Aluminum bismuth intermediate alloy and method for preparing the same

Abstract

The application relates to an aluminum-bismuth intermediate alloy and a preparation method thereof, wherein the preparation method of the aluminum-bismuth intermediate alloy comprises the following steps: S1, first, aluminum raw materials are added into a reaction container after the reaction container is preheated to 200-300 DEG C, the reaction container is heated to a first temperature, and heat preservation is carried out to prepare aluminum liquid; the first temperature is 680-720 DEG C; S2, then, bismuth raw materials are added into the aluminum liquid in batches to prepare alloy melt; S3, refining treatment is carried out to remove the dross on the surface of the alloy melt; S4, refinement treatment is carried out to uniformly disperse the refiner in the alloy melt after the refining treatment; S5, cooling treatment is carried out to prepare aluminum-bismuth intermediate alloy ingots; and the reaction container is kept in an inert gas atmosphere during the whole preparation process. The application has the beneficial effects that the aluminum-bismuth intermediate alloy prepared by the application has low burning loss rate of bismuth elements, no segregation and uniform dispersion, and can be stably prepared into high-purity, high-quality bismuth-containing aluminum intermediate alloy with excellent performance consistency.

Description

Technical Field

[0001] This invention relates to the field of aluminum-based alloy technology, and in particular to an aluminum-bismuth master alloy and its preparation method. Background Technology

[0002] Aluminum-bismuth master alloy is a preferred additive for manufacturing environmentally friendly lead-free free-machining aluminum alloys. It combines the high thermal and electrical conductivity of aluminum with the low shear strength and self-lubricating properties of bismuth. It has broad application prospects in the automotive industry, electronics, precision machinery and other fields. It can completely replace traditional lead-containing free-machining aluminum alloys and meet the mandatory requirements of global environmental regulations such as EU RoHS, REACH and China's "Regulations on the Restriction of Hazardous Substances in Electrical and Electronic Products" for lead-free manufacturing.

[0003] Aluminum-bismuth binary alloys are typical morphological alloys. Bismuth has extremely low room temperature solid solubility in aluminum (<0.01wt%), and the density difference between bismuth and aluminum is significant (bismuth density 9.8 g / cm³). 3 Aluminum density 2.7 g / cm³ 3 During the melting and solidification process, bismuth droplets are prone to settling due to density differences, resulting in severe gravity segregation. This ultimately leads to bismuth phase agglomeration and uneven distribution, directly deteriorating the machinability and mechanical properties of the alloy.

[0004] Currently, the industrial preparation of aluminum-bismuth master alloys mostly adopts high-temperature smelting processes. Typically, aluminum liquid is heated to 1000℃-1100℃ and then bismuth is added to complete the alloying. This process has the following drawbacks: First, the smelting temperature is much higher than the melting point of bismuth (271.4℃), resulting in high energy consumption. At the same time, bismuth is extremely volatile at high temperatures, with a measured burn-off rate as high as 5%-8%, which not only significantly increases production costs but also makes it difficult to accurately control the bismuth content of the final alloy, resulting in poor batch stability. Second, aluminum liquid is severely oxidized at high temperatures, producing a large number of Al2O3 oxide inclusions. Moreover, existing processes lack environmentally friendly refining steps adapted to low-temperature smelting, resulting in numerous inclusions and porosity defects in the melt, which reduces the purity and performance stability of the alloy. Summary of the Invention

[0005] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides an aluminum-bismuth master alloy and its preparation method, which solves the technical problems of bismuth burn-off, gravity segregation, many inclusions in the melt, and high energy consumption in the preparation methods of existing aluminum-bismuth master alloys.

[0006] (II) Technical Solution To achieve the above objectives, the main technical solutions adopted by the present invention include: In a first aspect, embodiments of the present invention provide a method for preparing an aluminum-bismuth master alloy, comprising the following steps: S1. First, preheat the reaction vessel to 200-300℃, then add the aluminum raw material to the reaction vessel, heat the reaction vessel to the first temperature, and hold it at the temperature to obtain molten aluminum; the first temperature is 680-720℃. S2. Then, the bismuth raw material is added to the aluminum liquid in batches. When adding the bismuth raw material, electromagnetic stirring is performed, and the reaction vessel is kept at the first temperature to obtain the alloy melt. S3. Refining process: An environmentally friendly refining agent is added to the reaction vessel using an inert gas as the carrier gas. After the environmentally friendly refining agent is added, electromagnetic stirring is performed and the reaction vessel is kept at the first temperature. Then, it is allowed to stand and the slag on the surface of the alloy melt is removed. S4. Refining treatment: Add a refining agent to the refined alloy melt, perform electromagnetic stirring and keep the reaction vessel at the first temperature, so that the refining agent is evenly dispersed in the refined alloy melt. S5. Cooling treatment: After the alloy melt refined by S4 is allowed to stand, it is cooled to room temperature by gradient cooling to obtain an aluminum-bismuth master alloy ingot; wherein, the reaction vessel is kept in an inert gas atmosphere throughout the preparation process.

[0007] The reaction vessel can be a medium-frequency furnace. The vessel is preheated to 200-300℃ to prevent excessive temperature difference between the reactants and the vessel, and to remove moisture from the vessel. Furthermore, an inert gas with a purity ≥99.99% is continuously introduced throughout the preheating and subsequent smelting process, with a stable flow rate of 0.3-0.5 L / min. This is controlled by an exhaust valve to maintain a slight positive pressure of 50-100 Pa within the furnace, ensuring that all air is exhausted throughout the process to isolate oxygen, prevent oxidation of the molten aluminum and bismuth, and reduce the formation of oxide inclusions.

[0008] The inert gases are either argon or nitrogen, with argon being preferred to avoid nitrogen reacting with molten aluminum at high temperatures to form aluminum nitride inclusions.

[0009] The first temperature is 680-720℃, which is slightly higher than the melting point of pure aluminum (660.3℃). This ensures that the aluminum ingot is completely melted. At the same time, the saturated vapor pressure of bismuth is extremely low within this temperature range, far below the critical temperature for large-scale volatilization of bismuth (760℃), thus suppressing the volatilization and burn-off of bismuth at the source. In addition, in S2, electromagnetic stirring maintains the temperature of the melt in the furnace at the first temperature of 680-720℃ throughout the process, allowing the bismuth to fully melt and be uniformly dispersed in the aluminum liquid, suppressing the sedimentation and agglomeration of bismuth droplets due to density differences, and controlling gravity segregation at the source.

[0010] In a preferred embodiment of the present invention, in the method for preparing the aluminum-bismuth master alloy, in S1, the aluminum raw material is an industrial pure aluminum ingot or electrolytic aluminum liquid with a purity ≥99.7%; the bismuth raw material is an industrial pure bismuth ingot with a purity ≥99.9% after removing the surface oxide layer; wherein, the bismuth ingot is polished to remove the oxide layer on the surface of 0.1-0.2 mm thick to prevent the oxide layer from entering the melt and introducing impurities.

[0011] The reaction vessel is heated to the first temperature using electromagnetic heating, with an operating frequency of 350-500Hz and a heating rate of 15-25℃ / min; the holding time is 5-10min. The heating rate of 15-25℃ / min avoids excessively rapid heating that could lead to localized overheating and oxidation of the aluminum raw material; the 5-10min holding time ensures a uniform temperature field in the molten aluminum, resulting in a stable aluminum liquid. The heating process is monitored in real-time using an infrared temperature control module, and the temperature deviation of the melt within the reaction vessel is controlled within ±8℃.

[0012] In a preferred embodiment of the present invention, in the method for preparing the aluminum-bismuth master alloy, in step S2, the mass ratio of bismuth raw material to alloy melt is 3-10:100. When adding bismuth raw materials to molten aluminum in batches, each batch of bismuth raw materials is added at a uniform speed along the side wall of the reaction vessel, with each batch taking 30-60 seconds and an interval of 2-3 minutes between batches.

[0013] The bismuth raw material is added in 2-3 batches. Each batch of bismuth raw material is added at a constant speed along the side wall of the reaction vessel. The aforementioned method of adding bismuth raw material can avoid the loss of bismuth element caused by molten splashing.

[0014] As a preferred embodiment of the present invention, in the preparation method of the aluminum-bismuth master alloy, in S2, during the later stage of electromagnetic stirring, a slag remover accounting for 0.06-0.08% of the alloy melt mass is added, stirring is continued for 1-2 minutes, and after standing, slag removal is performed. The slag remover is a mixture of equal masses of potassium chloride and sodium chloride. After adding it, stir for 1-2 minutes, let it stand for 30 seconds, and then perform slag removal to remove the oxidized slag on the surface of the melt and further improve the purity of the alloy.

[0015] In S2, S3 and S4, the magnetic field strength of the electromagnetic stirring is 0.1-0.3T and the stirring frequency is 50-100Hz.

[0016] In step S2, when adding bismuth raw material, electromagnetic stirring is continued for 5-15 minutes. In step S3, after adding the environmentally friendly refining agent, electromagnetic stirring is continued for 3-5 minutes to ensure that the environmentally friendly refining agent is fully dispersed and distributed in the melt, and fully reacts with oxide inclusions and dissolved hydrogen. In step S4, when adding the refining agent, electromagnetic stirring is continued for 3-5 minutes to ensure that the refining agent is uniformly dispersed in the melt.

[0017] In a preferred embodiment of the present invention, in the method for preparing the aluminum-bismuth master alloy, in step S3, the environmentally friendly refining agent is prepared by melt granulation. The environmentally friendly refining agent comprises, by mass fraction: 28-32 parts potassium chloride, 24-26 parts sodium chloride, 14-16 parts calcium fluoride, 19-21 parts sodium carbonate, and 9-11 parts rare earth oxides. The environmentally friendly refining agent has a melting point of ≤480℃, which also meets the low-temperature casting conditions for aluminum-bismuth master alloys.

[0018] The mass ratio of the environmentally friendly refining agent to the alloy melt in S2 is 0.1-0.35:100.

[0019] Preferably, the alloy comprises 30 parts potassium chloride, 25 parts sodium chloride, 15 parts calcium fluoride, 20 parts sodium carbonate, and 10 parts rare earth oxides. These environmentally friendly grain refiners refine the aluminum matrix grains through heterogeneous nucleation, and utilize the grain boundary pinning effect to limit the agglomeration and growth of the bismuth phase during solidification, further improving the uniformity of bismuth phase distribution and enhancing the alloy's mechanical properties. The rare earth oxides are a mixture of lanthanum oxide and cerium oxide, with a mass ratio of lanthanum oxide to cerium of 3:7.

[0020] Meanwhile, the aforementioned environmentally friendly refining agent does not contain toxic halogenated hydrocarbons such as hexachloroethane and carbon tetrachloride, and no toxic or harmful gases are released throughout the entire process. This composite refining agent has a melting point ≤640℃, making it perfectly suited for low-temperature melting environments of 680-720℃. It can melt rapidly and completely at low temperatures. Potassium chloride and sodium chloride serve as the base flux, regulating the system's melting point and melt viscosity, and improving the refining agent's dispersibility in molten aluminum. Calcium fluoride acts as a flux and adsorbent, enhancing its wetting and adsorption capacity for Al2O3 oxide inclusions. Sodium carbonate acts as a degassing agent, reacting with dissolved hydrogen in the melt to achieve dehydrogenation while reducing the melt's surface tension. Rare earth oxides further enhance the refining agent's adsorption capacity for fine inclusions, while purifying grain boundaries. It does not react with bismuth, thus not exacerbating bismuth volatilization and burn-off, making it highly compatible with the low-temperature process of this invention.

[0021] In a preferred embodiment of the present invention, in the preparation method of the aluminum-bismuth master alloy, in step S3, when adding the environmentally friendly refining agent, an inert gas is used as the carrier gas, and the environmentally friendly refining agent is sprayed into the alloy melt to a depth of 2 / 3 below the liquid surface. The carrier gas flow rate is 0.2-0.4 L / min, and the spraying time is 1-2 min.

[0022] The environmentally friendly refining agent injection process involves pre-loading the refining agent into a sealed refining tank. Using argon gas with a purity ≥99.99% as the carrier gas, the inert gas injection causes the refining agent to form dispersed micro-droplets within the melt, ensuring full contact with the entire melt surface and preventing the refining agent from floating on the melt surface and becoming ineffective. Simultaneously, the carrier gas bubbles further float inclusions and carry away dissolved hydrogen, significantly improving the refining effect at low temperatures. After adding the environmentally friendly refining agent and stirring, the mixture is allowed to stand for 5 minutes to allow inclusions, refining products, and bubbles to fully float to the surface. Floating slag is then removed from the melt surface, completing the low-temperature refining and impurity removal process.

[0023] In a preferred embodiment of the present invention, in the method for preparing the aluminum-bismuth master alloy, in step S4, the refining agent is an aluminum-based Ti-Zr composite. The mass ratio of the refining agent to the alloy melt in S2 is 0.1-0.3:100.

[0024] As a preferred embodiment of the present invention, the method for preparing the aluminum-bismuth master alloy uses aluminum as a carrier in the aluminum-based Ti-Zr composite, wherein the mass fraction of Ti is 4-6%, the mass fraction of Zr is 4-6%, and the balance is aluminum and unavoidable impurities.

[0025] In a preferred embodiment of the present invention, in the preparation method of the aluminum-bismuth master alloy, in S5, the gradient cooling method specifically involves: first cooling to 500°C at a cooling rate of 8-10°C / s to quickly pass through the temperature range of bismuth phase segregation and growth, thus suppressing bismuth phase coarsening; then cooling to 300°C at a cooling rate of 5-7°C / s to control the uniform growth of aluminum matrix grains; and finally cooling to room temperature at a cooling rate of 2-3°C / s to reduce the internal stress of the alloy and prevent cracks from forming in the ingot.

[0026] Before cooling, stirring is stopped, and the melt is allowed to stand for 2-5 minutes to allow any remaining small impurities and bubbles to rise fully. Then, a preheated cast iron mold at 200°C is used in conjunction with a water-cooled temperature control system to achieve the three-stage gradient cooling described above.

[0027] The process also includes post-processing steps, such as surface polishing of the cooled aluminum-bismuth master alloy ingot to remove oxide scale and burrs; cutting it into the corresponding specifications according to industrial requirements; using a direct-reading spectrometer to detect the composition, rejecting products with unqualified composition, and finally obtaining qualified bismuth-containing aluminum master alloy.

[0028] The qualified standards for the composition testing of bismuth-aluminum master alloys are as follows: the deviation between the measured content and the theoretical content of bismuth element is ≤5%, the content of a single impurity element meets the following requirements: Cu≤0.10%, Si≤0.20%, Fe≤0.20%, Zn≤0.10%, and the total impurity content is ≤0.80%.

[0029] Secondly, embodiments of the present invention provide an aluminum-bismuth master alloy, which is prepared using the preparation method described in the first aspect; In the aluminum-bismuth master alloy, the bismuth phase is uniformly distributed in the aluminum matrix in spherical or ellipsoidal shapes, the bismuth phase particle size is ≤5μm, the deviation between the measured and theoretical bismuth content is ≤5%, and the total impurity content is ≤0.80%. The aluminum-bismuth master alloy has a cast tensile strength of 130-160 MPa, an elongation after fracture of 14-19%, and a Brinell hardness of HBW 40-50.

[0030] (III) Beneficial Effects The beneficial effects of this invention are as follows: The aluminum-bismuth master alloy and its preparation method of this invention employ a low-temperature alloying process at 680-720℃. This temperature is only slightly higher than the melting point of pure aluminum and far below the critical temperature at which bismuth volatilizes in large quantities, thus suppressing the high-temperature volatilization of bismuth at its source. Combined with full-process inert gas micro-positive pressure protection, batch-by-batch uniform feeding, and a refining process at the same temperature as alloying, no additional heating is required, avoiding the aggravation of bismuth volatilization during refining. Ultimately, the bismuth burn-off rate is stably controlled within 1.5%, far lower than the 5-8% of traditional high-temperature processes. This not only significantly reduces raw material costs but also controls the bismuth content deviation within 5%, ensuring stable and controllable alloy batch composition.

[0031] This invention utilizes electromagnetic stirring, combined with environmentally friendly refining of a composite system, grain refinement with an aluminum-based Ti-Zr composite refining agent, and gradient cooling processes. This quadruple synergistic effect suppresses the gravitational segregation and agglomeration of bismuth. The refined melt purity is significantly improved, reducing the interference of oxide inclusions on bismuth phase nucleation. This results in a uniformly dispersed spherical or ellipsoidal bismuth phase distribution within the aluminum matrix, with a bismuth phase particle size ≤5μm and a standard deviation of bismuth phase position distribution ≤1.2, exhibiting excellent microscopic uniformity. This invention completely solves the problems of bismuth phase agglomeration and high oxide inclusion content leading to deterioration in alloy mechanical and machinability caused by traditional processes. The alloys prepared using this invention show significantly better tensile strength, elongation, and machinability than those produced using traditional processes. Compared to existing technologies, this invention achieves low burn-off rate, no segregation, and uniform dispersion of bismuth under the premise of low cost, low energy consumption, and environmental compliance throughout the entire process. It can stably prepare high-purity, high-performance, high-quality bismuth-containing aluminum master alloys, meeting the needs of large-scale industrial production. Attached Figure Description

[0032] Figure 1 The image shows the metallographic microstructure of the AlBi5 master alloy prepared in Example 2 of this invention. Figure 2 This is a process flow diagram of the method for preparing aluminum-bismuth master alloy according to the present invention. Detailed Implementation

[0033] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0035] Example 1 This embodiment provides a method for preparing an aluminum-bismuth master alloy (AlBi3 master alloy), see [link to documentation]. Figure 2 The process, specifically the steps, is as follows: (1) Raw material pretreatment: Select industrial pure aluminum ingots with a purity of 99.7% and industrial pure bismuth ingots with a purity of 99.9%. Polish the bismuth ingots to remove the 0.1mm thick oxide layer on the surface and set them aside. (2) Preheating and atmosphere protection of medium frequency furnace: Preheat the graphite crucible of medium frequency furnace to 200℃ for 30min; argon gas with a purity of 99.99% is continuously introduced throughout the preheating process and the subsequent melting process, with the flow rate stabilized at 0.3L / min, maintaining a slight positive pressure of 50Pa inside the furnace, and isolating air throughout the process; (3) Low temperature melting: Add the pretreated aluminum ingot into the crucible of the medium frequency furnace, turn on the medium frequency furnace to heat, control the working frequency of the equipment to 350Hz, the heating rate to 15℃ / min, heat to 680℃ (first temperature), hold for 5min to completely melt the aluminum ingot and obtain aluminum liquid with uniform temperature; the heating process is monitored in real time by an infrared temperature control module, and the temperature deviation of the melt is controlled within ±8℃; (4) Alloying treatment: Add the pretreated bismuth ingots to the aluminum liquid in two batches. The total amount of bismuth ingots added is 3% of the total mass of the alloy. The interval between each batch is 2 minutes. Each batch of bismuth ingots is added at a uniform speed along the side wall of the crucible. The feeding time for each batch is 30 seconds. At the same time as feeding, turn on the electromagnetic stirring of the medium frequency furnace, control the stirring magnetic field strength to be 0.1T and the stirring frequency to be 50Hz, and continue stirring for 5 minutes. Keep the temperature of the melt in the furnace stable at 680℃ throughout the stirring process. (5) Refining treatment: Keep the melt temperature constant at 680℃, and use argon as the carrier gas in a sealed refining tank to spray an environmentally friendly refining agent into the melt at a depth of 2 / 3 below the surface. The amount of refining agent added is 0.1% of the total mass of the melt. The refining agent is composed of 30% potassium chloride, 25% sodium chloride, 15% calcium fluoride, 20% sodium carbonate, and 10% rare earth oxides by mass fraction. The carrier gas flow rate is controlled at 0.2L / min, and the spraying time is 1min. After the spraying is completed, keep the electromagnetic stirring parameters of step (4) unchanged, stir for 3min, and then let it stand for 5min to remove the slag on the surface of the melt. (6) Refining treatment: Add aluminum-based Ti-Zr composite refining agent (Ti mass fraction 5%, Zr mass fraction 5%, balance is aluminum and unavoidable impurities) to the refined melt. The amount of refining agent added is 0.1% of the total mass of the melt. Keep the electromagnetic stirring parameters of step (4) and continue stirring for 3 minutes to make the refining agent evenly dispersed. (7) Gradient cooling: Stop stirring and let the melt stand for 2 minutes. Then, use a preheated cast iron metal mold at 200°C and a water-cooled temperature control system to perform three-stage gradient cooling according to the following parameters: first cool to 500°C at a cooling rate of 8°C / s, then cool to 300°C at a cooling rate of 5°C / s, and finally cool to room temperature at a cooling rate of 2°C / s to obtain AlBi3 intermediate alloy ingot. (8) Post-processing: The surface of the ingot is polished to remove oxide scale and burrs; it is cut into shape according to specifications; the composition is detected by direct reading spectrometer, and the AlBi3 intermediate alloy product is obtained after passing the test.

[0036] The prepared aluminum-bismuth master alloy was subjected to the following tests: (1) The formula for calculating the bismuth element burn-off rate described in this invention is as follows: Burn-off rate = [(Total mass of bismuth elements in the feed - Total mass of bismuth elements in the ingot) / Total mass of bismuth elements in the feed] × 100% The total mass of bismuth in the ingot was calculated by taking the average value of multiple measurements using a direct-reading spectrometer and combining it with the total mass of the ingot.

[0037] (2) Bismuth phase particle size and distribution uniformity were tested by metallographic microscope and scanning electron microscope (SEM) with image analysis software. The test method was as follows: three fields of view were taken at the center, 1 / 2 radius and edge of the cross-section of the ingot, and different magnifications were used for observation. The particle size of the bismuth phase in each field of view was counted, and the average value and distribution standard deviation were calculated. The distribution standard deviation ≤ 1.2 is considered uniform distribution.

[0038] (3) The mechanical properties described in this invention are tested using a unified standard: Room temperature tensile properties: Tested according to GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", with cast standard tensile specimens and tensile rate of 2 mm / min. Brinell hardness: Tested according to GB / T231.1-2018 "Metallic materials - Brinell hardness test - Part 1: Test method" with test conditions of φ2.5mm cemented carbide indenter, load of 62.5kgf, and holding time of 30s. Five points were tested for each sample and the average value was taken.

[0039] Referring to Table 1, the AlBi3 master alloy prepared in this embodiment was tested and found to have a bismuth burn-off rate of 1.2%, an average bismuth phase particle size of 3.7 μm, a maximum particle size ≤5 μm, and uniform distribution in the aluminum matrix; a cast tensile strength of 157 MPa, an elongation after fracture of 17%, and a Brinell hardness of HBW45; a total impurity content of 0.38%; and compositional analysis showed that the actual bismuth content was 2.96%, with a deviation of 1.33% from the theoretical bismuth content. The impurity element content met the qualified standards, and the contents of lead, mercury, cadmium, and hexavalent chromium all met the RoHS 2.0 directive limit requirements.

[0040] Example 2 This embodiment provides a method for preparing an aluminum-bismuth master alloy (AlBi5 master alloy), the specific steps of which are as follows: (1) Raw material pretreatment: Select aluminum ingots with a purity of 99.7% and industrial pure bismuth ingots with a purity of 99.9%. Polish the bismuth ingots to remove the oxide layer with a thickness of 0.15 mm on the surface and set them aside. (2) Preheating and atmosphere protection of medium frequency furnace: Preheat the graphite crucible of medium frequency furnace to 250℃ for 35min; argon gas with a purity of 99.99% is continuously introduced throughout the preheating process and the subsequent melting process, with the flow rate stabilized at 0.4L / min, maintaining a slight positive pressure of 80Pa inside the furnace, and isolating air throughout the process; (3) Low temperature melting: The pretreated aluminum ingot is added to the crucible of the medium frequency furnace, the medium frequency furnace is turned on for heating, the working frequency of the equipment is controlled at 400Hz, the heating rate is 20℃ / min, the temperature is heated to 700℃, and the temperature is held for 10min to completely melt the aluminum ingot and obtain aluminum liquid with uniform temperature; the heating process is monitored in real time by an infrared temperature control module, and the temperature deviation of the melt is controlled within ±8℃. (4) Alloying treatment: Pretreated bismuth ingots are added to the aluminum liquid in 3 batches. The total amount of bismuth ingots added is 5% of the total mass of the alloy. The interval between each batch is 2.5 min. Each batch of bismuth ingots is added at a uniform speed along the side wall of the crucible. The feeding time for each batch is 45 s. At the same time as feeding, the electromagnetic stirring of the medium frequency furnace is turned on. The stirring magnetic field strength is controlled at 0.2T and the stirring frequency is 75Hz. Stirring is continued for 10 min. The temperature of the melt in the furnace is kept stable at 700℃ throughout the stirring process. During the stirring process, 0.07% of the total mass of the melt is added as a slag remover (potassium chloride and sodium chloride are mixed in equal mass). Stir for 1.5 min. After standing for 30 s, slag removal is performed. (5) Refining treatment: Keep the melt temperature constant at 700℃, and inject environmentally friendly refining agent into the melt at a depth of 2 / 3 below the surface of the melt through a sealed refining tank with argon as the carrier gas. The amount of refining agent added is 0.25% of the total mass of the melt. The refining agent is composed of 30% potassium chloride, 25% sodium chloride, 15% calcium fluoride, 20% sodium carbonate, and 10% rare earth oxides by mass fraction. The carrier gas flow rate is controlled at 0.3L / min, and the injection time is 1.5min. After the injection is completed, keep the electromagnetic stirring parameters constant and stir for 3min. Then let it stand for 5min and remove the slag on the surface of the melt. (6) Refining treatment: Add aluminum-based Ti-Zr composite refining agent (Ti mass fraction 5.5%, Zr mass fraction 4.5%, balance is aluminum and unavoidable impurities) to the refined melt. The amount of refining agent added is 0.2% of the total mass of the melt. Keep the electromagnetic stirring parameters of step (4) and continue stirring for 4 minutes to make the refining agent evenly dispersed. (7) Gradient cooling: Stop stirring and let the melt stand for 3.5 min. Then, use a preheated cast iron metal mold to 200℃ and a water-cooled temperature control system to perform three-stage gradient cooling according to the following parameters: first cool to 500℃ at a cooling rate of 9℃ / s, then cool to 300℃ at a cooling rate of 6℃ / s, and finally cool to room temperature at a cooling rate of 2.5℃ / s to obtain AlBi5 intermediate alloy ingot; (8) Post-processing: The surface of the ingot is polished to remove oxide scale and burrs; it is cut into shape according to specifications; the composition is detected by direct reading spectrometer, and the AlBi5 intermediate alloy product is obtained after passing the test.

[0041] See Table 1 and Figure 1Testing revealed that the AlBi5 master alloy prepared in this embodiment had a bismuth burn-off rate of 1.0%, an average bismuth phase particle size of 3.9 μm, a maximum particle size ≤5 μm, and was uniformly distributed in the aluminum matrix. The as-cast tensile strength was 151 MPa, the elongation after fracture was 18%, and the Brinell hardness was HBW43. The total impurity content was 0.30%. Composition analysis showed that the actual bismuth content was 5.14%, with a deviation of 2.80% from the theoretical bismuth content. The impurity element content met the acceptable standards, and the contents of lead, mercury, cadmium, and hexavalent chromium all met the RoHS 2.0 directive limits. Figure 1 The uniformly distributed black dots are bismuth phase.

[0042] Example 3 This embodiment provides a method for preparing an aluminum-bismuth master alloy (AlBi10 master alloy), the specific steps of which are as follows: (1) Raw material pretreatment: Select aluminum ingots with a purity of 99.85% and industrial pure bismuth ingots with a purity of 99.9%. Grind the bismuth ingots to remove the 0.2mm thick oxide layer on the surface and set them aside. (2) Preheating and atmosphere protection of medium frequency furnace: Preheat the graphite crucible of medium frequency furnace to 300℃ for 40min; argon gas with a purity of 99.99% is continuously introduced throughout the preheating process and the subsequent melting process, with the flow rate stabilized at 0.5L / min, maintaining a slight positive pressure of 100Pa inside the furnace, and isolating air throughout the process. (3) Low temperature melting: The pretreated aluminum ingot is added to the crucible of the medium frequency furnace, the medium frequency furnace is turned on for heating, the working frequency of the equipment is controlled at 500Hz, the heating rate is 25℃ / min, the temperature is heated to 720℃, and the temperature is held for 10min to completely melt the aluminum ingot and obtain aluminum liquid with uniform temperature; the heating process is monitored in real time by an infrared temperature control module, and the temperature deviation of the melt is controlled within ±8℃. (4) Alloying treatment: Pretreated bismuth ingots are added to the aluminum melt in 3 batches. The total amount of bismuth ingots added is 10% of the total mass of the alloy. Each batch is added 3 minutes apart. Each batch of bismuth ingots is added at a uniform speed along the side wall of the crucible. The feeding time for each batch is 60 seconds. At the same time as feeding, the electromagnetic stirring of the medium frequency furnace is turned on. The stirring magnetic field strength is controlled at 0.3T and the stirring frequency is 100Hz. Stirring is continued for 15 minutes. The temperature of the melt in the furnace is kept stable at 720℃ throughout the stirring process. During the stirring process, 0.08% of the total mass of the melt is added as a slag remover (potassium chloride and sodium chloride are mixed in equal mass). Stir for 2 minutes, let stand for 30 seconds, and then remove the slag. (5) Refining treatment: Keep the melt temperature constant at 720℃, and inject environmentally friendly refining agent into the melt at a depth of 2 / 3 below the surface of the melt through a sealed refining tank using argon as the carrier gas. The amount of refining agent added is 0.35% of the total mass of the melt. The refining agent is composed of 30% potassium chloride, 25% sodium chloride, 15% calcium fluoride, 20% sodium carbonate, and 10% rare earth oxides by mass fraction. The carrier gas flow rate is controlled at 0.4L / min, and the injection time is 2min. After the injection is completed, keep the electromagnetic stirring parameters constant and stir for 3min. Then let it stand for 5min and remove the slag on the surface of the melt. (6) Refining treatment: Add aluminum-based Ti-Zr composite refining agent (Ti mass fraction 6.0%, Zr mass fraction 4.0%, balance is aluminum and unavoidable impurities) to the refined melt. The amount of refining agent added is 0.3% of the total mass of the melt. Keep the electromagnetic stirring parameters of step (4) and continue stirring for 5 minutes to make the refining agent evenly dispersed. (7) Gradient cooling: Stop stirring and let the melt stand for 5 minutes. Then, use a preheated cast iron metal mold to 200°C and a water-cooled temperature control system to perform three-stage gradient cooling according to the following parameters: first cool to 500°C at a cooling rate of 10°C / s, then cool to 300°C at a cooling rate of 7°C / s, and finally cool to room temperature at a cooling rate of 3°C / s to obtain AlBi10 intermediate alloy ingot. (8) Post-processing: The surface of the ingot is polished to remove oxide scale and burrs on the edges; it is cut into shape according to specifications; the composition is detected by direct reading spectrometer, and the AlBi10 intermediate alloy product is obtained after passing the test.

[0043] Referring to Table 1, the AlBi10 master alloy prepared in this embodiment has a bismuth burn-off rate of 1.3%, an average bismuth phase particle size of 4.2 μm, a maximum particle size ≤5 μm, and is uniformly distributed in the aluminum matrix; the as-cast tensile strength is 148 MPa, the elongation after fracture is 15%, and the Brinell hardness is HBW43; the total impurity content is 0.33%; the composition analysis shows that the actual bismuth content is 9.96%, which is 4.00% different from the theoretical bismuth content. The impurity element content meets the qualified standard, and the contents of lead, mercury, cadmium, and hexavalent chromium all meet the RoHS 2.0 directive limit requirements.

[0044] Comparative Example 1 This comparative example provides a method for preparing an aluminum-bismuth master alloy (AlBi5 master alloy), which uses a conventional high-temperature melting process in an industrial frequency furnace as a reference. The specific steps are as follows: (1) Raw material pretreatment: Same as in Example 2; (2) Preheating of the industrial frequency furnace: Preheat the graphite crucible of the industrial frequency furnace to 250°C without inert gas protection; (3) High-temperature melting: Add the pretreated aluminum ingot into the crucible of the industrial frequency furnace, heat it to 1000℃, hold it for 10 minutes, and let the aluminum ingot melt completely; (4) Alloying treatment: Add 5% of the total mass of bismuth ingots at one time, use mechanical stirring, stirring speed 60 rpm, stirring time 10 min, and maintain the melt temperature in the furnace at 1000℃ during the process; after stirring, add the same amount of slag remover as in Example 2, and remove the slag after stirring. (5) Refining treatment: Keep the melt temperature at 1000℃, add traditional hexachloroethane refining agent, the amount added is 0.3% of the total mass of the melt, stir for 3 minutes and then let stand to remove slag; (6) Cooling: After the alloy liquid has been allowed to stand for 5 minutes, it is cooled to room temperature in the furnace using the same metal mold as in Example 2; (7) Post-processing: Same as in Example 2.

[0045] The product performance test results are shown in Table 1.

[0046] Comparative Example 2 This comparative example provides a method for preparing an aluminum-bismuth master alloy (AlBi5 master alloy). The difference from Example 2 is that step (6) of the refining process is removed, while the remaining steps are the same as in Example 2 to prepare the AlBi5 master alloy.

[0047] The product performance test results are shown in Table 1.

[0048] Comparative Example 3 This comparative example provides a method for preparing an aluminum-bismuth master alloy (AlBi5 master alloy). The difference from Example 2 is that the gradient cooling in step (7) is replaced by direct cooling to room temperature in a mold. The remaining steps are the same as in Example 2 to prepare the AlBi5 master alloy.

[0049] The product performance test results are shown in Table 1.

[0050] Table 1 Performance test results of products from different embodiments and comparative examples

[0051] As shown in Table 1, the bismuth-containing aluminum master alloys prepared by the methods described in Examples 1 to 3 of this invention are significantly superior to the conventional high-temperature smelting process using an industrial frequency furnace in Comparative Example 1 in terms of bismuth element burn-off control, microstructure uniformity, melt purity, mechanical properties, machinability, energy consumption control, and environmental friendliness. Specific analysis is as follows: This invention utilizes the full-range electromagnetic stirring inherent in the medium-frequency furnace, combined with environmentally friendly refining of the composite system, grain refinement of aluminum-based Ti-Zr composite refining agent, and a three-stage gradient cooling process. The four-fold synergistic effect inhibits the gravitational segregation and agglomeration growth of bismuth. After refining, the purity of the melt is greatly improved, reducing the interference of oxide inclusions on the nucleation of the bismuth phase. This results in the bismuth phase being uniformly dispersed in the aluminum matrix in spherical or ellipsoidal shapes, with a bismuth phase particle size ≤5μm and a standard deviation of bismuth phase position distribution ≤1.2. This completely solves the performance degradation problem caused by bismuth phase agglomeration and numerous inclusions in traditional processes.

[0052] The bismuth-containing aluminum master alloy prepared by this invention contains 3-10% bismuth by mass, with the balance being aluminum and unavoidable impurities, the total impurity content being ≤0.80%. The bismuth content deviation is controlled within 5%, ensuring stable and controllable batch composition of the alloy. The lead, mercury, cadmium, and hexavalent chromium contents in the alloy all meet the RoHS 2.0 directive limits, making it lead-free and environmentally friendly. The bismuth burn-off rate is ≤1.5%, far lower than the 5-8% burn-off rate of traditional high-temperature processes, significantly reducing raw material costs.

[0053] The alloy prepared by this invention has a tensile strength of 130-160 MPa in the as-cast state, an elongation after fracture of 14-19%, and a Brinell hardness of HBW40-HBW50. It can be used to replace lead-based free-machining aluminum alloys (grades such as 2011 and 6262) in manufacturing. The alloy prepared by this invention has significantly better tensile strength, elongation, and machinability than products made by traditional processes (Comparative Example 1). It is ultimately applied in automotive engine bearings, electronic radiators, precision mechanical parts, and other scenarios, and has extremely high promotional value in the manufacturing fields of automotive, electronics, and precision mechanical aluminum alloy materials.

[0054] The medium-frequency induction melting technology used in this invention achieves a thermal efficiency of 60-80%, reducing energy consumption by more than 30% compared to traditional industrial frequency furnace melting. It also eliminates the high-cost vacuum melting step, achieving low-oxidation melting solely through inert gas micro-positive pressure protection, thus reducing equipment investment and process complexity. The medium-frequency furnace offers high temperature control precision and a uniform temperature field within the furnace. Its refining process is compatible with existing general-purpose blowing equipment for industrial aluminum melting, allowing for direct adaptation to semi-continuous industrial production. The single-furnace capacity can be flexibly adjusted to meet the needs of large-scale applications.

[0055] Compared to Example 2, Comparative Example 2 removed the refining process, resulting in a significant coarsening of the bismuth phase particle size and a substantial decrease in the tensile strength and elongation of the alloy. This demonstrates that the aluminum-based Ti-Zr composite refining process is one of the core steps in suppressing bismuth phase agglomeration and improving the mechanical properties of the alloy.

[0056] Compared to Example 2, Comparative Example 3 removed the gradient cooling step, resulting in severe gravity segregation and coarsening of the bismuth phase. The particle size was close to that of traditional high-temperature processes, and the mechanical properties deteriorated significantly. This demonstrates that the three-stage gradient cooling is a key process to suppress the segregation and growth of the bismuth phase during solidification and to achieve a uniform dispersion distribution.

[0057] Through multiple comparative examples, it has been verified that the four core processes of alloying, refining, fine refining, and gradient cooling in this invention work synergistically and are indispensable, together achieving the technical effects of low bismuth burn-off, no segregation, and high alloy performance.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

[0059] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0060] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0061] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0062] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, for example, sequentially. For instance, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For instance, the method may also include step (c), indicating that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0063] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0064] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

Claims

1. A method for preparing an aluminum-bismuth master alloy, characterized in that, Includes the following steps: S1. First, preheat the reaction vessel to 200-300℃, then add the aluminum raw material to the reaction vessel, heat the reaction vessel to the first temperature, and hold it at the temperature to obtain molten aluminum; the first temperature is 680-720℃. S2. Then, add the bismuth raw material to the aluminum liquid in batches. When adding the bismuth raw material, perform electromagnetic stirring and keep the reaction vessel at the first temperature to obtain the alloy melt. S3. Refining process: An environmentally friendly refining agent is added to the reaction vessel using an inert gas as the carrier gas. After the environmentally friendly refining agent is added, electromagnetic stirring is performed and the reaction vessel is kept at the first temperature. Then, it is allowed to stand and the slag on the surface of the alloy melt is removed. S4. Refining treatment: Add a refining agent to the refined alloy melt, perform electromagnetic stirring and keep the reaction vessel at the first temperature, so that the refining agent is evenly dispersed in the refined alloy melt. S5. Cooling treatment: After the alloy melt refined by S4 is allowed to stand, it is cooled to room temperature by gradient cooling to obtain an aluminum-bismuth master alloy ingot; the reaction vessel is kept in an inert gas atmosphere throughout the preparation process.

2. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S1, the aluminum raw material is industrial pure aluminum ingot or electrolytic aluminum liquid with a purity of ≥99.7%; the bismuth raw material is industrial pure bismuth ingot with a purity of ≥99.9% after removing the surface oxide layer. The reaction vessel is heated to the first temperature by electromagnetic heating, with an operating frequency of 350-500Hz and a heating rate of 15-25℃ / min. The heat preservation time is 5-10 minutes.

3. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S2, the mass ratio of bismuth raw material to alloy melt is 3-10:100; When adding bismuth raw materials to molten aluminum in batches, each batch of bismuth raw materials is added at a uniform speed along the side wall of the reaction vessel, with each batch taking 30-60 seconds and an interval of 2-3 minutes between batches.

4. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S2, during the later stage of electromagnetic stirring, add 0.06-0.08% of the slag remover by mass of the alloy melt, continue stirring for 1-2 minutes, and then remove the slag after standing. In S2, S3 and S4, the magnetic field strength of the electromagnetic stirring is 0.1-0.3T and the stirring frequency is 50-100Hz.

5. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S3, the environmentally friendly refining agent is prepared by melt granulation. The environmentally friendly refining agent comprises, by mass fraction: 28-32 parts potassium chloride, 24-26 parts sodium chloride, 14-16 parts calcium fluoride, 19-21 parts sodium carbonate, and 9-11 parts rare earth oxides. The mass ratio of the environmentally friendly refining agent to the alloy melt in S2 is 0.1-0.35:

100.

6. The method for preparing the aluminum-bismuth master alloy as described in claim 5, characterized in that, In S3, when adding environmentally friendly refining agent, inert gas is used as the carrier gas. The environmentally friendly refining agent is sprayed into the alloy melt to a depth of 2 / 3 below the liquid surface. The carrier gas flow rate is 0.2-0.4 L / min, and the spraying time is 1-2 min.

7. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S4, the refining agent is an aluminum-based Ti-Zr composite; The mass ratio of the refining agent to the alloy melt in S2 is 0.1-0.3:

100.

8. The method for preparing the aluminum-bismuth master alloy as described in claim 7, characterized in that, The aluminum-based Ti-Zr composite uses aluminum as a carrier, wherein the mass fraction of Ti is 4-6%, the mass fraction of Zr is 4-6%, and the balance is aluminum and unavoidable impurities.

9. The method for preparing the aluminum-bismuth master alloy as described in claim 1, characterized in that, In S5, the gradient cooling method is specifically as follows: first, cool to 500℃ at a cooling rate of 8-10℃ / s, then cool to 300℃ at a cooling rate of 5-7℃ / s, and finally cool to room temperature at a cooling rate of 2-3℃ / s.

10. An aluminum-bismuth master alloy, characterized in that, It was prepared by the preparation method according to any one of claims 1-9; In the aluminum-bismuth master alloy, the bismuth phase is uniformly distributed in the aluminum matrix in spherical or ellipsoidal shapes, the bismuth phase particle size is ≤5μm, the deviation between the measured and theoretical bismuth content is ≤5%, and the total impurity content is ≤0.80%. The aluminum-bismuth master alloy has a cast tensile strength of 130-160 MPa, an elongation after fracture of 14-19%, and a Brinell hardness of HBW 40-50.

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