Multi-element low-alloying high-speed extrusion flame-retardant magnesium alloy and preparation method thereof

Abstract

The invention discloses a multi-element low-alloying high-speed extrusion flame-retardant magnesium alloy and a preparation method thereof. The alloy comprises the following components in percentage by mass: 0.5 to 2.0 percent of Al, 0.2 to 1.0 percent of Zn, 0.1 to 1.0 percent of Mn, 0.2 to 0.8 percent of Ca, 0.2 to 1.0 percent of heavy rare earth element Y, Er or Gd and the balance of Mg and inevitable impurities. Through the synergistic effect of non-rare earth elements Al, Zn, Mn and Ca and trace heavy rare earth elements, matching of low cost, good mechanical property and flame retardant property is realized. After the alloy is subjected to optimized bipolar homogenization heat treatment, the alloy can be subjected to extrusion production at the extrusion rate of 10-30 m / min. The room-temperature ultimate tensile strength is larger than or equal to 270 MPa, the yield strength is larger than or equal to 240 MPa, the ductility is larger than or equal to 12%, and the ignition point is larger than or equal to 850 DEG C. Important support is provided for propelling the magnesium alloy extrusion material to achieve equipment lightweight and low-carbon manufacturing in the fields of rail transit, aerospace and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnesium alloy, and particularly relates to a multi-element low-alloyed high-speed extruded fire-retardant magnesium alloy and a preparation method thereof. BACKGROUND

[0002] Magnesium alloy has great development potential in high-end equipment manufacturing fields such as rail transit and aerospace due to its excellent lightweight characteristics. However, the current commercial magnesium alloy materials still face many technical bottlenecks in actual engineering applications, which limits their further promotion and application.

[0003] Firstly, the high activity of magnesium alloy makes it prone to spontaneous combustion during smelting and use, and once it catches fire, it is extremely difficult to extinguish. This characteristic requires additional protective measures during smelting and use to avoid oxidation and burning. Secondly, magnesium has an HCP structure, which makes its extrudability relatively weak and cannot be compared with aluminum alloys with an FCC structure, resulting in that the current lightweight alloy profiles are basically aluminum alloys. Especially in common commercial Mg-Al and Mg-Zn alloys, due to the presence of a large amount of low-melting-point Mg-Al and Mg-Zn phases, it is difficult to produce at an extrusion speed of more than 5 m / min, which makes it difficult for the alloy to realize high-efficiency extrusion and greatly increases the production cost.

[0004] In order to solve these technical problems, researchers have proposed two main improvement methods: one is low alloying; for example, CN 119392063 A discloses a low-alloyed high-strength and high-toughness magnesium alloy that can maintain excellent extrudability at an extrusion speed of 12-60 m / min. The second is to replace low-melting-point phases with high-melting-point phases; for example, CN 109338187 B proposes a low-cost high-speed extrudable high-strength and high-toughness deformed Mg-Bi-Sn-Al-Ca magnesium alloy, which contains a large amount of high-melting-point Mg3Bi2 and Mg2CaBi2 phases, ensuring that it can still maintain good mechanical properties at an extrusion speed of 0.1-30 m / min. However, the existing technology has limited effect on the improvement of mechanical properties after extrusion and heat treatment, and the fire-retardant performance does not meet the application standard, which makes it difficult for the existing magnesium alloy to meet the high requirements in the fields of rail transit, aerospace, etc.

[0005] In summary, the development of a new low-cost magnesium alloy material that can be high-speed extruded, age-hardened and highly fire-retardant has become a technical problem that needs to be solved by those skilled in the art. The breakthrough of this technology will be expected to promote the application of magnesium alloy in more fields and contribute to the realization of equipment lightweight and low-carbon manufacturing. SUMMARY

[0006] In view of the above-mentioned deficiencies existing in the prior art of magnesium alloy, the present application aims to provide a multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy and a preparation method thereof, so as to solve the problem of mismatching of extrudability-mechanical property-flame-retardant property of the magnesium alloy.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: A multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy, characterized in that the mass percentage of each component is as follows: Al: 0.5-2.0 %, Zn: 0.2-1.0 %, Mn: 0.1-1.0 %, Ca: 0.2-0.8 %, heavy rare earth elements Y, Er or Gd: 0.2-1.0 %, and the balance is Mg and inevitable impurities.

[0008] Further, the sum of the contents of the four elements of Al, Zn, Mn and Ca is ≤3.0 wt %.

[0009] Further, the sum of the contents of Ca and heavy rare earth is 0.5 wt %≤Ca and heavy rare earth content≤1.5 wt %.

[0010] Further, a continuous and dense REO-CaO-MgO composite oxide film can be formed on the surface of the alloy above 500 ℃.

[0011] Further, after being subjected to double-pole homogenization annealing, the alloy can be extruded at an extrusion rate of 10-30 m / min.

[0012] Further, after being subjected to solid solution and aging treatment, the extruded alloy exhibits excellent comprehensive mechanical and flame-retardant properties: ultimate tensile strength at room temperature ≥270 MPa, yield strength ≥240 MPa, elongation ≥12 %, and ignition point ≥850 ℃.

[0013] The present application also provides a preparation method of the multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy, which comprises the following steps: 1) The raw materials of high-purity Mg ingot, pure Al ingot, pure Zn ingot, Mg-Mn intermediate alloy ingot, Mg-Ca intermediate alloy ingot and Mg-RE intermediate alloy ingot are cut into blocks for easy melting, and the surface of the raw materials is polished to remove the surface oxide scale; then, the weighed raw materials are placed in an electric resistance furnace for preheating, and the preheating temperature is 200-400 ℃, and the time is 10-30 min; at the same time, the 304 stainless steel crucible for melting is cleaned with room temperature tap water to remove oil stains and impurities, and the surface residual liquid is dried; 2) Put the pure Mg ingot, pure Al ingot and pure Zn ingot preheated sufficiently into the crucible, and put the crucible into the pit-type resistance furnace preheated to 700-740 ℃; then, keep it in the CO2 and SF6 mixed protective atmosphere for 60-120 min, to obtain the liquid alloy; then, heat the melt to 740-760 ℃, add the Mg-Ca intermediate alloy and the Mg-Mn intermediate alloy, and keep it for 10-30 min; after the keeping, use the stainless steel stirrer for the first stirring, the stirring time is 2-5 min, and the scum formed on the surface of the melt is salvaged; then, the melt temperature is raised to 760-780 ℃, the Mg-RE intermediate alloy is added, and the keeping is continued for 10-30 min; after the keeping, the second stirring is carried out, the stirring time is 2-5 min, and the scum formed on the surface of the melt is salvaged; after the salvaging, the melt is kept still, and the keeping is carried out for 5-20 min; then, 0.5-1.5 g of the refining agent is added, and the refining is carried out for 10-30 min; after the refining, the third stirring is carried out, the stirring time is 2-5 min, and the scum on the surface of the melt is salvaged again; finally, the melt is kept still, and the keeping is carried out for 10-20 min before being taken out; the whole smelting process of adding the intermediate alloy, stirring and salvaging the scum is carried out in the protective atmosphere of CO2 and SF6, and the power supply needs to be disconnected at the same time; 3) Take out the magnesium melt and the crucible after the smelting in step 2), and slowly stretch the crucible into the room temperature tap water at a uniform speed under the protective atmosphere of CO2 and SF6 for water cooling; after the melt is completely solidified, the protective atmosphere is removed, and the air cooling to room temperature is taken out; then, the crucible is cut off, and is turned into an alloy ingot with a diameter of 75-83 mm and a height of 30-60 mm; 4) The alloy ingot obtained in step 3) is subjected to homogenization annealing treatment; the annealing heat treatment system is kept at 450-500 ℃ for 2-8 h, and then kept at 350-400 ℃ for 10-24 h; after the annealing treatment, the water cooling to room temperature is adopted, and the surface oxide layer is polished off by using a grinding wheel; 5) The annealed alloy ingot in step 4) is preheated in the resistance furnace, the preheating temperature is 250-450 ℃, and the preheating time is 20-60 min; 6) The alloy ingot preheated in step 5) is subjected to hot extrusion, the extrusion ratio is 9:1-80:1; the extrusion temperature is 250-450 ℃, the extrusion speed is 10-30 m / min, and the extruded material is obtained; 7) The extruded material in step 6) is subjected to solid solution at 400-500 ℃ for 5-30 min, and then is subjected to aging at 150-200 ℃ for 30-240 min.

[0014] Further, in step 2), the CO2 and SF6 are all commercially available products, the SF6 volume ratio is 0.1-1.0%.

[0015] Further, in step 2), the refining agent is hexachloroethane, RJ series or chloro salt flux.

[0016] The application also provides an application of the multi-element low-alloyed high-speed extruded fire-resistant magnesium alloy, and the magnesium alloy prepared by the method is used to prepare a fire-resistant magnesium alloy extrusion material in the fields of rail transit, aerospace and automobiles.

[0017] Compared with the prior art, the application has the following beneficial effects: 1. The application creatively utilizes the interaction and synergistic effect of multi-alloy elements, adopts non-rare earth Al, Mn, Zn and Ca and rare earth elements Y, Er or Gd to interact, forms rich high-density uniform dispersed Al-RE phase, Al-Mn phase, Al-Mn-RE phase and Mg-Ca phase, these phases can play a good precipitation strengthening effect, and inhibit the grain growth in the high-speed extrusion and T6 (solid solution + aging) process. Among them, the Ca element acts as a "third element", effectively improves the activity of heavy rare earth elements Y, Er and Gd, and significantly improves the oxidation resistance and fire resistance of the alloy, and a dense REO-CaO-MgO oxide film is formed on the surface above 500 ℃, so that the alloy has a burning point of more than 850 ℃ in the as-cast, extruded, solid solution and aging states.

[0018] 2. The application adopts a multi-element low-alloying method, adds a small amount of non-rare earth elements Al, Mn, Zn and Ca and heavy rare earth elements (Heavy Rare Earth Elements, HREE) Y, Er or Gd in pure Mg, so that the alloy can be extruded at an extrusion speed of 10-30 m / min, and exhibits an extrusion formability comparable to aluminum alloy; and the optimized double-pole homogenization heat treatment and solid solution aging heat treatment process are combined to jointly optimize the cost, mechanical properties, fire resistance and extrudability of the new type of magnesium alloy material.

[0019] 3. The multi-element low-alloyed high-speed extruded fire-resistant magnesium alloy developed by the application has a designed double-pole homogenization heat treatment system: 450-500 ℃ for 2-8 h, and then 350-400 ℃ for 10-24 h. The system aims to dissolve the low-melting point phase in the alloy, and promote the precipitation of Al-RE and Al-Mn-RE nano phases in the organization before extrusion, so as to effectively improve the organization morphology and extrudability.

[0020] 4、The multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy developed by the application has excellent aging strengthening effect, and after aging, high-density Al-Ca and Zn-Ca G.P. zones (Guinier-Preston Zones) are precipitated in the structure, so that the alloy exhibits excellent comprehensive mechanical properties and flame-retardant properties after T6 heat treatment: ultimate tensile strength ≥ 270 MPa at room temperature, yield strength ≥ 240 MPa, elongation ≥ 12%, ignition point ≥ 850 ℃, which provides important support for promoting the lightweight and low-carbon manufacturing of magnesium alloy extruded materials in the fields of rail transportation, aerospace and the like. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 SEM photos of Example 4 before and after bipolar homogenization treatment; Figure 2 Macroscopic photos of extruded rods of Examples 4-6 and Comparative Examples 1-4; Figure 3 OM photos of T6 treated Examples 4-6 and Comparative Examples 1-4. DETAILED DESCRIPTION

[0022] In order for those skilled in the art to better understand the technical solutions of the present application, the present application will be further described below in conjunction with specific examples, but the embodiments of the present application are not limited thereto.

[0023] Based on solving the problem of unbalanced comprehensive performance in the prior art, the present application creatively utilizes the interaction of multiple alloy elements, and provides a multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy and a preparation method thereof, which solves the problem of mismatching of extrudability-mechanical properties-flame-retardant properties of magnesium alloys. The developed new alloy is expected to meet the new demands of magnesium alloy materials in high-tech fields such as rail transportation, aerospace and the like.

[0024] I. The present application provides a multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy and a preparation method thereof The mass percentage of each component is: Al: 0.5-2.0 %, Zn: 0.2-1.0 %, Mn: 0.1-1.0 %, Ca: 0.2-0.8 %, heavy rare earth elements Y, Er or Gd: 0.2-1.0 %, and the balance is Mg and unavoidable impurities; wherein the sum of the contents of Al, Zn, Mn and Ca is ≤ 3.0 wt%; 0.5 wt% ≤ the sum of Ca and heavy rare earth content ≤ 1.5 wt%.

[0025] Table 1 Element contents (wt%) of magnesium alloys in Examples 1-3 and Comparative Examples 1-4 Examples Al Zn Mn Ca HREE Mg Impurities Example 1 0.57 0.26 0.22 0.29 0.31 98.34 0.01 Example 2 0.87 0.49 0.55 0.47 0.49 97.11 0.02 Example 3 1.64 0.54 0.27 0.54 0.76 96.24 0.01 Comparative Example 1 0.35 0.14 0.06 0.16 0.14 99.14 0.01 Comparative Example 2 2.44 1.43 1.12 0.95 1.23 92.81 0.02 Comparative Example 3 1.82 0.94 0.85 0.71 - 95.67 0.01 Comparative Example 4 2.89 0.49 0.48 - - 96.13 0.01 From Table 1, in Examples 1-3, the contents of Al, Zn, Mn, Ca and heavy rare earth elements are controlled within the range designed in the application. In Comparative Example 1, the contents of all elements are lower than the design requirements, in Comparative Example 2, the contents of all alloying elements exceed the design requirements, in Comparative Example 3, no heavy rare earth element is added, and in Comparative Example 4, it is a commercial AZ31B alloy.

[0026] In the application, the alkaline earth metal Ca element acts as the effect of "third element", effectively improves the activity of heavy rare earth elements and oxygen, promotes the formation of dense composite REO-CaO-MgO oxide film on the surface of the alloy, and significantly improves the oxidation resistance and flame retardant performance of the alloy. However, increasing a large amount of rare earth elements or Ca elements alone to improve the ignition point will increase the cost of the alloy and deteriorate the extrudability and aging effect.

[0027] The addition of Al, Zn, Ca and Mn elements is mainly to introduce high-density second phases such as Al-Mn phase, Al-Mn-RE phase, Al-RE phase and Mg-Ca phase, which can effectively inhibit the grain growth. At the same time, during the double-pole homogenization process, high-density Al-Mn-RE and Al-RE nanophases are precipitated in the alloy, which can effectively provide precipitation strengthening and inhibit grain growth. These high-density second phases can provide precipitation strengthening effect at room temperature, and at high temperature, they will decompose and release flame-retardant elements Ca and RE, further effectively improving the ignition point. In addition, the segregation of Zn, Ca and RE elements at the grain boundary can effectively reduce the migration rate of the grain boundary, thereby hindering the grain growth during high-speed extrusion and inhibiting the grain growth during T6 process. During the aging process, the precipitation of Zn-Ca and Al-Ca G.P. zones will further improve the strength of the alloy, especially the yield strength.

[0028] The alloying elements added in the multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy of the application are designed within a reasonable range, and there is no low-melting-point phase in the structure that deteriorates the extrudability, and the solidus temperature of the alloy is not significantly reduced, ensuring that the alloy can be extruded at an extrusion speed of 10-30 m / min.

[0029] Secondly, the application also provides a preparation method of the multi-element low-alloyed high-speed extruded flame-retardant magnesium alloy Example 4 The ingredients are prepared according to the composition and proportion shown in Example 1 in Table 1, and the following method is used for preparation, and the specific steps are as follows: ①Cut the raw materials, high-purity Mg ingot, pure Al ingot, pure Zn ingot, Mg-Mn intermediate alloy ingot, Mg-Ca intermediate alloy ingot and Mg-Y intermediate alloy ingot into blocks for easy smelting, and polish the surface of the raw materials to remove the surface oxide scale; then, place the weighed raw materials in the resistance furnace for preheating, the preheating temperature is 200 ℃, and the time is 30 min; at the same time, use room temperature tap water to clean the 304 stainless steel crucible for smelting, remove oil stains and impurities, and dry the surface residual liquid; ②Put the preheated pure Mg ingot, pure Al ingot and pure Zn ingot into the crucible, and place the crucible in the pit-type resistance furnace preheated to 700 ℃; then, keep it in the CO2 and SF6 mixed protective atmosphere for 120 min to obtain a liquid alloy; then, heat the melt to 740 ℃, add Mg-Ca intermediate alloy and Mg-Mn intermediate alloy, and keep it for 10 min; after the heat preservation is completed, use a stainless steel stirrer for the first stirring, the stirring time is 2 min, and the scum formed on the surface of the melt is salvaged; then, the melt temperature is raised to 760 ℃, Mg-Y intermediate alloy is added, and continues to be kept for 10 min; after the heat preservation is completed, the second stirring is carried out, the stirring time is 2 min, and the scum formed on the surface of the melt is salvaged; after the salvaging is completed, the melt is kept still, and the heat preservation is kept for 5 min; then, 0.5 g of refining agent is added, and refined for 10 min; after the refining is completed, the third stirring is carried out, the stirring time is 2 min; and the scum on the surface of the melt is salvaged again; finally, the melt is kept still, and the heat preservation is kept for 10 min before taking out; the whole smelting process of adding intermediate alloy, stirring and salvaging scum is carried out in the protective atmosphere of CO2 and SF6, and the power supply needs to be disconnected at the same time; ③Take out the magnesium melt and the crucible after smelting in step ②, and slowly stretch the crucible into room temperature tap water under the protection of CO2 and SF6; after the melt is completely solidified, the protective atmosphere is removed, and the air cooling to room temperature is taken out; then, the crucible is cut off, and turned into an alloy ingot with a diameter of 75 mm and a height of 60 mm; ④The alloy ingot obtained in step ③ is subjected to homogenization annealing treatment; the annealing heat treatment system is kept at 450 ℃ for 8 h, and then kept at 350 ℃ for 24 h; after the heat treatment is completed, the water is cooled to room temperature, and the surface oxide layer is polished off using a grinding wheel; ⑤Preheat the annealed alloy ingot in step ④ in the resistance furnace, the preheating temperature is 250 ℃, and the preheating time is 20 min; ⑥Hot extrusion is carried out on the preheated alloy ingot in step ⑤, the extrusion ratio is 9:1; the extrusion temperature is 250 ℃, and the extrusion speed is 10, 20 and 30 m / min respectively, and an extruded material is obtained; ⑦The extruded material in step ⑥ is solid-solved at 400 ℃ for 30 min, and then aged at 150 ℃ for 240 min; ⑧The tensile sample is taken from the center of the extruded rod, and the 10 mm x 10 mm x 10 mm cuboid is taken from the random position for OM observation and flame retardant performance test; the temperature rising rate of the ignition test is 10 ℃ / min, and the tensile machine tensile rate of the tensile mechanical property test is 1 mm / min.

[0030] Example 5 The ingredients are prepared according to the composition and proportion shown in Example 2 in Table 1, and the following method is used for preparation, and the specific steps are as follows: ①The raw materials high-purity Mg ingot, pure Al ingot, pure Zn ingot, Mg-Mn intermediate alloy ingot, Mg-Ca intermediate alloy ingot and Mg-Er intermediate alloy ingot are cut into blocks for easy melting, and the surface of the raw materials is polished to remove the surface oxide scale; then, the weighed raw materials are placed in the resistance furnace for preheating, the preheating temperature is 300 ℃, and the time is 20 min; at the same time, the 304 stainless steel crucible for melting is cleaned with room temperature tap water to remove oil stains and impurities, and the surface residual liquid is dried; ②The preheated pure Mg ingot, pure Al ingot and pure Zn ingot are put into the crucible, and the crucible is placed in the preheated to 720 ℃ well type resistance furnace; then, it is kept for 80 min under the mixed protective atmosphere of CO2 and SF6, and the liquid alloy is obtained; then, the melt is heated to 750 ℃, and the Mg-Ca intermediate alloy and the Mg-Mn intermediate alloy are added, and kept for 20 min; after the end of the heat preservation, the first stirring is carried out using a stainless steel stirrer, the stirring time is 3 min, and the dross formed on the surface of the melt is salvaged; then, the melt temperature is raised to 770 ℃, and the Mg-Er intermediate alloy is added, and the heat preservation is continued for 20 min; after the end of the heat preservation, the second stirring is carried out, the stirring time is 4 min, and the dross formed on the surface of the melt is salvaged; after the salvaging is completed, the melt is kept still, and the heat preservation is carried out for 10 min; then, 1.0 g of refining agent is added, and the refining is carried out for 20 min; after the refining is completed, the third stirring is carried out, the stirring time is 4 min, and the dross on the surface of the melt is salvaged again; finally, the melt is kept still, and the heat preservation is carried out for 15 min before taking out; the whole melting process of adding intermediate alloy, stirring and salvaging dross is carried out under the protective atmosphere of CO2 and SF6, and the power supply needs to be disconnected at the same time; ③The magnesium melt and the crucible after melting in step ② are taken out together, and the crucible is slowly and uniformly stretched into room temperature tap water for water cooling under the protective atmosphere of CO2 and SF6; after the melt is completely solidified, the protective atmosphere is removed, and the air cooling to room temperature is taken out; then, the crucible is cut off, and turned into an alloy ingot with a diameter of 80 mm and a height of 60 mm; IV. The alloy ingot obtained in step III is subjected to homogenization annealing treatment; the annealing heat treatment system is 480 °C for 5 h, followed by 370 °C for 16 h; after the heat treatment is completed, the alloy ingot is cooled to room temperature using tap water, and the surface oxide layer is removed by grinding with a grinding wheel; V. The annealed alloy ingot in step IV is preheated in an electric resistance furnace, the preheating temperature is 350 °C, and the preheating time is 40 min; VI. The preheated alloy ingot in step V is subjected to hot extrusion, the extrusion ratio is 45:1; the extrusion temperature is 350 °C, and the extrusion speed is 10, 20, and 30 m / min, respectively, to obtain an extruded material; VII. The extruded material in step VI is subjected to solid solution at 450 °C for 15 min, followed by aging at 170 °C for 120 min; VIII. The extruded rod is subjected to tensile test and OM observation of the center part and 10 mm x 10 mm x 10 mm cuboid at random positions, respectively; the temperature rising rate of the ignition test is 10 °C / min, and the tensile machine tensile rate of the tensile mechanical property test is 1 mm / min.

[0031] Example 6 The ingredients are prepared according to the composition and proportion shown in Example 3 in Table 1, and the following method is used for preparation, and the specific steps are as follows: I. The raw materials high-purity Mg ingot, pure Al ingot, pure Zn ingot, Mg-Mn intermediate alloy ingot, Mg-Ca intermediate alloy ingot and Mg-Gd intermediate alloy ingot are cut into blocks for easy melting, and the surface of the raw materials is polished to remove the surface oxide scale; then, the weighed raw materials are placed in an electric resistance furnace for preheating, the preheating temperature is 400 °C, and the time is 10 min; at the same time, the 304 stainless steel crucible for melting is cleaned using room temperature tap water to remove oil stains and impurities, and the surface residual liquid is dried; ② Put the preheated pure Mg ingot, pure Al ingot and pure Zn ingot into the crucible, and put the crucible into the pit-type resistance furnace preheated to 740℃; then, heat preservation for 60 min under the mixed protective atmosphere of CO2 and SF6, to obtain a liquid alloy; then, heat the melt to 760℃, add Mg-Ca master alloy and Mg-Mn master alloy, and heat preservation for 30 min; after the heat preservation, first stirring is performed using a stainless steel stirrer, the stirring time is 5 min, and the dross formed on the surface of the melt is salvaged; then, the melt temperature is raised to 780℃, Mg-Gd master alloy is added, and heat preservation for 30 min is continued; after the heat preservation, second stirring is performed, the stirring time is 5 min, and the dross formed on the surface of the melt is salvaged; after the salvaging is completed, the melt is kept still, and heat preservation for 20 min is performed; then, 1.5 g of a refining agent is added, and refining for 30 min is performed; after the refining is completed, third stirring is performed, the stirring time is 5 min, and the dross on the surface of the melt is salvaged again; finally, the melt is kept still, heat preservation for 20 min is performed, and then it is taken out; the whole melting process of adding master alloy, stirring and salvaging dross is performed under the protective atmosphere of CO2 and SF6, and the power supply needs to be disconnected at the same time; ③ The magnesium melt and the crucible after the melting in step ② are taken out together, and the crucible is slowly and uniformly stretched into room temperature tap water under the protective atmosphere of CO2 and SF6 for water cooling; after the melt is completely solidified, the protective atmosphere is removed, and the air-cooled alloy ingot is taken out to room temperature; then, the crucible is cut off, and the alloy ingot is turned into an alloy ingot with a diameter of 83 mm and a height of 30 mm by turning; ④ The alloy ingot obtained in step ③ is subjected to homogenization annealing treatment; the annealing heat treatment system is heat preservation for 2 h at 500℃, and then heat preservation for 10 h at 400℃; after the heat treatment is completed, the alloy ingot is cooled to room temperature using tap water, and the surface oxide layer is removed by grinding using a grinding wheel; ⑤ The annealed alloy ingot in step ④ is preheated in a resistance furnace, the preheating temperature is 450℃, and the preheating time is 60 min; ⑥ The preheated alloy ingot in step ⑤ is subjected to hot extrusion, the extrusion ratio is 80:1; the extrusion temperature is 450℃, and the extrusion speed is 10, 20 and 30 m / min, to obtain an extruded material; ⑦ The extruded material in step ⑥ is subjected to solid solution at 500℃ for 5 min, and then aging at 200℃ for 30 min; ⑧ The extruded rod is subjected to tensile test and OM observation on the center part and random position of 10 mm×10 mm×10 mm cuboid, respectively; the temperature rising rate of the ignition test is 10℃ / min, and the tensile machine tensile rate of the tensile mechanical property test is 1 mm / min.

[0032] Comparative Example 1 The ingredients were proportioned according to the composition proportions shown in Table 1 for Comparative Example 1, and the preparation process was consistent with that of Example 4, except that the homogenization treatment was only at 400 ℃ for 10 h, and the extrusion was only at an extrusion speed of 30 m / min. In addition, the same experimental procedures and methods as in Example 1 were used for OM observation, mechanical property testing, and flame retardant property testing of Comparative Example 1.

[0033] Comparative Example 2: The ingredients were proportioned according to the composition proportions shown in Table 1 for Comparative Example 2, and the preparation process and flow were consistent with those of Example 5, except that the homogenization treatment was only at 400 ℃ for 10 h, and the extrusion was only at an extrusion speed of 10 m / min. In addition, the same experimental procedures and methods as in Example 1 were used for OM observation, mechanical property testing, and flame retardant property testing of Comparative Example 1.

[0034] Comparative Example 3: The ingredients were proportioned according to the composition proportions shown in Table 1 for Comparative Example 3, and the preparation process was consistent with that of Example 6, except that the Mg-Y intermediate alloy was not included in the raw materials, the homogenization treatment was only at 400 ℃ for 10 h, and the extrusion was only at an extrusion speed of 10 m / min. In addition, the same experimental procedures and methods as in Example 1 were used for OM observation, mechanical property testing, and flame retardant property testing of Comparative Example 1.

[0035] Comparative Example 4: The magnesium alloy of this comparative example was a commercial AZ31B alloy, and the extrusion parameters, OM observation, room temperature mechanical property testing, and flame retardant property testing were consistent with those of Example 3, except that the homogenization treatment was only at 400 ℃ for 10 h, and the extrusion was only at an extrusion speed of 20 m / min.

[0036] III. Performance Testing Table 2 is a table of room temperature mechanical properties and ignition points of the magnesium alloys obtained in Examples 4-6 and Comparative Examples 1-4 after aging, as well as a table of extrusion speeds, for comparison of the microstructure and properties of the comparative examples and the comparative examples.

[0037] Table 2 As can be seen from Table 2, the alloys of Examples 4-6 are formed at extrusion speeds of 10, 20 and 30 m / min, and after aging treatment, the mechanical properties of the alloys can all meet the design requirements, and the ignition points are all above 850 ℃. In comparison, Comparative Example 1 can be extruded at an extrusion speed of up to 30 m / min, and has good plasticity, but the strength and ignition point are both lower than the design requirements; the strength and ignition point of Comparative Example 2 are higher, but the plasticity is low, and the extrusion speed is limited to below 10 m / min, and the cost also exceeds the design range; the strength and ignition point of Comparative Example 3 are both lower than the design requirements, and the extrusion speed is limited to below 10 m / min; the strength, plasticity and ignition point of Comparative Example 4 are all lower than the design requirements, and the extrusion speed is limited to below 20 m / min.

[0038] The SEM photos of Example 4 before and after bipolar homogenization in the present application are shown in Figure 1 . It can be seen that after the adopted bipolar homogenization heat treatment system, high-density Al-Ce and Al-Mn-Ce nano phases are precipitated in the alloy, which can play an excellent role in precipitation strengthening and grain growth inhibition in the subsequent extrusion process. The macro photos of the surface quality of the extruded rods of Examples 4-6 and Comparative Examples 1-4 at an extrusion speed of 30 m / min are shown in Figure 2 . As can be seen from Figure 2 , the surfaces of the extruded rods of Examples 4-6 are smooth and flat, without macro defects such as burrs, peeling or cracking; the surface of the extruded rod of Comparative Example 1 has no obvious macro defects; the surface of the extruded rod of Comparative Example 2 is rough, and there are a large number of annular cracks perpendicular to the extrusion direction; the surface of the extruded rod of Comparative Example 3 is rough; and the surface of the extruded rod of Comparative Example 4 has a large number of peeling defects.

[0039] The metallographic structure photos of Examples 4-6 and Comparative Examples 1-4 after T6 heat treatment at an extrusion speed of 30 m / min in the present application are shown in Figure 3 . As can be seen from Figure 3 , the grain sizes of the alloys of Examples 4-6 remain small after T6 heat treatment, and are all about 10 μm; the second phases are distributed in the form of dispersed fine particles in the structure. However, the grains of the alloys of Comparative Examples 1 and 3-4 are relatively coarse after T6 treatment; although a large number of second phases are contained in Comparative Example 2, which makes the grains relatively small after aging, the large number of second phases reduces the plasticity of the alloy.

[0040] In summary, the extruded bars of Examples 4-6 not only have excellent surface quality, but also the fine second phase in the microstructure effectively inhibits the grain growth during T6 process, thereby maintaining the fine-grained structure. In addition, the reasonable combination of alloying elements endows the material with excellent flame retardant properties, and finally achieves an effective balance between extrudability, mechanical properties, flame retardant properties and cost. As can be seen, through the synergistic effect of non-rare earth elements Al, Zn, Mn and Ca and trace heavy rare earth elements, the present application realizes the matching of low cost, good mechanical and flame retardant properties. After optimized double-stage homogenization heat treatment, the alloy can be extruded at an extrusion rate of 10-30 m / min. After T6 treatment, it shows excellent mechanical and flame retardant properties: ultimate tensile strength at room temperature ≥ 270 MPa, yield strength ≥ 240 MPa, elongation ≥ 12%, ignition point ≥ 850 ℃, which provides important support for promoting the realization of equipment lightweight and low-carbon manufacturing of magnesium alloy extruded materials in the fields of rail transportation, aerospace, etc.

[0041] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and not to limit the technical solutions. Those of ordinary skill in the art should understand that modifications or equivalent replacements to the technical solutions of the present application without departing from the spirit and scope of the technical solutions should be covered in the scope of the claims of the present application.

Claims

1. A flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials, characterized in that, The mass percentages of each component are as follows: Al: 0.5-2.0%, Zn: 0.2-1.0%, Mn: 0.1-1.0%, Ca: 0.2-0.8%, heavy rare earth elements Y, Er or Gd: 0.2-1.0%, with the balance being Mg and unavoidable impurities.

2. The flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials according to claim 1, characterized in that, The sum of the contents of the four elements Al, Zn, Mn and Ca is ≤3.0 wt%.

3. The flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials according to claim 1, characterized in that, 0.5 wt% ≤ the sum of Ca and heavy rare earth elements ≤ 1.5 wt%.

4. The flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials according to claim 1, characterized in that, A continuous and dense REO-CaO-MgO composite oxide film can be formed on the alloy surface at temperatures above 500 °C.

5. The flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials according to claim 1, characterized in that, After bipolar homogenization annealing, it can be extruded at an extrusion rate of 10-30 m / min.

6. The flame-retardant magnesium alloy produced by high-speed extrusion of multi-element low-alloy materials according to any one of claims 1-4, characterized in that, Extruded alloys exhibit excellent comprehensive mechanical and flame-retardant properties after solution treatment and aging: room temperature ultimate tensile strength ≥270MPa, yield strength ≥240MPa, elongation ≥12%, and ignition point ≥850℃.

7. A method for preparing a multi-element low-alloy high-speed extruded flame-retardant magnesium alloy, characterized in that, The raw materials are prepared according to the components described in claim 1, 2 or 3, comprising the following steps: 1) Cut the raw materials, including high-purity Mg ingots, pure Al ingots, pure Zn ingots, Mg-Mn master alloy ingots, Mg-Ca master alloy ingots, and Mg-RE master alloy ingots, into blocks that are easy to melt. Grind the surface of the raw materials to remove the oxide scale. Then, place the weighed raw materials in a resistance furnace for preheating at a temperature of 200-400 ℃ for 10-30 min. At the same time, clean the 304 stainless steel crucible for melting with room temperature tap water to remove oil and impurities, and dry the residual liquid on the surface. 2) Place preheated pure Mg, pure Al, and pure Zn ingots into a crucible, and then place the crucible into a pit-type resistance furnace preheated to 700-740℃. Hold the mixture under a mixed protective atmosphere of CO2 and SF6 for 60-120 min to obtain a liquid alloy. Next, raise the melt temperature to 740-760℃, add Mg-Ca and Mg-Mn master alloys, and hold for 10-30 min. After holding, perform a first stirring with a stainless steel stirrer for 2-5 min, and skim off any scum forming on the melt surface. Subsequently, raise the melt temperature to 760-780℃, add Mg-RE master alloy, and continue holding for 10-30 min. After holding, perform a second stirring for 2-5 min, and skim off any scum forming on the melt surface. After skimming, keep the melt still and hold for 5-20 min. Then, add 0.5-1.5 g of refining agent and refine for 10-30 minutes. min; after refining, a third stirring is carried out for 2-5 min; and the slag on the surface of the melt is skimmed off again; finally, the melt is kept still and heated for 10-20 min before being taken out; the entire smelting process, including adding intermediate alloys, stirring and skimming slag, is carried out under a protective atmosphere of CO2 and SF6, and the power supply must be disconnected at the same time. 3) Take out the molten magnesium and crucible from step 2) together, and under the protective atmosphere of CO2 and SF6, slowly and uniformly immerse the crucible in room temperature tap water for water cooling; after the melt has completely solidified, remove the protective atmosphere and air cool to room temperature; then, cut off the crucible and turn it into an alloy ingot with a diameter of 75-83 mm and a height of 30-60 mm. 4) The alloy ingot obtained in step 3) is subjected to homogenization annealing. The annealing heat treatment regime is to hold at 450-500 ℃ for 2-8 h, followed by holding at 350-400 ℃ for 10-24 h. After the annealing treatment is completed, it is cooled to room temperature with tap water and the oxide layer on the surface is removed by grinding with a grinding wheel. 5) Preheat the annealed alloy ingot from step 4) in a resistance furnace at a temperature of 250-450 ℃ for 20-60 min. 6) The preheated alloy ingot from step 5) is hot-extruded with an extrusion ratio of 9:1-80:1; the extrusion temperature is 250-450℃ and the extrusion speed is 10-30 m / min to obtain extruded material; 7) Solution treat the extruded material from step 6) at 400-500 ℃ for 5-30 min, and then age it at 150-200 ℃ for 30-240 min.

8. The method for preparing a flame-retardant magnesium alloy by multi-element low-alloy high-speed extrusion according to claim 7, characterized in that, In step 2), both CO2 and SF6 are commercially available products, with SF6 accounting for 0.1-1.0% of the volume.

9. The method for preparing a flame-retardant magnesium alloy by multi-element low-alloy high-speed extrusion according to claim 7, characterized in that, Step 2) The refining agent is hexachloroethane, RJ series or chloride salt flux.

10. An application of a multi-element low-alloy high-speed extruded flame-retardant magnesium alloy, characterized in that, The magnesium alloy prepared by the method described in claims 6-9 is used to prepare flame-retardant magnesium alloy extrusions for use in rail transportation, aerospace and automotive fields.

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