Lightweight high-strength heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles

Abstract

The invention relates to a lightweight high-strength heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles and a preparation method thereof, a proper amount of rare earth elements Nd and Sm are added into an Mg-Al alloy, and the magnesium alloy comprises the following chemical components in percentage by mass: 87-95.5% of Mg, 3-7% of Al, 1-4% of Nd and 0.5-2% of Sm. By adopting an alloying design thought and a preparation mode of gravity casting and hot rolling under the pulsed magnetic field, the mechanical property of the alloy can be remarkably improved, and the magnesium alloy with excellent room-temperature and high-temperature mechanical properties is obtained, so that the magnesium alloy is expected to be widely applied to preparation of structural parts for automobiles.

Description

Technical Field

[0001] The present invention belongs to the field of design and preparation of magnesium alloys for automobiles, and specifically relates to a lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles and a preparation method thereof. Background Art

[0002] There are two main advantages of magnesium alloys in the application of automotive parts: on the one hand, it has a good vibration reduction and noise reduction effect, thereby improving the safety and comfort of the car; on the other hand, it plays a role in lightweighting the car, thereby saving energy consumption, reducing CO2 emissions, and improving the performance of the car. At present, the magnesium alloys used in automobiles are mainly Mg-Al series magnesium alloys, which are mainly used in some shells (instrument panels, clutch housings, engine front covers) and brackets (brake brackets, mirror brackets, steering wheels, etc.) parts. With the continuous development of automobiles, the development of new products and the upgrading of existing products have put forward higher requirements for the room temperature and high temperature mechanical properties of materials. Therefore, it is necessary to find an effective strengthening and toughening method to improve the organization and mechanical properties of Mg-Al series magnesium alloys, break their application bottlenecks, and make them more and more widely used in automobiles. Summary of the invention

[0003] Studies have shown that rare earth elements are the most effective elements in improving the performance of magnesium alloys. They mainly play the role of deoxidation and dehydrogenation, improving creep resistance, refining grains, and improving alloy strength. Compared with magnesium alloys without rare earth elements, magnesium alloys containing a small amount of one or several rare earth elements have better mechanical properties, and the improvement of high-temperature performance is particularly obvious. Among the rare earth elements, neodymium (Nd) has a higher limit solid solubility in magnesium, which can produce a better aging strengthening effect, and it is the most effective in improving the high-temperature strength of magnesium alloys, far exceeding elements such as Ce, La and Pr; Sm and Nd are both light rare earth elements of the cerium group, and the limit solid solubility of Sm in the magnesium matrix is ​​higher than that of Nd, so the precipitation strengthening effect brought by Sm is better than that of Nd. At the same time, the Mg5 (Nd, Sm) second phase formed by Nd, Sm and Mg can also play a role in refining grains and strengthening the second phase, thereby improving mechanical properties.

[0004] In recent years, applying pulsed magnetic field during metal solidification process has been well developed as a method to effectively control the solidification structure and performance of materials. Pulsed magnetic field treatment has the advantages of no pollution, easy operation and significant effect. The main mechanism of action of pulsed magnetic field on alloy solidification process is to improve its comprehensive mechanical properties by refining the metal solidification structure. Many studies have shown that pulsed magnetic field has the effect of refining the structure of pure metals, low melting point alloys (aluminum alloys, copper alloys, magnesium alloys, etc.), composite materials, etc. Therefore, with regard to the preparation of high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy in the present invention, it is an effective method to improve its mechanical properties by applying pulsed magnetic field during solidification process.

[0005] In addition, for cast magnesium alloys, hot deformation methods are often required to further improve the mechanical properties of the alloy. Studies have shown that after hot deformation of cast magnesium alloys, the recrystallized grain size of the material is significantly finer than the cast grain size, the elongation of the deformed magnesium alloy can reach about 12 to 30%, and the strength is also improved to a certain extent. Therefore, it is necessary to perform hot rolling on the Mg-Al-Nd-Sm magnesium alloy proposed in the present invention to further improve its mechanical properties.

[0006] In summary, the object of the present invention is to provide a lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles and a preparation method thereof; A lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles, wherein the chemical components of the alloy are as follows: Mg: 87-95.5%, Al: 3-7%, Nd: 1-4% and Sm: 0.5-2% by weight.

[0007] Preferably, the mass percentages of the chemical components of the alloy are: Mg: 89.5-93%, Al: 4-6%, Nd: 2-3%, Sm: 1-1.5%.

[0008] The preparation method of the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles comprises gravity casting + hot rolling under a pulsed magnetic field.

[0009] The first step is smelting. The raw materials are heated in sequence in the range of 720-780°C until they are melted. The order of adding the raw materials is pure Mg, Mg-30Nd master alloy, Mg-30Sm master alloy, and pure Al. After melting, they need to be stirred continuously and kept warm for 10 minutes to fully alloy the elements.

[0010] The second step is pouring. Before pouring, stir and remove the slag from the melt. When the temperature cools to 720°C, pour the melt into a graphite mold preheated to 200°C. The pulse magnetic field treatment experiment requires the graphite mold to be placed in a coil for pouring. The specific process is as follows: connect the coil to the pre-prepared magnetic field solidification device, take out the preheated graphite mold from the furnace and place it in the magnetic field coil. After the molten metal is poured into the mold, start the magnetic field immediately. Experiments are carried out under different magnetic field parameters. After the molten metal in the graphite mold solidifies, turn off the device, take out the graphite mold, and take it out after the ingot cools down. The entire smelting and pouring process is carried out under the protection of a mixed gas with a volume fraction of 99.5% CO2+0.5% SF6.

[0011] Furthermore, the pulse magnetic field coil consists of 3 layers of 6×2mm 2 The flat sandbag wire is wound with 60 turns and a total length of 100 mm, and is wound on a stainless steel cylinder, which is wrapped with asbestos cloth and has an inner diameter of 60 mm. When preparing Mg-Al-RE rare earth magnesium alloy, the voltage range is 100-500V and the pulse frequency range is 1-10Hz.

[0012] The preferred process parameters of the pulse magnetic field treatment are: voltage range is 300-400V, and pulse frequency range is 2.5-5Hz.

[0013] The preferred hot rolling process parameters are: temperature 350-400°C, roll speed 10 r / min, and the rolling deformation of each pass controlled at 5%~10%.

[0014] The characteristics of the present invention are: 1) The present invention adopts alloying design ideas, solidification technology under a pulsed magnetic field and hot rolling to prepare a lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy material for automobiles. This method has a significant effect on improving the mechanical properties of the alloy.

[0015] 2) The present invention selects Nb and Sm as alloying elements, and these two elements can play a significant second phase strengthening effect.

[0016] 3) The casting method of the present invention adopts the casting technology under pulse magnetic field. The pulse magnetic field treatment has the advantages of no pollution, easy operation and significant effect. It can refine the metal solidification structure and significantly improve the comprehensive mechanical properties of the alloy. Implementation

[0017] The following is a further description of the embodiments of the present invention in conjunction with specific examples: The following is the preparation method of lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy material for automobiles: Step 1: Prepare raw materials: Use a cutting machine to cut an appropriate amount of pure magnesium, Mg-30Nd, Mg-30Sm master alloy, and pure aluminum according to the alloy composition, then use a grinder to remove impurities such as oxide scale on the surface, and then place it in an electric heated blast drying oven at 180°C to dry.

[0018] Step 2: Treat the crucible: After cleaning the 304 stainless steel crucible, evenly apply refractory coating (talcum powder: water: water glass = 3:2.5:2) on the inner surface of the crucible 4 times, with an interval of 30 minutes each time, to prevent the crucible from reacting with the metal and ensure the purity of the molten metal. Finally, put the crucible into the above-mentioned drying oven for drying.

[0019] Step 3: Melting: Place the treated stainless steel crucible in a resistance furnace for heating, and put the raw materials into a 750°C crucible in the order of pure magnesium, Mg-30Nd, Mg-30Sm master alloy, and pure aluminum. After melting, stir continuously and keep warm for 10 minutes to fully alloy the elements. The fourth step is: pouring: before pouring, stir and slag the melt, and when its temperature cools to 720℃, pour the melt into the graphite mold preheated to 200℃. The pulse magnetic field treatment experiment requires the graphite mold to be placed in the coil. The specific process is as follows: connect the coil to the magnetic field solidification device prepared in advance, increase the magnetic field voltage after powering on, and feel the coil vibrate significantly, indicating that the device is normal, and then return the voltage to zero. Take out the preheated graphite mold from the furnace and place it in the magnetic field coil. After the molten metal is poured into the mold, start the magnetic field immediately. Experiments are carried out under different magnetic field parameters, with a pulse voltage range of 100-500V and a pulse frequency range of 1-10Hz. After the molten metal in the graphite mold solidifies, turn off the device, take out the graphite mold, and take it out after the ingot cools down. The entire smelting and pouring process is carried out under the protection of a mixed gas with a volume fraction of 99.5% CO2+0.5% SF6. During the pouring process, since the pouring temperature of the alloy melt is high, in order to prevent accidents caused by splashing, the following measures should be taken to protect the pouring process: ① When pouring, the crucible mouth should be as close to the mold as possible, and the molten liquid should be poured into the mold slowly to avoid the danger caused by splashing of the molten liquid. The pouring process cannot be interrupted or discontinued; ② The tools in the pouring process need to be preheated first, then painted, and immediately placed in the furnace after use to continue preheating and drying to prevent moisture on the tools from entering the molten liquid and causing dangers such as solution explosion; ③ Wear lab coats, protective masks and gloves.

[0020] Step 5: Hot rolling, the process parameters are as follows: before rolling deformation, the alloy is first subjected to homogenization treatment at 400℃ for 8 hours, the rolling temperature is 350-400℃, the roller speed is 10 r / min, and the rolling deformation of each pass is controlled at 5%~10%. Before each rolling, the alloy is kept at the rolling temperature for 30 minutes to fully preheat it; the initial thickness of the alloy is 10mm, the final thickness is 2mm, the rolling deformation of each pass is controlled at 5%~10%, and each rolling pass is placed in a heating furnace for 5 minutes. After the last rolling, the sample is placed in the air to cool naturally to obtain a rolled plate.

[0021] Examples 1 to 8 are the alloy composition and magnetic field treatment process parameters of a lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy material for automobiles, see Table 1.

[0022] The specific data of the alloy composition, magnetic field treatment process parameters and hot rolling treatment methods of Comparative Examples 1 to 10 are shown in Table 1.

[0023] Table 1 Main alloy components, pulse magnetic field treatment process and hot rolling process of magnesium alloys in Examples and Comparative Examples

[0024] 1. Hardness test The hardness of the alloy was evaluated by Vickers microhardness tester (Hv). The hardness test was performed by loading 1 kgf for 15 s on the surface of the metallographically polished sample. Each group of alloy sample surfaces was tested 10 times, and the arithmetic average of the multiple measurement results was taken. The relevant results are shown in Table 2.

[0025] 2. Room temperature tensile mechanical properties test The tensile test was carried out on a MTS810 tensile testing machine with a tensile speed of 0.5 mm / min and a test temperature of room temperature. The tensile samples were prepared according to the national standard GB / T228.1-2010. The relevant results are shown in Table 2.

[0026] Table 2 Mechanical properties of magnesium alloys

[0027] From the mechanical property test results in Table 2, it can be seen that the alloy element content is low (Comparative Example 1) or high (Comparative Examples 2, 3, 4), the magnetic field treatment voltage is low (Comparative Example 5) or high (Comparative Example 6), and the magnetic field treatment frequency is low (Comparative Example 7) or high (Comparative Example 8). The yield strength, tensile strength, elongation and hardness of the alloy are generally low. It can be obtained that the samples within the preferred composition range and the preferred magnetic field treatment parameters (Examples 1 to 8) have better mechanical property test results and can better meet the use requirements of automotive parts.

[0028] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A lightweight, high-strength, heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles, characterized in that: The chemical composition mass percentages of the alloy are: Mg: 87~95.5%, Al: 3~7%, Nd: 1~4%, Sm: 0.5~2%.

2. The lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to claim 1, characterized in that: The optimized chemical composition of the alloy is as follows: Mg: 89.5~93%, Al: 4~6%, Nd: 2~3%, Sm: 1~1.5%.

3. The method for preparing the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to any one of claims 1 to 2, characterized in that: The preparation process includes gravity casting + hot rolling under a pulsed magnetic field.

4. The method for preparing the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to claim 3, characterized in that: The gravity casting process under pulsed magnetic field is as follows: The first step is smelting. The raw materials are heated to melt in the range of 720~780℃ in sequence. The order of adding raw materials is pure Mg, Mg-30Nd master alloy, Mg-30Sm master alloy, and pure Al. After melting, they need to be stirred continuously and kept warm for 10 minutes to fully alloy the elements. The second step is pouring. Before pouring, the melt is stirred and slag removed. When its temperature cools to 720℃, the melt is poured into a graphite mold preheated to 200℃. The pulse magnetic field treatment experiment requires that the graphite mold be placed in a coil for pouring. The specific process is as follows: connect the coil to the magnetic field solidification device prepared in advance, take out the preheated graphite mold from the furnace and place it in the magnetic field coil, start the magnetic field immediately after the molten metal is poured into the mold, and conduct experiments under different magnetic field parameters. After the molten metal in the graphite mold solidifies, close the device, take out the graphite mold, and take it out after the ingot cools down. The entire smelting and pouring process is carried out under the protection of a mixed gas with a volume fraction of 99.5% CO2+0.5% SF6.

5. The method for preparing the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to claim 4, characterized in that: The pulse magnetic field coil consists of 3 layers of 6×2mm 2 The flat sandbag wire is wound with 60 turns and a total length of 100 mm, and is wound on a stainless steel cylinder, which is wrapped with asbestos cloth and has an inner diameter of 60 mm. When preparing Mg-Al-RE rare earth magnesium alloy, the voltage range is 100-500V and the pulse frequency range is 1-10Hz.

6. The method for preparing the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to claim 4, characterized in that: The optimized pulse magnetic field treatment process parameters are: voltage range is 300-400V, pulse frequency range is 2.5-5Hz.

7. The method for preparing the lightweight, high-strength and heat-resistant Mg-Al-Nb-Sm magnesium alloy for automobiles according to claim 4, characterized in that: The hot rolling process parameters are: temperature 350-400℃, roll speed 10 r / min, and the rolling deformation of each pass is controlled at 5%~10%.

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