A gradient structure magnesium alloy material and a preparation method thereof

Abstract

The application relates to a gradient structure magnesium alloy material and a preparation method thereof, and relates to the technical field of magnesium alloy plastic processing. The aforementioned preparation method comprises the following steps: taking a magnesium alloy ingot as a blank; preheating the blank and performing pre-extrusion deformation to obtain an extrusion material; performing cold treatment on the extrusion material; and performing multi-directional forging on the cold-treated extrusion material to obtain a finished material. The preparation process is simple, the magnesium alloy material with a surface bimodal structure and a core twin crystal structure can be obtained through an improved conventional hot working process, large-scale industrial production is suitable, and the thickness of the surface bimodal structure of the prepared magnesium alloy material can reach 8-20% of the total thickness. The coordinated deformation between the gradient structures makes the comprehensive mechanical properties of the material excellent.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of plastic processing of magnesium alloy, and particularly relates to a magnesium alloy material with gradient structure and a preparation method thereof. BACKGROUND

[0002] The critical shear stress of pure magnesium single crystal is only 4.8-4.9 MPa, and the crystal structure determines the low strength and hardness, so that the pure magnesium single crystal must be strengthened before being applied to industrial structural materials. Researchers at home and abroad have carried out a large number of researches and explorations on the strengthening mechanism and strengthening method of magnesium alloy. Researches show that the strengthening methods such as solid solution strengthening, dispersion strengthening, fine-grain strengthening and deformation strengthening can improve the strength of the alloy by increasing the resistance of dislocation movement. However, the traditional strengthening methods will lead to a decrease in plasticity. For example, the super-fine grain structure in the micron or nanometer level obtained by the large plastic deformation process (SPD) can significantly improve the strength and hardness of the magnesium alloy, but the work hardening capacity and uniform elongation are relatively low, and many super-fine grain materials even reach their fracture stress in the elastic stage of the tensile deformation process. Therefore, if the strength-plasticity product is used as the standard for evaluating the comprehensive mechanical properties, the super-fine grain alloy is not superior to the conventional material, which seriously limits the application of the super-fine grain alloy as a structural material. SUMMARY

[0003] The present application aims to provide a preparation method of a magnesium alloy material with gradient structure, and the magnesium alloy material prepared by the method has excellent comprehensive mechanical properties.

[0004] In order to achieve the above-mentioned purpose, the present application adopts the following technical scheme: a preparation method of a magnesium alloy material with gradient structure, comprising the following steps:

[0005] (1) Preparation of blank: taking a magnesium alloy ingot as a blank;

[0006] (2) Pre-extrusion deformation: preheating the blank and then performing pre-extrusion deformation to obtain an extruded material;

[0007] (3) Cold treatment: cold treating the extruded material;

[0008] (4) Multi-directional forging: multi-directionally forging the cold-treated extruded material to obtain a finished material.

[0009] In step (2), the preheating temperature is 250-320 DEG C, and the preheating holding time is 10-30 min. Preferably, the preheating temperature is 250 DEG C, 280 DEG C, 300 DEG C or 320 DEG C, and the preheating holding time is 20 min.

[0010] In step (2), the extrusion temperature (same as the preheating and holding temperature) is 250-320℃, the extrusion ratio is 30-50, and the extrusion speed is 3-5 mm / s. Preferably, the extrusion temperature is 250℃, 280℃, 300℃, or 320℃; the extrusion ratio is 30, 40, or 50; and the extrusion speed is 5 mm / s.

[0011] In step (4), the cold-treated extruded material has a polyhedral structure, and when subjected to multi-directional forging, the number of passes is not less than the total number of faces of the extruded material (i.e., each face is forged at least once), and the deformation amount of each pass is not more than 5%.

[0012] Further, in step (4), the cold-treated extruded material has a cubic structure, and when subjected to multi-directional forging, the number of passes is 6-12 passes, and the deformation amount of each pass is 3%-5%. Preferably, the number of deformation passes is 6, 9, or 12, and the deformation amount of each pass is 3% or 5%.

[0013] In step (4), before the cold-treated extruded material is subjected to multi-directional forging, it is first machined into a cubic shape.

[0014] In step (4), the number of forging times is not less than 200 times / min, the forging speed is not less than 3 m / s, and the forging temperature is 245-255℃. Preferably, the number of forging times is 200-260 times / min, more preferably 200 times / min, 230 times / min, or 260 times / min; the forging speed is 3-8 m / s, more preferably 3 m / s, 5 m / s, or 8 m / s; and the forging temperature (hot working temperature) is 250±5℃, such as 245℃, 250℃, or 255℃.

[0015] In step (1), the magnesium alloy can be a Mg-Al-based alloy, such as a wrought magnesium alloy in the AZ series (Mg-Al-Zn), AM series (Mg-Al-Mn), AS series (Mg-Al-Si), and AE series (Mg-Al-Re).

[0016] In step (1), if the magnesium alloy ingot used as the blank is made by ordinary means, its structure may not be uniform enough. In this case, it can be subjected to homogenization annealing to improve the deformation performance of the blank. Of course, if the magnesium alloy ingot used as the blank is obtained by water-cooled copper mold casting, no heat treatment is required. The specific steps are as follows: after the alloying elements are proportioned, it is cast by water-cooled copper mold at a melting temperature of 700℃-750℃. After melting, it is rapidly cooled and poured with a cooling water flow rate of 3±0.5L / min. After the ingot cools, it is machined into a pre-extruded blank. This method can obtain a uniform equiaxed crystal structure, and the alloying elements do not have time to precipitate during rapid cooling, basically obtaining a single-phase solid solution. Therefore, no heat treatment is required before hot working of the blank.

[0017] In step (2), the purpose of pre-extruding the blank is to induce twinning through large deformation extrusion, thereby obtaining a high-density twinned structure.

[0018] In step (3), the extruded material can be rapidly cooled, including but not limited to water cooling and liquid nitrogen cryogenic treatment. The purpose of this step is mainly to avoid recovery and recrystallization.

[0019] In step (4), multi-directional forging can be small-deformation multi-directional forging, specifically: hot working is carried out on an air hammer, the number of forgings by the air hammer is not less than 200 times / min, the forging speed is not less than 3m / s, the hot working temperature is 250±5℃, one-time heating is used, that is, no heating is carried out between passes, and three-directional forging process can be used for forging. Each forging of one surface is counted as one pass, and the number of deformation passes is 6-12 passes. In order to ensure the smooth progress of hot working, the deformation amount of each pass generally does not exceed 5%. After the hot working is completed, the sample can be air-cooled to retain the high-temperature deformed structure. The purpose of this step is: in the process of small-deformation multi-directional forging, due to the limited deformation time and deformation amount, the deformation heat is difficult to transfer to the core, thereby inducing multiple rounds of dynamic recrystallization on the surface of material A (extruded material) to form a bimodal structure, while the core still retains the twinned structure. In this process, controlling the deformation temperature, forging frequency, and deformation amount is particularly important. When the deformation temperature is too low, the forging frequency too high, and the deformation amount too low, the recrystallization driving force is weak, often making it difficult to induce dynamic recrystallization, or only inducing grain boundary bowing nucleation, making it difficult to form a bimodal structure. Conversely, when the deformation temperature is too high, the forging frequency too slow, and the deformation amount too large, the deformation heat will be transferred to the core, causing dynamic recrystallization in the core structure, thus failing to maintain the twinned structure. Therefore, only with a suitable combination of deformation temperature, forging frequency, and deformation amount can large-angle grain boundary migration be induced in the surface structure to form a bimodal structure without affecting the twinned structure in the core.

[0020] The magnesium alloy material obtained by the above method exhibits a gradient structure with a bimodal surface and a twinned core. The twinned core facilitates coordinated deformation and improves the alloy's plasticity. The ultrafine grains in the bimodal structure provide high strength, while the coarse grains ensure sufficient uniform elongation. Simultaneously, the deformation coordination and strain distribution among the multi-scale grains in the gradient material promote work hardening and delay shear band initiation, thus resulting in superior dynamic shear properties. Therefore, the magnesium alloy material prepared by this invention achieves excellent comprehensive mechanical properties.

[0021] Another object of the present invention is to provide a gradient structure magnesium alloy material, which is prepared by the above-described method for preparing gradient structure magnesium alloy materials.

[0022] Among them, the above-mentioned gradient structure magnesium alloy material has a bimodal structure on the surface and a twinned structure in the core. The thickness of the bimodal structure is 8% to 20% of the total thickness of the material, and the thickness of the twinned structure is 80% to 92% of the total thickness of the material. The bimodal structure is distributed on each surface of the material.

[0023] The bimodal structure is composed of network grains and island grains with varying average grain sizes.

[0024] Furthermore, the average grain size of the network grains is greater than the average grain size of the island grains.

[0025] Preferably, the average grain size of the network grains is not less than 10 μm, and the average grain size of the island grains is not more than 1 μm.

[0026] More preferably, the average grain size of the network grains is 10 μm, and the average grain size of the island grains is 1 μm. That is, the surface bimodal structure is composed of coarse network grains with an average grain size of 10 μm and ultrafine island grains with an average grain size of 1 μm.

[0027] The preparation process of this invention is simple. Magnesium alloy materials with a bimodal surface structure and a twinned core structure can be obtained through an improved conventional hot working process. This is suitable for large-scale industrial production. Moreover, the magnesium alloy materials prepared by this invention have a bimodal surface structure thickness of 8-20% of the total thickness. The coordinated deformation between the gradient structures results in excellent comprehensive mechanical properties of the material. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the multi-directional forging process;

[0029] Figure 2 This is a schematic diagram of the pre-extrusion process;

[0030] Figure 3 Metallographic image of the surface bimodal structure obtained in Example 1;

[0031] Figure 4 Metallographic photograph of the heart twin structure obtained in Example 1;

[0032] Figure 5 This is a metallographic photograph of the surface ultrafine grain structure obtained in Comparative Example 1. Detailed Implementation

[0033] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention. It should be noted in advance that the following embodiments were completed in a laboratory setting. Those skilled in the art should understand that the amounts of each component given in the embodiments only represent the ratio between the components, and are not specific limitations.

[0034] Currently, magnesium alloys still have significant room for improvement in terms of comprehensive mechanical properties and service performance. This invention provides a feasible solution for optimizing the comprehensive mechanical properties and improving the service performance of magnesium alloys by introducing a gradient structure. Furthermore, introducing coarse grains into an ultrafine grain structure to form a bimodal grain structure can effectively improve the elongation properties of the material. Magnesium alloys with a gradient structure can effectively improve strength while maintaining good plasticity and toughness. However, existing technologies often obtain gradient materials through surface mechanical grinding, high-energy shot peening, and torsional deformation. These methods are complex, have limitations in processing depth, and generally result in gradient structures of coarse surface grains to fine core grains or vice versa. This invention improves conventional hot deformation processes to achieve precise control over the microstructure of magnesium alloys, thereby enabling the stable production of gradient magnesium alloys with excellent comprehensive mechanical properties and service performance. Example 1

[0035] Magnesium alloy materials with gradient structures were prepared by the following method.

[0036] AZ31 magnesium alloy was selected as the base material and formed by water-cooled copper mold casting. The mold forming size was 150×150×150mm, the mold cooling water capacity was 300±10ml, and the cooling water flow rate was 3±0.5L / min. After the ingot cooled, it was machined into a pre-extruded billet with a length, width and height of 120×120×120mm.

[0037] Pre-extrusion deformation: The pre-extrusion process can be referenced. Figure 2 The blank is placed in a heat-preheating furnace at 250℃ for 20 minutes. After heat preservation, it is placed in an extruder for pre-extrusion at a speed of 5 mm / s and an extrusion ratio of 30 to obtain material A.

[0038] (3) Cold treatment: Material A is rapidly cooled with water.

[0039] (4) Small deformation multi-directional forging: After the cold-treated material A is machined into an 80×80×80mm cube, small deformation multi-directional forging is performed (using a three-directional forging process, such as...). Figure 1 As shown, the forging surfaces are forged in the order ABCA… to obtain finished material B. Hot forming is performed on an air hammer at a forging speed of 3 m / s and a forging frequency of 200 times / min. The hot forming temperature is 245℃, using a single-pass heating method (no heating between passes). Each forged surface is counted as one pass, with a deformation of 3% per pass, for a total of 12 passes. After hot forming, the sample is air-cooled to retain the high-temperature deformed structure, thus obtaining material B. Material B has a gradient structure with a bimodal surface and a twinned core, as shown… Figure 3 and Figure 4 As shown, the thickness of the surface bimodal structure is approximately 20 mm. Example 2

[0040] The difference between this embodiment and embodiment 1 is that the process parameters of steps (2) and (4) are different. In this embodiment, step (2) is: the billet is placed in a heat-preheating furnace at a temperature of 300°C and a holding time of 20 minutes. After heat preservation, it is placed in an extruder for pre-extrusion at a speed of 5 mm / s and an extrusion ratio of 40 to obtain material A. Step (4) is: after the cold-treated material A is processed into a 70×70×70mm cube, it is subjected to small deformation multi-directional forging (using a three-directional forging process, such as...). Figure 1 As shown, the forging surfaces are forged in the order ABCA… to obtain finished material B. Hot forming is performed on an air hammer at a forging speed of 5 m / s and a forging frequency of 230 times / min. The hot forming temperature is 250℃, using a single-pass heating method (no heating between passes). Each forged surface is counted as one pass, with a deformation of 5% per pass, for a total of 9 passes. After hot forming, the sample is air-cooled to retain the high-temperature deformed microstructure, resulting in material B. Material B has a gradient structure with a bimodal surface microstructure and a twinned core microstructure, with the bimodal surface microstructure being approximately 15 mm thick. Example 3

[0041] The difference between this embodiment and Embodiments 1 and 2 is that the process parameters of steps (2) and (4) are different. In this embodiment, step (2) is: the billet is placed in a heat-preheating furnace at a temperature of 320°C for 20 minutes. After heat preservation, it is placed in an extruder for pre-extrusion at a speed of 5 mm / s and an extrusion ratio of 50 to obtain material A. Step (4) is: after the cold-treated material A is processed into a 50×50×50mm cube, it is subjected to small deformation multi-directional forging (using a three-directional forging process, such as...). Figure 1 As shown, the forging surfaces are forged in the order ABCA… to obtain finished material B. Hot forming is performed on an air hammer at a forging speed of 8 m / s and a forging frequency of 260 times / min. The hot forming temperature is 255℃, using a single-pass heating method (no heating between passes). Each forged surface is counted as one pass, with a deformation of 5% per pass, for a total of 6 passes. After hot forming, the sample is air-cooled to retain the high-temperature deformed microstructure, resulting in material B. Material B has a gradient structure with a bimodal surface microstructure and a twinned core microstructure, with the bimodal surface microstructure being approximately 8 mm thick.

[0042] The microstructure of material B, obtained in Example 1, was observed. Figure 3 It is evident that its surface layer consists of a bimodal structure composed of coarse network grains and ultrafine island grains. Figure 4 As can be seen, its core exhibits a high-density twinned structure. These characteristics were also observed in the materials ultimately obtained in Examples 2 and 3.

[0043] The difference between this comparative example and Example 1 is that step (4) is different. In this comparative example, the deformation amount per pass in step (4) is 3%, and the total number of deformation passes is 3. The microstructure of material B obtained in Comparative Example 1 was observed, as follows: Figure 5 As shown, its surface layer has an ultrafine-grained structure. This is because the total deformation in step (4) is too small, the deformation time is limited, the initial dynamic recrystallization grains do not have enough time to grow, and secondary recrystallization hardly occurs. The final microstructure of material B is an ultrafine-grained structure on the surface, as shown in the figure. Figure 5 As shown; the core is a twinned structure.

[0044] The difference between this comparative example and Example 2 is that step (4) is different. In this comparative example, the deformation amount per pass in step (4) is 5%, and the total number of deformation passes is 24. The final microstructure of material B obtained in Comparative Example 2 is a bimodal microstructure with both the surface and the core. This is because the deformation amount in step (4) is too large, and the thermal deformation effect is transferred to the core, inducing multiple rounds of dynamic recrystallization.

[0045] The difference between this comparative example and Example 3 is that step (4) is different. In this comparative example, the heat treatment temperature in step (4) is 300°C. The final microstructure of material B obtained in Comparative Example 3 is a bimodal microstructure on the surface and a fine-grained microstructure in the core. This is because the deformation temperature in step (4) is too high, and the heat deformation effect on the surface is transferred to the core, inducing dynamic recrystallization. However, the deformation amount is small, and the recrystallization in the core does not have enough time to grow.

[0046] Mechanical properties were tested on the materials obtained in Examples 1-3 and Comparative Examples 1-3. The results are shown in Table 1. It can be seen that the materials obtained in Examples 1-3, which have a bimodal surface structure and a twinned core structure, have better comprehensive mechanical properties than the materials obtained in Comparative Examples 1-3.

[0047] Table 1 Comparison of Mechanical Properties

[0048] Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Uniform elongation 23.2% 32.7% 30.1% 21.4% 30.1% 28% Toughness at break 28.5 MPa m 1,2 ]] 29.8 MPa m 1,2 ]] 27.3 MPa m 1,2 ]] 23.9 MPa m 1,2 ]]> 14.9 MPa m 1,2 ]] 24.6 MPa m 1,2 ]]>

[0049] The above embodiments and comparative examples used the most common AZ31 wrought magnesium alloy, and the specific element ratios are common knowledge in the art and will not be repeated here. It should be noted that the present invention has also conducted experiments on other wrought magnesium alloys, including the AZ series (Mg-Al-Zn), AM series (Mg-Al-Mn), AS series (Mg-Al-Si), and AE series (Mg-Al-Re). The experimental results show that the method described in the present invention can also obtain a gradient structure magnesium alloy with a surface bimodal structure and a core twinned structure, and it has excellent mechanical properties.

[0050] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

[0051] Finally, it should be emphasized that, in order to make it easier for those skilled in the art to understand the improvements of the present invention compared with the prior art, some descriptions of the present invention have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements can also constitute the content of the present invention.

Claims

1. A method for preparing a gradient structure magnesium alloy material, characterized in that, Includes the following steps: (1) Raw material preparation: Magnesium alloy ingots are used as raw materials; (2) Pre-extrusion deformation: The billet is preheated and pre-extruded to obtain extruded material; (3) Cold treatment: The extruded material is subjected to cold treatment; (4) Multi-directional forging: The cold-treated extruded material is forged in multiple directions to obtain the finished material; In step (1), the magnesium alloy is a Mg-Al alloy; In step (4), the extruded material after cold treatment has a polyhedral structure. When it is forged in multiple directions, each forging of one face is counted as one pass. The number of passes is not less than the total number of faces of the extruded material, and the deformation of each pass does not exceed 5%. In step (4), the number of forgings is not less than 200 times / min, the forging speed is not less than 3m / s, and the forging temperature is 245~255℃; In step (4), the obtained finished material has a gradient structure with a surface bimodal structure and a core twin structure; the surface bimodal structure is composed of a network of coarse grains with an average grain size of not less than 10 μm and island-shaped ultrafine grains with an average grain size of not more than 1 μm; the thickness of the surface bimodal structure is 8% to 20% of the total thickness of the material, and the thickness of the core twin structure is 80% to 92% of the total thickness of the material.

2. The method for preparing gradient structure magnesium alloy material according to claim 1, characterized in that: In step (2), the extrusion temperature is 250-320℃, the extrusion ratio is 30-50, and the extrusion speed is 3-5mm / s.

3. The method for preparing gradient structure magnesium alloy material according to claim 1, characterized in that: In step (4), the extruded material after cold treatment has a cubic structure. When it is forged in multiple directions, the number of passes is 6 to 12, and the deformation amount of each pass is 3% to 5%.

4. A gradient structure magnesium alloy material, characterized in that: The material is prepared by the method for preparing gradient structure magnesium alloy material according to any one of claims 1-3.

5. The gradient structure magnesium alloy material according to claim 4, characterized in that: The bimodal structure is distributed on each facet of the material.

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