A multi-step controlled-rate pre-stretching residual stress reduction method

Abstract

The application discloses a kind of multi-step controlled-rate pre-stretching residual stress reduction methods, belong to metal material processing technical field.Aiming at the technical problems of stress release imbalance, low deformation rate control precision in the pre-stretching process of wide high-strength aluminum alloy thick plate, three-in-one technical scheme of step-by-step progressive stretching, double-reference dynamic calibration and intermediate support roof is used: first load to yield critical point and remove elastic deformation displacement, complete the first stage of stretching at 0.3% to 0.8% deformation rate, release the clamp and stand still, use the real-time stress value collected as the new yield reference to calibrate the stroke, and then complete the second stage of stretching of the remaining deformation amount.The application significantly improves the deformation rate control precision and the uniformity of residual stress reduction, and is suitable for residual stress reduction treatment of wide aluminum alloy thick plate in the fields of aerospace, rail transportation, etc.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a multi-step speed-controlled pre-stretching residual stress reduction method. Background Technology

[0002] High-strength aluminum alloys (2-series and 7-series) possess characteristics such as high specific strength, good corrosion resistance, and excellent formability, making them a core material in high-end manufacturing fields such as large integral structural components for aerospace, rail transit vehicle bodies, and structural components for new energy equipment. To achieve high strength and toughness in service, aluminum alloy sheets need to undergo solution hardening to strengthen the microstructure. However, during the hardening process, the sheet undergoes rapid and intense cooling, resulting in a significant difference in cooling rates between the surface and the core. This leads to the formation of high-amplitude, unevenly distributed residual stresses within the sheet.

[0003] Residual stress from quenching is a key factor causing dimensional distortion and deviations in form and position accuracy during subsequent CNC machining of aluminum alloy sheets, and in severe cases, can lead to the scrapping of components. Therefore, residual stress reduction treatment must be performed on aluminum alloy sheets after quenching. Pre-stretching is the most widely used residual stress reduction process in the industrial production of thick aluminum alloy sheets due to its high stress reduction efficiency, stable effect, and ability to simultaneously improve sheet shape.

[0004] Currently, the mainstream technology in the industry is a single-pass large deformation rate pre-stretching process. Specifically, the quenched aluminum alloy sheet is fed into the clamps of a pre-stretching machine at both ends and fixed. It is then loaded at a constant stretching speed to a preset total pre-stretching deformation rate (the industry standard range is 2.0%–3.0%) in one pass. After reaching the set deformation, the clamps are held for a period of time to allow the sheet to undergo uniform plastic deformation and release internal residual stress. After the pressure holding period, the clamps are released and the sheet is removed, completing the stress relief treatment. Existing conventional pre-stretching machines are only equipped with clamps at both ends and lack an auxiliary support structure in the middle of the sheet. The equipment control system calculates the stretching stroke based on the nominal yield strength of the corresponding grade of aluminum alloy.

[0005] As high-end equipment develops towards larger and more integrated designs, the demand for wide-width aluminum alloy sheets continues to increase. Sheet specifications are expanding from conventional narrow-width medium-thick plates to wide, high-strength, and high-toughness aluminum alloy sheets exceeding 2000mm in width, and even reaching 3500mm. The traditional single-stage, high-deformation-rate pre-stretching process's inadequacy in adapting to large-width, thick plates is becoming increasingly apparent, specifically manifested in the following ways:

[0006] (1) The stress release is unbalanced in a single large deformation, resulting in poor plate shape control. When a large deformation rate is applied in a single loading, the stress release process of the quenched and distorted plate is violent and uneven. The stress is concentrated in the parts that originally had plate shape defects, which can easily lead to increased local warping and excessive transverse unevenness.

[0007] (2) Insufficient precision in deformation rate control, resulting in large process execution errors. The existing process only uses the nominal yield strength in the material handbook as the basis for stroke calculation, without eliminating the displacement during the elastic deformation stage of the plate, and without considering the fluctuation of the actual yield strength of the plate in different batches and under different quenching conditions, resulting in a large deviation between the actual plastic deformation rate and the process setting value.

[0008] (3) Uneven stress distribution on large-format boards can easily lead to quality defects. For wide, long, and thick boards, without intermediate support structures, the boards will sag and bend due to their own weight. During the stretching process, it is impossible to ensure uniform stress along the center line, resulting in poor consistency of plastic deformation across the entire width.

[0009] (4) The end clamping stress cannot be eliminated, and there are local anomalies in the stress distribution. The clamping state is maintained throughout the single stretching process. The local end stress generated by the clamping will remain inside the plate during the stretching process, resulting in abnormal residual stress distribution in the end area.

[0010] In response to the aforementioned shortcomings of the single pre-stretching process, various improvement schemes have been explored in this field. For example, one proposed a method combining step-by-step stretching with a movable plate support frame to improve plate shape and stress uniformity. However, this method still has the following significant shortcomings after practical verification: First, although step-by-step stretching is used, the clamping state is maintained throughout the process. The process of completely releasing the clamps and leaving the plate stationary between the two stretching stages is not disclosed. The local additional stress at the end generated by the clamp engagement will be "frozen" inside the plate during the second stretching stage, resulting in abnormal residual stress distribution in the end area. Second, the dual-benchmark dynamic calibration mechanism of resetting the yield benchmark with the real-time stress value after the first stretching stage is not disclosed. The nominal yield strength of the material is still used as a single benchmark to calculate the stroke, which cannot eliminate the measurement error caused by material batch fluctuations and secondary clamping position offsets. The actual deformation rate deviation is still -0.3% to -0.5%. Third, the support frame is only a generalized lifting structure. The number of support sets, the support spacing, and the parallelism between the top plate support surface and the stretching benchmark surface are not specified. It also does not integrate the triple functions of horizontal support, lateral centering correction, and longitudinal guiding positioning, making it difficult to ensure uniform stress along the entire length of the ultra-wide thick plate and resulting in poor consistency of plastic deformation across the entire width. Another approach proposes adding heat treatment or auxiliary stress-relieving processes after a single pre-stretching to further reduce stress. However, this approach alters the material microstructure or adds extra heat treatment steps, posing risks of lengthy processes, reduced production efficiency, and fluctuations in material properties. Furthermore, it fails to address the issues of low deformation rate control accuracy and residual end stress. None of the aforementioned improvements simultaneously resolve the four major technical challenges in the pre-stretching process of wide, thick plates: unbalanced stress release, insufficient deformation rate control accuracy, uneven stress distribution, and residual end stress. Further improvements are still needed. Summary of the Invention

[0011] In view of the above, it is necessary to provide a multi-step speed-controlled pre-stretching residual stress reduction method. Through process innovation and control mechanism optimization, this method can systematically solve the problems of low precision, poor uniformity, and difficulty in plate shape control during the pre-stretching stress relief process of wide-width high-strength aluminum alloy thick plates.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] A multi-step speed-controlled pre-stretching residual stress reduction method is disclosed. The method is applied to quenched 2-series and 7-series high-strength aluminum alloy wide plates with a thickness of 30-120 mm, a width of 2000-3600 mm, and a length of 15000-25000 mm. Three sets of intermediate support top plates are installed within the stroke range of the pre-stretching machine. The support spacing of the intermediate support top plates is 3000-6000 mm, and the parallelism between the top plate support surface and the stretching reference surface is ≤0.1 mm / m. The intermediate support top plates provide horizontal support, lateral centering correction, and longitudinal guiding positioning along the entire length of the plate. The total pre-stretching deformation rate is 2.0%-3.0%.

[0014] The overall pre-stretching process includes the following steps in sequence.

[0015] (1) Loading at the yield critical point: Clamp both ends of the solution-quenched aluminum alloy plate, enter the nominal yield strength parameter of the corresponding alloy quenched state into the pre-stretching machine control system, and load to the yield critical point of the plate. The displacement of the plate during the elastic deformation stage is not included in the effective stroke of the pre-stretching.

[0016] (2) First stage pre-stretching and constraint release: The first stage pre-stretching is completed with a deformation rate of 0.3% to 0.8%. After the pressure holding is completed, the current real-time stress value is collected and recorded. Then the clamps are completely released, the end constraints of the plate are released and left to stand.

[0017] (3) Secondary clamping and system parameter reset: re-clamp the plate, and enter the real-time stress value collected in step (2) as the new yield strength into the pre-stretching machine system to calibrate the stroke calculation benchmark.

[0018] (4) Second stage pre-stretching: The remaining deformation after deducting the first stage deformation rate from the total pre-stretching deformation rate is used to complete the second stage pre-stretching.

[0019] In this invention, the initial transverse unevenness of the plate after quenching is 0-15 mm / m.

[0020] In this invention, the pre-stretching speed in steps (2) and (4) is 0.8 to 3.0 mm / s, and the stretching speeds in the two stages are the same or independently differentiated.

[0021] In this invention, further, in step (2), after the first stage of pre-stretching is in place, the engagement pressure is maintained for 15 to 60 seconds; in step (4), after the second stage of pre-stretching is in place, the engagement pressure is maintained for 20 to 90 seconds.

[0022] In this invention, the resting time after the clamps are completely released in step (2) is 10 to 90 seconds, which is used to dissipate the local additional stress generated at the end of the plate during the first stage of stretching.

[0023] Furthermore, in this invention, the deformation rate control accuracy deviation of the pre-stretching equipment is ≤ ±0.2%, and it has the function of real-time synchronous monitoring of tensile deformation and instantaneous tensile stress.

[0024] Furthermore, in this invention, the top surface of the intermediate support top plate is provided with a longitudinal guide groove and a transverse centering stop, which can simultaneously achieve horizontal support, transverse centering correction and longitudinal guiding positioning.

[0025] In this invention, after step (1) to eliminate elastic deformation displacement and step (3) to calibrate the yield benchmark with real-time stress value for the second time, the pre-stretching machine performs the second stage stretching with the calibrated stroke benchmark, so that the deviation between the actual stretching rate and the process set stretching rate is ≤ ±0.2%.

[0026] Compared with existing technologies, this invention addresses the four major technical challenges of pre-stretching wide and thick plates. Through the synergistic effect of core distinguishing features such as step-by-step progressive stretching, dual-benchmark dynamic calibration, and intermediate support top plate, it has at least the following beneficial effects.

[0027] 1. Addressing the technical problem of "uneven stress release and poor plate shape control during single large deformation," this invention employs a step-by-step progressive stretching approach: "small deformation pre-correction + intermediate constraint release + large deformation depth stress relief." The first stage uses a small deformation rate of 0.3% to 0.8% to induce gentle plastic flow on the surface and in areas of concentrated distortion, initially correcting quenching shape deviations. The intermediate stage completely releases the clamps and allows the plate to stand still, eliminating additional end-clamping stress to prevent it from being "frozen" in the second stage. The second stage uses the remaining deformation to achieve large deformation depth stress relief, fully releasing residual stress in the core while ensuring uniform stress distribution. This three-step synergy avoids stress wave reflection and localized stress concentration caused by single loading, achieving a smooth and orderly release of residual stress. Verified on two typical alloy plates, 7050 and 2024, the transverse unevenness after treatment can be controlled within 1.5 mm / m, representing reductions of 67% and 70% respectively compared to the control sample, meeting the stringent shape requirements for aerospace applications.

[0028] 2. To address the technical problems of "insufficient precision in deformation rate control and large process execution errors," this invention establishes a "dual-benchmark segmented calibration" control logic with distinct characteristics. The first calibration begins measuring the effective stroke after loading to the yield critical point, completely excluding displacement in the elastic deformation segment. The second calibration uses the real-time stress value collected after the first stage of stretching as the new yield benchmark for the second stage. This value accurately reflects the actual work-hardening state and mechanical properties of the plate after the first stage of plastic deformation, thereby eliminating measurement deviations caused by material batch fluctuations and secondary clamping position offsets. Experimental verification shows that after adopting this invention, the deviation between the actual deformation rate and the set value can be stably controlled within ±0.2% from -0.3% to -0.4% in the comparative example, fundamentally solving the core problem of measurement error in the pre-stretching deformation rate of wide, thick plates.

[0029] 3. Addressing the technical problem of "unremovable end clamping stress and localized anomalies in stress distribution," this invention features a process distinction between the two-stage stretching: the clamps are completely released and the plate is left to stand for 10-90 seconds after the first-stage stretching and pressure holding is completed. This allows the localized additional stress generated at the plate ends by the clamping mechanism to dissipate naturally through stress relaxation, preventing it from being retained inside the plate during the second-stage stretching process. Combined with step-by-step progressive stretching to ensure stress release proceeds systematically from the surface to the core, and the intermediate support plate guarantees synchronous and uniform stretching across the entire width, this triple mechanism results in a symmetrical distribution of near-surface compressive stress and central tensile stress in the thickness direction of the treated plate. The abnormal tensile stress state commonly seen in the comparative example no longer appears near the surface, and the maximum tensile stress reduction of residual stress exceeds 70%, effectively reducing the risk of dimensional deformation caused by stress imbalance during subsequent CNC machining.

[0030] 4. Addressing the technical problem of "uneven stress distribution and susceptibility to quality defects in large-format plates," this invention features a distinctive three-set intermediate support top plate system. This system achieves uniform support along the entire length to counteract the sagging effect of the wide, thick plate's own weight. The parallelism between the top plate support surface and the tensile reference surface is ≤0.1mm / m. Combined with lateral centering correction and longitudinal guiding positioning, this hardware-level hardware ensures consistent plastic deformation across the entire width of 2000–3600mm wide plates, effectively avoiding quality defects such as localized stress concentration, edge cracking, and plate distortion. This invention is stably adaptable to high-strength aluminum alloy wide plates with thicknesses of 30–120mm, widths of 2000–3600mm, and lengths of 15000–25000mm. It is compatible with workpieces exhibiting an initial lateral unevenness of 0–15mm / m across the entire distortion range. It can be implemented by modifying existing pre-stretching equipment and has promising prospects for industrial application. Attached Figure Description

[0031] Figure 1The diagram shows the residual stress distribution along the thickness direction of a 45mm thick 7050 aluminum alloy sheet in Example 1, illustrating the characteristic that the residual stress is symmetrically distributed and near-surface is compressive stress after treatment using the method of the present invention.

[0032] Figure 2 The residual stress distribution along the thickness direction of a 45mm thick 7050 aluminum alloy sheet in Comparative Example 1 shows the characteristics of asymmetrical residual stress distribution and abnormal tensile stress near the surface after traditional single pre-stretching process.

[0033] Figure 3 The diagram shows the residual stress distribution along the thickness direction of a 60mm thick 2024 aluminum alloy sheet in Example 2, illustrating the characteristic that the residual stress is symmetrically distributed and near-surface is compressive stress after treatment using the method of the present invention.

[0034] Figure 4 The residual stress distribution along the thickness direction of a 60mm thick 2024 aluminum alloy sheet in Comparative Example 2 shows the characteristics of asymmetrical residual stress distribution and abnormal tensile stress near the surface after traditional single pre-stretching process. Detailed Implementation

[0035] The core mechanism of this invention lies in the synergistic effect of four technical features: "step-by-step progressive stretching + dual-benchmark dynamic calibration + intermediate release and static setting + intermediate support top plate." This systematically addresses the technical challenges of pre-stretching wide and thick plates from four dimensions: stress release path, deformation rate measurement accuracy, end stress elimination, and uniform stress distribution across the entire width. Specifically, step-by-step progressive stretching ensures that stress release proceeds systematically from the surface to the core, avoiding stress wave reflection caused by a single large deformation. Dual-benchmark dynamic calibration eliminates measurement errors at the source by eliminating elastic deformation displacement and resetting the yield benchmark with real-time stress values. Intermediate release and static setting allow the additional stress from the end clamping to dissipate naturally before the second stage of stretching, preventing it from being "frozen" inside the plate. The intermediate support top plate ensures consistent plastic deformation across the entire width of the wide and thick plate through uniform support and centering guidance.

[0036] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0037] Example 1

[0038] This embodiment proposes a multi-step speed-controlled pre-stretching residual stress reduction method for processing 7050 high-strength aluminum alloy wide and thick plates, specifically including the following steps.

[0039] 1. Test workpiece and supporting equipment

[0040] 1.1 Test workpiece

[0041] This embodiment deals with a quenched 7050 high-strength aluminum alloy wide and thick plate, with dimensions of 45mm thickness, 3500mm width, and 20000mm length. After quenching, the plate exhibits significant shape distortion, with an initial maximum transverse unevenness of 12mm / m. The plate surface is free of processing defects such as cracks and deep scratches, meeting the pre-stretching processing requirements.

[0042] 1.2 Supporting Equipment

[0043] A wide-width aluminum alloy pre-stretching machine is used, with a deformation rate control accuracy deviation of ≤±0.2%. It can monitor tensile deformation and instantaneous tensile stress in real time. Three sets of intermediate support top plates are installed within the pre-stretching machine's stroke range, with a support spacing of 5000mm. The parallelism between all top plate support surfaces and the stretching reference surface is ≤0.1mm / m. This provides horizontal support, lateral centering correction, and longitudinal guiding positioning along the entire length of the sheet material, preventing sagging, lateral bending, and wavy deformation due to the sheet material's own weight.

[0044] 2. Step-by-step pre-stretching process

[0045] In this embodiment, the total pre-stretch deformation rate is set to 2.2%, and the overall process is carried out according to the following steps:

[0046] Step 1: Yield Critical Point Loading. The quenched 7050 aluminum alloy sheet is hoisted onto the pre-stretching machine's worktable. The clamps at both ends evenly engage and tighten the sheet ends, ensuring proper alignment and balanced clamping force. The nominal yield strength parameter of the quenched 7050 aluminum alloy is entered into the pre-stretching machine's control system. The machine is started to apply tensile force at a low speed. The point of abrupt change in the slope of the stress-strain curve is used to determine if the sheet has reached the overall yield critical point (i.e., when the tensile stress reaches 95% of the nominal yield strength, strain acceleration is monitored, and the point of sudden drop in strain acceleration is the yield critical point). During loading, the system automatically discards displacement during the elastic deformation stage, excluding it from the effective pre-stretching stroke, ensuring that all subsequently measured deformations are effective plastic deformations.

[0047] Step Two: First-Stage Pre-Stretching and Constraint Release. After completing the yield critical point positioning, set the stretching speed to a uniform 1.5 mm / s and execute the first-stage pre-stretching, controlling the deformation rate to reach 0.5%. After stretching to the target deformation rate, maintain the clamping state and hold the pressure for 30 seconds to allow the concentrated residual stress on the surface of the plate to fully relax and release. After the pressure holding is completed and before unloading, the system uses a force sensor to collect and record the instantaneous tensile stress value of the plate (this value reflects the actual work hardening state of the material after the first-stage plastic deformation). Then, completely release the clamps at both ends, release the clamping constraint at the end of the plate, and let it stand at room temperature without external force for 20 seconds to allow the local additional stress generated by the clamping mechanism at the end of the plate during the first-stage stretching process to dissipate naturally through stress relaxation.

[0048] Step 3: Secondary Clamping and System Parameter Reset. After the settling period, the clamps at both ends of the pre-stretching machine re-clamp the ends of the sheet metal evenly to ensure consistent alignment during the secondary clamping. The real-time stress value collected in Step 2 is entered into the pre-stretching machine control system as the new yield strength for the second stage of stretching (after the first stage of stretching, the material undergoes work hardening, and the actual yield strength is higher than the initial nominal value; using the real-time stress value as a benchmark allows for more accurate calculation of the elastic deformation displacement in the second stage). This calibrates the stroke calculation benchmark for the second stage of stretching, eliminating material property fluctuations and measurement offsets caused by secondary clamping, and ensuring the accuracy of deformation rate execution.

[0049] Step 4: Second-stage pre-stretching. After parameter calibration, maintain a uniform stretching speed of 1.5 mm / s and perform the second-stage pre-stretching to complete the remaining 1.7% deformation, thus supplementing the total plastic deformation. After stretching to the target remaining deformation, maintain the clamped state and hold the pressure for 30 seconds to fully release the residual stress in the core of the sheet and achieve uniform stress across the entire surface. After holding the pressure, slowly unload the stretching force, completely release the clamps, and remove the sheet to complete the residual stress reduction process.

[0050] 3. Effects of the pre-stretching process

[0051] 3.1 Plate Shape Detection

[0052] After processing, the maximum lateral unevenness of the sheet metal was measured according to the aluminum alloy sheet metal shape inspection standard. The test results showed that the maximum lateral unevenness of the sheet metal was ≤1.5mm / m, which meets the technical requirement of ≤2.0mm / m for aerospace sheet metal. There were no new defects such as warping, indentations, or scratches on the sheet surface. The overall sheet shape was flat and stable, and the sheet shape correction effect was significant.

[0053] 3.2 Residual stress detection

[0054] According to the layer-by-layer peeling method in YS / T 1634-2023 "Evaluation Method for Residual Stress of Aluminum and Aluminum Alloy Products", residual stress was detected by taking L-axis samples along the thickness direction of the sheet. The sheet was divided into 20 layers along the thickness direction and peeled off layer by layer. The thickness of each layer was approximately 2.25 mm. After peeling, strain gauges were used to measure the released strain of each layer, and the residual stress value of each layer was calculated according to the formula of elasticity.

[0055] Test results show that the near-surface layer of the board exhibits compressive stress, while the central layer exhibits tensile stress. The residual stress distribution on the upper and lower surfaces is basically symmetrical. The maximum compressive stress value is -13.7 MPa, and the maximum tensile stress value is +8.4 MPa. The overall residual stress amplitude is low and the distribution is uniform (see [reference]). Figure 1 It possesses excellent low residual stress characteristics, which can effectively reduce the risk of dimensional deformation in subsequent CNC machining.

[0056] Example 2

[0057] This embodiment proposes a multi-step speed-controlled pre-stretching residual stress reduction method for processing 2024 high-strength aluminum alloy wide and thick plates, specifically including the following steps.

[0058] 1. Test workpiece and supporting equipment

[0059] 1.1 Test workpiece

[0060] This embodiment deals with a quenched 2024 high-strength aluminum alloy wide and thick plate with dimensions of 60mm thickness, 2200mm width, and 17000mm length. After quenching, the plate exhibits significant shape distortion, with an initial maximum transverse unevenness of 11mm / m. The plate surface is free of processing defects such as cracks and deep scratches, meeting the pre-stretching processing requirements.

[0061] 1.2 Supporting Equipment

[0062] A wide-width aluminum alloy pre-stretching machine is used, with a deformation rate control accuracy deviation of ≤±0.2%. It can monitor tensile deformation and instantaneous tensile stress in real time. Three sets of intermediate support top plates are installed within the pre-stretching machine's stroke range, with a support spacing of 4250mm. The parallelism between all top plate support surfaces and the stretching reference surface is ≤0.1mm / m. This provides horizontal support, lateral centering correction, and longitudinal guiding positioning along the entire length of the sheet metal, preventing sagging, lateral bending, and wavy deformation due to the sheet metal's own weight.

[0063] 2. Step-by-step pre-stretching process

[0064] In this embodiment, the total pre-stretch deformation rate is set to 2.5%, and the overall process is carried out according to the following steps.

[0065] Step 1: Yield Critical Point Loading. The quenched 2024 aluminum alloy sheet is hoisted onto the pre-stretching machine's worktable. The clamps at both ends evenly engage and tighten the sheet ends, ensuring proper alignment and balanced clamping force. The nominal yield strength parameter of the quenched 2024 aluminum alloy is entered into the pre-stretching machine's control system. The machine is started and a low-speed tensile force is applied. The point of abrupt change in the slope of the stress-strain curve is used to determine if the sheet has reached the overall yield critical point. During loading, the system automatically discards displacement during the elastic deformation stage, excluding it from the effective pre-stretching stroke, ensuring that all subsequent deformation measurements are effective plastic deformation.

[0066] Step Two: First-Stage Pre-Stretching and Constraint Release. After completing the yield critical point positioning, set the stretching speed to a uniform 1.2 mm / s and execute the first-stage pre-stretching, controlling the deformation rate to reach 0.6%. After stretching to the target deformation rate, maintain the clamping state and hold the pressure for 40 seconds to allow the concentrated residual stress on the surface of the plate to fully relax and release. After the pressure holding is completed and before unloading, the system uses force sensors to collect and record the instantaneous tensile stress value of the plate. Then, completely release the clamps at both ends, release the clamping constraint at the end of the plate, and let it stand at room temperature without external force for 30 seconds to allow the local additional stress generated by the clamping mechanism at the end of the plate during the first-stage stretching process to dissipate naturally through stress relaxation.

[0067] Step 3: Secondary Clamping and System Parameter Reset. After the settling period, the clamps at both ends of the pre-stretching machine re-clamp the ends of the sheet metal evenly to ensure consistent alignment during the secondary clamping. The real-time stress value collected in Step 2 is entered into the pre-stretching machine control system as the new yield strength for the second stage of stretching (after the first stage of stretching, the material undergoes work hardening, and the actual yield strength is higher than the initial nominal value; using the real-time stress value as a benchmark allows for more accurate calculation of the elastic deformation displacement in the second stage). This calibrates the stroke calculation benchmark for the second stage of stretching, eliminating material property fluctuations and measurement offsets caused by secondary clamping, and ensuring the accuracy of deformation rate execution.

[0068] Step 4: Second-stage pre-stretching. After parameter calibration, maintain a uniform stretching speed of 1.2 mm / s and perform the second-stage pre-stretching to complete the remaining 1.9% of the deformation, thus supplementing the total plastic deformation. After stretching to the target remaining deformation, maintain the clamped state and hold the pressure for 60 seconds to fully release the residual stress in the core of the sheet and achieve uniform stress across the entire surface. After holding the pressure, slowly unload the tensile force, completely release the clamps, and remove the sheet to complete the residual stress reduction process.

[0069] 3. Effects of the pre-stretching process

[0070] 3.1 Plate Shape Detection

[0071] After the pre-stretching process to reduce residual stress is completed, the maximum transverse unevenness of the sheet is measured according to the aluminum alloy sheet shape inspection standard. The test results show that the maximum transverse unevenness of the sheet is ≤1.5mm / m, which meets the company's internal quality control index of ≤3.0mm / m. There are no new defects such as warping, indentations, or scratches on the sheet surface. The overall sheet shape is flat and stable, and the sheet shape correction effect is significant.

[0072] 3.2 Residual stress detection

[0073] According to the layer-by-layer peeling method in YS / T 1634-2023 "Evaluation Method for Residual Stress of Aluminum and Aluminum Alloy Products", residual stress was detected by taking L-axis samples along the thickness direction of the sheet. The sheet was divided into 20 layers along the thickness direction and peeled off layer by layer. The thickness of each layer was approximately 3.0 mm. After peeling, strain gauges were used to measure the released strain of each layer, and the residual stress value of each layer was calculated according to the formula of elasticity.

[0074] Test results show that the near-surface layer of the board exhibits compressive stress, while the central layer exhibits tensile stress. The residual stress distribution on the upper and lower surfaces is basically symmetrical. The maximum compressive stress value is -11.5 MPa, and the maximum tensile stress value is +6.6 MPa (see [reference]). Figure 3 The overall residual stress amplitude is low and the distribution is uniform, exhibiting excellent low residual stress characteristics, which can effectively reduce the risk of dimensional deformation in subsequent CNC machining.

[0075] Comparative Example 1

[0076] This comparative example uses a conventional single pre-stretching process to treat a wide, thick 7050 high-strength aluminum alloy plate, serving as a control for Example 1.

[0077] 1. Test workpiece and supporting equipment

[0078] 1.1 Test workpiece

[0079] The comparative example selected the same test workpiece as in Example 1, which was a quenched 7050 high-strength aluminum alloy wide and thick plate with a thickness of 45 mm, a width of 3500 mm, and a length of 20000 mm. The initial maximum transverse unevenness of the plate after quenching was 12 mm / m, and the plate surface had no processing defects such as cracks or deep scratches, which was completely consistent with the initial state of Example 1.

[0080] 1.2 Supporting Equipment

[0081] The same wide aluminum alloy pre-stretching machine as in Example 1 was used. The equipment deformation rate control accuracy deviation was ≤ ±0.2%, and the tensile deformation and instantaneous tensile stress could be monitored in real time. This comparative example was operated in the traditional conventional process mode, without using the intermediate support top plate device of the equipment. The plate was positioned by clamping at both ends.

[0082] 2. Pre-stretching process

[0083] The total pre-stretch deformation rate in this comparative example was set at 2.2%, which is completely consistent with the total deformation set in Example 1. To ensure fairness in the comparison, the total holding time in this comparative example was kept consistent with that in Example 1 to eliminate the interference of holding time differences on the test results.

[0084] This comparative example adopts the existing conventional single pre-stretching process, which lacks three core technical features of the present invention: (1) There is no staged pre-stretching and intermediate clamp release process, and a stretching mode of loading to the target deformation rate is adopted in one go; (2) There is no staged yield stress dynamic calibration mechanism, and the stretching stroke is calculated only based on the nominal yield strength of the material, without secondary parameter reset; (3) The intermediate support top plate auxiliary device is not used, and there is no full-length uniform support and lateral centering correction.

[0085] The specific process is as follows: The quenched 7050 aluminum alloy sheet is hoisted onto the pre-stretching machine's worktable. The clamps at both ends evenly engage and tighten the sheet ends. The nominal yield strength parameter of the quenched 7050 aluminum alloy is entered into the pre-stretching machine's control system. The machine is then started and stretched at a uniform speed of 1.5 mm / s in a single pass. Although the equipment's deformation rate monitoring accuracy can reach ±0.2%, because only the fixed nominal yield strength is used as the basis for stroke calculation, the interference of elastic deformation displacement is not eliminated, and there is no segmented calibration mechanism. This systematic deviation at the control logic level results in an actual deformation rate of 1.9% after processing, failing to reach the process-set target value of 2.2%, corresponding to a deviation of -0.3%, exceeding the internal quality control tolerance requirement of ±0.2% between the actual and target deformation rates.

[0086] After stretching to the desired position, maintain the clamped state and hold the pressure for 60 seconds to allow the sheet to undergo plastic deformation and release residual stress. After holding the pressure, slowly unload the tensile force, completely release the clamps, and remove the sheet to complete the residual stress reduction process.

[0087] 3. Effects of the pre-stretching process

[0088] 3.1 Plate Shape Detection

[0089] After the pre-stretching process to reduce residual stress is completed, the maximum transverse unevenness of the board is measured according to the same testing standards as in Example 1. The test results show that the maximum transverse unevenness of the board is 4.5 mm / m, which does not meet the technical requirement of ≤2.0 mm / m. Furthermore, there is a significant increase in warping in some areas of the board, and the board shape correction effect is poor, failing to meet the pre-processing board shape requirements for high-precision processing.

[0090] 3.2 Residual stress detection

[0091] According to the layer-by-layer peeling method in YS / T 1634-2023 "Evaluation Method for Residual Stress of Aluminum and Aluminum Alloy Products", residual stress was detected by taking L-direction samples along the thickness direction of the sheet, and the detection method was completely consistent with that in Example 1. The sheet was divided into 20 layers along the thickness direction for layer-by-layer peeling characterization.

[0092] The test results show that the surface layer of the board exhibits compressive stress, but an abnormal tensile stress state occurs near the surface layer, and the residual stress on the upper and lower surfaces of the board is asymmetrically distributed; the maximum compressive stress value is -34.0 MPa, and the maximum tensile stress value is +63.4 MPa (see [reference]). Figure 2 The results show that the residual stress amplitude of the plate after the comparative treatment is high and the distribution is uneven. The residual stress reduction effect does not meet the technical requirements and cannot effectively control the risk of dimensional deformation in subsequent CNC machining.

[0093] Comparative Example 2

[0094] This comparative example uses a conventional single pre-stretching process to treat a wide and thick 2024 high-strength aluminum alloy plate, serving as a control for Example 2.

[0095] 1. Test workpiece and supporting equipment

[0096] 1.1 Test workpiece

[0097] The comparative example selected the same test workpiece as in Example 2, which was a quenched 2024 high-strength aluminum alloy wide and thick plate with a thickness of 60 mm, a width of 2200 mm, and a length of 17000 mm. The initial maximum transverse unevenness of the plate after quenching was 11 mm / m, and the plate surface had no processing defects such as cracks or deep scratches, which was completely consistent with the initial state of Example 2.

[0098] 1.2 Supporting Equipment

[0099] The same wide aluminum alloy pre-stretching machine as in Example 2 was used. The equipment deformation rate control accuracy deviation was ≤ ±0.2%, and the tensile deformation and instantaneous tensile stress could be monitored in real time. This comparative example was operated in the traditional conventional process mode, without using the intermediate support top plate device of the equipment. The plate was positioned by clamping at both ends.

[0100] 2. Pre-stretching process

[0101] The total pre-stretch deformation rate in this comparative example was set at 2.5%, which is completely consistent with the total deformation set in Example 2. To ensure fairness in the comparison, the total holding time in this comparative example was kept consistent with that in Example 2 to eliminate the interference of holding time differences on the test results.

[0102] This comparative example adopts the existing conventional single pre-stretching process, which lacks three core technical features of the present invention: (1) There is no staged pre-stretching and intermediate clamp release process, and a stretching mode of loading to the target deformation rate is adopted in one go; (2) There is no staged yield stress dynamic calibration mechanism, and the stretching stroke is calculated only based on the nominal yield strength of the material, without secondary parameter reset; (3) The intermediate support top plate auxiliary device is not used, and there is no full-length uniform support and lateral centering correction.

[0103] The specific process is as follows: The quenched 2024 aluminum alloy sheet is hoisted onto the pre-stretching machine's worktable. The clamps at both ends evenly engage and tighten the sheet ends. The nominal yield strength parameter of the quenched 2024 aluminum alloy is entered into the pre-stretching machine's control system. The machine is then started and stretched at a uniform speed of 1.2 mm / s in a single pass. Although the equipment's deformation rate monitoring accuracy can reach ±0.2%, because only the fixed nominal yield strength is used as the stroke calculation benchmark, the interference of elastic deformation displacement is not eliminated, and there is no segmented calibration mechanism. Systematic deviations at the control logic level result in the actual pre-stretching deformation rate reaching only 2.1% after processing and verification. This falls short of the process-set target value of 2.5%, and the deformation rate deviation exceeds the tolerance control requirement of ≤±0.2% for the actual pre-stretching deformation rate.

[0104] After stretching to the desired position, maintain the clamped state and hold the pressure for 100 seconds to allow the sheet to undergo plastic deformation and release residual stress. After holding the pressure, slowly unload the tensile force, completely release the clamps, and remove the sheet to complete the residual stress reduction process.

[0105] 3. Effects of the pre-stretching process

[0106] 3.1 Plate Shape Detection

[0107] After the pre-stretching process to reduce residual stress is completed, the maximum transverse unevenness of the board is measured according to the same testing standards as in Example 2. The test results show that the maximum transverse unevenness of the board is 5.0 mm / m, which does not meet the company's internal quality control index of ≤3.0 mm / m. Furthermore, there is obvious warping in some areas of the board, and the board shape correction effect is poor, failing to meet the pre-processing board shape requirements for high-precision processing.

[0108] 3.2 Residual stress detection

[0109] According to the layer-by-layer peeling method in YS / T 1634-2023 "Evaluation Method for Residual Stress of Aluminum and Aluminum Alloy Products", residual stress was detected by taking L-axis samples along the thickness direction of the sheet, and the detection method was completely consistent with that in Example 2. The sheet was divided into 20 layers along the thickness direction for layer-by-layer peeling characterization.

[0110] The test results show that the surface layer of the board exhibits compressive stress, but an abnormal tensile stress state occurs near the surface layer, and the residual stress on the upper and lower surfaces of the board is asymmetrically distributed; the maximum compressive stress value is -46.1 MPa, and the maximum tensile stress value is +27.3 MPa (see [reference]). Figure 4 The results show that the residual stress amplitude of the plate after the comparative treatment is relatively high, the residual stress reduction effect does not meet the technical requirements, and the risk of dimensional deformation in subsequent CNC machining cannot be effectively controlled.

[0111] Summary of implementation results:

[0112] The detection results of Examples 1-2 and Comparative Examples 1-2 are summarized and compared, as shown in Table 1.

[0113] Table 1 Comparison of the implementation effects of the examples and comparative examples

[0114]

[0115] As shown in Table 1, Examples 1 and 2, employing the technical solution of this invention, exhibit significantly better plate flatness, residual stress amplitude and distribution symmetry, and deformation rate control accuracy than Comparative Examples 1 and 2, which utilize the traditional single-stage pre-stretching process. Specifically, the transverse unevenness of the 7050 plate is reduced by 67% after treatment compared to existing processes, and by 70% for the 2024 plate; the maximum tensile stress reduction exceeds 70%, decreasing from +63.4 MPa to +8.4 MPa for the 7050 plate and from +27.3 MPa to +6.6 MPa for the 2024 plate; furthermore, the near-surface abnormal tensile stress state is eliminated, and the stress distribution on the upper and lower surfaces is essentially symmetrical; the deformation rate deviation narrows from -0.3% to -0.4% to within ±0.2%. These data fully demonstrate the effectiveness and significant advancement of the three-in-one technical solution of this invention: "step-by-step progressive stretching + dual-benchmark dynamic calibration + intermediate support top plate."

[0116] The multi-step speed-controlled pre-stretching residual stress reduction method provided by this invention can stably adapt to high-strength aluminum alloy wide sheets with thicknesses of 30–120 mm, widths of 2000–3600 mm, and lengths of 15000–25000 mm, and is compatible with workpieces with initial transverse unevenness ranging from 0–15 mm / m across the entire distortion range. The entire process can be implemented by modifying existing pre-stretching equipment, with adjustable parameters, stable production efficiency, and the ability to reliably mass-produce high-end aluminum alloy sheets that meet standards, demonstrating broad industrial application prospects.

[0117] The above embodiments only illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention.

Claims

1. A multi-step speed-controlled pre-stretching residual stress reduction method, characterized in that, The method is applied to quenched 2-series and 7-series high-strength aluminum alloy wide plates with a thickness of 30-120 mm, a width of 2000-3600 mm, and a length of 15000-25000 mm. Three sets of intermediate support top plates are set within the stroke range of the pre-stretching machine. The support spacing of the intermediate support top plates is 3000-6000mm. The parallelism between the support surface of the top plate and the stretching reference surface is ≤0.1mm / m. The intermediate support top plates provide horizontal support, lateral centering correction and longitudinal guiding positioning along the entire length of the plate. The total pre-stretch deformation rate is 2.0%–3.0%; The overall pre-stretching process includes the following steps in sequence: (1) Loading at the yield critical point: Clamp both ends of the solution-quenched aluminum alloy plate, enter the nominal yield strength parameter of the corresponding alloy quenched state into the pre-stretching machine control system, and load to the yield critical point of the plate. The displacement of the plate during the elastic deformation stage is not included in the effective stroke of the pre-stretching. (2) First stage pre-stretching and constraint release: The first stage pre-stretching is completed with a deformation rate of 0.3% to 0.8%. After the pressure holding is completed, the current real-time stress value is collected and recorded. Then, the clamps are completely released, the end constraints of the plate are released and the plate is left to stand. (3) Secondary clamping and system parameter reset: re-clamp the plate, and enter the real-time stress value collected in step (2) as the new yield strength into the pre-tensioning machine system to calibrate the stroke calculation benchmark; (4) Second stage pre-stretching: The remaining deformation after deducting the first stage deformation rate from the total pre-stretching deformation rate is used to complete the second stage pre-stretching.

2. The method according to claim 1, characterized in that, The initial transverse unevenness of the plate after quenching is 0-15 mm / m.

3. The method according to claim 1, characterized in that, The pre-stretching speed in steps (2) and (4) is 0.8 to 3.0 mm / s, and the stretching speeds in the two stages are the same or independently differentiated.

4. The method according to claim 1, characterized in that, In step (2), after the first stage of pre-stretching is in place, the engagement pressure is maintained for 15-60 seconds; in step (4), after the second stage of pre-stretching is in place, the engagement pressure is maintained for 20-90 seconds.

5. The method according to claim 1, characterized in that, The resting time after the clamps are completely released in step (2) is 10 to 90 seconds, which is used to dissipate the local additional stress generated at the end of the plate during the first stage of stretching.

6. The method according to any one of claims 1 to 5, characterized in that, The deformation rate control accuracy of the pre-stretching equipment is ≤ ±0.2%, and it has the function of real-time synchronous monitoring of tensile deformation and instantaneous tensile stress.

7. The method according to claim 1, characterized in that, The top surface of the intermediate support plate is provided with a longitudinal guide groove and a transverse centering stop, which can simultaneously achieve horizontal support, transverse centering correction and longitudinal guiding positioning.

8. The method according to claim 1, characterized in that, After eliminating elastic deformation displacement in step (1) and calibrating the yield benchmark a second time with real-time stress value in step (3), the pre-stretching machine performs the second stage of stretching with the calibrated stroke benchmark, so that the deviation between the actual stretching rate and the process-set stretching rate is ≤ ±0.2%.

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