Preparation method of large-size high-fatigue-resistance Al-Zn-Mg-Cu alloy structural component

Abstract

The invention relates to the technical field of aluminum alloy processing and preparation, and discloses a preparation method of a large-size high-fatigue-resistance Al-Zn-Mg-Cu alloy structural member, in the method, a round cast ingot with the diameter of 800-1000 mm is prepared through casting, the round cast ingot is subjected to die forging in die forging deformation to be deformed into a cylindrical forged piece with the diameter of 1300-1500 mm, and the cylindrical forged piece is subjected to die forging in the die forging deformation process to form the large-size high-fatigue-resistance Al-Zn-Mg-Cu alloy structural member. During extrusion deformation, a cylindrical forge piece is extruded into a round bar with the diameter being 600-800 mm, and then the high-performance aluminum alloy structural part is prepared through free forging, die forging and heat treatment. The invention innovatively provides a composite technical route of semi-continuous ingot casting, die forging deformation, extrusion, free forging and die forging, as-cast structures can be thoroughly broken, internal defects can be welded, large-deformation die forging blank making and the extrusion procedure with strong three-way pressure stress are introduced, deep structure transformation of the large-specification round ingot casting is achieved, and the large-specification round ingot casting quality is improved. And the source of fatigue crack initiation is fundamentally eliminated or reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of aluminum alloy processing and preparation, and particularly relates to a preparation method of a large-size Al-Zn-Mg-Cu alloy structural piece with high fatigue resistance. BACKGROUND

[0002] With the rapid development of the aviation industry, especially the increasing demand for large-size and high-load helicopters, unprecedented requirements are put forward for the performance of key structural pieces (such as main rotor joints, fuselage frames, central transmission system support structures, etc.) of the helicopters. These components not only need to bear high static loads, but also need to bear long-time, high-frequency and high-stress-amplitude alternating loads during the service of the aircrafts such as helicopters. Therefore, the fatigue resistance of the material, especially the tensile-compressive fatigue limit under high stress ratio (R=-1), becomes a core index for determining the service life, flight safety and reliability of the components. At present, in order to improve the performance of the aircrafts, the aviation design units put forward higher standards for such key aluminum alloy components, and require that the fatigue limit of the components is not less than 200 MPa under the conditions of stress concentration coefficient Kt=1 and stress ratio R=-1 without failure after 100 million cycles.

[0003] Al-Zn-Mg-Cu series (7000 series) high-strength aluminum alloys are one of the preferred materials for manufacturing the above-mentioned key aviation structural pieces, because they have high specific strength, good toughness and processing performance. In order to balance the high strength, stress corrosion resistance and fatigue resistance, the overaging state (such as T74) has become a general choice in the industry. However, there is a huge gap between "theoretically selecting T74 state" and "stably realizing the top fatigue performance of this state on large-size complex components in practice". This gap mainly comes from the organization control problems in the whole process of large-scale component preparation. For large-size die forgings required by helicopters and other aircrafts, the preparation process involves large-scale ingot melting, multi-pass hot deformation, complex heat treatment and other links, which brings great difficulties to the uniformity and stability of the material performance: (1) Metallurgical defects of large-scale ingots: large-size aluminum alloy structural pieces must use large-size aluminum alloy ingots as raw materials due to their demand for metal quantity. However, during the semi-continuous casting of aluminum alloy, it is easy to produce grain coarsening, composition segregation, porosity and inclusions, etc. These defects are difficult to completely eliminate in the subsequent processing, and will become the source of fatigue crack initiation, seriously deteriorating the fatigue performance of the material.

[0004] (2) Challenge of complex thermal deformation process: From large ingot to final forging, a series of thermal deformation processes such as forging, extrusion, multi-fire free forging and die forging are needed. If the process parameters are not properly controlled, it is easy to cause the internal flow line of the forging to be discontinuous, the structure to be uneven, the local grain to be coarse or abnormally grown, which will also significantly reduce the fatigue limit.

[0005] (3) Difficulty of heat treatment of large size components: For large die forgings with rib height exceeding 100 mm and web thickness exceeding 45 mm, there is a huge difference in cooling rate between the core and the surface of the component during solid solution quenching. This difference will cause huge residual internal stress and may cause deformation of the component. At the same time, uneven cooling will make the supersaturation solid solubility of different parts of the component inconsistent, directly affecting the uniformity of subsequent aging precipitation behavior, and finally leading to uneven fatigue performance of the whole component and difficulty in meeting the standard. Although the conventional quenching process (such as room temperature water quenching) has strong cooling capacity, the residual stress generated is too large; while the graded quenching or boiling water quenching can reduce the stress, but the cooling rate may not be sufficient to cause grain boundary precipitation phase to coarsen, which is also not conducive to fatigue resistance.

[0006] Currently, the industry mainly relies on the following two traditional technical paths to manufacture such large specification Al-Zn-Mg-Cu alloy die forgings (structural components with length and width reaching 3 meters): Path 1 "ingot-hot rolling-forging" path: First, prepare a square ingot with a thickness of 400-500 mm through semi-continuous casting, then roll the ingot into a plate with a thickness of 150-170 mm using hot rolling technology, and finally form the final structure through free forging and die forging.

[0007] Path 2 "ingot-direct forging" path: Similarly, use a square ingot with a thickness of 400-500 mm, without rolling, directly process it into the final shape through multi-fire free forging and die forging.

[0008] However, both of the above two traditional paths have inherent limitations when pursuing a high "tension-compression" fatigue limit of more than 200 MPa. In path 1, although hot rolling can provide large deformation, its deformation mode is mainly two-dimensional extension, and the welding ability of three-dimensional defects such as shrinkage and holes in the center of the ingot is limited, and the subsequent forging fire and deformation are relatively insufficient, making it difficult to completely break and heal all the inherent micro-defects in the as-cast structure (such as micro-holes, composition segregation). Also, although hot rolling can obtain uniform plate structure, the subsequent forging fire and deformation may be insufficient to completely break and heal the inherent micro-defects (such as micro-holes, composition segregation) in the as-cast structure, which are prone to become crack initiation sources under subsequent high-cycle fatigue load.

[0009] Path 2 lacks a high-strength, large deformation intermediate blanking process to completely transform the as-cast structure, and the deformation of direct forging is difficult to fully penetrate to the center of the large cross-section ingot, resulting in that the center of the ingot cannot be fully broken and homogenized. That is, due to the lack of a large deformation intermediate blanking process, the center of the ingot cannot be fully broken and homogenized, resulting in poor density and uniformity of the center structure of the final forging, and the overall fatigue performance, especially the stability of the fatigue limit, is difficult to guarantee.

[0010] The above two technical paths not only fail to completely eliminate as-cast structure defects, but also are prone to leaving fatigue crack sources, and are difficult to obtain overall uniform and dense forged flow line structure. The forged blanks prepared by the above two paths have a high risk of discontinuous or incomplete internal metal flow lines. Path 1 changes the flow line direction sharply from the plate to the forged piece, which is prone to stress concentration at the flow line turning point; Path 2 has insufficient deformation, and the center flow line is not obvious or even no flow line, and the structure density is poor. Such non-uniform structure will significantly reduce the overall fatigue performance of the material.

[0011] Due to the above defects, the forged blanks provided by the traditional path have "inherent deficiencies" in microstructure, and even if optimized solid solution and aging heat treatment is used subsequently, these inherent micro-defects and non-uniformity will become the "short board" of fatigue performance, making it difficult to stabilize the fatigue limit of the final component, especially the high-cycle "tension-compression" fatigue limit under the condition of Kt=1, R=-1, to exceed the stringent index of 200MPa.

[0012] Therefore, there is an urgent need for a completely new technical path that can achieve more complete organization optimization and defect control. SUMMARY

[0013] In view of the above deficiencies, the present application provides a preparation method of a large-specification high-fatigue-resistance Al-Zn-Mg-Cu alloy structural part.

[0014] To achieve the above object, the present application adopts the following technical scheme: A preparation method of a large-specification high-fatigue-resistance Al-Zn-Mg-Cu alloy structural part, comprising the following steps: Batching and smelting; Casting: using a semi-continuous casting process to prepare a circular ingot with a diameter of 800-1000mm; Die forging deformation: die forging the circular ingot to deform into a cylindrical forged piece with a diameter of 1300-1500mm; Extrusion: extruding the cylindrical forged piece into a round bar with a diameter of 600-800mm; Free forging and die forging: the obtained extruded round bar is subjected to free forging and die forging through multiple heating times to obtain a die forging piece; Solution quenching heat treatment; Cold pressing to reduce residual stress; Aging heat treatment.

[0015] Further, in the multiple heating times of free forging, the thickness of the forged blank obtained by forging is 100-150 mm. In the multiple heating times of die forging, the rib height of the obtained die forging piece is 100-120 mm, and the web thickness is 45-60 mm.

[0016] Further, in the multiple heating times of die forging, the preheating temperature of the die is 390-420°C.

[0017] Further, the mass percentage of the elements of the Al-Zn-Mg-Cu alloy structural piece is: Si≤0.06%, Fe≤0.08%, Zn: 7.2-8.5%, Cu: 1.7-2.3%, Mg: 1.5-2.3%, Zr: 0.09-0.12%, Ti: 0.02-0.04%, and the balance is Al and unavoidable impurities.

[0018] Further, in the solution quenching heat treatment, the die forging piece is first held at 440-460°C for 3-6 h, then held at 470-480°C for 5-8 h, and then quenched in a vertical quenching furnace, with water as the quenching medium, the water temperature being controlled at 50-60°C, and the quenching transfer time being controlled at 10-13 seconds; after solution quenching, cold pressing is performed to reduce residual stress at a deformation rate of 2-4%.

[0019] Further, in the aging heat treatment, the die forging piece is first held at 110-120°C for 5-7 h, and then held at 145-155°C for 14-22 h.

[0020] Further, the circular ingot prepared by the semi-continuous casting method is subjected to homogenization heat treatment, with the system being 400°C / 10h+468°C / 50h.

[0021] Further, after the homogenization heat treatment, the circular ingot is subjected to rail removal of the segregation shell layer, with the diameter being 880-950 mm; after cutting off the top and bottom of the circular ingot, the length is 2000-2600 mm.

[0022] Further, the water temperature after quenching is increased by no more than 5°C.

[0023] Further, the melting uses multi-stage melt purification equipment combining in-furnace refining, online refining, and multiple filtration for melt purification.

[0024] The application provides a preparation method of a large-scale helicopter key load-bearing structure with super-high fatigue performance, excellent strength and matching toughness.

[0025] The core of the application is to break through the traditional "ingot casting-hot rolling-forging" or "ingot casting-direct forging" technical path, and creatively adopt a composite preparation technical path of "semi-continuous ingot casting-die forging deformation-extrusion-free forging-die forging", and the systematic synergy of the path is as follows: First, a large-diameter round ingot with a diameter of 800-1000 mm is prepared by semi-continuous casting to meet the metal quantity requirement of the large component; The round ingot is subjected to first die forging to be deformed into a cylindrical forging with a larger diameter (1300-1500 mm), and the purpose of this step is: First, preliminary densification: closing the shrinkage and holes in the center of the ingot by large pressure; Second, shape conversion: preparing a suitable size and shape blank for the subsequent extrusion process; Then, large deformation extrusion, which is the most critical link for realizing the optimization of the organization in this scheme, the blank after die forging is extruded to obtain a round bar with a diameter of 600-800 mm, and the extrusion process can achieve the following effects due to its unique strong three-dimensional stress state: First, complete defect welding: effectively welding the micro-porosity and holes that may still exist after the ingot and die forging; Second, deep organization crushing: fully crushing the as-cast dendrites, coarse second phase and compounds to make them dispersed; Third, form ideal streamline: obtain uniform extrusion organization with continuous streamline, high density and grain refinement; This process provides an extremely excellent blank for the subsequent forging, and fundamentally eliminates most microstructure obstacles leading to the decrease of fatigue performance; Then, through multi-fire free forging and die forging, the extruded bar is used as a blank, and finally a die forging part with complex ribs is formed through multi-fire free forging and die forging.

[0026] Among them, the ingot needs to be removed from the car to remove the surface segregation layer, and the head and tail shrinkage and impurity-rich parts are cut off to ensure the internal quality of the blank.

[0027] Among them, the die preheating temperature in the die forging process is accurately controlled at 390-420℃, and the purpose is to reduce the quenching effect on the surface of the forging, ensure smooth metal flow and full filling, and at the same time avoid cracking or uneven organization caused by large temperature difference.

[0028] The material basis of the application is a high-purity, micro-alloyed Al-Zn-Mg-Cu alloy. The key component design is as follows: Strict control of impurities (Si≤0.06%, Fe≤0.08%) to minimize the formation of coarse brittle impurity phases (such as FeAl3, α(AlFeSi) and the like). These brittle phases are extremely prone to becoming the initiation core of fatigue cracks under alternating loads, and strict control of them is a prerequisite for achieving high "tension-compression" fatigue limits (≥200MPa).

[0029] The main strengthening elements (Cu: 1.7~2.3%, Mg: 1.5~2.3%, Zn: 7.2~8.5%) are proportioned within a certain range to ensure that the alloy can form a sufficient number of nanoscale age precipitation strengthening phases after subsequent T74 aging, thereby obtaining a high strength basis (yield strength ≥515MPa).

[0030] The grain structure control element (Zr: 0.09~0.12%) will form a dispersed distribution of Al3Zr nanoparticles during homogenization and hot working. These particles can strongly pin grain boundaries and subgrain boundaries, effectively inhibiting the recrystallization process and grain growth during hot working and solid solution, promoting the formation of unrecrystallized fibrous grain structure, thereby improving the fracture toughness and fatigue crack propagation resistance of the material.

[0031] The grain refining element (Ti: 0.02~0.04%) acts as a nucleating agent during casting, refining the as-cast grains, and helps to obtain a uniform, fine initial structure, laying a good foundation for subsequent processing.

[0032] To achieve a balance between high strength and high fatigue performance, the present invention also configures a set of heat treatment system coordinated with the above processing path: Two-stage solid solution treatment: using a stepwise temperature solid solution system of 440~460℃ / 3~6h + 470~480℃ / 5~8h.

[0033] The first stage is lower temperature holding, which aims to fully dissolve the low melting point eutectic phase (such as η phase), while preventing overburning.

[0034] The second stage is higher temperature holding, which aims to maximize the solid solubility of strengthening elements (Zn, Mg, Cu) in the aluminum matrix, preparing for subsequent age precipitation, thereby obtaining higher strength.

[0035] Precise control of warm water quenching: quenching is a key link between solid solution and aging, and its parameters are crucial.

[0036] Quenching water temperature (50~60℃), this temperature interval is the key to balance the "cooling intensity" and "residual stress". It can provide a cooling rate sufficient to suppress the grain boundary coarse balance phase out, while the thermal stress and organizational stress generated by room temperature water quenching (~20℃) is greatly reduced, thereby effectively reducing the harmful residual internal stress, which is extremely beneficial to improve the fatigue performance.

[0037] Strict quenching delay (within 10~13 seconds) ensures that the forging from the furnace to the complete immersion in quenching medium is extremely short, preventing excessive temperature drop during the transfer process, resulting in preferential precipitation of strengthening phase at grain boundaries, affecting the final performance.

[0038] Strict water temperature control (temperature rise ≤5℃), through the circulating system to ensure the uniformity of water temperature in the quenching tank, avoid the cooling capacity due to local water temperature too high, to ensure the uniformity of performance from the surface to the core of large cross-section components.

[0039] T74 overaging treatment: using 110~120℃ / 5~7h + 145~155℃ / 14~22h double-stage aging system, this system makes the strengthening phase moderately coarsening, and forms a widened unprecipitation zone at the grain boundary. In this way, both high strength (yield strength ≥515MPa) and significantly improved stress corrosion resistance and fatigue resistance of the alloy are maintained, and the best balance of strength, toughness and fatigue limit is ultimately achieved.

[0040] Through the above systematic design and control of composition, processing and heat treatment, the large-scale Al-Zn-Mg-Cu alloy structural parts prepared by the method of the present application can stably achieve the following excellent comprehensive performance: Super high fatigue resistance: under the condition of Kt=1, R=-1, the 1000 million cycle "tension-compression" fatigue limit is not less than 200MPa.

[0041] High strength and high toughness: excellent tensile strength and plasticity at room temperature, and the fracture toughness in L-T and T-L directions are maintained at a high level (K IC ≥25.0 MPa•m 1 / 2 ), meeting the design requirements of damage tolerance of aviation components.

[0042] Excellent performance uniformity: thanks to the whole process organization control from ingot to forming, the components show good and consistent mechanical properties in different orientations.

[0043] In summary, compared with the prior art, the beneficial effects of the present application are: The application innovatively proposes a "semi-continuous ingot-cold forging deformation-extrusion-forging-cold forging" composite technical route, which can thoroughly break the as-cast structure, weld internal defects, introduce a large deformation amount of cold forging billet and an extrusion process with strong three-dimensional compressive stress, realize deep organizational transformation of large-scale round ingots, effectively weld the internal porosity and holes of the ingot, break the coarse second phase and dendritic structure, and fundamentally eliminate or reduce the source of fatigue crack initiation.

[0044] Through the extrusion process, a fine-grained extruded rod with highly dense and uniform flow lines is obtained. This high-quality rod is used as the billet for subsequent free forging and cold forging, ensuring that the overall microstructure of the component remains highly uniform and dense even after multiple forging, laying a solid organizational foundation for high and stable fatigue performance.

[0045] The large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural part prepared by the application has highly uniform microstructure, strength and toughness, and can stably achieve the harsh technical requirement of "tension-compression" fatigue limit ≥200MPa under the condition of Kt=1, R=-1, 1000 million cycles, meeting the requirements of large-scale aviation components with ultra-high fatigue performance. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the application will be described below in a clear and complete manner. Obviously, the described embodiments are only a part of the embodiments of the application, not all. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0047] The conventional large-scale Al-Zn-Mg-Cu alloy structural part and its preparation process in the prior art cannot meet the harsh index of "tension-compression" fatigue limit ≥200MPa under the condition of Kt=1, R=-1, 1000 million cycles while ensuring high strength for large-scale helicopter key components. Therefore, it is urgent to develop a new comprehensive preparation method throughout the whole process of alloy preparation, thermal deformation and heat treatment, to fundamentally realize the perfect combination of high strength and high fatigue resistance of the material by precisely controlling the microstructure of the alloy, especially the grain morphology, precipitate size and distribution, and residual stress state.

[0048] Embodiment 1 The exemplary embodiments provide a preparation method of a large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural part and performance analysis of the large-scale Al-Zn-Mg-Cu alloy structural part prepared by the method.

[0049] I. Raw material preparation and melting and casting

[0050] Batching: The raw materials such as high-purity aluminum ingot, cathode copper, high-purity magnesium ingot, zinc ingot, Al-Zr intermediate alloy, Al-Ti intermediate alloy, etc. were used to batch according to the elemental mass percentage shown in Table 1.

[0051] Table 1 Chemical composition of alloy in Example 1 (mass percentage, wt%) Melting and casting: The prepared raw materials were melted in a melting furnace at 740℃±10℃, after refining and degassing, the melt purification equipment combined with in-furnace refining, online refining and multiple filtration was used for melt purification, and semi-continuous casting process was adopted to cast into a circular ingot with a diameter of φ880mm, then homogenization heat treatment was carried out with a system of 400℃ / 10h+468℃ / 50h. The length of the ingot was 2200mm. The carbody treatment was carried out on the ingot to completely remove the surface segregation layer, and the top and bottom parts with poor metallurgical quality were cut off, finally the high-quality ingot billet with a length of 2000mm was obtained.

[0052] II. Hot mechanical working

[0053] Die forging: The above-mentioned circular ingot was heated to 420℃ and kept for 20 hours. Then die forging was carried out on a large die forging press to deform the φ880mm ingot into a cylindrical forging with a diameter of φ1400mm and a height of 790mm.

[0054] Extrusion forming: The cylindrical forging was heated to 430℃ and kept for 8 hours. Then it was extruded into a round bar with a diameter of φ710mm on a extrusion press with a capacity of more than 10,000 tons. After cutting off the extrusion residue, an extruded bar with a length of about 1900mm was obtained.

[0055] Free forging: The extruded bar was heated to 400℃ and forged into a rectangular forging blank with a thickness of about 120mm through multiple heating times of free forging.

[0056] Die forging: The free forging blank was heated to 390℃, and under the condition that the die preheating temperature was 400℃, the large die forging with a rib height of 110mm and a web thickness of 56mm was finally formed through multiple heating times of die forging.

[0057] III. Heat treatment

[0058] Solution treatment: The die forging was placed in a large air-circulating solution furnace for two-stage solution treatment: first at 450℃ for 4 hours, then at 475℃ for 6 hours.

[0059] Quenching: After the end of solid solution, the die forgings are quickly transferred to a vertical quenching furnace. The quenching transfer time is strictly controlled to be 12 seconds. The quenching medium is water, the water temperature is set to 55°C, and the water temperature is uniformly ensured by a strong circulation device. After the die forgings enter the water, the maximum water temperature at the end of quenching is 58°C, and the temperature rise is controlled within 3°C.

[0060] Cold pressing: After solid solution and quenching, cold pressing with a deformation rate of 3% is performed to reduce residual stress.

[0061] Aging treatment: The quenched die forgings are placed in an aging furnace to perform a T74 two-stage aging system: first, heat preservation at 115°C for 6 hours, and then heat preservation at 150°C for 18 hours. After treatment, air cooling to room temperature.

[0062] After aging treatment, large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural parts are obtained, and the performance of the parts is detected.

[0063] From the die forgings prepared by the above complete process, samples are taken in different directions (L, LT, ST) according to the standard, and mechanical property tests are performed, and the results are as follows: Fatigue performance: Under the conditions of stress concentration coefficient Kt = 1 and stress ratio R = -1, axial tension-compression high-cycle fatigue test is performed. The test results show that the 100 million cycle "tension-compression" fatigue limit of the component reaches 220 MPa, meeting and exceeding the technical requirement of ≥200 MPa.

[0064] Room temperature tensile properties: The test results are shown in Table 2, which fully meet the expected indicators.

[0065] Table 2 Room temperature tensile properties of the die forgings prepared in Example 1 Fracture toughness: The test results are shown in Table 3, which shows excellent damage tolerance performance.

[0066] Table 3 Fracture toughness (KIC) of the die forgings prepared in Example 1 Example 2 The exemplary embodiments provide a preparation method of large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural parts and performance analysis of large-scale Al-Zn-Mg-Cu alloy structural parts prepared by the method.

[0067] I. Raw material preparation and melting and casting.

[0068] Batching: The elements are batched according to the mass percentage content shown in Table 4, and within the reasonable range defined in the present application, the alloying element content is appropriately increased to enhance the strength potential.

[0069] Table 4 Alloy chemical composition in Example 2 (mass percentage, wt%) Melting and casting: The prepared raw materials were melted in a melting furnace at 745℃±10℃, the melt purification equipment combined with in-furnace refining, online refining and multiple filtration was used for melt purification, and semi-continuous casting process was adopted to cast a round ingot with a diameter of φ900mm. Then, homogenization heat treatment was carried out with a system of 400℃ / 10h+468℃ / 50h. The length of the ingot was 2250mm. The car treatment was carried out on the ingot to completely remove the surface segregation layer, and the top and bottom parts with poor metallurgical quality were cut off, and finally a high-quality ingot blank with a length of 1950mm was obtained.

[0070] II. Hot mechanical working.

[0071] Die forging blanking: the above-mentioned round ingot was heated to 425℃ and kept for 18 hours. Die forging was carried out on a large die forging press to deform the φ900mm ingot into a cylindrical forged piece with a diameter of φ1450mm and a height of 750mm.

[0072] Extrusion forming: the forged piece was heated to 435℃ and kept for 7 hours. Then, it was extruded into a round bar with a diameter of φ650mm on a ten-thousand-ton extruder or above. After cutting off the extrusion residue, an extruded bar with a length of about 2300mm was obtained.

[0073] Free forging: the extruded bar was heated to 400℃ and forged into a rectangular blank with a thickness of about 110mm through multiple free forging.

[0074] Die forging forming: the free forging blank was heated to 390℃, and under the condition that the die preheating temperature was 390℃, the blank was die forged into a die forged piece with a rib height of 105mm and a web thickness of about 50mm through multiple die forging.

[0075] III. Heat treatment.

[0076] Solution treatment: the die forged piece was placed in a large air-circulating solution furnace for two-stage solution treatment: first, it was kept at 450℃ for 5 hours, and then it was heated to 475℃ for 7 hours.

[0077] Quenching: after the solution treatment, the die forged piece was quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled to be 11 seconds. The quenching medium was water, the water temperature was set to 52℃, and the water temperature was uniform through a strong circulation device. After the die forged piece entered the water, the maximum water temperature at the end of quenching was 56℃, and the temperature rise was controlled within 3℃.

[0078] Cold pressing: after solution quenching, cold pressing with a deformation rate of 3% was carried out to reduce residual stress.

[0079] Ageing treatment: the quenched die forgings were put into an ageing furnace to perform a T74 two-stage ageing regime: first at 115°C for 6 hours, then at 150°C for 14 hours. After the treatment, air cooling to room temperature was performed.

[0080] After the ageing treatment, the large-size high fatigue resistance Al-Zn-Mg-Cu alloy structural member was obtained, and its performance was detected.

[0081] From the die forgings prepared by the complete process, samples were taken in different directions (L, LT, ST) according to the standard, and mechanical property tests were performed, and the results were as follows: Fatigue performance: under the conditions of stress concentration coefficient Kt = 1 and stress ratio R = -1, axial tensile-compressive high-cycle fatigue tests were performed. The test results showed that the 1000 million cycle tensile-compressive fatigue limit of the member reached 224 MPa, meeting and exceeding the technical requirement of ≥ 200 MPa.

[0082] Room temperature tensile properties: the test results are shown in Table 5, and it can be seen that the strength is higher than that of Example 1, and the elongation is slightly reduced.

[0083] Table 5 Room temperature tensile properties of the die forgings prepared in Example 2 Fracture toughness: the test results are shown in Table 6, which is slightly lower than that of Example 1, but still higher than the minimum requirement.

[0084] Table 6 Fracture toughness (K IC ) Example 3 The exemplary embodiment provides a preparation method of a large-size high fatigue resistance Al-Zn-Mg-Cu alloy structural member and performance analysis of the large-size Al-Zn-Mg-Cu alloy structural member prepared thereby.

[0085] I. Raw material preparation and melting and casting.

[0086] Batching: the elements were batched according to the mass percentage content shown in Table 7, and within the reasonable range defined in the present application, the alloying element content was appropriately reduced to optimize the toughness and plasticity.

[0087] Table 7 Chemical composition of the alloy in Example 3 (mass percentage, wt%) Melting, casting: The prepared raw materials are melted in a melting furnace at 735°C ± 10°C, after refining and degassing, the multi-stage melt purification equipment combining in-furnace refining, on-line refining and multiple filtration is used for melt purification, the semi-continuous casting process is adopted to cast a round ingot with a diameter of φ850 mm, then homogenization heat treatment is carried out, the system is 400°C / 10h + 468°C / 50h. The length of the ingot is 2300 mm. The carbody treatment is carried out on the ingot to completely remove the surface segregation layer, and the top and bottom parts with poor metallurgical quality are cut off, finally the high-quality ingot billet with a length of 2050 mm is obtained.

[0088] II. Hot mechanical working.

[0089] Die forging billet: the above-mentioned round ingot is heated to 415°C and kept for 22 hours. Die forging is carried out on a large die forging press to deform the φ850 mm ingot into a cylindrical forged piece with a diameter of φ1350 mm and a height of 800 mm.

[0090] Extrusion forming: the forged piece is heated to 425°C and kept for 9 hours. Then it is extruded into a round bar with a diameter of φ750 mm on a ten-thousand-ton or more extruder. After cutting off the extrusion residue, an extruded bar with a length of about 1600 mm is obtained.

[0091] Free forging: the extruded bar is heated to 400°C and forged into a rectangular forged billet with a thickness of about 140 mm through multiple fire times of free forging.

[0092] Die forging forming: the free forging billet is heated to 390°C, and under the condition that the die preheating temperature is 410°C, the forged billet is die forged into a die forged piece with a rib height of 115 mm and a web thickness of about 55 mm through multiple fire times of die forging.

[0093] III. Heat treatment.

[0094] Solution treatment: the die forged piece is placed in a large air-circulating solution furnace for two-stage solution treatment: first at 450°C for 4 hours, then at 475°C for 6 hours.

[0095] Quenching: after the solution treatment is completed, the die forged piece is quickly transferred to a vertical quenching furnace. The quenching transfer time is strictly controlled to be 12 seconds. The quenching medium is water, the water temperature is set to 58°C, and the water temperature is uniformly ensured by a strong circulation device. After the die forged piece enters the water, the maximum water temperature at the end of quenching is 61°C, and the temperature rise is controlled within 3°C.

[0096] Cold pressing: after solution quenching, cold pressing with a deformation rate of 3% is carried out to reduce residual stress.

[0097] Aging treatment: The quenched die forgings were put into an aging furnace to perform a T74 two-stage aging regime: first at 115°C for 6 hours, then at 150°C for 22 hours. By setting the second stage aging time as the upper limit of the range (22h), the moderate coarsening of strengthening phase and the optimization of the characteristics of the precipitate-free zone near the grain boundary were promoted, which significantly improved the fracture toughness and elongation of the material while sacrificing a small amount of strength.

[0098] After aging treatment, large-scale Al-Zn-Mg-Cu alloy structural parts with high fatigue resistance were obtained, and their performance was detected.

[0099] From the die forgings prepared by the above complete process, samples were taken in different directions (L, LT, ST) according to the standard, and mechanical property tests were conducted, with the following results: Fatigue performance: Under the conditions of stress concentration coefficient Kt = 1 and stress ratio R = -1, axial tensile-compressive high-cycle fatigue test was conducted. The test results showed that the 100 million cycle fatigue limit of the component reached 210 MPa, meeting and exceeding the technical requirement of ≥200 MPa.

[0100] Room temperature tensile properties: The test results are shown in Table 8, which shows that the strength is slightly lower than that of Example 1, while the elongation is significantly improved.

[0101] Table 8 Room temperature tensile properties of die forgings prepared in Example 3 Fracture toughness: The test results are shown in Table 9, which are significantly higher than those of Example 1 and Example 2, showing the best damage tolerance performance.

[0102] Table 9 Fracture toughness (KIC) of die forgings prepared in Example 3 Comparative Example 1 This comparative example uses the "path 1" mentioned in the background art, i.e. the traditional process of "ingot casting-hot rolling-forging", to prepare die forgings of the same size, for comparison with the method of the present application.

[0103] I. Raw material preparation and melting and casting.

[0104] Batching: The same alloy chemical composition (see Table 1) as in Example 1 was used for batching and melting.

[0105] Melting and casting: The prepared raw materials were melted in a melting furnace at 740°C ± 10°C, and after refining and degassing, a multi-stage melt purification device combining in-furnace refining, online refining and multiple filtration was used for melt purification. A semi-continuous casting method was used to prepare a flat ingot with a thickness of 440 mm and a width of 1850 mm. After homogenization heat treatment, the ingot was cut off at the head and tail and the surface was milled. After milling, the thickness of the ingot was 400 mm and the width was 1820 mm.

[0106] II. Hot mechanical working.

[0107] Hot rolling: The milled ingot was heated to 420°C and held for 6 hours. Then the ingot was rolled from 400 mm thick to 160 mm thick plate on a hot rolling mill, and the head, tail and edge were cut.

[0108] Forging: The hot rolled plate was used as a blank and subjected to 4 heating times of free forging, followed by 3 heating times of die forging. Finally, a die forging piece with a similar specification to Example 1 was prepared, with a rib height of about 110 mm and a web thickness of 55 mm. The die forging mold temperature was also controlled at 400°C.

[0109] III. Heat treatment.

[0110] For fair comparison, the same heat treatment process as Example 1 was used in this comparative example.

[0111] Solution treatment: The die forging piece was placed in a large air-circulating solution furnace and subjected to two-stage solution treatment: first at 450°C for 4 hours, then at 475°C for 6 hours.

[0112] Quenching: After solution treatment, the die forging piece was quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled at 12 seconds. The quenching medium was water, with a water temperature set at 55°C and a strong circulation device to ensure uniform water temperature. After the die forging piece was put into the water, the maximum water temperature at the end of quenching was 58°C, with a temperature rise controlled within 3°C.

[0113] Cold pressing: After solution and quenching, cold pressing was performed with a deformation rate of 3% to reduce residual stress.

[0114] Ageing treatment: The quenched die forging piece was placed in an ageing furnace and subjected to a T74 two-stage ageing regime: first at 115°C for 6 hours, then at 150°C for 18 hours. After treatment, it was air cooled to room temperature.

[0115] After ageing treatment, a large-scale Al-Zn-Mg-Cu alloy structural piece was obtained, and its performance was detected.

[0116] From the die forging piece prepared by the above complete process, samples were taken in different directions (L, LT, ST) according to the standard for mechanical property testing, and the results were as follows: Fatigue performance: axial tension-compression high-cycle fatigue test was conducted under the condition of stress concentration factor Kt = 1 and stress ratio R = -1. The test results show that the 100 million cycle "tension-compression" fatigue limit of the component reaches 194 MPa, which fails to meet the index requirement of 200 MPa.

[0117] Room temperature tensile and fracture toughness: the test results (see Tables 10 and 11) show that the strength (tensile strength, yield strength), elongation and fracture toughness are equivalent to those of Example 1, and are at an excellent level.

[0118] Table 10 Comparison of room temperature tensile properties of Comparative Example 1 and Example 1 Table 11 Comparison of fracture toughness of Comparative Example 1 and Example 1 Result analysis: The die forgings prepared by the method of Comparative Example 1 have the same chemical composition and final heat treatment system as Example 1, and their conventional static mechanical properties (strength, plasticity and toughness) are equivalent. However, their fatigue performance fails to meet the standard. This fully proves that the "ingot- hot rolling-forging" path has inherent shortcomings: although hot rolling provides large deformation, its two-dimensional extension deformation mode has limited ability to weld certain oriented shrinkage holes and three-dimensional defects in the center of the ingot; at the same time, the subsequent forging heating times and deformation are relatively insufficient, and it is difficult to completely break and heal all the inherent micro defects in the as-cast structure. These small defects become the source of fatigue cracks under alternating load, resulting in a decrease in the ultimate bearing capacity of the component under high-cycle fatigue conditions. This result indirectly confirms the necessity and superiority of the introduction of the "die forging-extrusion" path in the present application for improving fatigue performance.

[0119] Comparative Example 2 This comparative example uses the "path 2" mentioned in the background art, i.e. the traditional "ingot-direct forging" process, to prepare die forgings of the same specification, in order to form a comparison with the method of the present application.

[0120] I. Raw material preparation and melting and casting.

[0121] Batching: the same alloy chemical composition (see Table 1) as Example 1 was used for batching and melting.

[0122] Melting and casting: The prepared raw materials were melted in a melting furnace at 740°C ± 10°C, and after refining and degassing, a multi-stage melt purification device combining in-furnace refining, online refining and multiple filtration was used for melt purification. A semi-continuous casting method was used to prepare a flat ingot with a thickness of 440 mm and a width of 1850 mm. After homogenization heat treatment, the ingot was cut off at the head and tail and the surface was milled. After milling, the thickness of the ingot was 400 mm and the width was 1820 mm.

[0123] II. Hot mechanical working.

[0124] Direct forging: The milled ingot was subjected to 6 heating times of free forging, and was forged to a thickness of about 160 mm.

[0125] Die forging: The free forging blank was subjected to 3 heating times of die forging, and finally a die forging piece with a similar specification to Example 1 was prepared, with a rib height of about 110 mm and a web thickness of 56 mm. The die forging mold temperature was also controlled at 400°C.

[0126] III. Heat treatment.

[0127] For fair comparison, the same heat treatment process as Example 1 was used in this comparative example.

[0128] Solution treatment: The die forging piece was placed in a large air-circulating solution furnace and subjected to two-stage solution treatment: first at 450°C for 4 hours, then at 475°C for 6 hours.

[0129] Quenching: After solution treatment, the die forging piece was quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled at 12 seconds. The quenching medium was water, with a water temperature set at 55°C and a strong circulation device to ensure uniform water temperature. After the die forging piece was put into the water, the maximum water temperature at the end of quenching was 58°C, with a temperature rise controlled within 3°C.

[0130] Cold pressing: After solution quenching, cold pressing was performed with a deformation rate of 3% to reduce residual stress.

[0131] Ageing treatment: The quenched die forging piece was placed in an ageing furnace and subjected to a T74 two-stage ageing regime: first at 115°C for 6 hours, then at 150°C for 18 hours. After treatment, it was air-cooled to room temperature.

[0132] After ageing treatment, a large-scale Al-Zn-Mg-Cu alloy structural piece was obtained, and its performance was detected.

[0133] From the die forging piece prepared by the above complete process, samples were taken in different directions (L, LT, ST) according to the standard for mechanical property testing, and the results were as follows: Fatigue limit performance: axial tension-compression high cycle fatigue test was conducted under the condition of stress concentration factor Kt = 1 and stress ratio R = -1. The test results show that the 100 million cycle fatigue limit of the component reaches 182 MPa, which fails to reach the index requirement of 200 MPa.

[0134] Room temperature tensile and fracture toughness: the test results (see Tables 12 and 13) show that the strength (tensile strength, yield strength) and elongation are equivalent to those of Example 1, but the fracture toughness decreases slightly.

[0135] Table 12 Comparison of room temperature tensile properties of Comparative Example 2 and Example 1 Table 13 Comparison of fracture toughness of Comparative Example 2 and Example 1 Result analysis: The die forgings prepared by the method of Comparative Example 2, although having the same chemical composition and final heat treatment system as Example 1, have obvious decrease in performance, especially fatigue performance and fracture toughness. The reason is that the "ingot-direct forging" technical path lacks an intermediate blanking process (such as extrusion) with high strength and large deformation, so the deformation of direct forging cannot fully penetrate to the center of the large cross-section ingot. As a result, the metallurgical defects (such as composition segregation and coarse second phase) in the center of the ingot and the coarse grains cannot be fully broken and homogenized, and these unimproved organizational defects seriously deteriorate the dynamic performance (fatigue) and damage tolerance performance (fracture toughness) of the material. This comparative example further highlights the necessity of introducing the "die forging-extrusion" path in the present application to obtain overall excellent performance, especially ultra-high fatigue limit.

[0136] The fatigue limit performance of the examples of the present application is significantly improved compared with the comparative examples. In order to explain the core of the superior high fatigue resistance performance of the present application from the physical essence (total deformation), the total deformation rates of the three examples and two comparative examples are counted and calculated. In order to scientifically represent the total deformation rate, the cumulative true strain (ε) is used as the quantitative index. True strain is more capable of reflecting the degree of internal organizational change of the material, and its calculation formula is: ε = ln (initial height / final height) or equivalent strain. For multiple types of cumulative deformation, the total true strain can be approximately equal to the sum of the true strains of each main deformation step.

[0137] Table 14 True strain statistical analysis of the deformation path of the examples of the present application Table 15 True strain statistical analysis of the traditional deformation path of the comparative examples The main purpose of the subsequent die forging pass of Comparative Example 1 is shape forming, which causes a further thickness reduction in the thickness direction far less than the free forging step in the present application, and thus its additional true strain contribution is small.

[0138] The core index of the component prepared by the method of the present application, i.e., the "tension-compression" fatigue limit under the condition of Kt=1, R=-1, reaches 200 MPa or more, meeting the stringent requirements of the new generation of large-scale helicopters for key structural components.

[0139] In contrast, the fatigue limit of Comparative Example 1 (hot rolling-forging path) is only 194 MPa, and that of Comparative Example 2 (direct forging path) is as low as 182 MPa, both of which fail to meet the requirements. This fully proves that the present application successfully solves the 200 MPa fatigue performance bottleneck that is difficult to overcome by traditional methods.

[0140] The method of the present application innovatively introduces "die forging-extrusion-free forging" as the core deformation chain, so that the cumulative true strain (Σε) of the material in the entire processing process reaches 4.8 to 5.4, which is more than 3 times that of the traditional path (Σε<1.5) of the comparative examples. This huge, multi-directional deformation, combined with the unique three-dimensional compressive stress state of the extrusion process, can effectively weld the inherent loose, hole and other three-dimensional defects inside the ingot, and completely break the coarse as-cast structure and the second phase. This maximizes the elimination of micro-crack initiation sources that lead to fatigue failure, thereby laying the most solid organizational foundation for achieving ultra-high fatigue limit.

[0141] The method of the present application does not sacrifice other properties in exchange for high fatigue performance. As shown in the examples, while achieving ultra-high fatigue limit, the component still maintains high strength (tensile strength ≥ 560 MPa), good plasticity (elongation ≥ 10.5%) and excellent fracture toughness (K IC ≥ 26.5 MPa·m 1 / 2 ), achieving a perfect balance of strength, toughness, plasticity and fatigue resistance.

[0142] Comparative Example 2 shows that even under the condition of comparable static performance, the fatigue performance and fracture toughness of the traditional path are often short boards. The present application effectively avoids this problem through organizational homogenization.

[0143] The method of the present application obtains a highly uniform and dense preform through extrusion, providing the best quality raw material for subsequent free forging and die forging. This significantly reduces the risk of defects such as organizational inhomogeneity and discontinuous flow lines in the final component, thereby improving the performance consistency and yield rate in mass production, and having good industrial application prospects.

[0144] In summary, the application solves the industry problem of insufficient fatigue performance of large-size Al-Zn-Mg-Cu alloy components caused by internal organizational defects by a new composite thermal mechanical processing path with high deformation, can prepare high-end aviation components with overall performance better than traditional methods, and has significant technical progress and great application value.

[0145] The above is only a specific embodiment of the application, but the protection scope of the application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the application, which should be covered within the protection scope of the application. Therefore, the protection scope of the application should be subject to the protection scope of the claims.

Claims

1. A method for producing a large-size high fatigue resistance Al-Zn-Mg-Cu alloy structural member, characterized by, It comprises the following steps: batching, smelting; casting: using semi-continuous casting process to prepare round ingot with diameter of 800-1000mm; die forging deformation: the round ingot is die forged to become cylindrical forgings with diameter of 1300-1500mm; extrusion: the cylindrical forgings are extruded into round bars with diameter of 600-800mm; free forging and die forging: the obtained extruded round bars are subjected to multiple times of free forging and die forging to obtain die forgings; solid solution quenching heat treatment; cold pressing to reduce residual stress; aging heat treatment.

2. The preparation method according to claim 1, wherein in the multiple times of free forging, the thickness of the forged blank obtained by forging is 100-150mm; in the multiple times of die forging, the rib height of the obtained die forgings is 100-120mm, and the web thickness is 45-60mm.

3. The preparation method according to claim 2, wherein in the multiple times of die forging, the die preheating temperature is 390-420℃.

4. The preparation method according to claim 1, wherein the mass percentage of elements of the Al-Zn-Mg-Cu alloy structural member is: Si≤0.06%, Fe≤0.08%, Zn: 7.2-8.5%, Cu: 1.7-2.3%, Mg: 1.5-2.3%, Zr: 0.09-0.12%, Ti: 0.02-0.04%, and the balance is Al and inevitable impurities.

5. The preparation method according to claim 1, wherein in the solid solution quenching heat treatment, the die forgings are first kept at 440-460℃ for 3-6h, then kept at 470-480℃ for 5-8h, and then quenched in a vertical quenching furnace, the quenching medium is water, the water temperature is controlled at 50-60℃, and the quenching transfer time is controlled at 10-13s; after the solid solution quenching, the cold pressing to reduce residual stress is performed at a deformation rate of 2-4%.

6. The preparation method according to claim 1, wherein in the aging heat treatment, the die forgings are first kept at 110-120℃ for 5-7h, and then kept at 145-155℃ for 14-22h.

7. The preparation method according to claim 1, wherein the round ingot prepared by the semi-continuous casting method is subjected to homogenization heat treatment, and the system is 400℃ / 10h+468℃ / 50h.

8. The preparation method according to claim 1, wherein after the homogenization heat treatment, the round ingot is subjected to wagon removal of segregation shell layer, and the diameter is 880-950mm; after the top and bottom of the round ingot are cut off, the length is 2000-2600mm.

9. The preparation method according to claim 5, wherein the water temperature after quenching is increased by no more than 5℃.

10. The preparation method according to claim 1, wherein the smelting uses multi-stage melt purification equipment combined with in-furnace refining, online refining and multiple filtration for melt purification. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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