High-performance flame-retardant wrought magnesium alloy containing fine grain structure and preparation method thereof

Abstract

The high-performance flame-retardant wrought magnesium alloy comprises the following components in percentage by weight: 4.0 to 5.0 percent of Al, 1.0 to 1.6 percent of Zn, 0.2 to 2.3 percent of Ca, 0.2 to 1.8 percent of Y, less than or equal to 0.1 percent of inevitable impurities and the balance of Mg. And the elements also need to meet the following conditions: 1.5% < = Ca + Y < = 2.5%, and Al / (Ca + Y): 1.8-2.5. According to the magnesium alloy, only an Al2Ca phase, an Al2Y phase and a Ca2Mg6Zn3 phase are formed in the magnesium alloy, fine recrystallized grains are formed in an alloy structure to wrap long-strip-shaped deformed grains, it is guaranteed that the average grain size of the fine recrystallized grains is 1-3 microns, and the mechanical property and the flame retardant property of the magnesium alloy are synergistically improved. In the preparation method, heat treatment is not needed before extrusion, so that the process operation is greatly simplified, and the cost is greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of magnesium alloy materials, and in particular to a high-performance flame-retardant deformable magnesium alloy containing a fine-grained structure and a preparation method thereof. Background Art

[0002] In the context of the rapid development of rail transit equipment, in order to meet the goals of further improving the operating speed, carrying capacity and reducing operating costs of high-speed trains, subway trains, urban rail trains and other train groups, new lightweight materials are urgently needed to be promoted and applied in the field of rail transit. Lightweight materials help to significantly reduce the weight of trains, save energy and reduce emissions. At the same time, due to the weight reduction, the operation control of trains in harsh environments such as high-speed driving, turning and ups and downs is more stable. At present, rail transit vehicles mainly use aluminum alloy materials, and the 7 series aluminum alloy materials used in key load-bearing components adopt the mechanical properties specified in "GB / T 26494 Aluminum Alloy Extrusion Profiles for Rail Transit Vehicle Structures", with tensile strength ≥310MPa, yield strength ≥260MPa, and elongation ≥10%.

[0003] For the rail transit field, magnesium alloy materials have more comprehensive characteristics. As the lightest metal structural material, magnesium alloy has a density of only 2 / 3 of that of aluminum alloy and has the characteristics of high specific strength and high specific stiffness. At the same time, compared with aluminum alloy materials, magnesium alloy also has excellent functional characteristics such as vibration reduction and noise reduction. It is a new type of lightweight structural material with the most development potential in the rail transit field.

[0004] Mg-Al-Zn alloys are the most commonly used commercial magnesium alloys. They have been used in rail transit vehicle interior parts and non-critical load-bearing components. However, they face bottleneck problems such as low tensile strength (<250MPa) and ignition temperature (<600℃), which limit their large-scale application in key load-bearing components of rail transit vehicles.

[0005] According to a large number of studies, alloying methods can simultaneously improve the room temperature strength and flame retardancy of magnesium alloys. For example, the addition of Ca can form strengthening phases such as Al2Ca in Mg-Al-Zn alloys, which helps to improve the strength of the alloy; in addition, the addition of Ca helps to eliminate the low melting point β-Mg 17 Al 12 phase, and at the same time forms a protective CaO film layer, which helps to increase the ignition temperature of the alloy. In addition to Ca, some rare earth elements such as Y and Gd can also play a role in simultaneously improving the mechanical properties and ignition temperature of the alloy. However, in the Mg-Al system magnesium alloy with dual flame retardant elements that have been disclosed, it is difficult to achieve a synergistic improvement in mechanical properties, flame retardancy, and forming properties.

[0006] Chinese patent 2019800995010 discloses "aerospace-grade magnesium alloy with high strength, flame retardancy and extrudable tubes". The Mg-Al-Zn-Ca-Y-Mn series alloys developed therein can be used for certain aerospace components. They are flame resistant and have high tensile strength, but have not overcome the problem of low yield strength, which is less than 200MPa.

[0007] Chinese patent 2011800055846 discloses "non-flammable magnesium alloy with excellent mechanical properties and preparation method thereof". The disclosed Mg-Al-Zn-Ca-Y-(Mn / Zr) alloy still faces the problem of yield strength less than 250 MPa.

[0008] Chinese patent 2020104330345 discloses "a high-strength Mg-Ca-Mn-Al-Zn deformable magnesium alloy containing gadolinium and yttrium rare earth elements and its preparation method", and its components by weight percentage are: calcium: 0.50-2.00%; aluminum: 0.10-2.00%; zinc: 0.30-2.00%; manganese: 0.10-3.00%; rare earth (gadolinium or yttrium): 0.20-3.00%, and the balance is magnesium and unavoidable impurities. The disclosed deformable magnesium alloy has good mechanical properties, but the reverse extrusion process used is difficult to achieve continuous preparation.

[0009] The deformed magnesium alloy materials reported in the prior art have the problems of low strength, easy flammability and difficult forming. Therefore, it is urgently necessary to carry out research on the composition design of new deformed magnesium alloys and new forming technologies to develop new highly flame retardant deformed magnesium alloys to meet the use requirements of key load-bearing components of rail transit vehicles. Summary of the invention

[0010] The purpose of the present invention is to provide a high-performance flame-retardant deformed magnesium alloy with fine-grained structure and a preparation method thereof, so as to achieve the synergistic improvement of the room temperature strength, flame retardancy and forming ability of the deformed magnesium alloy; the room temperature tensile strength of the deformed magnesium alloy is ≥320MPa, the yield strength is ≥280MPa, the elongation is ≥10%, and the ignition point is ≥850°C, which meets the use requirements of key load-bearing components of rail transit vehicles; and, combined with a short-process deformation process, rapid extrusion can be performed without long-term heat treatment.

[0011] To achieve the above object, the technical solution of the present invention is:

[0012] A high-performance flame-retardant deformable magnesium alloy with fine-grained structure, the weight percentage of which is: Al: 4.0-5.0%, Zn: 1.0-1.6%, Ca: 0.2-2.3%, Y: 0.2-1.8%, the balance includes Mg and other inevitable impurities, wherein the total amount of inevitable impurities is ≤0.1%; and the above elements also need to meet the following requirements:

[0013] 1.5%≤Ca+Y≤2.5%;

[0014] Al / (Ca+Y):1.8~2.5.

[0015] Preferably, the balance is Mg and other inevitable impurities.

[0016] The deformable magnesium alloy structure of the present invention is fine recrystallized grains wrapped in long deformed grains, dispersed granular Al2Y second phase and Al2Ca+Ca2Mg6Zn3 second phase dispersed along the extrusion direction; wherein the average particle size of the fine recrystallized grains is 1 to 3 μm.

[0017] The room temperature tensile strength of the deformed magnesium alloy of the present invention is ≥320MPa, the yield strength is ≥280MPa, and the elongation is ≥10%.

[0018] The ignition point of the deformable magnesium alloy of the present invention is ≥850°C.

[0019] The magnesium alloy composition of the present invention is to add Ca and Y to the Mg-Al-Zn system magnesium alloy, and control 1.5%≤Ca+Y≤2.5%, Al / (Ca+Y): 1.8-2.5, Ca and Y elements will consume Al element in the alloy, and avoid Al element and Mg element reacting to form low melting point Mg 17 Al 12 phase, and form Al2Ca phase and Al2Y phase, and the remaining Ca elements form Ca2Mg6Zn3 phase, ensuring that only Al2Ca, Al2Y, and Ca2Mg6Zn3 second phases are generated in the magnesium matrix.

[0020] After extrusion deformation, the Al2Ca phase is broken and distributed in strips along the extrusion direction. Dynamic recrystallization behavior of grains occurs near the enriched strip-shaped Al2Ca phase, while long strip-shaped deformed grains are formed near the poor Al2Ca phase, ensuring the formation of a structure in which the recrystallized grains wrap the deformed grains. The addition of Y element can refine the recrystallized grains and ensure that the average grain size of the recrystallized grains is 1 to 3 μm. The fine recrystallized grains have high-density grain boundaries, which help to hinder the slip of dislocations and are beneficial to the improvement of the strength of the magnesium alloy. At the same time, the fine recrystallized grains have a strong ability to coordinate deformation, which is beneficial to the improvement of elongation. The long strip-shaped deformed grains have a better hardening effect, further ensuring that the magnesium alloy has a higher strength. In addition, the Al2Y phase is dispersed in the alloy in a granular form, mainly playing the role of second phase dispersion strengthening.

[0021] In magnesium alloys, Ca element can not only form a strengthening phase containing Ca to improve the mechanical properties of magnesium alloys, but also has a flame retardant effect. In the present invention, Ca and Y elements are added in a Mg-Al-Zn system magnesium alloy in a coordinated manner. Since Y element has a large solid solubility in the magnesium alloy matrix, it can react in situ to generate Y2O3. Ca element has a low solid solubility in the alloy matrix. By forming a low thermal stability second phase such as Ca2Mg6Zn3, it can be used as a Ca source to provide flame retardant Ca element to the magnesium alloy matrix, thereby promoting the formation of CaO on the surface of the magnesium alloy. Under the joint action of Ca / Y, by changing the surface oxide film structure of the magnesium alloy, a Y2O3 / CaO / MgO composite oxide layer is formed on the surface of the magnesium alloy, thereby improving the ignition point of the magnesium alloy.

[0022] The present invention ensures that the magnesium alloy has high room temperature mechanical properties by controlling the composition and utilizing the second phase strengthening effect of Al2Y and Al2Ca, the organization grain strengthening and the hardening effect. At the same time, due to the presence of Ca and Y elements in the alloy matrix and the effect of the Ca2Mg6Zn3 phase, the alloy can form a Y2O3 / CaO / MgO composite oxide layer on the surface of the magnesium alloy under high temperature conditions, thereby protecting the matrix from severe oxidation and improving the ignition point of the magnesium alloy, thereby achieving a synergistic improvement in the room temperature mechanical properties and flame retardant properties of the magnesium alloy.

[0023] The method for preparing the high-performance flame-retardant deformable magnesium alloy containing fine-grained structure of the present invention comprises the following steps:

[0024] 1) Ingredients

[0025] Using pure Mg ingots, pure Zn ingots, pure Al ingots, Mg-Ca master alloys, and Mg-Y master alloys as raw materials, batching and preheating are performed according to the above magnesium alloy composition;

[0026] 2) Melting

[0027] Put the preheated pure Mg ingot into a crucible, heat it to 740-750℃, melt it with protective gas, and wait until it is completely melted; add pure Al ingot, pure Zn ingot, Mg-Ca, Mg-Y master alloy, wait until the alloy is completely melted, and remove the oxide and slag on the surface of the melt;

[0028] 3) Refining

[0029] Adding a refining agent to the obtained melt, stirring it fully, and then keeping it warm for 15 to 20 minutes; cooling the refined melt to room temperature with water to obtain a magnesium alloy ingot;

[0030] 4) Preheating before extrusion

[0031] The magnesium alloy ingot is preheated at 300-320°C for 60-120 minutes;

[0032] 5) Extrusion

[0033] The preheated ingot is hot extruded at 320-340° C. to obtain a magnesium alloy rod or a magnesium alloy profile, with an extrusion speed of 10-20 mm / s and an extrusion ratio of 16-18.

[0034] Preferably, in step 3), the preheating temperature is 220-250° C., and the preheating time is 10-30 min.

[0035] Preferably, in step 3), the refining agent is hexachloroethane.

[0036] Preferably, in step 3), the protective gas is a mixed gas of N2+SF6, wherein the volume proportion of SF6 is 0.5-1.0%.

[0037] Based on the composition described in the present invention, only Al2Ca, Al2Y, and Ca2Mg6Zn3 second phases are formed in the magnesium alloy matrix. The Al2Ca, Al2Y, and Ca2Mg6Zn3 second phases formed in the alloy are the key to the high performance of the magnesium alloy, so the present invention ensures the existence of these second phases in the magnesium alloy after extrusion. Moreover, the formed Al2Ca, Al2Y, and Ca2Mg6Zn3 second phases have good thermal stability, will not melt after rapid extrusion, and will not affect the forming performance. Therefore, the present invention does not need to perform short-range or long-range heat treatment before deformation to eliminate these second phases.

[0038] In the extrusion process of the present invention, the extrusion temperature is controlled at 320-340°C, which can ensure that the alloy structure obtains a structure in which fine recrystallized grains wrap long strip-shaped deformed grains. Extrusion temperature that is too high will cause the deformed grains to disappear and the recrystallized grains to coarsen. Extrusion temperature that is too low cannot form enough recrystallized grains. The extrusion speed is 10-20 mm / s, which is also to ensure that the structure in which fine recrystallized grains wrap long strip-shaped deformed grains can be obtained. The extrusion ratio is controlled at 16-20, which can ensure that the Al2Y phase is dispersed in the matrix, and the Al2Ca phase is broken after extrusion deformation and distributed in strips along the extrusion direction.

[0039] Beneficial effects of the present invention:

[0040] 1. In terms of composition design, the present invention adds Ca and Y elements to the Mg-Al-Zn magnesium alloy system in a coordinated manner, and controls 1.5%≤Ca+Y≤2.5%, Al / (Ca+Y): 1.8-2.5, to ensure that only Al2Ca phase, Al2Y phase and Ca2Mg6Zn3 phase are formed in the magnesium alloy, and the alloy structure forms fine recrystallized grains encapsulating long strip-shaped deformed grains, ensuring that the average grain size of the fine recrystallized grains is 1-3 μm, and utilizing the second phase strengthening effect, organizational grain strengthening and hardening effect to improve the mechanical properties of the magnesium alloy. At the same time, due to the presence of Ca and Y elements in the alloy matrix, plus the effect of the Ca2Mg6Zn3 phase, the alloy can form a Y2O3 / CaO / MgO composite oxide layer on the surface of the magnesium alloy under high temperature environment, thereby protecting the matrix from severe oxidation and improving the ignition point of the magnesium alloy. The mechanical properties and flame retardant properties of the magnesium alloy are synergistically improved.

[0041] 2. Based on the composition design, the magnesium alloy ingot obtained in the present invention is directly subjected to rapid extrusion after simple preheating, which can also ensure that the obtained bars and profiles have good forming properties. Compared with the traditional preparation method, it is not necessary to perform heat treatment before extrusion to eliminate the second phase in the matrix, which greatly simplifies the process operation and cost.

[0042] 3. The room temperature tensile strength of the deformed magnesium alloy obtained by the present invention is ≥320MPa, the yield strength is ≥280MPa, the elongation is ≥10%, and the ignition point is ≥850℃, achieving the synergistic improvement of the room temperature strength, flame retardancy and forming ability of the deformed magnesium alloy. On the basis of meeting the mechanical properties, the ignition point of the magnesium alloy is increased to above 850℃, while the ignition point of the existing commercial flame retardant magnesium alloys (such as AZ31, AZ80, AZ91, ZK60) is 550-600℃. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a microstructure photograph of the deformed magnesium alloy of Example 1 of the present invention.

[0044] Figure 2 This is a microstructure photograph of the deformed magnesium alloy of Example 2 of the present invention.

[0045] Figure 3 This is a microstructure photograph of the deformed magnesium alloy of Example 3 of the present invention.

[0046] Figure 4 This is a microstructure photograph of the deformed magnesium alloy of Example 4 of the present invention.

[0047] Figure 5 This is a microstructure photograph of the deformed magnesium alloy of Example 5 of the present invention.

[0048] Figure 6 This is a microstructure photograph of the deformed magnesium alloy of Example 6 of the present invention.

[0049] Figure 7 This is a microstructure photograph of the deformed magnesium alloy of Example 7 of the present invention.

[0050] Figure 8 This is a microstructure photograph of the deformed magnesium alloy of Example 8 of the present invention.

[0051] Fig. 9 This is a photo of the deformed magnesium alloy structure of comparative example 4. DETAILED DESCRIPTION

[0052] The technical solution of the present invention is described in detail below through examples. This example is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given, but the protection scope of the present invention is not limited to the following examples.

[0053] The magnesium alloy composition of the embodiment of the present invention is shown in Table 1, and the rest is Mg and inevitable impurities. The specific process is shown in Table 2. The properties of the obtained magnesium alloy part are shown in Table 3.

[0054] The ignition point performance test and tensile mechanics test were carried out with a DSC heating rate of 20°C / min and a tensile testing machine tensile rate of 1mm / min. The ignition point test was carried out in accordance with GB / T 13464-2008 Thermal Analysis Test Method for Thermal Stability of Materials, and the tensile mechanics test was carried out in accordance with GB / T 16865-2013 Specimens and Methods for Tensile Test of Deformed Aluminum, Magnesium and Their Alloy Processing Products.

[0055] Comparative Example 1 is a "high-strength, flame-resistant, extrudable aviation-grade magnesium alloy" disclosed in Chinese Patent 2019800995010, with a Zn content of 0.1-0.8wt%, an Al content of 7-11wt%, and Ca and Y element contents of 0.75-1.50wt%, all of which are not within the scope of the present invention. The highest performance of the extruded tube or rectangular strip is a yield strength of 208MPa, a tensile strength of 306MPa, and an elongation of 9.46%, which does not meet the performance indicators of the present invention.

[0056] Comparative Example 2 is the "Non-combustible magnesium alloy with excellent mechanical properties and its preparation method" disclosed in Chinese Patent 2011800055846. The Mg alloy contains by weight: more than 1.0% but less than 7.0% Al, 0.05%-2.0% Ca, 0.05%-2.0% Y, more than 0% but not more than 6.0% Zn, the balance of Mg and other inevitable impurities. Based on the total weight of the magnesium alloy, the total content of Ca and Y is equal to or higher than 0.1% but less than 2.5%. The invention Al / (Ca+Y) is not within the scope of the rights of the present invention. The yield strength of the magnesium alloy extrusion material prepared by the method does not exceed 250MPa. The yield strength represents the ability of the material to resist permanent plastic deformation, indicating that the material is more easily deformed than the present invention.

[0057] Comparative Example 3 is "A high-strength Mg-Ca-Mn-Al-Zn deformable magnesium alloy containing gadolinium and yttrium rare earth elements and its preparation method" disclosed in 2020104330345, and its components are as follows by mass percentage: calcium: 0.50-2.00%; aluminum: 0.10-2.00%; zinc: 0.30-2.00%; manganese: 0.10-3.00%; rare earth (gadolinium or yttrium): 0.20-3.00%, the balance is magnesium and unavoidable impurities, and the composition is not within the scope of the present invention. The homogenization treatment holding time is 20 to 60 hours, and the process cost is relatively high. The reverse extrusion process used cannot achieve the continuous preparation of bars and profiles.

[0058] In Comparative Example 4, only Ca element is added, and the deformed magnesium alloy prepared by the same process as the present invention has poorer mechanical properties and flame retardant properties than those of the present invention.

[0059] Figures 1 to 8 They are respectively photographs of the deformed magnesium alloy tissues obtained in Examples 1 to 8 of the present invention. It can be seen from the pictures that the grain characteristics after extrusion are fine recrystallized grains wrapping long deformed grains, dispersed granular Al2Y second phase and Al2Ca+Ca2Mg6Zn3 second phase dispersed along the extrusion direction; wherein, the average particle size of the fine recrystallized grains is 1 to 3 μm. The above tissue characteristics are the key to obtaining high performance.

[0060] It can be seen from Table 2 that the deformed magnesium alloy prepared according to the composition and process of the present invention has good strength, elongation and high ignition point, and can maintain good forming performance and comprehensive performance at high extrusion speed without the need for a pre-extrusion heat treatment process.

[0061] Table 2

[0062] Table 3

[0063]

Claims

1. A high-performance flame-retardant deformable magnesium alloy with fine-grained structure, the weight percentage of which is: Al: 4.0-5.0%, Zn: 1.0-1.6%, Ca: 0.2-2.3%, Y: 0.2-1.8%, and the balance includes Mg and other inevitable impurities; wherein, The total amount of unavoidable impurities is ≤ 0.1%; and the above elements must also meet the following requirements: 1.5%≤Ca+Y≤2.5%; Al / (Ca+Y):1.8~2.

5.

2. The high performance flame retardant deformable magnesium alloy with fine grain structure according to claim 1, characterized in that: The balance is Mg and other inevitable impurities.

3. The high performance flame retardant deformable magnesium alloy with fine grain structure according to claim 1 or 2, characterized in that: The deformed magnesium alloy has an alloy structure of fine recrystallized grains enveloping long deformed grains, dispersed granular Al2Y second phase and Al2Ca+Ca2Mg6Zn3 second phase dispersed along the extrusion direction; wherein the average particle size of the fine recrystallized grains is 1 to 3 μm.

4. The high performance flame retardant deformable magnesium alloy with fine grain structure as claimed in claim 1, 2 or 3, characterized in that: The room temperature tensile strength of the deformed magnesium alloy is ≥320 MPa, the yield strength is ≥280 MPa, and the elongation is ≥10%.

5. The high performance flame retardant deformable magnesium alloy with fine grain structure as claimed in claim 1 or 2 or 3 or 4, characterized in that: The ignition point of the deformable magnesium alloy is ≥850°C.

6. The method for preparing a high performance flame retardant deformable magnesium alloy with fine grain structure according to any one of claims 1 to 5, characterized in that: The following steps are involved: 1) Ingredients Using pure Mg ingots, pure Zn ingots, pure Al ingots, Mg-Ca master alloys, and Mg-Y master alloys as raw materials, batching and preheating are performed according to the magnesium alloy composition of claim 1; 2) Melting Put the preheated pure Mg ingot into the crucible, raise the temperature to 740-750°C, melt it with protective gas, and wait until it is completely melted; add pure Al ingot, pure Zn ingot, Mg-Ca, Mg-Y master alloy, wait until the alloy is completely melted, and remove the oxides and slag on the surface of the melt; 3) Refining Add a refining agent to the obtained melt, stir it thoroughly, and then keep it warm for 15 to 20 minutes; Cooling the refined melt to room temperature with water to obtain a magnesium alloy ingot; 4) Preheating before extrusion The magnesium alloy ingot is preheated at 300-320°C for 60-120 minutes; 5) Extrusion The preheated ingot is hot extruded at 320-340° C. to obtain a magnesium alloy rod or a magnesium alloy profile, with an extrusion speed of 10-20 mm / s and an extrusion ratio of 16-18.

7. The preparation method according to claim 6, characterized in that: In step 3), the preheating temperature is 220-250° C., and the preheating time is 10-30 minutes.

8. The preparation method according to claim 6, characterized in that: In step 3), the refining agent is hexachloroethane.

9. The preparation method according to claim 6, characterized in that: In step 3), the protective gas is a mixed gas of N2+SF6, wherein the volume proportion of SF6 is 0.5-1.0%.

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