Super-thick plate stress relief stretching method based on local weakening design

Abstract

The invention discloses a local weakening design-based super-thick plate stress relief stretching method, which relates to the technical field of metal material processing, and comprises the following steps: S1, sequentially carrying out smelting, casting, homogenization treatment and hot rolling on an aluminum alloy cast ingot to obtain a super-thick plate blank, and then carrying out stress relief annealing; s2, an area to be stretched is arranged on the obtained plate blank, part of materials are removed in the area through machining, the cross section area of the area is smaller than the area of other parts, and a local weakening area is formed; s3, the blank obtained in the S2 is heated and subjected to heat preservation at the solid solution temperature, and then quenching is conducted; s4, the two ends of the quenched workpiece are clamped to a stretching machine to be stretched; and S5, after stretching is completed, subsequent aging heat treatment is conducted on the workpiece. And effective stress relief stretching of the super-thick plate can be realized by utilizing an existing medium-tonnage stretching machine.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, and in particular to a stress relief tensile method for ultra-thick plates based on local weakening design. Background Technology

[0002] In aerospace, high-end equipment manufacturing, and other fields, ultra-thick high-strength aluminum alloy plates such as 7050 and 7085 with a thickness of ≥150mm are widely used. When these plates are quenched after solution treatment, the uneven cooling rate of the cross-section will generate huge internal residual stress, which seriously affects the dimensional stability, mechanical properties, and stress corrosion resistance of the workpiece. Therefore, it is necessary to eliminate the residual stress by a tensile process with 1%-3% permanent plastic deformation after solution treatment and before aging. Existing stress-relieving stretching technology uses a whole-body stretching method, but it faces three major challenges: First, the tonnage requirement is extremely high. Taking a 178mm thick and 1400mm wide 7050 aluminum alloy plate as an example, its theoretical tensile force after solution treatment reaches 87220kN, far exceeding the capacity of most manufacturers' stretching machines below 80MN. Second, the equipment investment is huge. The equipment, foundation construction, and maintenance costs for manufacturing giant stretching machines with a capacity of 10,000 tons are high. Third, the production flexibility is poor. Due to the limitations of equipment capacity, factories cannot produce thicker or higher-strength alloy plates, and outsourcing will lead to long production cycles and high costs. Summary of the Invention

[0003] The purpose of this invention is to provide a stress relief stretching method for ultra-thick plates based on local weakening design, which can effectively relieve stress in ultra-thick plates using existing medium-tonnage stretching machines.

[0004] To achieve the above objectives, the technical solution of the present invention is as follows: A stress relief tensile method for ultra-thick plates based on local weakening design includes the following steps: S1. The aluminum alloy ingot is successively melted, cast, homogenized and hot rolled to obtain an ultra-thick plate billet, and then stress-relief annealing is performed. S2. A stretching area is set on the slab obtained above. Part of the material is removed from the area by mechanical processing, so that the cross-sectional area of ​​the area is smaller than the area of ​​other parts, forming a local weakening area. S3. The billet obtained in S2 is heated and held at the solution temperature, and then quenched. S4. Clamp both ends of the quenched workpiece onto the stretching machine and stretch it. S5. After stretching, the workpiece undergoes subsequent aging heat treatment.

[0005] Preferably, the localized weakening zone is designed in a non-final use area according to the final shape of the product, so as to facilitate subsequent removal; the localized weakening zone is set at the end or middle section of the blank, and includes one or more.

[0006] Preferably, the local weakening zone is set in the middle section, and the modified billet shape is any one of dog bone shape, I-shape or dumbbell shape.

[0007] Preferably, the load required for the cross-sectional area of ​​the locally weakened zone to yield under tensile stress is less than or equal to the safe operating load of the target tensile testing machine; it needs to satisfy: F machine / σ b ≤A local ≤F machine / σ s F machine The maximum safe load for the stretching machine; A local σ is the effective cross-sectional area of ​​the weakened region; s The measured or estimated yield strength of the material after solution quenching; σ b This refers to the tensile strength of the material.

[0008] Preferably, the effective length of the local weakening region needs to satisfy L local ≥5t local ; among which, L local t represents the effective length of the weakened region. local The thickness of the weakened region.

[0009] Preferably, a large-radius arc is used for a smooth transition between the locally weakened area and the clamping area to avoid stress concentration leading to tearing in the transition area; the arc radius R satisfies R≥max(2.8t). local 0.1W local );t local W represents the thickness of the weakened region. local This is for the width of the weakened region.

[0010] Preferably, in step S4, the stretching is completed before the material undergoes natural aging, with a stretching amount of 1%-3%.

[0011] Preferably, in the initial stretching stage, the billet undergoes 0-0.5% overall deformation, and a low stretching speed of 0.5-1.5 mm / s is used; in the stable stage, the billet undergoes 0.5%-3% overall deformation, and a medium stretching speed of 1.5-3 mm / s is used.

[0012] Preferably, stress testing is performed on the stretched billet.

[0013] After adopting the above technical solution, the beneficial effects of the present invention are: 1. This invention breaks through the dependence of existing technologies on giant stretching machines by using "local weakening design". By designing local weakening zones to reduce the load required for stretching, medium-tonnage stretching machines can handle ultra-thick plates without the need to purchase giant equipment with a capacity of tens of thousands of tons.

[0014] 2. This method eliminates the costs of purchasing, constructing and maintaining giant stretching machines. The costs associated with such equipment typically account for more than 30% of the total investment in a production line. This method can directly avoid these high expenses.

[0015] 3. Achieve uniform elimination of residual stress throughout the ultra-thick plate, rather than limiting it to a localized area. This avoids problems such as dimensional instability, fluctuations in mechanical properties, and poor resistance to stress corrosion caused by residual stress, enabling ultra-thick plates to meet the stringent quality requirements of aerospace, high-end equipment manufacturing, and other fields.

[0016] 4. It can be adapted to ultra-thick plates of different thicknesses and strengths (such as 7000 series and 2000 series aluminum alloys), and the weakened area can be designed in the non-use area according to the final shape of the product (which can be cut off later) without affecting the structure and performance of the finished product. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of a structure where the localized weakening zone is located at the end of the billet; Figure 2 This is a schematic diagram of a structure where the localized weakening zone is located in the middle of the blank (dog bone, I-shaped); Figure 3 This is a schematic diagram of the structure of a billet with multiple weakened zones; Figure 4 yes Figure 3 Side view. Detailed Implementation

[0018] The invention will now be further described with reference to the accompanying drawings.

[0019] The orientations mentioned in this specification are based on the orientation of the stress relief tensile method for ultra-thick plates based on local weakening design of this invention during normal operation. They do not limit the orientation during storage and transportation, but only represent relative positional relationships, not absolute positional relationships.

[0020] This invention applies to ultra-thick high-strength aluminum alloy plates of the 7000 series and 2000 series with a thickness greater than 150mm. After casting and hot rolling, ultra-thick plates (especially high-strength aluminum alloys) develop residual stresses due to temperature gradients and uneven microstructure (e.g., surface compression and core tension), which are in a state of "mutually canceling equilibrium." Existing methods release these internal stresses through tensile deformation, where the tensile force reaches the material's yield strength (σ). SDuring stretching, the material enters the plastic deformation stage (rather than elastic deformation). At this point, the grains that were previously "compressed" or "stretched" will slip, and the stored internal stress will be released as the grains slip, forming a new low-stress equilibrium. Current stretching methods generally control the stretching deformation to 1%-3%, which allows most of the residual stress (about 80%-90%) to be released without causing excessive deformation of the material. After stretching, a "new equilibrium state with extremely low stress levels" will be formed inside the material, making it difficult to deform due to the release of internal stress during subsequent processing or use.

[0021] The key design element of this invention is to artificially reduce the cross-sectional area of ​​the middle section (or non-critical section) of the billet through machining, forming a "locally weakened zone." According to mechanical principles, during stretching, stress preferentially concentrates in the area with the smallest cross-sectional area; therefore, plastic deformation is forcibly confined to the weakened zone. This design ensures that even using a small-tonnage stretching machine, 1%-3% of the specified permanent deformation can be achieved within the weakened zone, providing a prerequisite for subsequent stress release.

[0022] When a stretching machine applies tension to both ends of the billet, the weakened zone undergoes plastic deformation and elongation first. Due to this plastic deformation, the stiffness of the weakened zone decreases, leading to a redistribution of stress throughout the entire component. The elastic contraction of the unweakened region (elastic zone) is "constrained" by the plastically elongated weakened region (plastic zone), creating an internal force between them. This internal force reduces the residual stress level in the unweakened region. Throughout this process, "strain coordination" and "stress redistribution" are at play, rather than simple "stress transfer." As the weakened zone continues to undergo plastic deformation, its residual stress is released through dislocation slip and other mechanisms. Simultaneously, the residual stress in the end regions is continuously transferred to the weakened zone and is "consumed" and released along with its deformation. This process continues until stretching stops, resulting in a new, stable state with extremely low and uniform stress distribution within the billet, thus eliminating residual stress across the entire ultra-thick plate. In simple terms, this method is equivalent to creating a "stress release channel" (weakened zone) on the billet. By first deforming and releasing the stress in this channel, the stress of the entire billet is then "pulled" and "guided" to concentrate and release at this point, ultimately achieving the goal of eliminating overall stress.

[0023] A stress relief tensile method for ultra-thick plates based on local weakening design includes the following steps: S1, Preceding process.

[0024] The aluminum alloy ingot is sequentially smelted, cast, homogenized, and hot rolled to obtain an ultra-thick slab. Then, stress-relief annealing is performed to eliminate the macroscopic internal stress introduced by hot working and ensure the dimensional stability of subsequent machining.

[0025] S2, Modification of billet shape.

[0026] A stretching area is set on the slab obtained above. Part of the material is removed from the area by mechanical processing, so that the cross-sectional area of ​​the area is smaller than the area of ​​other parts, forming a local weakened area.

[0027] Preferably, the localized weakening zone can be designed in a non-final use area according to the final shape of the product. For example, the localized weakening zone is set at the end or middle section of the blank, and may include one or more.

[0028] Among them, the localized weakening zone is set at the end of the billet (e.g., Figure 1 As shown in the figure, it is suitable for situations where one or both ends of the final product need to be milled significantly. It can combine the machining allowance with the weakened area to reduce material waste.

[0029] If set in the middle section, the modified billet shape will be dog bone, I-shaped, or dumbbell-shaped (e.g., Figure 2 (As shown). 1) Dog bone shape is suitable for most rectangular blanks. Deformation and stress release are the most uniform and symmetrical, and the process is the simplest. 2) I-shaped, which reduces both width and thickness, can minimize the cross-sectional area and is the preferred design for maximizing "load reduction". 3) Dumbbell shape, which mainly reduces the width direction, is suitable for scenarios where machining in the thickness direction is difficult or where it is necessary to retain the complete thickness direction structure.

[0030] The principle for setting the location of the weakened area is to not affect the product, facilitate processing, and promote deformation. Its core principles are threefold: 1. Prioritize "non-final use areas" to avoid affecting product performance. The weakened area should be placed in areas that can be removed later or do not participate in product function: 1) Products with machining allowances; for example, when producing a 150mm thick panel, with a 50mm machining allowance at both ends (to be milled off later), the weakened area can be placed here. After stretching, the allowance can be milled off, and the finished product will not be affected. 2) Non-critical parts of the product; for example, when producing T-shaped extra-thick parts, the weakened area can be placed at the "web edge" of the T-shape (non-load-bearing part). Small deformations at the web edge after stretching will not affect the overall load-bearing capacity.

[0031] 2. Next, select a "symmetrical or centered position" to ensure uniform deformation. 1) Symmetrical position (e.g., the middle of a dog bone shape): Deformation will be symmetrically distributed along the center, and the deformation at both ends of the non-weakened area will be consistent (the deformation of the non-weakened area on the left is the same as that on the right, avoiding bending of the board due to uneven deformation and ensuring the flatness of the final product. 2) Avoid eccentric positions; if the weakened area is set at one end (e.g., 200mm from the left end and 800mm from the right end), the deformation of the non-weakened area on the left end will be much greater than that on the right end during stretching (e.g., 0.5% on the left end and 0.1% on the right end), and the board will bend to the right, making subsequent correction difficult.

[0032] 3. Finally, consider "processing convenience" to reduce process costs. 1) Avoid complex structural areas; if the sheet has holes, grooves, or other complex structures, the weakened area should be far away from these areas (at least ≥3t) to avoid damaging the complex structure and increasing processing difficulty when processing the weakened area. 2) Proximity to the clamping end; the weakened area should not be too far from the clamping end of the stretching machine (recommended ≤1m) to avoid the sheet wobbling due to "excessive lever arm" during stretching, which would affect the stretching stability.

[0033] The core design concept of this invention is to artificially create a locally weakened zone with a small cross-section, so that during subsequent tensile testing, plastic deformation and stress release will concentrate within this predetermined weakened zone. Because the cross-sectional area A of this region... local Smaller than the cross-sectional area A of the original billet original Therefore, the required tensile force F local =σ s ×A local It also decreased significantly year-on-year. Determined by reverse calculation: A local ≤F machine / σ s Among them, A local F represents the effective cross-sectional area of ​​the weakened region. machine The maximum safe load for the stretching machine is usually taken as the rated maximum load F of the stretching machine. max 80%-90%; σ s For the measured or estimated yield strength of materials after solution quenching, the measured value within the aging period after solution quenching should be used first, and the sampling location should be the non-weakened area adjacent to the weakened area (sampling size 10mm × 50mm); if an estimate is required, it should be based on the σ of the same batch of ingots. s Statistical data (at least 3 parallel samples, with a deviation ≤ ±3%), the prediction formula is σ s预 =σ s实测均值 ×(1+2%), with a safety margin reserved.

[0034] Taking 7050 aluminum alloy as an example: plate thickness 200mm, width 1500mm, σ s =350MPa, existing equipment F max =2500 tons (approximately 25000 kN). Take F machine =0.85 * 25000 kN = 21250 kN. Therefore, A local ≤21250000N / 350MPa, A local ≤60714mm 2 .

[0035] Taking 2024 aluminum alloy as an example: plate thickness 100mm, width 800mm, σ s =280MPa, existing equipment F max =1000 tons (approximately 10000 kN). Take Fmachine =8000kN. Then A local ≤8000000N / 280MPa, approximately equal to A local ≤28571mm 2 .

[0036] But if A local If the value is too small, the stress in the weakened region will far exceed σ. s This directly achieves the tensile strength σ of the material. b This causes the weakened zone to break before completing 1%-3% of plastic deformation, thus preventing stress release. Therefore, the stress in the weakened zone is ≤ tensile strength σ. b To avoid premature fracture (core lower limit); the weakened region must first enter plastic deformation (σ≥σ s Furthermore, before 1%-3% of permanent deformation is achieved, the stress must not exceed the tensile strength σ of the material. b (Otherwise it will break). Combining the relationship between tensile force and stress, we can deduce A. local Minimum allowable value: A local ≥F machine / σ b ; where σ b Tensile strength of material after solution quenching (needs to be measured, usually σ) b >σ s For example, 7050 aluminum alloy σ s Approximately 350 MPa, σ b Approximately 480 MPa); F machine This is the maximum safe load for the stretching machine.

[0037] When the tensioning machine applies the maximum safe load F machine At that time, the maximum stress σ in the weakened region max =F machine / A local To avoid σ max Exceeding σ b A needs to be guaranteed local ≥F machine / σ b .

[0038] In conclusion, A local The reasonable range is F machine / σ b ≤A local ≤F machine / σ s。

[0039] Let's take 7050 aluminum alloy as an example for verification: Upper limit: A local_max =F machine / σ s=21250000 / 350≈60714mm²; Lower limit: A local_min =F machine / σ b =21250000 / 480≈44270mm²; Reasonable range: 44270mm²≤A local ≤60714mm² (within this range, the weakened zone can preferentially undergo plastic deformation without premature fracture).

[0040] Although theoretically A local It is feasible within the "lower limit - upper limit" range, but it is recommended to use A in engineering practice. local The value is chosen "near the midpoint of the interval" for the following reasons: to avoid the risk of material performance fluctuations: the σ of the same batch of materials... b There may be fluctuations of ±3%, if A local Approaching the lower limit, once σ b The measured value is too low, which may lead to breakage; A local When the stress is too large, the stress increase is more gradual, and the stretching machine can more accurately control the deformation by 1%-3%, avoiding over-deformation.

[0041] To ensure that the weakened region has sufficient length to develop uniform plastic deformation and to avoid complicating the stress state due to the "end constraint effect," the effective length of the local weakened region needs to satisfy L. local ≥5t local ; among which, L local t represents the effective length of the weakened region. local To determine the thickness of the weakened area, five detection points (one at each end and three in the middle) need to be evenly selected along the length of the weakened area using a coordinate measuring machine. The minimum thickness of these five points is then used as the basis for calculation.

[0042] The weakened zone needs to ensure at least 2 tons in the middle. local The length is under uniaxial tension (strain gradient ≤ 0.1% / mm) to ensure uniform development of 1%-3% plastic deformation. According to the "boundary constraint stress attenuation law" in elasticity, the attenuation distance of the three-dimensional stress constraint along the length of the clamping end and transition zone is 1.5t. local (i.e., 1.5t from the transition zone) local The area within the range is affected by constraints. Therefore, the total length of the weakened region must cover the constrained areas at both ends (1.5t each). local ) + intermediate uniaxial tension zone (2t) local ), that is: L local ≥1.5t local +2t local +1.5t local =5t local .

[0043] For 2024 aluminum alloy samples (t) local Comparison test (=100mm): When L local =500mm (5t) local When the middle part is 200mm (2t) local Strain deviation within the specified range is ≤0.2%; When L local =400mm (4t) local When the strain deviation in the middle reaches 0.5%, the constrained areas at both ends overlap, resulting in local strain exceeding the limit. When L local =600mm (6t) local At 5t, the strain uniformity is similar to that at 5t. local The results are consistent, but the material utilization rate is reduced by 10%, hence 5t local This is the optimal value.

[0044] This length is designed to ensure that the stress state in the central part of the weakened zone is close to ideal uniaxial tension, avoiding the influence of three-dimensional stress constraints at the clamping ends or transition zone on the full development of plastic deformation. This area is the core region for controllably and uniformly absorbing 1%-3% of plastic strain. If the length is insufficient, the entire weakened zone will be under the influence of end constraints, making effective stress release impossible and potentially leading to uncontrolled deformation. The thickness tolerance of the weakened zone is ≤±0.5mm, and the width tolerance is ≤±2mm. Dimensional accuracy is ensured using a five-axis machining center.

[0045] The effective length L of the aforementioned weakened region local ≥5t local (To ensure uniform deformation), t needs to be confirmed. local The corresponding L local Is it within the allowable range of billet length (e.g., t)? local =30mm, then L local ≥150mm. If the billet length is sufficient, no adjustment is needed; if the length is insufficient, the t can be appropriately reduced. local， To reduce L local ).

[0046] A local =t local ×W local , where t local To reduce the thickness of the weakened region, W local The width of the weakened zone. Since the width of the weakened zone is usually the same as the original width of the blank (no additional milling width is required, making processing simpler and cheaper), the thickness of the weakened zone can be determined by t. local =A local / W local Direct calculation is the most basic derivation logic.

[0047] In engineering, prioritize "do not change width", i.e., W local =The original width of the billet is used for the following reasons: only the thickness direction needs to be milled (such as surface grooving), and there is no need to cut along the width direction, so as to avoid damaging the overall structure of the billet; keeping the width unchanged can ensure that the stress is evenly distributed along the width direction, and avoid introducing additional stress concentration due to changes in width.

[0048] Combined with the already determined A local Upper and lower limits (A) local_max =F machine / σ s A local_min =F machine / σ b ), to obtain a reasonable range of thickness: t local_min =A local_min / W local ;t local_max =A local_max / W local Taking 7050 aluminum alloy as an example, continuing with the previous parameters: A local Range: 44270mm²≤Alocal≤60714mm²; Original width W original =1500mm (take W) original =1500mm); t local_min =44270 / 1500≈29.5mm; t local_max =60714 / 1500≈40.5mm; Therefore, the thickness of the weakened zone should be selected between 29.5mm and 40.5mm.

[0049] Taking into account processing capacity and deformation uniformity, the final t is determined. local The calculated t local The tolerance range needs to be adjusted based on actual processing conditions. If a five-axis machining center is used, the thickness tolerance must be ≤ ±0.5mm. It is necessary to confirm whether the equipment can stably process to the target thickness (e.g., 29.5mm is acceptable). If the minimum processing thickness of the equipment is 30mm, then take t. local =30mm.

[0050] t local Too thin a material can cause deformation during processing; if t local For plates <20mm (ultra-thick plates), chatter is prone to occur during milling, causing thickness tolerances to exceed ±0.5mm. W needs to be appropriately reduced. local to increase t local . t local Excessive thickness leads to insufficient stress; if t local >tlocalmax (If it exceeds 40.5mm), it will cause A local >A local_max If the stress in the weakened zone is less than σs, it cannot enter the plastic deformation stage and needs to be readjusted.

[0051] The transition zone between the locally weakened area and the clamping area is achieved using a large-radius circular arc to smoothly transition the area and avoid stress concentration that could lead to tearing. The radius R of the circular arc should satisfy R ≥ max(2.8t). local 0.1W local );t local W represents the thickness of the weakened region. local This refers to the width of the weakened zone. This formula is the design principle for the radius R of the transition zone arc. The core idea is to ensure that the transition zone avoids stress concentration while also accommodating the width of the weakened zone by "taking the larger of two key values." This fundamentally prevents cracking in the transition zone during stretching and ensures smooth stress transfer. The specific breakdown is as follows: 1. Simultaneously consider "2.8t" local "and "0.1W local The essence is "double insurance"—"2.8t". local "It prevents stress concentration from the 'thickness dimension', 0.1W" local "It adapts the weakened area size from the "width dimension" and takes the maximum value to meet the security requirements of both dimensions at the same time.

[0052] The design of the transition zone radius uses a stress concentration factor Kt ≤ 1.2 as the critical index (this value is determined based on the plasticity reserve of 7000 and 2000 series aluminum alloys; when Kt ≤ 1.2, the stress in the transition zone will not exceed 1.2 times the material's yield strength, thus avoiding cracking under tension). According to the "Stress Concentration Factor Chart for Stepped Shaft Fillet Transitions" in the theory of "Stress Concentration Factor for Stepped Shaft Fillet Transitions" in mechanics of materials, Kt is related to the fillet radius R and the thickness t of the weakened zone. local The relationship satisfies: Kt = 1 + 0.8 × (t) local When Kt = 1.2, substituting into the equation, we get: 1.2 = 1 + 0.8 × (t)². local Solving for R, we get R≥2.8t. local .

[0053] For the width direction, finite element simulation was used (model parameters: weakened region width W). local =500-1500mm, thickness t local =100-200mm, material 7050 aluminum alloy, σ s =350MPa) found: when R≥0.1W localWhen the stress gradient along the width of the weakened zone is ≤5MPa / mm, it ensures uniform stress diffusion along the width direction and avoids localized stress accumulation at the edges. Therefore, the maximum value of both is taken as the design criterion. "For 7050 aluminum alloy samples (t)" local =120mm, W local A comparative experiment was conducted using a diameter of 500mm. When R = 336 mm (2.8 t) local When the stress in the transition zone is 380 MPa (Kt=1.09), no cracks appear after tension. When R = 200 mm (1.7 t) local When the stress in the transition zone is 455 MPa (Kt=1.30), microcracks appear after tension. When R=50mm (0.1W) local When R = 40 mm (0.08 W), the edge stress in the width direction is 8% higher than that at the center; local When the edge stress is 15% higher than the center stress, it verifies the 0.1W... local The necessity of it. First layer of protection: R ≥ 2.8t local —Avoiding stress concentration and preventing cracking in the transition zone by addressing the "thickness dimension" aims to "allow stress to be smoothly transmitted along the thickness direction." The principle is that the degree of stress concentration in the transition zone is inversely proportional to the "radius R" of the arc and to the "thickness t of the weakened zone." local "Proportional, t" local The larger the value, the more drastic the change in cross-section along the thickness direction, and the more prone it is to stress concentration. When R ≥ 2.8t local At this time, the stress concentration factor can be reduced to below 1.2 (safe range), avoiding the transition zone from being torn due to excessive stress, while ensuring that the stress can "penetrate" from the surface of the weakened zone to the core and then be transferred to the working area.

[0054] For example, in Case 1 above, the width W of the weakened region... local Take 480mm, 0.1W local =48mm, 2.8t local =336mm, and finally take the maximum value of R, then R≥336mm.

[0055] Second layer of insurance: R ≥ 0.1W local —Adapting the weakened area along its width dimension avoids localized stress buildup; the aim is to ensure uniform stress diffusion along the width direction. The principle is: the width W of the weakened area… local The larger the value, the more pronounced the "change in the area of ​​force application" in the width direction (e.g., W). local =1000mm weakening zone, compared to W local =500mm is more difficult to ensure uniform stress diffusion). When R≥0.1Wlocal At this point, the arc can cover 1 / 10 of the width of the weakened zone, allowing the stress to "gradually transition" in the width direction, preventing localized stress accumulation at the edges of the weakened zone. For example, W local =500mm, 0.1W local =50mm, R≥50mm can ensure uniform stress diffusion in the width direction.

[0056] For example, in Case 1 above, if the width of the weakened region W local =500mm, then 0.1W local =50mm——At this point, R should be at least 50mm to meet the stress diffusion requirements in the width direction.

[0057] 2. Finally, "take the maximum value": ensure that the security requirements of both dimensions are met. In the formula, "take max(2.8t)" local 0.1W local ")" is because "2.8t" local "and "0.1W local The calculation results for "" often differ significantly, and the larger one must be selected to simultaneously cover the safety requirements for both thickness and width: when the weakened region is thicker and the width is smaller (e.g., t) local =120mm, W local =500mm): 2.8t local =336mm, 0.1W local =50mm, the maximum value is 336mm - at this time, R is 336mm, which can not only meet the crack prevention in the thickness direction, but also far exceed the 50mm requirement in the width direction.

[0058] Simply put, the essence of this formula is to "not overlook any dimension that may cause stress concentration". By taking the maximum value, the arc design of the transition zone is "both thick and wide enough" to ensure that the stress can be smoothly transmitted during stretching without cracking or stress interruption.

[0059] The surface roughness of the transition zone must be ≤Ra1.6μm. Five-axis milling is used to ensure the continuity of the arc and avoid stress concentration sources caused by machining tool marks.

[0060] Multiple weakening zones are mainly applicable to ultra-long billets, when the billet length is extremely large (e.g., greater than 15m). Figure 3 and Figure 4As shown in the diagram, designing multiple weakened zones can avoid deformation instability and clamping difficulties caused by an excessively long single weakened zone. The principles for setting multiple weakened zones are: 1) Rationally design the width and length of the weakened zones, as well as the width of the connection area between adjacent weakened zones. For example, keeping the ratio of the width of one weakened zone to the width of the connection area between adjacent weakened zones within a suitable range, such as 1:1 or higher, can make the tensile force more evenly transmitted to each weakened zone. 2) Ensure that the weakened zones are evenly distributed on the material, avoiding situations where local weakened zones are too dense or sparse. For example, in some elastic composites, the weakened zones are distributed in multiple rows, each row extending along the transverse direction of the web, and these rows are evenly distributed along the direction of the web, which helps to evenly transmit the force during tension. 3) Optimize the shape of the weakened zones so that they can evenly bear and transmit stress during tension. For example, using regular-shaped grooves such as circles and ellipses as weakened zones is more conducive to the even distribution of stress than irregular shapes. In addition, the reason for choosing a groove shape instead of a through hole is that the blank is an ultra-thick plate, which is extremely difficult to process through. A groove is easier to achieve by milling only the surface. In addition, if the finished product requires the full thickness (such as a large mold substrate), a through hole cannot be made. The cross-sectional area can only be reduced by cutting the local width through a groove.

[0061] If the billet length exceeds 15m, 3-4 weakened zones can be used, with an adjacent spacing of ≥5m. Synchronous stretching control is employed, ensuring the strain difference between each weakened zone is ≤0.2%, thus resolving the end stress accumulation problem caused by stretching a single weakened zone. However, for multi-weakened zone stretching of ultra-long billets, improper control can lead to excessive local strain, such as 0.8% strain in one weakened zone and 0.5% in another, resulting in new residual stress. Without control parameters, repeated implementation is impossible. Therefore, two extensometers (along the length direction) are installed in the middle of each weakened zone to collect strain data in real time. The two extensometers are symmetrically arranged along the length of the weakened zone, with a spacing of 2t (t being the minimum thickness of the weakened zone). The center point of the extensometer must coincide with the center point of the weakened zone to avoid proximity to the transition zone. A "master-slave control" approach is adopted, using the strain of the middle weakened zone as a benchmark (target strain 1%-3%). The tension of the cylinders at both ends is adjusted through the servo system of the stretching machine to ensure that the strain difference between each weakened zone is ≤0.2%. The strain adjustment response time of the hydraulic cylinder of the stretching machine is ≤100ms. When the strain difference between a certain weakened zone and the reference strain exceeds 0.1%, the servo system needs to start pre-adjustment to ensure that the final strain difference is ≤0.2%.

[0062] S3, solution treatment.

[0063] The blank obtained in S2 is heated and held at the solution temperature, and then quenched.

[0064] 7050 aluminum alloy: solution treatment temperature 480±5℃, holding time 1.5h, quenching medium is 20-30℃ deionized water, cooling rate ≥250℃ / min.

[0065] 2024 aluminum alloy: solution treatment temperature 495±5℃, holding time 1h, quenching medium is 20-30℃ deionized water, cooling rate ≥200℃ / min.

[0066] S4, stretching.

[0067] The two ends of the quenched workpiece are clamped onto a stretching machine with a matching tonnage capacity for stretching. After solution quenching, the stretching should be completed before the material undergoes natural aging, preferably within 1 hour. If it exceeds 1 hour, the tensile strength (σ) needs to be retested. S And adjust A local Because aluminum alloys undergo "natural aging" after quenching (e.g., 7050 quenched and left at room temperature for 1 hour, σ...). S It will increase by 5%-10%); if the time interval is too long, σ S Increasing this will cause the originally calculated A to... local Insufficient, unable to achieve the expected deformation amount. Controlling the elongation of the stretching machine to induce 1%-3% permanent plastic deformation in the billet, thereby achieving stress relief for the entire ultra-thick plate.

[0068] The elongation rate E = (length after stretching - length before stretching) / length before stretching × 100%, or E = (L - L0) / L0 × 100%. The method for determining whether 1%-3% has been achieved is: compare the dimensions before and after stretching, i.e., measure the length L0 before stretching and the length L after stretching, then calculate the elongation rate E according to E = (L - L0) / L0 × 100%.

[0069] During the stretching process, the stretching rate is controlled. In the initial stage of stretching, when the thick plate undergoes 0-0.5% overall deformation, a low stretching speed of 0.5-1.5 mm / s is used to ensure that the stress steadily exceeds σ. S During the stabilization phase, the thick plate undergoes an overall deformation of 0.5%-3%, and a medium speed of 1.5-3 mm / s is used to balance deformation efficiency and dynamic recovery.

[0070] S5. After stretching, perform subsequent aging heat treatment on the workpiece. If the "locally weakened area" is a non-final use area of ​​the product, it can be milled away in a subsequent process.

[0071] Testing methods after tensile stress release: The residual stress in the core is measured by combining the peeling method with strain gauges (blind hole method), or the stress distribution in the thickness direction is measured by neutron diffraction.

[0072] To verify the stress relief effect of the present invention, residual stress was measured on the treated workpiece. Detailed data and effect analysis are provided in the subsequent experimental examples section.

[0073] Example 1: 7050 aluminum alloy ultra-thick plate (dog bone shaped weakening zone) Specimen parameters: Material: 7050 aluminum alloy Original blank dimensions: 200mm thick × 1500mm wide × 4000mm long Weakening zone design: Located in the middle of the billet, shaped like a dog bone. Weakening zone thickness t local =120mm, width W local =500mm, effective length L local =600mm(5t local The transition arc radius R = 336 mm.

[0074] Tensile process: Apply 1.8% permanent plastic deformation within 1 hour after solution quenching.

[0075] Residual stress measurement results (unit: MPa): The measurement location is at the middle section of the billet along the length direction, and the measurement is carried out along the thickness direction.

[0076] Measurement location Stress type Residual stress before treatment (MPa) Residual stress after treatment (MPa) Stress reduction Surface layer (5mm from the surface) Axial stress -185 -25 86.5% Core (center of thickness) Axial stress +165 +18 89.1% 1 / 4 thickness Axial stress +85 +10 88.2% Data Analysis and Conclusions: Stress state transformation: Before treatment, the billet exhibits a typical post-hot working stress distribution—compressive stress on the surface (-185MPa) and tensile stress in the core (+165MPa), with the internal and external stresses in equilibrium. After treatment using this method, the compressive stress on the surface and the tensile stress in the core are eliminated to a low stress level close to zero (within ±25MPa).

[0077] Overall stress relief effect: Data shows that not only did stress release occur in the weakened area itself, but the residual stress in the measured section far from the weakened area (located in the non-weakened area) was also uniformly and synchronously reduced. This proves that the plastic deformation of the local weakened area successfully triggered stress redistribution and overall stress release throughout the entire component, rather than a local effect.

[0078] Example 2: 2024 aluminum alloy ultra-thick plate (end weakened zone) Specimen parameters: Material: 2024 aluminum alloy Original blank dimensions: 100mm thick × 800mm wide × 3000mm long Weakening zone design: Located at one end of the billet (in the subsequent machining allowance area). Weakening zone thickness t local =70mm, width W local =400mm, effective length Llocal =350mm(5t local ).

[0079] Tensile process: Apply 2.2% permanent plastic deformation within 1 hour after solution quenching.

[0080] Residual stress measurement results (unit: MPa): To examine the uniformity of stress relief, three typical cross-sections were selected on the billet for measurement: cross-section A (adjacent to the weakened zone), cross-section B (middle of the billet), and cross-section C (the other end furthest from the weakened zone). Cross-section A (near the weakened zone) is located outside the "exit end" of the weakened zone, with a distance ≤ 1 times the length of the weakened zone (i.e., ≤ 350 mm), typically taken as "the end of the weakened zone + 50-100 mm". In this embodiment, the billet length is 3000 mm, and the specific location of cross-section A can be set at 400 mm (from the weakened zone to 350 mm, extending 50 mm to the right). Cross-section B is divided according to 1 / 2 of the total length of the billet, i.e., 3000 mm × 1 / 2 = 1500 mm. Cross-section C (far end) is located near the rightmost end of the billet, with a distance ≥ 2 / 3 of the total length of the billet from the weakened zone, typically taken as 2900 mm (extending 100 mm to the left from the right end of 3000 mm).

[0081] Cross-section position Measurement points Residual stress before treatment (MPa) Residual stress after treatment (MPa) Stress reduction Section A (near weakened region) Heart +150 +15 90.0% Section B (middle of the billet) Heart +145 +20 86.2% Section C (far end) Heart +140 +22 84.3% Data Analysis and Conclusions: Spatial homogeneity: The residual stress in the core of the three different cross-sections was at a similar level before treatment. After treatment with this method, the stress in all cross-sections was synchronously reduced to a similar low level (15-22 MPa).

[0082] Consistent effect: Although the weakened zone is located at the end, the residual stress along the entire length of the billet is effectively and uniformly eliminated. This proves that stress release is achieved through the overall mechanical coordination of the component, eliminating the doubt that "it is only effective near the weakened zone" and fully demonstrating the global effectiveness of the method of this invention.

[0083] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A stress-relief tensile method for ultra-thick plates based on local weakening design, characterized in that: Includes the following steps: S1. The aluminum alloy ingot is successively melted, cast, homogenized and hot rolled to obtain an ultra-thick plate billet, and then stress-relief annealing is performed. S2. A stretching region is set on the slab obtained above. Part of the material is removed from the region by mechanical processing, so that the cross-sectional area of ​​the region is smaller than the cross-sectional area of ​​the non-stretching region, forming a local weakening region. S3. The billet obtained in S2 is heated and held at the solution temperature, and then quenched. S4. Clamp both ends of the quenched workpiece onto the stretching machine and stretch it. S5. After stretching, the workpiece undergoes subsequent aging heat treatment.

2. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: The localized weakening zone is designed in a non-final use area according to the final shape of the product, so that it can be removed later; the localized weakening zone is set at the end or middle section of the blank, and includes one or more.

3. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 2, characterized in that: The local weakening zone is set in the middle section, and the modified billet shape is any one of dog bone shape, I-shape or dumbbell shape.

4. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: The load required for the cross-sectional area of ​​the locally weakened zone to yield under tension is less than or equal to the safe working load of the target tensioning machine. It needs to meet the following: F machine / σ b ≤A local ≤F machine / σ s F machine The maximum safe load for the stretching machine; A local σ is the effective cross-sectional area of ​​the weakened region; s The measured or estimated yield strength of the material after solution quenching; σ b This refers to the tensile strength of the material.

5. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: The effective length of the local weakening region needs to satisfy L. local ≥5t local Among them, L local t represents the effective length of the weakened region. local The thickness of the weakened region.

6. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: A large-radius circular arc is used for a smooth transition between the locally weakened zone and the clamping zone to avoid stress concentration that could lead to tearing in the transition zone; the radius R of the circular arc satisfies R≥max(2.8t). local 0.1W local );t local W represents the thickness of the weakened region. local This is for the width of the weakened region.

7. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: In S4, the stretching is completed before the material undergoes natural aging, with a stretching amount of 1%-3%.

8. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 7, characterized in that: In the initial stretching stage, the billet undergoes 0-0.5% overall deformation, and a low stretching speed of 0.5-1.5 mm / s is used. In the stable stage, the billet undergoes 0.5%-3% overall deformation, and a medium stretching speed of 1.5-3 mm / s is used.

9. The method for stress relief tensioning of ultra-thick plates based on local weakening design according to claim 1, characterized in that: Stress testing is performed on the stretched billet.

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