calculated based on the plastic strain ε of the cross-sectional material and the forming pressure P, and it is used as the initial process parameter for the experiment;

[0029] S4, segmented forming

[0030] S41, main body contour forming

[0031] According to the calculated average pressurization rate P, the forming die is inflated to deform the pipe placed in the die cavity of the forming die. In this process, the main contour area and small feature deformation rate algorithm calculates the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor, and compares it with the value obtained by the forming limit model to determine whether it can be formed under this condition. If it can be formed, the pressurization rate is increased while ensuring the forming, and the pressurization rate is adjusted along the optimal hardening rate path until the main contour is attached to the die. Finally, it is determined whether the large-size contour is attached to the film according to the displacement sensor reading, thereby obtaining the process parameters with the best forming performance;

[0032] S42, local small feature forming

[0033] According to the geometric shape and material model of the small fillet, the process window is established, and the pressurization rate corresponding to the minimum strain rate is selected for the experiment. Then, the main contour area and small feature deformation rate algorithm is used to calculate the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor, and it is compared with the value obtained by the forming limit model to determine whether it can be formed under this condition. If it can be formed, the pressurization rate is reduced while ensuring the forming until the contour of the local small feature is attached to the film. Finally, the displacement sensor reading is used to determine whether the contour of the local small feature is attached to the film, and finally the process parameters with the best forming performance are obtained;

[0034] S5. Stop pressurizing, open the forming mold, and take out the pipe fitting.

[0035] Preferably, the material model in step S3 is established by the following method:

[0036] Establishing the phenomenological FB model: and the JC model:

[0037] Alternatively, establish a physically meaningful constitutive model

[0038] Where ε is the strain, is the strain rate, ρ is the dislocation density, is the hardening rate, w d is the damage, T is the forming temperature, K is the strength coefficient, A is the initial yield stress at the reference strain rate and reference temperature; B and n are the strain hardening modulus and hardening exponent of the material, respectively, C is the strain rate strengthening parameter of the material, m is the thermal softening index of the material, ε eq is the equivalent strain, is the dimensionless equivalent plastic strain rate, the homologous temperature T * =(TTr )(T m -T r ), T r 、T m are the reference temperature and melting temperature respectively, and T is the current temperature.

[0039] Preferably, the calculation process of the actual strain rate of the tube blank in step S41 is as follows:

[0040] Define the cross-sectional perimeter of the original tube after deformation:

[0041] l i =πD i (1)

[0042] Where D i is the diameter of the original tube after deformation;

[0043] The cross-sectional strain of the original tube after deformation:

[0044]

[0045] Where l0 is the original cross-sectional circumference of the tube blank;

[0046] The cross-sectional strain rate of the original tube after deformation:

[0047]

[0048] Where t0 is the initial time; t i is the deformation time;

[0049] Combining equations (1), (2) and (3), we can get the actual strain rate of the tube:

[0050]

[0051] Where D i is the diameter of the original tube after deformation; D0 is the original tube diameter;

[0052] When the local small feature in step S42 is the petal fillet area, the actual tube strain rate is calculated by the following method:

[0053] The deformation process of the petal fillet area is approximately regarded as an ellipse, whose major axis remains unchanged and the minor axis changes continuously with the deformation of the tube blank;

[0054] Set the cross-sectional perimeter of the original tube after deformation:

[0055]

[0056] Where a is the length of the long axis of the original tube, b is i is the length of the short axis of the original tube after deformation;

[0057] Combining equations (2) and (3), the actual strain rate of the tube is obtained:

[0058]

[0059] Where b0 is the length of the minor axis of the original tube blank;

[0060] When the local small feature in step S42 is a rectangular small rounded corner, the actual strain rate of the tube blank is calculated by the following method:

[0061] The rectangular rounded corners are divided into two parts. The first part approximates the deformation process as a trapezoid, and the variable is the height of the trapezoid; the second part approximates it as an ellipse, and the variable is the length of the minor axis of the ellipse.

[0062] Set the perimeter of the first section of the original tube after deformation:

[0063]

[0064] Where c is the distance between the adjacent contact points between the tube and the die at the fillet, h is i It is the height of the original tube after deformation;

[0065] Combining equations (2) and (3), the actual strain rate of the tube blank in the first part is obtained:

[0066]

[0067] Where h0 is the height of the original tube;

[0068] The petal fillet area calculation process is repeated to calculate the actual strain rate of the second portion of the original tube after deformation.

[0069] Therefore, the present invention has the following beneficial effects:

[0070] 1. Forming quality: Compared with the single pressurization rate of traditional hot air forming, a segmented forming method is adopted. A fast pressurization rate is used in the early stage to ensure the uniformity of wall thickness. In the later stage, the pressurization rate is changed according to the needs of small features, which reduces the pressure required for forming small fillets and reduces the difficulty of the experiment.

[0071] 2. Production efficiency: Adjust the pressurization rate to load at the highest possible strain rate without cracking to improve production efficiency.

[0072] 3. Process stability: The material thermal deformation behavior model is used to predict the stress and strain at various temperatures and strain rates during the bulging process. By adjusting the process parameters in real time, the material is always in a stable forming area, thereby avoiding forming defects and improving stability.

[0073] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 It is a structural schematic diagram of the present invention;

[0075] Figure 2 is a flow chart of the method of the present invention;

[0076] Figure 3 It is a schematic diagram of a uniaxial stretching curve of the present invention;

[0077] Figure 4 is the forming limit diagram of the present invention;

[0078] Figure 5 It is a schematic diagram of the forming mold structure of the present invention;

[0079] Figure 6 A cross-sectional layout diagram of a displacement sensor of the present invention;

[0080] Figure 7 It is another cross-sectional arrangement diagram of a displacement sensor of the present invention;

[0081] Figure 8 It is a diagram of the strain rate calculation method when the local small feature of the present invention is the petal fillet area;

[0082] Fig. 9 It is a diagram of the strain rate calculation method when the local small feature of the present invention is a rectangular small rounded corner area;

[0083] Fig.10 It is a schematic diagram of the main body contour shaping and adjusting system of the present invention;

[0084] Fig.11 It is a schematic diagram of the local small feature forming adjustment system of the present invention.

[0085] Among them: 1. Multi-channel recorder; 2. Displacement sensor; 3. Forming die; 4. Induction heating wire; 5. Air expansion forming system; 6. Induction heater; 7. Temperature sensor; 8. Punch; 9. Data processing system; 10. Insulation board; 11. Press. DETAILED DESCRIPTION

[0086] The present invention will be further described below in conjunction with the accompanying drawings. It should be noted that this embodiment is based on the technical solution and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to this embodiment.

[0087] The hot air bulging forming control device for special-shaped light alloy pipes based on thermal deformation includes:

[0088] The forming mold 3 is used to form the pipe fittings and transfer temperature to the pipe fittings; the upper mold in the forming mold 3 in this embodiment is connected to the press 11 for mold closing, and the lower mold is fixed to the table of the press 11. After mold closing, the upper mold is pressurized to ensure full contact between the pipe fittings and the mold in the subsequent inflation process.

[0089] The mold material can be selected from commonly used mold materials for hot air pressing, such as low carbon steel, stainless steel, Ni7N, etc. A heat preservation plate 10 is provided on the outer surface of the mold to prevent temperature fluctuations caused by heat loss.

[0090] An induction heating system for heating the pipe through the forming die 3;

[0091] Temperature measurement and control system to ensure uniform temperature of the forming die 3 and the pipe fittings;

[0092] Mechanical control system for expanding and sealing both ends of the pipe;

[0093] The gas expansion forming system 5 is used to introduce compressed gas into the interior of the pipe;

[0094] Displacement measurement system, used to monitor the displacement changes of the pipe section;

[0095] The data processing system 9 is used to establish a material model and a forming limit model that can predict the stress-strain curve of the material under different temperatures and strain rates, and calculate the strain rate at each cross-sectional position of the pipe fitting in each time period based on the displacement signal collected by the displacement measurement system, and compare it with the forming limit derived from the material model, thereby judging whether the strain rate at this position meets the material forming requirements, and then feeding back to the inflation forming system 5 for adjustment.

[0096] Among them, the induction heating system includes an induction heating wire 4 spirally arranged in the forming mold 3 around the mold cavity of the forming mold 3 and an induction heater 6 electrically connected to the induction heating wire 4 (the induction heating wire 4 is placed around the pipe axially), which is used to output power current and control the coil output power; the induction heating wire 4 and the forming mold 3 are isolated by a high-temperature ceramic ring to avoid direct contact with the forming mold. The principle is: the induction heater 6 converts the three-phase industrial frequency AC into direct current after rectification, and then converts the direct current into an adjustable current, generating high-density magnetic lines in the induction coil, and cutting the metal material contained in the induction coil, generating eddy currents in the metal material, thereby generating heat by using the eddy current effect, and in this process, the heating rate can be adjusted by adjusting the current and frequency. The rated output voltage of the power supply in this embodiment is 380V, and the rated output current is 50A. For heat-resistant steel, the pipe fittings are usually heated in the range of 600-1100℃.

[0097] Preferably, the displacement measurement system includes a quartz rod with one side in contact with the outer side of the large convex part of the pipe, and the other side of the quartz rod in contact with a plurality of displacement sensors 2 arranged along the axial direction of the pipe, and the plurality of displacement sensors 2 are electrically connected to the multi-channel recorder 1. Preferably, the small characteristic deformation area of ​​the forming mold 3 is provided with a displacement measurement hole; the gap in the displacement measurement hole is matched with a quartz rod and a displacement sensor 2. The displacement sensor 2 in this embodiment has a resolution of 5 μm and a measurement range of 0 to 125 mm. It is not resistant to high temperatures, so the quartz rod is required to isolate the heat of the tube blank while transmitting the bulging displacement of the tube blank and transmitting the signal to the multi-channel recorder 1 for monitoring the degree of hot air pressure bulging of the pipe. The multi-channel recorder 1 in this embodiment is a paperless multi-channel recorder 1. In this embodiment, the initial undeformed moment is set to 0, and the paperless multi-channel recorder 1 records the displacement changes in the whole process, so as to judge the forming situation according to the displacement changes at each position.

[0098] Preferably, the temperature measurement and control system includes a temperature sensor 7, a multi-channel recorder 1 and a temperature control box; the temperature sensor 7 is arranged on the outside of the pipe symmetrical to the displacement sensor 2, and the temperature sensor 7 is connected to the multi-channel recorder 1 and the temperature control box respectively, and the temperature sensor 7 and the displacement sensor 2 are located in the same horizontal plane; the temperature measuring end of the temperature sensor 7 is in direct contact with the metal pipe, so as to transmit the temperature signal to the multi-channel recorder 1 and the temperature control box. In this embodiment, the temperature sensor 7 uses a thermocouple with a temperature measurement range of 0 to 800°C, and the temperature control box is connected to the induction heater 6. According to the thermocouple feedback of the temperature signal inside the mold, the power of the induction heater 6 is controlled, and the temperature in the mold cavity of the forming mold 3 is adjusted to keep the temperature of the metal pipe and the forming mold 3 uniform. In this embodiment, the mold temperature and the pipe forming temperature are the same, generally 0.3 to 0.6Tm. More specifically, the heating temperature range of aluminum alloy pipe fittings is 300-500°C; for titanium alloy and high-strength steel, the heating temperature range of their pipe fittings is usually 700-1000°C; for high-temperature alloys, the heating temperature range of their pipe fittings is usually 800-1200°C.

[0099] Preferably, the mechanical control system includes a press 11 connected to the outside of the forming mold 3 and a punch 8 coaxially arranged at both ends of the mold cavity of the forming mold 3, the punch 8 is connected to the horizontal servo cylinder, and the press 11 and the horizontal servo cylinder are both electrically connected to the mechanical control cabinet for controlling the stroke and pressure of the horizontal servo cylinder and the press 11. The holding pressure of the press 11 in this embodiment is greater than 10kN, and the holding pressure of the horizontal servo cylinder is greater than 2kN.

[0100] Preferably, the pneumatic forming system 5 includes a compressed gas source and a gas source control cabinet. The compressed gas source is connected to the inside of the forming mold 3 through the gas source control cabinet by a connecting gas pipe. A pressure sensor and a one-way valve are arranged in the connecting gas pipe. The pressure sensor and the one-way valve are electrically connected to the gas source control cabinet. The gas source control cabinet can control the pressure and pressurization rate of the gas flowing out of the compressed gas source, and can control the gas to be inflated into the tube blank at a fixed value or a segmented pressurization rate, so that the gas pressure inside the tube can be kept constant. The compressed gas source in this embodiment is a high-pressure gas cylinder that can be filled with different types of high-pressure inert gases such as nitrogen and argon, which can provide 0-30MPa of high-pressure gas; the gas pressure of the pipe fitting can be kept at 0.1-20MPa.

[0101] The method of the hot air bulging forming control device of the special-shaped light alloy pipe based on thermal deformation comprises the following steps:

[0102] S1. Load materials and enter initial conditions

[0103] Inputting the material, geometry, forming temperature and forming requirements of the special-shaped hollow pipe into the data processing system 9;

[0104] Step S1 also includes controlling the mold temperature to T using a temperature measurement and control system and an induction heating system to ensure that the pipe has a uniform temperature field after heating;

[0105] S2. According to the geometric shape of the special-shaped hollow pipe, the main contour area and small feature deformation rate algorithm are established, and the plastic strain ε and forming pressure P of the material at each typical cross section of the pipe are calculated according to the material shaping;

[0106] S3. First, based on the uniaxial tensile test data and forming limit data of the given pipe material, a material model that can predict the stress-strain curve of the material at different temperatures and strain rates is established (used to collect the pipe temperature by thermocouple and determine the forming temperature, so as to facilitate the next test after the pipe temperature reaches the forming temperature and stabilizes) and a forming limit model (used to reflect the corresponding relationship between different process parameters (temperature, pressurization rate) and the high-temperature hardening, elongation and forming limit of the material); then, the forming process window is preliminarily determined in combination with the cross-sectional strain calculated according to the geometric shape of the pipe; finally, the minimum strain rate in the forming process window is selected, and the average pressurization rate P corresponding to the minimum strain rate is calculated in combination with the plastic strain ε of the cross-sectional material and the forming pressure P, and it is used as the initial process parameter for experiment;

[0107] Preferably, the material model in step S3 is established by the following method:

[0108] Establishing the phenomenological FB model: and the JC model:

[0109] Alternatively, establish a physically meaningful constitutive model

[0110] Where ε is the strain, is the strain rate, ρ is the dislocation density, is the hardening rate, w d is the damage, T is the forming temperature, K is the strength coefficient, A is the initial yield stress at the reference strain rate and reference temperature; B and n are the strain hardening modulus and hardening exponent of the material, respectively, C is the strain rate strengthening parameter of the material, m is the thermal softening index of the material, ε eq is the equivalent strain, is the dimensionless equivalent plastic strain rate, the homologous temperature T * =(TT r )(T m -T r ), T r , T m are the reference temperature and melting temperature respectively, and T is the current temperature.

[0111] S4, segmented forming

[0112] It is known from the common knowledge in this field that the greater the pressurization rate, the higher the degree of deformation and hardening of the material, and the better the uniformity of the overall wall thickness, while the slower the pressurization rate, the better the elongation of the material. Therefore, it is necessary to change the process parameters such as the pressurization rate and the amount of gas in real time to control the output of the forming parameters of the device for the optimal process path with the maximum gradient of the material hardening performance or the highest elongation, so as to achieve real-time online regulation of the complex parts forming process and ensure the optimal forming quality of the final parts. Moreover, the uniformity of wall thickness is the key to the main contour of the formed parts, and the cracking defect is the key to the forming of small features. In order to ensure that the formed parts meet the requirements of uniform overall wall thickness and do not have defects such as small features that cannot be formed, the entire forming process is divided into two stages;

[0113] S41, main body contour forming

[0114] After the pipe reaches the forming temperature, the forming die 3 is inflated according to the calculated average pressurization rate P, so that the pipe placed in the mold cavity of the forming die 3 is deformed. In this process, the main contour area and the small feature deformation rate algorithm calculate the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor 2, and compare it with the value obtained by the forming limit model to obtain the forming limit of the pipe, and judge whether it can be formed under this condition (that is, whether the current strain rate meets the plastic condition). If it can be formed, the pressurization rate is increased under the premise of ensuring the forming, and the pressurization rate is regulated along the optimal path of the hardening rate (because the larger the pressurization rate, the more obvious the strain hardening, which can prevent the weak parts from plastic deformation and improve the overall deformation uniformity. Therefore, the pressurization rate is increased under the premise of ensuring the forming, and the wall thickness uniformity is improved) until the main contour is attached to the mold, and finally the large size contour is judged according to the indication of the displacement sensor 2, so as to obtain the process parameters with the best forming performance; if it cannot be formed, the pressurization rate is changed by adjusting the inflation forming system 5, and then the strain rate is changed to form it;

[0115] Preferably, the calculation process of the actual strain rate of the tube blank in step S41 is as follows:

[0116] Define the cross-sectional perimeter of the original tube after deformation:

[0117] l i =πD i (1)

[0118] Where D i is the diameter of the original tube after deformation;

[0119] The cross-sectional strain of the original tube after deformation:

[0120]

[0121] Where l0 is the original cross-sectional circumference of the tube blank;

[0122] The cross-sectional strain rate of the original tube after deformation:

[0123]

[0124] Where t0 is the initial time; t i is the deformation time;

[0125] Combining equations (1), (2) and (3), we can get the actual strain rate of the tube:

[0126]

[0127] Where D i is the diameter of the original tube after deformation; D0 is the original tube diameter;

[0128] S42, local small feature forming

[0129] According to the geometric shape and material model of the small fillet, the process window is established, and the pressurization rate corresponding to the minimum strain rate is selected for the experiment. Then, the main contour area and the small feature deformation rate algorithm are used to calculate the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor 2, and it is compared with the value obtained by the forming limit model to determine whether it can be formed under this condition. If it can be formed, the pressurization rate is reduced under the premise of ensuring the forming until the contour of the local small feature is attached to the film. Finally, the displacement sensor 2 reading is used to determine whether the contour of the local small feature is attached to the film, and finally the process parameters with the best forming performance are obtained. For general metals, the smaller the strain rate, the greater the elongation, so the forming limit can be improved. Therefore, the use of a slow pressurization rate can reduce the material strain rate and reduce the material flow stress, thereby achieving the purpose of reducing the shaping pressure. For example, for the high-temperature deformation behavior of light alloy materials, the lower the material strain rate is usually, the better the plasticity is. Therefore, in terms of process parameter control, the pressurization rate is reduced and the material plasticity is improved under the premise of satisfying the strain amount of the small feature until the material plasticity reaches the optimal state. If it cannot be formed, the pressure rate is changed by adjusting the inflation forming system 5, thereby changing the strain rate to form it;

[0130] When the local small feature in step S42 is the petal fillet area, the actual tube strain rate is calculated by the following method:

[0131] The deformation process of the petal fillet area is approximately regarded as an ellipse, whose major axis remains unchanged and the minor axis changes continuously with the deformation of the tube blank;

[0132] Set the cross-sectional perimeter of the original tube after deformation:

[0133]

[0134] Where a is the length of the long axis of the original tube, b is i is the length of the short axis of the original tube after deformation;

[0135] Combining equations (2) and (3), the actual strain rate of the tube is obtained:

[0136]

[0137] Where b0 is the length of the minor axis of the original tube blank;

[0138] When the local small feature in step S42 is a rectangular small rounded corner, the actual strain rate of the tube blank is calculated by the following method:

[0139] The rectangular rounded corners are divided into two parts. The first part approximates the deformation process as a trapezoid, and the variable is the height of the trapezoid; the second part approximates it as an ellipse, and the variable is the length of the minor axis of the ellipse.

[0140] Set the perimeter of the first section of the original tube after deformation:

[0141]

[0142] Where c is the distance between the adjacent contact points between the tube and the die at the fillet, h is i It is the height of the original tube after deformation;

[0143] Combining equations (2) and (3), the actual strain rate of the tube blank in the first part is obtained:

[0144]

[0145] Where h0 is the height of the original tube;

[0146] The petal fillet area calculation process is repeated to calculate the actual strain rate of the second portion of the original tube after deformation.

[0147] S5, stop pressurizing, open the forming mold 3, and take out the pipe fitting.

[0148] This embodiment is described as follows using aluminum alloy as an example, and includes the following steps:

[0149] Step 1: Use the single-pulling data of aluminum alloy to establish a unified uniaxial viscoplastic constitutive model to obtain the stress and strain of the material under different process parameters. Use the forming limit of the material under different conditions to derive the forming limit model to obtain the forming limit of the material under different process conditions. Combine the values ​​derived from the material model and the contour dimensions of the formed part to preliminarily determine the forming process range. The temperature range is 400-500°C, and the pressurization rate is 0.1-1MPa / s. (This embodiment takes 450°C and 0.1MPa / s as an example)

[0150] Step 2: Heat the mold to 450°C, put the pipe into the mold and close the mold, and the punch 8 seals both ends.

[0151] Step 3: After the pipe is kept warm for 5 minutes, argon gas is injected into the mold at a pressurization rate of 0.1 MPa / s until the displacement sensor 2 shows that the film is attached at the large contour position. During this period, the data processing system 9 calculates the actual strain rate of the pipe blank according to the indication of the displacement sensor 2, and compares it with the value obtained by the forming limit model to determine whether it can be formed under this condition.

[0152] Step 4: After confirming that the large contour is formed, start forming the small fillet. From the shaping pressure formula, it can be seen that when the fillet radius is very small, the required forming pressure is very large. In order to reduce the shaping pressure, a slow pressurization rate is used to reduce the material strain rate and reduce the material flow stress, thereby achieving the purpose of reducing the shaping pressure. Using a small pressurization rate of 0.01MPa / s, the strain rate at the forming fillet is calculated based on the reading of displacement sensor 2, and the forming limit at this position is calculated using the actual strain rate. Based on the forming limit, it is determined whether the shaping of the material at the fillet can meet the forming requirements.

[0153] Step 5: After multiple experiments, the forming process parameters with good wall thickness uniformity and economical applicability are finally obtained.

[0154] Therefore, the present invention adopts the above-mentioned hot air expansion forming control device and method for special-shaped light alloy pipe fittings based on thermal deformation, preliminarily infers the process parameters through the material model, reduces the range of experimental parameters, and greatly improves the experimental efficiency; and adopts the segmented pressurization method, which not only improves the uniformity of the overall wall thickness of the formed pipe fittings, but also reduces the pressure required for forming small fillets, thereby reducing the experimental difficulty and cost; at the same time, a displacement sensor is used to monitor the deformation of each typical cross-section in real time, so that it can clearly understand how the pipe fittings are deformed during the whole process, and obtain the strain rate of each cross-section through the approximate calculation method, and judge whether the forming conditions can be met in combination with the material model, so that the forming process parameters can be accurately obtained.

[0155] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A hot air bulging forming control method for special-shaped light alloy pipes based on thermal deformation, characterized in that: The following steps are involved: S1. Load materials and enter initial conditions Input the material, geometry, forming temperature and forming requirements of the special-shaped hollow pipe into the data processing system; S2. According to the geometric shape of the special-shaped hollow pipe, the main contour area and small feature deformation rate algorithm are established, and the plastic strain ε and forming pressure P of the material at each typical cross section of the pipe are calculated according to the plasticity of the material; S3. First, based on the uniaxial tensile test data and forming limit data of the given pipe material, a material model and a forming limit model are established to predict the stress-strain curve of the material at different temperatures and strain rates; then, the forming process window is preliminarily determined based on the cross-sectional strain calculated according to the geometric shape of the pipe; finally, the minimum strain rate in the forming process window is selected, and the average pressurization rate corresponding to the minimum strain rate is calculated based on the plastic strain ε of the cross-sectional material and the forming pressure P. This was used as the initial process parameter for the experiment; S4, segmented forming S41, main body contour forming The average pressurization rate calculated Inflate the forming die to deform the pipe placed in the die cavity of the forming die. In this process, the main contour area and small feature deformation rate algorithm calculates the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor, and compares it with the value obtained by the forming limit model to determine whether it can be formed under this condition. If it can be formed, the pressurization rate is increased while ensuring the forming, and the pressurization rate is regulated along the optimal hardening rate path until the main contour is attached to the die. Finally, it is determined whether the large-size contour is attached to the film according to the displacement sensor reading, thereby obtaining the process parameters with the best forming performance; S42, local small feature forming According to the geometric shape and material model of the small fillet, the process window is established, and the pressurization rate corresponding to the minimum strain rate is selected for the experiment. Then, the main contour area and small feature deformation rate algorithm is used to calculate the strain rate of the actual tube blank according to the displacement data collected by the displacement sensor, and it is compared with the value obtained by the forming limit model to determine whether it can be formed under this condition. If it can be formed, the pressurization rate is reduced while ensuring the forming until the contour of the local small feature is attached to the film. Finally, the displacement sensor reading is used to determine whether the contour of the local small feature is attached to the film, and finally the process parameters with the best forming performance are obtained; S5. Stop pressurizing, open the forming mold, and take out the pipe fitting.

2. The hot air bulging forming control method for special-shaped light alloy pipes based on thermal deformation according to claim 1 is characterized in that: The material model in step S3 is established by the following method: Establishing the phenomenological FB model: and the JC model: Alternatively, a physically meaningful constitutive model can be established Where ε is the strain, is the strain rate, ρ is the dislocation density, is the hardening rate, w d is the damage, T is the forming temperature, K is the strength coefficient, A is the initial yield stress at the reference strain rate and reference temperature; B and n are the strain hardening modulus and hardening exponent of the material, respectively, C is the strain rate strengthening parameter of the material, m is the thermal softening index of the material, ε eq is the equivalent strain, is the dimensionless equivalent plastic strain rate, the homologous temperature T * =(TT r )(T m -T r ), T R , T m are the reference temperature and melting temperature respectively, and T is the current temperature.

3. The hot air bulging forming control method for special-shaped light alloy pipes based on thermal deformation according to claim 1 is characterized in that: The calculation process of the actual strain rate of the tube blank in step S41 is as follows: Define the cross-sectional perimeter of the original tube after deformation: l i ＝πD i (1) Where D i is the diameter of the original tube after deformation; The cross-sectional strain of the original tube after deformation: Where l0 is the original cross-sectional circumference of the tube blank; The cross-sectional strain rate of the original tube after deformation: Where t0 is the initial time; t i is the deformation time; Combining equations (1), (2) and (3), we can get the actual strain rate of the tube: Where D i is the diameter of the original tube after deformation; D0 is the original tube diameter; When the local small feature in step S42 is the petal fillet area, the actual tube strain rate is calculated by the following method: The deformation process of the petal fillet area is approximately regarded as an ellipse, whose major axis remains unchanged and the minor axis changes continuously with the deformation of the tube blank; Set the cross-sectional perimeter of the original tube after deformation: Where a is the length of the long axis of the original tube, b is i is the length of the short axis of the original tube after deformation; Combining equations (2) and (3), the actual strain rate of the tube is obtained: Where b0 is the length of the minor axis of the original tube blank; When the local small feature in step S42 is a rectangular small rounded corner, the actual strain rate of the tube blank is calculated by the following method: The rectangular rounded corners are divided into two parts. The first part approximates the deformation process as a trapezoid, and the variable is the height of the trapezoid; the second part approximates it as an ellipse, and the variable is the length of the minor axis of the ellipse. Set the perimeter of the first section of the original tube after deformation: Where c is the distance between the adjacent contact points between the tube and the die at the fillet, h is i It is the height of the original tube after deformation; Combining equations (2) and (3), the actual strain rate of the tube blank in the first part is obtained: Where h0 is the height of the original tube; The petal fillet area calculation process is repeated to calculate the actual strain rate of the second portion of the original tube after deformation.

4. A device for executing the hot air bulging control method for special-shaped light alloy pipes based on thermal deformation as described in any one of claims 1 to 3, characterized in that: include: Forming dies, used to form the pipe and transfer temperature to the pipe; an induction heating system for heating the pipe through the forming die; A temperature measurement and control system, used to ensure uniform temperature of the forming die and the pipe fittings; Mechanical control system for expanding and sealing both ends of the pipe; The gas expansion forming system is used to introduce compressed gas into the interior of the pipe; Displacement measurement system, used to monitor the displacement changes of the pipe section; The data processing system is used to establish a material model and a forming limit model that can predict the stress-strain curve of the material under different temperatures and strain rates, and calculate the strain rate at each cross-sectional position of the pipe fitting in each time period based on the displacement signal collected by the displacement measurement system, and compare it with the forming limit derived from the material model, thereby judging whether the strain rate at this position meets the material forming requirements, and then feeding back to the inflation forming system for adjustment.

5. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 4 is characterized in that: The induction heating system comprises an induction heating wire spirally arranged in the forming mold around the mold cavity of the forming mold, and an induction heater electrically connected to the induction heating wire; The induction heating wire is isolated from the forming mold by a high-temperature ceramic ring.

6. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 5 is characterized in that: The displacement measurement system includes a quartz rod with one side in contact with the outer side of the large convex part of the pipe, and the other side of the quartz rod in contact with a plurality of displacement sensors arranged along the axial direction of the pipe, and the plurality of displacement sensors are electrically connected to a multi-channel recorder.

7. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 6 is characterized in that: The small characteristic deformation areas of the forming mold are all provided with displacement measurement holes; The quartz rod and the displacement sensor are fitted in the clearance of the displacement measuring hole.

8. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 6 is characterized in that: The temperature measurement and control system includes a temperature sensor, a multi-channel recorder and a temperature control box; The temperature sensor is arranged on the outside of the pipe symmetrically with respect to the displacement sensor. The temperature sensor is respectively connected to the multi-channel recorder and the temperature control box, and the temperature control box is connected to the induction heater.

9. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 4 is characterized in that: The mechanical control system includes a press connected to the outside of the forming mold and punches coaxially arranged at both ends of the mold cavity of the forming mold, the punches are connected to a horizontal servo cylinder, and the press and the horizontal servo cylinder are both electrically connected to a mechanical control cabinet.

10. The device for controlling the hot air bulging forming of special-shaped light alloy pipes based on thermal deformation according to claim 4 is characterized in that: The inflatable forming system includes a compressed air source and an air source control cabinet. The compressed air source is connected to the interior of the forming mold via the air source control cabinet using a connecting air pipe. A pressure sensor and a one-way valve are provided in the connecting air pipe. The pressure sensor and the one-way valve are both electrically connected to the air source control cabinet.

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