Composite extrusion method for preparing high-performance grain-refined magnesium alloy

Abstract

A combined extrusion method for preparing high-performance grain-refined magnesium alloy comprises the following steps that a mold is cleaned, and a molybdenum disulfide solid lubricant is evenly sprayed to an extrusion channel to reduce friction force; the tightening mold is tightly wrapped with refractory cotton after being placed at a proper extrusion position of a workbench; induction heating plates are placed on the two sides of the mold, a thermocouple is placed in an extrusion channel for temperature measurement, and heat preservation is conducted for 30 min after heating is conducted to the experimental temperature; polishing the sample to remove an oxide layer, and then uniformly spraying a MoS2 solid lubricant; putting into a muffle furnace, heating to 300-350 DEG C along with the furnace, and preserving heat for 15 minutes; coordinating the heat preservation time of the sample and the mold; after the heat preservation time is finished, the heating and temperature measuring equipment is moved away, and the sample is put into an extrusion channel and extruded at the speed of 2-4 mm / s; after extrusion is finished, the sample and the mold are air-cooled together at the original position of a workbench; after cooling to room temperature, opening the mold and taking out the sample. The tensile strength, the ductility and the product of strength and elongation of the sample extruded by the method are 298.02 MPa, 23.42% and 6.98 GPa% respectively, and the performance of the magnesium alloy is greatly enhanced.

Description

Technical Field

[0001] The invention belongs to the technical field of magnesium alloy processing technology, and in particular relates to a composite extrusion method for preparing a high-performance grain-refined magnesium alloy. Technical Background

[0002] The forming process of magnesium alloys can be divided into two categories: liquid forming and plastic processing forming. The properties of magnesium alloys prepared by different forming processes are significantly different. Although cast magnesium alloys are favored for their high production efficiency, low cost and relatively simple production process, they are prone to defects such as coarse grains, uneven structural composition and poor mechanical properties during the casting process, which limits their application potential in a wider range of fields. In contrast, magnesium alloys that have been deformed usually exhibit better mechanical properties than cast magnesium alloys. Due to the poor plastic deformation ability at room temperature, deformed magnesium alloys usually require high-temperature plastic deformation, which not only reduces production efficiency but also increases the consumption of resources and energy. In order to improve the performance of deformed magnesium alloys while reducing processing procedures and production costs, it is necessary to explore new processing techniques and technologies. Based on this idea, high-performance deformed magnesium alloys developed through the comprehensive development of technologies such as fine grain strengthening, precipitation strengthening and texture strengthening have greatly improved the development potential and application prospects of deformed magnesium alloys. Therefore, the preparation of high-performance deformed magnesium alloys through new processes and technologies has become a new development trend in the magnesium alloy industry at home and abroad. Among the many strategies for improving the performance of magnesium alloys, grain refinement technology stands out as a key means to improve its performance. At present, the commonly used methods for grain refinement of magnesium alloys include alloying, powder metallurgy, continuous plastic deformation, low-temperature plastic deformation, and large plastic deformation. These technologies improve the strength and plasticity of magnesium alloys by increasing the number of grain boundaries and improving the characteristics of grain boundaries, providing new opportunities for the further application of magnesium alloys.

[0003] Severe Plastic Deformation (SPD) process refers to the process of applying a large amount of plastic strain to bulk metal during metal forming to form ultrafine-grained metal for the manufacture of high-strength and lightweight parts. In order to apply extremely large strains without changing the shape of the material, people have developed a variety of SPD processes. The ultrafine-grained metals created by the SPD process exhibit extremely high strength, so they can be used as ultra-high-strength metals. Among them, the SPD processes based on extrusion technology include equal channel angular pressing (Equal Channel Angular Pressing, ECAP), high-pressure torsion (High-Pressure Torsion, HPT), rotary die equal channel angular pressing (Rotary-die Equal Channel Angular Pressing, Rotary-die ECAP), cyclic extrusion compression (Cyclic Extrusion-Compression, CEC), cyclic closed-die forging (Cyclic Closed-Die Forging, CCDF) and parallel equal channel angular pressing (Parallel Channel Equal Channel Angular Pressing, PC-ECAP). Many new SPD processes that have emerged in recent years are formed by combining traditional SPD processes (such as ECAP and HPT) with other technologies. The benefits of this composite process include simplified tool design, reduced load, reduced material loss, the ability to process larger workpieces, automated processing, and possible continuous operation. Relevant experimental results have shown that the composite process based on ECAP can simultaneously improve the strength and elongation of the material.

[0004] In the 1990s, Valiev et al. experimentally demonstrated the great potential of ECAP technology in preparing ultrafine-grained materials with unique properties. Since ECAP is simpler and less expensive than other SPD processes, it can produce ultrafine-grained materials with uniform and dense structures without changing the shape of the material, and is considered to be one of the most promising SPD methods. ECAP refers to the process of lubricating the material and then subjecting it to large plastic deformation at the corner of the extrusion channel in an approximately pure shear manner through external pressure, thereby preparing submicron or even nanometer-level ultrafine-grained structures with almost no change in the cross-sectional area of ​​the material. After ECAP treatment, the grain basal plane of the magnesium alloy forms a certain angle with the extrusion axis, which is conducive to the activation of more slip systems. Therefore, ECAP is more suitable for magnesium and its alloys, which usually have only limited slip systems at room temperature.

[0005] The grain size of magnesium alloy treated by ECAP process has been significantly refined, which greatly improves the strength and plasticity of the material. The research of Martynenko et al. shows that the grain size of magnesium alloy WE43 treated by 12 ECAP passes can be refined to 0.7-1μm, and the texture of the alloy is transformed from basal surface texture to prismatic surface texture. The transformation of its microstructure and texture significantly improves the strength and plasticity of the alloy. Huang et al. treated Mg-Al-Ca-Mn alloy by 12 ECAP passes, and achieved a tensile strength of 372MPa and an elongation at break of 8%. They believed that the formation of fine dynamic recrystallized grains and the dynamic precipitation of rich nanoscale precipitation phases not only enhanced the strength of the alloy, but also improved the ductility of the alloy by refining and homogenizing the microstructure and weakening the texture. Yuan et al. used pre-solution heat treatment on ZK60 magnesium alloy to precipitate fine metastable phase particles (MgZn 2 ), these precipitated particles improve the work hardening of the alloy during the SPD process, thereby improving the strength and ductility of the alloy at the same time. Lu et al.'s research shows that the Mg-1.8Gd-1Zn-0.1Zr alloy prepared by solution treatment and 16 passes of ECAP treatment has a yield strength of 334.4MPa and an elongation of 22.5%, which significantly improves the comprehensive mechanical properties of the alloy. In addition, the alloy exhibits good plastic forming ability during the rolling process after ECAP, and can be successfully rolled into a thinner magnesium sheet while maintaining a high tensile strength and appropriate elongation.

[0006] Nevertheless, the ECAP technology is still limited in its widespread application in large-scale industrial production due to its low processing efficiency and poor single-pass grain refinement effect.

[0007] The factors affecting ECAP technology are as follows:

[0008] (1) Influence of mode angles ψ and Φ

[0009] The reason why ECAP can produce plastic deformation close to pure shear is the result of the combined effect of the extrusion pass number N, the die inner angle Φ, and the die outer angle ψ. During the experiment, the material is first placed vertically in the die and extruded from the right channel by the punch under the action of force. Then, when the material passes through the corner, a close to ideal pure shear deformation occurs inside. Since the size and shape of the material do not change during the extrusion process, multiple extrusions can be performed to accumulate the deformation of each time, and finally a large total strain is obtained to obtain the required grain size.

[0010] (2) Influence of extrusion path

[0011] According to the direction and angle of sample rotation during ECAP, it can be divided into the following four paths:

[0012] 1) Path A: After extrusion, the sample is directly placed into the mold without any rotation for the next process;

[0013] 2) BA path: the sample is rotated 90° after extrusion and enters the next pass, with the rotation direction changing alternately;

[0014] 3) BC path: the sample is rotated 90° after extrusion and enters the next pass, and the rotation direction remains unchanged;

[0015] 4) Path C: After extrusion, the sample is rotated 180° and enters the next pass.

[0016] (3) Influence of extrusion passes

[0017] According to the total formula of ECAP equivalent cumulative strain, the total equivalent cumulative strain increases exponentially with the increase of extrusion passes, and the grain size of the material becomes smaller and smaller. However, the actual experimental results do not match this. Within a certain range, with the increase of ECAP passes, the material accumulates a large amount of strain during the ECAP process, the proportion of high-angle grain boundaries gradually increases, the grain size decreases, and the grain refinement effect is obvious. However, after exceeding this range, although the high-angle grain boundaries still gradually increase with the increase of extrusion passes, the grain size hardly changes. At the same time, after too many extrusion passes, the defects inside the material will increase, and even cracks will appear inside the material, which seriously affects the performance of the material.

[0018] (4) Influence of extrusion temperature

[0019] According to thermodynamic theory, during the plastic deformation of metal materials, the higher the temperature, the higher the internal energy of the atoms, the greater the kinetic energy of the atoms, the stronger the activity of the dislocations, and the easier it is for the grains to recover and recrystallize, thereby refining the grains. However, if the temperature continues to rise, the grains will increase in size, and the same is true for the ECAP deformation of the material.

[0020] (5) Influence of extrusion speed

[0021] Within a certain range, the extrusion speed has little effect on the grain refinement of the material, but too fast an extrusion speed will affect the grain refinement of the ECAP structure. The results show that within a certain range, the extrusion speed has no obvious effect on the grain size of the material, but it affects the uniformity of the grain structure distribution; when the extrusion speed exceeds a certain range, the grains are severely damaged, the grain size becomes uneven, and the proportion of equiaxed crystals inside the grains decreases. Summary of the invention

[0022] The purpose of the present invention is to provide a composite extrusion method for preparing high-performance grain-refined magnesium alloy in view of the limitations of the existing magnesium alloy grain-refining ECAP technology.

[0023] In recent years, more and more scholars have devoted themselves to studying the use of large plastic deformation technology to process ultrafine grained materials. Among the many large plastic deformation technologies, equal-channel angular pressing (Equal-Channel Angular Pressing, ECAP) is one of the most studied methods for preparing ultrafine grains (Ultrafine Grains, UFG) metal materials. Many related research results show that magnesium alloys that have undergone large plastic deformation have excellent comprehensive mechanical properties and have great potential for industrial application. However, due to the shortcomings of low ECAP processing efficiency and limited single-pass grain refinement, it is currently difficult to further apply it on a large scale. To this end, the present invention proposes a composite extrusion method for preparing high-performance grain-refined magnesium alloys.

[0024] To achieve the purpose of the present invention, the technical solution of the present invention is as follows:

[0025] A composite extrusion method for preparing a high-performance grain-refined magnesium alloy, the specific process method is as follows:

[0026] (1) Clean the mold and evenly spray molybdenum disulfide (MoS) on the extrusion channel 2 ) solid lubricant to reduce friction; close the mold and place it at a suitable extrusion position on the workbench and wrap it tightly with refractory cotton; place induction heating plates on both sides of the mold and put thermocouples into the extrusion channel to measure the temperature, heat it to the experimental temperature and keep it warm for 30 minutes.

[0027] (2) Grind the sample to remove the oxide layer and then spray MoS uniformly 2 Solid lubricant: place it in a muffle furnace, heat it to 300-350°C, and then keep it warm for 15 minutes.

[0028] (3) Coordinate the insulation time of the sample and the mold; after the insulation time is over, remove the heating and temperature measuring equipment, put the sample into the extrusion channel, and extrude it at a speed of 2-4 mm / s; after the extrusion is completed, the sample and the mold are air-cooled together in the original position on the workbench; after cooling to room temperature, open the mold and take out the sample.

[0029] The present invention has the following remarkable effects:

[0030] (1) With the process of the present invention, the tensile strength, elongation and strength-ductility product of the magnesium alloy sample increase first and then decrease with the increase of deformation temperature. When the deformation temperature is 325 degrees, the tensile strength, elongation and strength-ductility product of the sample reach the maximum value, which are 298.02MPa, 23.42% and 6.98GPa respectively, which are 184.3%, 413.6% and 1354.2% higher than the original sample respectively. It shows that the performance of the magnesium alloy is greatly enhanced by the extrusion method of the present invention.

[0031] (2) Compared with the traditional multi-pass ECAP technology, the present invention adopts the equal channel angular extrusion-forward extrusion composite extrusion technology, which has the advantages of short processing flow, no need to heat treat the raw materials before extrusion, low equipment requirements, high equivalent strain degree and uniform distribution.

[0032] (3) The composite extrusion method of the present invention is to add a forward extrusion convergence structure to the traditional ECAP structure by ECAP-FE, further improve the extrusion ratio to promote grain refinement, and significantly improve the problem of poor uniformity of grain refinement effect after single-pass ECAP. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of conventional magnesium alloy extrusion SPD process;

[0034] Figure 2 Schematic diagram of conventional magnesium alloy extrusion ECAP process;

[0035] Figure 3 This is a schematic diagram of the conventional magnesium alloy extrusion ECAP process route;

[0036] Figure 4 Schematic diagram of the magnesium alloy ECAP-FE extrusion die of the present invention;

[0037] Figure 5 This is a schematic diagram of a nano-metal processing test bench for the magnesium alloy ECAP-FE extrusion process of the present invention;

[0038] Figure 6 Schematic diagram of SEM images of the cross section and longitudinal section of the ECAP-FE extruded sample of the magnesium alloy of the present invention.

[0039] Figure 1 Middle: (a) Equal channel angular pressing; (b) high pressure torsion; (c) rotary die equal channel angular pressing; (d) cyclic extrusion compression; (f) parallel equal channel angular pressing.

[0040] Figure 4 Middle: The magnesium alloy ECAP-FE extrusion die consists of a working auxiliary cylinder, a working main cylinder, a column, a temperature control system equipped with an induction heater, and a hydraulic control system.

[0041] Figure 6 Middle: (a) cross section; (b) longitudinal section. Figure 6 In Figure a, β-Mg 17 Al 12 The phase is obviously refined, and the overall shape is small strips or small-area particles, accounting for a small area. Figure 6 In the sample of Figure b, β-Mg 17 Al 12The phases are evenly distributed and the phase area accounts for a low proportion. They are mostly granular or short rods along the ED direction, without large flake structures. 17 Al 12 The phase area accounts for less than 5% and 6% respectively. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be fully described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0043] A composite extrusion method for preparing a high-performance grain-refined magnesium alloy, the specific process method is as follows:

[0044] (1) Clean the mold and evenly spray molybdenum disulfide (MoS) on the extrusion channel 2 ) solid lubricant to reduce friction; close the mold and place it at a suitable extrusion position on the workbench and wrap it tightly with refractory cotton; place induction heating plates on both sides of the mold and put thermocouples into the extrusion channel to measure the temperature, heat it to the experimental temperature and keep it warm for 30 minutes.

[0045] (2) Grind the sample to remove the oxide layer and then spray MoS uniformly 2 Solid lubricant: place it in a muffle furnace, heat it to 300-350°C, and then keep it warm for 15 minutes.

[0046] (3) Coordinate the insulation time of the sample and the mold; after the insulation time is over, remove the heating and temperature measuring equipment, put the sample into the extrusion channel, and extrude it at a speed of 2-4 mm / s; after the extrusion is completed, the sample and the mold are air-cooled together in the original position on the workbench; after cooling to room temperature, open the mold and take out the sample.

[0047] The research results show that the Equal Channel Angular Pressing-Forward Extrusion (ECAP-FE) composite extrusion technology of the present invention has the advantages of short processing flow, no need to heat treat the raw materials before extrusion, low equipment requirements, high equivalent strain degree and uniform distribution compared with the traditional multi-pass ECAP technology. ECAP-FE adds the convergence structure of forward extrusion to the traditional ECAP structure, further improving the extrusion ratio and promoting grain refinement, while significantly improving the problem of poor uniformity of grain refinement effect after single-pass ECAP.

[0048] The cast AZ91D magnesium alloy was extruded by ECAP-FE composite extrusion technology. Commercial cast AZ91D magnesium alloy bars were selected and machined into cylindrical specimens with a size of Φ19.7×130mm. The specific chemical composition is shown in Table 1.

[0049] Table 1 Main element composition of AZ91D magnesium alloy

[0050]

[0051]

[0052] The internal structure and physical object of ECAP-FE composite extrusion die Figure 4 The die consists of an ECAP part and a FE part. The extrusion channel of the ECAP part is Φ20mm, the inner angle of the die is Φ90° (without arc transition), and the outer angle of the die is ψ45°; the inlet of the extrusion channel of the FE part is Φ20mm, the outlet is Φ10mm, and the cone angle is 5°. The organization orientation of the extruded product is shown in Figure 4 As shown in ND, ED and TD, the ND-TD direction constitutes the cross section, and the ND-ED direction constitutes the longitudinal section.

[0053] After the untreated commercial cast AZ91D magnesium alloy rod is subjected to the single-pass ECAP-FE composite extrusion of the present invention, the tensile strength and elongation of the sample reach 290 MPa and more than 23% respectively.

Claims

1. A composite extrusion method for preparing a high-performance grain-refined magnesium alloy, characterized in that: The composite extrusion method is as follows: (1) Clean the mold and spray molybdenum disulfide (MoS2) solid lubricant evenly on the extrusion channel to reduce friction; tighten the mold and place it at a suitable extrusion position on the workbench and wrap it tightly with refractory cotton; place induction heating plates on both sides of the mold and put thermocouples into the extrusion channel to measure the temperature, heat it to the experimental temperature and keep it warm for 30 min; (2) After grinding the sample to remove the oxide layer, evenly spray MoS2 solid lubricant on it; place it in a muffle furnace and heat it to 300-350°C and keep it warm for 15 min; (3) Coordinate the insulation time of the sample and the mold; after the insulation time is over, remove the heating and temperature measuring equipment, place the sample in the extrusion channel, and extrude it at a speed of 2-4 mm / s; after the extrusion is completed, the sample and the mold are air-cooled together in the original position on the workbench; after cooling to room temperature, open the mold and take out the sample.

2. The composite extrusion method for preparing a high-performance grain-refined magnesium alloy according to claim 1, characterized in that: The die consists of an ECAP part and a FE part. The extrusion channel of the ECAP part is Φ20 mm, the inner angle Φ of the die is 90°, and there is no arc transition; the outer angle ψ of the die is 45°; the inlet of the extrusion channel of the FE part is Φ20 mm, the outlet is Φ10 mm, and the cone inclination angle is 5°.

3. The composite extrusion method for preparing a high-performance grain-refined magnesium alloy according to claim 1, characterized in that: After single-pass ECAP-FE composite extrusion of the present invention, the tensile strength and elongation of the sample reached 290 MPa and more than 23%, respectively.

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