Preparation method of corner aluminum cross arm and corner aluminum cross arm

Abstract

The application discloses a preparation method of an aluminum angle cross arm and the aluminum angle cross arm and relates to the field of cross arms, and comprises the following steps: S1, preparing an aluminum alloy melt composed of the following elements: Zn: 4-5 wt%, Mg: 1.2-2 wt%, Cu: 0.01-0.2 wt%, Mn: 0.2-0.5 wt%, Zr: 0.08-0.2 wt%, and the balance of Al and inevitable impurity elements; S2, casting the aluminum alloy melt to obtain an aluminum alloy ingot; S3, homogenizing the aluminum alloy ingot; S4, hot extruding the aluminum alloy ingot subjected to the homogenization treatment to form an aluminum angle cross arm blank with an angle-shaped cross section; and S5, subjecting the aluminum angle cross arm blank to solid solution treatment and aging treatment to obtain a high-strength corrosion-resistant aluminum angle cross arm. The aluminum angle cross arm prepared by the application has high strength and good corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of crossarms, and specifically to a method for preparing an angle aluminum crossarm and the angle aluminum crossarm itself. Background Technology

[0002] Transmission line towers are crucial infrastructure in power transmission systems. Crossarms, as load-bearing components within the tower structure, primarily support insulators and transmission conductors, and must withstand long-term conductor loads, wind loads, and environmental factors. Therefore, crossarm materials typically require high mechanical strength, good corrosion resistance, and good service stability.

[0003] In existing technologies, angle aluminum crossarms are typically made of aluminum alloy. Aluminum alloys have advantages such as low density, good corrosion resistance, and ease of processing and forming, and are therefore widely used in the field of power transmission equipment. Currently, the preparation of angle aluminum crossarms usually involves processes such as aluminum alloy smelting, forming, and heat treatment to improve material properties and meet the requirements of the crossarm's use.

[0004] However, existing aluminum alloy crossarms still suffer from the problem of balancing strength and corrosion resistance during long-term service, making it difficult for their overall performance to meet the requirements of high-performance transmission line equipment. Summary of the Invention

[0005] This invention aims to address one of the technical problems in related technologies to a certain extent. To this end, this invention provides a method for preparing an angle aluminum crossarm and an angle aluminum crossarm thereof, the resulting angle aluminum crossarm exhibiting high strength and good corrosion resistance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing an aluminum angle crossarm includes the following steps: S1, prepare an aluminum alloy melt with the following elemental composition: Zn: 4-5 wt%, Mg: 1.2-2 wt%, Cu: 0.01-0.2 wt%, Mn: 0.2-0.5 wt%, Zr: 0.08-0.2 wt%, with the balance being Al and unavoidable impurity elements; S2, the aluminum alloy melt is cast to obtain an aluminum alloy ingot; S3, homogenize the aluminum alloy ingot; S4, the aluminum alloy ingot that has undergone the homogenization treatment is hot extruded to form an angle aluminum crossbeam blank with an angular cross section; S5, the angle aluminum crossarm blank is subjected to solution treatment and aging treatment to obtain a high-strength corrosion-resistant angle aluminum crossarm.

[0007] In this application, by adding specific amounts of Zn, Mg, Cu, Mn, and Zr to the Al matrix, Zn, Mg, and Cu play a strengthening role during subsequent solution treatment and aging treatment, while Mn and Zr are utilized to stabilize the alloy microstructure, resulting in an aluminum alloy with both high strength and good corrosion resistance. Homogenization treatment of the cast aluminum alloy ingot reduces compositional segregation and microstructure inhomogeneity formed during solidification, providing a more uniform billet state for subsequent hot extrusion. Hot extrusion directly forms angle aluminum crossarm billets with angular cross-sections from the aluminum alloy ingots. This improves the material microstructure and mechanical properties through extrusion deformation and creates a continuous, integral structure along the crossarm's length, enhancing its bending resistance and dimensional stability. Subsequent solution treatment and aging treatment further enhance the strengthening effect of the alloying elements, resulting in stable mechanical properties for the angle aluminum crossarm. Therefore, this invention can improve the strength, bending resistance, corrosion resistance and long-term service stability of angle aluminum crossarms while giving full play to the lightweight characteristics of aluminum alloys, and is suitable for continuous and mass production of angle aluminum crossarms.

[0008] Optionally, in step S1, the impurity elements in the aluminum alloy melt include Fe and Si, wherein the mass percentage of Fe is less than 0.3 wt%, the mass percentage of Si is less than 0.3 wt%, and the total mass percentage of other impurity elements besides Fe and Si is less than 0.15 wt%.

[0009] Optionally, in step S1, after melting the Al raw material, Zn raw material, Cu raw material, Al-Mn master alloy and Al-Zr master alloy are added. After the Zn raw material, Cu raw material, Al-Mn master alloy and Al-Zr master alloy are melted, Mg raw material is added for smelting to obtain the aluminum alloy melt.

[0010] Optionally, in step S1, the melting temperature of the aluminum alloy melt is 700-750°C; after the Mg raw material is melted, the aluminum alloy melt is refined and kept at a constant temperature for 20-40 minutes.

[0011] Optionally, the refining process includes: refining the aluminum alloy melt with C2Cl6, or refining the aluminum alloy melt with nitrogen and / or argon, for a refining time of 5 to 10 minutes.

[0012] Optionally, in step S3, the homogenization process includes: heating the aluminum alloy ingot to 440-465°C and holding it at that temperature for 20-40 hours, then cooling it in the furnace to below 220°C after the holding period, and finally air-cooling it after it is removed from the furnace.

[0013] Optionally, in step S4, the temperature of the hot extrusion molding is 380–420°C, and the extrusion ratio is 10–60.

[0014] Optionally, in step S5, the solution treatment includes: heating the angle aluminum crossarm blank to 450-490°C and holding it at that temperature for 1-4 hours; the aging treatment is artificial aging, performed at a temperature of 100-130°C for 15-30 hours.

[0015] Optionally, in step S5, after the solution treatment is completed, the angle aluminum crossarm blank is first subjected to water quenching, and then the angle aluminum crossarm blank after water quenching is subjected to the aging treatment.

[0016] Furthermore, the present invention also provides an angle aluminum crossarm, which is prepared by any of the aforementioned preparation methods. The reasoning process for the beneficial effects of the angle aluminum crossarm provided by the present invention is similar to that of the aforementioned preparation methods, and will not be repeated here.

[0017] These features and advantages of the present invention will be disclosed in detail in the following specific embodiments and accompanying drawings. The preferred embodiments or means of the present invention will be shown in detail in conjunction with the accompanying drawings, but are not intended to limit the technical solutions of the present invention. In addition, each of these features, elements and components appearing in the following text and drawings is a plurality of, and different symbols or numbers are used for convenience of representation, but all represent parts with the same or similar construction or function. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings: Figure 1 This is a schematic flowchart of the preparation method of the angle aluminum crossarm in this invention. Detailed Implementation

[0019] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described are intended to explain the present invention and should not be construed as limiting the invention.

[0020] The terms "an embodiment," "example," or "example" used in this specification refer to a particular feature, structure, or characteristic described in connection with the embodiment itself that may be included in at least one embodiment disclosed in this application. The phrase "in an embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment.

[0021] It should be understood that the following content is used to provide a complete and clear description of the technical solution of the present invention, and is not intended to limit the present invention to a specific raw material grade, equipment model, product size, or single process parameter. Without changing the core technical concept of the present invention—using an aluminum alloy melt with a specific elemental composition as the material basis and sequentially implementing casting, homogenization treatment, hot extrusion molding, solution treatment, water quenching, and artificial aging treatment—those skilled in the art can make adaptive adjustments to the specific operation methods in each step according to the production equipment, the specifications of the angle aluminum crossarm, and the actual installation requirements.

[0022] This invention provides a method for preparing an angle aluminum crossarm and the angle aluminum crossarm obtained by this method. The method first prepares an aluminum alloy melt containing Zn, Mg, Cu, Mn, and Zr, controlling the content of unavoidable impurity elements; then, the aluminum alloy melt is cast into an aluminum alloy ingot, which is homogenized; next, the homogenized aluminum alloy ingot is hot-extruded to form an angle aluminum crossarm blank with an angular cross-section; finally, the angle aluminum crossarm blank is subjected to solution treatment, water quenching, and artificial aging treatment in sequence to obtain a high-strength, corrosion-resistant angle aluminum crossarm. The above composition design and the sequential connection of each process step form a complete process chain for the preparation, forming, and heat treatment of the aluminum alloy material, centered around the load-bearing capacity and outdoor service requirements of the angle aluminum crossarm.

[0023] In this invention, the mass percentages of each element in the aluminum alloy melt are as follows: Zn 4-5 wt%, Mg 1.2-2 wt%, Cu 0.01-0.2 wt%, Mn 0.2-0.5 wt%, Zr 0.08-0.2 wt%, with the balance being Al and unavoidable impurity elements. It should be noted that wt% here represents the mass percentage, that is, the percentage of the corresponding element's mass to the total mass of the aluminum alloy. The element content can be controlled by weighing different masses of pure metal raw materials and intermediate alloy raw materials, or it can be corrected during the smelting process by sampling, analyzing, and adding the corresponding raw materials, as long as the final aluminum alloy melt meets the above elemental composition.

[0024] Al is the matrix element of aluminum alloys and also constitutes the main material of angle aluminum crossarms. Compared with steel crossarms, using Al-based materials can reduce the mass of components of the same volume, providing a lightweight basis for crossarm handling, hoisting, and high-altitude installation. Zn, Mg, and Cu are the main alloying elements in this embodiment. Their content, in conjunction with subsequent solution treatment, quenching, and artificial aging processes, enables the aluminum alloy to improve its strength through heat treatment. Mn and Zr are added in lower amounts to adjust the microstructure of the aluminum alloy during casting, homogenization, and hot extrusion, allowing the material to maintain good microstructure stability even after prolonged holding and significant plastic deformation. The above elements do not act in isolation but, under given content ranges and with the coordination of subsequent processes, jointly form a material system suitable for angle aluminum crossarms.

[0025] The Zn content is limited to 4–5 wt%. Within this range, Zn can combine with elements such as Mg to provide a compositional basis for subsequent age hardening. If the Zn content is too low, the strength obtained by the aluminum alloy after solution treatment and aging may be insufficient; if the Zn content is too high, although it may increase the strength level to some extent, it will increase the difficulty of controlling the smelting composition and subsequent processing, and may affect the balance between strength, plasticity, and corrosion resistance. Therefore, limiting the Zn content to 4–5 wt% achieves a suitable balance between the required strength and processing stability for angle aluminum crossarms.

[0026] The mass percentage of Mg is limited to 1.2–2 wt%. Mg and Zn together constitute the main heat treatment strengthening element combination. Controlling Mg within this range ensures sufficient compositional response in subsequent solution treatment and artificial aging treatment, while avoiding excessive Mg content that would lead to increased oxidation loss during melting and increased extrusion molding difficulty. Therefore, the Mg content range is not only related to the final strength but also coordinated with the melting charging sequence and hot extrusion molding process.

[0027] The mass percentage of Cu is limited to 0.01–0.2 wt%. While the Cu content is low, it is not entirely Cu-free; Cu can participate in regulating the strength and heat treatment response of the aluminum alloy. Since the angle aluminum crossarm needs to be exposed to outdoor environments for extended periods, this invention does not use excessively high Cu content, but rather controls it within a narrow, low range. This is to improve the material's mechanical properties while minimizing the adverse effects of excessive Cu content on corrosion resistance. In other words, this Cu content range corresponds to the high-strength, corrosion-resistant product positioning of this invention.

[0028] The mass percentage of Mn is limited to 0.2–0.5 wt%, and the mass percentage of Zr is limited to 0.08–0.2 wt%. In this embodiment, Mn is preferably added in the form of an Al-Mn master alloy, and Zr is preferably added in the form of an Al-Zr master alloy. The melting points of Mn and Zr, as well as the conditions for direct addition to the aluminum melt, are different from those of raw materials such as Al, Zn, and Mg. Using an aluminum-based master alloy as a carrier allows Mn and Zr to form a pre-alloyed state with Al upon addition, reducing the melting difficulties caused by direct addition of elements and improving the operability of feeding and the uniformity of element distribution. The addition of Mn and Zr can also be combined with homogenization treatment and hot extrusion deformation to improve the continuity and stability of the microstructure of the angle aluminum crossarm billet.

[0029] In addition to the elements actively added as described above, Fe, Si, and other trace impurities are inevitably introduced into aluminum alloy raw materials and smelting equipment. In a preferred embodiment, the mass percentage of Fe in the aluminum alloy melt is less than 0.3 wt%, the mass percentage of Si is less than 0.3 wt%, and the individual content of other unavoidable impurity elements besides Fe and Si can be controlled to be less than 0.05 wt%, with the total content of other unavoidable impurity elements being less than 0.15 wt%. The aforementioned "total content of other impurity elements less than 0.15 wt%" at least covers the total control of other unavoidable impurities besides Fe and Si. By selecting Al raw materials with low Fe content and Zn, Cu, and Mg raw materials with sufficient purity, and by avoiding the introduction of excessive impurities into the melt by smelting tools, furnace linings, and transfer tools, the above-mentioned impurity content can be made to meet the requirements. Among them, other impurity elements include, but are not limited to, one or more of Cr, Ni, Ti, V, Ga, Pb, Sn, Bi, B, Na, and Ca. Other impurity elements are elements that are unavoidably introduced during the raw material or preparation process, excluding Zn, Mg, Cu, Mn, and Zr added as alloy design components.

[0030] Controlling the Fe and Si content can reduce the chance of coarse or brittle impurity-related structures forming in aluminum alloys, and reduce the adverse effects of impurities on the material's plasticity, toughness, and corrosion resistance. For angle aluminum crossarms, they not only bear static loads but may also withstand wind vibrations, installation preload, and localized stresses near bolt holes.

[0031] like Figure 1As shown, in step S1, an aluminum alloy melt with the above-mentioned elemental composition is prepared. This step includes raw material preparation, Al raw material melting, addition of Zn and Cu raw materials and intermediate alloys, subsequent addition of Mg raw material, melting temperature control, refining treatment, and static holding. This step is not only used to convert solid raw materials into liquid metal, but more importantly, it determines the order of addition according to the melting characteristics and activity of each element, and obtains a melt with relatively uniform composition and low gas and inclusion content through refining and static holding, thus providing conditions for casting ingots.

[0032] In the raw material preparation stage, industrially pure Al with low Fe content or other Al raw materials that meet impurity control requirements can be selected as the base material. Pure Zn, pure Cu, and pure Mg can be selected as the sources of the corresponding elements, and Al-10Mn master alloy and Al-5Zr master alloy can be selected as the sources of Mn and Zr, respectively. It should be noted that Al-10Mn means that the nominal mass percentage of Mn in the master alloy is about 10%, and the remainder is mainly Al; Al-5Zr means that the nominal mass percentage of Zr in the master alloy is about 5%, and the remainder is mainly Al. The above master alloy contents are only preferred examples. Other aluminum-based master alloys with different Mn or Zr contents can also be used, and the feed mass should be recalculated according to the actual content of the target elements in the master alloy.

[0033] During batching, the amount of each raw material to be added is calculated based on the total mass of the target melt and the content of the target elements. For example, when the total mass of the target aluminum alloy is M, the target Zn mass can be determined by multiplying M by the percentage of the target Zn mass. The amounts of Mn and Zr to be added also need to be converted based on the actual contents of Mn and Zr in the Al-Mn master alloy and Al-Zr master alloy. The Al mass brought in by each master alloy should also be taken into account in the calculation to ensure that the content of each element in the final aluminum alloy meets the requirements. In addition, for raw materials that may be lost during the smelting process, a reasonable compensation amount can be set based on existing production experience, but the compensation should still be based on the final melt composition meeting the aforementioned limits.

[0034] During smelting, Al raw materials are first added to the smelting furnace and heated. The smelting furnace can be a crucible furnace, induction furnace, resistance furnace, or other conventional smelting equipment capable of melting aluminum alloys. This invention does not depend on a specific furnace type; the smelting equipment only needs to be able to stably control the melt temperature at 700–750°C and avoid significant contamination of the melt. Melting the Al raw materials first forms a continuous aluminum liquid matrix, providing a liquid medium for the subsequent melting of Zn, Cu, and Al-Mn and Al-Zr master alloys.

[0035] After the Al raw material has formed a continuous molten aluminum, Zn raw material, Cu raw material, Al-Mn master alloy, and Al-Zr master alloy are added. These raw materials can be added in batches or sequentially, ensuring complete melting. Preferably, after each batch of raw material is added, the melt is maintained at 700–750°C and moderately stirred to ensure sufficient contact between the newly added raw material and the molten aluminum. Stirring can be achieved using conventional tools for aluminum-resistant melts or by utilizing melt circulation formed by a gas refining device. The purpose is to promote the melting of raw materials and homogeneity of composition; a specific stirring speed is not required.

[0036] After the Zn, Cu, Al-Mn, and Al-Zr master alloys have melted, the Mg raw material is added. "After melting" here means that the added raw material no longer exists as a clearly unmelted solid mass and can form a continuous melt with the aluminum liquid. Individual testing of each raw material particle is not required in industrial production. Mg is added later because it is more reactive in high-temperature aluminum melt. If added too early, before the Al raw material has completely melted or when other master alloys require prolonged holding time for melting, the increased residence time of Mg in the high-temperature environment makes it prone to oxidation and burn-off, causing the final Mg content to deviate from the target range. By adding Mg after the melting of other main raw materials, the high-temperature exposure time of Mg before refining can be shortened, improving the stability of composition control.

[0037] The temperature throughout the smelting process is controlled at 700–750℃. This temperature range allows Al, Zn, Cu, Mg, and aluminum-based master alloys to form a melt with sufficient fluidity for casting. If the temperature is below this range, the melting rate of some raw materials, especially master alloys, may be slow, and the melt fluidity may be insufficient. If the temperature is significantly higher than this range, the oxidation loss of reactive elements such as Mg may increase, leading to higher energy consumption. Therefore, 700–750℃ is not only the temperature condition for melting the raw materials but also a balance between melting efficiency, composition retention, and subsequent casting fluidity.

[0038] After the Mg raw material is melted, the aluminum alloy melt undergoes refining. Refining can be achieved by adding a C2Cl6 refining agent or by introducing nitrogen and / or argon gas into the melt. Both methods are intended to promote the removal of dissolved gases and inclusions from the melt, reducing the likelihood of defects such as porosity and inclusions in the ingot. The refining method can be selected based on the environmental requirements of the production site, equipment configuration, and melt batch size; it is not required to use both methods simultaneously.

[0039] In a preferred embodiment, when refining with C2Cl6, the amount of C2Cl6 added is 0.6% of the total mass of the aluminum alloy melt. C2Cl6 can be added in the form of a refining agent suitable for aluminum melt refining, ensuring sufficient contact with the melt. An addition amount of 0.6% of the total mass of the aluminum alloy melt achieves a stable melt treatment effect. When refining with nitrogen and / or argon, high-purity nitrogen, argon, or a mixture of both can be introduced from the bottom or interior of the melt, allowing bubbles to carry away gases and inclusions from the melt as they rise.

[0040] Regardless of whether C2Cl6 refining or gas refining is used, the refining time is preferably 5–10 minutes. If the refining time is too short, gases and inclusions may not have fully migrated out of the melt; if the refining time is too long, it will prolong the high-temperature residence time of the melt and reduce production efficiency. Controlling the refining time to 5–10 minutes allows the melt processing to be coordinated with the subsequent addition of Mg, completing melt purification in a shorter time and reducing additional Mg loss at high temperatures.

[0041] After refining, the molten aluminum alloy is held at a static temperature for 20–40 minutes. This static holding period involves stopping vigorous agitation while maintaining the melt in a castable state, allowing residual bubbles and inclusions from the refining process to rise or sink further, and enabling the temperature and composition of the melt to become more uniform across its various regions. A static holding time of less than 20 minutes may result in the melt not being sufficiently stable; a holding time exceeding 40 minutes usually does not provide a significant benefit commensurate with the extended time, and may instead increase element loss and energy consumption. A static holding period of 20–40 minutes ensures that the melt has a relatively stable temperature, fluidity, and composition before entering the mold.

[0042] In step S2, the molten aluminum alloy is cast to obtain an aluminum alloy ingot. After static holding, the molten aluminum alloy is poured into a mold and allowed to solidify. The mold can be a cast iron mold or other metal mold suitable for aluminum alloy ingot preparation. The internal dimensions of the mold are determined based on the barrel size of the extrusion equipment, the specifications of the target angle aluminum crossarm, and the production batch. In this specific embodiment, a cast iron mold is used, which has the advantages of simple structure, easy reuse, and the ability to obtain ingots that meet the requirements of subsequent extrusion.

[0043] During the casting process, the melt gradually solidifies from a liquid state to form a cast aluminum alloy ingot. Step S2 transforms the homogeneous melt obtained in step S1 into a solid billet that can be transported, heat-treated, and loaded into extrusion equipment. Because temperature gradients and elemental distribution differences inevitably exist during the solidification process, the resulting ingot has a cast structure and still requires subsequent homogenization treatment. Therefore, this invention does not directly extrude the cast ingot, but instead incorporates a homogenization treatment between step S2 and hot extrusion to improve the internal state of the ingot.

[0044] In step S3, the aluminum alloy ingot undergoes a homogenization treatment. This homogenization treatment involves heating the aluminum alloy ingot to 440–465°C and holding it at that temperature for 20–40 hours. After holding, the ingot is cooled in the furnace to below 220°C and then air-cooled. This homogenization treatment can also be called homogenization annealing, and it is applied to the as-cast aluminum alloy ingot obtained in step S2, rather than the already extruded angle aluminum crossarm billet.

[0045] Heating the ingot to 440–465℃ and holding it at that temperature for 20–40 hours allows the ingot to diffuse elements within the as-cast microstructure without undergoing overall melting. This higher temperature and longer holding time reduces localized component segregation and alleviates internal stresses generated during casting and solidification. The homogenized ingot exhibits a more uniform internal composition and microstructure, reducing molding fluctuations caused by significant differences in deformation capacity between different regions during subsequent hot extrusion.

[0046] When the homogenization temperature is below 440℃, the element diffusion rate is slow, and it may be difficult to fully improve the inhomogeneity of the casting within a given holding time. When the temperature is above 465℃, higher requirements are placed on the accuracy of temperature control, and the risk of local overheating may increase. When the holding time is less than 20 hours, especially for ingots with large cross-sections, the center and surface of the ingot may not have achieved sufficient homogenization. When the holding time exceeds 40 hours, the process cycle and energy consumption increase significantly. Therefore, the parameter combination of 440–465℃ and 20–40 hours is used to achieve a balance between homogenization effect, process safety, and production efficiency.

[0047] After the heat preservation period, the ingot is cooled in the furnace to below 220°C before being removed from the furnace and air-cooled. Furnace cooling refers to gradually lowering the ingot temperature within the furnace as the heating power is turned off or reduced. This cooling method avoids the excessive temperature gradient caused by immediately exposing the homogenized ingot to a lower ambient temperature. Cooling to below 220°C before air-cooling reduces subsequent furnace dwell time and lowers the risk of thermal stress and surface oxidation caused by high-temperature removal from the furnace. Step S3, consisting of heating and heat preservation, furnace cooling, and air-cooling, constitutes a continuous process, all three working together to bring the ingot to a state suitable for hot extrusion.

[0048] Step S4 involves hot extruding the homogenized aluminum alloy ingot to form an angle aluminum crossarm blank with a angular cross-section. During hot extrusion, the homogenized ingot is heated to 380–420°C, placed into the extrusion cylinder of the extrusion equipment, and pressure is applied by the extrusion rod, causing the aluminum alloy to flow out through the extrusion die with a angular outlet. The extruded profile is continuously formed along its length, and its cross-section is determined by the shape of the die outlet.

[0049] The hot extrusion temperature is 380–420℃. Within this temperature range, aluminum alloy ingots possess suitable plastic flow capabilities, enabling the formation of long, continuous profiles through the die. If the temperature is too low, the material's deformation resistance increases, potentially leading to increased extrusion load, surface cracking, or insufficient cross-sectional filling. If the temperature is too high, the dimensional control and microstructure stability of the profile after demolding may be affected. Limiting the extrusion temperature to 380–420℃ helps achieve a balance between forming capacity, surface quality, and microstructure control.

[0050] In addition, the extrusion ratio is 10–60. The extrusion ratio is the ratio of the effective cross-sectional area of ​​the ingot before extrusion to the cross-sectional area of ​​the profile after extrusion, which reflects the overall degree of deformation of the material during the extrusion process. When the extrusion ratio is low, the material deformation is small, which is suitable for angle aluminum crossarms with relatively large cross sections; when the extrusion ratio is high, it is possible to obtain profiles with smaller cross sections or larger lengths, but the requirements for extrusion equipment capacity and process control are higher. Limiting the extrusion ratio to 10–60 allows this method to cover a variety of angle aluminum crossarm specifications, while ensuring that the material undergoes sufficient plastic deformation to form a continuous angle cross section.

[0051] An angular cross-section refers to a cross-section consisting of at least two interconnected plate-like portions forming an included angle. In a preferred embodiment, the angular cross-section is an L-shaped cross-section, comprising a first plate segment and a second plate segment, which extend continuously along the length of the angle aluminum crossarm and are interconnected. The first and second plate segments can be of equal width to form an equilateral angle aluminum crossarm, or they can be set to different widths according to the tower connection position and load-bearing requirements to form an unequal angle aluminum crossarm. The aforementioned "angular cross-section" can cover the different specific cross-sections mentioned above, and the L-shaped cross-section is a preferred and applicable specific form for power crossarms.

[0052] Angle aluminum crossarm blanks are formed by integral hot extrusion. The first and second plate segments are continuously formed from the same aluminum alloy material at the joint, eliminating the need for welding to assemble the two plates into an angled structure. This continuous structure reduces local performance differences caused by welds or splicing interfaces. Therefore, step S4 not only completes the product shape manufacturing but also enables the material to form a continuous microstructure and structure along its length during thermoplastic deformation, providing a uniform blank for subsequent overall heat treatment.

[0053] In step S5, the angle aluminum crossarm blank is subjected to solution treatment and aging treatment to obtain a high-strength, corrosion-resistant angle aluminum crossarm. In a preferred embodiment, step S5 is performed in the order of "solution treatment - water quenching - artificial aging treatment". Water quenching is placed between the two to quickly retain the high-temperature state after solution treatment to a lower temperature and to provide a supersaturated solution state basis for artificial aging.

[0054] Solution treatment involves heating the angle aluminum crossarm blank to 450–490℃ and holding it at that temperature for 1–4 hours. Solution treatment allows alloying elements that can dissolve into the Al matrix in the hot-extruded material to fully enter a solution state during the high-temperature holding process, and it also makes the heat treatment state of different areas more consistent. Since the angle aluminum crossarm has already formed its final or near-final angle cross-section, solution treatment of the entire blank or section ensures that the first section, the second section, and the area connecting them receive the same thermal history, thereby reducing performance differences at different locations along the cross-section.

[0055] When the solution temperature is below 450℃, the degree to which alloying elements enter the solution state may be insufficient, limiting the degree of supersaturation available for subsequent artificial aging. When the solution temperature is above 490℃, the temperature control margin decreases, potentially increasing the risk of localized overheating or shape changes. When the holding time is less than 1 hour, the angle aluminum crossarm blank, especially the thicker sections, may not have fully reached the set temperature. When the holding time exceeds 4 hours, the heat treatment cycle and energy consumption increase. Therefore, it can be demonstrated that the parameter range of 450–490℃ and 1–4 hours can accommodate angle aluminum crossarms with different wall thicknesses and provide a stable basis for subsequent quenching and aging treatments.

[0056] After solution treatment, the angle aluminum crossarm billet undergoes water quenching. Water quenching involves rapidly transferring the billet, which has undergone high-temperature holding, into a water medium for rapid cooling. Rapid cooling inhibits the premature precipitation of alloying elements during slow cooling, preserving the high-temperature solution state at a lower temperature and forming a supersaturated solution state suitable for artificial aging. Water quenching is not solution heating and holding itself, but rather an independent cooling step that follows solution treatment and precedes artificial aging.

[0057] In terms of operation, water quenching should be carried out promptly after solution treatment and heat preservation to avoid the billet remaining in the air for a long time, causing a slow temperature drop. The water medium can be ambient temperature water capable of rapid cooling or circulating water with a controllable temperature on the production site. In addition, those skilled in the art can select an appropriate water tank and transfer method according to the length of the angle aluminum crossarm, wall thickness, and layout of the heat treatment equipment, as long as rapid water cooling after solution treatment can be achieved.

[0058] After water quenching, the angle aluminum crossarm blanks undergo artificial aging treatment. The artificial aging temperature is 100–130℃, and the time is 15–30 hours. Artificial aging refers to actively setting the temperature and holding time using equipment such as an aging furnace, allowing alloying elements in a supersaturated state after quenching to gradually precipitate under controlled conditions, thereby forming a stable strengthened state. Compared with room temperature natural aging, artificial aging allows for more precise control of the treatment cycle and final performance, making it suitable for mass production of angle aluminum crossarms.

[0059] When the artificial aging temperature is below 100℃, the time required to reach the target strengthened state may be significantly prolonged; when the temperature is above 130℃, the precipitation process accelerates, but its controllability decreases, and the balance between strength, plasticity, and corrosion resistance may change. When the aging time is less than 15 hours, the strengthening process may be insufficient; when the time exceeds 30 hours, the production cycle is prolonged, and over-aging may occur. Therefore, the artificial aging time is limited to 100–130℃ and 15–30 hours to match the Zn, Mg, and Cu content and the aforementioned solution quenching regime of this invention.

[0060] After step S5 is completed, the angle aluminum crossarm can be allowed to cool naturally to room temperature to obtain a high-performance angle aluminum crossarm. This angle aluminum crossarm maintains the angled cross-section formed in S4 and obtains a material state suitable for service through S5. Since solution treatment, quenching, and aging are all performed after profile forming, the heat treatment effect can directly act on the first plate segment, the second plate segment, and their connecting area of ​​the angle aluminum crossarm, avoiding the damage to the strengthened state that may be caused by heat treatment followed by large deformation processing.

[0061] Specifically, the angle aluminum crossarm obtained by this invention is preferably an equilateral L-shaped integral profile, but it can also be an unequal-sided L-shaped integral profile. Its length, segment width, and thickness are determined according to the installation dimensions of the power poles and towers. Mounting holes for connecting insulators, towers, and conductor fittings can be machined at both ends and the stress-bearing area of ​​the crossarm according to installation standards. The mounting holes can be formed using conventional machining methods such as drilling after extrusion and heat treatment, thus avoiding the need to fix the specific hole positions and diameters as a necessary limitation of the material and preparation method of this invention.

[0062] Because the angle aluminum crossarm is integrally extruded from an Al-based alloy, its main body does not need to rely on a hot-dip galvanized layer for basic corrosion protection, nor does it need to be welded together to form an L-shaped cross-section. The lower density gives the crossarm a lower self-weight for the same external dimensions, the integral extrusion structure makes the connection area between the two plate sections continuous, and the specific alloy composition and heat treatment process give the material high strength and good corrosion resistance.

[0063] The present invention is illustrated below through specific embodiments and comparative examples. In the embodiments, industrial pure Al, pure Zn, pure Cu, pure Mg, Al-5Zr master alloy, and Al-10Mn master alloy are used as raw materials, and casting is performed using cast iron molds. Unless otherwise specified, conventional equipment used in the fields of aluminum alloy smelting, heat treatment, and extrusion can be employed.

[0064] Example 1 This embodiment provides a method for preparing an aluminum angle crossarm. Using industrially pure Al, pure Zn, pure Cu, pure Mg, Al-5Zr master alloy, and Al-10Mn master alloy as raw materials, the raw materials are batched according to the target alloy composition, resulting in an aluminum alloy with the following mass percentages: Zn 4 wt%, Mg 1.2 wt%, Cu 0.1 wt%, Mn 0.3 wt%, Zr 0.15 wt%, and the balance being Al and unavoidable impurities. Specifically, the mass percentages of Fe and Si are less than 0.3 wt%, and the total mass percentage of other impurity elements besides Fe and Si is less than 0.15 wt%.

[0065] Industrial-grade pure Al is added to a melting furnace and heated to 720°C until it is completely melted, forming an aluminum melt. Then, pure Zn, pure Cu, Al-10Mn master alloy, and Al-5Zr master alloy are added to the aluminum melt, and the melting temperature is maintained until all the pure Zn, pure Cu, Al-10Mn master alloy, and Al-5Zr master alloy are completely melted. After the above raw materials are completely melted, pure Mg is added and smelted, allowing the pure Mg to melt and dissolve in the aluminum melt, resulting in an aluminum alloy melt composed of the above elements.

[0066] After pure Mg is melted, the aluminum alloy melt is refined using C2Cl6. The amount of C2Cl6 added is 0.6% of the total mass of the aluminum alloy melt, and the refining time is 8 minutes. After refining, the aluminum alloy melt is held at a constant temperature for 30 minutes to stabilize it. Then, the aluminum alloy melt is poured into a cast iron mold for cooling and solidification. After complete solidification, it is demolded to obtain an aluminum alloy ingot.

[0067] The aluminum alloy ingot was placed in a heat treatment furnace for homogenization. The homogenization temperature was 450℃, and the holding time was 20 hours. After the holding time, the aluminum alloy ingot was allowed to cool with the furnace. When the temperature of the aluminum alloy ingot dropped below 220℃, the ingot was removed from the heat treatment furnace and allowed to continue cooling in air.

[0068] The homogenized aluminum alloy ingot is hot-extruded. The hot extrusion temperature is 400℃ and the extrusion ratio is 30:1. The aluminum alloy ingot undergoes plastic deformation through an extrusion die with an angular exit to form an angle aluminum crossbeam billet with an angular cross section.

[0069] The angle aluminum crossarm blanks were subjected to solution treatment, water quenching, and artificial aging treatment in sequence. Specifically, the angle aluminum crossarm blanks were heated to 465℃ and held for 1 hour for solution treatment; after solution treatment, the angle aluminum crossarm blanks were removed from the heat treatment furnace and water quenched; subsequently, the water-quenched angle aluminum crossarm blanks were heated to 120℃ and held for 24 hours for artificial aging treatment. After artificial aging treatment, the angle aluminum crossarms were cooled to room temperature to obtain the angle aluminum crossarms.

[0070] The mechanical properties of the angle aluminum crossarm prepared in this embodiment were tested, and its yield strength was measured to be 380 MPa, tensile strength to be 430 MPa, and elongation to be 15%. An immersion test was conducted on the angle aluminum crossarm using a 3.5% NaCl solution. After immersion for 60 days, the alloy surface remained bright, and no obvious signs of corrosion were observed, indicating that the angle aluminum crossarm prepared in this embodiment possesses both high mechanical strength and good corrosion resistance.

[0071] Example 2 This embodiment provides a method for preparing an aluminum angle crossarm. Using industrially pure Al, pure Zn, pure Cu, pure Mg, Al-5Zr master alloy, and Al-10Mn master alloy as raw materials, the raw materials are batched according to the target alloy composition, resulting in an aluminum alloy with the following mass percentages: Zn 4.5 wt%, Mg 1.8 wt%, Cu 0.1 wt%, Mn 0.3 wt%, Zr 0.15 wt%, and the balance being Al and unavoidable impurity elements. Specifically, the mass percentages of Fe and Si are less than 0.3 wt%, and the total mass percentage of other impurity elements besides Fe and Si is less than 0.15 wt%.

[0072] Industrial-grade pure Al is added to a melting furnace and heated to 720°C until it is completely melted, forming an aluminum melt. Then, pure Zn, pure Cu, Al-10Mn master alloy, and Al-5Zr master alloy are added to the aluminum melt, and the melting temperature is maintained until all the pure Zn, pure Cu, Al-10Mn master alloy, and Al-5Zr master alloy are completely melted. After the above raw materials are completely melted, pure Mg is added and smelted, allowing the pure Mg to melt and dissolve in the aluminum melt, resulting in an aluminum alloy melt with the aforementioned elemental composition.

[0073] After pure Mg is melted, the aluminum alloy melt is refined using C2Cl6. The amount of C2Cl6 added is 0.6% of the total mass of the aluminum alloy melt, and the refining time is controlled to be 5-10 minutes. After the refining process, the aluminum alloy melt is held at a constant temperature for 30 minutes. Subsequently, the aluminum alloy melt is poured into a cast iron mold for cooling and solidification. After the aluminum alloy melt has completely solidified, it is demolded to obtain an aluminum alloy ingot.

[0074] The aluminum alloy ingot was placed in a heat treatment furnace for homogenization. The homogenization temperature was 450℃, and the holding time was 20 hours. After the holding time, the aluminum alloy ingot was allowed to cool with the furnace. When the temperature of the aluminum alloy ingot dropped below 220℃, the ingot was removed from the heat treatment furnace and allowed to continue cooling in air.

[0075] The homogenized aluminum alloy ingot is hot-extruded. The hot extrusion temperature is 400℃ and the extrusion ratio is 30:1. The aluminum alloy ingot is plastically deformed through an extrusion die with an angular exit to form an angle aluminum crossbeam billet with an angular cross section.

[0076] The angle aluminum crossarm blanks were subjected to solution treatment, water quenching, and artificial aging treatment in sequence. Specifically, the angle aluminum crossarm blanks were heated to 465℃ and held for 1 hour for solution treatment; after solution treatment, the angle aluminum crossarm blanks were removed from the heat treatment furnace and water quenched; subsequently, the water-quenched angle aluminum crossarm blanks were heated to 120℃ and held for 24 hours for artificial aging treatment. After artificial aging treatment, the angle aluminum crossarms were cooled to room temperature to obtain the angle aluminum crossarms.

[0077] The mechanical properties of the angle aluminum crossarm prepared in this embodiment were tested, and its yield strength was found to be 408 MPa, tensile strength to be 465 MPa, and elongation to be 13%. An immersion test was conducted on the angle aluminum crossarm using a 3.5% NaCl solution. After immersion for 60 days, the alloy surface remained bright, and no obvious signs of corrosion were observed, indicating that the angle aluminum crossarm prepared in this embodiment has high mechanical strength and good corrosion resistance.

[0078] Comparative Example 1 This comparative example provides a method for preparing an angle aluminum crossarm without the addition of Mn and Zr.

[0079] Using industrially pure Al, pure Zn, pure Cu, and pure Mg as raw materials, the alloy was formulated according to the target alloy composition, resulting in an aluminum alloy with a Zn mass percentage of 4 wt%, a Mg mass percentage of 1.2 wt%, and a Cu mass percentage of 0.1 wt%, with the balance being Al and unavoidable impurity elements. This comparative example did not include Al-10Mn master alloys or Al-5Zr master alloys, meaning the resulting aluminum alloy did not contain Mn and Zr, which were added as part of the design composition.

[0080] Industrial-grade pure Al is added to a melting furnace and heated to 720°C until it is completely melted, forming an aluminum melt. Then, pure Zn and pure Cu are added to the aluminum melt, and the melting temperature is maintained until the pure Zn and pure Cu are completely melted. After the pure Zn and pure Cu have completely melted, pure Mg is added and smelted, causing the pure Mg to melt and dissolve in the aluminum melt, resulting in an aluminum alloy melt.

[0081] After pure Mg is melted, the aluminum alloy melt is refined using C2Cl6, with the amount of C2Cl6 added being 0.6% of the total mass of the aluminum alloy melt. After refining, the aluminum alloy melt is held at a constant temperature for 30 minutes. Subsequently, the aluminum alloy melt is poured into a cast iron mold for cooling and solidification. After the aluminum alloy melt has completely solidified, it is demolded to obtain an aluminum alloy ingot.

[0082] The aluminum alloy ingot was placed in a heat treatment furnace for homogenization treatment at a temperature of 450℃ for 20 hours. After the homogenization treatment, the aluminum alloy ingot was allowed to cool with the furnace. When the temperature of the aluminum alloy ingot dropped below 220℃, the aluminum alloy ingot was removed from the heat treatment furnace and allowed to continue cooling in air.

[0083] The homogenized aluminum alloy ingot is hot extruded at a temperature of 400°C and an extrusion ratio of 30:1, so that the aluminum alloy ingot is formed into an angle aluminum crossbeam blank with an angled cross section through an extrusion die with an angled exit.

[0084] The angle aluminum crossarm blank was heat-treated. Specifically, the angle aluminum crossarm blank was heated to 465℃ and held for 1 hour for solution treatment; after solution treatment, the angle aluminum crossarm blank was water-quenched; subsequently, the water-quenched angle aluminum crossarm blank was heated to 120℃ and held for 24 hours for artificial aging treatment. After artificial aging treatment, the angle aluminum crossarm was cooled to room temperature to obtain the angle aluminum crossarm of this comparative example.

[0085] The mechanical properties of the aluminum angle crossarm prepared in this comparative example were tested, and its yield strength was 290 MPa, tensile strength was 345 MPa, and elongation was 21%. An immersion test was conducted using a 3.5% NaCl solution. After immersion for 15 days, signs of intergranular corrosion appeared on the alloy surface.

[0086] Comparing this comparative example with Example 1, it can be seen that both use the same Zn, Mg, and Cu contents, as well as the same smelting, refining, homogenization, hot extrusion, and heat treatment processes. The main difference is that Example 1 further added 0.3 wt% Mn and 0.15 wt% Zr. The yield strength of Example 1 increased from 290 MPa in this comparative example to 380 MPa, and the tensile strength increased from 345 MPa to 430 MPa. Meanwhile, this comparative example showed signs of intergranular corrosion after 15 days of immersion, while Example 1 showed no obvious signs of corrosion after 60 days of immersion. This indicates that, under the alloy composition and preparation process adopted in this invention, the addition of Mn and Zr is beneficial to improving the mechanical strength and corrosion resistance of the angle aluminum crossarm.

[0087] As can be seen from Examples 1, 2, and Comparative Example 1, the technical effect of the present invention is not solely produced by a single heat treatment temperature or a single element. The Examples and Comparative Examples used the same melting temperature of 720℃, refining with 0.6% C2Cl6, homogenization at 450℃ for 20 hours, extrusion at 400℃, an extrusion ratio of 30:1, solution treatment at 465℃ for 1 hour, and artificial aging at 120℃ for 24 hours. However, the presence or absence of Mn and Zr, as well as the different contents of Zn and Mg, resulted in significant differences in the final performance. This demonstrates a synergistic relationship between the component system and the complete process route.

[0088] In addition to Al raw materials, pure Zn, pure Cu, and pure Mg can be replaced with aluminum-based master alloys that can provide the corresponding elements. When using master alloys to replace pure metals, the amount added should be recalculated based on the actual composition of the master alloy, and the mass percentages of Zn, Mg, Cu, Mn, and Zr in the final melt should be kept within the aforementioned limits.

[0089] In addition, for production lines equipped with chemical refining capabilities, C2Cl6 can be used for refining; for production lines equipped with rotary jet or bottom-blowing devices, nitrogen, argon, or a mixture of nitrogen and argon can be used for refining. When using gas refining, the gas purity, flow rate, and injection position can be determined based on the melt volume and equipment, as long as the melt can be effectively processed within 5–10 minutes. It should be noted that the common goal of different refining methods is to reduce the gas and inclusion content of the melt, rather than to change the alloy element composition.

[0090] Casting molds are not limited to cast iron molds. Depending on the extrusion press barrel size, steel molds, semi-continuous casting devices, or other casting equipment capable of producing extrusion ingots can be used. If molds with different cooling capacities are used, the as-cast microstructure of the ingot may differ, but the internal composition and microstructure of the ingot can be adjusted through subsequent homogenization treatment at 440–465℃ for 20–40 hours. Therefore, the mold material and specific ingot size can be selected according to production conditions.

[0091] Homogenization treatment can be performed within the same temperature and time range, with specific parameters selected based on the ingot size. For ingots with smaller diameters or cross-sectional dimensions, treatment conditions close to 440℃ and 20 hours can be chosen; for ingots with larger cross-sections or where a more thorough homogenization effect is desired, conditions close to 465℃ and 40 hours can be selected. It should be noted that regardless of the combination used, the ingots should be cooled in the furnace to below 220℃ after the holding period before being removed from the furnace and air-cooled.

[0092] In addition, for thinner angle aluminum profiles, solution treatment can be carried out at 450–470℃ for 1–2 hours, followed by aging at 100–120℃ for a longer period. For thicker profiles, treatment can be carried out at higher solution temperatures and longer holding times to ensure that the center of the cross-section reaches the treated state. The above selection should still fall within the range of 450–490℃ for 1–4 hours of solution treatment and 100–130℃ for 15–30 hours of artificial aging, with water quenching performed between the two.

[0093] The method of this invention can also be used to prepare other extruded profiles using the same material system as the angle aluminum crossarm. By changing the extrusion die, H-shaped, channel-shaped, or other continuous cross-section aluminum alloy profiles can be formed.

[0094] Compared to traditional Q355 angle steel crossarms, this invention uses an Al-based alloy, which significantly reduces the component's weight under the same specifications, reducing the burden of handling and high-altitude installation. Aluminum alloys inherently possess good resistance to atmospheric corrosion, eliminating the need to rely entirely on the hot-dip galvanized layer on the steel surface. Simultaneously, the combination of low Fe, low Si, and Mn / Zr microalloying with refining, homogenization, and heat treatment ensures that the embodiment maintains a good surface condition even under long-term immersion in a 3.5% NaCl solution. Therefore, the aluminum angle crossarm of this invention achieves a balance of lightweight, high strength, and corrosion resistance.

[0095] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the contents described in the accompanying drawings and the specific embodiments above. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A method for preparing an aluminum angle crossbar, characterized in that, Includes the following steps: S1, prepare an aluminum alloy melt composed of the following elements: Zn: 4-5 wt%, Mg: 1.2-2 wt%, Cu: 0.01-0.2 wt%, Mn: 0.2-0.5 wt%, Zr: 0.08-0.2 wt%, with the balance being Al and unavoidable impurity elements; S2, the aluminum alloy melt is cast to obtain an aluminum alloy ingot; S3, homogenize the aluminum alloy ingot; S4, the aluminum alloy ingot that has undergone the homogenization treatment is hot extruded to form an angle aluminum crossbeam blank with an angular cross section; S5, the angle aluminum crossarm blank is subjected to solution treatment and aging treatment to obtain a high-strength corrosion-resistant angle aluminum crossarm.

2. The method for preparing the angle aluminum crossarm according to claim 1, characterized in that, In step S1, the impurity elements in the aluminum alloy melt include Fe and Si, wherein the mass percentage of Fe is less than 0.3 wt%, the mass percentage of Si is less than 0.3 wt%, and the total mass percentage of other impurity elements besides Fe and Si is less than 0.15 wt%.

3. The method for preparing the angle aluminum crossarm according to claim 1, characterized in that, In step S1, after melting the Al raw material, Zn raw material, Cu raw material, Al-Mn master alloy and Al-Zr master alloy are added. After the Zn raw material, Cu raw material, Al-Mn master alloy and Al-Zr master alloy are melted, Mg raw material is added for smelting to obtain the aluminum alloy melt.

4. The method for preparing the angle aluminum crossarm according to claim 3, characterized in that, In step S1, the melting temperature of the aluminum alloy melt is 700-750℃; after the Mg raw material is melted, the aluminum alloy melt is refined and kept at a constant temperature for 20-40 minutes.

5. The method for preparing the angle aluminum crossarm according to claim 4, characterized in that, The refining process includes: refining the aluminum alloy melt with C2Cl6, or refining the aluminum alloy melt with nitrogen and / or argon, for a refining time of 5 to 10 minutes.

6. The method for preparing the angle aluminum crossarm according to claim 1, characterized in that, In step S3, the homogenization process includes: heating the aluminum alloy ingot to 440-465°C and holding it at that temperature for 20-40 hours, cooling it to below 220°C in the furnace after the holding period, and then air-cooling it after it is taken out of the furnace.

7. The method for preparing the angle aluminum crossarm according to claim 1, characterized in that, In step S4, the temperature of the hot extrusion molding is 380-420°C, and the extrusion ratio is 10-60.

8. The method for preparing the angle aluminum crossarm according to claim 1, characterized in that, In step S5, the solution treatment includes: heating the angle aluminum crossbeam blank to 450-490°C and holding it at that temperature for 1-4 hours; the aging treatment is artificial aging, which is carried out at a temperature of 100-130°C for 15-30 hours.

9. The method for preparing the angle aluminum crossarm according to claim 8, characterized in that, In step S5, after the solution treatment is completed, the angle aluminum crossarm blank is first subjected to water quenching, and then the angle aluminum crossarm blank after water quenching is subjected to the aging treatment.

10. An angle aluminum crossbeam, characterized in that, The aluminum angle crossbar is prepared by the preparation method described in any one of claims 1 to 9.

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