CN101850376B - Method and die for forward extrusion and variable diameter bending extrusion of magnesium alloy semi-solid billets - Google Patents

Abstract

The invention discloses a method and a die for the forward extrusion and variable diameter bending extrusion of magnesium alloy semi-solid billets. The preparation method comprises a predeformation process and an isothermal spheroidizing heat treatment process, wherein in the predeformation process, uniform deformation of large deformation amount is realized by the die for the forward extrusion and variable diameter bending extrusion; cutting a magnesium alloy bar stock which is subjected to large deformation by a fixed size; heating the magnesium alloy bar stock cut by the fixed size at the temperature of between 520 and 580 DEG C under the protection of argon, and keeping the temperature of the obtained magnesium alloy bar stock for 10 to 30 minutes; and processing a bending extrusion angle of the die to be over 90 degrees. The method has the advantages of small predeformation resistance, large and uniform deformation, safety, reliability and no three-waste pollution. The grains of the prepared magnesium alloy semi-solid billets have fine size, are uniform and have a shape close to that of a sphere. The method solves the problem that the conventional SIMA method cannot continuously prepare the large-size semi-solid billets and meets the requirements of the semi-solid thixoforming workpieces for large-scale continuous production.

Description

Forward extrusion and variable diameter bending extrusion method and mold for preparing magnesium alloy semi-solid billet

technical field

The invention belongs to the field of extrusion deformation of magnesium alloys, in particular to an extrusion deformation processing method and a processing die for improving the mechanical properties of magnesium alloys.

Background technique

Magnesium alloys have become an important component of automobiles, aerospace and electronic communications due to their high specific strength, specific stiffness, shock absorption, wear resistance, thermal conductivity, electromagnetic shielding, easy cutting and easy recycling and other good comprehensive properties. Important new materials in other industries. Today, with increasingly severe energy and resource issues and increasingly prominent environmental protection issues, magnesium alloys are known as "green engineering structural materials in the 21st century", and lightweight magnesium alloys with good comprehensive properties are becoming a global focus. With the increasing application proportion of magnesium alloy structural parts, the requirements for magnesium alloy forming technology are also getting higher and higher.

The forming methods of magnesium alloys are mainly divided into two categories: liquid casting and plastic deformation. Since magnesium belongs to the close-packed hexagonal crystal structure, there are few slip systems, and the plasticity at room temperature is poor. Now about 90% of magnesium alloy engineering structural parts are manufactured by casting. Magnesium alloy casting methods include: sand casting, metal gravity casting, investment casting, squeeze casting, low pressure casting and pressure casting. These casting methods all require a special magnesium alloy melting furnace, and need to use a covering agent or protective gas to prevent the oxidative combustion of magnesium during the melting process, so the capital investment is large, and there are serious environmental protection and safety hazards. At the same time, for the main forming method of magnesium alloy - die casting method, although complex parts can be formed, there are defects such as shrinkage cavity, shrinkage porosity, and pores when forming thick and uneven castings, which reduces the mechanical properties of the parts. performance. The plastic processing of magnesium alloy can improve the performance of parts, but due to the limitation of its plastic deformation ability, it can only produce parts with extremely simple shapes.

Semi-solid forming technology has the advantages of both casting and plastic processing, that is, it can use a small force to form a high-quality magnesium alloy with a small force (only a small amount of processing or no processing can be used as a mechanical component). pieces.

The preparation of semi-solid blank is the basis and key of semi-solid forming. At present, its main preparation methods include: mechanical stirring method, electromagnetic stirring method, near-liquidus casting method, strain-induced melting activation method (SIMA) and so on. Due to the easy oxidation characteristics of magnesium alloys, it is difficult to prepare magnesium alloy semi-solid billets by liquid method, while SIMA method has simple process and low equipment investment, which avoids the disadvantage of easy oxidation of melt at high temperature, so SIMA method is used in magnesium alloys. The preparation of alloy semi-solid billets has been paid more and more attention. The existing SIMA methods are mainly upsetting and equal channel angular extrusion, in which the upsetting method not only deforms unevenly, but also greatly limits the size of the billet; the magnesium alloy semi-solid billet prepared by equal channel angular extrusion is subject to Due to the limitations of molds and equipment, only short billets can be prepared each time, and multiple passes of extrusion are required to obtain large equivalent strains.

Chinese patent document CN100360700C (patent application number 03132471.1) discloses a composite preparation method of magnesium alloy unequal-diameter bend extrusion-shear-induced isothermal spheroidization semi-solid billet, the billet is subjected to unequal-diameter bend extrusion, extrusion The ratio is kept between 1.5 and 2.0, the corner extrusion angle is 90°, and the cycle is repeated 3 to 4 times. However, this method requires multiple passes of extrusion to refine the magnesium alloy grains to a smaller size, with many procedures, low production efficiency, and high production costs; and the extrusion time of each pass is relatively long. Extrusion speed will reduce the quality of the product.

Contents of the invention

The purpose of the present invention is to provide a forward extrusion and variable diameter bending extrusion method and die which can continuously prepare magnesium alloy semi-solid billets.

The technical solution for realizing the purpose of the present invention is a forward extrusion and variable diameter bending extrusion die for preparing a magnesium alloy semi-solid billet, including an upper die, a punch, a die, a die cover, a spacer, a lower die and a pressing plate; The die sleeve is located and fixed on the lower template, and the lower surface of the die sleeve is in contact with the upper surface of the lower template; the lower part of the space in the middle of the die sleeve is placed with a pad; the die is provided with a feed port at the upper end; the die It is set above the pad and located in the middle space of the die sleeve; the punch is stepped, and its upper end is fixedly connected with the upper template. When in use, the lower end of the punch is aligned with the feed port of the die; the center of the pressing plate is provided with a The through hole, the pressure plate is fixed on the upper surface of the die sleeve and the die, and the through hole in the center is aligned with the feed port of the die; the cavity of the die includes a positive extrusion channel and a variable diameter curved extrusion channel ;The positive extrusion channel includes the first straight road and the second straight road, and the feeding port of the first straight road is also the feeding port of the die; The shape transition section is connected with the second straight road, wherein the cross-sectional area of the first straight road is larger than that of the second straight road; the variable-diameter curved extrusion channel is an extrusion bend, and the extrusion angle of the bend is 90°-150° , the cross-sectional area of the variable-diameter curved extrusion channel gradually becomes smaller from the head end of the upper part to the tail end of the lower part; an equal-diameter outlet channel is set in the die sleeve, and the equal-diameter outlet channel is a straight outlet channel, which runs along the Extending outward along the extension direction of the variable-diameter curved extrusion channel, its cross-sectional area is the same as the minimum cross-sectional area of the variable-diameter curved extrusion channel.

The left part and the right part of the above-mentioned die cover respectively have a through hole through the upper and lower sides, and a ceramic tube with the same height as the through hole is placed in the through hole, and the heating wire is arranged in the ceramic tube.

A forward extrusion and variable diameter bending extrusion method for preparing a magnesium alloy semi-solid billet, wherein the extrusion die used includes an upper die, a punch, a die, a die set, a pad, a lower die and a pressing plate; The mold sleeve is located and fixed on the lower template, and the lower surface of the die sleeve is in contact with the upper surface of the lower template; the lower part of the middle space of the die sleeve is placed with a pad; the die is provided with a feed port at the upper end; the die is set It is above the pad and located in the middle space of the die sleeve; the punch is stepped, and its upper end is fixedly connected with the upper template. When in use, the lower end of the punch is aligned with the feed port of the die; the center of the pressure plate is provided with a Hole, the pressure plate is fixed on the upper surface of the die sleeve and the die, and the through hole in the center is aligned with the feed port of the die; the cavity of the die includes a positive extrusion channel and a variable diameter curved extrusion channel; The positive extrusion channel includes a first straight road and a second straight road. The feed port of the first straight road is also the feed port of the die; the first straight road extends axially downward from the top of the die and passes through the The transition section is connected with the second straight road, wherein the cross-sectional area of the first straight road is larger than that of the second straight road; the variable-diameter curved extrusion channel is an extrusion bend, and the extrusion angle of the bend is 90°-150°, The cross-sectional area of the variable-diameter curved extrusion channel gradually decreases from the head end of the upper part to the tail end of the lower part; an equal-diameter outlet channel is arranged in the die sleeve, and the equal-diameter outlet channel is a straight outlet channel, which runs along the The extension direction of the variable-diameter curved extrusion channel extends outwards, and its cross-sectional area is the same as the minimum cross-sectional area of the variable-diameter curved extrusion channel; the extrusion method includes the following steps: ①Machining of the magnesium casting billet: the magnesium alloy The cylindrical billet is processed into a cylindrical billet corresponding to the inner diameter of the upper end of the above-mentioned die; ② Pre-deformation of the billet: Preheat the billet that has been machined in step ① to raise the temperature to 260 ° C ~ 300 ° C; The extrusion die is fixed on the worktable of the general extruder, and the extrusion die is preheated so that the temperature of the extrusion die is 40°C to 60°C lower than the billet preheating temperature; put the preheated billet into the extrusion die In the die, under the drive of the hydraulic press, the punch of the extrusion die presses the blank into the through hole in the middle of the pressing plate of the extrusion die. The first straight path and cross-sectional area of the positive extrusion channel of the die are smaller The extrusion ratio is 15-30; the punch continues to go down, and the billet after being extruded is pressed into the variable-diameter curved extrusion channel for curved extrusion and shearing with a rotation angle greater than 90°. shearing deformation; the next piece of blank is pressed into the process of the forward extrusion channel and the reduced-diameter curved extrusion channel, and the previous blank that was retained in the die is pressed into the equal-diameter outlet channel of the die sleeve of the extrusion die. Extrude the die, and continue to extrude the billet out of the die sleeve to obtain a pre-deformed magnesium alloy bar; the above process is carried out continuously; ③cut the pre-deformed magnesium alloy bar through step ② according to the required length;④ Isothermal heat treatment: Heating the magnesium alloy rods cut to length in step ③ to a semi-solid temperature of 520°C-580°C in an argon atmosphere in a resistance furnace, and keeping the temperature for 10min-30min to obtain a magnesium alloy semi-solid billet, wherein the furnace The temperature is heated to the semi-solid temperature at a fast and then slow heating rate.

In the above step ④, the temperature is raised quickly and then slowly to the semi-solid temperature of the billet, which is to raise the furnace temperature to 20 °C to 30 °C before the semi-solid temperature of the magnesium alloy billet at a rate of 13 °C to 15 °C per minute. , and then slowly raise the furnace temperature to the semi-solid temperature at a rate of 1°C to 2°C per minute.

During the pre-deformation process of the blank in the above step ②, the extrusion speed of the punch is 30 millimeters per minute.

The invention has positive effects: (1) The grain size of the magnesium alloy semi-solid billet produced by the invention is small, uniform and close to spherical. (2) The present invention has small pre-deformation resistance, large and uniform deformation, safety and reliability, and no three-waste pollution. (3) In the present invention, a large plastic deformation is realized in the forward extrusion part, and the deformation is further increased in the subsequent bend extrusion and shearing with a turning angle greater than 90 degrees, and the deformation in the entire cross section tends to It is uniform and consistent, which solves the problem that the traditional SIMA method cannot continuously prepare large-size semi-solid billets, and meets the needs of large-scale continuous production of semi-solid thixoformed parts. (4) The method of the present invention can also realize the industrialized production of low-temperature extrusion molding, which not only improves the production efficiency, but also the extruded magnesium alloy material has high extrusion strength, high yield strength, and good mechanical properties of elongation.

Description of drawings

Fig. 1 is the schematic diagram of the die of forward extrusion and variable diameter bending extrusion of the present invention; Fig. 2 is the A-A sectional view of Fig. 1; Fig. 3 is the B-direction view of the half die of Fig. 2; Fig. 4 is the embodiment 2 Heating temperature curve during isothermal treatment of AZ91D magnesium alloy billet; Fig. 5 is the edge microstructure diagram of the AZ91D magnesium alloy billet after pre-deformation in embodiment 2; Fig. 6 is the core part of the AZ91D magnesium alloy billet after pre-deformation in embodiment 2 Microstructure diagram; Fig. 7 is the grain refinement effect figure produced by the extrusion of the present invention for the AZ91D magnesium alloy billet in embodiment 2; Fig. 9 is a grain refinement effect diagram produced by extrusion of the present invention for the AZ91D magnesium alloy billet in embodiment 4; Fig. 10 is a grain refinement effect map produced by the extrusion of the present invention for the AZ91D magnesium alloy billet in embodiment 5 Effect diagram.

The marks in the above drawings are as follows: upper template 1, punch 2, die 3, half die 30, positive extrusion channel 31, first straight 31-1, second straight 31-2, variable diameter curved extrusion Pressure channel 32, die set 4, equal-diameter outlet channel 41, heating wire 42, ceramic tube 43, cushion block 5, lower template 6, and pressing plate 7.

Detailed ways

(Example 1, forward extrusion and variable diameter bending extrusion die for preparing magnesium alloy semi-solid billet) see Fig. 1 to Fig. 3, the forward extrusion and variable diameter bending extrusion for preparing magnesium alloy semi-solid billet of this embodiment The processing mold comprises an upper template 1, a punch 2, a die 3, a die cover 4, a heating wire 42, a ceramic tube 43, a pad 5, a lower template 6 and a pressing plate 7.

The die cover 4 is seated and fixed on the lower template 6, and the lower surface of the die cover 4 is in contact with the upper surface of the lower template 6; the lower part of the space in the middle of the die cover 4 places a spacer 5. The die 3 is provided with a feeding port located at the upper end, the die 3 is arranged above the cushion block 5, and is located in the middle space of the die set 4, and the die 3 and the die set 4 are in clearance fit; the punch 2 is stepped , its upper end is fixedly connected with the upper template 1, and the lower end of the punch 2 is aligned with the feed port of the die 3 during use, and the gap between the die 3 and the punch 2 is selected as 0.05mm on one side. The center of the pressing plate 7 is provided with a through hole, the pressing plate 7 is fixed on the upper surface of the die cover 4 and the die 3 , and the through hole in the center is aligned with the feeding port of the die 3 .

The left part and the right part of the above-mentioned die cover 4 respectively have a through hole through up and down, and a ceramic tube 43 with the same height as the through hole is placed in the through hole, and the heating wire 42 is arranged in the ceramic tube 43 .

The overall cavity of the above-mentioned die 3 is made up of two completely symmetrical semi-concaves 30, each semi-concave 30 is provided with half an extrusion channel, and its shape is a semicircular groove, and the two semi-concaves 30 is fitted oppositely by the side that is respectively provided with groove, constitutes a complete die 3, then forms with bolt fixing, two half dies 30 are fixed with die cover 4 by pressing plate 7.

The die cavity of the die 3 includes a positive extrusion channel 31 and a diameter-reducing curved extrusion channel 32 , the positive extrusion channel 31 is above the variable-diameter curved extrusion channel 32 and the two channels communicate. The positive extrusion channel 31 comprises a first straight road 31-1 and a second straight road 31-2, and the feeding port of the first straight road 31-1 is also the feeding port of the die 3, and the first straight road 31-1 starts from The top of the die 3 extends downward in the axial direction, and connects with the second straight road 31-2 through a truncated conical transition section, wherein the cross-sectional area of the first straight road 31-1 is greater than that of the second straight road 31-2; The curved extrusion channel 32 is an extrusion bend, extending from the lowermost end of the second straight channel 31-2 to the obliquely downward direction of the die 3 until it connects with the equal-diameter outlet channel 41 of the die sleeve 4. The cross-sectional area of extrusion channel 32 gradually becomes smaller from the head end of the uppermost part, that is, the lowermost end of the second straight road 31-2, to the tail end connected with die sleeve 4, and its bend extrusion angle (that is, the angle of bend) Bending angle) is 120°; the equal-diameter outlet passage 41 of the above-mentioned die cover 4 is a straight outlet passage, and extends outward along the extension direction of the variable-diameter bending extrusion passage 32 in the die cover 4, and its cross-sectional area is the same as The minimum cross-sectional areas of the variable-diameter curved extrusion channels 32 are the same.

(Example 2, forward extrusion and variable diameter bending extrusion method for preparing magnesium alloy semi-solid billet) see Figure 4 to Figure 7, the forward extrusion and variable diameter corner extrusion for preparing magnesium alloy semi-solid billet in this embodiment The used extrusion die of the method adopts the die obtained in Example 1, and what the present embodiment deals with is the AZ91D magnesium alloy billet, and the method for preparing the magnesium alloy semi-solid billet comprises the following steps: 1. the machining of the magnesium billet: The magnesium alloy cylindrical billet is processed into a cylindrical blank corresponding to the inner diameter of the upper end of the die 3, the diameter of the blank is 50mm, and the height is 120mm.

② Pre-deformation of the billet: Preheat the cylindrical billet machined in step ① in a box-type resistance furnace at a preheating temperature of 280°C and keep it warm for 20 minutes. The extrusion die of Example 1 is fixed on the workbench of a general-purpose extruder, so that the male die 2 can slide up and down relative to the female die 3 . Then the electric heating wire 42 arranged in the mold is switched on to power, and the extrusion mold is preheated so that the temperature of the mold reaches 220°C. Put the preheated cylindrical billet into the concave die 3 of the extrusion die, and the punch 2 of the extrusion die presses the billet into the positive extrusion channel through the through hole in the middle of the pressing plate 7 of the extrusion die under the drive of the hydraulic press In the first straight road 31-1 of 31 and the second straight road 31-2 whose cross-sectional area is smaller than the first straight road 31-1, the extrusion speed is 30 mm per minute, thereby producing a large compression deformation, and the extrusion ratio (extrusion The ratio of the total cross-sectional area of the product before pressing to the total cross-sectional area of the product after extrusion) is 15 to 30 (17 in this embodiment); The same direction) after the billet is pressed into the variable-diameter curved extrusion channel 32, further added in the variable-diameter deformation and 120-degree curved extrusion shear deformation (bending deformation that occurs while traveling in a channel) Large deformation; when the punch 2 goes down to the lower end of the first straight road 31-1 and can no longer go down, the punch 2 goes up, puts the next cylindrical blank into the die 3, and then the punch 2 goes down and squeezes it down One piece of cylindrical billet, the next piece of cylindrical billet is pressed into the die of the extrusion die when the billet retained in the die 3 is pressed into the process of being pressed into the forward extrusion channel 31 and the variable-diameter curved extruding passage 32 The equal-diameter outlet channel 41 of the sleeve 4 is extruded from the die 3, and the blank is extruded from the die sleeve 4 to obtain a pre-deformed magnesium alloy rod by continuing extrusion; During the deformation process, the temperature of the billet was maintained at 280°C, and the equivalent strain during pre-deformation (the strain equivalent to the unidirectional strain formed by the appropriate combination of each strain component in the general strain state) was 5.1.

③ Cutting the pre-deformed magnesium alloy bar in step ② to a certain length as required (cutting to a certain length in the axial direction).

④ Isothermal heat treatment: heat the magnesium alloy bar after step ③ cut to length in an argon atmosphere in a resistance furnace in a fast and then slow manner to a semi-solid temperature of 565°C, and then keep it warm for 20 minutes to obtain a semi-solid magnesium alloy billet; the specific heating method is: when heating, the furnace temperature is controlled by a voltage regulator to change the voltage, and the furnace temperature is raised to 20°C to 30°C before the semi-solid temperature of the magnesium alloy billet at a rate of 13.5°C per minute (this embodiment is 30°C), and then the furnace temperature is slowly raised to the semi-solid temperature, that is, the isothermal temperature, at a rate of 1°C per minute.

See Figure 5 and Figure 6, the purpose of the pre-deformation in the above step ② is to obtain sufficient induced strain, this step is the basis and key to obtain a uniform and fine near-spherical semi-solid billet; in the pre-deformation process, extrusion, diameter reduction Extrusion and shear deformation The extrusion technology is skillfully combined, and the forward extrusion deformation and the variable-diameter shear deformation with a rotation angle greater than 90° are realized in one extrusion, and the equivalent strain can reach more than 5. (SIMA strain-induced melting activation method process principle: use the traditional casting method to obtain castings, carry out hot extrusion deformation on the metal billet in the temperature range of recovery and recrystallization, break the as-cast dendrite structure, and store part in the structure of the billet Finally, the deformed metal billet can be cut into a certain size according to the needs, quickly heated to the solid-liquid two-phase region and properly kept warm, through the release of deformation energy, a spherical semi-solid slurry with thixotropy can be obtained Therefore, the above-mentioned sufficient induced strain can be understood as storing sufficient change energy in the blank structure).

As shown in Figure 4, the purpose of the above step ④ isothermal heat treatment is to obtain the required solid-liquid phase ratio and make the primary phase spherical. The heating rate of the semi-solid isothermal treatment adopts the principle of first fast and then slow, because although the fast heating rate can effectively improve the production efficiency, it will cause uneven temperature distribution in the billet, and the liquid does not have enough time to infiltrate the crystal grains Boundary, leading to liquid outflow, making the fluidity of the semi-solid billet worse; and when the heating rate is slow, not only affects the production efficiency, but also the grains are easy to coarsen and grow, when the heating temperature is high, it is difficult for the billet to maintain itself shape. Therefore, the selection principle of the heating speed should be heated to the set temperature at the fastest speed while ensuring uniform heating, and can avoid unfavorable factors such as grain growth caused by keeping the billet at high temperature for a long time. Adopt the principle of first fast and then slow.

The invention combines the advantages of large extrusion ratio extrusion deformation and variable-diameter bending extrusion deformation. During the flow extrusion deformation process of the magnesium alloy billet in the variable-diameter bending extrusion channel of the die, the internal structure of the metal presents a certain directionality. , so that the grains are elongated or even broken, increasing the recrystallization nucleation rate (the number of crystal nuclei formed in the parent phase per unit time and unit volume), and refining the grains.

(Example 3, forward extrusion and variable diameter bending extrusion method for preparing magnesium alloy semi-solid billet) see Fig. 8, the difference between this embodiment and embodiment 2 is: in step ②, through step ① mechanical processing The billet is preheated in a box-type resistance furnace. The preheating temperature is 300°C, and the temperature is kept for 15 minutes. When the extrusion die is preheated, the mold temperature is 240°C. ℃.

Compared with embodiment 2, the present embodiment increases the pre-deformation temperature, which will make the average grain diameter larger, but the present embodiment reduces the pre-deformation holding time, and correspondingly makes the average grain diameter smaller; compared with embodiment 1, The raising of present embodiment pre-deformation temperature and the reduction of pre-deformation holding time all can make grain shape factor (the ratio of the outer surface area of the outer surface of solid particle and volume and the identical ball of solid particle; Because volume is identical but shape is different Among the objects, the outer area of the sphere is the smallest, so the shape coefficient is always greater than 1; the size of the shape coefficient indicates the degree of difference between the particle collision and the sphere, and the closer the value is to 1, the closer its shape is to Ball) becomes larger, that is, the shape of the grain is more irregular.

(Example 4, the forward extrusion and variable-diameter bending extrusion method of preparing magnesium alloy semi-solid billet) see Figure 9, the difference between this embodiment and Example 3 is: step ② in the pre-deformation process, after step ① The machined blank is preheated in a box-type resistance furnace at a temperature of 300°C and held for 10 minutes.

During the isothermal heat treatment process of the blank in step ④, the blank cut to length in step ④ is heated to 580°C in a resistance furnace in an argon atmosphere, and kept for 20 minutes; the specific method is: the furnace temperature is changed by a voltage regulator during heating Voltage control, the method of raising the furnace temperature to 580°C is: first raise the furnace temperature to 560°C at a rate of 13.5°C per minute, and then slowly increase the furnace temperature to the semi-solid temperature at a rate of 1°C per minute , that is, the isothermal temperature.

Compared with Example 3, the shape factor of the crystal grains in this example is close to 1; because although the pre-deformation holding time in this example is shorter, the shape factor has a certain increase, but the temperature of the isothermal heat treatment is higher. During the isothermal holding time, the higher the heating temperature of the billet, the smaller the shape factor of the grains, which greatly reduces the shape factor.

(Example 5, the forward extrusion and variable-diameter bending extrusion method of preparing magnesium alloy semi-solid billet) see Fig. 10, the difference between this embodiment and embodiment 4 is: step ④ during the isothermal heat treatment process of billet , heat the billet cut to length in step ③ to 580° C. in a resistance furnace in an argon atmosphere, and keep it warm for 30 minutes.

Compared with Example 4, the shape factor of the grains in this example is closer to 1; because of the same isothermal temperature, the longer the isothermal time, the smaller the shape factor of the grains.

Claims (5)

1. A forward extrusion and variable-diameter bending extrusion die for preparing magnesium alloy semi-solid billets, comprising an upper template (1), a punch (2), a die (3), a die cover (4), and a pad (5), the lower template (6) and the pressure plate (7); the die cover (4) is located and fixed on the lower template (6), and the lower surface of the die cover (4) and the upper surface of the lower template (6) In contact with each other; the lower part of the space in the middle of the die cover (4) places a pad (5); the die (3) is provided with a feed port at the upper end; It is located in the middle space of the die sleeve (4); the punch (2) is stepped, and its upper end is fixedly connected with the upper template (1). When in use, the lower end of the punch (2) is aligned with the feeding material of the die (3). Mouth; the center of the pressing plate (7) is provided with a through hole, the pressing plate (7) is fixed on the upper surface of the die cover (4) and the die (3), and the through hole in the center of the die (3) and the inlet of the die (3) The feed openings are aligned; it is characterized in that: the cavity of the die (3) includes a positive extrusion channel (31) and a variable diameter curved extrusion channel (32); the positive extrusion channel (31) includes a first straight channel (31 -1) and the second straight road (31-2), the feed port of the first straight road (31-1) is also the feed port of the die (3); the first straight road (31-1) from the die (3) The top end extends downward and axially, and connects with the second straight road (31-2) through a truncated conical transition section, wherein the cross-sectional area of the first straight road (31-1) is larger than that of the second straight road (31-2) The cross-sectional area of the variable-diameter curved extrusion channel (32) is an extrusion bend, and its bend extrusion angle is 90 ° ~ 150 °, and the cross-sectional area of the variable-diameter curved extrusion channel (32) is from the head end of the top The tail end of the lower part gradually becomes smaller; the equal-diameter outlet passage (41) is arranged in the die sleeve (4), and the equal-diameter outlet passage (41) is a straight outlet passage. The extension direction of the curved extrusion channel (32) extends outwards, and its cross-sectional area is the same as the minimum cross-sectional area of the variable-diameter curved extrusion channel (32). 2. The forward extrusion and variable-diameter bending extrusion die for preparing magnesium alloy semi-solid billets according to claim 1, characterized in that: the left and right parts of the die cover (4) are each provided with a through upper and lower A through hole, a ceramic tube (43) with the same height as the through hole is placed in the through hole, and the heating wire (42) is arranged in the ceramic tube (43). 3. A forward extrusion and variable diameter bending extrusion method for preparing a magnesium alloy semi-solid billet, characterized in that: the extrusion die used therein comprises an upper template (1), a punch (2), a die (3 ), die set (4), pad (5), lower formwork (6) and pressure plate (7); die set (4) is located and fixed on the lower formwork (6), and the die set (4) The lower surface is in contact with the upper surface of the lower formwork (6); the cushion block (5) is placed in the lower part of the space in the middle of the die sleeve (4); the die (3) is provided with a feed port at the upper end; the die (3) It is arranged above the cushion block (5) and is located in the middle space of the die sleeve (4); the punch (2) is stepped, and its upper end is fixedly connected with the upper template (1). When in use, the punch (2) The lower end is aligned with the feeding port of the die (3); the center of the pressing plate (7) is provided with a through hole, and the pressing plate (7) is fixed on the upper surface of the die cover (4) and the die (3), and its center The through hole of the die is aligned with the feed port of the die (3); the cavity of the die (3) includes a positive extrusion channel (31) and a variable diameter curved extrusion channel (32); the positive extrusion channel (31) Comprise the first straight road (31-1) and the second straight road (31-2), the feed inlet of the first straight road (31-1) is also the feed inlet of the die (3); the first straight road ( 31-1) extends downward from the top of the die (3) in the axial direction, and connects to the second straight road (31-2) through a truncated conical transition section, wherein the cross-sectional area of the first straight road (31-1) is larger than that of the first straight road (31-1). The cross-sectional area of two straight roads (31-2); the variable diameter curved extrusion channel (32) is an extrusion bend, and its bend extrusion angle is 90 ° ~ 150 °, and the variable diameter curved extrusion channel (32) The cross-sectional area gradually decreases from the head end of the upper part to the tail end of the lower part; an equal-diameter outlet channel (41) is arranged in the die cover (4), and the equal-diameter outlet channel (41) is a straight outlet channel, and the die cover ( 4) extending outward along the extension direction of the variable-diameter curved extrusion channel (32), and its cross-sectional area is the same as the minimum cross-sectional area of the variable-diameter curved extrusion channel (32); the extrusion method includes the following steps: 1. Machining of magnesium casting slab: processing the magnesium alloy cylindrical casting slab into a cylindrical blank corresponding to the inner diameter of the upper end of the above-mentioned die (3); ② Pre-deformation of the billet: Preheat the billet processed by step ① to raise the temperature to 260°C to 300°C; fix the above-mentioned extrusion die on the workbench of the general extrusion machine, and pre-heat the extrusion die. heat, so that the temperature of the extrusion die is 40°C to 60°C lower than the preheating temperature of the billet; put the preheated billet into the die (3) of the extrusion die, and the punch of the extrusion die is driven by the hydraulic press (2) The blank is pressed into the first straight path (31-1) of the positive extrusion passage (31) of the die (3) through the through hole in the middle of the pressing plate (7) of the extrusion die and the cross-sectional area is smaller than the first straight path (31-1). The second straight track (31-2) of the track (31-1) is compressed and deformed, and the extrusion ratio is 15 to 30; the punch (2) continues to descend, and the billet after being extruded is pressed into the variable-diameter curved extrusion channel (32) carry out bending extrusion and shearing deformation with a turning angle greater than 90°; the next piece of blank is pressed into the forward extrusion channel (31) and the variable diameter bending extrusion channel (32) in the process, the former The billet retained in the die (3) is pressed into the equal-diameter outlet passage (41) of the die sleeve (4) of the extrusion die and extruded from the die (3), and the billet is extruded from the die sleeve by continuing extrusion (4) and obtain the magnesium alloy bar stock of pre-deformation; Above-mentioned process is carried out continuously; ③Cutting the pre-deformed magnesium alloy bar in step ② according to the required length; ④ Isothermal heat treatment: heat the magnesium alloy bar after step ③ cut to length in an argon atmosphere in a resistance furnace to a semi-solid temperature of 520°C-580°C, and keep it warm for 10min-30min to obtain a magnesium alloy semi-solid billet, wherein The furnace temperature is heated to the semi-solid temperature at a fast and then slow heating rate. 4. The forward extrusion and variable-diameter bending extrusion method for preparing magnesium alloy semi-solid billet according to claim 3, characterized in that: step 4. described first rapidly and then slowly warming up to the semi-solid temperature of the billet, is first Raise the furnace temperature at a rate of 13°C to 15°C per minute to 20°C to 30°C before the semi-solid temperature of the magnesium alloy billet, and then slowly increase the furnace temperature at a rate of 1°C to 2°C per minute to semi-solid temperature. 5. the forward extrusion and variable diameter bending extrusion method of preparing magnesium alloy semi-solid billet according to claim 3, it is characterized in that: step 2. in the pre-deformation process of billet, the extruding speed of punch (2) is 30mm per minute.

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