High-toughness magnesium alloy and preparation method thereof

Abstract

The invention discloses a high-toughness magnesium alloy and a preparation method thereof, and belongs to the field of metal materials. The magnesium alloy comprises the following components in percentage by mass: 6.0 to 7.5 percent of Gd, 2.0 to 3.0 percent of Sm, 0.1 to 0.4 percent of Zr and 0.1 to 0.2 percent of Ho. The balance is magnesium and inevitable impurities, and the inevitable impurities are less than or equal to 0.05 wt%. The preparation process of the alloy comprises the steps of smelting, casting, extruding, pre-straining and aging treatment. In the prior art, the interior of the magnesium alloy is mainly composed of equiaxed grain structures, the obdurability of the material is changed by regulating and controlling the sizes of equiaxed grains, and the magnesium alloy obtained through the method is composed of a layered heterogeneous structure composed of deformed grains and fine and uniform recrystallized grains and precipitated phases evenly dispersed in the deformed grain area. The area fraction of recrystallized grains is greater than or equal to 70%, the average size of the recrystallized grains is less than or equal to 1.2 microns, and the size of a precipitated phase in deformed grains is less than or equal to 50nm; and the yield strength of the alloy is larger than or equal to 390 MPa, and the ductility of the alloy is larger than or equal to 8.5%.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing, and specifically relates to a high-strength and high-toughness magnesium alloy and its preparation method. Background Technology

[0002] Magnesium alloys, as excellent lightweight structural materials, are widely used in the automotive industry, electronics (3C products), aerospace, and medical devices due to their low density, high specific strength, excellent damping and vibration reduction properties, and good biocompatibility. Meanwhile, my country has abundant magnesium resources, and the preparation and application of magnesium alloys align with the national green and low-carbon development concept. Therefore, in the past two decades, research related to magnesium alloys has received widespread attention from both academia and industry.

[0003] However, traditional magnesium alloys still have certain limitations in terms of strength and toughness: the room temperature absolute strength of conventional magnesium alloys is relatively low. For example, the yield strength of AZ31B alloy (rolled / extruded state) at room temperature is about 250-290 MPa, and the elongation is about 5-7% (and the anisotropy is significant, with the elongation in the transverse state often being less than 6%). For the heat-treatable ZK60 alloy, after conventional extrusion and T5 aging, its yield strength is often less than 300 MPa, and the elongation is only about 5%. Under the same conditions, the yield strength of AZ80 alloy is less than 300 MPa and the elongation is ~6%, meaning that it is difficult to improve strength and toughness simultaneously.

[0004] More importantly, existing technologies, in order to overcome the technical bottleneck of simultaneously improving strength and toughness, typically employ complex deformation and thermomechanical treatment processes. However, these methods generally suffer from a significant drawback: while increasing strength, they severely sacrifice the material's plasticity, or vice versa. For example, multi-pass equal diameter angular extrusion (ECAP) treatment of AZ31 alloy can increase its yield strength several times, but its elongation drops sharply to below 5%, exhibiting obvious brittle characteristics. Applying multi-cycle rolling-intermediate annealing treatment to ZK60 alloy can achieve a high yield strength of 350 MPa, but its elongation is only ~6%. These complex processes are not only energy-intensive and time-consuming, but also require demanding equipment, making it difficult to achieve simultaneous improvement in alloy strength and toughness for industrial production.

[0005] Therefore, how to achieve the industrialized production of high-strength and high-toughness magnesium alloys while simplifying the process and reducing energy consumption has become a pressing technical challenge for the development of magnesium alloy materials. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a high-strength and high-toughness magnesium alloy and its preparation method. The alloy composition, by mass percentage, consists of 6.0-7.5 wt.% Gd, 2.0-3.0 wt.% Sm, 0.1-0.4 wt.% Zr, and 0.1-0.2 wt.% Ho, with the remainder being magnesium and unavoidable impurities, the unavoidable impurities being ≤0.05 wt.%.

[0007] The method for preparing the high-strength and high-toughness magnesium alloy includes the following steps:

[0008] (1) Under the protection of a CO2 and SF6 mixed gas with a volume ratio of 97-99:3-1, pure magnesium, magnesium gadolinium master alloy, magnesium samarium master alloy, magnesium holmium master alloy, and magnesium zirconium master alloy are heated and melted at 710-730℃. After complete melting, the mixture is stirred and slag is removed for 5-20 minutes. Argon gas is introduced and the mixture is kept at a constant temperature for 15-30 minutes to obtain the alloy melt. The alloy melt is then injected into a mold preheated to 150-250℃ using gravity casting to obtain a magnesium alloy ingot.

[0009] (2) The magnesium alloy ingot obtained in step (1) is subjected to solution treatment at 505-515℃ for 6.5-7.5h, and after being rolled into a sheet, a magnesium alloy billet is obtained. Then, it is subjected to extrusion treatment, grinding, tensile pre-strain treatment and aging treatment along the extrusion direction at room temperature to obtain a high-strength and high-toughness magnesium alloy.

[0010] The extrusion process is as follows: the magnesium alloy ingot and the extrusion die are preheated at 320-400℃ for 0.5-3 hours, and then extruded at 320-400℃ with an extrusion speed of 0.55-0.95 mm / s and an extrusion ratio of 5.5-8.5:1.

[0011] The tensile pre-strain treatment along the extrusion direction is as follows: the deformation is 2-5%, and the tensile pre-strain rate is 0.4-0.8 mm / min;

[0012] The aging treatment is as follows: heat preservation at 190-220℃ for 8-12 hours;

[0013] The high-strength and high-toughness magnesium alloy has a layered heterogeneous structure composed of deformed grains and fine, uniform recrystallized grains, as well as precipitates uniformly dispersed within the deformed grain regions. The recrystallized grains account for ≥70% of the area, the average size of the recrystallized grains is ≤1.2μm, the size of the precipitates in the deformed grains is ≤50nm, the alloy yield strength is ≥390MPa, and the elongation is ≥8.5%.

[0014] Furthermore, the alloy composition, by mass percentage, consists of 6.5-7.2 wt.% Gd, 2.3-2.8 wt.% Sm, 0.2-0.35 wt.% Zr, 0.12-0.15 wt.% Ho, with the remainder being magnesium and unavoidable impurities, the unavoidable impurities being ≤0.03 wt.%.

[0015] Further, the extrusion process described in step (2) involves preheating the magnesium alloy ingot and the extrusion die at 325-395℃ for 1-2 hours, and then extruding at 325-395℃ with an extrusion speed of 0.6-0.9 mm / s and an extrusion ratio of 6-8:1.

[0016] Further, in step (2), a tensile pre-strain treatment is performed along the extrusion direction: the deformation is 3-4%, and the tensile pre-strain rate is 0.5-0.7 mm / min.

[0017] Further, the aging treatment described in step (2) involves keeping the temperature at 200-210℃ for 9-11 hours.

[0018] Compared with existing technologies that suffer from complex processes, high energy consumption, and poor strength and toughness, this invention simplifies the process, reduces energy consumption, and simultaneously improves the strength and toughness of the alloy by selecting the best alloying elements, precisely optimizing the composition ratio, and synergistically controlling the process and process parameters. Its main advantages include:

[0019] (1) This invention achieves a short process, a simple process flow, reduced energy consumption, and has excellent prospects for industrial production. Compared with the complex processes in the prior art that rely on multiple passes of "ECAP" and "cyclic rolling-annealing" which are energy-intensive, have long cycles, and require demanding equipment, the process route provided by this invention is greatly simplified, significantly reducing production energy consumption, time costs, and dependence on special equipment. It avoids the structural damage and performance fluctuations that are easily generated in complex processes, and is easier to achieve large-scale and stable industrial production. At the same time, this invention obtains an alloy with higher strength and toughness than the prior art, thus improving the alloy's strength and toughness simultaneously.

[0020] (2) The alloy obtained by the present invention has a strong and toughened microstructure that combines fine grains, nano-precipitations and heterogeneous structure: The present invention is composed of a layered heterogeneous structure consisting of deformed grains and fine and uniform recrystallized grains, as well as precipitates uniformly dispersed in the deformed grain region. The area fraction of recrystallized grains is ≥70%, the average size of recrystallized grains is ≤1.2μm, the size of precipitates in deformed grains is ≤50nm, the alloy yield strength is ≥390MPa, and the elongation is ≥8.5%. The layered heterogeneous microstructure obtained by the present invention enables multiple mechanisms such as fine grain strengthening, dislocation strengthening and precipitation strengthening to work synergistically. In contrast, most magnesium alloys in the prior art are composed of equiaxed grains and do not form this layered heterogeneous structure. Moreover, they are strengthened only through a single pathway, and their strengthening ability is weaker than that of the present invention.

[0021] (3) The present invention successfully achieves simultaneous improvement of strength and toughness: If the yield strength of magnesium alloy obtained by the prior art is to be increased to ~300MPa, in addition to complex deformation process, high temperature and long time heat treatment are required. Even so, the elongation will be reduced to ~5%. However, the yield strength of magnesium alloy obtained by the present invention is higher than that of alloy obtained by the prior art, reaching ≥390MPa, and at the same time, the elongation is significantly improved, with an elongation of ≥8.5%. Detailed Implementation

[0022] Example 1

[0023] The high-strength and high-toughness alloy, by mass percentage, comprises the following magnesium alloy components: Gd: 6.5 wt.%, Sm: 2.3 wt.%, Zr: 0.2 wt.%, Ho: 0.12 wt.%, with the remainder being magnesium and unavoidable impurities, the unavoidable impurity content being 0.02 wt.%; its preparation method includes the following steps:

[0024] (1) Under the protection of a mixture of CO2 and SF6 with a volume ratio of 97:3, pure magnesium, gadolinium magnesium master alloy, samarium magnesium master alloy, holmium magnesium master alloy, and zirconium magnesium master alloy are heated and melted at 710°C. After complete melting, the mixture is stirred and slag is removed for 10 minutes. Argon gas is introduced and the mixture is kept at a constant temperature for 15 minutes to obtain the alloy melt. The alloy melt is then injected into a mold preheated to 150°C using gravity casting to obtain a magnesium alloy ingot.

[0025] (2) The magnesium alloy ingot obtained in step (1) is subjected to solution treatment at 505℃ for 6.5h, and after being machined, a magnesium alloy billet is obtained. Then, it is subjected to extrusion treatment, grinding, tensile pre-strain treatment and aging treatment along the extrusion direction at room temperature to obtain a high-strength and high-toughness magnesium alloy.

[0026] The extrusion process is as follows: the magnesium alloy ingot and the extrusion die are preheated at 330°C for 1 hour, and then extruded at 330°C with an extrusion speed of 0.65 mm / s and an extrusion ratio of 6.5:1.

[0027] The tensile pre-strain treatment along the extrusion direction is as follows: the deformation is 3% and the tensile pre-strain rate is 0.5 mm / min;

[0028] The aging treatment is as follows: heat preservation at 200℃ for 9 hours;

[0029] The obtained high-strength and high-toughness magnesium alloy has a yield strength of 434 MPa and an elongation of 8.5% at room temperature. It has a layered heterogeneous structure, with recrystallized grains accounting for 70% of the area, an average recrystallized grain size of 0.98 μm, and an average precipitated phase size of 45 nm in the deformed grains.

[0030] Example 2

[0031] The high-strength and high-toughness alloy, by mass percentage, comprises the following magnesium alloy components: Gd: 7.0 wt.%, Sm: 2.5 wt.%, Zr: 0.3 wt.%, Ho: 0.13 wt.%, with the remainder being magnesium and unavoidable impurities, the unavoidable impurity content being 0.02 wt.%. The preparation method includes the following steps:

[0032] (1) Under the protection of a mixture of CO2 and SF6 with a volume ratio of 98:2, pure magnesium, magnesium gadolinium master alloy, magnesium samarium master alloy, magnesium holmium master alloy, and magnesium zirconium master alloy are heated and melted at 720°C. After complete melting, the mixture is stirred and slag is removed for 15 minutes. Argon gas is introduced and the mixture is kept at a constant temperature for 20 minutes to obtain the alloy melt. The alloy melt is then injected into a mold preheated to 200°C using gravity casting to obtain a magnesium alloy ingot.

[0033] (2) The magnesium alloy ingot obtained in step (1) is subjected to solution treatment at 510℃ for 7 hours, and after being machined, a magnesium alloy billet is obtained. Then, it is subjected to extrusion treatment, grinding, tensile pre-strain treatment and aging treatment along the extrusion direction at room temperature to obtain a high-strength and high-toughness magnesium alloy.

[0034] The extrusion process is as follows: the magnesium alloy ingot and the extrusion die are preheated at 340°C for 1.5 hours, and then extruded at 340°C with an extrusion speed of 0.7 mm / s and an extrusion ratio of 7:1.

[0035] The tensile pre-strain treatment along the extrusion direction is as follows: the deformation is 3.5%, and the tensile pre-strain rate is 0.6 mm / min;

[0036] The aging treatment is as follows: heat preservation at 205℃ for 10 hours;

[0037] Tensile tests showed that the obtained high-strength and high-toughness magnesium alloy had a yield strength of 467 MPa and an elongation of 12.3% at room temperature. It had a layered heterogeneous structure, with recrystallized grains accounting for 80% of the area, an average recrystallized grain size of 0.90 μm, and an average precipitated phase size of 42 nm in the deformed grains.

[0038] Example 3

[0039] The high-strength and high-toughness alloy, by mass percentage, comprises the following magnesium alloy components: Gd: 7.2 wt.%, Sm: 2.8 wt.%, Zr: 0.35 wt.%, Ho: 0.15 wt.%, with the remainder being magnesium and unavoidable impurities, the unavoidable impurity content being 0.02 wt.%. The preparation method includes the following steps:

[0040] (1) Under the protection of a mixture of CO2 and SF6 with a volume ratio of 99:1, pure magnesium, magnesium gadolinium master alloy, magnesium samarium master alloy, magnesium holmium master alloy, and magnesium zirconium master alloy are heated and melted at 730°C. After complete melting, the mixture is stirred and slag is removed for 20 minutes. Argon gas is introduced and the mixture is kept at a constant temperature for 25 minutes to obtain the alloy melt. The alloy melt is then injected into a mold preheated to 250°C using gravity casting to obtain a magnesium alloy ingot.

[0041] (2) The magnesium alloy ingot obtained in step (1) is subjected to solution treatment at 515℃ for 7.5h, and after being machined, a magnesium alloy billet is obtained. Then, it is subjected to extrusion treatment, grinding, tensile pre-strain treatment and aging treatment along the extrusion direction at room temperature to obtain a high-strength and high-toughness magnesium alloy.

[0042] The extrusion process is as follows: the magnesium alloy ingot and the extrusion die are preheated at 390°C for 2 hours, and then extruded at 390°C with an extrusion speed of 0.8 mm / s and an extrusion ratio of 8:1.

[0043] The tensile pre-strain treatment along the extrusion direction is as follows: the deformation is 4%, and the tensile pre-strain rate is 0.7 mm / min;

[0044] The aging treatment is as follows: heat preservation at 210℃ for 11 hours;

[0045] Tensile tests showed that the obtained high-strength and high-toughness magnesium alloy had a yield strength of 393 MPa and an elongation of 10.7% at room temperature. It had a layered heterogeneous structure, with recrystallized grains accounting for 78% of the area, an average recrystallized grain size of 1.13 μm, and an average precipitated phase size of 50 nm in the deformed grains.

[0046] In summary, the high-strength and high-toughness magnesium alloy and its preparation method provided by this invention have successfully achieved a breakthrough in comprehensive performance through the synergistic control of alloying elements, precise optimization of component ratios, and process and process parameters. Compared with the prior art, this invention has the following outstanding advantages:

[0047] The process is simple and efficient: it adopts a short-process route, eliminating complex steps such as multiple ECAP passes and cyclic rolling-annealing in existing technologies. This process has a clear parameter window, short production cycle, low energy consumption, and is easier to scale up for industrial production.

[0048] This invention successfully prepared a structure with a "layered heterogeneous structure," consisting of ultrafine recrystallized grains (average size ≤ 1.2 μm, area fraction ≥ 70%) and deformed grains containing nano-precipitates (size ≤ 50 nm). This structure achieves multiple synergistic effects of grain refinement strengthening, dislocation strengthening, and precipitation strengthening.

[0049] Significantly Improved Mechanical Properties: The results of the examples show that the magnesium alloys prepared in the embodiments of the present invention have a room temperature yield strength of 393-467 MPa and an elongation of 8.5-12.3%, achieving an excellent match between strength and plasticity. This performance index is significantly superior to various traditional magnesium alloys mentioned in the prior art and alloys prepared by complex processes. In addition, the component ratios and process parameters used in each embodiment of the present invention are different. Among them, the component ratio of Example 3 is not the highest, but its performance is the best. This shows that the best effect of the present invention is not determined by a certain element, element ratio, process or process parameter, but is achieved through the synergistic regulation of component interaction, ratio, process and process parameters. Moreover, the best technical effect can only be achieved within the protection scope of the claims of the present invention.

[0050] This invention, through synergistic innovation in composition and process, simplifies the process flow while significantly improving the strength and toughness of magnesium alloys, providing an efficient and economical solution for developing high-performance magnesium alloy products, and has promising industrial application prospects.

Claims

1. A high-strength and high-toughness magnesium alloy, characterized in that, The alloy composition, by mass percentage, consists of 6.0-7.5 wt.% Gd, 2.0-3.0 wt.% Sm, 0.1-0.4 wt.% Zr, and 0.1-0.2 wt.% Ho, with the remainder being magnesium and unavoidable impurities, the unavoidable impurities being ≤0.05 wt.%. The method for preparing the high-strength and high-toughness magnesium alloy includes the following steps: (1) Under the protection of a CO2 and SF6 mixed gas with a volume ratio of 97-99:3-1, pure magnesium, magnesium gadolinium master alloy, magnesium samarium master alloy, magnesium holmium master alloy, and magnesium zirconium master alloy are heated and melted at 710-730℃. After complete melting, the mixture is stirred and slag is removed for 5-20 minutes. Argon gas is introduced and the mixture is kept at a constant temperature for 15-30 minutes to obtain the alloy melt. The alloy melt is then injected into a mold preheated to 150-250℃ using gravity casting to obtain a magnesium alloy ingot. (2) The magnesium alloy ingot obtained in step (1) is subjected to solution treatment at 505-515℃ for 6.5-7.5h, and after being rolled into a sheet, a magnesium alloy billet is obtained. Then, it is subjected to extrusion treatment, grinding, tensile pre-strain treatment and aging treatment along the extrusion direction at room temperature to obtain a high-strength and high-toughness magnesium alloy. The extrusion process is as follows: the magnesium alloy ingot and the extrusion die are preheated at 320-400℃ for 0.5-3 hours, and then extruded at 320-400℃ with an extrusion speed of 0.55-0.95 mm / s and an extrusion ratio of 5.5-8.5:

1. The tensile pre-strain treatment along the extrusion direction is as follows: the deformation is 2-5%, and the tensile pre-strain rate is 0.4-0.8 mm / min; The aging treatment is as follows: heat preservation at 190-220℃ for 8-12 hours; The high-strength and high-toughness magnesium alloy has a layered heterogeneous structure composed of deformed grains and fine, uniform recrystallized grains, as well as precipitates uniformly dispersed within the deformed grain regions. The recrystallized grains account for ≥70% of the area, the average size of the recrystallized grains is ≤1.2μm, the size of the precipitates in the deformed grains is ≤50nm, the alloy yield strength is ≥390MPa, and the elongation is ≥8.5%.

2. The high-strength and high-toughness magnesium alloy according to claim 1, characterized in that: The alloy composition, by mass percentage, consists of 6.5-7.2 wt.% Gd, 2.3-2.8 wt.% Sm, 0.2-0.35 wt.% Zr, 0.12-0.15 wt.% Ho, with the remainder being magnesium and unavoidable impurities, the unavoidable impurities being ≤0.03 wt.%.

3. The high-strength and high-toughness magnesium alloy according to claim 1, characterized in that: The extrusion process described in step (2) involves preheating the magnesium alloy ingot and the extrusion die at 325-395℃ for 1-2 hours, and then extruding at 325-395℃ with an extrusion speed of 0.6-0.9 mm / s and an extrusion ratio of 6-8:

1.

4. A high-strength and high-toughness magnesium alloy according to claim 1, characterized in that: Step (2) involves tensile pre-strain treatment along the extrusion direction: the deformation is 3-4%, and the tensile pre-strain rate is 0.5-0.7 mm / min.

5. A high-strength and high-toughness magnesium alloy according to claim 1, characterized in that: The aging treatment described in step (2) involves keeping the temperature at 200-210℃ for 9-11 hours.