A method for molding a hollow subframe of aluminum matrix composite material using integral low-pressure casting

Abstract

This invention discloses a method for forming an integral low-pressure cast aluminum-based composite hollow subframe. Inorganic salts are selected as reactants and, after being dried and kept at a constant temperature, are added to molten aluminum alloy for reaction. The reaction is carried out under the action of electromagnetic and ultrasonic fields to generate in-situ nanoparticles. The composite melt is then subjected to rare earth alloying, slag removal, degassing, refining, and fine-refining treatments to obtain a composite melt ready for use. The molten aluminum-based composite material is then pressed from bottom to top into an integral hollow subframe metal mold for low-pressure casting. During the solidification process, circulating cooling water is applied to the metal mold until the casting is completely solidified. Finally, the casting is subjected to a stepped energy-saving heat treatment to obtain an integral aluminum-based composite hollow subframe with good grain structure and excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of low-pressure casting technology of metal matrix composites, and relates to a method for forming an integral low-pressure cast aluminum matrix composite hollow subframe. Background Technology

[0002] The subframe, as a key structural component bearing the weight of the vehicle body, is one of the key structural components in the automotive parts industry's focus on lightweight technology applications. Due to its low density, good formability, and excellent corrosion resistance, aluminum alloy has become an important means of achieving lightweight chassis. As a safety component supporting the front and rear axles and suspension, the chassis requires high strength, rigidity, and fatigue resistance; lightweight manufacturing is currently a major challenge for further lightweighting in new energy vehicles.

[0003] Chinese patent document CN106925756A describes a "low-pressure casting preparation method for an aluminum alloy subframe." This method includes the design of the aluminum alloy material's chemical composition, alloy melting and modification refinement treatment, sand mold assembly, pressure-time curve analysis, low-pressure casting, and heat treatment of the casting. However, the material used is ZL101A + 0-0.5% RE. With the increasing performance requirements for aluminum alloy subframes, this alloy composition is no longer sufficient to meet development needs. It is necessary to develop new aluminum-based composite materials with higher strength, higher toughness, and fatigue resistance. Furthermore, the document mentions using a T6 heat treatment process, with a total solution treatment and aging time of 9-13 hours. This excessively long heat treatment time not only causes thermal deformation of the subframe, affecting the product yield, but also results in significant energy consumption, contradicting national energy-saving requirements. Therefore, developing new aluminum-based composite materials for low-pressure casting of integral subframes to improve the high strength, toughness, and fatigue resistance of the castings is particularly important. Optimizing the heat treatment process of the castings and developing energy-saving heat treatment technologies are also urgently needed.

[0004] A search of published patents revealed that Chinese patent document CN107881377A describes a "gravity tilt casting method for aluminum alloy subframes." This method involves assembling a sand mold from melted and refined aluminum alloy material, placing the sand mold within a gravity casting mold, and obtaining a gravity-cast aluminum alloy subframe according to preset tilting parameters and solidification time. However, the castings produced by this method have low surface finish, are prone to pitting after shot blasting, and have low production efficiency, thus increasing production costs. The process is relatively simple and unsuitable for the production of hollow, thin-walled, integral subframes. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a novel, simple, and low-cost method for forming a hollow subframe of an integral low-pressure cast aluminum-based composite material that achieves high strength, high toughness, and fatigue resistance.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for molding an integral low-pressure cast aluminum-based composite hollow subframe includes the following steps:

[0008] (1) Preparation of inorganic salts: Fluorine salts were selected as reactants, placed in a forced-air drying oven for heat preservation and drying to remove water of crystallization. After drying, they were mixed, cooled, ground, and wrapped in aluminum foil for later use.

[0009] (2) Preparation of aluminum-based composite material melt: The aluminum alloy is melted in a crucible melting furnace. After the aluminum alloy is melted and heated to the reaction temperature, the fluoride salt prepared in step (1) is added. The melt is pressed into the furnace with a bell jar and stirred. Then, the melt is reacted under the action of electromagnetic field and ultrasonic field. After the reaction generates in-situ nanoparticles, the composite material melt is subjected to slag removal, degassing, refining and refining treatment to obtain the composite material melt for use.

[0010] (3) Rare earth element addition: Add rare earth erbium to process the melt in step (2);

[0011] (4) Low pressure casting: First, the mold is assembled. After the mold is preheated, the molten aluminum-based composite material in step (3) is injected into the metal mold from bottom to top. During the solidification process, circulating cooling water is applied to the metal mold until the casting is completely solidified.

[0012] (5) Energy-saving heat treatment: The castings in step (4) are subjected to high-frequency vibration to remove the sand core, cut off the gating and riser and the runner, and X-ray non-destructive testing. After no defects are found, solution treatment and step-age treatment are carried out, and finally fluorescent testing is carried out.

[0013] Preferably, the inorganic salt in step (1) is an industrial-grade fluoride salt K2ZrF6 and KBF4 mixed together, both with a purity greater than 99 wt.%, the drying oven temperature is 150-300℃, the particle size of both is greater than 200 mesh, and the drying time is 2-4 hours; the mass ratio of K2ZrF6 to KBF4 is 0.75-1.15.

[0014] The reaction formulas of K2ZrF6, KBF4 with the melt are as follows:

[0015] 3K2ZrF6+6KBF4+10Al=3ZrB2+9KAlF4+K3AlF6

[0016] Preferably, the aluminum alloy composition in step (2) is Si: 6.5-7.5% wt.%, Mg: 0.25-0.45% wt.%, Ti: 0.08-0.2% wt.%, Zr: 0.1-0.2% wt.%, Fe: <0.2% wt.%, Zn <0.1% wt.%, Cu: <0.1% wt.%, Mn: <0.1% wt.%, Sn: <0.01% wt.%, Pb: <0.03% wt.%, the reaction temperature of the aluminum alloy with the fluoride salt is 820-870℃, the reaction time is 20-30 min, the electromagnetic field setting parameters are magnetic field frequency 10-15 Hz, electromagnetic stirring time is 5-30 min, and ultrasonic power is 1-2.5 kW.

[0017] Preferably, the rare earth erbium in step (3) is an Al-10Er-5Sr alloy. After the melt temperature in step (2) drops to 700-725℃, Al-10Er-5Sr alloy is added to the melt in step (2) and stirred evenly. The amount added is 0.3-0.8% by mass. Then, it is kept at 720-740℃ for 15-30 minutes.

[0018] Preferably, the mold in step (4) is a sand core and a metal mold, which are assembled on a low-pressure casting machine and the upper and lower molds are closed; the pouring temperature of the molten aluminum-based composite material is controlled at 710-740℃.

[0019] As a preferred option, the injection process includes six steps: 1) Pre-pressing treatment: applying pressure to the molten aluminum-based composite material, controlling the distance between the liquid level in the riser tube and the mold gate, which is 50-150 mm, and automatically compensating for the pressure.

[0020] 2) Filling stage: Filling time is 10-30 seconds, and pressure is 20-30 kPa;

[0021] 3) Pressurization stage: Pressurization time is 10-90 seconds, and pressure is 30-70 kPa;

[0022] 4) Pressure holding stage: The pressure holding time is 40-200 seconds, and the pressure is 30-70 kPa;

[0023] 5) Depressurization stage: Depressurization time is 1 to 10 seconds;

[0024] 6) Delayed cooling phase: The delayed cooling time is 50 to 300 seconds.

[0025] Preferably, the application of circulating cooling water to the metal mold during the solidification process in step (4) includes two stages: the first stage is to cool the subframe body, and the second stage is to cool and harden the casting as a whole, including cooling of the gating system and runner.

[0026] Preferably, in step (5), the solution treatment involves raising the temperature from room temperature to 520–550°C and holding it for 3–5 hours, followed by quenching treatment, with the quenching medium water temperature maintained at 20–80°C; the aging treatment includes a two-stage aging process, first raising the temperature from room temperature to 120–140°C and holding it for 1.5–2.5 hours, then raising it to 145–170°C and holding it for 1.5–2.5 hours.

[0027] Compared with existing technologies, the advantages of this invention are: This invention utilizes a direct melt reaction method, where the nano-reinforcement is generated within the matrix, resulting in a strong bond between the matrix and the reinforcement, and a clean interface. The reinforcement particles are small in size. After electromagnetic / ultrasonic field modulation, the in-situ reaction efficiency is significantly improved, particle clusters are clearly dispersed and diffusely distributed within the matrix, and the particles and interface are well bonded. In the Al-Zr-B system, ZrB2 particles have a near-spherical morphology with a size of 10-80 nm. The magnetic field is more effective in improving particle yield, and the ultrasonic field has a more significant impact on particle distribution, morphology, and size. The electromagnetic field can improve the wettability of the reactants and the melt, promoting mass and heat transfer and increasing the number of nuclei during the in-situ reaction. The combined addition of Er and Zr promotes the precipitation of grain boundary precipitates, enhancing the material's strength. Simultaneously, it causes the grain boundary precipitates to distribute discontinuously, interrupting corrosion channels and improving corrosion resistance. In the low-pressure casting process, this invention employs a metal mold and a circulating water cooling system. The system is simple in structure and easy to operate. The water cooling system uses a tightly fitted metal mold cooling water jacket, utilizing circulating water for cooling, which is energy-saving and environmentally friendly, improves production efficiency, results in a small grain structure, and enhances the subframe performance. During heat treatment, a two-stage aging process is employed. The first-stage low-temperature aging treatment provides convenient nucleation conditions for the subsequent second-stage aging process, creating the GP zone. These favorable conditions effectively promote the precipitation of the transition phase β'', achieving the performance level of long-term aging treatments compared to direct aging in a shorter aging time, thus achieving energy-saving effects. Attached Figure Description

[0028] Figure 1 This is a flow chart of the molding process according to an embodiment of the present invention.

[0029] Figure 2 Scanning electron microscope (SEM) images of the microstructure of a monolithic aluminum-based composite hollow subframe.

[0030] Figure 3 Image showing the microstructure of reinforcing particles inside the integral aluminum-based composite hollow subframe.

[0031] Figure 4 This is a drawing of a hollow subframe casting made of integral aluminum-based composite material. Detailed Implementation

[0032] Example 1

[0033] Industrial-grade fluoride salts K2ZrF6 and KBF4 in a mass ratio of 1:0.9 were selected as reactants. They were placed in a forced-air drying oven and dried at 150℃ for 4 hours to remove the water of crystallization. After drying, they were mixed, cooled, ground, and wrapped in aluminum foil for later use. The reactants were then prepared with the following composition: Si: 6.5–7.5% wt.%, Mg: 0.25–0.45% wt.%, Ti: 0.08–0.2% wt.%, Zr: 0.1–0.2% wt.%, Fe: An aluminum alloy with <0.2% wt.%, Zn <0.1% wt.%, Cu <0.1% wt.%, Mn <0.1% wt.%, Sn <0.01% wt.%, and Pb <0.03% wt.% was melted in a crucible melting furnace. After the aluminum alloy melted and was heated to a reaction temperature of 820℃, a prepared fluoride salt was added, and the melt was pressed into the furnace using a bell jar. The reaction was then carried out for 30 minutes under the action of a 10Hz electromagnetic field and a 1kW ultrasonic field. After the reaction generated in-situ nanoparticles, the composite material melt was subjected to slag removal, degassing, refining, and refining treatment to obtain a composite material melt for later use. After the melt temperature dropped to 700℃, 0.3% Al-10Er-5Sr alloy was added to the melt and stirred uniformly, and then held at 720℃ for 30 minutes. The sand core and metal mold are then assembled onto the low-pressure casting machine, and the upper and lower molds are closed. After the mold is preheated at 200°C, molten aluminum-based composite material at 740°C is pressed into the metal mold from bottom to top. The distance between the liquid level in the riser pipe and the mold gate is controlled at 80mm. The pressure is automatically compensated. The filling stage lasts for 30 seconds with a pressure of 20KPa, the pressurization stage lasts for 50 seconds with a pressure of 50KPa, the holding stage lasts for 80 seconds with a pressure of 50KPa, the depressurization stage lasts for 3 seconds, and the delayed cooling stage lasts for 200 seconds. During the solidification process, circulating cooling water is applied to the metal mold. The casting underwent high-frequency vibration to remove sand cores, cut off gating gates and runners, and X-ray non-destructive testing. After confirming no defects, it underwent solution treatment and stepped aging treatment. The solution treatment involved heating from room temperature to 550℃ and holding for 5 hours, followed by quenching with the quenching medium water temperature maintained at 70℃. The aging treatment was a two-stage process: first heating from room temperature to 120℃ and holding for 2 hours, then heating to 155℃ and holding for 2 hours, finally followed by fluorescent testing. In summary, a monolithic aluminum-based composite hollow subframe was obtained.

[0034] Example 2

[0035] Industrial-grade fluoride salts K2ZrF6 and KBF4 in a mass ratio of 1:1.1 were selected as reactants and dried in a forced-air drying oven at 200℃ for 3 hours to remove the water of crystallization. After drying, they were mixed, cooled, ground, and wrapped in aluminum foil for later use. The reactants were then prepared with the following composition: Si: 6.5–7.5% wt.%, Mg: 0.25–0.45% wt.%, Ti: 0.08–0.2% wt.%, Zr: 0.1–0.2% wt.%, Fe: An aluminum alloy with <0.2% wt.%, Zn <0.1% wt.%, Cu <0.1% wt.%, Mn <0.1% wt.%, Sn <0.01% wt.%, and Pb <0.03% wt.% was melted in a crucible melting furnace. After the aluminum alloy melted and was heated to a reaction temperature of 850℃, a prepared fluoride salt was added, and the melt was pressed into the furnace using a bell jar. The reaction was then carried out for 25 minutes under the action of an electromagnetic field of 15 Hz and an ultrasonic field of 2 kW. After the reaction generated in-situ nanoparticles, the composite material melt was subjected to slag removal, degassing, refining, and refining treatment to obtain a composite material melt for later use. After the melt temperature dropped to 725℃, 0.6% Al-10Er-5Sr alloy was added to the melt and stirred uniformly, and then held at 730℃ for 20 minutes. The sand core and metal mold are then assembled onto the low-pressure casting machine, and the upper and lower molds are closed. After the mold is preheated at 200°C, molten aluminum-based composite material at 740°C is pressed into the metal mold from bottom to top. The distance between the liquid level in the riser pipe and the mold gate is controlled at 100mm, and the pressure is automatically compensated. The filling stage lasts for 20 seconds with a pressure of 25KPa, the pressurization stage lasts for 30 seconds with a pressure of 45KPa, the holding stage lasts for 100 seconds with a pressure of 45KPa, the depressurization stage lasts for 3 seconds, and the delayed cooling stage lasts for 150 seconds. During the solidification process, circulating cooling water is applied to the metal mold. The casting underwent high-frequency vibration to remove sand cores, cut off gating gates and runners, and X-ray non-destructive testing. After confirming the absence of defects, it underwent solution treatment and stepped aging treatment. The solution treatment involved heating from room temperature to 540℃ and holding for 3 hours, followed by quenching with the quenching medium water temperature maintained at 65℃. The aging treatment was a two-stage process: first, heating from room temperature to 130℃ and holding for 2 hours, then heating to 160℃ and holding for 2 hours, and finally performing fluorescent testing. In summary, a monolithic aluminum-based composite hollow subframe was obtained.

[0036] Example 3

[0037] Industrial-grade fluoride salts K2ZrF6 and KBF4 in a mass ratio of 1:1.15 were selected as reactants. They were placed in a forced-air drying oven and dried at 250℃ for 2 hours to remove the water of crystallization. After drying, they were mixed, cooled, ground, and wrapped in aluminum foil for later use. The reactants were then prepared with the following composition: Si: 6.5–7.5% wt.%, Mg: 0.25–0.45% wt.%, Ti: 0.08–0.2% wt.%, Zr: 0.1–0.2% wt.%, Fe: < An aluminum alloy with 0.2% wt.%, Zn < 0.1% wt.%, Cu < 0.1% wt.%, Mn < 0.1% wt.%, Sn < 0.01% wt.%, and Pb < 0.03% wt.% was melted in a crucible melting furnace. After the aluminum alloy melted and was heated to a reaction temperature of 855℃, a prepared fluoride salt was added, and the melt was pressed into the furnace using a bell jar. The reaction was then carried out for 30 minutes under the action of a 12Hz electromagnetic field and a 1.5kW ultrasonic field. After the reaction generated in-situ nanoparticles, the composite material melt was subjected to slag removal, degassing, refining, and refining treatment to obtain a composite material melt for later use. After the melt temperature dropped to 720℃, 0.6% Al-10Er-5Sr alloy was added to the melt and stirred uniformly, and then held at 725℃ for 20 minutes. The sand core and metal mold are then assembled onto the low-pressure casting machine, and the upper and lower molds are closed. After the mold is preheated at 200°C, molten aluminum-based composite material at 720°C is pressed into the metal mold from bottom to top. The distance between the liquid level in the riser pipe and the mold gate is controlled at 120mm. The pressure is automatically compensated. The filling stage lasts for 10 seconds with a pressure of 30KPa, the pressurization stage lasts for 20 seconds with a pressure of 40KPa, the holding stage lasts for 120 seconds with a pressure of 40KPa, the depressurization stage lasts for 5 seconds, and the delayed cooling stage lasts for 120 seconds. During the solidification process, circulating cooling water is applied to the metal mold. The casting underwent high-frequency vibration to remove sand cores, cut off gating gates and runners, and X-ray non-destructive testing. After confirming no defects, it underwent solution treatment and stepped aging treatment. The solution treatment involved heating from room temperature to 535℃ and holding for 3 hours, followed by quenching with the quenching medium water temperature maintained at 60℃. The aging treatment was a two-stage process: first heating from room temperature to 135℃ and holding for 2 hours, then heating to 170℃ and holding for 2 hours, finally followed by fluorescent testing. In summary, a monolithic aluminum-based composite hollow subframe was obtained.

[0038] Based on the above results, the integral aluminum-based composite hollow subframe prepared by this invention has good in-situ wettability and uniform distribution of nanoparticles with the matrix. At the same time, it can improve the strength, toughness and fatigue resistance of the integral aluminum-based composite hollow subframe. This invention can reduce the curb weight of automobiles, achieve high output power, and achieve high safety and reliability. It can also achieve low economic cost, energy saving, fuel saving and environmental protection.

Claims

1. A method for molding an integral low-pressure cast aluminum-based composite hollow subframe, characterized in that, Includes the following steps: (1) Inorganic salt preparation: Fluoride salts were selected as reactants, placed in a forced-air drying oven for heat preservation and drying to remove crystal water, and after drying, they were mixed, cooled, ground, and wrapped in aluminum foil for later use; The inorganic salts in step (1) were industrial grade fluoride salts K2ZrF6 and KBF4, which were a mixture with particle sizes greater than 200 mesh, and the mass ratio of K2ZrF6 to KBF4 was 0.75-1.15; (2) Preparation of aluminum-based composite material melt: The aluminum alloy is melted in a crucible melting furnace. After the aluminum alloy is melted and heated to the reaction temperature, the fluoride salt prepared in step (1) is added. The melt is pressed into the bell jar and stirred. Then it reacts under the action of electromagnetic field and ultrasonic field. After the reaction generates in-situ nanoparticles, the composite material melt is deslag-removed, degassed, refined and fined to obtain the composite material melt for use. The reaction temperature of aluminum alloy and fluoride salt is 820~870℃, the reaction time is 20min~30min, the electromagnetic field setting parameters are magnetic field frequency 10~15Hz, electromagnetic stirring time is 5-30min, and ultrasonic power is 1~2.5kW. (3) Rare earth element addition: Add rare earth erbium to process the melt in step (2); the rare earth erbium in step (3) is Al-10Er-5Sr alloy. After the temperature of the melt in step (2) drops to 700-725℃, add Al-10Er-5Sr alloy to the melt in step (2) and stir evenly. The amount added is 0.3~0.8% of the mass fraction. Then keep it at 720-740℃ for 15-30 minutes. (4) Low-pressure casting: First, the mold is assembled. After the mold is preheated, the molten aluminum-based composite material in step (3) is injected into the metal mold from bottom to top. During the solidification process, circulating cooling water is applied to the metal mold until the casting is completely solidified. (5) Energy-saving heat treatment: The castings in step (4) are subjected to high-frequency vibration to remove the sand core, cut off the gating and riser and the runner, and X-ray non-destructive testing. After the defects are removed, solution treatment and step-age treatment are carried out. In step (5), the solution treatment is carried out by raising the temperature from room temperature to 520~550°C and holding it for 3~5 hours. Then, quenching treatment is carried out, and the water temperature of the quenching medium is kept at 20~80°C. The aging treatment includes a two-stage aging process. First, the temperature is raised from room temperature to 120~140°C and held for 1.5~2.5 hours. Then, the temperature is raised to 145~170°C and held for 1.5~2.5 hours. Finally, fluorescent testing is carried out.

2. The method for forming an integral low-pressure cast aluminum-based composite hollow subframe according to claim 1, characterized in that, The inorganic salts in step (1) are industrial-grade fluoride salts K2ZrF6 and KBF4, which are a mixture with a purity greater than 99 wt.%. The drying oven is kept at a temperature of 150-300℃ and the drying time is 2-4 hours.

3. The method for forming an integral low-pressure cast aluminum-based composite hollow subframe according to claim 1, characterized in that, The aluminum alloy composition in step (2) is Si: 6.5~7.5%wt.%, Mg: 0.25~0.45%wt.%, Ti: 0.08~0.2%wt.%, Zr: 0.1~0.2%wt.%, Fe: <0.2%wt.%, Zn<0.1%wt.%, Cu: <0.1%wt.%, Mn: <0.1%wt.%, Sn: <0.01%wt.%, Pb: <0.03%wt.%.

4. The method for forming an integral low-pressure cast aluminum-based composite hollow subframe according to claim 1, characterized in that, In step (4), the mold is a combination of a sand core and a metal mold, which are assembled on a low-pressure casting machine and the upper and lower molds are closed; the pouring temperature of the molten aluminum-based composite material is controlled at 710~740℃.

5. The method for forming an integral low-pressure cast aluminum-based composite hollow subframe according to claim 4, characterized in that, The injection process includes six steps: 1) Pre-pressing treatment: Apply pressure to the molten aluminum-based composite material, control the distance between the liquid level in the riser tube and the mold gate, which is 50~150mm, and automatically compensate for the pressure. 2) Filling stage: Filling time is 10~30 seconds, pressure is 20~30 kPa; 3) Pressurization stage: Pressurization time is 10~90 seconds, and pressure is 30~70 kPa; 4) Pressure holding stage: The pressure holding time is 40~200 seconds, and the pressure is 30~70 kPa; 5) Depressurization stage: Depressurization time is 1~10 seconds; 6) Delayed cooling phase: The delayed cooling time is 50~300 seconds.

6. The method for forming an integral low-pressure cast aluminum-based composite hollow subframe according to claim 1, characterized in that, The application of circulating cooling water to the metal mold during solidification in step (4) includes two stages: the first stage is to cool the subframe body, and the second stage is to cool and harden the casting as a whole, including cooling of the gating gate and runner.

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