High thermal conductivity flame-retardant wrought magnesium alloy material and preparation method thereof

Abstract

The application discloses a high-thermal-conductivity flame-retardant wrought magnesium alloy material and a preparation method thereof, and the magnesium alloy material comprises the following components in percentage by weight: 2-6% of Al, 1.5-5% of Ca, 0.3-1% of Zn, 0.3-0.8% of Mn, 0.3-1.5% of RE, and the balance of Mg and other inevitable impurities, wherein Al / Ca is less than or equal to 2, and RE is one to three elements selected from Ce, La, Y, Nd, Gd and Sm. The magnesium alloy material has good room-temperature mechanical properties, flame-retardant properties and excellent thermal conductivity, the material has a room-temperature thermal conductivity of greater than or equal to 123 W / m*K, a yield strength of greater than or equal to 200 MPa, a tensile strength of greater than or equal to 250 MPa, an elongation of greater than or equal to 8%, and a burning point of greater than or equal to 850 DEG C; and the material can be prepared into a rod, a pipe, a plate, a complex profile or a structural member and the like by using a hot working method, and the process applicability is wide, so the material can be popularized on a large scale.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy materials technology, and in particular to a high thermal conductivity and flame retardant deformable magnesium alloy material and its preparation method. Background Technology

[0002] Magnesium alloys possess excellent properties such as low density, high specific stiffness and strength, good thermal and electrical conductivity, electromagnetic shielding, and environmental compatibility, resulting in significant structural weight reduction. With the continuous development of magnesium alloy forming processes, the performance of magnesium alloy products is becoming increasingly superior, meeting the needs of more application scenarios. Although castings remain the mainstream product for magnesium alloys, the exploration of diverse properties and the demand for rapid, large-scale production indicate that wrought magnesium alloys have great potential for widespread application in the automotive, aerospace, and oil and gas extraction industries.

[0003] In critical components such as aviation, high-speed rail, and battery pack casings, materials not only need to have sufficiently strong mechanical properties, but also require special physical properties such as flame retardancy and thermal conductivity. However, existing commercially available wrought magnesium alloys suffer from problems such as low ignition points and easy oxidation and combustion, as well as reduced thermal conductivity in common high-strength and high-toughness magnesium alloy materials.

[0004] Key factors influencing the flame retardancy of magnesium alloys include the Pilling-Bedworth ratio, the Gibbs free energy for oxide film formation, and the solid solubility of the alloying elements. Therefore, flame-retardant alloying elements typically require that the added alloying elements react with oxygen preferentially over magnesium, and that the resulting oxides have high density. For half a century, sufficient attention has been paid to the research and development of flame-retardant magnesium alloys both domestically and internationally. Research and development of flame-retardant magnesium alloys containing Ca, Be, Zn, and RE have been carried out, and certain achievements have been made.

[0005] The thermal conductivity of pure magnesium at room temperature is approximately 160 W / m·K. Alloy design requires balancing strength and thermal conductivity; however, adding alloying elements generally reduces the thermal conductivity of magnesium alloys, and the degree of influence varies significantly among different alloying elements. Mg-Zn alloys are common thermally conductive magnesium alloys; through the control of alloying elements, the room temperature thermal conductivity of Mg-Zn alloys can exceed 120 W / m·K. In contrast, at an Al content of 2 wt%, the thermal conductivity drops to 80 W / (m·K), and when the Al content increases to 6 wt%, the thermal conductivity significantly decreases to 50 W / (m·K).

[0006] With the booming development of the magnesium alloy industry, the demand for magnesium alloys in lightweight fields such as aviation, aerospace, automotive, and rail transportation is rising sharply, requiring magnesium alloys to have superior comprehensive properties such as mechanical properties, flame retardancy, and thermal conductivity. Therefore, in-depth exploration of magnesium alloy materials with excellent comprehensive properties is of great significance for expanding the applications of magnesium alloys.

[0007] For example, the Chinese patent publication CN113811629A discloses a "flame-retardant magnesium alloy and its manufacturing method," which is a Mg-Ca-Al-Si-RE flame-retardant magnesium alloy material. Its chemical composition has a Ca mass fraction of <9%, an Al mass fraction of 0.5-5.7%, a Si mass fraction of <1.3%, rare earth elements of 0.4-1.3%, and Al+8Ca≥20.5%, with an Al / Ca ratio of <1.7. However, this magnesium alloy material has a three-dimensional network of continuous (Mg,Al)2Ca phase, resulting in poor room temperature strength and plasticity, and a thermal conductivity greater than 80 W / m·K. The patent does not disclose the specific flame-retardant effect and room temperature mechanical properties obtained by this method.

[0008] Another example is US Patent Publication No. US20190112693A1, "PLASTIC DEFORMATION MAGNESIUMALLOY HAVING EXCELLENT THERMAL CONDUCTIVITY AND FLAME RETARDANY, AND PREPARATION METHOD THEREFOR," which discloses a Mg-Zn-Sn-Ca / Si / Mn alloy with a Zn mass fraction of 0.5–5%, a Sn mass fraction of 0.6–3.5%, and at least one of Ca, Si, and Mn with a mass fraction <1.5%, and all alloying elements with a mass fraction of 2.5–6.3%. The prepared magnesium alloy materials, underwent flame retardant material testing by the US Federal Aviation Administration (FAA), all of which resulted in combustion. Furthermore, the mechanical properties were not disclosed, indicating poor flame retardant performance.

[0009] Chinese Patent Publication No. CN111286658A discloses a "die-castable high thermal conductivity and flame-retardant magnesium alloy and its preparation method," which is a Mg-Al-RE-Ca high thermal conductivity and flame-retardant cast magnesium alloy. Its chemical composition includes 2.5-4.5% Al by mass, 2-6% RE1 by mass, 0.05-0.5% RE2 by mass, and 0.01-0.45% Ca by mass; wherein RE1 is one or more of La and Ce, and RE2 is one or more of Sm and Y. This magnesium alloy has a thermal conductivity ≥120 W / m·K at room temperature, a yield strength, tensile strength, and elongation of ≥146 MPa, ≥223 MPa, and ≥6.5%, respectively, and an ignition point of ≥840℃. However, this material is a die-cast magnesium alloy, and its yield strength is less than 150 MPa, which cannot meet the mechanical properties required for certain components. Summary of the Invention

[0010] The purpose of this invention is to provide a high thermal conductivity and flame retardant deformable magnesium alloy material and its preparation method. This magnesium alloy has good room temperature mechanical properties, flame retardant properties and excellent thermal conductivity. Its room temperature thermal conductivity is ≥123W / m·K, yield strength is ≥200MPa, tensile strength is ≥250MPa, elongation is ≥8%, and ignition point is ≥850℃. This material can be prepared into bars, tubes, plates, complex profiles or structural parts by conventional hot working methods, and has wide process applicability.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows:

[0012] A high thermal conductivity and flame retardant deformable magnesium alloy material, the composition by weight percentage is: Al 2-6%; Ca 1.5-5%; Zn 0.3-1%; Mn 0.3-0.8%; RE 0.3-1.5%, the balance being Mg and other unavoidable impurities; and Al / Ca≤2; RE is 1-3 elements selected from Ce, La, Y, Nd, Gd and Sm.

[0013] Preferably, the balance is Mg and other unavoidable impurities.

[0014] The high thermal conductivity and flame retardant deformable magnesium alloy material of the present invention has a room temperature thermal conductivity ≥123W / m·K, a yield strength ≥200MPa, a tensile strength ≥250MPa, an elongation ≥8%, and an ignition point ≥850℃.

[0015] In the composition design of the high thermal conductivity and flame retardant deformable magnesium alloy material described in this invention:

[0016] As a surface-active element, Ca not only improves the flame-retardant properties of magnesium alloys, but also preferentially accumulates on the surface of magnesium alloys at high temperatures. Ca forms a composite oxide film with rare earth elements, rapidly repairing cracks in the oxide layer on the surface of liquid magnesium alloys. Furthermore, Ca reduces the solid solubility of Al, thereby reducing the impact of Al on the thermal conductivity of magnesium alloys. In the alloy described in this invention, Ca, Al, Zn, and rare earth elements form composite phases in magnesium, such as MgAlCa, Ca2Mg6Zn3, Mg2Ca, Mg2RE, and Mg4AlRE. The combined effect of these phases further enhances the flame-retardant effect of the magnesium alloy.

[0017] The addition of Zn improves the machinability of the alloy and reduces the influence of Ca on machinability, making the magnesium alloy material of the present invention easier to process. The composite phase formed by Ca and RE significantly reduces the grain size of the magnesium alloy after processing, thereby improving the mechanical properties of the flame-retardant magnesium alloy material and achieving the desired strength and toughness.

[0018] The preparation method of the high thermal conductivity and flame retardant deformable magnesium alloy material of the present invention includes the following steps:

[0019] 1) Alloy smelting and refining

[0020] Smelting according to the above composition, the order of adding alloy raw materials is Mg ingot, Al ingot, Zn ingot, Mg-Mn master alloy, Mg-Ca master alloy, Mg-RE master alloy; after all raw materials are added, refining is carried out; then standing is carried out at a temperature of 670-690℃.

[0021] 2) Casting

[0022] Cast alloy billets;

[0023] 3) Solution treatment

[0024] Solution treatment temperature: 450–530℃; solution treatment time: 8–24 h.

[0025] 4) Peeling the billet

[0026] 5) Hot working

[0027] Deformed magnesium alloy components were prepared by hot working method, with a hot working temperature of 360-460℃ and a deformation ratio of 2-45.

[0028] Furthermore, it also includes step 6) heat treatment of the hot-deformed magnesium products, with a heat treatment temperature of 150-280℃ and a time of 30-1200min.

[0029] Preferably, in step 1), the melting and refining temperature is 710–730°C, the protective gas is 1.5–2.5% SF6 + CO2, the inert gas is argon, and the blowing and stirring time is 2–5 min.

[0030] Preferably, before alloy smelting in step 1), Mg ingots, Al ingots, Zn ingots, Mg-Ca master alloy, Mg-Mn master alloy, and 1 to 3 kinds of Mg-RE master alloys are quantitatively prepared according to the composition of claim 1, and the above materials are preheated and dried; preferably, the preheating temperature is 170 to 250°C and the time is 3 to 6 hours.

[0031] Preferably, in step 1) alloy smelting, the prepared Mg ingot and alloy element ingot are melted, a protective gas is introduced, a magnesium alloy smelting refining agent is added, an inert gas is introduced and stirred, and the mixture is kept at a constant temperature for 25 to 40 minutes.

[0032] Preferably, in step 2), the casting adopts a semi-continuous casting process, the bar pulling speed is 35-60 mm / min, and the casting temperature is 670-700℃.

[0033] Preferably, the peeling thickness in step 4) is 5-10 mm.

[0034] Preferably, the hot working method in step 5) is extrusion, rolling or forging.

[0035] In the preparation method of the high thermal conductivity and flame retardant deformable magnesium alloy material described in this invention:

[0036] Mg-Ca master alloys and Mg-RE master alloys are added last during the smelting process to ensure the yield of Ca and RE elements.

[0037] The solution treatment temperature is 450-530℃ and the solution treatment time is 8-24h. This is to ensure that the composite continuous equiaxed dendritic precipitates formed by the continuous MgAlCa phase, Ca2Mg6Zn3 phase, Mg2Ca phase, Mg2RE phase and Mg4AlRE phase in the alloy are dissolved into point-like discontinuous phases, which is convenient for subsequent hot working.

[0038] The casting temperature is 670-700℃, which can ensure the fluidity of the magnesium alloy liquid affected by Ca and rare earth elements and prevent cracking during the casting process.

[0039] The deformation ratio of hot working is 2 to 45, and the temperature is 360 to 460°C. This is to ensure that hot deformation can be achieved and that the grains and precipitates of the flame-retardant magnesium alloy material can be fully broken up, thereby greatly improving the mechanical properties of the prepared alloy.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] There is currently no magnesium alloy material that simultaneously possesses high thermal conductivity, flame retardancy, and excellent mechanical properties, in contrast to the present invention.

[0042] Existing high thermal conductivity magnesium alloys are mostly materials developed for cast magnesium alloys, and their mechanical properties cannot meet the requirements of high strength and toughness components.

[0043] The preparation methods used in this invention, such as semi-continuous casting and extrusion, are simple in process and easy to control in terms of process parameters. Compared with existing high-strength and high-toughness magnesium alloy materials, they only use a small amount of rare earth elements, resulting in low production costs and making them suitable for large-scale industrial production. Detailed Implementation

[0044] The present invention will be further described below with reference to the embodiments.

[0045] The chemical composition of the magnesium alloys in the embodiments and comparative examples of this invention is shown in Table 1, with the remainder being Mg and other unavoidable impurities. The process parameters of the embodiments are shown in Table 2. The performance of the embodiments and comparative examples is shown in Table 3.

[0046] The preparation process of the magnesium-based alloy in this invention is as follows: raw material preparation, alloy smelting, billet preparation, billet solution treatment, billet peeling, hot working, and heat treatment of deformed magnesium alloy components to obtain the final product. Specifically, the billet preparation method in Examples 1-4 and Examples 8-10 is semi-continuous casting; the billet preparation method in Examples 5-7, Examples 11-13, and the comparative examples is gravity casting using metal molds.

[0047] The ignition point was determined using 0.2 mm thick foil via thermogravimetric analysis; the ignition points listed in Table 3 are measured values. Flammability tests were conducted according to standard DOT-FAA-AR-00-12_Chapter 25; Table 3 lists the weight loss rate, ignition time, and self-extinguishing time. Thermal conductivity was tested at room temperature. Room temperature tensile tests were conducted according to standard GB / T 228.1-2010.

[0048] As can be seen from Tables 1 to 3, the flame-retardant magnesium alloy prepared by this invention did not burn under aviation kerosene flame baking at 920–980℃, exhibited excellent mechanical properties, and had significantly higher thermal conductivity than conventional Mg-Al magnesium alloy materials. Therefore, this invention simultaneously achieves high strength and toughness, flame retardancy, and high thermal conductivity in magnesium alloy materials.

[0049] Comparative Example 1 uses a commonly used AZ41 magnesium alloy, which is extruded to obtain the final product. Its properties are tested, and the results are shown in Table 3. The comparison reveals the difference in alloy quality when Ca, Zn, Mn, and rare earth elements are lacking.

[0050] Comparative Example 2 is US Patent US20180030578A1. The alloy system of Comparative Example 2 is Mg-Zn system, which is a common thermally conductive magnesium alloy material. However, its ignition point and flame retardant properties are lower than those of this invention, and its mechanical properties are not mentioned.

[0051] Comparative Example 3 is Chinese Patent Publication No. CN113811629A. Comparative Example 3 uses an oxide film formed by rare earth elements (RE) to inhibit liquid ablation, and its thermal conductivity is less than 100 W / m·K. The magnesium alloy of the present invention has higher thermal conductivity, flame retardancy and mechanical properties than Comparative Example 3.

[0052] Comparative Example 4 is Chinese Patent Publication No. CN111286658A, in which the rare earth content is as high as 6.5%; the rare earth content used in this invention is less than 2.3%, the material cost is significantly lower than that of Comparative Example 4, and since Comparative Example 4 is a die-cast alloy, its mechanical properties are far lower than those of this invention.

[0053]

[0054]

[0055]

[0056]

Claims

1. A high thermal conductivity and flame retardant wrought magnesium alloy material, wherein the weight percentage of its composition is: Al 4~6%; Ca 1.5~5%; Zn 0.3~1%; Mn 0.3~0.8%; RE 0.3~1.5%, with the balance containing Mg and other unavoidable impurities; and Al / Ca≤2; RE is 1~3 elements selected from Ce, La, Y, Nd, Gd and Sm; the wrought magnesium alloy material has a room temperature thermal conductivity ≥123W / m•K, yield strength ≥200MPa, tensile strength ≥250MPa, elongation ≥8%, and ignition point ≥850℃.

2. The high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 1, characterized in that, The balance consists of Mg and other unavoidable impurities.

3. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Alloy smelting and refining The alloy raw materials are smelted according to the composition described in claim 1 or 2, and the order of addition of the alloy raw materials is Mg ingot, Al ingot, Zn ingot, Mg-Mn master alloy, Mg-Ca master alloy, and Mg-RE master alloy; after all raw materials are added, the alloy is refined and allowed to stand at a temperature of 670~690℃. 2) Casting Cast alloy billets; 3) Solution treatment Solution treatment temperature: 450~530℃, solution treatment time: 8~24h; 4) Peeling the billet 5) Hot working Deformed magnesium alloy components were prepared by hot working method, with a hot working temperature of 360~460℃ and a deformation ratio of 2~45.

4. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, It also includes step 6) heat treatment of the hot-deformed magnesium products, with a heat treatment temperature of 150~280℃ and a time of 30~1200min.

5. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, In step 1), the melting and refining temperature is 710~730℃, the protective gas is 1.5~2.5%SF6+CO2, the inert gas is argon, and the blowing and stirring time is 2~5min.

6. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, Step 1) Before alloy smelting, prepare Mg ingots, Al ingots, Zn ingots, Mg-Ca master alloy, Mg-Mn master alloy, and 1 to 3 kinds of Mg-RE master alloys according to the composition of claim 1 or 2. Preheat and dry the above materials at a temperature of 170 to 250°C for 3 to 6 hours.

7. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, Step 1) In the alloy smelting process, the prepared Mg ingot and alloy element ingot are melted, a protective gas is introduced, a magnesium alloy smelting refining agent is added, an inert gas is introduced and stirred, and the mixture is kept at a constant temperature for 25~40 minutes.

8. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, In step 2), the casting adopts a semi-continuous casting process, the bar pulling speed is 35~60mm / min, and the casting temperature is 670~700℃.

9. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, Step 4) Peel the skin to a thickness of 5~10mm.

10. The method for preparing the high thermal conductivity and flame retardant deformable magnesium alloy material as described in claim 3, characterized in that, In step 5), hot working is performed by extrusion, rolling or forging.

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