Fine-grained heat-stable high-strength high-bake-hardening magnesium alloy and method for producing same

Abstract

This invention provides a fine-grained, heat-stable, high-strength, and bake-hardening magnesium alloy and its preparation method, relating to the field of metal processing. This invention modifies the coarse Al-Mn phase into a high-density nanoscale Al-Mn-RE phase through RE microalloying. The heat resistance of the nanophase is improved through elemental phase boundary segregation, effectively enhancing the pinning effect of the nanoscale Al-Mn-RE phase. Co-segregation at coupled elemental grain boundaries further stabilizes the grain boundaries, suppressing grain coarsening during solid solution treatment. The average grain size can be stabilized at 2-15 micrometers. Furthermore, the density of the strengthening phase precipitated in the high-solid-solution microstructure after baking is extremely high, with a bake-hardening value ≥70 MPa, and some reaching over 90 MPa. The mechanical properties after baking are excellent, with a yield strength ≥260 MPa and an elongation ≥10%.

Description

Technical Field

[0001] This invention relates to the field of metal material processing, specifically to a fine-grained, thermally stable, high-strength, high-bake-hardening magnesium alloy and its preparation method. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials, possess advantages such as low density, high specific strength, and strong damping properties, making them promising candidates for lightweight automotive applications. Baking hardening performance is a key performance indicator for magnesium alloys used in automotive panels and other covering components after stamping, painting, and low-temperature baking. It refers to the material's ability to further enhance its strength after deformation through short-term aging treatment at a specific temperature (typically aging at 170-200°C for 20-30 minutes).

[0003] Compared to typical bake-hardening materials used in automotive body panels, such as low-carbon steel and 6xxx series aluminum alloys, magnesium alloys have relatively low initial absolute strength (strength difference exceeding 100 MPa), necessitating higher bake-hardening values ​​to compensate for this weakness. Currently, conventional bake-hardening magnesium alloys suffer from an inherent contradiction: fine-grained microstructure and high-solution microstructure are incompatible. High-temperature solution treatment (up to 500°C) is typically required before bake-hardening, resulting in poor thermal stability of the fine-grained microstructure, which easily coarsens to tens or even hundreds of micrometers, leading to severe loss of fine-grained strengthening. To stabilize the fine-grained microstructure, a common strategy is to perform incomplete solution treatment before grain coarsening, sacrificing some age-hardening effects to achieve a balance between age-hardening and fine-grained strengthening. After 2% pre-strain and paint heat treatment, the bake-hardening value of magnesium alloys is typically ≤60 MPa. Therefore, achieving high bake-hardening properties of magnesium alloys based on high-solution thermal stability fine-grained microstructure control is of great significance for the widespread application of magnesium alloys in automotive production. Summary of the Invention

[0004] To address the technical problems of coarse and low-density Al-Mn phases in traditional magnesium alloys, which cannot effectively pin grains, and grains easily coarsen to over 20 micrometers after high-temperature solution treatment, and the strengthening effect of precipitated phases after baking is weak, with bake-hardening values ​​typically ≤60MPa, this invention provides a fine-grained, thermally stable, high-strength, and high-baked-hardening magnesium alloy and its preparation method. Through RE microalloying, the coarse Al-Mn phase is modified into a high-density nanoscale Al-Mn-RE phase. Segregation at elemental phase boundaries improves the heat resistance of the nanoscale phase, effectively enhancing the pinning effect of the nanoscale Al-Mn-RE phase. Co-segregation at coupled elemental grain boundaries further stabilizes the grain boundaries, suppressing grain coarsening during solution treatment. The average grain size can be stabilized at 2-15 micrometers. Furthermore, the density of the strengthening phase precipitated in the high-solid-solution microstructure after baking is extremely high, with bake-hardening values ​​≥70MPa, and some reaching over 90MPa. The mechanical properties after baking are excellent, with a yield strength ≥260MPa and an elongation ≥10%.

[0005] Therefore, the present invention provides the following technical solution:

[0006] On the one hand, the present invention provides a fine-grained, heat-stable, high-strength, high-bake-hardening magnesium alloy, the alloy composition by mass percentage being: Al: 0.5-1.5%, Ca: 0.2-0.6%, Mn: 0.5-2.0%, Zn: 0.01-1.0%, RE: 0.1-0.7%, the remainder being Mg and unavoidable impurities, the total impurity content not exceeding 0.05%; wherein 0.3≤Al / Mn≤2.5, 0.1≤RE / Mn≤1.2.

[0007] Furthermore, the alloy composition by mass percentage is: Al: 0.6-1.4%, Ca: 0.2-0.5%, Mn: 0.6-1.8%, Zn: 0.01-0.8%, RE: 0.2-0.6%.

[0008] Furthermore, the RE component is one or any combination of Gd, Y, Nd, Ce, and Sm.

[0009] Furthermore, the fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy has an average grain size ≤15 micrometers, a bake-hardening value ≥70MPa, a yield strength ≥260MPa after baking, and an elongation ≥10%.

[0010] On the other hand, the present invention also provides a method for preparing the above-mentioned fine-grained, heat-stable, high-strength, and bake-hardening magnesium alloy, comprising the following steps: Under inert gas protection, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-RE master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 690-760°C, the mixture is stirred evenly. Then, the temperature is lowered to 680-720°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots. The magnesium alloy ingot is subjected to a step homogenization treatment of holding at 300-350°C for 1-2 hours and at 450-500°C for 3-6 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot. The homogeneous alloy ingot is extruded or rolled in multiple passes to form magnesium alloy sheets. The magnesium alloy sheet is subjected to a solution treatment to obtain a solution-treated magnesium alloy sheet. The solid solution magnesium alloy sheet is first pre-deformed, and then subjected to short-time aging treatment.

[0011] Furthermore, in the extrusion deformation, the extrusion temperature is 400-450°C, the extrusion ratio is 20-50, and the extrusion speed is 4-5 m / min.

[0012] Furthermore, the multi-pass rolling is 4-13 passes, with a pass reduction of 10-40% and a total reduction of ≥80%. Each pass is held at 280-350℃ for 5-20 minutes before rolling.

[0013] Furthermore, the solution treatment is performed by holding at 480-520℃ for 10-30 minutes.

[0014] Furthermore, the short-time aging treatment is aging at 180-200℃ for 20-30 minutes.

[0015] Furthermore, the process of pre-deforming the obtained solid solution magnesium alloy sheet includes: pre-deforming the obtained solid solution magnesium alloy sheet by 2%.

[0016] Compared with existing technologies, this invention has the following advantages through the synergistic regulation of components, processes, and structures: (1) Compared with the prior art, the alloy composition of the present invention has the characteristics of multi-element micro-alloying and low cost, reducing the amount of single alloying element added. The amount of rare earth element RE added is less than 1 wt.%, which has industrialization prospects. The composition ratio was optimized according to 0.3≤Al / Mn≤2.5 and 0.1≤RE / Mn≤1.2. The alloy composition is suitable for the control of microstructure and macro properties of fine-grained, thermally stable, high-strength, and high-baking-hardening magnesium alloys.

[0017] (2) The preparation process of this invention is simple. The alloy ingot can be extruded / rolled after simple homogenization treatment. It has good processing formability, no edge cracks in the alloy plate, and high yield. It is suitable for the industrial production of large-size magnesium alloy plates. Detailed Implementation

[0018] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0019] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0020] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0021] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0022] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0023] Example 1 Taking a magnesium alloy of Mg-1.0Al-0.4Ca-0.6Mn-0.3Gd-0.01Zn (wt.%) as an example, where Al / Mn=1.7 and RE / Mn=0.5, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-Gd master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 740°C, the mixture is stirred evenly. Then the temperature is lowered to 720°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0024] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 330°C for 1 hour and 500°C for 3 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0025] (3) The homogeneous alloy ingot obtained in step (2) is extruded and deformed into magnesium alloy sheet. The extrusion temperature is 450°C, the extrusion ratio is 35, and the extrusion speed is 4.5m / min.

[0026] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 510℃ for 30 minutes to obtain a solution-treated magnesium alloy sheet.

[0027] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 190℃ for 30 minutes.

[0028] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 510℃ for 30 min, with an average grain size of 13 micrometers. After a 2% pre-deformation and a short-time aging treatment at 190℃ for 30 min, the bake-hardening value was 94 MPa, the yield strength after baking was 281 MPa, and the elongation was 12%.

[0029] Example 2 Taking a magnesium alloy of Mg-1.2Al-0.3Ca-0.5Mn-0.2Nd-0.4Zn (wt.%) as an example, where Al / Mn=2.4 and RE / Mn=0.4, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-Nd master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 730°C, the mixture is stirred evenly. Then the temperature is lowered to 710°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0030] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 300°C for 2 hours and at 480°C for 4 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0031] (3) The homogeneous alloy ingot obtained in step (2) is rolled into magnesium alloy sheet through multiple passes. The multiple passes are 10 passes with a reduction of 15-17% per pass and a total reduction of 85%. Before each pass, the sheet is held at 300℃ for 10 minutes.

[0032] (4) The magnesium alloy plate obtained in step (3) is subjected to a solution treatment at 500℃ for 10 minutes to obtain a solution-treated magnesium alloy plate.

[0033] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 180℃ for 30 minutes.

[0034] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 500℃ for 10 min, resulting in an average grain size of 9 micrometers. After a 2% pre-deformation and a short-term aging treatment at 180℃ for 30 min, the bake-hardening value was 80 MPa, the yield strength after baking was 275 MPa, and the elongation was 15%.

[0035] Example 3 Taking a magnesium alloy of Mg-1.4Al-0.5Ca-0.6Mn-0.3Gd-0.2Y-0.1Sm-0.2Zn (wt.%) as an example, where Al / Mn=2.3 and RE / Mn=1, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy, Mg-Gd master alloy, Mg-Y master alloy and Mg-Sm master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 760°C, the mixture is stirred evenly, then cooled to 700°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0036] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 310°C for 1 hour and 500°C for 6 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0037] (3) The homogeneous alloy ingot obtained in step (2) is rolled into magnesium alloy sheet through multiple passes. The multiple passes are 13 passes with a reduction of 10-12% per pass and a total reduction of 80%. Before each pass, the sheet is held at 330℃ for 10 minutes.

[0038] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 480℃ for 20 minutes to obtain a solution-treated magnesium alloy sheet.

[0039] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 200℃ for 30 minutes.

[0040] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 480℃ for 20 minutes, resulting in an average grain size of 7 micrometers. After a 2% pre-deformation and a short-term aging treatment at 200℃ for 30 minutes, the bake-hardening value was 91 MPa, the yield strength after baking was 292 MPa, and the elongation was 14%.

[0041] Example 4 Taking a magnesium alloy of Mg-0.6Al-0.25Ca-0.5Mn-0.2Gd-0.2Ce-0.1Nd-0.4Zn (wt.%) as an example, where Al / Mn=1.2 and RE / Mn=1, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy, Mg-Gd master alloy, Mg-Ce master alloy and Mg-Nd master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 710°C, the mixture is stirred evenly, then cooled to 690°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0042] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 320°C for 2 hours and 450°C for 6 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0043] (3) The homogeneous alloy ingot obtained in step (2) is extruded and deformed into magnesium alloy sheet. The extrusion temperature is 440°C, the extrusion ratio is 20, and the extrusion speed is 5m / min.

[0044] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 490℃ for 30 minutes to obtain a solution-treated magnesium alloy sheet.

[0045] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 190℃ for 30 minutes.

[0046] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 490℃ for 30 minutes, resulting in an average grain size of 8 micrometers. After a 2% pre-deformation and a short-time aging treatment at 190℃ for 30 minutes, the bake-hardening value was 70 MPa, the yield strength after baking was 268 MPa, and the elongation was 16%.

[0047] Example 5 Taking a magnesium alloy of Mg-0.8Al-0.3Ca-1.8Mn-0.2Y-0.5Zn (wt.%) as an example, where Al / Mn=0.4 and RE / Mn=0.1, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-Y master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 750°C, the mixture is stirred evenly. Then the temperature is lowered to 700°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0048] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 330°C for 2 hours and 480°C for 2 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0049] (3) The homogeneous alloy ingot obtained in step (2) is extruded and deformed into magnesium alloy sheet. The extrusion temperature is 400°C, the extrusion ratio is 50, and the extrusion speed is 4m / min.

[0050] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 480℃ for 10 minutes to obtain a solution-treated magnesium alloy sheet.

[0051] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 180℃ for 30 minutes.

[0052] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 480℃ for 10 min, with an average grain size of 3 micrometers. After a 2% pre-deformation and a short-time aging treatment at 180℃ for 30 min, the bake-hardening value was 77 MPa, the yield strength after baking was 305 MPa, and the elongation was 16%.

[0053] Example 6 Taking a magnesium alloy of Mg-0.7Al-0.25Ca-0.9Mn-0.3Sm-0.8Zn (wt.%) as an example, where Al / Mn=0.8 and RE / Mn=0.3, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-Sm master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 730°C, the mixture is stirred evenly. Then the temperature is lowered to 720°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0054] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 340°C for 1 hour and 460°C for 2 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0055] (3) The homogeneous alloy ingot obtained in step (2) is rolled into magnesium alloy sheet through multiple passes. The multiple passes are 7 passes with a reduction of 20-25% per pass and a total reduction of 85%. Before each pass, the sheet is held at 280℃ for 20 minutes.

[0056] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 510℃ for 30 minutes to obtain a solution-treated magnesium alloy sheet.

[0057] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 200℃ for 30 minutes.

[0058] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 510℃ for 30 minutes, resulting in an average grain size of 5 micrometers. After a 2% pre-deformation and a short-time aging treatment at 200℃ for 30 minutes, the bake-hardening value was 72 MPa, the yield strength after baking was 283 MPa, and the elongation was 18%.

[0059] Example 7 Taking a magnesium alloy of Mg-1.3Al-0.5Ca-1.2Mn-0.2Ce-0.6Zn (wt.%) as an example, where Al / Mn=1.1 and RE / Mn=0.2, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-Ce master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 740°C, the mixture is stirred evenly. Then the temperature is lowered to 700°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0060] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 350°C for 2 hours and 490°C for 4 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0061] (3) The homogeneous alloy ingot obtained in step (2) is extruded and deformed into magnesium alloy sheet. The extrusion temperature is 420°C, the extrusion ratio is 45, and the extrusion speed is 4.5m / min.

[0062] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 500℃ for 20 minutes to obtain a solution-treated magnesium alloy sheet.

[0063] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 200℃ for 20 minutes.

[0064] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 500℃ for 20 minutes, resulting in an average grain size of 12 micrometers. After a 2% pre-deformation and a short-time aging treatment at 200℃ for 20 minutes, the bake-hardening value was 96 MPa, the yield strength after baking was 284 MPa, and the elongation was 17%.

[0065] Example 8 Taking a magnesium alloy of Mg-0.9Al-0.4Ca-1.4Mn-0.4Ce-0.2Sm-0.5Zn (wt.%) as an example, where Al / Mn=0.6 and RE / Mn=0.4, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy, Mg-Ce master alloy and Mg-Sm master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 750°C, the mixture is stirred evenly. Then the temperature is lowered to 710°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0066] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 320°C for 2 hours and 500°C for 5 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0067] (3) The homogeneous alloy ingot obtained in step (2) is rolled into magnesium alloy sheet through multiple passes. The multiple passes are 4 passes with a reduction of 30-40% per pass and a total reduction of 80%. Before each pass, the sheet is held at 350℃ for 5 minutes.

[0068] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 520℃ for 10 minutes to obtain a solution-treated magnesium alloy sheet.

[0069] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 180℃ for 20 minutes.

[0070] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 520℃ for 10 min, resulting in an average grain size of 7 micrometers. After a 2% pre-deformation and a short-time aging treatment at 180℃ for 20 min, the bake-hardening value was 75 MPa, the yield strength after baking was 276 MPa, and the elongation was 15.5%.

[0071] Example 9 Taking a magnesium alloy of Mg-0.5Al-0.3Ca-0.5Mn-0.1Gd-0.1Nd-0.05Zn (wt.%) as an example, where Al / Mn=1 and RE / Mn=0.4, the preparation method is as follows: (1) Under the protection of inert gas, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy, Mg-Gd master alloy and Mg-Nd master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 690°C, the mixture is stirred evenly. Then the temperature is lowered to 680°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots.

[0072] (2) The magnesium alloy ingot obtained in step (1) is subjected to a step homogenization treatment of holding at 330°C for 1 hour and 470°C for 1 hour, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot.

[0073] (3) The homogeneous alloy ingot obtained in step (2) is extruded and deformed into magnesium alloy sheet. The extrusion temperature is 430°C, the extrusion ratio is 40, and the extrusion speed is 4m / min.

[0074] (4) The magnesium alloy sheet obtained in step (3) is subjected to a solution treatment at 480℃ for 30 minutes to obtain a solution-treated magnesium alloy sheet.

[0075] (5) The solid solution magnesium alloy plate obtained in step (4) is first subjected to 2% pre-deformation, and then subjected to short-time aging treatment at 190℃ for 20 minutes.

[0076] The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy obtained was subjected to a solution treatment at 480℃ for 30 minutes, resulting in an average grain size of 10 micrometers. After a 2% pre-deformation and a short-time aging treatment at 190℃ for 20 minutes, the bake-hardening value was 85 MPa, the yield strength after baking was 277 MPa, and the elongation was 13%.

[0077] Comparative Example 1 Comparative paper: Authors Yi-Jia Li et al., Journal *Materials Science and Engineering:A*, Volume (Publication Year), Page: 831 (2022) 142239. The paper reports a Mg-2Zn-0.5Ca (wt.%) bake-hardening magnesium alloy with the following composition: 2 wt.% Zn, 0.5 wt.% Ca, and the remainder Mg. Preparation process: sub-rapid solidification, 3-pass rolling, solution treatment at 450°C for 60 min, 2% pre-deformation + baking at 175°C for 30 min. Average grain size: 89 μm, bake-hardening value: 60 MPa, yield strength after baking: 200 MPa, elongation: 17%.

[0078] Comparative Example 1 used two alloying elements: a relatively high content of 2 wt% Zn and a low content of 0.5 wt.% Ca. The composition design only considered bake hardening, and the preparation process did not involve extrusion. Its rolled microstructure lacked fine-grained thermal stability, and the grains coarsened to 89 micrometers after high-temperature solution treatment. In addition, the bake hardening value was low, only 60 MPa; the yield strength after baking was also low, only 200 MPa. This invention adopts a multi-element microalloying composition design strategy of Al, Ca, Mn, RE, and Zn, and optimizes the composition ratio (0.3≤Al / Mn≤2.5, 0.1≤RE / Mn≤1.2). The preparation process can be both rolled and extruded. Through the control of composition, ratio, process, and process parameters, the rolled / extruded microstructure of this invention has significantly higher fine-grained thermal stability, and the bake hardening value and strength after baking are also significantly higher than those of Comparative Example 1: the average grain size after solution treatment is ≤15 micrometers, the bake hardening value is ≥70 MPa, and the yield strength after baking is ≥260 MPa.

[0079] Furthermore, each embodiment of the present invention uses different components, proportions, processes, and process parameters, but the resulting material properties are different. This indicates that the effect obtained by the present invention is not determined by a single component, proportion, process, or process parameter, but is achieved through the interaction of components, the synergistic regulation of proportions, processes, and process parameters. Moreover, the best technical effect can only be achieved within the scope of the claims.

Claims

1. A fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy, characterized in that, The alloy composition by mass percentage is as follows: Al: 0.5-1.5%, Ca: 0.2-0.6%, Mn: 0.5-2.0%, Zn: 0.01-1.0%, RE: 0.1-0.7%, with the remainder being Mg and unavoidable impurities, the total impurity content not exceeding 0.05%; wherein 0.3≤Al / Mn≤2.5, 0.1≤RE / Mn≤1.

2.

2. The fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to claim 1, characterized in that, The alloy composition by mass percentage is: Al: 0.6-1.4%, Ca: 0.2-0.5%, Mn: 0.6-1.8%, Zn: 0.01-0.8%, RE: 0.2-0.6%.

3. The fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to claim 1, characterized in that, The RE component is one or any combination of Gd, Y, Nd, Ce, and Sm.

4. A fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to any one of claims 1 to 3, characterized in that, The fine-grained, thermally stable, high-strength, and bake-hardening magnesium alloy has an average grain size of ≤15 micrometers, a bake-hardening value of ≥70MPa, a yield strength of ≥260MPa after baking, and an elongation of ≥10%.

5. A method for preparing a fine-grained, heat-stable, high-strength, bake-hardening magnesium alloy as described in any one of claims 1 to 4, characterized in that, Includes the following steps: Under inert gas protection, the raw materials of pure Mg, pure Al, pure Zn, Mg-Mn master alloy, Mg-Ca master alloy and Mg-RE master alloy of the prepared quality are added to the melting furnace in sequence. After melting at 690-760°C, the mixture is stirred evenly. Then, the temperature is lowered to 680-720°C for refining, degassing and slag removal to obtain alloy liquid, which is then cast into magnesium alloy ingots. The magnesium alloy ingot is subjected to a step homogenization treatment of holding at 300-350°C for 1-2 hours and at 450-500°C for 3-6 hours, then air-cooled to room temperature, and the alloy surface is mechanically polished to obtain a homogeneous alloy ingot. The homogeneous alloy ingot is extruded or rolled in multiple passes to form magnesium alloy sheets. The magnesium alloy sheet is subjected to a solution treatment to obtain a solution-treated magnesium alloy sheet. The solid solution magnesium alloy sheet is first pre-deformed, and then subjected to short-time aging treatment.

6. The method for preparing a fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to claim 5, characterized in that, In the extrusion deformation, the extrusion temperature is 400-450°C, the extrusion ratio is 20-50, and the extrusion speed is 4-5m / min.

7. The method for preparing a fine-grained, heat-stable, high-strength, bake-hardening magnesium alloy according to claim 5, characterized in that, The multi-pass rolling process consists of 4-13 passes, with a pass reduction of 10-40% and a total reduction of ≥80%. Each pass is held at 280-350℃ for 5-20 minutes before each pass.

8. The method for preparing a fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to claim 5, characterized in that, The solution treatment involves holding the solution at 480-520℃ for 10-30 minutes.

9. The method for preparing a fine-grained, heat-stabilized, high-strength, bake-hardening magnesium alloy according to claim 5, characterized in that, The short-time aging treatment is aging at 180-200℃ for 20-30 minutes.

10. The method for preparing a fine-grained, heat-stable, high-strength, bake-hardening magnesium alloy according to claim 5, characterized in that, The process of pre-deforming the obtained solid solution magnesium alloy sheet includes: pre-deforming the obtained solid solution magnesium alloy sheet by 2%.