Magnesium alloy plate thickness gradient grading differential extrusion die

Abstract

The invention discloses a magnesium alloy plate thickness gradient grading differential extrusion die which comprises a forming die and a process die, and one side of the forming die is detachably and fixedly connected with one side of the process die; a process hole is formed in the process die, a forming hole is formed in the forming die, and the forming hole is communicated with the process hole; an outlet area is arranged at the end, close to the auxiliary hole, of the forming hole, a shear deformation area is arranged at the end, away from the auxiliary hole, of the forming hole, a working belt is further arranged between the outlet area and the shear deformation area, and a first step-shaped differential shear belt and a second step-shaped differential shear belt are arranged on the two opposite side surfaces of the shear deformation area in the width direction of the shear deformation area respectively. The first stepped differential shear belt and the second stepped differential shear belt are both of a multi-stage stepped structure; the first step-shaped differential shear belt and the second step-shaped differential shear belt are of an asymmetric structure in the direction of the central axis of the working belt.

Description

Technical Field

[0001] The invention relates to the technical field of magnesium alloy plastic processing, and in particular to a magnesium alloy plate thickness-direction gradient graded differential speed extrusion die. Background Art

[0002] Magnesium alloy has the advantages of low density, high specific strength, good damping and vibration reduction, good electromagnetic shielding, excellent thermal conductivity and abundant resources. It is the most promising lightweight material with integrated structure and function, which can meet the needs of major equipment upgrades in the fields of aerospace, rail transportation, electronic communications, etc. However, due to the close-packed hexagonal crystal structure of magnesium alloy, it is easy to produce strong basal texture during plastic processing, resulting in a significant decrease in the elongation and processing performance of the material, which seriously restricts the deep processing and application of magnesium alloy sheets.

[0003] In order to solve the problem of strong basal texture generated during the plastic processing of magnesium alloy, some studies have introduced shear force to achieve texture control of magnesium alloy extruded sheet through asymmetric extrusion die structure design. Different from the traditional symmetric extrusion process of magnesium alloy, asymmetric extrusion of magnesium alloy sheet forms flow velocity difference due to shear force, which makes the hard-to-deform second phase in the alloy fully broken under the action of complex constraints, thereby promoting the dynamic recrystallization process to weaken the texture; at the same time, the asymmetric extrusion die structure changes the strain path of the alloy, causing the direction of part of the basal texture to deflect, thereby reducing the texture strength.

[0004] However, in the prior art, the magnesium alloy asymmetric extrusion die designed based on the above ideas still has some problems in the actual forming process, and the effect of weakening the base surface texture of the extruded magnesium alloy sheet is still unsatisfactory. Patent CN219985767 discloses an asymmetric extrusion die for weakening the texture of the magnesium alloy. It only adopts the design of asymmetric distribution of the end face along the central axis where the shear deformation zone communicates with the working belt, but does not use the multi-stage thinning deformation zone design. In actual extrusion forming, the magnesium alloy will have a large deformation resistance and high load, which affects the forming efficiency. Patent CN114406025 discloses a magnesium alloy extruded sheet and its extrusion device and extrusion method. A torsional deformation zone is set between the feed port and the discharge port so that the upper and lower surfaces of the sheet have a flow rate difference, thereby weakening the texture, improving plasticity and forming performance, but it has higher requirements and greater difficulty in processing molds of different specifications. Patent CN105032964 discloses a continuous extrusion processing device and extrusion processing method for magnesium alloy thin strip with weak base texture, which adopts an asymmetric design of the shear deformation zone along the central axis, and has the effect of step-by-step thinning and grain refinement, but its stepped differential shear band angle is small and only considers magnesium alloy thin strip, and the texture weakening effect is insufficient for thicker plates. Patent CN115592022 discloses a double asymmetric large extrusion ratio gradient extrusion die, based on the setting of the asymmetric shear deformation zone, the working zone and the exit zone axis are offset along the radial direction of the die to correct the asymmetric coefficient ratio to obtain a magnesium alloy plate with uniform structure, but the deformation zone is still essentially a symmetrical design, and the differential shear effect is weak; it can be seen that the above patents have some problems in the actual forming process and texture weakening effect. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a magnesium alloy sheet material with a thickness gradient graded differential extrusion die. Based on the forming characteristics of the magnesium alloy sheet material, a shear deformation zone is set in the forming hole of the die, and an asymmetric first step differential shear band and a second step differential shear band are further set in the shear deformation zone. During the extrusion process, the magnesium alloy forms an asymmetric flow velocity difference along the thickness gradient direction on both sides of the central axis, changes the alloy strain path, and achieves the effect of weakening the magnesium alloy texture, so as to solve the problem of the existence of strong basal surface texture of magnesium alloy after traditional extrusion in the prior art. Ultimately, while solving the actual production pain points, the basal surface texture of the magnesium alloy is effectively weakened and the mechanical properties of the material are improved.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A magnesium alloy plate thickness gradient graded differential extrusion die, the die comprising a forming die and a process die, one side of the forming die is detachably fixedly connected to one side of the process die; the process die is provided with a process hole, the two ends of the process hole penetrate the opposite side walls of the process die; the forming die is provided with a forming hole, the two ends of the forming hole penetrate the opposite side walls of the forming die, and the forming hole is connected with the process hole; an exit zone is provided at one end of the forming hole close to the process hole, a shear deformation zone is provided at one end of the forming hole away from the process hole, and a working belt is further provided between the exit zone and the shear deformation zone, one end of the shear deformation zone is connected with one end of the working belt, the other end of the working belt is connected with one end of the exit zone, and the other end of the exit zone is connected with the process hole; the opposite side surfaces of the shear deformation zone along the width direction are respectively a first step differential shear belt and a second step differential shear belt; the first step differential shear belt and the second step differential shear belt are both multi-step structures; the first step differential shear belt and the second step differential shear belt are asymmetric structures along the central axis direction of the working belt.

[0008] Preferably, calculated along the length direction of the forming hole, the height of the forming die is H0, the height of the shear deformation zone is H1, and H0 and H1 satisfy the following relationship:

[0009] H1>H0÷2.

[0010] Preferably, calculated along the width direction of the forming hole, the thickness of the shear deformation zone is D1, the thickness of the working zone is D2, and the thickness of the outlet zone is D3, and the following relationship is satisfied:

[0011] D1>D3>D2.

[0012] Preferably, calculated along the width direction of the forming hole, the second stepped differential shear band is a double-step structure, and the thickness of a single step is D0; D0 and D1, D2 satisfy the following relationship:

[0013] D0<(D1-D2)÷2.

[0014] Preferably, calculated along the length direction of the forming hole, when the first step-shaped differential shear band is an n-step structure, the straight-line distance between the shear deformation zone and the working band is L0, and the length of each step is L1, L2, L3...Ln-1, Ln, and L0≤L1≤L2≤L3...Ln-1≤Ln.

[0015] Preferably, the end of the working belt away from the process hole is bent in a direction away from the central axis of the working belt and then continues to extend to form a first end face and a second end face respectively; wherein, the first end face is located on the first step-shaped differential shear band, and the second end face is located on the second step-shaped differential shear band; the angle between the first end face and the central axis of the working belt is 60 to 90°, and the angle between the second end face and the central axis of the working belt is 60 to 90°.

[0016] Preferably, the extrusion coefficient of the mold is 9-51.

[0017] The invention provides a method for thickness-direction gradient differential extrusion of a magnesium alloy plate, and the above-mentioned die is used to extrude the magnesium alloy plate to obtain the magnesium alloy plate.

[0018] Preferably, the specific steps are as follows:

[0019] Step 1: homogenizing the as-cast magnesium alloy billet and then water quenching; wherein the homogenizing temperature is 320° C. to 520° C. and the holding time is 8 to 20 hours;

[0020] Step 2: Heat the die and the magnesium alloy billet to an extrusion temperature of 300°C to 450°C, respectively, and keep them warm for 10 to 20 minutes;

[0021] Step 3: Install the preheated die on the extruder, and the extrusion barrel is detachably fixedly connected to the forming die in the die, so that the discharge end of the extrusion barrel is connected to the shear deformation zone on the forming hole in the forming die, and then put the preheated magnesium alloy billet into the extrusion barrel;

[0022] Step 4: The magnesium alloy billet is extruded by an extruder. The magnesium alloy billet passes through the shear deformation zone and the working zone in the forming die in sequence under the thrust of the extrusion rod, and finally a magnesium alloy sheet is obtained. The extrusion speed is 0.1-5 mm / s.

[0023] Preferably, the magnesium alloy sheet extruded by the extrusion method has a thickness of 2 to 5 mm and a width of 56 to 112 mm.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention conducts in-depth research and improvement on the structure of the mold. By designing and improving the molding hole structure of the molding die, a shear deformation zone is formed in the plastic deformation channel of the molding die, and then different stepped differential shear bands are arranged on both sides of the shear deformation zone. At the same time, the symmetry between the shear deformation zone and the end surface of the working band is adjusted, so that the flow velocity of the magnesium alloy billet during extrusion forming is asymmetrically distributed along the central axis, forming shear deformation, thereby effectively crushing the second phase in the magnesium alloy and achieving the effect of grain refinement. At the same time, the asymmetrical distribution design of the stepped differential shear band changes the strain path of the material, deflects the base surface texture direction to a certain extent, thereby weakening the texture, which is conducive to further deep processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the structure of the thickness-direction gradient graded differential speed extrusion die for the magnesium alloy sheet of the present invention.

[0027] Figure 2 It is a schematic diagram of the parts corresponding to various sizes in the mold described in the present invention.

[0028] Figure 3 It is a schematic diagram of the mold size and structure used in Example 1; wherein a is the mold size diagram used in Example 1, and b is a schematic diagram of the structure of the mold forming hole.

[0029] Figure 4 It is a schematic diagram of the mold size and structure used in Example 2; wherein a is the mold size diagram used in Example 2, and b is a schematic diagram of the structure of the mold forming hole.

[0030] Figure 5 It is a simulation result diagram of the flow velocity in the shear deformation zone and working zone of the forming die; among them, a is a traditional die, b is embodiment 1, and c is embodiment 2.

[0031] Figure 6 3 is a comparison diagram of the experimental results of the {0001} pole figure of the alloy after extrusion; wherein a is a traditional die, b is Example 1, and c is Example 2.

[0032] Figure 7 Schematic diagram of the mold used in Comparative Examples 1 and 2; wherein a is the mold size diagram used in Comparative Examples 1 and 2, and b is the structural schematic diagram of the mold.

[0033] In the figure, the forming die 1, the process die 2, the process hole 3, the outlet area 4, the shear deformation area 5, the working zone 6, the first step differential shear zone 7, the second step differential shear zone 8, the first end face 9, and the second end face 10. DETAILED DESCRIPTION

[0034] The present invention will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the present invention belong to the scope of protection of the present invention.

[0035] Unless otherwise indicated in specific cases in the present invention, the numerical ranges listed herein include the upper and lower limits, and all integers and fractions within the range, and are not limited to the specific values ​​listed when defining the range.

[0036] 1. A magnesium alloy plate thickness gradient graded differential extrusion die

[0037] The mold includes a forming mold 1 and a process mold 2, one side of the forming mold 1 is detachably fixedly connected to one side of the process mold 2; a process hole 3 is provided on the process mold 2, and both ends of the process hole 3 penetrate the opposite side walls of the process mold 2; a forming hole is provided on the forming mold 1, and both ends of the forming hole penetrate the opposite side walls of the forming mold 1, and the forming hole is connected to the process hole 3, and the axis of the forming hole coincides with the axis of the process hole. An outlet zone 4 is provided at one end of the forming hole close to the process hole 3, a shear deformation zone 5 is provided at one end of the forming hole away from the process hole 3, a working belt 6 is also provided between the outlet zone 4 and the shear deformation zone 5, one end of the shear deformation zone 5 is connected with one end of the working belt 6, the other end of the working belt 6 is connected with one end of the outlet zone 4, and the other end of the outlet zone 4 is connected with the process hole 3; the opposite side surfaces of the shear deformation zone 5 along its width direction are respectively a first step differential shear band 7 and a second step differential shear band 8; the first step differential shear band 7 and the second step differential shear band 8 are both multi-step structures; the first step differential shear band 7 and the second step differential shear band 8 are asymmetric structures along the central axis direction of the working band 6.

[0038] The present invention found that, when studying the existing magnesium alloy extrusion die, magnesium alloys are prone to form a strong basal texture (0002) during plastic processing due to their unique hexagonal close-packed (HCP) crystal structure, which not only reduces the elongation and processing performance of the material, but also limits the further deep processing and application of magnesium alloy sheets. Although the prior art introduces shear deformation to regulate the texture through the asymmetric extrusion die structure design, it is still easy to form a basal texture, and the weakening effect is not good. The present invention conceives to redesign the structure of the die, and adopts a multi-stage stepped differential shear band design so that the material undergoes continuous changes when passing through different parts of the die, thereby realizing the step-by-step regulation of the magnesium alloy structure. This step-by-step regulation helps to refine the grains and further improve the mechanical properties and forming properties of the material. At the same time, the asymmetrically distributed working band changes the alloy strain path, causing the direction of part of the basal texture to deflect, thereby reducing the texture strength. Compared with the prior art, the present invention can weaken the basal texture of the magnesium alloy sheet to a greater extent, and improve the plasticity and forming properties of the material. When the magnesium alloy billet is extruded, due to the asymmetric distribution of the flow rate along the central axis, significant shear deformation is formed, which effectively breaks up the difficult-to-deform second phase in the magnesium alloy and promotes the occurrence of the dynamic recrystallization process, thereby achieving the purpose of weakening the texture. The design of the mold described in the present invention takes into account the flow characteristics of the material during the entire extrusion process, ensuring that the ideal texture regulation effect can be achieved regardless of the product specifications. Therefore, the mold described in the present invention is not only suitable for the production of thin plates and strips, but also can be used to produce magnesium alloy plates of different thicknesses and sizes. Furthermore, the present invention can not only improve the plasticity and forming properties of the product by effectively regulating the texture of the magnesium alloy plate, but also reduce defects caused by strong base surface texture, such as cracks and delamination. In practical applications, the mold described in the present invention is suitable for the extrusion process of any magnesium alloy material.

[0039] In some embodiments of the present invention, Figures 1 to 4 As shown, the inlet of the forming hole, that is, the end of the forming hole away from the outlet area, is elliptical from the perspective of the inlet of the forming hole. Calculated along the length direction of the forming hole, the height of the forming die is H0, and the height of the shear deformation zone is H1. In order to reduce the extrusion load and improve the forming efficiency, H0 and H1 satisfy the following relationship:

[0040] H1>H0÷2.

[0041] In some embodiments of the present invention, Figures 1-2 As shown, calculated along the width direction of the forming hole, the thickness of the shear deformation zone is D1, the thickness of the working zone is D2, and the thickness of the outlet zone is D3. In order to facilitate the subsequent demoulding process and ensure the working life of the mold, the thickness of the outlet zone D3 should satisfy the following relationship:

[0042] D1>D3>D2.

[0043] In some embodiments of the present invention, Figures 1-2 As shown, calculated along the width direction of the forming hole, the second step-shaped differential shear band is a double-step structure, and the thickness of a single step is D0; to ensure the continuity of the forming process, D0 and D1, D2 satisfy the following relationship:

[0044] D0<(D1-D2)÷2.

[0045] In some embodiments of the present invention, Figures 1-2 As shown, calculated along the length direction of the forming hole, when the first step-shaped differential shear band is an n-level step structure, the straight-line distance between the shear deformation zone and the working band is L0, that is, the distance between the end faces where the shear deformation zone and the working band are connected is L0, and the lengths of each step are L1, L2, L3...Ln-1, Ln, which means that the distance between the end face where the shear deformation zone and the working band are connected and the end face of the first-level step-shaped differential shear band is L1, the distance between the end faces of the first-level and second-level step-shaped differential shear bands is L2, and the distance between the end faces of the second-level and third-level step-shaped differential shear bands is L3. By analogy, the distance between the end faces of the n-1th and nth-level step-shaped differential shear bands is Ln; at the same time, in order to ensure the forming efficiency and achieve the effect of step-by-step thinning, the following relationship L0≤L1≤L2≤L3...Ln-1≤Ln should be satisfied.

[0046] In some embodiments of the present invention, Figures 1-2 As shown, the end of the working belt away from the process hole is bent in a direction away from the central axis of the working belt and then continues to extend to form a first end face 9 and a second end face 10 respectively; wherein, the first end face is located on the first step differential shear band, and the second end face is located on the second step differential shear band; to ensure the effect of differential shearing, the angle between the first end face and the central axis of the working belt is 60 to 90°, and the angle between the second end face and the central axis of the working belt is 60 to 90°.

[0047] In some embodiments of the present invention, Figures 1-2 As shown, the extrusion coefficient of the die is 9-51.

[0048] 2. A method for thickness-direction gradient differential extrusion of magnesium alloy sheets

[0049] The above die is used to extrude the magnesium alloy sheet to obtain the magnesium alloy sheet. The specific steps are as follows:

[0050] Step 1: homogenizing the as-cast magnesium alloy billet and then water quenching; wherein the homogenizing temperature is 320° C. to 520° C. and the holding time is 8 to 20 hours;

[0051] Step 2: Heat the die and the magnesium alloy billet to an extrusion temperature of 300°C to 450°C, respectively, and keep them warm for 10 to 20 minutes;

[0052] Step 3: Install the preheated die on the extruder, and the extrusion barrel is detachably fixedly connected to the forming die in the die, so that the discharge end of the extrusion barrel is connected to the shear deformation zone on the forming hole in the forming die, and then put the preheated magnesium alloy billet into the extrusion barrel;

[0053] Step 4: The magnesium alloy billet is extruded by an extruder. The magnesium alloy billet passes through the shear deformation zone and the working zone in the forming die in sequence under the thrust of the extrusion rod, and finally a magnesium alloy sheet is obtained. The extrusion speed is 0.1-5 mm / s.

[0054] In some embodiments of the present invention, the magnesium alloy sheet extruded by the extrusion method has a thickness of 2 to 5 mm and a width of 56 to 112 mm.

[0055] In some embodiments of the present invention, the method is applicable to extrusion processes of all magnesium alloy materials.

[0056] 3. Examples and Comparative Examples

[0057] Example 1

[0058] The specific dimensions of the thickness gradient dual-stage differential extrusion die for the magnesium alloy plate in this embodiment are as follows: Figure 3 As shown, the dual-stage differential extrusion die includes a forming die and a process die, the forming die and the process die have the same central axis, wherein the process die height H10 is 40.5 mm, and the process hole diameter D10 is 19.1 mm; the forming die height H11 is 55.0 mm, the shear deformation zone diameter D11 is 27.1 mm, the distance H12 from the end surface S11 of the stepped differential shearing zone above the shear deformation zone to the end surface S10 of the shear deformation zone is 21.0 mm, and the shear deformation zone The distance H13 from the end face S12 of the lower stepped differential shear band to the end face S10 of the shear deformation zone is 15.0 mm, the height D12 of the end face of the step differential shear band in the shear deformation zone is 4.0 mm, the upper and lower end faces S13 and S14 communicating with the working band and the shear deformation zone are aligned and the distance H14 from the lower stepped differential shear band S12 is 18.0 mm; the working band height H15 is 8.5 mm; the extrusion ratio of the magnesium alloy thickness gradient double-stage differential die is 20.

[0059] In specific implementation, the forming mold and the process mold are both made of 4Cr5MoSiV1 mold steel, the roughness of the surface and mold cavity of the forming mold and the process mold are Ra0.08-0.16μm, the process fillet is R1.0, and the unfilled chamfer is 45°; the mold heat treatment process is secondary tempering after quenching, and the hardness after heat treatment reaches HRC48-52.

[0060] Based on the above-mentioned magnesium alloy thickness gradient double-stage differential extrusion die, an extrusion forming experiment was conducted on the magnesium alloy, and the experimental material was in the cast state. The implementation steps are as follows:

[0061] S1, homogenization treatment of the cast Mg-4Al-1.8Ca-0.3Mn alloy at 400 °C for 12 h, the purpose is to eliminate residual stress and homogenize the structure, reduce deformation resistance and improve the alloy's forming ability;

[0062] S2, heating the die and the blank to the extrusion temperature of 400°C and keeping the temperature for 20 minutes;

[0063] S3, assembling the preheated mold onto the extruder, connecting the extrusion barrel with the shear deformation zone of the forming die; placing the preheated billet in the extrusion barrel;

[0064] S4, the billet is extruded by a XJ-500T horizontal extruder, the extrusion speed is set to 1 mm / s, and the billet enters the forming die under the thrust of the extrusion rod, and finally a magnesium alloy sheet with the same shape and size as the working belt is obtained.

[0065] The extrusion forming process was simulated by DEFORM finite element simulation software to analyze the flow behavior of the billet. Figure 5 b is the simulation result of the flow velocity of the blank in the shear deformation zone and working zone of the thickness gradient double-stage differential extrusion die. Figure 5 a is the simulation result of the flow velocity in the shear deformation zone and working zone of the conventional forward extrusion die under the same extrusion ratio. By comparing and analyzing the simulation results, it can be seen that the setting of the step-shaped differential shear zone significantly changes the flow velocity of the billet about the central axis. The difference in flow velocity introduces shear deformation, and the second phase in the billet is more thoroughly broken under the complex stress-strain deformation conditions, which refines the grains and deflects the basal texture, greatly weakening the basal texture.

[0066] Figure 6 a is the {0001} pole figure of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion based on the above-mentioned magnesium alloy thickness gradient double-stage differential extrusion die. Figure 6b is the {0001} pole figure of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion by the conventional forward extrusion die. Under the same extrusion ratio, the maximum texture strength of the alloy after conventional forward extrusion is 49.96, while the maximum texture strength after extrusion using the die of the present invention is 28.78, and the texture direction shifts toward the ND direction. Compared with conventional forward extrusion forming, the maximum texture strength of the present invention is reduced by about 32%.

[0067] Example 2

[0068] The specific dimensions of the thickness gradient three-stage differential extrusion die for the magnesium alloy plate in this embodiment are as follows: Figure 4 As shown, the thickness gradient three-stage differential extrusion die includes a forming die and a process die, the forming die and the process die have the same central axis, wherein the process die height H20 is 40.5 mm, and the process hole diameter D20 is 19.1 mm; the forming die height H21 is 55.0 mm, the shear deformation zone diameter D21 is 27.1 mm, the distance H22 from the end face S21 of the stepped differential shear band above the shear deformation zone to the end face S20 of the shear deformation zone is 21.0 mm, and the distance H22 from the end face S22 of the stepped differential shear band below the shear deformation zone to the end face S20 of the shear deformation zone is 21.0 mm. The distance H23 between the end face S20 of the shear deformation zone is 15.0 mm, the height D22 of the stepped differential shear band in the shear deformation zone is 4.0 mm, the distance H24 between the upper end face S23 of the working band communicating with the shear deformation zone and the upper end face S21 of the stepped differential shear band is 6.0 mm, and the distance H25 between the lower end face S24 of the working band communicating with the shear deformation zone and the lower end face S22 of the stepped differential shear band is 18.0 mm; the height H26 of the working band is 8.5 mm; and the extrusion ratio of the thickness-oriented gradient three-stage differential die is 20.

[0069] In specific implementation, the forming mold and the process mold are both made of 4Cr5MoSiV1 mold steel, the roughness of the surface and mold cavity of the forming mold and the process mold are Ra0.08-0.16μm, the process fillet is R1.0, and the unfilled chamfer is 45°; the mold heat treatment process is secondary tempering after quenching, and the hardness after heat treatment reaches HRC48-52.

[0070] Based on the above-mentioned magnesium alloy thickness gradient three-stage differential speed extrusion die, an extrusion forming experiment was conducted on the magnesium alloy, and the experimental material was in the cast state. The implementation steps are as follows:

[0071] S1, homogenization treatment of the cast Mg-4Al-1.8Ca-0.3Mn alloy at 400 °C for 12 h, the purpose is to eliminate residual stress and homogenize the structure, reduce deformation resistance and improve the alloy's forming ability;

[0072] S2, heating the die and the blank to the extrusion temperature of 400°C and keeping the temperature for 20 minutes;

[0073] S3, assembling the preheated mold onto the extruder, connecting the extrusion barrel with the shear deformation zone of the forming die; placing the preheated billet in the extrusion barrel;

[0074] S4, the billet is extruded by a XJ-500T horizontal extruder, the extrusion speed is set to 1 mm / s, and the billet enters the forming die under the thrust of the extrusion rod, and finally a magnesium alloy sheet with the same shape and size as the working belt is obtained.

[0075] The extrusion process is simulated by DEFORM finite element simulation software to analyze the flow behavior of the billet Figure 5 c is the simulation result of the flow velocity of the billet in the shear deformation zone and working zone of the three-stage differential extrusion die. Figure 5 a is the simulation result of the flow velocity in the shear deformation zone and working zone of the conventional forward extrusion die under the same extrusion ratio. By comparing and analyzing the simulation results, it can be seen that the setting of the step-shaped differential shear zone and the asymmetric distribution of the working zone significantly change the flow velocity of the billet about the central axis. The difference in flow velocity introduces multiple shear deformations, and the second phase in the billet is more thoroughly broken under the complex stress-strain deformation conditions, which refines the grains and deflects the basal texture, greatly weakening the basal texture.

[0076] Figure 6 b is the {0001} pole figure of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion using the above-mentioned magnesium alloy double-stage differential extrusion die. Figure 6 a is the {0001} pole figure of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion by a conventional forward extrusion die. Under the same extrusion ratio, the maximum texture strength of the alloy after conventional forward extrusion is 49.96, while the maximum texture strength after extrusion using the die of the present invention is 12.13, and the texture direction shifts toward the ND direction. Compared with conventional forward extrusion forming, the maximum texture strength of the present invention is reduced by about 75%, and compared with the above-mentioned embodiment 1, it is reduced by about 42%. By increasing the number of thickness gradient flow rate differences during magnesium alloy extrusion, it helps to improve the plasticity of magnesium alloy sheets and the deep processing capabilities of products.

[0077] Comparative Example 1

[0078] The specific dimensions of the extrusion die for the magnesium alloy sheet in this comparative example are as follows: Figure 7As shown, the extrusion die includes a forming die and a process die, the forming die and the process die have the same central axis, wherein the process die height H00 is 40.5 mm, and the process hole diameter D00 is 19.1 mm; the forming die height H01 is 55.0 mm, the shear deformation zone diameter D01 is 27.1 mm, the upper and lower end faces of the stepped shear band in the shear deformation zone are aligned, the distance H02 from the feed inlet end face S01 of the shear deformation zone to the step shear band end face S00 of the deformation zone is 15.0 mm, the height D02 of the step shear band end face of the shear deformation zone is 4.0 mm, the upper and lower end faces S02 and S03 of the working band communicating with the shear deformation zone are aligned and the distance H03 from the step shear band S00 is 18.0 mm; the working band height H04 is 8.5 mm; the extrusion ratio of the magnesium alloy thickness gradient double-stage differential die is 20.

[0079] In specific implementation, the forming mold and the process mold are both made of 4Cr5MoSiV1 mold steel, the roughness of the surface and mold cavity of the forming mold and the process mold are Ra0.08-0.16μm, the process fillet is R1.0, and the unfilled chamfer is 45°; the mold heat treatment process is secondary tempering after quenching, and the hardness after heat treatment reaches HRC48-52.

[0080] Based on the above-mentioned magnesium alloy sheet extrusion die, an extrusion forming experiment was conducted on the magnesium alloy, and the experimental material was in the cast state. The implementation steps are as follows:

[0081] S1, homogenization treatment of the cast Mg-4Al-1.8Ca-0.3Mn alloy at 400 °C for 12 h, the purpose is to eliminate residual stress and homogenize the structure, reduce deformation resistance and improve the alloy's forming ability;

[0082] S2, heating the die and the blank to the extrusion temperature of 400°C and keeping the temperature for 20 minutes;

[0083] S3, assembling the preheated mold onto the extruder, connecting the extrusion barrel with the deformation zone of the forming die; placing the preheated billet in the extrusion barrel;

[0084] S4, the billet is extruded by a XJ-500T horizontal extruder, the extrusion speed is set to 1 mm / s, and the billet enters the forming die under the thrust of the extrusion rod, and finally a magnesium alloy sheet with the same shape and size as the working belt is obtained.

[0085] The extrusion process is simulated by DEFORM finite element simulation software to analyze the flow behavior of the billet. Figure 5 a is the simulation result of the flow velocity in the shear deformation zone and working zone of the conventional forward extrusion die under the same extrusion ratio. The streamlines are symmetrically distributed along the central axis, and the flow velocity is uniform. Figure 6a is the {0001} pole figure of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion by the conventional forward extrusion die, and the maximum texture strength of the alloy after conventional forward extrusion is 49.96. Table 1 shows the mechanical properties of the Mg-4Al-1.8Ca-0.3Mn alloy after extrusion by the above-mentioned magnesium alloy sheet extrusion die. The yield strength of the Mg-4Al-1.8Ca-0.3Mn alloy after conventional forward extrusion is 245MPa, the tensile strength is 315MPa, and the elongation is 10%.

[0086] Comparative Example 2

[0087] The magnesium alloy sheet extrusion die used in Comparative Example 1 was used to perform an extrusion forming experiment on the magnesium alloy, and the experimental material was in a cast state. The implementation steps are as follows:

[0088] S1, homogenization treatment of the cast Mg-4Al-1.8Ca-0.3Mn alloy at 400 °C for 12 h, the purpose is to eliminate residual stress and homogenize the structure, reduce deformation resistance and improve the alloy's forming ability;

[0089] S2, heat the blank to 380℃, heat the mold to 320℃, and keep warm for 20min;

[0090] S3, assembling the preheated mold onto the extruder, connecting the extrusion barrel with the deformation zone of the forming die; placing the preheated billet in the extrusion barrel;

[0091] S4, the billet is extruded by a XJ-500T horizontal extruder, the extrusion speed is set to 1 mm / s, and the billet enters the forming die under the thrust of the extrusion rod, and finally a magnesium alloy sheet with the same shape and size as the working belt is obtained.

[0092] Mechanical tests were performed on the magnesium alloy plates prepared in Examples 1-2 and Comparative Examples 1-2. The results are shown in Table 1.

[0093] Table 1

[0094] Sheet Type Yield strength (MPa) Tensile strength(MPa) Elongation(%) Example 1 240 310 15 Example 2 235 300 18 Comparative Example 1 245 315 10 Comparative Example 2 190 280 7

[0095] It can be seen from Table 1 that:

[0096] (1) Under the same extrusion ratio and process conditions, the embodiment of the die of the present invention can significantly improve the elongation of the magnesium alloy sheet, whether it is a two-stage or three-stage die, and the improvement effect is very significant; at the same time, the magnesium alloy sheet can also maintain excellent strength. When preparing magnesium alloy sheets in the prior art, there is often a relationship between strength and elongation, but the die of the present invention can achieve a significant improvement in the elongation of the magnesium alloy sheet without significantly weakening the strength of the magnesium alloy sheet. This is an unexpected discovery in the actual application of the die of the present invention.

[0097] (2) Comparative Examples 1 and 2 both adopt the extrusion process of the conventional forward extrusion die, and in particular, Comparative Example 1 is the optimal process under the conventional forward extrusion die. Compared with the embodiment, it can be seen that even Comparative Example 1 is not as good as the embodiment in improving the elongation of the magnesium alloy sheet; in particular, compared with Embodiments 1-2 and Comparative Example 1 and Comparative Example 2, Embodiments 1-2 greatly improve the elongation of the magnesium alloy sheet, and also ensure the strength of the magnesium alloy sheet, which shows that the die of the present invention can achieve the strength that can be achieved by the optimal process of the conventional forward extrusion die, and can also greatly improve the elongation of the magnesium alloy sheet, which can expand the application range of the magnesium alloy sheet and has a good industrial application prospect.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A magnesium alloy plate thickness gradient graded differential extrusion die, the die comprising a forming die (1) and a process die (2), one side of the forming die is detachably fixedly connected to one side of the process die; characterized in that: The process mold is provided with a process hole (3), the two ends of which penetrate the two opposite side walls of the process mold; the molding mold is provided with a molding hole, the two ends of which penetrate the two opposite side walls of the molding mold, and the molding hole is connected with the process hole; An outlet zone (4) is provided at one end of the forming hole close to the process hole, a shear deformation zone (5) is provided at one end of the forming hole away from the process hole, a working belt (6) is provided between the outlet zone and the shear deformation zone, one end of the shear deformation zone is connected to one end of the working belt, the other end of the working belt is connected to one end of the outlet zone, and the other end of the outlet zone is connected to the process hole; The surfaces of the shear deformation zone on opposite sides along its width direction are respectively a first step-shaped differential shear band (7) and a second step-shaped differential shear band (8); both the first step-shaped differential shear band and the second step-shaped differential shear band are multi-step structures; The first step-shaped differential shearing zone and the second step-shaped differential shearing zone are asymmetric structures along the central axis direction of the working zone.

2. The mold according to claim 1, characterized in that: Calculated along the length direction of the forming hole, the height of the forming die is H0, the height of the shear deformation zone is H1, and H0 and H1 satisfy the following relationship: H1>H0÷2.

3. The mold according to claim 2, characterized in that: Calculated along the width direction of the forming hole, the thickness of the shear deformation zone is D1, the thickness of the working zone is D2, and the thickness of the outlet zone is D3, and the following relationship is satisfied: D1>D3>D2.

4. The mold according to claim 3, characterized in that: Calculated along the width direction of the forming hole, the second stepped differential shear band is a double-step structure, and the thickness of a single step is D0; D0 and D1, D2 satisfy the following relationship: D0<(D1-D2)÷2.

5. The mold according to claim 4, characterized in that: Calculated along the length direction of the forming hole, when the first step-shaped differential shear zone is an n-step structure, the straight-line distance between the shear deformation zone and the working zone is L0, and the length of each step is L1, L2, L3...Ln-1, Ln, and L0≤L1≤L2≤L3...Ln-1≤Ln.

6. The mold according to claim 1, characterized in that: The end of the working belt away from the process hole is bent in a direction away from the central axis of the working belt and then continues to extend to form a first end face (9) and a second end face (10); wherein the first end face is located on the first step differential shear band, and the second end face is located on the second step differential shear band; the angle between the first end face and the central axis of the working belt is 60 to 90°, and the angle between the second end face and the central axis of the working belt is 60 to 90°.

7. The mold according to claim 1, characterized in that: The extrusion coefficient of the die is 9-51.

8. A method for thickness-direction gradient differential extrusion of a magnesium alloy sheet, characterized in that: The magnesium alloy sheet is extruded by using the die described in any one of claims 1 to 7.

9. The extrusion method according to claim 8, characterized in that: The specific steps are as follows: Step 1: homogenizing the as-cast magnesium alloy billet and then water quenching; wherein the homogenizing temperature is 320° C. to 520° C. and the holding time is 8 to 20 hours; Step 2: Heat the die and the magnesium alloy billet to an extrusion temperature of 300°C to 450°C, respectively, and keep them warm for 10 to 20 minutes; Step 3: Install the preheated die on the extruder, and the extrusion barrel is detachably fixedly connected to the forming die in the die, so that the discharge end of the extrusion barrel is connected to the shear deformation zone on the forming hole in the forming die, and then put the preheated magnesium alloy billet into the extrusion barrel; Step 4: The magnesium alloy billet is extruded by an extruder. The magnesium alloy billet passes through the shear deformation zone and the working zone in the forming die in sequence under the thrust of the extrusion rod, and finally a magnesium alloy sheet is obtained. The extrusion speed is 0.1-5 mm / s.

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