A low-alloyed high-corrosion-resistant magnesium alloy and a preparation method thereof

Abstract

This invention belongs to the field of metal material processing and provides a low-alloy, high-corrosion-resistant magnesium alloy and its preparation method. The magnesium alloy composition, by mass percentage, includes: Ca: 0.05-0.5%, In: 0-1.0%, rare earth elements: 0-2.0%, unavoidable impurities ≤0.03%, and the balance being Mg; the rare earth elements are one or any combination of Gd, Dy, Ho, Tm, Er, and Sc. The magnesium alloy preparation method includes atmosphere-protected melting, sub-rapid solidification, homogenization, multi-pass rolling, and recrystallization annealing. This invention reduces impurity phases and second phases, obtains a fully recrystallized fine-grained structure, effectively eliminates dislocations and residual stress, ultimately reduces galvanic corrosion and cathodic hydrogen evolution reaction, significantly improves the passivation ability of the film, promotes uniform corrosion, and enhances film continuity, thereby achieving excellent corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing and provides a low-alloy high corrosion-resistant magnesium alloy and its preparation method. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials, also possess high specific strength and high specific stiffness, making them ideal for achieving lightweighting goals in the automotive, high-speed rail, aerospace, and defense industries. Furthermore, magnesium alloys exhibit excellent electromagnetic shielding properties and rapid thermal conductivity, offering broad application prospects in the 3C product sector. Some magnesium alloys also demonstrate good biocompatibility and mechanical properties compatible with the human body, making them highly promising biodegradable biomedical materials. However, magnesium has a very low standard electrode potential, making it susceptible to severe galvanic corrosion when the matrix and precipitated phases come into contact. Additionally, magnesium has a low Pilling-Bedworth ratio (PBR, the volume ratio of oxide to the metal consumed in forming the oxide) of less than 1, resulting in a porous and easily permeable surface corrosion film that is easily penetrated by corrosive media. This leads to poor corrosion resistance in magnesium alloys, severely limiting their practical applications.

[0003] It is well known that unavoidable Fe impurities in magnesium alloys are a key reason for their rapid corrosion. A large potential difference exists between elemental Fe and Fe-rich precipitates and the magnesium matrix, easily initiating severe galvanic corrosion. Fe also has a high electrocatalytic effect on the hydrogen evolution reaction, thus significantly reducing the corrosion resistance of magnesium alloys. Fe has extremely low solid solubility in magnesium; even Fe impurities with a mass percentage below 0.005% are easily precipitated during solidification and deformation heat treatment, significantly negatively impacting the corrosion resistance of magnesium alloys. Therefore, controlling Fe impurity precipitation is a crucial technical problem that urgently needs to be solved to improve the corrosion resistance of magnesium alloys. Furthermore, the large grain size, precipitate size, or eutectic phase size of magnesium alloys also contributes to their poor corrosion resistance. Existing technologies improve the corrosion resistance of alloys by increasing the amount of magnesium alloy components and rare earth content, and by using high-temperature, long-term treatment, which increases the alloy production cost. Therefore, how to improve the corrosion resistance of magnesium alloys by reducing alloy production costs, simplifying processes, avoiding the formation of Fe impurity precipitates, refining grains, reducing the size of precipitates or eutectic phases, and improving the protective and stable properties of the corrosion film is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] To address the aforementioned technical challenges, this invention provides a low-alloy, high-corrosion-resistant magnesium alloy, comprising the following components by mass percentage: Ca: 0.05-0.5%, In: 0-1.0%, rare earth elements: 0-2.0%, unavoidable impurities ≤0.03%, and the balance being Mg; wherein the rare earth elements are one or any combination of Gd, Dy, Ho, Tm, Er, and Sc; its preparation method includes the following steps:

[0005] (1) Under a protective gas atmosphere, pure Mg and Mg-Ca master alloys are heated and melted at 680-700℃; then, rare earth element master alloys are added at 700-750℃, and after complete melting, pure In is added at 690-720℃. After stirring for 5-10 minutes, the mixture is held at this temperature for 10-20 minutes, and after refining and slag removal, an alloy melt is obtained; the rare earth element master alloy is one or any combination of Mg-Gd, Mg-Dy, Mg-Ho, Mg-Tm, Mg-Er, and Mg-Sc master alloys.

[0006] (2) The alloy melt obtained in step (1) is poured into a copper mold by gravity casting or subjected to twin-roll casting and rolling. Under the condition of a cooling rate of 250-650K / s, the alloy billets in the sub-rapid solidification as-cast state or cast-rolled state are obtained respectively.

[0007] (3) The cast or cast-rolled alloy billet obtained in step (2) is subjected to homogenization heat treatment and water-cooled to room temperature to obtain homogenized billet. The homogenization heat treatment is: holding at 250-450℃ for 0.5-4 hours.

[0008] (4) The homogenized billet obtained in step (3) is rolled in multiple passes to obtain a rolled billet, and then a recrystallization annealing treatment is performed to obtain a low-alloy high corrosion-resistant magnesium alloy.

[0009] The twin-roll casting process described in step (2) is as follows: the casting speed is 3-8 m / min, the casting temperature is 620-700℃, and the casting roll gap is 3-12 mm;

[0010] The multi-pass rolling in step (4) is as follows: the total number of passes is 5-12, and the temperature is maintained at 200-400℃ for 5-20 minutes before each pass, and the rolling temperature is 250-380℃; the reduction per pass is 20-50%, and the total reduction is 60-90%; the recrystallization annealing treatment is as follows: the temperature is maintained at 200-350℃ for 5-120 minutes.

[0011] Furthermore, by mass percentage, the Ca content is 0.1-0.3%, and the In content is 0.1-0.8%.

[0012] Furthermore, the rare earth alloying elements comprise 0.1-1.8% by mass percentage.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] Compared with existing technologies, this invention reduces the amount of alloy or rare earth alloy added (even without any rare earth), and reduces the high-temperature, long-term processing. The alloy addition is ≤3.5%, belonging to a low-alloy system. This invention reduces impurities and precipitates, lowers galvanic corrosion, and forms a fully recrystallized fine-grained structure, promoting uniform corrosion of the magnesium matrix and improving film continuity. It also eliminates dislocations and residual stress in the alloy after rolling, improving film stability. Compared with alloys obtained by existing technologies, the alloy obtained by this invention has an average hydrogen evolution rate ≤0.072 mL / cm² after immersion in 5.5 wt.% sodium chloride solution for 14 days. 2 / day, average rate of weight loss ≤0.096mg / cm³ 2 The average corrosion rate is ≤0.201 mm / y, which is lower than that of alloys obtained by existing technologies. Therefore, the alloys obtained by this invention have better corrosion resistance than those obtained by existing technologies. At the same time, this invention also achieves mechanical properties comparable to those of alloys obtained by existing technologies. In other words, this invention ensures high mechanical properties while also improving corrosion resistance. Detailed analysis is as follows:

[0015] 1. Existing magnesium alloys contain Fe-rich precipitates such as Al-Fe, Mn-Fe, and Zr-Fe, as well as a large amount of coarse secondary phases, which can cause severe localized corrosion. This invention effectively avoids the formation of Al-Fe, Mn-Fe, and Zr-Fe precipitates by forcibly dissolving Fe impurities, and reduces coarse secondary phases through sub-rapid solidification. In other words, it effectively prevents the precipitation of high-potential Fe-rich phases and secondary phases, suppressing galvanic corrosion. Simultaneously, it effectively weakens the cathodic hydrogen evolution reaction and significantly reduces the anodic dissolution rate of magnesium.

[0016] 2. In existing technologies, rare earth elements in magnesium mostly exist as precipitated phases, which have limited effect on improving the PBR (Pilling-Bedworth ratio, the volume ratio of oxides to the metal consumed to form the oxide). The surface corrosion film is loose and porous, easily penetrated by corrosive media, resulting in poor corrosion resistance of magnesium alloys and severely restricting their practical applications. In contrast, the magnesium alloy obtained by this invention has rare earth elements dissolved in the magnesium matrix, significantly improving the PBR. The rare earth oxides / hydroxides generated during corrosion can fully participate in the film formation process, which is beneficial to improving the film density and thus hindering the contact between corrosive media and the magnesium matrix. In addition, it effectively eliminates dislocations and residual stress, ultimately reducing galvanic corrosion and cathodic hydrogen evolution reaction, and significantly improving the passivation ability and continuity of the film. At the same time, the second phase of the alloy obtained by this invention has a small size (<1 micrometer) and a small number, which can dissolve quickly and completely. The calcium ions generated after dissolution combine with the carbon dioxide dissolved in the solution to form calcium compounds that accumulate in the corrosion product layer, thereby improving the protective properties of the film and effectively preventing further corrosion of the alloy surface. Detailed Implementation

[0017] Example 1

[0018] Taking the Mg-0.12Ca alloy as an example, it comprises the following components by mass percentage: Ca is 0.12%, total unavoidable impurities ≤ 0.03%, and the balance is Mg. Its preparation method is as follows:

[0019] (1) Alloy raw materials are prepared according to the mass percentage, and pure Mg and Mg-Ca master alloy are added in sequence under argon protection. The temperature is raised to 690℃ and then kept at the temperature for melting. After all the materials are melted, they are stirred at 700℃ for 5 minutes and then kept at the temperature for 10 minutes. Then, they are refined and slag is removed to obtain alloy melt.

[0020] (2) The alloy melt obtained in step (1) is poured into a plate-shaped copper mold preheated to 200°C by gravity casting to obtain a sub-rapid solidification cast billet; the sub-rapid solidification means that the cooling rate is 300K / s.

[0021] (3) The as-cast billet obtained in step (2) is kept at 300°C for 0.5 hours for homogenization treatment, and then cooled to room temperature by water to obtain a homogenized billet;

[0022] (4) The homogenized billet obtained in step (3) is subjected to multi-pass rolling. The rolling process is as follows: rolling temperature is 275℃, rolling is performed in 6 passes, the total reduction is 83%, and the billet is held at 300℃ for 10 minutes before each pass. Then, recrystallization annealing heat treatment is performed. The annealing process is as follows: temperature is 225℃, time is 30 minutes, to obtain a high corrosion-resistant Mg-0.12Ca alloy. The average hydrogen evolution rate of the alloy after immersion in 5.5wt.% sodium chloride solution for 14 days is 0.072mL / cm. 2 / day, the average rate of weight loss was 0.096 mg / cm³. 2 / d, with an average corrosion rate of 0.201 mm / y.

[0023] Example 2

[0024] Taking the Mg-0.2Ca-0.5In alloy as an example, it comprises the following components by mass percentage: Ca 0.2%, In 0.5%, unavoidable impurities ≤ 0.03%, and the balance being Mg. Its preparation method is as follows:

[0025] (1) Alloy raw materials are prepared according to the mass percentage, and pure Mg and Mg-Ca master alloy are added sequentially under the protection of carbon dioxide / sulfur hexafluoride with a volume ratio of 9:1. The temperature is raised to 690℃ and then kept at the temperature for melting. After complete melting, pure In is added at 700℃, stirred for 5 minutes and kept at the temperature for 15 minutes. Then, refining and slag removal are carried out to obtain alloy melt.

[0026] (2) The alloy melt obtained in step (1) is poured into a plate-shaped copper mold preheated to 200°C by gravity casting to obtain a sub-rapid solidification cast billet; the sub-rapid solidification means that the cooling rate is 350 K / s.

[0027] (3) The as-cast billet obtained in step (2) is kept at 400°C for 2 hours for homogenization treatment, and then cooled to room temperature by water to obtain a homogenized billet;

[0028] (4) The homogenized billet obtained in step (3) is subjected to multi-pass rolling. The rolling process is as follows: rolling temperature is 325℃, rolling is performed in 5 passes, the total reduction is 80%, and the billet is held at 350℃ for 15 minutes before each pass. Then, recrystallization annealing heat treatment is performed. The annealing process is as follows: temperature is 300℃, time is 10 minutes, to obtain a high corrosion-resistant Mg-0.2Ca-0.5In alloy. The average hydrogen evolution rate of the alloy after immersion in 5.5wt.% sodium chloride solution for 14 days is 0.055mL / cm. 2 / day, the average rate of weight loss was 0.078 mg / cm³. 2 / d, with an average corrosion rate of 0.164 mm / y.

[0029] Example 3

[0030] Taking the Mg-0.1Ca-0.5In-1.0Sc alloy as an example, it comprises the following components by mass percentage: Ca 0.1%, In 0.5%, Sc 1.0%, unavoidable impurities ≤ 0.03%, and the balance being Mg. Its preparation method is as follows:

[0031] (1) Alloy raw materials are prepared according to the mass percentage, and pure Mg and Mg-Ca master alloy are added in sequence under argon protection. The temperature is raised to 690℃ and then kept at the temperature for melting. Then, Mg-Sc master alloy is added at 750℃. After complete melting, pure In is added at 700℃. After stirring for 5 minutes, the temperature is kept for 20 minutes. Then, refining and slag removal are carried out to obtain alloy melt.

[0032] (2) The alloy melt obtained in step (1) is subjected to twin-roll casting and rolling to obtain a cast-rolled billet; the twin-roll casting and rolling process is as follows: casting speed is 6 m / min, casting temperature is 650℃, casting roll gap is 4 mm, and cooling rate is 500 K / s.

[0033] (3) The cast-rolled billet obtained in step (2) is kept at 400°C for 2 hours for homogenization treatment, and then cooled to room temperature by water to obtain a homogenized billet;

[0034] (4) The homogenized billet obtained in step (3) is subjected to multi-pass rolling. The rolling process is as follows: rolling temperature is 300℃, rolling is performed in 8 passes, the total reduction is 83%, and the billet is held at 320℃ for 20 minutes before each pass. Then, recrystallization annealing heat treatment is performed. The annealing process is as follows: temperature is 320℃, time is 8 minutes, to obtain a high corrosion-resistant Mg-0.1Ca-0.5In-1Sc alloy. The average hydrogen evolution rate of the alloy after immersion in 5.5wt.% sodium chloride solution for 14 days is 0.043mL / cm. 2 / day, the average rate of weight loss was 0.065 mg / cm³. 2 / d, with an average corrosion rate of 0.137 mm / y.

[0035] Example 4

[0036] Taking the Mg-0.1Ca-0.8In-1.5Dy alloy as an example, it comprises the following components by mass percentage: Ca 0.1%, In 0.8%, Dy 1.5%, unavoidable impurities ≤ 0.03%, and the balance being Mg. Its preparation method is as follows:

[0037] (1) Alloy raw materials are prepared according to the mass percentage, and pure Mg and Mg-Ca master alloy are added sequentially under the protection of carbon dioxide / sulfur hexafluoride with a volume ratio of 95:5. The temperature is raised to 690°C and then held for melting. Then, Mg-Dy master alloy is added at 740°C. After complete melting, pure In is added at 700°C. The mixture is stirred for 10 minutes and held for 20 minutes. Then, it is refined and slag is removed to obtain alloy melt.

[0038] (2) The alloy melt obtained in step (1) is poured into a plate-shaped copper mold preheated to 200°C by gravity casting to obtain a sub-rapid solidification cast billet; the sub-rapid solidification means that the cooling rate is 380 K / s.

[0039] (3) The as-cast billet obtained in step (2) is kept at 400°C for 4 hours for homogenization treatment, and then cooled to room temperature by water to obtain a homogenized billet;

[0040] (4) The homogenized billet obtained in step (3) is subjected to multi-pass rolling. The rolling process is as follows: rolling temperature is 325℃, rolling is performed in 8 passes, the total reduction is 83%, and the billet is held at 350℃ for 20 minutes before each pass. Then, recrystallization annealing heat treatment is performed. The annealing process is as follows: temperature is 350℃, time is 5 minutes, to obtain a high corrosion-resistant Mg-0.1Ca-0.8In-1.5Dy alloy. The average hydrogen evolution rate of the alloy after immersion in 5.5wt.% sodium chloride solution for 14 days is 0.037mL / cm. 2 / day, the average rate of weight loss was 0.056 mg / cm³. 2 / d, with an average corrosion rate of 0.118 mm / y.

[0041] Example 5

[0042] Taking the Mg-0.2Ca-0.8In-0.8Gd-1.0Er alloy as an example, it comprises the following components by mass percentage: Ca 0.2%, In 0.8%, Gd 0.8%, Er 1.0%, unavoidable impurities ≤ 0.03%, and the balance being Mg. Its preparation method is as follows:

[0043] (1) Alloy raw materials are prepared according to the mass percentage, and pure Mg and Mg-Ca master alloy are added sequentially under the protection of carbon dioxide / sulfur hexafluoride with a volume ratio of 92:8. The temperature is raised to 690°C and then held for melting. Then, Mg-Gd and Mg-Er master alloys are added at 750°C. After complete melting, pure In is added at 700°C. After stirring for 10 minutes, the temperature is held for 20 minutes. Then, refining and slag removal are carried out to obtain alloy melt.

[0044] (2) The alloy melt obtained in step (1) is subjected to twin-roll casting and rolling to obtain a cast-rolled billet; the twin-roll casting and rolling process is as follows: the casting and rolling speed is 5 m / min, the casting and rolling temperature is 660℃, the casting and rolling roll gap is 5 mm, and the cooling rate is 600 K / s.

[0045] (3) The cast-rolled billet obtained in step (2) is kept at 450°C for 4 hours for homogenization treatment, and then cooled to room temperature by water to obtain a homogenized billet.

[0046] (4) The homogenized billet obtained in step (3) is subjected to multi-pass rolling. The rolling process is as follows: rolling temperature is 375℃, rolling is performed in 12 passes, the total reduction is 90%, and the billet is held at 400℃ for 20 minutes before each pass. Then, recrystallization annealing heat treatment is performed. The annealing process is as follows: temperature is 350℃, time is 10 minutes, to obtain a high corrosion-resistant Mg-0.2Ca-0.8In-0.8Gd-1.0Er alloy. The average hydrogen evolution rate of the alloy after immersion in 5.5wt.% sodium chloride solution for 14 days is 0.042mL / cm. 2 / day, the average rate of weight loss was 0.063 mg / cm³. 2 / d, with an average corrosion rate of 0.132 mm / y.

[0047] Comparative Example 1

[0048] In the prior art, Luo et al. published a journal article entitled "The microstructure and corrosion resistance of as-extruded Mg-6Gd-2Y-(0–1.5)Nd-0.2Zr alloys" in Materials and Design 186(2020)108289, which disclosed a Mg-6Gd-2Y-1Nd-0.2Zr alloy prepared by melting, casting, solution homogenization, hot water quenching and hot extrusion. In this comparative example, the homogenization treatment temperature was 520℃ and the time was 12 hours. The alloy obtained had a minimum weight loss corrosion rate of 0.85 mm / y after being immersed in a 5 wt% sodium chloride solution saturated with magnesium hydroxide for 3 days. Compared with the comparative example, the homogenization heat treatment used in this invention has a lower temperature (<450℃), a shorter holding time (<4 hours), and a lower total amount of rare earth elements (<2wt.%), and can even be done without adding rare earth elements, which reduces processing energy consumption and saves production costs. In all the embodiments, the corrosion resistance of the alloy in Example 1 ranks low. This example (Mg-0.12Ca alloy) did not add rare earth elements. Even with the amount of alloy added being much less than that in Comparative Example 1, the corrosion rate of the alloy in Example 1 after immersion in 5.5wt.% sodium chloride solution for 14 days was 0.201 mm / y (see Example 1 for details), which is much lower than the 0.85 mm / y disclosed in Comparative Example 1. Therefore, the corrosion resistance of the alloy obtained by this invention is far superior to that of the alloy obtained in Comparative Example 1.

[0049] In summary, compared with existing technologies such as Comparative Example 1, this invention simplifies the process by reducing the amount of alloy and rare earth elements added (and even eliminating the need for any rare earth elements), while achieving superior alloy corrosion resistance compared to existing technologies. Furthermore, the components, proportions, processes, and process parameters used in all embodiments of this invention are different. Although the amount of alloy components added in Example 4 is not the highest, the alloy corrosion resistance obtained in Example 4 is the best. This demonstrates that the significant improvement achieved by this invention is not determined by a single component or process, but rather by the synergistic regulation of the interaction of alloy components, proportions, processes, and process parameters, which enables the alloy to achieve optimal corrosion resistance.

Claims

1. A low-alloy, high-corrosion-resistant magnesium alloy, characterized in that: By mass percentage, it comprises the following components: Ca: 0.05-0.3%, In: 0-1.0%, rare earth elements: 0-2.0%, unavoidable impurities ≤0.03%, and the balance being Mg; the rare earth elements are one or any combination of Gd, Dy, Ho, Tm, Er, and Sc; its preparation method includes the following steps: (1) Under a protective gas, pure Mg and Mg-Ca master alloys are heated and melted at 680-700℃; then, rare earth element master alloys are added at 700-750℃. After complete melting, pure In is added at 690-720℃. After stirring for 5-10 minutes, the mixture is kept at this temperature for 10-20 minutes. After refining and slag removal, the alloy melt is obtained. The rare earth element master alloy is one or any combination of Mg-Gd, Mg-Dy, Mg-Ho, Mg-Tm, Mg-Er, and Mg-Sc master alloys. (2) The alloy melt obtained in step (1) is poured into a copper mold by gravity casting or subjected to twin-roll casting and rolling. Under the condition of a cooling rate of 250-650K / s, the sub-rapid solidification as-cast or cast-rolled alloy billets are obtained respectively. (3) The cast or cast-rolled alloy billet obtained in step (2) is subjected to homogenization heat treatment and water-cooled to room temperature to obtain homogenized billet. The homogenization heat treatment is: holding at 250-450℃ for 0.5-4 hours. (4) The homogenized billet obtained in step (3) is rolled in multiple passes to obtain a rolled billet, and then a recrystallization annealing treatment is performed to obtain a low-alloy high corrosion-resistant magnesium alloy. The twin-roll casting process described in step (2) is as follows: the casting speed is 3-8 m / min, the casting temperature is 620-700℃, and the casting roll gap is 3-12 mm; The multi-pass rolling in step (4) is as follows: the total number of passes is 5-12, and each pass is held at 200-400℃ for 5-20 minutes before rolling, and the rolling temperature is 250-380℃; the reduction per pass is 20-50%, and the total reduction is 60-90%; the recrystallization annealing treatment is as follows: held at 200-350℃ for 5-120 minutes. The low-alloyed high-corrosion-resistant magnesium alloy has an average hydrogen evolution rate of ≤0.072 mL / cm after being immersed in a 5.5 wt.% sodium chloride solution for 14 days 2 / d, an average weight loss rate of ≤0.096 mg / cm 2 / d, and an average corrosion rate of ≤0.201 mm / y.

2. The low-alloy, high-corrosion-resistant magnesium alloy according to claim 1, characterized in that: The Ca, expressed as a percentage by mass: 0.1-0.3%, In: 0.1-0.8%.

3. The low-alloy, high-corrosion-resistant magnesium alloy according to claim 1, characterized in that: The rare earth alloying elements are 0.1-1.8% by mass percentage.