7xxx-series aluminum alloy wire rod and method of making, aluminum alloy article and method of making

Abstract

This invention provides a 7xxx series aluminum alloy wire and its preparation method, as well as aluminum alloy parts and their preparation methods. By mass percentage, the 7xxx series aluminum alloy wire comprises the following elements: 7.0~9.0% Zn, 1.5~2.5% Mg, 0.5~1.5% Cu, 0.05~0.35% Sc, 0.05~0.25% Zr, Ti ≤0.04%, Fe ≤0.15%, Si ≤0.10%, unavoidable impurities ≤0.15%, and the balance being Al; the total mass content of Sc and Zr is 0.20~0.45%. The 7xxx series aluminum alloy wire of this invention, after arc additive manufacturing and heat treatment, exhibits high strength and is less prone to cracking, better meeting the mechanical property requirements of aluminum alloys in aerospace and other fields.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy technology, and more specifically, to a 7xxx series aluminum alloy wire and its preparation method, and aluminum alloy parts and their preparation methods. Background Technology

[0002] Aluminum alloys are widely used due to their low density and good mechanical properties. Arc additive manufacturing technology uses an electric arc as a heat source and metal wire as raw material. By depositing the wire layer by layer along a planned path, arc additive manufacturing components are obtained. It can efficiently produce small batches of products with complex structures, thus enabling better applications in aerospace, weaponry, rail transportation and other fields.

[0003] Currently, the aluminum alloys used in arc additive manufacturing are mainly Al-Si, Al-Mg, and some Al-Cu alloy wires. Their strength after arc additive manufacturing is generally below 300 MPa, and after heat treatment, it reaches around 400 MPa. Furthermore, cracking is prone to occur during the arc additive manufacturing process, which cannot meet the future equipment development requirements of the aerospace field. Therefore, there is a need to develop high-strength aluminum alloy wires suitable for arc additive manufacturing. Summary of the Invention

[0004] The main objective of this invention is to provide a 7xxx series aluminum alloy wire and its preparation method, as well as an aluminum alloy part and its preparation method, to solve the problems of low strength and easy cracking of aluminum alloy wires used in additive manufacturing in the prior art.

[0005] To achieve the above objectives, according to one aspect of the present invention, a 7xxx series aluminum alloy wire is provided, comprising, by mass percentage, the following elements: 7.0~9.0% Zn, 1.5~2.5% Mg, 0.5~1.5% Cu, 0.05~0.35% Sc, 0.05~0.25% Zr, Ti ≤0.04%, Fe ≤0.15%, Si ≤0.10%, and the total content of unavoidable impurities ≤0.15%, with the balance being Al; the total mass content of Sc and Zr is 0.20~0.45%; the volume fraction of Al3(Sc, Zr) nanoparticles in the 7xxx series aluminum alloy wire is 0.25~0.49%; and the volume fraction of MgZn2 phase in the 7xxx series aluminum alloy wire is 1.50~2.20%.

[0006] Furthermore, by mass percentage, the aforementioned 7xxx series aluminum alloy wire comprises the following elements: 7.5~8.8% Zn, 1.7~2.5% Mg, 1.0~1.5% Cu, 0.10~0.35% Sc, 0.10~0.25% Zr, 0.01~0.04% Ti, 0.03~0.10% Fe, 0.02~0.08% Si, with unavoidable impurities totaling ≤0.15%, and the balance being Al.

[0007] Furthermore, the mass ratio of Zn to Mg is 3~4.5:1; and / or the mass ratio of Sc to Zr is 0.5~3.5:1; and / or the diameter of the 7xxx series aluminum alloy wire is 1.0~1.6mm.

[0008] According to another aspect of the present invention, a method for preparing the above-mentioned 7xxx series aluminum alloy wire is provided. The preparation method includes: step S1, after batching the raw materials corresponding to the aluminum alloy, performing melting, refining, casting, homogenization heat treatment and hot rolling in sequence to obtain a bar; step S2, performing multiple annealing, drawing and wire scraping on the bar in sequence to obtain 7xxx series aluminum alloy wire.

[0009] Further, step S1 above also includes: after the pure aluminum ingot is first melted, Al-Sc master alloy, Al-Zr master alloy, Al-Cu master alloy and pure zinc ingot are added for a second melting to obtain a first alloy liquid; the first alloy liquid is stirred, allowed to stand and first degassed and refined in sequence, and then pure magnesium ingot is added for a third melting to obtain a second alloy liquid; the second alloy liquid is subjected to a second degassing and refining and casting in sequence to obtain an ingot; the ingot is subjected to homogenization heat treatment, cooling and hot rolling in sequence to obtain a bar; In the process, the first melting temperature is 850~870℃, and the first melting time is 2~6h; the second melting temperature is 780~800℃, and the second melting time is 1~4h; the third melting temperature is 750~770℃, and the third melting time is 10~20min; the first degassing and refining temperature is 720~760℃, and the first degassing and refining time is 15~35min; the second degassing and refining temperature is 710~750℃, and the second degassing and refining time is 18~36min.

[0010] Further, in step S1 above, the homogenization heat treatment temperature is 470~475℃, and the homogenization heat treatment time is 24~30h; and / or, hot rolling includes multiple passes of hot roughing and multiple passes of hot finishing, the deformation per pass of hot roughing is 2.7~28.0%, and the total deformation of hot roughing is 95~98.0%; the deformation per pass of hot finishing is 10.0~30.0%, and the total deformation of hot finishing is 82~85%; the hot rolling temperature is 430~450℃; and / or, step S2 above further includes: cold rolling the bar to obtain cold-rolled wire, and subjecting the cold-rolled wire to multiple passes of annealing, drawing, and wire scraping to obtain 7xxx series aluminum alloy wire; and / or, the temperature of the multiple passes of annealing is 410~450℃.

[0011] According to another aspect of the present invention, a method for preparing an aluminum alloy part is provided, the method comprising: performing arc additive manufacturing on the above-mentioned 7xxx series aluminum alloy wire to obtain an arc additive manufacturing component; and performing heat treatment on the arc additive manufacturing component to obtain an aluminum alloy part.

[0012] Furthermore, the voltage of the arc additive manufacturing is 9.8~12.5V, and the current of the arc additive manufacturing is 90~140A; the volume fraction of Al3(Sc, Zr) nanoparticles in the arc additive manufactured component is 0.15~0.30%; and / or, the density of the arc additive manufactured component is ≥99%, the average grain size of the arc additive manufactured component is 20~60μm; the yield strength of the arc additive manufactured component is ≥240MPa, the tensile strength of the arc additive manufactured component is ≥350MPa, and the elongation of the arc additive manufactured component is ≥6%.

[0013] Furthermore, the heat treatment includes solution treatment and aging performed sequentially; wherein the solution treatment temperature is 470~475℃ and the solution treatment time is 4~8h; the aging temperature is 110~121℃ and the aging time is 20~30h; the volume fraction of MgZn2 phase in the aluminum alloy part is 3.10~5.05%; and / or, the yield strength of the aluminum alloy part is ≥460MPa, the tensile strength of the aluminum alloy part is ≥550MPa, and the elongation of the aluminum alloy part is ≥8%.

[0014] According to another aspect of the present invention, an aluminum alloy part is provided, which is prepared by the above-described preparation method.

[0015] By applying the technical solution of this invention, the 7xxx series aluminum alloy wire of this application, with the addition of Sc and Zr elements and the control of the total mass of Sc and Zr elements within the aforementioned range, can form Al3(Sc, Zr) multi-component composite nanoparticles with the aforementioned volume fraction. This refines the grains and suppresses cracking during the arc additive manufacturing process, thereby improving the additive manufacturing formability and mechanical properties of the 7xxx series aluminum alloy wire. Further controlling the mass content of Al3(Sc, Zr) nanoparticles within the aforementioned range enables the tensile strength of the 7xxx series aluminum alloy wire after additive manufacturing to be ≥350MPa, while simultaneously suppressing cracking. Adding the aforementioned mass content of Zn element can form a large amount of MgZn2 reinforcing phase, thereby improving the mechanical properties of the 7xxx series aluminum alloy wire. Controlling the mass content of Zn and Mg elements within the aforementioned range enables the tensile strength of the aluminum alloy after further heat treatment following additive manufacturing to be ≥550MPa. Adding the aforementioned amount of Cu promotes the precipitation of the MgZn2 strengthening phase after aging heat treatment of arc additive manufacturing components, thereby further improving their strength. Simultaneously, it also improves the intergranular structure, such as grain boundary precipitates and grain boundary non-precipitate zones, thus enhancing the corrosion resistance of the aluminum alloy. In summary, the 7xxx series aluminum alloy wire of this application, after arc additive manufacturing and heat treatment, exhibits high strength and is less prone to cracking, better meeting the mechanical property requirements of aluminum alloys in aerospace, weaponry, and rail transportation fields. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0017] Figure 1 The surface morphology of the 7xxx series aluminum alloy wire in Embodiment 5 of this application is shown;

[0018] Figure 2 A metallographic diagram of the arc additive manufacturing component in Embodiment 5 of this application is shown;

[0019] Figure 3 A grain structure diagram of the arc additive manufacturing component in Embodiment 5 of this application is shown;

[0020] Figure 4 SEM images of the aluminum alloy part in Embodiment 5 of this application are shown;

[0021] Figure 5 The surface morphology of the 7xxx series aluminum alloy wire in Comparative Example 8 of this application is shown;

[0022] Figure 6 The metallographic structure of the arc additive manufacturing component in Comparative Example 8 of this application is shown;

[0023] Figure 7 A grain structure diagram of the arc additive manufacturing component in Comparative Example 8 of this application is shown;

[0024] Figure 8 SEM images of the aluminum alloy parts in Comparative Example 8 of this application are shown. Detailed Implementation

[0025] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] As analyzed in the background section of this application, the aluminum alloy wires used in additive manufacturing in the prior art have the problems of low strength and easy cracking. In order to solve the above problems, this application provides a 7xxx series aluminum alloy wire and its preparation method, as well as an aluminum alloy part and its preparation method.

[0027] In a typical embodiment of this application, a 7xxx series aluminum alloy wire is provided, comprising, by mass percentage, the following elements: 7.0~9.0% Zn, 1.5~2.5% Mg, 0.5~1.5% Cu, 0.05~0.35% Sc, 0.05~0.25% Zr, Ti ≤0.04%, Fe ≤0.15%, Si ≤0.10%, unavoidable impurities ≤0.15%, and the balance being Al; the total mass content of Sc and Zr is 0.25~0.50%; the volume fraction of Al3(Sc, Zr) nanoparticles in the 7xxx series aluminum alloy wire is 0.25~0.49%; and the volume fraction of MgZn2 phase in the 7xxx series aluminum alloy wire is 1.50~2.20%.

[0028] The 7xxx series aluminum alloy wire of this application, with the addition of Sc and Zr elements and the control of the total mass of Sc and Zr elements within the aforementioned range, can form Al3(Sc, Zr) multi-component composite nanoparticles with the aforementioned volume fractions. This refines the grains and suppresses cracking during the arc additive manufacturing process, thereby improving the additive manufacturing formability and mechanical properties of the 7xxx series aluminum alloy wire. Further controlling the mass content of Al3(Sc, Zr) nanoparticles within the aforementioned range enables the tensile strength of the 7xxx series aluminum alloy wire after additive manufacturing to be ≥350 MPa, while simultaneously suppressing cracking. The addition of Zn elements with the aforementioned mass content forms a large amount of MgZn2 reinforcing phase, thereby improving the mechanical properties of the 7xxx series aluminum alloy wire. Controlling the mass content of Zn and Mg elements within the aforementioned range enables the tensile strength of the aluminum alloy after further heat treatment following additive manufacturing to be ≥550 MPa. Adding the aforementioned amount of Cu promotes the precipitation of the MgZn2 strengthening phase after aging heat treatment of arc additive manufacturing components, thereby further improving their strength. Simultaneously, it also improves the intergranular structure, such as grain boundary precipitates and grain boundary non-precipitate zones, thus enhancing the corrosion resistance of the aluminum alloy. In summary, the 7xxx series aluminum alloy wire of this application, after arc additive manufacturing and heat treatment, exhibits high strength and is less prone to cracking, better meeting the mechanical property requirements of aluminum alloys in aerospace, weaponry, and rail transportation fields.

[0029] To further improve the mechanical properties of 7xxx series aluminum alloy wire, in one embodiment of this application, the 7xxx series aluminum alloy wire comprises the following elements by mass percentage: 7.5~8.8% Zn, 1.7~2.5% Mg, 1.0~1.5% Cu, 0.10~0.35% Sc, 0.10~0.25% Zr, 0.01~0.04% Ti, 0.03~0.10% Fe, 0.02~0.08% Si, with the total content of unavoidable impurities ≤0.15%, and the balance being Al.

[0030] In one embodiment of this application, the mass ratio of Zn to Mg is 3~4.5:1; and / or, the mass ratio of Sc to Zr is 0.5~3.5:1; and / or, the diameter of the 7xxx series aluminum alloy wire is 1.0~1.6mm.

[0031] Preferably controlling the mass ratio of Zn and Mg within the above range helps promote the formation of the MgZn2 strengthening phase, thereby improving the mechanical properties of 7xxx series aluminum alloy wires. At the same time, it also helps promote the precipitation of the MgZn2 strengthening phase after heat treatment of aluminum alloys (arc additive manufacturing components) after arc additive manufacturing, thereby further improving the mechanical properties of aluminum alloy parts (arc additive manufacturing components after heat treatment).

[0032] Preferably controlling the mass ratio of Sc to Zr within the aforementioned range helps promote the formation of Al3(Sc, Zr) multi-component composite nanoparticles, thereby refining the grains and reducing cracking, thus improving the additive manufacturing formability and mechanical properties of 7xxx series aluminum alloy wires. Simultaneously, it also facilitates the re-precipitation of dispersed Al3(Sc, Zr) multi-component composite nanoparticles after arc additive manufacturing of 7xxx series aluminum alloy wires, thereby better controlling grain growth and dispersion strengthening after heat treatment, ultimately improving the strength of aluminum alloy parts.

[0033] Preferably controlling the diameter of 7xxx series aluminum alloy wire within the above range helps to achieve uniform melting and deposition during the arc additive manufacturing process, forming a good interlayer bond, thereby improving the density and overall performance of aluminum alloy parts.

[0034] In another typical embodiment of this application, a method for preparing the above-mentioned 7xxx series aluminum alloy wire is provided. The preparation method includes: step S1, after the raw materials corresponding to the aluminum alloy are batched, they are successively smelted, refined, cast, homogenized heat treatment and hot rolled to obtain a bar; step S2, the bar is successively annealed, drawn and scraped in multiple passes to obtain 7xxx series aluminum alloy wire.

[0035] The 7xxx series aluminum alloy wire obtained by the above preparation method in this application exhibits excellent mechanical properties and high surface quality. Specifically, after batching according to the elemental composition requirements of the 7xxx series aluminum alloy wire, the raw materials are completely melted through smelting. Refining removes gases and inclusions from the alloy melt, thereby improving the purity of the aluminum alloy. Preliminary forming through casting followed by homogenization heat treatment eliminates component segregation, thus improving the microstructure of the aluminum alloy. Hot rolling forms bars, preparing them for further processing. Multiple annealing passes on the bars eliminate work hardening, restoring the material's ductility and plasticity, which is beneficial for subsequent drawing processes and reduces potential cracks and fractures during drawing. Drawing gradually reduces the wire diameter to the required specifications. Scraping removes the oxide layer or other surface defects formed on the drawn wire surface, thereby improving the surface quality of the 7xxx series aluminum alloy wire. In summary, the preparation method of this application can produce high-quality, high-purity 7xxx series aluminum alloy wires, which can be better applied in arc additive manufacturing. The resulting aluminum alloy parts have excellent microstructure and mechanical properties, thus better meeting the requirements of aerospace and other fields.

[0036] In one embodiment of this application, step S1 further includes: step S11, after the pure aluminum ingot is first melted, Al-Sc master alloy, Al-Zr master alloy, Al-Mn master alloy and pure zinc ingot are added for a second melting to obtain a first alloy liquid; step S12, after the first alloy liquid is stirred, allowed to stand and first degassed and refined, pure magnesium ingot is added for a third melting to obtain a second alloy liquid; step S13, the second alloy liquid is second degassed and refined and cast to obtain an ingot; the ingot is then subjected to homogenization heat treatment, The material is cooled and hot-rolled to obtain bars; wherein the first melting temperature is 850~870℃ and the first melting time is 2~6h; the second melting temperature is 780~800℃ and the second melting time is 1~4h; the third melting temperature is 750~770℃ and the third melting time is 10~20min; the first degassing and refining temperature is 720~760℃ and the first degassing and refining time is 15~35min; the second degassing and refining temperature is 710~750℃ and the second degassing and refining time is 18~36min.

[0037] Preferably, the temperatures and times for the first and second melting processes are within the aforementioned ranges, which helps to fully melt the pure aluminum ingot, pure zinc ingot, and the aforementioned intermediate alloy, thereby contributing to the uniform distribution of the alloying elements in the molten aluminum. The first and second degassing refining processes each independently include: introducing pure argon for degassing and refining. Preferably, the temperature and time for the first degassing refining are within the aforementioned ranges, which helps to remove dissolved gases from the first alloy molten aluminum, reducing porosity and oxides in the material, thereby improving the density and mechanical properties of the 7xxx series aluminum alloy wire. Preferably, after the first degassing refining, pure magnesium ingots are added for a third melting process. Controlling the temperature and time of the third melting within the aforementioned ranges helps to ensure the magnesium element is fully dissolved and uniformly distributed in the first alloy molten aluminum, thereby improving the strength of the 7xxx series aluminum alloy wire. Preferably, the temperature and time for the second degassing refining are within the aforementioned ranges, which helps to further remove gases from the second alloy molten aluminum, thereby improving the quality of the 7xxx series aluminum alloy wire.

[0038] The preferred Al-Sc master alloy has a Sc content of 1.8-2.2% by mass, the preferred Al-Zr master alloy has a Zr content of 3.8-4.2% by mass, and the preferred Al-Cu master alloy has a Cu content of 48-52% by mass.

[0039] In one embodiment of this application, in step S1 above, the homogenization heat treatment temperature is 470~475℃, and the homogenization heat treatment time is 24~30h; and / or, hot rolling includes multiple passes of hot roughing and multiple passes of hot finishing, the deformation per pass of hot roughing is 2.7~28.0%, and the total deformation (total processing rate) of hot roughing is 95~98%; the deformation per pass of hot finishing is 10.0~30.0%, and the total deformation (total processing rate) of hot finishing is 82~85%; the hot rolling temperature is 430~450℃; and / or, step S2 further includes: cold rolling the bar to obtain cold-rolled wire, and subjecting the cold-rolled wire to multiple passes of annealing, drawing, and wire scraping to obtain 7xxx series aluminum alloy wire; and / or, the temperature of the multiple passes of annealing is 410~450℃.

[0040] Preferably controlling the temperature and time of the homogenization heat treatment within the aforementioned range not only helps promote grain size homogenization and reduce the formation of coarse grains, thereby improving the machinability of the aluminum alloy, but also helps reduce internal stress generated during casting, thus reducing the likelihood of cracking during subsequent processing, and ultimately improving the surface quality of 7xxx series aluminum alloy wire. Preferably controlling the hot rolling temperature within the aforementioned range, and controlling the deformation per pass and the total deformation within the aforementioned range, helps to give the aluminum alloy good plastic deformation capacity and obtain a uniform deformed microstructure. Preferably controlling the deformation per pass and the total deformation within the aforementioned range during hot finish rolling helps to improve the plastic deformation of 7xxx series aluminum alloy wire, thereby obtaining aluminum alloy wire with a uniform microstructure.

[0041] The preferred material is bar stock, which is then cold-rolled to obtain wire. The preferred multi-pass annealing temperature is within the aforementioned range, which helps reduce internal stress, restore the ductility and plasticity of the wire, and thus facilitates subsequent drawing and forming.

[0042] In another typical embodiment of this application, a method for preparing an aluminum alloy part is provided, the method comprising: step S1, performing arc additive manufacturing on the above-mentioned 7xxx series aluminum alloy wire to obtain an arc additive manufactured component; step S2, performing heat treatment on the arc additive manufactured component to obtain an aluminum alloy part.

[0043] This application utilizes the aforementioned 7xxx series aluminum alloy wire for arc additive manufacturing, enabling the aluminum alloy wire to be deposited layer by layer along a planned path to obtain arc additive manufactured components with complex structures. Simultaneously, arc additive manufacturing promotes the formation of Al3(Sc, Zr) multi-component composite nanoparticles, thereby refining grains and inhibiting solidification cracking, thus improving the strength of the arc additive manufactured components and reducing cracking. Heat treatment of the arc additive manufactured components promotes the redissolution and precipitation of strengthening phases such as MgZn2, forming fine and dispersed strengthening particles, thereby significantly improving the strength of the aluminum alloy parts and better meeting the mechanical property requirements of aluminum alloys in aerospace and other fields.

[0044] In one embodiment of this application, the voltage of the arc additive manufacturing is 9.8~12.5V, the current of the arc additive manufacturing is 90~140A; the volume fraction of Al3(Sc, Zr) nanoparticles in the arc additive manufactured component is 0.15~0.30%; the average grain size of the arc additive manufactured component is 20~60μm; and / or, the density of the arc additive manufactured component is ≥99%, the yield strength of the arc additive manufactured component is ≥240MPa, the tensile strength of the arc additive manufactured component is ≥350MPa, and the elongation of the arc additive manufactured component is ≥6%.

[0045] Preferably controlling the voltage and current in arc additive manufacturing within the aforementioned range not only helps to obtain a dense, crack-free structure and reduce defects such as pores, but also helps to promote the formation of Al3(Sc, Zr) nanoparticles, ensuring that their volume fraction reaches the aforementioned range. At the same time, controlling their average grain size within the aforementioned range helps to improve the strength of arc additive manufactured components, ensuring that their mechanical properties reach the aforementioned range.

[0046] In one embodiment of this application, the heat treatment includes solution treatment and aging performed sequentially; wherein the solution treatment temperature is 470~475℃ and the solution treatment time is 4~8h; the aging temperature is 110~121℃ and the aging time is 20~30h; the volume fraction of MgZn2 phase in the aluminum alloy part is 3.10~4.60%; and / or, the yield strength of the aluminum alloy part is ≥460MPa, the tensile strength of the aluminum alloy part is ≥550MPa, and the elongation of the aluminum alloy part is ≥8%.

[0047] Preferably controlling the solution temperature and time, and the aging temperature and time within the above ranges helps to promote the redissolution and precipitation of the MgZn2 phase, so that its volume fraction reaches the above range, thereby improving the strength of aluminum alloy parts and making their mechanical properties reach the above range, thus better meeting the requirements of aerospace and other fields for the mechanical properties of aluminum alloys.

[0048] In another typical embodiment of this application, an aluminum alloy part is provided, which is prepared by the above-described preparation method.

[0049] The aluminum alloy parts prepared by the above method have high strength and are not easy to crack, and can be better applied in aerospace, weaponry, rail transportation and other fields.

[0050] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0051] Example 1

[0052] Preparation of 7xxx series aluminum alloy wire:

[0053] The aluminum alloy raw materials are formulated by mass percentage as follows: 7.0% Zn, 2.0% Mg, 1.5% Cu, 0.05% Sc, 0.25% Zr, 0.08% Fe, 0.06% Si, 0.03% Ti, with the balance being Al.

[0054] Pure aluminum ingots were heated to 860℃ for the first melting. When the furnace temperature reached 790℃, Al-2Sc master alloy, Al-4Zr master alloy, Al-50Cu master alloy, and pure zinc ingots were added for the second melting, resulting in the first alloy liquid. This was then stirred and allowed to stand. When the standing temperature reached 740℃, the first degassing and refining process was performed, involving the introduction of pure argon for degassing and stirring for 25 minutes. At 760℃, pure magnesium ingots were added for the third melting, resulting in the second alloy liquid. This second alloy liquid underwent a second degassing and refining process, involving the introduction of pure argon for degassing and stirring for 25 minutes, and was then cast into ingots (round ingots) with a diameter of 162mm.

[0055] The round ingots were heated to 470℃ for homogenization heat treatment for 30 hours, then air-cooled after removal from the furnace. The ingots were then machined into round ingots with a diameter of 145mm. The round ingots were then hot-rolled at 440℃. The hot rolling process consisted of first performing multiple passes of hot rough rolling to obtain bars with a diameter of 20mm, with a pass deformation of 10% and a total deformation of 98.0%. Then, multiple passes of hot finish rolling were performed to obtain bars with a diameter of 8mm, with a pass deformation of 20% and a total deformation of 84.0%.

[0056] The bar material is subjected to multiple annealing, drawing, wire scraping and cleaning processes to obtain 7xxx series aluminum alloy wire with a diameter of 1.2mm. The annealing temperature is 430℃.

[0057] Preparation of aluminum alloy parts:

[0058] 7xxx series aluminum alloy wire was manufactured using CMT (Continuous Arc Technology) to obtain an arc-manufactured component. The arc-manufacture voltage was 12.1V and the arc-manufacture current was 120A. The arc-manufactured component was then subjected to heat treatment. The heat treatment consisted of solution treatment at 475℃ for 4 hours and aging at 121℃ for 24 hours.

[0059] Example 2

[0060] The difference from Example 1 lies in the preparation of the 7xxx series aluminum alloy wire: The aluminum alloy raw materials were proportioned by mass percentage as follows: 7.3% Zn, 1.8% Mg, 1.3% Cu, 0.08% Sc, 0.18% Zr, 0.09% Fe, 0.07% Si, 0.03% Ti, with the balance being Al. The homogenization heat treatment temperature was 472℃, ultimately yielding the 7xxx series aluminum alloy wire.

[0061] Preparation of aluminum alloy parts: The solution treatment temperature was 475°C and the time was 4 hours; the aging temperature was 121°C and the time was 24 hours, and finally aluminum alloy parts were obtained.

[0062] Example 3

[0063] The difference from Example 1 lies in the preparation of the 7xxx series aluminum alloy wire: The aluminum alloy raw materials were proportioned by mass percentage as follows: 7.5% Zn, 1.5% Mg, 1.2% Cu, 0.10% Sc, 0.16% Zr, 0.07% Fe, 0.05% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature was 473℃, ultimately yielding the 7xxx series aluminum alloy wire.

[0064] Preparation of aluminum alloy parts: The solution treatment temperature was 475°C and the time was 4 hours; the aging temperature was 121°C and the time was 24 hours, and finally aluminum alloy parts were obtained.

[0065] Example 4

[0066] The difference from Example 1 lies in the preparation of the 7xxx series aluminum alloy wire: The aluminum alloy raw materials were proportioned by mass percentage as follows: 7.8% Zn, 2.2% Mg, 1.1% Cu, 0.15% Sc, 0.15% Zr, 0.07% Fe, 0.05% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature was 475℃. The bar was cold-rolled to obtain wire with a diameter of 4mm, followed by multiple passes of annealing, drawing, scraping, and cleaning to obtain the 7xxx series aluminum alloy wire.

[0067] Preparation of aluminum alloy parts: The solution treatment temperature was 475°C and the time was 4 hours; the aging temperature was 121°C and the time was 24 hours, and finally aluminum alloy parts were obtained.

[0068] Example 5

[0069] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.0% Zn, 2.3% Mg, 1.0% Cu, 0.20% Sc, 0.10% Zr, 0.06% Fe, 0.04% Si, 0.01% Ti, with the balance being Al, to finally obtain the 7xxx series aluminum alloy wire, and ultimately the aluminum alloy parts.

[0070] Example 6

[0071] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.2% Zn, 2.4% Mg, 0.8% Cu, 0.25% Sc, 0.08% Zr, 0.07% Fe, 0.04% Si, 0.02% Ti, with the balance being Al, to finally obtain the 7xxx series aluminum alloy wire, and ultimately the aluminum alloy parts.

[0072] Example 7

[0073] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.5% Zn, 2.5% Mg, 0.7% Cu, 0.30% Sc, 0.10% Zr, 0.08% Fe, 0.04% Si, 0.02% Ti, with the balance being Al, to finally obtain the 7xxx series aluminum alloy wire, and ultimately the aluminum alloy parts.

[0074] Example 8

[0075] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 9.0% Zn, 2.5% Mg, 0.5% Cu, 0.35% Sc, 0.05% Zr, 0.06% Fe, 0.06% Si, 0.02% Ti, with the balance being Al, to finally obtain the 7xxx series aluminum alloy wire, and ultimately the aluminum alloy parts.

[0076] Example 9

[0077] The difference from Example 5 is that the total mass content of Mg and Zn elements is 10.3%, and the mass ratio of Zn to Mg elements is 3.12:1, ultimately yielding 7xxx series aluminum alloy wire and aluminum alloy parts.

[0078] Example 10

[0079] The difference from Example 5 is that the total mass content of Mg and Zn elements is 10.3%, and the mass ratio of Zn to Mg elements is 4.42:1, ultimately yielding 7xxx series aluminum alloy wire and aluminum alloy parts.

[0080] Example 11

[0081] The difference from Example 5 is that the total mass content of Mg and Zn elements is 10.3%, and the mass ratio of Zn to Mg elements is 5.06:1, ultimately yielding 7xxx series aluminum alloy wire, and finally aluminum alloy parts.

[0082] Example 12

[0083] The difference from Example 5 is that the total mass content of Sc and Zr elements is 0.35%, and the mass ratio of Sc to Zr elements is 0.52:1, ultimately yielding 7xxx series aluminum alloy wire and aluminum alloy parts.

[0084] Example 13

[0085] The difference from Example 5 is that the total mass content of Sc and Zr elements is 0.35%, the mass ratio of Sc to Zr elements is 3.38:1, and finally 7xxx series aluminum alloy wire is obtained, and finally aluminum alloy parts are obtained.

[0086] Example 14

[0087] The difference from Example 5 is that the total mass content of Sc and Zr elements is 0.35%, and the mass ratio of Sc to Zr elements is 0.17:1, ultimately obtaining 7xxx series aluminum alloy wire, and finally obtaining aluminum alloy parts.

[0088] Example 15

[0089] The difference from Example 5 is that the homogenization heat treatment temperature in the preparation of 7xxx series aluminum alloy wire is 470°C, and 7xxx series aluminum alloy wire is finally obtained.

[0090] Preparation of aluminum alloy parts: The voltage of electric arc additive manufacturing is 9.8V, the current of electric arc additive manufacturing is 90A, the solution temperature is 470°C and the time is 8h; the aging temperature is 110°C and the time is 30h, and finally aluminum alloy parts are obtained.

[0091] Example 16

[0092] The difference from Example 5 is that the homogenization heat treatment temperature in the preparation of 7xxx series aluminum alloy wire is 465°C, and 7xxx series aluminum alloy wire is finally obtained.

[0093] Preparation of aluminum alloy parts: The voltage of electric arc additive manufacturing is 12.8V, the current of electric arc additive manufacturing is 150A, the solution temperature is 468°C and the time is 10h; the aging temperature is 125°C and the time is 16h, and finally aluminum alloy parts are obtained.

[0094] Comparative Example 1

[0095] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: The aluminum alloy raw materials were proportioned by mass percentage as follows: 7.7% Zn, 1.4% Mg, 1.3% Cu, 0.18% Sc, 0.10% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature was 470℃, ultimately yielding the 7xxx series aluminum alloy wire, and finally, the aluminum alloy parts.

[0096] Comparative Example 2

[0097] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: The aluminum alloy raw materials were proportioned by mass percentage as follows: 6.9% Zn, 2.0% Mg, 1.3% Cu, 0.10% Sc, 0.15% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature was 472℃, ultimately yielding the 7xxx series aluminum alloy wire, and finally, the aluminum alloy parts.

[0098] Comparative Example 3

[0099] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 7.6% Zn, 1.9% Mg, 0.3% Cu, 0.13% Sc, 0.12% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, and the hot rolling temperature is 430°C, ultimately yielding the 7xxx series aluminum alloy wire, and finally, aluminum alloy parts.

[0100] Comparative Example 4

[0101] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.6% Zn, 2.8% Mg, 0.8% Cu, 0.15% Sc, 0.13% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, and the hot rolling temperature is 430°C, ultimately yielding the 7xxx series aluminum alloy wire, which in turn yields the aluminum alloy parts.

[0102] Comparative Example 5

[0103] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 9.3% Zn, 2.3% Mg, 1.0% Cu, 0.18% Sc, 0.10% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, and the hot rolling temperature is 430°C, ultimately yielding the 7xxx series aluminum alloy wire, which in turn yields the aluminum alloy parts.

[0104] Comparative Example 6

[0105] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are prepared according to the following proportions: 8.5% Zn, 2.3% Mg, 1.8% Cu, 0.13% Sc, 0.15% Zr, 0.08% Fe, 0.06% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, and finally, 7xxx series aluminum alloy wire is obtained, which in turn yields aluminum alloy parts.

[0106] Comparative Example 7

[0107] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.2% Zn, 2.1% Mg, 1.3% Cu, 0.10% Sc, 0.04% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, ultimately yielding the 7xxx series aluminum alloy wire, and finally, aluminum alloy parts.

[0108] Comparative Example 8

[0109] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 7.8% Zn, 2.0% Mg, 1.2% Cu, 0.04% Sc, 0.12% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, ultimately yielding the 7xxx series aluminum alloy wire, and finally, aluminum alloy parts.

[0110] Comparative Example 9

[0111] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 8.2% Zn, 2.1% Mg, 1.4% Cu, 0.32% Sc, 0.15% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, ultimately yielding the 7xxx series aluminum alloy wire, and finally, aluminum alloy parts.

[0112] Comparative Example 10

[0113] The difference from Example 4 lies in the preparation of the 7xxx series aluminum alloy wire: by mass percentage, the aluminum alloy raw materials are proportioned as follows: 7.8% Zn, 2.0% Mg, 1.3% Cu, 0.22% Sc, 0.25% Zr, 0.06% Fe, 0.04% Si, 0.02% Ti, with the balance being Al. The homogenization heat treatment temperature is 473°C, ultimately yielding the 7xxx series aluminum alloy wire, and finally, aluminum alloy parts.

[0114] Test method:

[0115] Average grain size test: Metallographic samples were taken from the wire arc additive manufacturing components. After anodic coating, the grain structure was observed using a metallographic microscope. The average grain size was calculated using the intercept method according to national standard GB / T 6394.

[0116] Volume fraction of Al3(Sc, Zr) nanoparticles: The distribution of Al3(Sc, Zr) particles in the alloy microstructure was observed using TEM, and the size and number of Al3(Sc, Zr) particles were statistically analyzed to calculate the volume fraction.

[0117] Volume fraction of MgZn2 phase: The distribution of MgZn2 precipitates in the alloy microstructure is observed using TEM, and the size and quantity of MgZn2 phase are counted to calculate the volume fraction.

[0118] Yield strength, tensile strength and elongation tests: Take plate-shaped tensile specimens along the printing direction of the arc additive forming part and perform tensile property tests in accordance with the "Methods and requirements for tensile testing of metallic materials at room temperature" (GB / T 228.1-2021).

[0119] The microstructure characteristics of the 7xxx series aluminum alloy wires are shown in Table 1. The microstructure and properties of the arc additively manufactured components in the printed state are shown in Table 2. The microstructure and properties of the heat-treated aluminum alloy parts are shown in Table 3.

[0120] Table 1

[0121]

[0122] Table 2

[0123]

[0124] Table 3

[0125]

[0126] As can be seen from the above, the content of Mg in Comparative Example 1 is relatively low, the content of Zn in Comparative Example 2 is relatively low, and the content of Cu in Comparative Example 3 is relatively low. After the heat treatment of the components manufactured by arc additive manufacturing, the amount of MgZn2 precipitation strengthening phase is reduced, which leads to the lower strength of the aluminum alloy parts.

[0127] Comparative Example 4 had a high Mg content, Comparative Example 5 had a high Zn content, and Comparative Example 6 had a high Cu content. These components, manufactured by arc additive manufacturing, exhibited a large amount of eutectic phase in their microstructure. Furthermore, the second phase in the microstructure was difficult to completely dissolve after heat treatment, leading to reduced plasticity. Therefore, the elongation of both the arc additive manufactured components and the aluminum alloy parts was relatively low.

[0128] The Zr content in Comparative Example 7 was low, and the Sc content in Comparative Example 8 was low. The total amount of both Sc and Zr was also low. As a result, there were not enough Al3(Sc, Zr) nanoparticles to refine the grains and suppress cracking during the arc additive manufacturing process. Therefore, the microstructure of the arc additive manufactured component contained cracks, and its strength and elongation were also low.

[0129] The total amounts of Sc and Zr elements were higher in Comparative Examples 9 and 10, while the total amounts of Sc and Zr elements were lower in Comparative Example 9. This resulted in the presence of primary Al3(Sc, Zr) phase particles in the microstructure of the arc additive manufacturing components. These particles were difficult to dissolve after heat treatment, leading to poorer plasticity and reduced elongation of the alloy.

[0130] in, Figure 1 This is a surface morphology image of the 7xxx series aluminum alloy wire in Example 5. Figure 1 As can be seen, the surface of the 7xxx series aluminum alloy wire is smooth and free of cracks.

[0131] Figure 2 This is a metallographic diagram of the arc additive manufacturing component in Example 5, from... Figure 2 As can be seen, the components manufactured by electric arc additive manufacturing have a dense structure, no cracks, and a small number of pores.

[0132] Figure 3 This is a grain structure diagram of the arc additive manufacturing component in Example 5, from... Figure 3 As can be seen, the printed structure of the arc additive manufacturing component consists of fine equiaxed grains with uniform size.

[0133] Figure 4 This is a SEM image of the aluminum alloy part in Example 5, from... Figure 4 As can be seen, after solution treatment, the coarse second phase in the microstructure of the arc additive manufacturing component is completely dissolved, which is conducive to the precipitation of the MgZn2 strengthening phase.

[0134] Figure 5 The image shows the surface morphology of the 7xxx series aluminum alloy wire in Comparative Example 8. Figure 5 As can be seen, the surface of the 7xxx series aluminum alloy wire has a small number of defects such as pits and scratches.

[0135] Figure 6 This is a metallographic diagram of the arc additive manufacturing component in Example 8. Figure 6 As can be seen, there are some microcracks in the microstructure of the arc additive manufacturing components, resulting in poor printing effect.

[0136] Figure 7 This is a grain structure diagram of the arc additive manufacturing component in Example 8. Figure 7 As can be seen, the printed structure of the arc additive manufacturing component is columnar crystal morphology with large grain size.

[0137] Figure 8 This is a SEM image of the aluminum alloy part in Comparative Example 8. Figure 8 As can be seen, after solution treatment, the second phase in the microstructure of the arc additive manufacturing component is fully dissolved, which is conducive to the precipitation of the MgZn2 strengthening phase.

[0138] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0139] The 7xxx series aluminum alloy wire of this application, with the addition of Sc and Zr elements and the control of the total mass of Sc and Zr elements within the aforementioned range, can form Al3(Sc, Zr) multi-component composite nanoparticles with the aforementioned volume fractions. This refines the grains and suppresses cracking during the arc additive manufacturing process, thereby improving the additive manufacturing formability and mechanical properties of the 7xxx series aluminum alloy wire. Further controlling the mass content of Al3(Sc, Zr) nanoparticles within the aforementioned range enables the tensile strength of the 7xxx series aluminum alloy wire after additive manufacturing to be ≥350 MPa, while simultaneously suppressing cracking. The addition of Zn elements with the aforementioned mass content forms a large amount of MgZn2 reinforcing phase, thereby improving the mechanical properties of the 7xxx series aluminum alloy wire. Controlling the mass content of Zn and Mg elements within the aforementioned range enables the tensile strength of the aluminum alloy after further heat treatment following additive manufacturing to be ≥550 MPa. Adding the aforementioned amount of Cu promotes the precipitation of the MgZn2 strengthening phase after aging heat treatment of arc additive manufacturing components, thereby further improving their strength. Simultaneously, it also improves the intergranular structure, such as grain boundary precipitates and grain boundary non-precipitate zones, thus enhancing the corrosion resistance of the aluminum alloy. In summary, the 7xxx series aluminum alloy wire of this application, after arc additive manufacturing and heat treatment, exhibits high strength and is less prone to cracking, better meeting the mechanical property requirements of aluminum alloys in aerospace, weaponry, and rail transportation fields.

[0140] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A 7xxx series aluminum alloy wire, characterized in that, The 7xxx series aluminum alloy wire comprises, by weight percentage, 7.0-9.0% Zn. The composition comprises: 1.5-2.5% Mg; 0.5-1.5% Cu; 0.05-0.35% Sc; 0.05-0.25% Zr; Ti ≤0.04%; Fe ≤0.15%; Si ≤0.10%; unavoidable impurities ≤0.15%; and the balance being Al. The total mass content of the Sc and Zr elements is 0.20-0.45%. The volume fraction of Al3(Sc, Zr) nanoparticles in the 7xxx series aluminum alloy wire is 0.25~0.49%; The volume fraction of MgZn2 phase in the 7xxx series aluminum alloy wire is 1.50~2.20%.

2. The 7xxx series aluminum alloy wire according to claim 1, characterized in that, The 7xxx series aluminum alloy wire comprises the following elements by weight percentage: The Zn element is 7.5% to 8.8%; 1.7~2.5% of the Mg element; 1.0~1.5% Cu; 0.10~0.35% Sc; 0.10~0.25% Zr; 0.01~0.04% Ti; 0.03~0.10% Fe; 0.02~0.08% Si, with unavoidable impurities totaling ≤0.15%, and the balance being Al.

3. The 7xxx series aluminum alloy wire according to claim 1 or 2, characterized in that, The mass ratio of Zn to Mg is 3~4.5:1; and / or the mass ratio of Sc to Zr is 0.5~3.5:1; And / or, the diameter of the 7xxx series aluminum alloy wire is 1.0~1.6mm.

4. A method for preparing the 7xxx series aluminum alloy wire according to any one of claims 1 to 3, characterized in that, The preparation method includes: Step S1: After the raw materials corresponding to the aluminum alloy are batched, they are successively smelted, refined, cast, homogenized heat treatment and hot rolled to obtain bars; Step S2: The bar is subjected to multiple annealing, drawing and wire scraping processes to obtain the 7xxx series aluminum alloy wire.

5. The preparation method according to claim 4, characterized in that, Step S1 further includes: After the pure aluminum ingot is first melted, Al-Sc master alloy, Al-Zr master alloy, Al-Cu master alloy and pure zinc ingot are added for a second melting to obtain the first alloy liquid. The first alloy liquid is stirred, allowed to stand, and then degassed and refined in the first stage. Pure magnesium ingots are then added for the third melting to obtain the second alloy liquid. The second alloy liquid is subjected to a second degassing refining and the casting process in sequence to obtain an ingot; the ingot is subjected to the homogenization heat treatment, cooling and hot rolling in sequence to obtain the bar stock; Wherein, the first melting temperature is 850~870℃, and the first melting time is 2~6h; the second melting temperature is 780~800℃, and the second melting time is 1~4h; the third melting temperature is 750~770℃, and the third melting time is 10~20min; The temperature of the first degassing and refining process is 720~760℃, and the time for the first degassing and refining process is 15~35min; the temperature of the second degassing and refining process is 710~750℃, and the time for the second degassing and refining process is 18~36min.

6. The preparation method according to claim 4 or 5, characterized in that, In step S1, the temperature of the homogenization heat treatment is 470~475℃, and the time of the homogenization heat treatment is 24~30h. And / or, the hot rolling includes multiple passes of hot roughing and multiple passes of hot finishing, wherein the deformation per pass of the hot roughing is 2.7-28.0%, and the total deformation of the hot roughing is 95-98.0%; the deformation per pass of the hot finishing is 10.0-30.0%, and the total deformation of the hot finishing is 82-85%; and the temperature of the hot rolling is 430-450°C. And / or, step S2 further includes: cold rolling the bar to obtain cold-rolled wire, and sequentially subjecting the cold-rolled wire to the multi-pass annealing, drawing, and wire scraping to obtain the 7xxx series aluminum alloy wire; and / or, the temperature of the multi-pass annealing is 410~450℃.

7. A method for preparing an aluminum alloy part, characterized in that, The preparation method includes: The 7xxx series aluminum alloy wire according to any one of claims 1 to 3 is subjected to electric arc additive manufacturing to obtain an electric arc additive manufacturing component; the electric arc additive manufacturing component is subjected to heat treatment to obtain the aluminum alloy part.

8. The preparation method according to claim 7, characterized in that, The voltage of the arc additive manufacturing is 9.8~12.5V, and the current of the arc additive manufacturing is 90~140A; the volume fraction of Al3(Sc, Zr) nanoparticles in the arc additive manufactured component is 0.15~0.30%; And / or, the density of the arc additive manufacturing component is ≥99%, and the average grain size of the arc additive manufacturing component is 20~60μm; The yield strength of the arc additive manufacturing component is ≥240MPa, the tensile strength of the arc additive manufacturing component is ≥350MPa, and the elongation of the arc additive manufacturing component is ≥6%.

9. The preparation method according to claim 8, characterized in that, The heat treatment includes solution treatment and aging performed sequentially; wherein the solution treatment temperature is 470~475℃ and the solution treatment time is 4~8h; the aging temperature is 110~121℃ and the aging time is 20~30h; the volume fraction of MgZn2 phase in the aluminum alloy part is 3.10~5.05%; And / or, the aluminum alloy part has a yield strength ≥460MPa, a tensile strength ≥550MPa, and an elongation ≥8%.

10. An aluminum alloy part, characterized in that, The aluminum alloy part is prepared by the preparation method according to any one of claims 7 to 9.

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