A low-alloy magnesium alloy with high corrosion resistance and high thermal stability and its preparation method

Abstract

This invention belongs to the field of metallic materials technology and provides a low-alloy magnesium alloy with high corrosion resistance and high thermal stability, and its preparation method. The magnesium alloy composition by mass percentage is: aluminum: 0.8-1.5%, calcium: 0.2-0.5%, manganese: 0.4-0.8%, with magnesium as the added element, unavoidable impurities, and balance. The added element is a rare earth element, with a mass percentage of 0.5-1.0%, and the rare earth element is one or a combination of gadolinium and yttrium. The preparation method includes: alloy melting, water-cooled copper mold casting, stepped homogenization heat treatment, multi-pass multi-directional rolling forming, and recrystallization annealing. This invention achieves high corrosion resistance and high-temperature grain thermal stability of the magnesium alloy under the premise of low alloy element content (≤3.8wt%), with a corrosion rate of <0.35mg·cm in a 3.5wt% sodium chloride aqueous solution. ‑2 ·d ‑1 Furthermore, it can maintain a grain size of <7.5μm under high-temperature heat treatment at 500℃ for 1 hour.

Description

Technical Field

[0001] This invention relates to the field of metallic materials, and in particular to a low-alloy magnesium alloy with high corrosion resistance and high thermal stability, and its preparation method. Background Technology

[0002] Magnesium alloys, as one of the most promising lightweight metallic structural materials, possess advantages such as low density, high specific strength, and good electromagnetic shielding, thus showing broad application prospects in the automotive, 3C product, and aerospace industries. However, the poor corrosion resistance of magnesium alloys restricts their industrial application, mainly due to two reasons: First, magnesium is chemically highly reactive, with a standard electrode potential of -2.37V, the lowest among all metallic structural materials. This results in the second-phase potential commonly found in magnesium alloys being significantly higher than that of the magnesium matrix, leading to intense microgalvanic corrosion and subsequent degradation of the magnesium matrix. Furthermore, the oxide film formed on the surface during traditional magnesium alloy corrosion is loose and porous, exhibiting poor stability and generally failing to provide effective protection. Additionally, the hydrogen evolution process further disrupts the density of the oxide film.

[0003] On the other hand, magnesium alloys have a relatively large Hall Page coefficient, and changes in grain size have a significant impact on their mechanical properties. For example, heat treatment can lead to excessively large grains, reducing the alloy's mechanical properties. In existing technologies, increasing the volume fraction of the second phase can improve the alloy's thermal stability, but this also results in more severe microgalvanic corrosion, making it difficult to simultaneously achieve high corrosion resistance and high thermal stability in magnesium alloys. Therefore, effectively reducing production costs while simultaneously improving the alloy's corrosion resistance and thermal stability is a pressing technical challenge that needs to be addressed. Summary of the Invention

[0004] To address the aforementioned technical challenges, this invention provides a low-alloy magnesium alloy with high corrosion resistance and high thermal stability. The alloy composition, by mass percentage, is: aluminum: 0.8-1.5%, calcium: 0.2-0.5%, manganese: 0.4-0.8%, with magnesium as the additive element, unavoidable impurities, and balance. The additive element is a rare earth element, with a mass percentage of 0.5-1.0%, and the rare earth element is one or a combination of gadolinium and yttrium. The total unavoidable impurities are ≤0.05%. Its preparation method includes the following steps:

[0005] (1) Under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 9:1 to 19:1, pure magnesium is heated to melt at 660-720℃; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy are added, and after heating at 675-715℃ for 10-20 minutes, one or a combination of magnesium-gadolinium master alloy and magnesium-yttrium master alloy is added, and after standing at 670-710℃ for 10-20 minutes, after stirring evenly, argon refining and slag skimming, magnesium alloy melt is obtained;

[0006] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0007] (3) The ingot obtained in step (2) is subjected to step-by-step homogenization heat treatment under argon protection, and then water quenched at room temperature to obtain a homogeneous alloy.

[0008] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy with high corrosion resistance and high thermal stability; the corrosion rate of the magnesium alloy is <0.35 mg·cm. -2 ·d -1 After high-temperature treatment, the grain size of the magnesium alloy is <7.5μm.

[0009] Furthermore, the aluminum content is 0.85-1.4%, the calcium content is 0.3-0.45%, and the manganese content is 0.45-0.75%.

[0010] Furthermore, the mass percentage of the rare earth elements is 0.52-0.95%.

[0011] Further, the stepped homogenization heat treatment described in step (3) is divided into a two-step stepped homogenization heat treatment or a three-step stepped homogenization heat treatment. The two-step stepped homogenization heat treatment is: holding at 400-450℃ for 2-6 hours, and then holding at 480-520℃ for 1-6 hours. The three-step stepped homogenization heat treatment is: holding at 300-350℃ for 2-7 hours, holding at 400-450℃ for 2-6 hours, and then holding at 480-520℃ for 1-6 hours.

[0012] Further, the multi-pass multi-directional rolling forming in step (4) is as follows: the roll temperature is 80-120℃, the roll speed is 10-17 revolutions per minute, the number of rolling passes is 2-12, the rolling direction of each pass of the alloy needs to be 30-180° with the rolling direction of the previous pass, and the alloy needs to be held at 300-400℃ for 10-20 minutes before each pass of rolling, and the total reduction of multi-pass multi-directional rolling is 72%-78%.

[0013] Furthermore, the recrystallization annealing treatment is performed at an annealing temperature of 350-400℃ for 5-20 minutes.

[0014] Furthermore, the corrosion rate of the magnesium alloy obtained in step (4) was 0.17-0.33 mg·cm⁻¹. -2 ·d -1 After high-temperature treatment, the grain size of the magnesium alloy is 5.3-7.3μm.

[0015] Compared with the existing technology, the present invention has the following characteristics:

[0016] 1) In existing technologies, to obtain magnesium alloys with high corrosion resistance and high thermal stability, it is often necessary to add a large amount of alloying elements, especially expensive rare earth elements, which greatly increases the cost of raw materials and also increases the density of the alloy. Compared with existing technologies, the alloying element addition amount of the present invention is lower, with a base magnesium content ≥96.2wt.% and a total alloying element content ≤3.8wt.%, achieving low alloying, saving production costs, and the resulting alloy has lightweight characteristics.

[0017] 2) In the existing magnesium alloy processing, if the reduction amount is too large, the alloy will crack. However, the present invention prevents the alloy from cracking under large reduction amounts.

[0018] 3) The elements added in this invention have a high saturation solid solubility in the magnesium matrix, forming a large number of high-density nanoscale precipitates. These nanoscale phases can effectively pin grain boundary migration, avoid abnormal grain growth, and at the same time play an in-situ pinning role on the film structure, enhancing the passivation ability of the film, improving the compactness of the film, realizing the homogenization of the structure and grain refinement, and simultaneously improving the high-temperature thermal stability and corrosion resistance of the alloy.

[0019] This invention, through the synergistic effect of component ratios, processes, and parameters, achieves an alloy with superior corrosion resistance, thermal stability, and mechanical properties compared to alloys obtained using existing technologies, while reducing the amount of alloy additives. Furthermore, existing reports indicate that magnesium alloys undergo grain growth after high-temperature heat treatment, but this invention, even at higher processing temperatures than existing technologies, does not significantly affect the grain size, maintaining a smaller grain size that is finer than that of alloys disclosed in existing technologies. Therefore, it achieves a simultaneous improvement in corrosion resistance, thermal stability, and high mechanical properties. The corrosion rate of the alloy obtained by this invention is <0.35 mg·cm⁻¹. -2 ·d -1 After high-temperature treatment, the grain size can still be <7.5μm (this grain size is smaller than the grain size of alloys in the prior art that have not undergone high-temperature treatment). Detailed Implementation

[0020] Example 1

[0021] Taking the Mg-1.0Al-0.3Ca-0.5Mn-0.52Gd alloy as an example (with the following composition by mass percentage: Al: 1.0%, Ca: 0.3%, Mn: 0.5%, Gd: 0.52%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0022] (1) Pure magnesium was heated to 660°C and melted under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 9:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 680°C for 10 minutes, magnesium-gadolinium master alloy was added. After standing at 670°C for 10 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0023] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0024] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 400℃ for 2 hours, heat treatment at 480℃ for 1 hour, and then water quenching at room temperature to obtain a homogeneous alloy.

[0025] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 1 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 80°C, the roll speed is 15 revolutions per minute, and there are 4 rolling passes. The rolling direction of the alloy in each pass must be 90° to the rolling direction of the previous pass. At the same time, the alloy needs to be held at 300°C for 10 minutes before each rolling pass. The total reduction of the 4 rolling passes is ~75%. The recrystallization annealing is performed as follows: the annealing temperature is 350°C, and the annealing time is 5 minutes.

[0026] The low-alloy magnesium alloy 1 obtained in step (4) has a grain size of ~5.4 μm and a corrosion rate of ~0.23 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 After being kept at 500℃ for 1 hour, the grain size can still be maintained at ~5.7μm.

[0027] Example 2

[0028] Taking the Mg-1.0Al-0.38Ca-0.6Mn-0.7Gd alloy as an example (with the following composition by mass percentage: Al: 1.0%, Ca: 0.38%, Mn: 0.6%, Gd: 0.7%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0029] (1) Pure magnesium was heated to 670°C and melted under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 11:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 685°C for 12 minutes, magnesium-gadolinium master alloy was added. After standing at 670°C for 12 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0030] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0031] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 420℃ for 2.5 hours, heat treatment at 490℃ for 2 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0032] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 2 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 90°C, the roll speed is 17 revolutions per minute, and there are 3 rolling passes. The rolling direction of the alloy in each pass must be 120° with the rolling direction of the previous pass. At the same time, the alloy needs to be held at 310°C for 12 minutes before each rolling pass. The total reduction of the 3 rolling passes is ~72%. The recrystallization annealing is performed as follows: the annealing temperature is 360°C, and the annealing time is 8 minutes.

[0033] The low-alloy magnesium alloy 2 obtained in step (4) has a grain size of ~5.3 μm and a corrosion rate of ~0.17 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 After being kept at 500℃ for 1 hour, the grain size can still be maintained at ~5.6μm.

[0034] Example 3

[0035] Taking the Mg-1.1Al-0.35Ca-0.56Mn-0.95Gd alloy as an example (with the following composition by mass percentage: Al: 1.1%, Ca: 0.35%, Mn: 0.56%, Gd: 0.95%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0036] (1) Pure magnesium was heated to 680°C and melted under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 12:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 675°C for 15 minutes, magnesium-gadolinium master alloy was added. After standing at 680°C for 15 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0037] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0038] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 300℃ for 2 hours, heat treatment at 425℃ for 3 hours, heat treatment at 495℃ for 2.5 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0039] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 3 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 95°C, the roll speed is 16 revolutions per minute, and there are 6 rolling passes. The rolling direction of the alloy in each pass must be 60° to the rolling direction of the previous pass. At the same time, the alloy needs to be held at 320°C for 15 minutes before each rolling pass. The total reduction of the 6 rolling passes is ~73%. The recrystallization annealing is performed as follows: the annealing temperature is 365°C, and the annealing time is 10 minutes.

[0040] The low-alloy magnesium alloy 3 obtained in step (4) has a grain size of ~5.7 μm and a corrosion rate of ~0.31 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 Furthermore, even after holding at 500℃ for 1 hour, the grain size can still be maintained at ~6.2μm.

[0041] Example 4

[0042] Taking the Mg-0.85Al-0.4Ca-0.65Mn-0.53Y alloy as an example (with the following composition by mass percentage: Al: 0.85%, Ca: 0.4%, Mn: 0.65%, Y: 0.53%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0043] (1) Pure magnesium was heated to 685°C under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 14:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 690°C for 16 minutes, magnesium-yttrium master alloy was added. After standing at 685°C for 16 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0044] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0045] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 430℃ for 3.5 hours, heat treatment at 500℃ for 3 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0046] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 4 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 100°C, the roll speed is 14 revolutions per minute, and there are 8 rolling passes. The rolling direction of the alloy in each pass must be 45° to the rolling direction of the previous pass. At the same time, the alloy needs to be held at 325°C for 16 minutes before each rolling pass. The total reduction of the 8 rolling passes is ~76%. The recrystallization annealing is performed as follows: the annealing temperature is 370°C, and the annealing time is 12 minutes.

[0047] The low-alloy magnesium alloy 4 obtained in step (4) has a grain size of ~6.5 μm and a corrosion rate of ~0.26 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 Furthermore, even after holding at 500℃ for 1 hour, the grain size can still be maintained at ~7.3μm.

[0048] Example 5

[0049] Taking the Mg-1.2Al-0.45Ca-0.55Mn-0.75Y alloy as an example (with the following composition by mass percentage: Al: 1.2%, Ca: 0.45%, Mn: 0.55%, Y: 0.75%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0050] (1) Pure magnesium was heated to 690°C under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 15:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 695°C for 16 minutes, magnesium-yttrium master alloy was added. After standing at 690°C for 17 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0051] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0052] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 435℃ for 4 hours, heat treatment at 503℃ for 4 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0053] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 5 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 105°C, the roll speed is 13 revolutions per minute, and there are 2 rolling passes. The rolling direction of the alloy in each pass must be 180° with the rolling direction of the previous pass. At the same time, the alloy needs to be held at 325°C for 17 minutes before each rolling pass. The total reduction of the 2-pass rolling is ~74%. The recrystallization annealing is performed as follows: the annealing temperature is 375°C, and the annealing time is 15 minutes.

[0054] The low-alloy magnesium alloy 5 obtained in step (4) has a grain size of ~5.5 μm and a corrosion rate of ~0.18 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 Furthermore, even after holding at 500℃ for 1 hour, the grain size can still be maintained at ~5.9μm.

[0055] Example 6

[0056] Taking the Mg-1.3Al-0.32Ca-0.75Mn-0.95Y alloy as an example (with the following composition by mass percentage: Al: 1.3%, Ca: 0.32%, Mn: 0.75%, Y: 0.95%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0057] (1) Pure magnesium was heated to 695°C under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 17:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 700°C for 18 minutes, magnesium-yttrium master alloy was added. After standing at 695°C for 18 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0058] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0059] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 320℃ for 3 hours, heat treatment at 440℃ for 4.5 hours, heat treatment at 505℃ for 4.5 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0060] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 6 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 110°C, the roll speed is 12 revolutions per minute, and there are 5 rolling passes. The rolling direction of the alloy in each pass must be 72° to the rolling direction of the previous pass. At the same time, the alloy needs to be held at 330°C for 18 minutes before each rolling pass. The total reduction of the 5 rolling passes is ~77%. The recrystallization annealing is performed as follows: the annealing temperature is 380°C, and the annealing time is 17 minutes.

[0061] The low-alloy magnesium alloy 6 obtained in step (4) has a grain size of ~5.9 μm and a corrosion rate of ~0.33 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 Furthermore, even after holding at 500℃ for 1 hour, the grain size can still be maintained at ~6.5μm.

[0062] Example 7

[0063] Taking the Mg-0.9Al-0.43Ca-0.7Mn-0.44Gd-0.5Y alloy as an example (with the following composition by mass percentage: Al: 0.9%, Ca: 0.43%, Mn: 0.7%, Gd: 0.44%, Y: 0.5%, unavoidable total impurities ≤ 0.05%, balance being magnesium), its preparation method is as follows:

[0064] (1) Pure magnesium was heated to 700°C and melted under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 19:1; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy were added, and after heating at 710°C for 19 minutes, magnesium-gadolinium and magnesium-yttrium master alloys were added. After standing at 700°C for 19 minutes, the mixture was stirred evenly, refined with argon gas and skimmed off to obtain magnesium alloy melt.

[0065] (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold;

[0066] (3) The ingot obtained in step (2) is subjected to a step-by-step homogenization process under argon protection. The step-by-step homogenization process is as follows: heat treatment at 445℃ for 5 hours, heat treatment at 510℃ for 5 hours, and then water quenching at room temperature to obtain a homogeneous alloy.

[0067] (4) The homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing to obtain a low-alloy magnesium alloy 7 with high corrosion resistance and high thermal stability. The multi-pass multi-directional rolling is performed as follows: the roll temperature is 115°C, the roll speed is 11 revolutions per minute, and there are 9 rolling passes. The rolling direction of the alloy in each pass must be 40° to the rolling direction of the previous pass. At the same time, the alloy needs to be held at 335°C for 19 minutes before each rolling pass. The total reduction of the 9 rolling passes is ~78%. The recrystallization annealing is performed as follows: the annealing temperature is 390°C, and the annealing time is 19 minutes.

[0068] The low-alloy magnesium alloy 7 obtained in step (4) has a grain size of ~6.4 μm and a corrosion rate of ~0.20 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 Furthermore, even after holding at 500℃ for 1 hour, the grain size can still be maintained at ~6.8μm.

[0069] Comparative Example 1

[0070] A research paper titled "Influence of texture on corrosion rate of AZ31 Mg alloy in 3.5 wt.% NaCl," published in Volume 32, pp. 4548-4552 of the journal *Materials and Design* in 2011, tested the corrosion rate of commercially available AZ31 rolled steel sheets. Immersion experiments showed that the sample with the best corrosion resistance exhibited a corrosion rate of 2.4 mg·cm⁻¹ in a 3.5 wt% sodium chloride solution. -2 ·d -1 The highest corrosion rate of this invention is only 0.33 mg·cm⁻¹. -2 ·d -1 The corrosion rate of the alloy in this invention is much lower than that of the alloy in Comparative Example 1. Therefore, the corrosion resistance of the alloy in this invention is significantly better than that of the alloy in Comparative Example 1.

[0071] Comparative Example 2

[0072] In a 2012 paper titled "Fundamental studies on the thermal stability and mechanical characteristics of AZ31 alloy" published in Volume 536, pp. 64-72 of Materials Science and Engineering A, the average grain size of a commercial AZ31 alloy after heat treatment at 450°C for 1 hour was ~18.2 μm. In contrast, the magnesium alloy of this invention, after holding at 500°C for 1 hour, had a maximum grain size of ~7.3 μm. According to prior art reports, the higher the heat treatment temperature of magnesium alloys, the larger the grain size. Based on prior art calculations, this invention, using a higher treatment temperature than existing technologies, should have yielded an alloy with a larger grain size than the alloy obtained in Comparative Example 2. However, the result is that this invention obtains finer grains than existing technologies. This demonstrates that, compared to existing technologies, the alloy obtained by this invention maintains a smaller grain size and better thermal stability even under high-temperature treatment.

[0073] In summary, according to existing technology reports, it is difficult to simultaneously improve corrosion resistance and thermal stability. However, compared with existing technologies, this invention achieves a simultaneous improvement in the corrosion resistance, thermal stability, and mechanical properties of the alloy. Therefore, compared with existing technologies, this invention has achieved unexpected technical effects. Furthermore, existing technologies improve the corrosion resistance of alloys by increasing alloying elements or rare earth content, with the total content of added alloying elements generally exceeding 6%, which increases the alloy production cost. This invention, however, simultaneously improves the corrosion resistance, thermal stability, and mechanical properties of the alloy while controlling the alloy addition amount to below 3.8%. Moreover, existing technologies report that magnesium alloys undergo significant grain growth after high-temperature heat treatment. This invention, even at higher treatment temperatures than existing technologies, maintains a smaller grain size, closely resembling the initial grain size, and is even finer than the grains disclosed in existing technologies. Therefore, it achieves high thermal stability and high mechanical properties in the alloy. Furthermore, as can be seen from the embodiments of this invention, the components, proportions, and process parameters differ in different embodiments. In Example 2, the total alloy addition and the amount of each alloying element added are not the highest among the embodiments, yet the alloy simultaneously achieves the best corrosion resistance and thermal stability (smallest grain size). This demonstrates that this invention does not achieve a significant improvement in the overall material performance simply by adding rare earth elements or a combination of one or more elements; in other words, a significant improvement in the overall material performance cannot be achieved through a simple combination of elements. In summary, the alloy with excellent comprehensive performance (simultaneously improving corrosion resistance, thermal stability, and mechanical properties) obtained by this invention is achieved through the interaction between alloying elements, the elemental proportions, and the synergistic control of process parameters.

Claims

1. A low-alloy magnesium alloy with high corrosion resistance and high thermal stability, characterized in that: The alloy composition, by mass percentage, is: aluminum: 0.8-1.5%, calcium: 0.2-0.5%, manganese: 0.4-0.8%, with added elements, unavoidable impurities, and the balance being magnesium; the added elements are rare earth elements, with a mass percentage of 0.5-1.0%, and the rare earth elements are one or a combination of gadolinium and yttrium; the total unavoidable impurities are ≤0.05%; its preparation method includes the following steps: (1) Under the protection of a mixed gas of carbon dioxide and sulfur hexafluoride with a volume ratio of 9:1 to 19:1, pure magnesium is heated to melt at 660-720℃; then pure aluminum, magnesium-manganese master alloy and magnesium-calcium master alloy are added, and after heating at 675-715℃ for 10-20 minutes, one or a combination of magnesium-gadolinium master alloy and magnesium-yttrium master alloy is added, and after standing at 670-710℃ for 10-20 minutes, after stirring evenly, argon refining and slag skimming, magnesium alloy melt is obtained; (2) The magnesium alloy melt obtained in step (1) is cast into an ingot by manual casting in a water-cooled copper mold; (3) The ingot obtained in step (2) is subjected to step-by-step homogenization heat treatment under argon protection, and then water quenched at room temperature to obtain a homogeneous alloy. (4) After the homogeneous alloy obtained in step (3) is subjected to multi-pass multi-directional rolling and recrystallization annealing, a low-alloy magnesium alloy with high corrosion resistance and high thermal stability is obtained; the corrosion rate of the magnesium alloy is <0.35mg·cm-2·d-1; after high temperature treatment at 500℃ for 1h, the grain size of the magnesium alloy is <7.5μm. The multi-pass multi-directional rolling forming in step (4) is as follows: the roll temperature is 80-120℃, the roll speed is 10-17 revolutions per minute, the rolling passes are 2-12, the rolling direction of the alloy in each pass is 30-180° with the rolling direction of the previous pass, and the alloy needs to be held at 300-400℃ for 10-20 minutes before each rolling pass, and the total reduction of multi-pass multi-directional rolling is 72%-78%; the recrystallization annealing treatment in step (4) is as follows: the annealing temperature is 350-400℃, and the annealing time is 5-20 minutes.

2. The low-alloy magnesium alloy with high corrosion resistance and high thermal stability according to claim 1, characterized in that: Aluminum as a percentage by mass: 0.85-1.4%, Calcium: 0.3-0.45%, Manganese: 0.45-0.75%.

3. The low-alloy magnesium alloy with high corrosion resistance and high thermal stability according to claim 1, characterized in that: The rare earth element mass percentage is 0.52-0.95%.

4. A low-alloy magnesium alloy with high corrosion resistance and high thermal stability according to any one of claims 1-3, characterized in that: The stepped homogenization heat treatment described in step (3) is divided into two-step or three-step stepped homogenization heat treatment. The two-step stepped homogenization heat treatment is: holding at 400-450℃ for 2-6 hours, and then holding at 480-520℃ for 1-6 hours. The three-step stepped homogenization heat treatment is: holding at 300-350℃ for 2-7 hours, holding at 400-450℃ for 2-6 hours, and then holding at 480-520℃ for 1-6 hours.

5. A low-alloy magnesium alloy with high corrosion resistance and high thermal stability according to any one of claims 1-3, characterized in that: The corrosion rate of the magnesium alloy obtained in step (4) was 0.17-0.33 mg·cm-2·d-1; after high temperature treatment at 500℃ for 1h, the grain size of the magnesium alloy was 5.3-7.3 μm.