A method for preparing a large-size Al-Zn-Mg-Cu alloy structural member with high fatigue resistance

Abstract

The application relates to the technical field of aluminum alloy processing and preparation, and discloses a preparation method of a large-size Al-Zn-Mg-Cu alloy structural member with high fatigue resistance, in which a circular ingot with a diameter of 800-1000 mm is prepared through casting, the circular ingot is die forged in die forging deformation to be deformed into a cylindrical forged piece with a diameter of 1300-1500 mm, the cylindrical forged piece is extruded into a round bar with a diameter of 600-800 mm in extrusion deformation, and then a high-performance aluminum alloy structural member is prepared through free forging, die forging and heat treatment. The application innovatively proposes a composite technical route of "semi-continuous ingot-die forging deformation-extrusion-free forging-die forging", can thoroughly break the as-cast structure and weld internal defects, introduces die forging with a large deformation amount and extrusion with strong three-way compressive stress, realizes deep structure modification of the large-size circular ingot, and fundamentally eliminates or reduces the source of fatigue crack initiation.

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 alternating loads with long time, high frequency and high stress amplitude during the service of the aircrafts such as helicopters. Therefore, the fatigue resistance of the material, especially the tension-compression fatigue limit under high stress ratio (R=-1), becomes the core index to determine 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 alloy is one of the preferred materials for manufacturing the above-mentioned key aviation structural pieces, because it has high specific strength, good toughness and processing performance. In order to balance 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 problem 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:

[0004] (1) Metallurgical defects of large-scale ingot: 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.

[0005] (2) Challenge of complex thermal deformation process: From the large ingot to the final die forging, a series of thermal deformation processes such as die 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.

[0006] (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.

[0007] 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):

[0008] Path 1 "ingot-hot rolling-forging" path: First, prepare a square ingot with a thickness of 400-500 mm by semi-continuous casting, then roll the ingot into a plate with a thickness of 150-170 mm by hot rolling, and finally form the final structural component by free forging and die forging.

[0009] 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 by multi-fire free forging and die forging.

[0010] 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. That is, although hot rolling can obtain uniform plate-like structure, the subsequent forging fire and deformation may be insufficient to completely break and heal the inherent micro-defects (such as micro-holes and composition segregation) in the as-cast structure, which are prone to become crack initiation sources under subsequent high-cycle fatigue load.

[0011] 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.

[0012] The above two technical paths not only fail to completely eliminate as-cast structure defects, but also are prone to leave 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 form stress concentration at the flow line turning point; Path 2 has no obvious flow line in the center due to insufficient deformation, and the structure density is poor. Such non-uniform structure will significantly reduce the overall fatigue performance of the material.

[0013] 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 target of 200MPa.

[0014] Therefore, there is an urgent need for a completely new technical path that can achieve more complete organization optimization and defect control. SUMMARY

[0015] 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.

[0016] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0017] A preparation method of a large-specification high-fatigue-resistance Al-Zn-Mg-Cu alloy structural part, comprising the following steps:

[0018] Batching and smelting;

[0019] Casting: using a semi-continuous casting process to prepare a circular ingot with a diameter of 800-1000mm;

[0020] Die forging deformation: the circular ingot is die forged to become a cylindrical forged piece with a diameter of 1300-1500mm;

[0021] Extrusion: extruding the cylindrical forging into a round bar with a diameter of 600-800 mm;

[0022] 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;

[0023] Solution quenching heat treatment;

[0024] Cold pressing to reduce residual stress;

[0025] Aging heat treatment.

[0026] Further, in the multiple heating times of free forging, the thickness of the forging blank obtained by forging is 100-150 mm;

[0027] In the multiple heating times of die forging, the rib height of the obtained die forging is 100-120 mm, and the web thickness is 45-60 mm.

[0028] Further, in the multiple heating times of die forging, the mold preheating temperature is 390-420℃.

[0029] Further, the mass percentage of elements of the Al-Zn-Mg-Cu alloy structural part 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.

[0030] Further, in the solution quenching heat treatment, the die forging is first heated at 440-460℃ for 3-6h, then heated at 470-480℃ for 5-8h, and then quenched in a vertical quenching furnace, with water as the quenching medium, the water temperature is controlled at 50-60℃, and the quenching transfer time is controlled at 10-13 seconds; after solution quenching, cold pressing is performed to reduce residual stress with a deformation rate of 2-4%.

[0031] Further, in the aging heat treatment, the die forging is first heated at 110-120℃ for 5-7h, and then heated at 145-155℃ for 14-22h.

[0032] Further, the round ingot prepared by the semi-continuous casting method is subjected to homogenization heat treatment, with a system of 400℃ / 10h+468℃ / 50h.

[0033] Further, after homogenization heat treatment, the round ingot is subjected to car removal of the segregation shell layer, with a diameter of 880-950mm; after cutting off the top and bottom of the round ingot, the length is 2000-2600mm.

[0034] Further, the water temperature after quenching increases by no more than 5℃.

[0035] Further, the smelting uses the multi-stage melt purification equipment combining in-furnace refining, on-line refining and multiple filtration to perform melt purification.

[0036] The application provides a preparation method of a large-scale helicopter key load-bearing structure part with super-high fatigue performance, excellent strength and matching toughness.

[0037] The core of the application is to break through the traditional technical path of "ingot casting-hot rolling-forging" or "ingot casting-direct forging", 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:

[0038] First, a large-scale circular ingot with a diameter of 800-1000 mm is prepared by semi-continuous casting to meet the metal quantity requirement of the large component;

[0039] The circular 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 the step is to:

[0040] Firstly, preliminary densification: closing the shrinkage and holes in the center of the ingot by large pressure;

[0041] Secondly, shape conversion: preparing a blank with suitable size and shape for the subsequent extrusion process;

[0042] Then, large deformation extrusion, which is the most critical link for realizing organization optimization in the scheme, is performed on the blank after die forging 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:

[0043] Firstly, complete defect welding: effectively welding the micro-porosity and holes that may still exist after the ingot and die forging;

[0044] Secondly, deep organization crushing: fully crushing the as-cast dendrites, coarse second phase and compounds to make them dispersed;

[0045] Thirdly, forming ideal streamline: obtaining uniform extrusion organization with continuous streamline, high density and refined grains;

[0046] The process provides a blank with extremely excellent organization for subsequent forging, and fundamentally eliminates most microstructure obstacles leading to the decrease of fatigue performance;

[0047] Then, the extruded bar is used as a blank to perform multi-fire free forging and die forging, and finally a die forging part with complex ribs is formed.

[0048] Wherein, the ingot needs to be removed from the surface segregation layer by car, and the head and tail shrinkage and enriched impurity part is cut off, to ensure the internal quality of the blank.

[0049] Wherein, the mold 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 forgings, ensure smooth metal flow and full filling, and at the same time avoid cracking or uneven organization caused by large temperature difference.

[0050] The material basis of the application is a high-purity, micro-alloyed Al-Zn-Mg-Cu alloy. The key component design is as follows:

[0051] Impurities (Si≤0.06%, Fe≤0.08%) are strictly controlled to minimize the formation of coarse brittle impurity phases (such as FeAl3, α(AlFeSi) and the like). These brittle phases are extremely prone to become the initiation core of fatigue cracks under alternating load, and strict control of them is a prerequisite for achieving high "tension-compression" fatigue limit (≥200MPa).

[0052] The main strengthening elements (Cu: 1.7~2.3%, Mg: 1.5~2.3%, Zn: 7.2~8.5%) are matched within a certain range, which ensures that the alloy can form a sufficient number of nanoscale aging precipitation strengthening phases after subsequent T74 aging, thereby obtaining a high strength basis (yield strength ≥515MPa).

[0053] 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 the grain boundaries and subgrain boundaries, effectively inhibit the recrystallization process and grain growth during hot working and solid solution process, and promote the formation of unrecrystallized fibrous grain structure, thereby improving the fracture toughness and fatigue crack propagation resistance of the material.

[0054] The grain refining element (Ti: 0.02~0.04%) acts as a nucleating agent during casting, refining the as-cast grains, and helping to obtain a uniform and fine initial structure, laying a good foundation for subsequent processing.

[0055] In order to achieve the balance between high strength and high fatigue performance, the application also configures a set of heat treatment system coordinated with the above processing path:

[0056] Two-stage solid solution treatment: using 440~460℃ / 3~6h +470~480℃ / 5~8h step-up solid solution system.

[0057] The first stage is lower temperature holding, which aims to make the low melting point eutectic phase (such as η phase) fully dissolved, while preventing overburning.

[0058] The second stage of higher temperature holding aims to maximize the solid solubility of the strengthening elements (Zn, Mg, Cu) in the aluminum matrix, to prepare for subsequent aging precipitation, and to obtain higher strength.

[0059] Precise control of warm water quenching: Quenching is a key link between solid solution and aging, and its parameters are crucial.

[0060] Quenching water temperature (50-60℃): This temperature range is the key to balancing "cooling intensity" and "residual stress". It can provide a cooling rate sufficient to suppress the precipitation of coarse grain boundary equilibrium phase, while significantly reducing thermal stress and microstructure stress generated by room temperature water quenching (~20℃), thereby effectively reducing harmful residual internal stress, which is extremely beneficial to improving fatigue performance.

[0061] Strict quenching delay (within 10-13 seconds) ensures that the time from furnace discharge to complete immersion in the quenching medium is extremely short, preventing excessive temperature drop during transfer and causing the strengthening phase to precipitate preferentially at the grain boundaries, affecting the final performance.

[0062] Strict water temperature control (temperature rise ≤5℃): Through the circulation system, the water temperature in the quenching tank is uniform, avoiding the decline of cooling capacity due to local high water temperature, and ensuring the uniformity of performance from the surface to the core of large cross-section components.

[0063] T74 overaging treatment: A two-stage aging system of 110-120℃ / 5-7h + 145-155℃ / 14-22h is adopted, which makes the strengthening phase moderately coarsened and forms a widened precipitation-free zone at the grain boundaries. This not only maintains high strength (yield strength ≥515MPa), but also significantly improves the stress corrosion resistance and fatigue resistance of the alloy, ultimately achieving the best balance between strength, toughness and fatigue limit.

[0064] 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:

[0065] Ultra-high fatigue resistance: Under the condition of Kt=1, R=-1, the 1000 million cycle "tension-compression" fatigue limit is not less than 200MPa.

[0066] High strength and high toughness: The room temperature tensile strength and plasticity are excellent, and the fracture toughness in L-T and T-L directions is maintained at a high level (K IC ≥25.0 MPa•m 1 / 2 ), meeting the design requirements of damage tolerance for aviation components.

[0067] Excellent performance uniformity: thanks to the full-process microstructure control from ingot to forming, the components exhibit good and consistent mechanical properties in different orientations.

[0068] In summary, compared with the prior art, the beneficial effects of the present application are:

[0069] The present application innovatively proposes a "semi-continuous casting ingot-die forging deformation-extrusion-free forging-die forging" composite technical route, which can completely break the as-cast structure, weld internal defects, introduce large deformation amount of die forging and extrusion process with strong three-dimensional compressive stress, realize deep microstructure modification of large-scale round ingot, effectively weld the internal porosity and holes of the ingot, break the coarse second phase and dendritic structure, and thus fundamentally eliminate or reduce the source of fatigue crack initiation.

[0070] Through the extrusion process, a fine-grained extruded rod with highly dense and uniform flow lines is obtained, which is used as the billet for subsequent free forging and die forging, ensuring that even after multiple forging, the overall component can still maintain a highly uniform and dense microstructure, laying a solid organizational foundation for high and stable fatigue performance.

[0071] The large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural part prepared by the present 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

[0072] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0073] The conventional large-scale Al-Zn-Mg-Cu alloy structural part and its preparation process in the prior art are difficult to meet the stringent index of "tension-compression" fatigue limit ≥200MPa under the condition of Kt=1, R=-1, 1000 million cycles while ensuring high strength, which is required for key components of a helicopter. Therefore, there is an urgent need to develop a completely 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.

[0074] Example 1

[0075] The present exemplary embodiment provides a preparation method of a large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural member and performance analysis of the large-scale Al-Zn-Mg-Cu alloy structural member prepared thereby.

[0076] I. Raw material preparation and melting and casting.

[0077] 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. are used to prepare the alloy according to the elemental mass percentage shown in Table 1.

[0078] Table 1 Chemical composition of the alloy in Example 1 (mass percentage, wt%)

[0079]

[0080] Melting and casting: The prepared raw materials are melted in a melting furnace at 740℃±10℃, and after refining and degassing, the melt purification equipment combining in-furnace refining, online 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 φ880mm, which is then subjected to homogenization heat treatment with a schedule of 400℃ / 10h+468℃ / 50h. The ingot length is 2200mm. The carbody treatment is performed on the ingot to completely remove the surface segregation layer, and the top and bottom parts with poor metallurgical quality are cut off, so that a high-quality ingot blank with a length of 2000mm is finally obtained.

[0081] II. Hot mechanical working.

[0082] Die forging blanking: the above-mentioned round ingot is heated to 420℃ and kept for 20 hours. Then, die forging is performed on a large die forging press to deform the ingot with a diameter of φ880mm into a cylindrical forged piece with a diameter of φ1400mm and a height of 790mm.

[0083] Extrusion forming: the cylindrical forged piece is heated to 430℃ and kept for 8 hours. Then, it is 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 is obtained.

[0084] Free forging: the extruded bar is heated to 400℃ and forged into a rectangular blank with a thickness of about 120mm through multiple fire times of free forging.

[0085] Die forging forming: the free forging blank is heated to 390℃, and under the condition that the die preheating temperature is 400℃, the large die forging piece with a rib height of 110mm and a web thickness of 56mm is finally formed through multiple fire times of die forging.

[0086] III. Heat treatment.

[0087] Solution treatment: The die forgings were placed in a large air-circulation solution furnace for two-stage solution treatment: first at 450°C for 4 hours, then at 475°C for 6 hours.

[0088] Quenching: After solution treatment, the die forgings were quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled at 12 seconds. The quenching medium was water, the water temperature was set at 55°C, and the water temperature was uniformized by a strong circulation device. After the die forgings entered the water, the maximum water temperature at the end of quenching was 58°C, and the temperature rise was controlled within 3°C.

[0089] Cold pressing: After solution quenching, cold pressing was performed to reduce residual stress with a deformation rate of 3%.

[0090] Ageing treatment: After quenching, the die forgings were placed in an ageing furnace to perform a T74 two-stage ageing regime: first at 115°C for 6 hours, then at 150°C for 18 hours. After treatment, air cooling was performed to room temperature.

[0091] After the ageing treatment, large-scale Al-Zn-Mg-Cu alloy structural parts with high fatigue resistance were obtained, and their performance was detected.

[0092] 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 performed, with the following results:

[0093] 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 component reached 220 MPa, meeting and exceeding the technical requirement of ≥200 MPa.

[0094] Room temperature tensile properties: The test results are shown in Table 2, which fully meet the expected indicators.

[0095] Table 2 Room temperature tensile properties of the die forgings prepared in Example 1

[0096]

[0097] Fracture toughness: The test results are shown in Table 3, showing excellent damage tolerance performance.

[0098] Table 3 Fracture toughness (KIC) of the die forgings prepared in Example 1

[0099]

[0100] Example 2

[0101] The present 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 by the method.

[0102] I. Raw material preparation and melting and casting.

[0103] Batching: the elements were 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 was appropriately increased to enhance the strength potential.

[0104] Table 4 Chemical composition of the alloy in Example 2 (mass percentage, wt%)

[0105]

[0106] 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 a semi-continuous casting process was used to cast a circular ingot with a diameter of φ900mm, followed by homogenization heat treatment with a system of 400℃ / 10h+468℃ / 50h. The ingot length was 2250mm. The car treatment was performed 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.

[0107] II. Hot mechanical processing.

[0108] Die forging blanking: the above-mentioned circular ingot was heated to 425℃ and kept for 18 hours. Die forging was performed 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.

[0109] Extrusion forming: the forged piece was heated to 435℃ and kept for 7 hours. Subsequently, it was extruded into a round bar with a diameter of φ650mm on a ten-thousand-ton or more extruder. After cutting off the extrusion residue, an extruded bar with a length of about 2300mm was obtained.

[0110] Free forging: the extruded bar was heated to 400℃, and through multiple fire times of free forging, it was forged into a rectangular forged blank with a thickness of about 110mm.

[0111] Die forging forming: the free forging blank was heated to 390℃, and under the condition that the die preheating temperature was 390℃, through multiple fire times of die forging, the forged blank was die forged into a die forged piece with a rib height of 105mm and a web thickness of about 50mm.

[0112] III. Heat treatment.

[0113] Solution treatment: The die forgings were put into a large air-circulation solution furnace for two-stage solution treatment: first at 450°C for 5 hours, then at 475°C for 7 hours.

[0114] Quenching: After the solution treatment, the die forgings were 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 be 52°C, and the water temperature was ensured to be uniform by a strong circulation device. After the die forgings were put into the water, the maximum water temperature at the end of quenching was 56°C, and the temperature rise was controlled to be within 3°C.

[0115] Cold pressing: After the solution quenching, cold pressing was performed to reduce the residual stress, with a deformation rate of 3%.

[0116] Aging treatment: After quenching, the die forgings were put into an aging furnace to perform a T74 two-stage aging system: first at 115°C for 6 hours, then at 150°C for 14 hours. After the treatment, the die forgings were air-cooled to room temperature.

[0117] After the aging treatment, large-scale high fatigue performance Al-Zn-Mg-Cu alloy structural parts were obtained, and their performance was detected.

[0118] 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 performed, with the following results:

[0119] Fatigue performance: Under the conditions of stress concentration coefficient Kt = 1 and stress ratio R = -1, axial tension-compression high-cycle fatigue tests were performed. The test results showed that the 1000 million cycle "tension-compression" fatigue limit of the component reached 224 MPa, meeting and exceeding the technical requirement of ≥200 MPa.

[0120] Room temperature tensile properties: The test results are shown in Table 5, which shows that the strength is higher than that of Example 1, while the elongation is slightly reduced.

[0121] Table 5 Room temperature tensile properties of die forgings prepared in Example 2

[0122]

[0123] 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.

[0124] Table 6 Fracture toughness (K IC )

[0125]

[0126] Example 3

[0127] The present exemplary embodiment provides a preparation method of 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 by the method.

[0128] I. Raw material preparation and melting and casting.

[0129] 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 toughness and plasticity.

[0130] Table 7 Chemical composition of the alloy in Example 3 (mass percentage, wt%)

[0131]

[0132] Melting and casting: the prepared raw materials were melted in a melting furnace at 735℃±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 the semi-continuous casting process was adopted to cast a circular ingot with a diameter of φ850mm, and then homogenization heat treatment was carried out with a system of 400℃ / 10h+468℃ / 50h. The length of the ingot was 2300mm. 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 the high-quality ingot billet with a length of 2050mm was obtained.

[0133] II. Hot mechanical processing.

[0134] Die forging: the above-mentioned circular ingot was heated to 415℃ and kept for 22 hours. Die forging was carried out on a large die forging press to deform the φ850mm ingot into a cylindrical forging with a diameter of φ1350mm and a height of 800mm.

[0135] Extrusion forming: the forging was heated to 425℃ and kept for 9 hours. Then, it was extruded into a round bar with a diameter of φ750mm on a ten-thousand-ton extruder or above. After cutting off the extrusion residue, an extruded bar with a length of about 1600mm was obtained.

[0136] Free forging: the extruded bar was heated to 400℃ and forged into a rectangular forging blank with a thickness of about 140mm through multiple fire times of free forging.

[0137] Die forging: the free forging blank was heated to 390℃, and under the condition that the die preheating temperature was 410℃, the forging blank was die forged into a die forging with a rib height of 115mm and a web thickness of about 55mm through multiple fire times of die forging.

[0138] III. Heat treatment.

[0139] Solution treatment: The die forgings were placed in a large air-circulation solution furnace for two-stage solution treatment: first at 450°C for 4 hours, then at 475°C for 6 hours.

[0140] Quenching: After the solution treatment, the die forgings were quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled at 12 seconds. The quenching medium was water, the water temperature was set at 58°C, and the water temperature was uniform through a strong circulation device. After the die forgings entered the water, the maximum water temperature at the end of quenching was 61°C, and the temperature rise was controlled within 3°C.

[0141] Cold pressing: After solution quenching, cold pressing was performed with a deformation rate of 3% to reduce residual stress.

[0142] Aging treatment: The quenched die forgings were placed in 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 holding time to the upper limit of the range (22h), the strengthening phase was promoted to moderate coarsening and the characteristics of the precipitate-free zone near the grain boundary were optimized, which significantly improved the fracture toughness and elongation of the material while sacrificing a small amount of strength.

[0143] After the aging treatment, large-scale high fatigue resistance Al-Zn-Mg-Cu alloy structural parts were obtained, and their performance was detected.

[0144] 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 performed, with the following results:

[0145] Fatigue performance: Under the conditions of stress concentration coefficient Kt=1 and stress ratio R=-1, axial tension-compression high-cycle fatigue tests were performed. The test results showed that the 1000 million cycle fatigue limit of the component reached 210 MPa, meeting and exceeding the technical requirement of ≥200 MPa.

[0146] 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.

[0147] Table 8 Room temperature tensile properties of die forgings prepared in Example 3

[0148]

[0149] 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.

[0150] Table 9 Fracture toughness (KIC) of die forgings prepared in Example 3

[0151]

[0152] Comparative Example 1

[0153] 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 the same size of die forgings for comparison with the method of the present application.

[0154] I. Raw material preparation and melting and casting.

[0155] Batching: The same alloy chemical composition (see Table 1) as in Example 1 was used for batching and melting.

[0156] Melting and casting: The prepared raw material was melted in a melting furnace at 740℃±10℃, and after refining and degassing, the melt purification equipment combining in-furnace refining, online refining and multiple filtration was used for melt purification. The semi-continuous casting method was used to prepare a flat ingot with a thickness of 440mm and a width of 1850mm. 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 400mm and the width was 1820mm.

[0157] II. Hot mechanical processing.

[0158] Hot rolling: The milled ingot was heated to 420℃ and held for 6 hours. Then the ingot was rolled on a hot rolling mill from 400mm thickness to 160mm thick plate, and the head and tail and edge were cut.

[0159] Forging: The hot rolled plate was used as a blank to perform free forging for 4 heats, and then die forging for 3 heats, finally preparing a die forging with a similar size to Example 1, with a rib height of about 110mm and a web thickness of 55mm. The die forging die temperature was also controlled at 400℃.

[0160] III. Heat treatment.

[0161] For fair comparison, the same heat treatment process as in Example 1 was used in this comparative example.

[0162] Solution treatment: The die forgings were 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.

[0163] Quenching: After solution treatment, the die forgings were quickly transferred to a vertical quenching furnace. The quenching transfer time was strictly controlled at 12 seconds. The quenching medium was water, the water temperature was set at 55℃, and the water temperature was uniform by a strong circulation device. After the die forgings entered the water, the maximum water temperature at the end of quenching was 58℃, and the temperature rise was controlled within 3℃.

[0164] Cold pressing: After solution and quenching, cold pressing was performed with a deformation rate of 3% to reduce residual stress.

[0165] 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 18 hours. After treatment, air cooling to room temperature.

[0166] After aging treatment, large-scale Al-Zn-Mg-Cu alloy structural parts were obtained, and their performance was detected.

[0167] From the die forgings prepared by the complete process described above, samples were taken in different directions (L, LT, ST) according to the standard, and mechanical property tests were performed, with the following results:

[0168] 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 component reached 194 MPa, which failed to meet the requirement of 200 MPa.

[0169] Room temperature tensile and fracture toughness: the test results (see Tables 10 and 11) showed that the strength (tensile strength, yield strength), elongation and fracture toughness were comparable to those of Example 1, and were at a good level.

[0170] Table 10 Comparison of room temperature tensile properties of Comparative Example 1 and Example 1

[0171]

[0172] Table 11 Comparison of fracture toughness of Comparative Example 1 and Example 1

[0173]

[0174] Result analysis:

[0175] The die forgings prepared by the method of Comparative Example 1 had the same chemical composition and final heat treatment regime as Example 1, and their conventional static mechanical properties (strength, plasticity, toughness) were comparable. However, their fatigue performance failed 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 orientations of shrinkage holes and other three-dimensional defects in the center of the ingot; at the same time, the subsequent forging passes 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 residual small defects become the source of fatigue crack initiation 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 introducing the "die forging-extrusion" path to improve fatigue performance.

[0176] Comparative Example 2

[0177] The comparative example uses the "path 2" mentioned in the background art, i.e. the traditional "ingot-cold forging" process to prepare the same specification of die forgings, to form a contrast with the method of the application.

[0178] I. Raw material preparation and melting and casting.

[0179] Batching: the same alloy chemical composition (see Table 1) as Example 1 is used for batching and melting.

[0180] Melting and casting: the prepared raw material is melted in a melting furnace at 740℃±10℃, after refining and degassing, the melt purification equipment combining in-furnace refining, online refining and multiple filtration is used for melt purification, and the semi-continuous casting method is used to prepare a flat ingot with a thickness of 440mm and a width of 1850mm. After homogenization heat treatment, the ingot is cut at the head and tail and the surface is milled. After milling, the thickness of the ingot is 400mm and the width is 1820mm.

[0181] II. Hot mechanical processing.

[0182] Direct forging: the milled ingot is subjected to 6 heating times of free forging, and is forged to a thickness of about 160mm.

[0183] Die forging: the free forging blank is subjected to 3 heating times of die forging, and finally a die forging piece with a rib height of about 110mm and a web thickness of 56mm is prepared. The die forging die temperature is also controlled at 400℃.

[0184] III. Heat treatment.

[0185] In order to make a fair comparison, the same heat treatment process as Example 1 is used in the comparative example.

[0186] Solution treatment: the die forging piece is 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.

[0187] Quenching: after solution treatment, the die forging piece is quickly transferred to a vertical quenching furnace. The quenching transfer time is strictly controlled at 12 seconds. The quenching medium is water, the water temperature is set at 55℃, and the water temperature is uniform through a strong circulation device. After the die forging piece enters the water, the maximum water temperature at the end of quenching is 58℃, and the temperature rise is controlled within 3℃.

[0188] Cold pressing: after solution and quenching, cold pressing is carried out with a deformation rate of 3% to reduce residual stress.

[0189] 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 18 hours. After treatment, air cooling to room temperature.

[0190] After aging treatment, large-scale Al-Zn-Mg-Cu alloy structural parts were obtained, and their performance was detected.

[0191] From the die forgings prepared by the complete process described above, samples were taken in different directions (L, LT, ST) according to the standard for mechanical property testing, and the results were as follows:

[0192] Fatigue limit performance: under the conditions of stress concentration coefficient Kt = 1 and stress ratio R = -1, axial tensile and compressive high-cycle fatigue tests were performed. The test results showed that the 1000 million cycle fatigue limit of the component reached 182 MPa, which failed to meet the requirement of 200 MPa.

[0193] Room temperature tensile and fracture toughness: the test results (see Tables 12 and 13) showed that the strength (tensile strength, yield strength) and elongation were comparable to those of Example 1, but the fracture toughness decreased slightly.

[0194] Table 12 Comparison of room temperature tensile properties of Comparative Example 2 and Example 1

[0195]

[0196] Table 13 Comparison of fracture toughness of Comparative Example 2 and Example 1

[0197]

[0198] Result analysis:

[0199] The die forgings prepared by the method of Comparative Example 2, although the chemical composition and final heat treatment regime were exactly the same as Example 1, but their performance, especially fatigue performance and fracture toughness, all showed a significant decrease. The reason is that the "ingot-direct forging" technical path lacks a high-strength, large-deformation intermediate billet-making process (such as extrusion), and the deformation of direct forging cannot fully penetrate to the center of the large-section ingot. As a result, the metallurgical defects (such as composition segregation, coarse second phase) in the center of the ingot and 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.

[0200] The fatigue limit performance of the embodiments of the present application is significantly improved compared with the comparative examples. In order to explain the core of the superior high fatigue performance of the present application from the physical essence (total deformation), the total deformation rates of three embodiments 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 a quantitative index. The 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 various and multiple cumulative deformations, the total true strain can be approximately equal to the sum of the true strains of each main deformation step.

[0201] Table 14 True strain statistical analysis of the deformation path of the embodiments of the present application

[0202]

[0203] Table 15 True strain statistical analysis of the deformation path of the comparative examples

[0204]

[0205] The main purpose of the subsequent die forging heat of Comparative Example 1 is shape forming, and the further thickness reduction caused by it is much smaller than the free forging step in the present application, so its additional true strain contribution is small.

[0206] The core index of the component prepared by the method of the present application, the "tension-compression" fatigue limit under the condition of Kt = 1, R = -1, is stably above 200 MPa, which meets the stringent requirements of the new generation of large-scale helicopters for key structural components.

[0207] 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 standard. This fully proves that the present application successfully solves the 200 MPa fatigue performance bottleneck that the traditional method cannot overcome.

[0208] 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 whole processing process is as high as 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, multidirectional deformation, combined with the unique three-dimensional compressive stress state of the extrusion process, can effectively weld the inherent porosity, holes 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 the micro-crack initiation source that leads to fatigue failure, thereby laying the most solid organizational foundation for achieving ultra-high fatigue limit.

[0209] The method of the present application does not sacrifice other performances for high fatigue performance. As shown in the examples, while obtaining an 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 ), realizing the perfect balance of strength, toughness, plasticity and fatigue resistance.

[0210] And Comparative Example 2 shows that the traditional path, even in the case of comparable static performance, its fatigue performance and fracture toughness are often short boards. The present application effectively avoids this problem through uniformization of the structure.

[0211] The method of the present application obtains a preform with highly uniform and dense structure by extrusion, providing the best quality raw material for subsequent free forging and die forging. This significantly reduces the risk of defects such as non-uniform structure and discontinuous streamline in the final component, thereby improving the consistency of performance and yield in mass production, and has good industrial application prospects.

[0212] In summary, the present application solves the industry problem of insufficient fatigue performance of large-scale Al-Zn-Mg-Cu alloy components caused by internal structural defects through a new and high-deformation composite thermal mechanical processing path, and can prepare high-end aviation components with comprehensive performance better than traditional methods. The technical progress is significant, and the application value is huge.

[0213] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, and should be covered within the protection scope of the present application. Therefore, the protection scope of the present 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, The method comprises the following steps: batching and smelting; casting: using a semi-continuous casting process to prepare a round ingot with a diameter of 800-1000 mm; die forging deformation: die forging the round ingot to deform into a cylindrical forging with a diameter of 1300-1500 mm; extrusion: extruding the cylindrical forging into a round bar with a diameter of 600-800 mm; free forging and die forging: obtaining a die forging by multiple times of free forging and die forging of the obtained extruded round bar; solid solution quenching heat treatment; cold pressing to reduce residual stress; aging heat treatment; wherein, the element mass percentage 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 unavoidable impurities; in the multiple times of free forging, the thickness of the forging blank obtained by forging is 100-150 mm; in the multiple times of die forging, the rib height of the obtained die forging is 100-120 mm, and the web thickness is 45-60 mm; in the solid solution quenching heat treatment, the die forging is 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 10-13 seconds; after the solid solution quenching, cold pressing is performed to reduce the residual stress with a deformation rate of 2-4%; in the aging heat treatment, the die forging is first kept at 110-120℃ for 5-7h, and then kept at 145-155℃ for 14-22h; the round ingot prepared by the semi-continuous casting method is subjected to homogenization heat treatment, and the system is 400℃ / 10h+468℃ / 50h.

2. The preparation method according to claim 1, wherein, in the multiple times of die forging, the die preheating temperature is 390-420℃.

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

4. The preparation method according to claim 1, the water temperature after quenching is increased by no more than 5℃.

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

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