Aluminum magnesium alloy bicycle frame pipe and preparation method thereof

Abstract

The invention discloses an aluminum-magnesium alloy bicycle frame pipe and a preparation method thereof, and particularly relates to the technical field of metal material machining, the aluminum-magnesium alloy bicycle frame pipe comprises the following alloy chemical components: Si, Mg, Cu, Mn, Fe, Cr and La + Ce, Fe and rare earth are defined as alloy elements, and the ratio of Mn / Fe and Mg / Si is optimized; the preparation method comprises the steps of refining and degassing, rare earth modification rod casting and hot extrusion forming. The problems that an existing aluminum alloy frame pipe is insufficient in strength, limited in light weight and uneven in performance are solved, through the scheme, the pipe achieves high plasticity, compact and uniform tissue, excellent welding performance and ultra-light weight, and the fatigue life is effectively prolonged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material processing, and particularly relates to an aluminum-magnesium alloy bicycle frame pipe and a preparation method thereof. BACKGROUND

[0002] The bicycle industry, as an important green travel and leisure sports field in the world, has never stopped pursuing the performance of frame materials for a long time. Among them, lightweight, high strength and high modulus are the key indicators for measuring the core performance of bicycle frame pipes, which are directly related to the riding efficiency, control experience and safety and reliability of the whole vehicle. Among many candidate materials, aluminum alloy, with its excellent mechanical properties, good formability and relatively low density, has gradually become one of the preferred materials for medium and high-end bicycle frames, and has been widely used and continuously iterated in the global range.

[0003] Looking at the existing technology, the current market mainstream bicycle frame pipes mostly adopt 6061, 6069, 6066 and other series of aluminum alloys with silicon, magnesium and copper as the main alloying elements. This kind of alloy belongs to heat treatable strengthening type aluminum alloy, and its original intention is to form fine and dispersed strengthening phases such as Mg2Si and Al2Cu through solid solution treatment and aging strengthening, so as to significantly improve the tensile strength and yield strength of the material. Specifically, when these alloys are prepared into bicycle frame pipes, the ingot is usually processed into a pipe with the required shape through hot extrusion process, and then welded to assemble into a frame. At a specific stage of historical development, 6XXX series aluminum alloy, with its relatively mature metallurgical process, moderate cost and comprehensive performance sufficient to meet market demand at that time, effectively promoted the process of bicycle lightweight, and realized significant weight reduction compared with traditional steel frame, laying a solid foundation for the improvement of bicycle performance.

[0004] However, with the increasing demand of consumers for bicycle performance, especially the pursuit of extreme light weight and higher riding safety, some inherent characteristics of existing aluminum alloys at the principle level gradually reveal their limitations in coping with new challenges, and evolve into deep technical contradictions that cannot be ignored. Specifically, although existing aluminum alloys have the potential for heat treatable strengthening, in the actual preparation process of bicycle frame tubes, especially for parts that are hot extruded and then need to be welded, the strength performance of the "hot extruded state" is often not optimal. The reason is that hot extrusion as a high-temperature deformation process will affect the solubility and precipitate phase state inside the alloy to some extent, resulting in difficulty in achieving the ultimate mechanical properties, especially the strength, without sufficient subsequent heat treatment (such as complete solution + aging). Correspondingly, in order to ensure the safety and structural stability of the frame during actual riding, a relatively large tube wall thickness must be maintained, for example, the minimum wall thickness of 0.8 mm is common in existing technology. Under this limitation, the further light weight space is severely compressed, making it difficult to meet the increasingly stringent light weight demand of the high-end market. Tube wall thickness less than 0.8 mm is often difficult to pass the rigorous bench test after being welded into a frame, thereby limiting its application in ultra-lightweight frame design.

[0005] Furthermore, the alloy composition ratio of existing aluminum alloys and their microstructure evolution during solidification and hot working make them prone to coarse grain rings and heterogeneous grain structures. This non-uniform grain structure essentially reflects the lack of control of grain growth and recrystallization behavior of the alloy under certain process conditions. Specifically, the formation of coarse grain rings is often related to the uneven distribution of strain and temperature of the outer layer of the tube during extrusion, resulting in recrystallized grain sizes in this region much larger than the inner region of the tube; while heterogeneous grain structure indicates that the grain size distribution inside the alloy is extremely uneven, which may be caused by incomplete recrystallization, abnormal grain growth or a large number of precipitates hindering grain growth, etc. These defects in microstructure severely damage the uniformity and isotropy of material properties. Macroscopically, coarse grain rings and heterogeneous grain structures significantly reduce the plasticity and toughness of the material, and become potential areas of stress concentration. In the dynamic working conditions of bicycle frames that bear alternating and impact loads, the non-uniformity of microstructure greatly weakens the fatigue strength and fracture resistance of the frame tube, thereby directly increasing the risk of fatigue fracture during long-term riding, posing a potential threat to the safety of riders. SUMMARY

[0006] The present application provides a new type of aluminum-magnesium alloy bicycle frame pipe and a preparation method thereof to solve the technical contradiction that the existing 6XXX series aluminum alloy bicycle frame pipe in a hot extruded state is insufficient in strength, difficult to achieve extreme lightweight, and easy to form coarse grain ring and heterogeneous structure, resulting in uneven performance and reduced fatigue life. The pipe effectively improves the comprehensive mechanical properties of the material in the hot extruded state, especially the strength and plasticity, and significantly improves the microstructure uniformity through precise control of the alloy composition ratio and optimized preparation process, so that the minimum wall thickness of the frame pipe can be less than 0.8mm, and the super-lightweight design of the bicycle frame is realized under the premise of meeting the riding safety and reliability.

[0007] To achieve the above-mentioned application purposes, the present application provides an aluminum-magnesium alloy bicycle frame pipe and a preparation method thereof. The chemical composition of the alloy pipe contains 0.5% to 0.8% of silicon, 3.5% to 5.0% of magnesium, 0.2% to 0.6% of copper, 0.15% to 0.35% of manganese, 0.10% to 0.20% of iron, 0.10% to 0.30% of chromium, and 0.02% to 0.20% of mixed rare earth of lanthanum and cerium, wherein the mass percentage content of a single impurity element is less than or equal to 0.10%, and the total mass percentage content of all impurity elements is less than 0.20%.

[0008] As a preferred embodiment of the present application, the chemical composition of the aluminum-magnesium alloy bicycle frame pipe is characterized in that the iron element is explicitly defined as an alloying element with a mass percentage content of 0.10% to 0.20%, rather than a harmful impurity as traditionally recognized. Further, the ratio of the manganese content to the iron content (mass percentage) in the alloy is controlled within the range of 1.0 to 3.0, which aims to regulate the formation of iron-based intermetallic compounds, inhibit the generation of needle-shaped or flaky brittle phases by optimizing the morphology and distribution of Al-Fe-Si phases, and promote the formation of non-needle-shaped compounds with smaller size and more globular or blocky morphology, thereby significantly reducing the contribution of these phases to stress concentration effects, effectively improving the plasticity and toughness of the alloy, and improving the processing performance during hot deformation.

[0009] As a preferred embodiment of the present application, the chemical composition of the aluminum-magnesium alloy bicycle frame tube is characterized in that the silicon element is explicitly defined as an alloying element with a mass percentage content of 0.5% to 0.8%. During the solidification and subsequent cooling of the alloy, the added silicon reacts preferentially with magnesium to form Mg2Si compounds. Part of the Mg2Si compounds forms a dispersed distribution of second-phase particles inside the ingot, and part of them is dissolved in the aluminum matrix and precipitates in the form of fine and dispersed particles during subsequent hot extrusion and cooling. These dispersed precipitated Mg2Si phases provide effective dispersion strengthening to the alloy by pinning dislocations and hindering grain boundary movement. Further, the ratio of the mass percentage of magnesium content to silicon content in the alloy is controlled within the range of 4 to 10. This specific ratio ensures that after the formation of the Mg2Si dispersion strengthening phase from silicon and part of the magnesium, there is still sufficient excess magnesium dissolved in the α-Al solid solution. These excess magnesium atoms significantly contribute to the solid solution strengthening effect by distorting the lattice and hindering dislocation movement, further improving the yield strength of the alloy. The synergistic effect of dispersion strengthening and solid solution strengthening enables the alloy of the present application to obtain excellent comprehensive mechanical properties in the hot extruded state.

[0010] As a preferred embodiment of the present application, the chemical composition of the aluminum-magnesium alloy bicycle frame tube is characterized in that it contains 0.02% to 0.20% of a mixture of lanthanum and cerium rare earths. The rare earth elements exhibit strong modification during the solidification of the alloy. Specifically, lanthanum and cerium act as effective heterogeneous nucleation agents, significantly increasing the number of nucleation cores of α-Al grains, thereby promoting the formation of finer and more uniform equiaxed crystal structures. The refined α-phase grains not only improve the tensile strength and yield strength of the alloy, but more importantly, by increasing the grain boundary area, effectively suppress the formation of coarse grain rings and heterogeneous crystal structures, improving the plasticity and toughness of the material, and thus improving the fatigue performance. In addition, rare earth elements can also form stable and high-melting-point compounds with harmful impurity elements in the alloy (such as Fe and Si), changing the morphology and distribution of these compounds, making them change from needle-shaped or flaky to spherical or blocky, further reducing the contribution to stress concentration. The synergistic effect of lanthanum and cerium makes the grain refinement effect more significant, and can optimize the size and distribution of precipitated phases, thereby exhibiting excellent strength and plasticity combination on a macroscopic scale.

[0011] As a preferred embodiment of the present application, the performance characteristics of the aluminum-magnesium alloy bicycle frame pipe material are that, after hot extrusion forming, the minimum wall thickness can reach 0.8 mm, and can be successfully welded into a bicycle frame. At room temperature, the tensile strength of the pipe material is not less than 290 MPa, the yield strength is not less than 150 MPa, and the elongation is maintained at a level of not less than 16%. Compared with the existing 5083 aluminum alloy, the alloy of the present application exhibits higher strength and plasticity in the hot extruded state, providing a solid material foundation for the lightweight design of the frame. Further, after the welding process such as argon arc welding, laser welding or high-frequency induction welding, the mechanical property decay rate of the weld area of the aluminum-magnesium alloy bicycle frame pipe material of the present application is controlled within the range of 5% to 10%. This performance decay rate is significantly lower than the weld performance decay rate of the existing 6061 aluminum alloy (usually more than 20%), with a reduction of more than 50%, indicating that the alloy of the present application has excellent welding performance and weld area strength retention capability, ensuring the overall structural integrity and riding safety of the welded frame.

[0012] To achieve the above-mentioned application purpose, the present application further provides a preparation method of an aluminum alloy pipe material for bicycle frames, which comprises the following steps: Step S1: preparing an aluminum alloy liquid.

[0013] The preparation method uses electrolytic aluminum liquid or aluminum ingots with a purity of not less than 99.7% as the base material. In the alloy melting furnace, the aluminum base material is heated to a temperature range of 750°C to 780°C to ensure that the aluminum base material is completely melted and has good fluidity. Subsequently, the following components are accurately added to every 1000 kg of aluminum liquid: 5 kg to 8 kg of crystalline silicon with a purity of not less than 99.9%; 1.5 kg to 3.5 kg of metallic manganese with a purity of not less than 99.8%; 35 kg to 50 kg of magnesium ingot with a purity of not less than 99.9%; 2 kg to 6 kg of electrolytic copper plate with a purity of not less than 99.99%. At the same time, according to the preset iron element content target, a proper amount of micro-iron powder with a purity of not less than 99.5% is added to accurately adjust the iron content in the alloy liquid to a mass percentage range of 0.10% to 0.20%. The addition sequence needs to be strictly controlled, usually the high-melting-point elements are added first, and then the low-melting-point elements are added after they are fully melted, to ensure the uniform dissolution of alloy elements. The accurate control of alloy liquid temperature and the accurate proportioning of components are the key prerequisites to ensure the microstructure and mechanical properties of the final alloy pipe material.

[0014] Step S2: refining and degassing.

[0015] After the addition of alloying elements is completed and the stirring is uniform, the aluminum alloy liquid is subjected to a refining and degassing treatment. This step aims to remove non-metallic inclusions and dissolved hydrogen from the alloy liquid. Specifically, 5 kg to 8 kg of a sodium-free refining agent, which mainly consists of hexachloroethane, chlorinated salts, fluorinated salts, and the like, is added to every 1000 kg of the alloy liquid of the present application. The refining agent decomposes at high temperatures to produce active gases and slag, and the inclusions and gas bubbles are floated to the liquid surface to form a dross by the floating method. The refining and degassing process lasts for a certain period of time, and mechanical stirring or inert gas (such as argon) blowing is performed during this period to facilitate the removal of impurities and gases. After the refining and degassing are completed, the content of inclusions in the alloy liquid is evaluated by a K-mold test, and the inclusions level is required to reach level 1 to ensure the purity of the ingot inside. At the same time, the gas content of the alloy liquid is measured by vacuum sampling or reduced pressure solidification method, and the gas content is required to be not higher than 0.06 ml / 100 g to effectively avoid the pore defects caused by gas precipitation during the subsequent casting process, and to ensure the density of the cast rod.

[0016] Step S3: modification treatment and continuous casting rod.

[0017] After the refining and degassing are completed, the temperature of the alloy liquid is accurately adjusted to a suitable casting temperature range of 710°C to 750°C. Subsequently, 0.2 kg to 2 kg of aluminum foil lanthanum cerium mixed rare earth is added to every 1000 kg of the aluminum alloy liquid. The aluminum foil lanthanum cerium is a special form of rare earth addition, in which the lanthanum cerium alloy is tightly wrapped with a thin aluminum foil. This wrapping structure can effectively prevent the oxidation and burning loss of rare earth elements before or during the addition to the alloy liquid, significantly improving the recovery rate of rare earth elements, and has a lower cost advantage compared to traditional rare earth intermediate alloy. After the addition of rare earth, the alloy liquid is thoroughly stirred to ensure that the rare earth elements are uniformly dispersed in the alloy liquid and play their heterogeneous nucleation role to refine the α-Al grains. After uniform stirring, the alloy liquid is cast into a rod by a continuous casting process. During the continuous casting process, the casting speed and cooling rate are accurately controlled to further optimize the solidification structure of the cast rod, so that the grains are small and uniform, and the generation of casting defects such as shrinkage, center segregation and macroscopic cracks is maximally inhibited, ensuring that the cast rod has excellent internal quality and uniform chemical composition.

[0018] Step S4: hot extruded pipe.

[0019] The casting rod obtained in step S3 is heated to a temperature range of 460℃ to 500℃, and is sufficiently kept warm to make the internal temperature of the casting rod uniform, reaching a "hot penetration" state, to ensure good plasticity in the subsequent hot extrusion process. Subsequently, the hot penetration casting rod is formed on a hot extrusion machine to prepare various specifications of bicycle frame pipes. The hot extrusion process of the present application can prepare pipes with wall thicknesses of 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, 1.8mm, etc., and especially importantly, it can achieve the preparation of ultra-thin pipes with a minimum wall thickness of 0.6mm. During the extrusion process, the extrusion ratio is controlled in the range of 15 to 70, and the extrusion speed is controlled in the range of 3m / s to 12m / s. The precisely controlled extrusion temperature, extrusion ratio and extrusion speed can induce dynamic recovery and dynamic recrystallization of the alloy during deformation. By optimizing these parameters, the present application can effectively control the grain size and distribution of the pipe in the extruded state, inhibit the formation of coarse grain ring and heterogeneous structure, and obtain fine and uniform equiaxed crystal structure, thereby significantly improving the comprehensive mechanical properties of the pipe, especially the strength and fatigue performance, to provide high-quality raw materials for subsequent welded frame manufacturing. The prepared pipe can be directly used for welding bicycle frames.

[0020] Compared with the prior art, the present application has the following remarkable beneficial effects: Excellent corrosion resistance: The alloy of the present application belongs to the aluminum-magnesium alloy system, and this alloy system is known for its good resistance to seawater corrosion and general atmospheric corrosion. Precise control of the alloy composition, especially the synergistic effect of magnesium, manganese and chromium elements, forms a dense oxide film on the surface of the pipe, providing excellent corrosion resistance for the bicycle frame pipe, prolonging the service life of the frame and reducing maintenance costs.

[0021] Dense and uniform casting rod structure: Due to the addition of appropriate amounts of silicon, iron and rare earth elements in the alloy of the present application, and through strict control of the refining degassing and modification treatment steps during preparation, the fluidity of the alloy liquid is greatly improved, and the nucleation of fine and uniform α-Al grains is promoted. Therefore, the obtained casting rod structure is dense and uniform, effectively avoiding defects such as shrinkage, center segregation and cracks commonly found in traditional casting processes. The uniformity of the internal structure of the casting rod is the basis for the consistency of the subsequent extrusion molding and the final pipe performance.

[0022] High strength and ductility combination: The present alloy realizes effective solid solution strengthening by precisely controlling the contents of magnesium, manganese and chromium. Further, the formation of Mg2Si phase by silicon and magnesium, and the grain refinement effect of rare earth elements and the optimization of harmful phase morphology, together build a multi-level strengthening mechanism. The synergistic effect of these mechanisms enables the present alloy to obtain excellent strength and ductility combination in the hot extruded state. The strength-ductility product (the product of tensile strength and elongation) of the pipe is greater than 4.5 GPa%, which is significantly higher than that of the existing 6061 aluminum alloy frame pipe, with an increase of more than 20%. High strength and ductility means that the material still has good deformation ability while bearing large load, greatly improving the safety redundancy of the frame under extreme working conditions.

[0023] Ultra-lightweight design capability: The present alloy can exhibit higher comprehensive mechanical properties than traditional 5XXX series alloys in the hot extruded state, and effectively suppresses the formation of coarse grain ring and mixed crystal structure, ensuring the uniformity and reliability of the material performance. This enables the present alloy pipe to be prepared into an ultra-thin specification with a minimum wall thickness of 0.6 mm. A bicycle frame welded from a pipe with a wall thickness of 0.6 mm can still meet the stringent bench test requirements, ensuring riding safety. Compared with a frame made of traditional 6061 aluminum alloy with the same size, the present frame can achieve significant weight reduction while maintaining performance, for example, the weight of a typical mountain bike frame (e.g., a medium-sized frame, with a size range of 17-19 inches) can be reduced by about 15% to 25%, thereby greatly improving the lightweight level of the entire vehicle.

[0024] Excellent welding performance: In the alloy design of the present aluminum-magnesium alloy pipe, the contents of copper, manganese, iron and other elements are precisely controlled, and rare earth elements are added to optimize the solidification interval and microstructure stability during welding, effectively suppressing the grain coarsening and softening in the heat-affected zone. Therefore, the frame pipe can be connected using various welding techniques such as argon arc welding, laser welding, high-frequency induction welding, etc., and the mechanical property decay rate of the weld area is controlled at a low level of 5% to 10%. Compared with the weld performance decay rate of existing 6061 aluminum alloy (usually more than 20%), the weld performance decay of the present alloy is reduced by more than 50%, which greatly enhances the overall strength and fatigue life of the welded frame, ensuring the structural integrity and riding safety of the frame during long-term service, effectively solving the potential safety hazards caused by the weld performance decay of traditional aluminum alloy frames. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a flowchart of a method for preparing an aluminum-magnesium alloy bicycle frame pipe according to the present application.

[0026] The reference signs are as follows: 101 preparation of aluminum alloy liquid, 102 refining and degassing, 103 modification treatment and continuous casting of a rod, and 104 hot extrusion of a pipe. DETAILED DESCRIPTION

[0027] The present detailed description aims to elaborate the engineering implementation details, process parameters and expected performance data of an aluminum magnesium alloy bicycle frame pipe and its preparation method, so as to ensure that those skilled in the art can fully understand and implement the present application. The pipe, with its unique and precise alloy composition design and optimized preparation process, solves the key technical problems of the existing 6XXX series aluminum alloy in the hot extruded state, such as insufficient strength, easy formation of coarse grain ring and heterogeneous crystal structure, etc., thereby realizing the ultimate lightweight design of the bicycle frame while ensuring excellent mechanical properties and reliability.

[0028] The application provides an aluminum-magnesium alloy bicycle frame pipe and a preparation method thereof. The chemical composition of the alloy pipe is accurately proportioned, and the following core elements are covered according to the mass percentage: the content of silicon is strictly controlled in the range of 0.5% to 0.8%, which mainly forms Mg2Si compounds with magnesium in the alloy matrix to form fine dispersion strengthening phase, and is partially dissolved in the aluminum matrix to provide strengthening potential for subsequent heat treatment. The content of magnesium is set to 3.5% to 5.0%, and the high magnesium content is the key to realize significant solid solution strengthening, which effectively hinders dislocation movement through lattice distortion and ensures sufficient magnesium to combine with silicon to form Mg2Si. The addition amount of copper is 0.2% to 0.6%, and the copper element can promote the formation of precipitated phase to further improve the strength of the alloy, especially in the subsequent solid solution aging treatment. The content of manganese (Mn) is between 0.15% and 0.35%, and manganese, as an important grain refiner and impurity element regulator, can form compounds with iron to optimize its morphology and distribution, and can also form a dispersion phase to hinder grain growth. The content of iron is limited to a specific range of 0.10% to 0.20%, and in the application, iron is considered as an important alloy element rather than a simple impurity, and its content is accurately controlled to promote the formation of spherical or block-shaped Fe-Al-Si intermetallic compounds, rather than harmful needle-shaped or flaky phases, thereby reducing the negative impact on plasticity and toughness. The content of chromium is 0.10% to 0.30%, and chromium is also a grain refiner and can enhance the corrosion resistance of the alloy to form fine Cr-Al compounds as heterogeneous nucleation points. The content of lanthanum and cerium mixed rare earth is 0.02% to 0.20%, and the addition of rare earth elements is the key to realizing grain ultra-fining and structure homogenization in the application, which can significantly improve the nucleation rate of alpha-Al grains, inhibit the generation of coarse grain rings, and further optimize the morphology of intermetallic compounds. At the same time, the application strictly controls the mass percentage content of a single impurity element to be less than or equal to 0.10%, and the total mass percentage content of all impurity elements is less than 0.20%, so as to ensure the overall purity of the alloy and avoid the adverse effects of harmful impurities on mechanical properties and processing performance.

[0029] Further, the present application designs the interaction relationship of part alloy elements in detail. Specifically, the iron element in the present application is not strictly controlled at a low limit as a harmful impurity in traditional aluminum alloy, but is precisely defined as a functional alloy element, and the mass percentage content is controlled at 0.10% to 0.20%. Through the fine control of the iron content, combined with the effect of manganese element, the present application controls the ratio of manganese content to iron content (mass percentage) in the alloy to be in the optimized interval of 1.0 to 3.0. The design of this specific ratio aims to effectively control the formation mechanism and morphology characteristics of iron-based intermetallic compounds (such as AlFeSi phase) in the alloy during solidification and subsequent thermal deformation. By maintaining this ratio, the formation of brittle Al-Fe-Si phase in the form of needles or sheets in traditional 6XXX series aluminum alloy can be inhibited, and instead, the formation of non-needle-shaped intermetallic compounds with smaller size and more globular or blocky morphology is promoted. These morphologically optimized compounds are more dispersed and have lower interface energy with the matrix, thereby significantly reducing their contribution to the plasticity and toughness of the alloy as stress concentrators. During thermal deformation, the presence of these optimized intermetallic compounds in the matrix can effectively pin the grain boundaries, inhibit abnormal grain growth, and provide additional nucleation sites for dynamic recrystallization, thereby improving the thermal processing performance of the alloy and reducing the cracking tendency during hot extrusion.

[0030] As a preferred embodiment of the present application, the silicon element is explicitly defined as an alloying element with a mass percentage content of 0.5% to 0.8%. During the solidification and subsequent cooling of the alloy, the added silicon will preferentially react with the high content of magnesium in the alloy to form thermodynamically stable Mg2Si compounds. A portion of the Mg2Si forms fine and dispersed second-phase particles inside the ingot, which can effectively pin dislocations during subsequent hot extrusion, hindering dislocation movement and thus providing significant dispersion strengthening effects. Another portion of the silicon and magnesium exists in the form of a solid solution in the α-Al matrix. During subsequent hot extrusion and cooling, with the decomposition of the supersaturated solid solution, fine and dispersed particles are precipitated from the matrix, further enhancing the dispersion strengthening effect. Further, the present application controls the ratio of the magnesium content to the silicon content (mass percentage) in the alloy to be in the range of 4 to 10. This specific ratio is crucial, as it ensures that after sufficient Mg2Si dispersion strengthening phases are formed, there is still a sufficient amount of excess magnesium element that can exist in the form of a solid solution in the α-Al solid solution. These excess magnesium atoms, due to the difference in atomic radius with aluminum atoms, introduce local lattice distortion in the aluminum lattice. This lattice distortion can effectively hinder the movement of dislocations, thus producing significant solid solution strengthening effects, further improving the yield strength of the alloy. Therefore, by precisely controlling the Mg / Si ratio, the alloy of the present application can simultaneously utilize the synergistic effect of dispersion strengthening and solid solution strengthening, so that the material can obtain excellent comprehensive mechanical properties in the hot extruded state, showing a combination of high strength and good plasticity.

[0031] As a preferred embodiment of the present application, the alloy contains 0.02% to 0.20% of lanthanum and cerium mixed rare earth elements. The rare earth elements play multiple key roles in the present application. First, during the alloy solidification process, lanthanum and cerium act as effective heterogeneous nucleation agents, which can significantly increase the number of nucleation cores of a-Al grains. Specifically, the rare earth elements and their compounds (such as Al-RE intermetallic compounds or RE oxides) provide nucleation interfaces that match well with the aluminum matrix lattice, thereby promoting the formation of uniform and large number of a-Al crystal nuclei in the liquid metal. This leads to the formation of finer and more uniform equiaxed crystal structure after solidification is completed. The refined a-phase grains not only improve the tensile strength and yield strength of the alloy according to the Hall-Petch relationship, but more importantly, by greatly increasing the grain boundary area, effectively suppresses the formation of coarse grain rings and heterogeneous crystal structures that are prone to form in traditional aluminum alloys during hot working. The reduction or elimination of these macroscopic defects greatly improves the plasticity and toughness of the material, and significantly improves the fatigue performance, prolonging the service life of the component. Second, the rare earth elements can also form stable and high melting point compounds with harmful impurity elements in the alloy (such as iron, silicon). By combining with elements such as iron and silicon, the rare earth elements change the morphology and distribution of these intermetallic compounds, making them change from the needle-shaped or flaky brittle form commonly seen in traditional aluminum alloys to spherical or blocky form. This optimization of the morphology further reduces the contribution of these compounds to stress concentration, thereby improving the toughness of the alloy. The synergistic effect of lanthanum and cerium not only enhances the grain refinement effect of a single rare earth element, but also optimizes the size, distribution and morphology of precipitates, thereby macroscopically making the alloy exhibit excellent strength and plasticity combination, providing a more excellent material basis for bicycle frames.

[0032] The aluminum magnesium alloy bicycle frame pipe prepared by the present application has performance characteristics significantly superior to the prior art. After hot extrusion molding, the minimum wall thickness of the pipe can be stabilized to 0.8 mm, and it can be successfully welded into a bicycle frame under strict industrial welding conditions without cracking or significant performance degradation. Under room temperature environment, the pipe has a tensile strength of not less than 290 MPa, a yield strength of not less than 150 MPa, and an elongation of not less than 16% through standard tensile test. The combination of these performance indicators, especially the balance of high strength and high plasticity in the hot extruded state, provides a solid material guarantee for the ultra-lightweight design of the bicycle frame. Further, after being connected by mainstream welding processes such as argon arc welding, laser welding or high-frequency induction welding, the mechanical property decay rate of the weld area of the aluminum magnesium alloy bicycle frame pipe of the present application can be stably controlled in a relatively low range of 5% to 10%. Compared with the case that the weld performance decay rate of the conventional 6061 aluminum alloy is usually higher than 20%, the weld performance decay rate of the alloy of the present application is reduced by more than 50%, which fully shows that the alloy has excellent welding performance and weld area strength retention capability, ensuring the overall structural integrity, reliability and riding safety of the welded frame, and effectively overcoming the problem of softening and strength loss of conventional high-strength aluminum alloy during the welding process.

[0033] The present application further provides a preparation method of an aluminum alloy pipe for bicycle frame, which comprises the following series of optimization steps: Step S1: preparing an aluminum alloy liquid.

[0034] The core of the preparation method lies in the selection and precise proportioning of raw materials. First, electrolytic aluminum liquid or aluminum ingots with a purity of not less than 99.7% are selected as the base material to ensure the starting purity of the alloy liquid. In an alloy melting furnace equipped with an advanced temperature control system, the aluminum base material is heated to a precise temperature range of 750°C to 780°C. This temperature range can ensure complete melting of the aluminum base material and good fluidity, which is beneficial to the uniform dissolution of subsequent alloying elements, and can also avoid excessive temperature leading to element burning loss and increased energy consumption. Subsequently, the following high-purity alloy components are precisely added to every 1000 kg of aluminum liquid according to the preset mass percentage: 5 kg to 8 kg of crystalline silicon with a purity of not less than 99.9% to introduce the formation element of Mg2Si strengthening phase; 1.5 kg to 3.5 kg of metallic manganese with a purity of not less than 99.8% for grain refinement and optimization of iron-based intermetallic compounds; 35 kg to 50 kg of magnesium ingot with a purity of not less than 99.9% as the main strengthening element; and 2 kg to 6 kg of electrolytic copper plate with a purity of not less than 99.99% to provide precipitation strengthening potential. It is particularly noteworthy that according to the preset iron element content target (0.10% to 0.20%), the present application needs to add an appropriate amount of micro-iron powder with a purity of not less than 99.5%. The use of micro-iron powder facilitates precise control of the iron content in the alloy liquid, ensuring that it plays a role as an alloying element rather than a harmful impurity. The addition sequence needs to strictly follow the metallurgical principle, usually adding high melting point elements (such as silicon, manganese, iron) first, and then adding low melting point elements (such as magnesium, copper) after they are fully melted and dissolved. This sequence can effectively prevent low melting point elements from being oxidized or evaporated too early in the high-temperature aluminum liquid, and ensure the uniform dissolution of alloying elements, avoiding local segregation. Precise control of alloy liquid temperature and precise proportioning of components are prerequisites for ensuring the uniformity of microstructure and excellent mechanical properties of the final alloy pipe.

[0035] Step S2: Refining degassing.

[0036] After all alloying elements are added and well stirred, the aluminum alloy melt is subjected to a critical refining and degassing treatment. This step aims to efficiently remove non-metallic inclusions and dissolved hydrogen from the alloy melt, which are the main causes of casting defects and performance degradation. Specifically, 5kg to 8kg of sodium-free refining agent is added to every 1000kg of the alloy melt of the present application. The refining agent mainly consists of hexachloroethane (C2Cl6), chlorinated salts (such as NaCl, KCl), and fluorinated salts (such as NaF, KAlF4), etc. In the high-temperature aluminum melt, hexachloroethane decomposes to produce active chlorine gas, which combines with hydrogen in the aluminum melt to form hydrogen chloride gas that escapes, and reacts with non-metallic inclusions to form a molten slag that is easy to float. Chlorinated salts and fluorinated salts lower the melting point and surface tension of the molten slag, promoting the coalescence and floating of the slag and inclusions, and making the inclusions and bubbles float to the surface of the liquid to form a dross by the floating method. The refining and degassing process lasts for about 20 to 40 minutes, during which mechanical stirring or inert gas (such as argon) blowing is carried out. Mechanical stirring can accelerate the reaction of the refining agent with the aluminum melt, promoting the floating of impurities; inert gas blowing uses bubbles to adsorb hydrogen and inclusions when they move in the aluminum melt and carry them out of the surface of the liquid. After the refining and degassing are completed, the K-mold test is used to evaluate the content of non-metallic inclusions in the alloy melt, and the inclusion level is required to reach level 1 or higher in the ISO 18884-2:2018 standard, to ensure the extremely high purity of the ingot inside. At the same time, the vacuum sampling or reduced pressure solidification method is used to determine the gas content of the alloy melt, and the gas content is required to be not higher than 0.06ml / 100g (at standard atmospheric pressure), to effectively avoid the pinhole defects caused by gas precipitation in the subsequent casting process, and to ensure the density and uniformity of the cast rod.

[0037] Step S3: modification treatment and continuous casting rod.

[0038] After the completion of the refining and degassing, the temperature of the alloy melt is precisely adjusted to a suitable casting temperature range of 710°C to 750°C to ensure the stability of the alloy melt during the addition of rare earth and the casting process. Subsequently, 0.2 kg to 2 kg of aluminum foil lanthanum cerium mixed rare earth is added to every 1000 kg of aluminum alloy melt. The aluminum foil lanthanum cerium is an innovative form of rare earth addition, in which the lanthanum cerium alloy is tightly wrapped by a layer of thin aluminum foil. This wrapping structure can effectively isolate air before and during the addition of rare earth elements to the alloy melt, significantly preventing the oxidation and burning loss of rare earth elements, thereby greatly improving the recovery rate of rare earth elements (usually more than 90%). Compared with traditional bare rare earth master alloy, the aluminum foil wrapped form not only improves the efficiency, but also has a lower cost advantage. After the addition of rare earth, the alloy melt is stirred for at least 10 minutes to ensure that the rare earth elements are uniformly dispersed in the alloy melt and fully play their role as heterogeneous nucleating agents to refine the a-Al grains. After uniform stirring, the alloy melt is cast into a rod using an advanced continuous casting process. During the continuous casting process, the casting speed (for example, 200 mm to 400 mm per minute) and the cooling rate (for example, by water-cooled copper mold) are precisely controlled to further optimize the solidification structure of the cast rod. This precise control can induce the formation of fine and uniform equiaxed crystal structure and maximally suppress common casting defects such as shrinkage (by feeding effect), center segregation (by uniform solidification front) and macroscopic cracks (by controlling cooling stress). These measures together ensure that the cast rod has excellent internal quality and uniform chemical composition, providing high-quality raw materials for the subsequent hot extrusion process.

[0039] Step S4: Hot extruding the pipe.

[0040] The cast rod obtained in step S3 is heated to a temperature range of 460-500℃ and is kept at this temperature for a sufficient time to homogenize the temperature inside the rod and reach a "hot-soaked" state. This "hot-soaked" state is crucial to ensure that the alloy has good plasticity, uniform deformability and suppresses local overheating during subsequent hot extrusion. Subsequently, the hot-soaked rod is shaped on an advanced hot extrusion machine to produce various specifications of bicycle frame tubes. The hot extrusion process used in the present invention has significant flexibility and precision, and can produce tubes with wall thicknesses of 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, 1.8mm, etc. Importantly, it has the technical capability to produce ultra-thin tubes with a minimum wall thickness of 0.6mm, which is crucial for the extreme lightweight design of bicycle frames. During extrusion, the extrusion ratio (i.e. the ratio of the cross-sectional area of the cast rod to that of the extruded tube) is controlled in the range of 15-70 to ensure sufficient deformation and microstructure refinement. The extrusion speed is controlled in the range of 3-12m / s, which can effectively control the extrusion heat effect while ensuring production efficiency. The precisely controlled extrusion temperature, extrusion ratio and extrusion speed can induce sufficient dynamic recovery and dynamic recrystallization of the alloy during deformation. By optimizing these parameters, the present invention can effectively control the grain size and distribution of the tube in the as-extruded state, obtaining fine, uniform equiaxed crystal structure and effectively suppressing coarse grain ring and heterogeneous crystal structure, etc. which may lead to performance degradation. The refinement of the grain and the homogenization of the structure significantly improve the comprehensive mechanical properties of the tube, especially the tensile strength, yield strength and fatigue performance. The prepared tube can be directly used for welding bicycle frames without additional solid solution treatment or aging treatment, greatly simplifying the production process and reducing costs.

Claims

1. An aluminium magnesium alloy bicycle frame tube characterised in that, Chemical composition by mass percentage: 0.5% to 0.8% silicon, 3.5% to 5.0% magnesium, 0.2% to 0.6% copper, 0.15% to 0.35% manganese, 0.10% to 0.20% iron, 0.10% to 0.30% chromium, 0.02% to 0.20% mixed rare earth of lanthanum and cerium, wherein the mass percentage content of each individual impurity element is less than or equal to 0.10%, and the total mass percentage content of all impurity elements is less than 0.20%.

2. The aluminum-magnesium alloy bicycle frame tube according to claim 1, characterized in that: The ratio of the content of manganese to the content of iron in the alloy, Mn / Fe ratio, is controlled in the range of 1.0 to 3.0, which is designed to regulate the formation mechanism and morphological characteristics of iron-based intermetallic compounds, to inhibit the generation of needle-like or flaky brittle phases by optimizing the morphology and distribution of Al-Fe-Si phases, and to promote the formation of smaller-sized, more globular or block-shaped non-needle intermetallic compounds, thereby significantly reducing the contribution of these phases to stress concentration effects, effectively improving the plasticity and toughness of the alloy, and improving the processing performance during hot deformation.

3. The aluminum-magnesium alloy bicycle frame tube according to claim 1, characterized in that: The ratio of the content of magnesium to the content of silicon in the alloy, Mg / Si ratio, is controlled in the range of 4 to 10, which is crucially important, as it ensures that after forming sufficient Mg2Si dispersion strengthening phases, there is still sufficient excess magnesium element in the form of solid solution in the α-Al solid solution, which significantly contributes to the solid solution strengthening effect by distorting the lattice and hindering dislocation movement, further improving the yield strength of the alloy, so that the synergistic effect of dispersion strengthening and solid solution strengthening can be achieved; and the tube can obtain excellent comprehensive mechanical properties in the hot extruded state, with a tensile strength of not less than 290 MPa, a yield strength of not less than 150 MPa, and an elongation of not less than 16%, and after welding processes such as argon arc welding, laser welding or high-frequency induction welding, the mechanical property decay rate of the weld area is controlled in a relatively low range of 5% to 10%.

4. A method for producing an aluminum-magnesium alloy bicycle frame tube, which is applied to the aluminum-magnesium alloy bicycle frame tube according to any one of claims 1 to 3, characterized by, Comprising the following steps: Step S1: Preparation of aluminum alloy liquid (101): Selecting electrolytic aluminum liquid or aluminum ingot with purity not less than 99.7% as base material, heating the aluminum base material to an alloy liquid initial temperature of 750-780℃ in an alloy melting furnace to ensure complete melting of the aluminum base material and good fluidity; then, accurately adding the following components to each 1000 kg of the aluminum liquid: 5-8 kg of crystalline silicon with a purity of not less than 99.9%, 1.5-3.5 kg of metallic manganese with a purity of not less than 99.8%, 35-50 kg of magnesium ingot with a purity of not less than 99.9%, 2-6 kg of electrolytic copper plate with a purity of not less than 99.99%, and an appropriate amount of micro-iron powder with a purity of not less than 99.5% to accurately adjust the iron content in the aluminum liquid to a mass percentage range of 0.10%-0.20%; the adding sequence is to add high-melting-point elements first, and then add low-melting-point elements after they are fully melted; Step S2: Refining and degassing (102): After the addition of alloy elements and sufficient stirring, the aluminum alloy liquid obtained in step S1 is subjected to refining and degassing treatment, and 5-8 kg of sodium-free refining agent is added to each 1000 kg of the alloy liquid; Step S3: Modification and continuous casting rod (103): The alloy liquid casting temperature of the alloy liquid is accurately adjusted to a suitable casting temperature range of 710-750℃, then 0.2-2 kg of aluminum foil lanthanum cerium mixed rare earth is added to each 1000 kg of the aluminum alloy liquid, the rare earth is tightly wrapped with a thin aluminum foil to effectively prevent oxidation and burning loss before or during the addition of the alloy liquid, and the alloy liquid is subjected to sufficient stirring after the addition to ensure uniform dispersion of the rare earth elements and full play of their heterogeneous nucleation effect, then the alloy liquid is cast into a rod by using a continuous casting process; Step S4: Hot extrusion pipe (104): The cast rod obtained in step S3 is heated to a cast rod heating temperature of 460-500℃ and is subjected to sufficient heat preservation to make the internal temperature uniform and reach a hot penetration state, then it is formed on a hot extrusion machine to prepare various specifications of bicycle frame pipes.

5. The preparation method according to claim 4, wherein: in the refining and degassing step (102), the refining and degassing time of the refining and degassing process lasts for 20-40 minutes, during which mechanical stirring or inert gas blowing is performed to promote the floating of impurities and bubbles to the liquid surface to form dross, thereby accelerating the removal of impurities and gas; after the completion of refining and degassing, the content of non-metallic inclusions in the alloy liquid is evaluated by K-mode inspection to reach level 1 or above in the ISO18884-2:2018 standard, and the gas content of the alloy liquid is determined by vacuum sampling or reduced pressure solidification method to be not higher than 0.06 ml / 100g.

6. The preparation method according to claim 4, wherein: ​ In the modification treatment and continuous casting rod (103) steps, after adding the aluminum foil lanthanum cerium mixed rare earth, the alloy liquid is subjected to sufficient stirring of rare earth elements for at least 10 minutes to ensure that the rare earth elements are uniformly dispersed in the alloy liquid and fully play their role as heterogeneous nucleating agents, significantly increase the number of nucleation cores of α-Al grains, and realize the refinement of α-Al grains, thereby inhibiting the formation of coarse grain rings and heterogeneous structures; in the continuous casting process, by accurately controlling the casting speed to be 200 mm / min to 400 mm / min and the cooling rate, the solidification structure of the cast rod is further optimized, so that the grains are small and uniform, and the generation of casting defects such as shrinkage, center segregation and macroscopic cracks is maximally inhibited, thereby ensuring that the cast rod has excellent internal quality and uniform chemical composition.

7. The preparation method of claim 4, wherein: In the hot extrusion pipe (104) step, the cast rod is heated to a cast rod heating temperature and then subjected to sufficient heat preservation for at least 4 hours to ensure that the internal temperature of the cast rod is uniform and reaches a hot penetration state, thereby providing a basis for good plasticity, uniform deformation ability and inhibition of local overheating in the subsequent hot extrusion process; in the hot extrusion forming process, the extrusion ratio is controlled in the range of 15 to 70, and the extrusion speed is controlled in the range of 3 m / s to 12 m / s; the accurately controlled extrusion temperature, extrusion ratio and extrusion speed can induce the alloy to undergo sufficient dynamic recovery and dynamic recrystallization during deformation, thereby effectively controlling the grain size and distribution of the pipe in the extruded state and obtaining a fine and uniform equiaxed grain structure; the minimum pipe wall thickness of the prepared pipe can reach 0.6 mm.

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