A method for preparing a high-strength cast magnesium alloy

Abstract

The application discloses a preparation method of a high-strength cast magnesium alloy, the alloy containing two non-rare-earth strengthening alloy elements of Sr and Ti, wherein the Ti element is preferentially combined with Al to form AlTi or Al3Ti intermetallic compounds and is gathered along the grain boundary, hindering the growth of alpha-Mg grains, the remaining Al element is combined with Mg to form a beta phase, the grain size is reduced, the morphology of the beta phase is improved, and thus the mechanical properties are improved; the Sr element can strengthen the beta phase on one hand, and can consume the Al element at the solidification front by forming a new Al4Sr phase, so that the volume fraction of the beta phase is reduced; in addition, the Sr element can form an adsorption film of strontium on the grain growth interface as a surface active element, so that the grain growth speed is reduced, the alloy has more time to generate more crystal nuclei when solidifying, and the grains are refined; under the synergistic effect of the two elements, the strength of the magnesium alloy is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of magnesium alloy material preparation technology, specifically to a method for preparing a high-strength cast magnesium alloy. Background Technology

[0002] The demand for lightweighting has led to the increasingly widespread application of magnesium alloys in industries such as automotive, aerospace, and telecommunications. Compared to aluminum alloys, the lower strength of magnesium alloys limits their application, making the design and synthesis of higher-strength magnesium alloys a hot international topic. Refining the as-cast microstructure and improving the as-cast mechanical properties of magnesium alloys using alloying elements is a new approach to synthesizing high-strength magnesium alloys. A common method is to add a certain proportion of rare earth elements, such as La, Ce, Y, and Gd, to magnesium alloys; however, their high cost limits their widespread application. Alkaline earth element Sr and transition element Ti are inexpensive and possess excellent properties such as refining the microstructure and generating high-melting-point strengthening phases during the alloying process of magnesium alloys, thus attracting widespread attention both domestically and internationally. Existing research has shown that excessive addition of alloying elements Sr or Ti can form coarse second phases that disrupt the matrix, leading to a decrease in the performance of magnesium alloys.

[0003] CN116103551A discloses a method for preparing a high-temperature, high-strength, and high-toughness cast magnesium alloy. By mass percentage, it contains 5%-6% highly soluble rare earth elements, 2%-3% rare earth element Y, 1.5%-3% zinc, and the balance is magnesium. The process involves melting the raw materials, casting, and obtaining an ingot. After removing the outer layer of the ingot, a solution treatment is performed, followed by air cooling to room temperature, completing the preparation. The cast magnesium alloy ultimately prepared by this invention achieves the following properties: room temperature yield strength of 205-212 MPa, tensile strength of 257-273 MPa, and elongation of 14%-17%. The 7%-9% rare earth element used in this process, due to the high cost of rare earth elements and the low yield during casting, will significantly increase the preparation cost. CN114836663A discloses a high-strength cast magnesium alloy and its preparation method. The alloy comprises, by mass percentage, 7% Zn, 3%-5% Al, 0.3%-0.5% Mn, 0.5%-1% Re, with a total unavoidable impurities of ≤0.04%, and the balance being Mg. The Re alloy includes La and Ce, with La and Ce accounting for 35% and 65% of the total Re alloy addition, respectively. Mn, La, and Ce are added as Mg-5wt.%Mn, Mg-30wt.%La, and Mg-30wt.%Ce master alloys, respectively. This process uses a small amount of rare earth elements to purify the melt, but a heat treatment process is still required to obtain high strength.

[0004] In order to overcome the shortcomings and deficiencies of the existing technology, the purpose of this invention is to refine the grains of magnesium alloys by using the synergistic refining effect of Sr and Ti elements, thereby improving the strength of magnesium alloys. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0006] A high-strength magnesium alloy material, wherein the composition and mass percentage of the cast magnesium alloy are: Al 7.5%-9.5%, Zn 0.2%-0.8%, Mn 0.2%-0.8%, Sr 0.1%-0.5%, Ti 0.05%-0.15%, unavoidable impurities totaling less than or equal to 0.04%, and the balance being Mg. Mn, Sr, and Ti are added in the form of MnCl2, Mg-30%Sr master alloy, and titanium powder, respectively. The preparation method of this cast magnesium alloy includes:

[0007] (1) Batching: According to the weight percentage, take Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots, and dry them;

[0008] (2) Preheating: Before entering the site, Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots are placed in a heat treatment furnace for preheating and drying at a temperature of 200-250℃ for a time of not less than 30 minutes. If water vapor is found on the surface, they cannot be put into the furnace.

[0009] (3) Loading the furnace: Clean the crucible before loading it into the furnace. The order of addition is Mg ingot-MnCl2-Al ingot-Zn ingot-Mg-30%Sr master alloy and titanium powder. During the melting process, bridging of the furnace charge is prohibited. If it is found, it should be dealt with in time.

[0010] (4) Melting: Set the furnace temperature to 750℃-780℃, add Mg ingots, and add a heat-insulating cover or other heat-insulating device during the melting process to prevent heat loss and oxidation; carefully operate the melting of the furnace charge, and use CO2+SF6 mixed gas for protection throughout the melting process. If white smoke and bright light are found, it means that the furnace charge is burning, and immediately sprinkle No. 2 solvent; after all the furnace charge has melted, remove the surface slag, remove the bottom slag, and evenly sprinkle a layer of No. 2 solvent on the upper surface to prepare for alloying;

[0011] (5) Alloying: When the temperature is raised to 750℃, refine for 3 minutes, making sure to stir the bottom of the pot thoroughly; after refining, let stand for 3 minutes, then take a sample for spectral analysis and record the results (including time, temperature, and amount added). Based on the results, prepare the alloying materials and keep a small sample for subsequent retesting; when the alloy liquid temperature is 750℃, sprinkle the weighed MnCl2 onto the surface of the melt. After the MnCl2 melts, use a stirring tool to add No. 2 flux while stirring for 3 minutes. After addition, take a sample for spectral analysis. Record the following steps (including time, temperature, and amount added): At a melt temperature of 740℃, place the weighed aluminum and zinc in the center of the melt surface to melt. After adding aluminum and zinc, extract the sample for spectral analysis and record the results (including time, temperature, and amount added). Keep the sample for future retesting. At a melt temperature of 760℃, place the weighed magnesium-strontium alloy and titanium powder in the center of the melt surface to melt. After adding magnesium and lanthanum, extract the sample for spectral analysis and record the results (including time, temperature, and amount added). Keep the sample for future retesting.

[0012] (6) Refining: The refining temperature is 760℃; after adding aluminum, zinc and lanthanum, the refining should be stirred in time; refine for 3-5 minutes, until the alloy liquid is shiny after refining;

[0013] (7) Reduce iron content: Cool the alloy liquid to 680-700℃, and sprinkle a small amount of flux to cover it while stirring, so as not to damage the surface of the alloy liquid.

[0014] (8) Standing: After refining, remove the slag on top and evenly sprinkle a layer of No. 2 flux; let stand for 25-35 minutes, and prepare for casting after the composition is qualified.

[0015] (9) Casting: Before casting, clean the inner wall of the mold and blow it with SF6 for protection; when pouring the liquid using hydraulic pressure, let it flow slowly into the mold's inner retaining shovel to prevent splashing and injury. Stop casting when the molten liquid flows to the edge of the large diameter, indicating that casting is complete. After casting the crucible solution, clean the pot and wait for the next batch of melting.

[0016] Furthermore, the impurity element requirements for Mg ingots in the raw materials are: Fe≤0.004%, Si≤0.03%, Ni≤0.0006%, Cu≤0.002%; for Al ingots, Fe≤0.12%, Si≤0.06%, Ni≤0.015%, Cu≤0.004%; for Zn ingots, Fe≤0.001%, Cu≤0.002%; and for MnCl2, the purity is ≥96%, and Ni≤0.01%.

[0017] Furthermore, the titanium powder used has a size of less than 10 μm and a purity of greater than 99%.

[0018] Furthermore, the basic composition of the No. 2 flux is 38±3% MnCl2, 37±3% KCl, 9±3% BaCl2, 5±1% CaCl2, and 10±3% NaCl.

[0019] Furthermore, the No. 2 flux needs to be sieved through a 100-mesh sieve before use, and the sieve passing rate is ≥50%.

[0020] Furthermore, the smelting process is protected by a CO2+SF6 mixture, wherein the CO2 content is 99% and the SF6 content is 1%.

[0021] The beneficial effects of this invention are as follows:

[0022] The cast magnesium alloy produced by the method of this invention contains two non-rare earth strengthening alloying elements, Sr and Ti. Ti preferentially forms AlTi or Al3Ti intermetallic compounds with Al and aggregates along grain boundaries, hindering α-Mg grain growth. The remaining Al combines with Mg to form the β phase, thus reducing grain size and improving the morphology of the β phase, thereby enhancing mechanical properties. Sr strengthens the β phase, enhancing bonding. Furthermore, it consumes Al elements at the solidification front by forming a new Al4Sr phase, reducing the volume fraction of the β phase. In addition, as a surface-active element, Sr forms a strontium adsorption film at the grain growth interface, leading to a slower grain growth rate and allowing more time for the alloy to solidify, resulting in more nuclei and grain refinement. The simultaneous addition of Ti and Sr elements revealed new Al3Ti and Al4Sr phases in the microstructure. In AZ80, the addition of Sr preferentially formed the more chemically stable Al-Sr compound, while the precipitation of the β phase and Al3Ti was potentially suppressed. This not only refined the matrix and β phase but also increased the Ti yield, reducing the disruptive effect of Al3Ti on the matrix. The synergistic effect of these two elements significantly improved the strength of the magnesium alloy. Attached Figure Description

[0023] Figure 1 Here are metallographic images of alloys obtained in Comparative Example 1;

[0024] Figure 2 The curve shows the relationship between strain and standard load for alloy 1 (Comparative Example).

[0025] Figure 3 The image shows the metallographic structure of alloy 2 as shown in the comparative example.

[0026] Figure 4 The strain versus standard load curve is shown for alloy 2 in Comparative Example.

[0027] Figure 5 The image shows the metallographic structure of alloy 3 as shown in the comparative example.

[0028] Figure 6 The curve showing the strain versus standard load relationship of alloy 3 in Comparative Example 3 is shown.

[0029] Figure 7 The image shows the metallographic structure of the alloy used in the experiment.

[0030] Figure 8 The figure shows the relationship between strain and standard load for the test alloy. Detailed Implementation

[0031] The following examples illustrate the specific technical solutions of the present invention in detail. It should be understood that the following examples are only used to explain the present invention and should not be construed as limiting the present invention.

[0032] Example 1

[0033] The composition used in this embodiment is: Al 8%, Zn 0.2%, Mn 0.2%, Sr 0.1%, Ti 0.05%, with unavoidable impurities totaling less than or equal to 0.04%, and the balance being Mg. Mn, Sr, and Ti are added in the form of MnCl2, Mg-30%Sr master alloy, and titanium powder, respectively.

[0034] Preparation method:

[0035] (1) Batching: According to the weight percentage, take Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots, and dry them;

[0036] (2) Preheating: Before entering the site, Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots are placed in a heat treatment furnace for preheating and drying at 200℃ for 35 minutes. If water vapor is found on the surface, they cannot be put into the furnace.

[0037] (3) Loading the furnace: Clean the crucible before loading it into the furnace. The order of addition is Mg ingot-MnCl2-Al ingot-Zn ingot-Mg-30%Sr master alloy and titanium powder. During the melting process, bridging of the furnace charge is prohibited. If it is found, it should be dealt with in time.

[0038] (4) Melting: Set the furnace temperature to 780℃, add Mg ingots, and add a heat-insulating cover or other heat-insulating device during the melting process to prevent heat loss and oxidation; carefully operate the melting of the furnace charge, and use CO2+SF6 mixed gas for protection throughout the melting process. If white smoke and bright light are found, it means that the furnace charge is burning, and immediately sprinkle No. 2 flux; after all the furnace charge has melted, remove the surface slag, remove the bottom slag, and evenly sprinkle a layer of No. 2 flux on the upper surface to prepare for alloying;

[0039] (5) Alloying: When the temperature is raised to 750℃, refine for 3 minutes, making sure to stir the bottom of the pot thoroughly; after refining, let stand for 3 minutes, then take a sample for spectral analysis and record the results (including time, temperature, and amount added). Based on the results, prepare the alloying mixture and keep a small sample for subsequent retesting; when the alloy liquid temperature is 750℃, sprinkle the weighed MnCl2 onto the surface of the flux. After the MnCl2 melts, use a stirring tool to add No. 2 flux while stirring for 3 minutes. After addition, take a sample for spectral analysis. Record the following steps (including time, temperature, and amount added): At a melt temperature of 740℃, place the weighed aluminum and zinc in the center of the melt surface to melt. After adding aluminum and zinc, extract the sample for spectral analysis and record the results (including time, temperature, and amount added). Keep the sample for future retesting. At a melt temperature of 760℃, place the weighed magnesium-strontium alloy and titanium powder in the center of the melt surface to melt. After adding magnesium and lanthanum, extract the sample for spectral analysis and record the results (including time, temperature, and amount added). Keep the sample for future retesting.

[0040] (6) Refining: The refining temperature is 760℃; after adding aluminum, zinc and lanthanum, the refining should be stirred in time; the refining time is about 5 minutes, until the alloy liquid is shiny after refining;

[0041] (7) Reduce iron content: Cool the alloy liquid to 690°C, and sprinkle a small amount of flux to cover it while stirring, so as not to damage the surface of the alloy liquid.

[0042] (8) Standing: After refining, remove the slag on top and evenly sprinkle a layer of No. 2 flux; let stand for 25-35 minutes, and prepare for casting after the composition is qualified.

[0043] (9) Casting: Before casting, clean the inner wall of the mold and blow it with SF6 for protection; when pouring the liquid using hydraulic pressure, pour it slowly so that the liquid flows into the inner shovel of the mold to prevent splashing and injury. Stop casting when the molten liquid flows to the edge of the large diameter, and the casting is complete. After the crucible solution has been cast, clean the pot and wait for the next batch of melting.

[0044] Example 2

[0045] The differences between this embodiment and Embodiment 1 are as follows:

[0046] The proportions used in this embodiment are Al 8.5%, Zn 0.6%, Mn 0.5%, Sr 0.2%, Ti 0.1%, with unavoidable impurities totaling less than or equal to 0.04%, and the balance being Mg.

[0047] In the second step of the preparation method, the temperature in the heat treatment furnace during preheating is 220℃, and the preheating time is 40 minutes.

[0048] In the fourth step of the preparation method, the furnace temperature is set to 750℃;

[0049] The iron-reducing temperature in the seventh step of the preparation method is set to 700℃;

[0050] The eighth step of the preparation method involves letting the mixture stand for 30 minutes. Once the composition is deemed acceptable, it is ready for casting.

[0051] Example 3

[0052] The differences between this embodiment and Embodiment 1 are as follows:

[0053] The composition used in this embodiment is Al 9%, Zn 0.8%, Mn 0.8%, Sr 0.5%, Ti 0.15%, with an unavoidable impurity content of less than or equal to 0.04%, and the balance being Mg.

[0054] In the second step of the preparation method, the temperature in the heat treatment furnace is 250℃ for 45 minutes during preheating.

[0055] In the fourth step of the preparation method, the furnace temperature is set to 760℃;

[0056] The iron-reducing temperature in the seventh step of the preparation method is set to 680℃;

[0057] The eighth step of the preparation method involves letting the mixture stand for 35 minutes. Once the composition is deemed acceptable, it is ready for casting.

[0058] Comparative Example 1

[0059] The formulation used in this comparative example was Al 8.5%, Zn 0.6%, and Mn 0.5%, prepared using the same method as in Example 1. The microstructure was observed using a metallographic microscope, and the results are as follows: Figure 1 As shown; the mechanical properties of the material were tested using a universal tensile testing machine, and the results are as follows. Figure 2 As shown. The alloy obtained by this method has an average grain diameter of 597 μm, a yield strength of 82.6 MPa, a tensile strength of 149 MPa, and an elongation of 2.9%.

[0060] Comparative Example 2

[0061] The formulation used in this comparative example was Al 8.5%, Zn 0.6%, Mn 0.5%, and Ti 0.05%. The preparation method was the same as in Example 1. The microstructure was observed using a metallographic microscope, and the results are as follows: Figure 3 As shown; the mechanical properties of the material were tested using a universal tensile testing machine, and the results are as follows. Figure 4 As shown. The alloy obtained by this method has an average grain diameter of 392 μm, a yield strength of 87.6 MPa, a tensile strength of 187.9 MPa, and an elongation of 6.3%.

[0062] Comparative Example 3

[0063] The formulation used in this comparative example was Al 8.5%, Zn 0.6%, Mn 0.5%, Sr 0.1%, and the preparation method was the same as in Example 1. The microstructure was observed using a metallographic microscope, and the results are as follows: Figure 5 As shown; the mechanical properties of the material were tested using a universal tensile testing machine, and the results are as follows. Figure 6 As shown. The alloy obtained by this method has an average grain diameter of 368 μm, a yield strength of 88.5 MPa, a tensile strength of 192.8 MPa, and an elongation of 5.4%.

[0064] Test case

[0065] The formulation used in this experiment was Al 8.5%, Zn 0.6%, Mn 0.5%, Ti 0.05%, Sr 0.1%, and the preparation method was the same as in Example 1. The microstructure was observed using a metallographic microscope, and the results are as follows: Figure 7 As shown; the mechanical properties of the material were tested using a universal tensile testing machine, and the results are as follows. Figure 8 As shown, the alloy obtained by this method has an average grain diameter of 211 μm, a yield strength of 106.1 MPa, a tensile strength of 205 MPa, and an elongation of 5.3%. The comparison shows that the alloy refined by the synergistic use of non-rare earth alloying elements Sr and Ti exhibits significant grain refinement, and a marked increase in yield strength and tensile strength.

[0066] Metallographic results show that the addition of non-rare earth elements Sr and Ti can refine the grain size. This is mainly because Ti has a close-packed hexagonal structure below 882℃ (a = 0.29503 nm, c = 0.46831 nm), which can serve as heterogeneous nucleation sites for α-Mg, increasing the number of nucleation sites. Furthermore, Ti readily forms compounds with Al, and its aggregation at the solidification front hinders grain growth, while also consuming Al in the liquid phase, resulting in a finer β phase. The addition of Sr preferentially forms Al4Sr compounds, similarly reducing the β phase content. In terms of mechanical properties, the coarse β phase is refined, and the addition of Al-Ti and Al-Sr compounds in the matrix provides reinforcement, thus improving performance.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a high-strength magnesium alloy material, comprising: (1) Batching: According to the weight percentage, take Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots, and dry them; Al 7.5%-9.5%, Zn 0.2%-0.8%, Mn 0.2%-0.8%, Sr 0.1%-0.5%, Ti 0.05%-0.15%, total unavoidable impurities ≤0.04%, and the balance Mg; Mn, Sr and Ti are added in the form of MnCl2, Mg-30%Sr master alloy and titanium powder respectively; titanium powder size <10μm, purity >99%; (2) Preheating: Al ingots, Zn ingots, MnCl2, Mg-30%Sr master alloy, titanium powder and Mg ingots are preheated and dried for later use. (3) Loading the furnace: Mg ingots, MnCl2, Al ingots, Zn ingots, Mg-30%Sr master alloy and titanium powder are loaded into the furnace in sequence; (4) Melting: Set the furnace temperature to 780℃, put in Mg ingots and keep them warm to melt. After all the furnace charge has melted, remove the surface slag, remove the bottom slag, and evenly sprinkle a layer of No. 2 flux on the upper surface to prepare for alloying. (5) Alloying: When the temperature is raised to 750℃, refine for 3 minutes. After standing for 3 minutes, when the alloy liquid temperature is 750℃, sprinkle MnCl2 onto the surface of the melt. After the MnCl2 melts, stir and sprinkle No. 2 flux to carry out alloying. Stir for 3 minutes. When the alloy liquid temperature is 740℃, place Al ingot and Zn ingot into the melt to melt. When the alloy liquid temperature is 760℃, place Mg-30%Sr master alloy and titanium powder into the melt to melt, and obtain a rough alloy sample. (6) Refining: Set the refining temperature to 760℃ and continue refining for 5 minutes until the alloy liquid becomes shiny; (7) Reduce iron content: Cool the alloy liquid to 680-700℃, and sprinkle a small amount of No. 2 flux to cover it while stirring; (8) Standing: After refining, remove the surface slag, evenly sprinkle a layer of No. 2 flux, let stand for 25-35 minutes, and cast after the composition is qualified.

2. The preparation method according to claim 1, wherein: The impurity element requirements for the Mg ingot in step (1) are as follows: Fe≤0.004%, Si≤0.03%, Ni≤0.0006%, Cu≤0.002%; The impurity element requirements for Al ingots are as follows: Fe≤0.12%, Si≤0.06%, Ni≤0.015%, Cu≤0.004%; The requirements for impurity elements in Zn ingots are as follows: Fe ≤ 0.001%, Cu ≤ 0.002%; The purity of MnCl2 is ≥96%, and Ni is ≤0.01%.

3. The preparation method according to claim 1, wherein: The preheating temperature in step (2) is 200℃, and the preheating time is not less than 30 minutes.

4. The preparation method according to claim 1, wherein: In step (4), a CO2+SF6 mixture is used for protection throughout the melting process.

5. The preparation method according to claim 4, wherein: The mixed gas contains 99% CO2 by volume and 1% SF6 by volume.

6. The preparation method according to claim 1, wherein: The flux #2 mentioned in step (5) includes: MgCl2: 38±3%, KCl: 37±3%, BaCl2: 9±3%, CaCl2: 5±1%, NaCl: 10±3%.

7. The preparation method according to claim 1, wherein: The No. 2 flux needs to be sieved through a 100-mesh sieve before use, and the sieve passing rate must be ≥50%.

8. A high-strength cast magnesium alloy prepared by any of the preparation methods according to claims 1 to 7.

Images