Method for preparing fine-grain weak-texture magnesium alloy plate through differential extrusion

Abstract

The invention belongs to the technical field of magnesium alloy extrusion processing, and particularly relates to a method for preparing a fine-grain weak-texture magnesium alloy plate through differential extrusion. The method comprises the steps that a magnesium alloy cast ingot is prepared, the magnesium alloy cast ingot is subjected to homogenization treatment, the cast ingot subjected to homogenization treatment and a differential extrusion die are preheated, then extrusion is conducted at different temperatures, and the grain refinement and texture weakening of the obtained asymmetric extrusion plate are achieved along with the reduction of the temperature. The process design is reasonable, the structure is simple, and the defects of insufficient plasticity and the like of magnesium alloy plates can be effectively overcome. Large-scale production is easy to realize, and the magnesium alloy application range is expanded.

Description

Technical Field

[0001] This invention belongs to the field of magnesium alloy extrusion processing technology, specifically relating to a method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion. Background Technology

[0002] Magnesium alloys are the least dense structural materials among metals, possessing advantages such as high specific strength and specific stiffness, and easy recyclability. Therefore, they occupy an important position in transportation, aerospace, and other fields, and are hailed as "green energy materials of the 21st century." Magnesium alloys have a close-packed hexagonal crystal structure, resulting in few independent slip systems at room temperature. Room temperature deformation is mainly characterized by basal plane slip, leading to relatively poor formability.

[0003] Invention patent CN202210964881.3 discloses a high-strength, low-texture, low-alloy-content Mg-Zn-Y-Ca-Zr magnesium alloy and its preparation method. This method primarily involves adding high-content alloying elements Zn and rare-earth element Zr, dissolved into the matrix, to weaken the texture and improve the strength and plasticity of the magnesium alloy. Invention patent CN202311347160.9 discloses an apparatus and method for preparing fine-grained, low-texture magnesium alloy sheets using torsion-shear composite extrusion. During extrusion deformation, the alloy material undergoes torsional extrusion with flow diversion. The different frictional forces on the surfaces of the alloy contacting the punch and die allow for differential torsion, thereby weakening the texture and refining the grain size of the sheet. Adding other alloying elements to alter the texture characteristics is too costly for large-scale industrial production. Traditional extrusion or rolling processes still produce a strong matrix texture, leading to anisotropy and tensile-compressive asymmetry in the sheet material, limiting the application of magnesium alloy sheets. Therefore, there is an urgent need to develop a deformation process that is simple in structure, easy to operate, and effectively weakens the texture of magnesium alloy plates and refines the grains, which is of great significance for further promoting the application of magnesium alloys.

[0004] Currently, the main deformation methods for weakening the texture of magnesium alloy sheets include equal-channel angular extrusion, transverse gradient extrusion, asymmetric split-die extrusion, and asymmetric rolling. The aim of all these methods is to introduce additional stress by utilizing different angles and speeds, causing the crystal orientation to deflect during deformation, thereby weakening the texture of the magnesium alloy sheet and giving it good deformability. Invention patent CN202011352901.9 discloses a method for weakening the texture of ATX-based magnesium alloy sheets through asymmetric extrusion. After preparing the sheet through conventional extrusion, it is placed in a self-made asymmetric extrusion die. The proportion of the billet on the left and right sides of the forming channel hole in the die cavity is different, and the excess portion undergoes additional upsetting deformation, resulting in a combined superposition of upsetting and extrusion deformation, achieving a significant weakening of the sheet texture compared to the initially extruded sheet. Invention patent CN102154597A discloses a processing method for refining the grain size and improving the texture of a magnesium alloy sheet / strip on both sides. This method involves placing the magnesium alloy sheet / strip between bending rollers, causing it to bend and deform as it passes through the rollers. The shear stress on the surface layer causes the base surface to deflect, thus improving the original texture. While these deformation methods can effectively weaken the texture and refine the grain size, they still present a series of problems: high equipment costs, complex structural components, and difficulty in mass production. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion. This method employs a differential extrusion die to refine the grain size and weaken the texture of the magnesium alloy sheet, effectively solving the problems of low processing efficiency, complex operation, and high cost associated with existing deformation processes. It also provides new ideas and options for asymmetric extrusion deformation.

[0006] The technical solution of this invention is: A method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion includes the following steps: Step 1: Preparation of magnesium alloy ingots; Step 2: Homogenize the magnesium alloy ingot from Step 1. Step 3: Preheat the ingot and differential extrusion die after homogenization treatment in Step 2, and then perform differential extrusion at 250℃-350℃. As the temperature decreases, the obtained differential extruded sheet has finer grains and weaker texture. The differential extrusion die has different parallel flow channel lengths in the middle, with the lower flow channel being 4mm longer than the upper flow channel, and the extrusion cylinder diameter is 80-90mm. The magnesium alloy mentioned in step one has the following composition by mass percentage: Al: 5~7wt.%, Ca: 2~4wt.%, Zn: 0.5~1.5wt.%, Mn: 0.2~0.5wt.%, with the balance being magnesium (Mg) and unavoidable impurities. The method for preparing the magnesium alloy ingot is as follows: Under a protective gas, pure magnesium, pure aluminum, pure zinc, magnesium-calcium master alloy and magnesium-manganese master alloy are used as raw materials and melted at 730℃-740℃ according to the above mass ratio. After complete melting, the mixture is stirred for 5-10 minutes, then slag is removed, and the mixture is kept at a constant temperature for 5-10 minutes. Finally, the mixture is water-cooled to room temperature to obtain the magnesium alloy ingot.

[0007] Furthermore, in the above-mentioned method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion, the homogenization process in step two is divided into two stages. In the first stage, the temperature is maintained at 390℃-410℃ for 15-17 hours, and in the second stage, the temperature is raised to 500℃-520℃ and maintained for 47-49 hours. During the homogenization process, graphite powder is used to cover the magnesium alloy ingot to prevent excessive oxidation of the magnesium alloy ingot surface. Subsequently, the alloy ingot is quenched in hot water at 60-80℃ to room temperature. Furthermore, in the above-mentioned method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion, in step three, the preheating temperature of the ingot and the differential extrusion die is 330℃-360℃, and the holding time is 0.5-1.5h. The ingot is then placed in the differential extrusion die for extrusion; the extrusion ratio is 15-20, and the extrusion speed is 5-10mm / s. Furthermore, in the above-mentioned method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion... Furthermore, in the above-mentioned method for preparing fine-grained, weakly textured magnesium alloy sheets by differential extrusion, the resulting differentially extruded sheets have a width of 50-60 mm and a thickness of 1-5 mm.

[0008] Using a conventional extrusion die with parallel flow channels of the same length in the middle and at the top and bottom, and an extrusion cylinder diameter of 80-90mm, magnesium alloy sheets were prepared. A comparison was made between magnesium alloy sheets prepared using a differential extrusion die and those prepared using a conventional extrusion die. With the same dimensions, the differential extruded sheet obtained using the differential extrusion die exhibited grain refinement and weakened texture as the temperature decreased. Compared to the conventional extruded sheet, the differential extruded sheet showed finer grains and a weaker texture.

[0009] Advantages and beneficial effects of the present invention: 1. Based on the screening principle of adding Ca to magnesium alloys to improve their ignition point, this invention designs and develops a high-calcium-content flame-retardant magnesium alloy. Fine-grained, weakly textured, high-plasticity flame-retardant magnesium alloy sheets are prepared using conventional extrusion and differential extrusion deformation processes. First, homogenization treatment dissolves Zn into the matrix of the alloy, which is beneficial for second-phase strengthening. Then, the (Mg,Al)₂Ca phase is transformed into the Al₂Ca phase. Finally, differential extrusion is used to prepare fine-grained, weakly textured magnesium alloy sheets, achieving high plasticity. The differential extrusion die has parallel runners of different lengths, with the upper runner being longer than the lower runner. This results in different flow velocities on the upper and lower surfaces of the alloy billet as it passes through the runners during extrusion, creating a velocity difference. The velocity difference introduces additional asymmetric shear stress into the die in the thickness direction, enabling differential extrusion of the material. This promotes the coordination between grains during magnesium alloy extrusion, inducing dynamic recrystallization and resulting in grain refinement of the sheet. The shear stress causes a deflection of the c-axis of the magnesium alloy, with the basal texture shifting by 10°-15° along the extrusion direction, improving the original texture and enhancing the room-temperature mechanical properties of the magnesium alloy. This invention features a rational process design and simple structure, effectively addressing the shortcomings of insufficient plasticity in magnesium alloy sheets. It is easily achievable for large-scale production, expanding the application range of magnesium alloys.

[0010] 2. The differential extrusion process of this invention can not only effectively weaken the base texture and improve room temperature formability, but also further promote recrystallization, refine the grains, make the structure more uniform, and thus improve the performance.

[0011] 3. The extrusion die design of this invention is simpler and easier to operate than the existing asymmetric extrusion die, with higher productivity. It effectively shortens the process of preparing sheet metal with weakened texture and has good prospects for industrial application. Attached Figure Description

[0012] Figure 1 (a) is the reverse pole diagram of the conventional extruded sheet in Example 3, and (b) is the reverse pole diagram of the differential extruded sheet in Example 1; Figure 2 (a) and (b) are the (0001) pole figures of the alloy differentially extruded sheet at 340°C in Example 1 and the alloy differentially extruded sheet at 350°C in Example 2, respectively. Figure 3 (a) and (b) are IPF diagrams of the alloy in Example 1, which was differentially extruded at 340°C, and the alloy in Example 2, which was differentially extruded at 350°C, respectively. (c) is the IPF diagram of the alloy in Example 3, which was conventionally extruded at 340°C.

[0013] Figure 4 This is a schematic diagram of a differential extrusion die. Detailed Implementation

[0014] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0015] The differential extrusion die used in the following embodiments is as follows: Figure 4 As shown, the mold has parallel runners of different lengths in the middle, with the lower runner being 4mm longer than the upper runner, and the extrusion cylinder diameter is 80mm.

[0016] In the following examples, pure magnesium, pure aluminum, pure zinc, magnesium-calcium master alloy and magnesium-manganese master alloy were used as raw materials and melted at 740°C. After complete melting, the mixture was stirred for 10 minutes, then slag was removed, and the mixture was kept at a constant temperature for 10 minutes. Finally, it was water-cooled to room temperature to obtain Mg-6Al-3Ca-1Zn-0.3Mn magnesium alloy ingot.

[0017] Example 1. The Mg-6Al-3Ca-1Zn-0.3Mn magnesium alloy ingot was machined into a cylinder with a diameter of 80 mm and a height of 40 mm, and then subjected to homogenization treatment: first, it was heated to 400℃ and held for 16 h, and then heated to 515℃ and held for 48 h. During the treatment, graphite powder was used to cover the magnesium alloy ingot to prevent excessive oxidation of the surface of the magnesium alloy ingot. Subsequently, the alloy ingot was quenched in hot water at 80℃ to room temperature. The homogenized ingot and differential extrusion die were preheated at 340℃ for 1 hour and then hot extruded to form a sheet with a width of 56mm and a thickness of 5mm. The extrusion temperature was 340℃, the extrusion speed was 10mm / s, and the extrusion ratio was 18:1.

[0018] Example 2 The Mg-6Al-3Ca-1Zn-0.3Mn magnesium alloy ingot was machined into a cylinder with a diameter of 80 mm and a height of 40 mm, and then subjected to homogenization treatment: first, it was heated to 400℃ and held for 16 h, and then heated to 515℃ and held for 48 h. During the treatment, graphite powder was used to cover the magnesium alloy ingot to prevent excessive oxidation of the surface of the magnesium alloy ingot. Subsequently, the alloy ingot was quenched in hot water at 80℃ to room temperature. The homogenized ingot and differential extrusion die were preheated at 350℃ for 1 hour and then hot extruded to form a sheet with a width of 56mm and a thickness of 5mm. The extrusion temperature was 350℃, the extrusion speed was 10mm / s, and the extrusion ratio was 18:1.

[0019] Comparative Example 1 The Mg-6Al-3Ca-1Zn-0.3Mn magnesium alloy ingot was machined into a cylinder with a diameter of 80 mm and a height of 40 mm, and then subjected to homogenization treatment: first, it was heated to 400℃ and held for 16 h, and then heated to 515℃ and held for 48 h. During the treatment, graphite powder was used to cover the magnesium alloy ingot to prevent excessive oxidation of the surface of the magnesium alloy ingot. Subsequently, the alloy ingot was quenched in hot water at 80℃ to room temperature. The homogenized ingot and conventional extrusion die were preheated at 340℃ for 1 hour and then hot extruded to form a sheet with a width of 56mm and a thickness of 5mm. The extrusion temperature was 340℃, the extrusion speed was 10mm / s, and the extrusion ratio was 18:1.

[0020] The difference between Example 1 and Example 2 is the extrusion temperature of differential extrusion. The extrusion temperature of Example 1 is 340°C, and the extrusion temperature of Example 2 is 350°C. All other process parameters are the same. The difference between Example 1 and Comparative Example 1 is the extrusion die. Example 1 uses a differential extrusion die, while Comparative Example 1 uses a conventional extrusion die. All other process parameters are the same.

[0021] Inverse pole figures were obtained by performing texture analysis on the differential extruded sheet material prepared in Example 1 and the conventional extruded sheet material prepared in Comparative Example 1 using EBSD technology. The results are as follows: Figure 1 As shown in (a) and (b).

[0022] from Figure 1 As can be seen from (a) and (b), after differential extrusion and conventional extrusion at 340℃, the pole density of the two extruded plates is relatively dispersed between the <-12-10> and <01-10> poles. Figure 1 (b) The extreme point density is more concentrated in the medium differential extrusion. The texture strength of the conventional extruded sheet is 5.27, while that of the differential extruded sheet is 4.73.

[0023] Texture analysis of the differential extrusion sheets prepared in Examples 1 and 2 was performed using EBSD technology to obtain (0001) pole figures, and the results are as follows: Figure 2 As shown in (a) and (b).

[0024] from Figure 2 As shown in (a) and (b), the alloy exhibits a typical basal texture after differential extrusion at 340℃ and 350℃, with the basal texture deflected by 10°-15° along the extrusion direction. Compared to the sheet extruded at 350℃, the texture strength of the sheet extruded at 340℃ decreased from 12.62 to 8.18, a decrease of 4.44. The texture strength is more dispersed, indicating that differential extrusion and cooling extrusion can significantly weaken the texture of conventionally extruded sheets.

[0025] from Figure 3As can be seen from (a) and (b), differential extrusion at 340℃, compared to differential extrusion at 350℃, results in finer grains and a composition of dynamic recrystallization. From Figure 3 As can be seen from (a) and (c), differential extrusion produces finer grains compared to conventional extrusion. The texture strength of differentially extruded sheets decreases and the grains become finer because differential extrusion introduces additional shear stress, refining the grains and weakening the texture.

[0026] Performance tests were conducted on Example 1, Example 2, and Comparative Example 1, and the test results are shown in Table 1: Table 1. Performance test results of magnesium alloys in the examples and comparative examples. serial number Ultimate tensile strength (MPa) Yield strength (MPa) Elongation (%) Example 1 279 187 16 Example 2 259 153 15 Comparative Example 1 263 157 13 As shown in Table 1, the fine-grained, weakly textured magnesium alloy of the present invention has a maximum tensile strength of 279 MPa at room temperature and a maximum yield strength of 187 MPa. It also has high plasticity and an elongation of up to 16%.

[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method of producing a fine-grained, weakly textured magnesium alloy sheet by differential extrusion, characterized in that, It comprises the following steps: Step one, preparation of magnesium alloy ingot; Step two, homogenization treatment of magnesium alloy ingot in step one; Step three, preheating of the homogenization treated ingot in step two and differential speed extrusion die, then differential speed extrusion at 250-350℃, the obtained differential speed extruded plate is grain refined and texture weakened as the temperature decreases; the differential speed extrusion die has different parallel channel lengths in the middle of the die, the lower channel length is 4mm longer than the upper channel, and the extrusion cylinder diameter is 80-90mm; In step one, the magnesium alloy comprises A1: 5-7wt.%, Ca: 2-4wt.%, Zn: 0.5-1.5wt.%, Mn 0.2-0.5wt.%, and the balance is magnesium Mg and unavoidable impurities; The preparation method of the magnesium alloy ingot is as follows: under the protection of gas, pure magnesium, pure aluminum, pure zinc, magnesium calcium intermediate alloy and magnesium manganese intermediate alloy are used as raw materials for smelting at 730-740℃ according to the above mass ratio, after complete melting, stirring for 5-10min, then slagging, standing for 5-10min, then water cooling to room temperature to obtain magnesium alloy ingot.

2. The method of claim 1, wherein the method is characterized by: The homogenization treatment process in step two is divided into two stages, stage one at 390-410℃ for 15-17h, and stage two at 500-520℃ for 47-49h, graphite powder is used to cover the magnesium alloy ingot during homogenization treatment to prevent excessive oxidation of the magnesium alloy ingot surface, then the alloy ingot is quenched to room temperature with hot water at 60-80℃.

3. The method of claim 1, wherein the method is characterized by: In step three, the preheating temperature of the ingot and differential speed extrusion die is 330-360℃, and the holding time is 0.5-1.5h, the ingot is put into the differential speed extrusion die for extrusion; the extrusion ratio during extrusion is 15-20, and the extrusion speed is 5-10mm / s.

4. The method of claim 1, wherein the method is characterized by: The obtained differential speed extruded plate is 50-60mm wide and 1-5mm thick.

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