A low-cost cast magnesium alloy with both high corrosion resistance and high strength and its preparation method.

Abstract

This invention discloses a low-cost cast magnesium alloy with both high corrosion resistance and high strength, and its preparation method. The magnesium alloy comprises the following components by mass percentage: Zn 1%~4%, Al 6%~9%, Y 0.05%~2%, Mn 0.05%~2%, with the balance being magnesium and unavoidable impurities; wherein the sum of the mass percentages of Zn and Al is 9%~11%. This invention, through Al and Zn multi-element alloying design and coupling with a small amount of rare earth elements, induces the formation of a dense corrosion film layer with "passivation-secondary passivation" characteristics on the alloy surface, resulting in an average hydrogen evolution corrosion rate as low as 0.04 mL / cm². 2 / day (0.10 mm / a). Simultaneously, the optimized composition ratio enables the second phase to be dispersed and significantly refines the grains, resulting in a tensile strength of 250 MPa and an elongation of 7.1% in the as-cast alloy. This invention achieves a synergistic balance of high corrosion resistance, high strength, and low cost without relying on high rare earth additions or subsequent deformation processing, and has broad prospects for industrial application.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and in particular to a low-cost cast magnesium alloy with both high corrosion resistance and high strength, and a method for preparing the same. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials currently available, have broad application prospects in fields such as automotive lightweighting and aerospace. However, the generally poor corrosion resistance and mechanical properties of magnesium alloys have become the main bottleneck restricting their large-scale application.

[0003] Currently, existing technologies for improving the performance of magnesium alloys mainly suffer from the following problems: High rare earth alloying improves corrosion resistance by introducing a large amount of rare earth elements (RE) to construct a dense RE2O3 barrier on the surface, but this not only leads to a significant increase in raw material costs but also causes a deterioration in alloy plasticity. Traditional microalloying, by introducing elements such as Ca and Mn, can control costs, but its improvement in corrosion resistance is limited, and due to the lack of effective precipitates, the mechanical properties of the alloy remain at a low level. Melt purification methods reduce harmful impurities such as Fe, Ni, and Cu through metallurgical refining to mitigate galvanic corrosion, but the process cost is high. Surface modification technology, while providing a temporary barrier, cannot change the intrinsic chemical stability of the matrix.

[0004] In addition, existing studies generally rely on extrusion, rolling and other methods to introduce fine grains, dislocations and texture reinforcement to compensate for the shortcomings of cast magnesium alloys, which leads to a longer material preparation process and increased energy consumption; and it is difficult to achieve integrated molding of complex components.

[0005] In summary, traditional alloy design optimizes corrosion resistance and strength separately, creating a dilemma where it is difficult to simultaneously achieve "high corrosion resistance, high strength, and low cost." Therefore, developing a low-cost alloying scheme that can synergistically achieve high corrosion resistance and high strength under as-cast conditions is key to breaking through the current bottleneck in the industrialization of magnesium alloys. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a low-cost cast magnesium alloy with both high corrosion resistance and high strength and its preparation method, so as to solve the problems of poor intrinsic corrosion resistance and mechanical properties of existing cast magnesium alloys, and the long process flow, high cost and high energy consumption caused by the general reliance on plastic deformation processes such as extrusion and rolling to introduce fine grains and strengthening structure (to compensate for the lack of cast performance).

[0007] To address the aforementioned technical problems, this invention employs the following technical solution: a low-cost cast magnesium alloy possessing both high corrosion resistance and high strength. The magnesium alloy comprises the following components by mass percentage: Zn 1-4%, Al 6-9%, Y 0.05-2%, Mn 0.05-2%, with the balance being magnesium and unavoidable impurities; wherein the sum of the mass percentages of Zn and Al is 9-11%. Through a multi-element composite alloying design of Al, Zn, Y, and Mn, the total amount of Al and Zn is controlled within a specific range of 9-11%, balancing the matrix solid solubility and the amount of second-phase precipitation. This avoids the coarse formation of brittle phases due to excessive alloying elements. The addition of trace amounts of Y effectively refines the grains and promotes the formation of a dense passivation film on the surface, while Mn purifies the melt and reduces galvanic corrosion from harmful impurities. Without the need for high rare earth additions or subsequent deformation processing, a dense corrosion barrier is reconstructed on the matrix surface, simultaneously strengthening the alloy microstructure, resulting in excellent intrinsic chemical stability and mechanical properties.

[0008] Preferably, the magnesium alloy comprises the following components by mass percentage: Zn 1-3%, Al 7-9%, Y 0.1-1%, Mn 0.1-1%, with the balance being magnesium and unavoidable impurities; wherein the sum of the mass percentages of Zn and Al is 10%. In this way, through precise component ratio, the Mg in the matrix... 17 Al 12 The phase distribution is more uniform, avoiding the formation of continuous network or coarse blocky brittle phases, thereby reducing the initiation point of galvanic corrosion. Furthermore, this formulation can induce the formation of a film with "passivation-secondary passivation" characteristics on the surface, significantly reducing the average hydrogen evolution corrosion rate to 0.04 mL / cm². 2 / day, and to achieve a tensile strength of 250MPa.

[0009] Furthermore, the impurity content in the alloy is less than 0.02%, with the mass percentages of harmful impurities Fe, Cu, and Ni limited to: Fe ≤ 0.005%, Cu ≤ 0.005%, and Ni ≤ 0.001%, respectively. This solves the problem of micro-cell galvanic corrosion caused by impurity segregation. Combined with the iron removal effect of Mn, the passivation film modification effect of Y, and the melt purification process, the intrinsic chemical stability of the magnesium alloy matrix is ​​significantly improved.

[0010] Furthermore, in the microstructure of the magnesium alloy, the magnesium alloy grains exist in the form of dendritic crystals, and Mg... 17 Al 12 The phases are distributed in a fine and dispersed manner, with small amounts of Al8Mn4Y and Al2Y phases present in the microstructure. Thus, through the synergistic design of the proportions of trace rare earth element Y with the main alloying elements Al and Zn, the Mg... 17 Al 12The refinement and modification of the phase transforms it from a traditional coarse network or block into a diffusely distributed, isolated small particle, which weakens the initiation point of galvanic corrosion and hinders dislocation movement, thereby synergistically improving toughness and corrosion resistance.

[0011] Another object of the present invention is to provide a method for preparing the above-mentioned cast magnesium alloy with both high corrosion resistance and high strength, comprising the following steps: S1: Pure aluminum, pure zinc, pure magnesium, Mg-Y master alloy and Mg-Mn master alloy are used as raw materials for component mixing. Then, pure Mg is heated to melt under a protective atmosphere. Pure aluminum, pure zinc Mg-Y master alloy and Mg-Mn master alloy are then added. After complete melting, an alloy melt is obtained.

[0012] S2: The alloy melt obtained in step S1 is stirred and kept at a constant temperature, and then cooled to obtain the cast magnesium alloy.

[0013] This simplifies the preparation process, eliminating the complex extrusion, rolling and other plastic deformation steps in traditional magnesium alloy production. Microstructure optimization can be achieved through simple melting and casting, significantly reducing energy consumption and equipment costs.

[0014] Preferably, the weight percentage of Y in the Mg-Y master alloy is 20-30%, and the weight percentage of Mn in the Mg-Mn master alloy is 10-30%. This ensures that trace elements diffuse rapidly and uniformly in the melt, improving compositional accuracy.

[0015] Preferably, the protective atmosphere is a mixture of CO2 and SF6 in a volume ratio of 99:1; the heating temperature is 680~760℃. This specific gas mixture ratio and temperature range can form a dense surface protective film, reducing inclusions in the melt, ensuring melt purity, and solving the problems of easy oxidation and safety during magnesium alloy smelting.

[0016] Preferably, the stirring method in step S2 is mechanical stirring, and the stirring time is 2-5 minutes; the holding time is 30-50 minutes. This enhances the consistency of the internal structure of the alloy and reduces performance fluctuations caused by component segregation.

[0017] Preferably, slag removal is performed before stirring and during the heat preservation process. This multi-stage refining process solves the problem of incomplete removal of non-metallic inclusions. Slag removal eliminates oxide scale and solvent residue, removing potential corrosion nuclei and performance degradation points at the source.

[0018] Preferably, the cooling process is water cooling at a rate of 110-130°C / min. This controlled high-speed water cooling achieves rapid solidification, significantly refines the grains, and induces the second phase to precipitate in the form of dispersed micro-elements. Utilizing the grain refinement and dispersion strengthening effects, the yield strength and ductility of the alloy in the as-cast state are further optimized.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes the synergistic design of the proportions of rare earth elements (Y) and main alloying elements (Al, Zn) to enhance the Mg content. 17 Al 12 The uniform distribution of the phase within the magnesium matrix prevents continuous network or coarse blocky brittle phases from becoming crack initiators. This allows the alloy to form a dense corrosion film during corrosion, exhibiting a unique "passivation-secondary passivation" phenomenon, effectively blocking further erosion by the corrosive medium. Simultaneously, the grain-refining strengthening effect of Zn significantly enhances the alloy's strength and toughness. The as-cast magnesium alloy prepared by this invention achieves a tensile strength of 250 MPa, an elongation of 7.1%, and an average hydrogen evolution rate as low as 0.04 mL / cm². 2 / day (0.10 mm / a) has achieved the goal of improving its corrosion resistance and overall strength under as-cast conditions, providing a new approach for the development of magnesium alloys that balance economy and high performance.

[0020] 2. Without requiring additional heat treatment, plastic deformation treatment (such as extrusion or rolling), or surface protection treatment, the alloy exhibits excellent mechanical and corrosion resistance in the as-cast state, significantly shortening the process and reducing energy consumption. Because high performance can be achieved in the as-cast state, this invention is particularly suitable for realizing the integrated casting of complex lightweight components, providing a feasible solution for large-scale applications in the automotive, aerospace, and other fields.

[0021] 3. This invention adds only trace amounts of rare earth elements, effectively reducing the raw material content and avoiding the problem of plasticity deterioration caused by high rare earth content. This invention can be achieved through conventional smelting and water cooling processes, requiring minimal equipment, with simple processes, good repeatability, and easy large-scale industrial production. Attached Figure Description

[0022] Figure 1 The corrosion rate of the cast magnesium alloy prepared by this invention is shown.

[0023] Figure 2 This is an optical morphology image of the cast magnesium alloy obtained by the present invention after corrosion.

[0024] Figure 3 The images show the SEM (A) and EDS (B) images of the cast magnesium alloy prepared according to this invention.

[0025] Figure 4 This is a polarization curve of the cast magnesium alloy prepared according to the present invention.

[0026] Figure 5 The tensile mechanical properties of the cast magnesium alloy prepared by this invention are shown. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods; and unless otherwise specified, the materials used are commercially available.

[0028] I. A method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength Example 1 This example uses the following steps: (1) Ingredients: Al 9.0 wt.%, Zn 1.0 wt.%, Y 0.5 wt.%, Mn 0.3 wt.%, with the balance being magnesium and unavoidable impurities; Prepare the raw materials according to the above composition and weigh them after polishing them to a metallic luster. The raw materials used are high-purity magnesium (99.99%), pure aluminum (99.99%), pure zinc (99.99%), Mg-30%Y master alloy and Mg-10%Mn master alloy.

[0029] (2) Put pure magnesium into an electric resistance furnace and heat it to 680~760°C under a protective atmosphere of CO2 and SF6 mixed gas (ratio of 99:1). After melting, the pure magnesium melt is slag-removed for the first time. Then, preheated pure aluminum, pure zinc, Mg-30%Y master alloy and Mg-10%Mn master alloy are added and melted at 680~760°C. After complete melting, the alloy melt is obtained.

[0030] (3) The melt obtained in step (2) is slag removed for the second time, and then mechanically stirred for 2 to 5 minutes. After stirring, the alloy melt needs to be kept warm for 30 to 50 minutes. When the holding time is 20 to 30 minutes, the alloy melt is slag removed for the third time.

[0031] (4) The alloy melt after step (3) is placed in water for cooling to room temperature at a rate of 110~130℃ / min to obtain the cast magnesium alloy.

[0032] The component ratios of Examples 2-8 and Comparative Examples 1-4 are different, as shown in Table 1. Other steps are the same as in Example 1.

[0033] Table 1 II. Performance Verification 1. The cast magnesium alloys of Examples 1-8 and Comparative Examples 1-4 were placed in a 3.5 wt.% NaCl solution for corrosion performance testing. The corrosion time was 15 days, and the experimental temperature was room temperature (25°C). The collected hydrogen gas was recorded using the water displacement method to calculate the corrosion rate. After the corrosion test, the corrosion condition of the magnesium alloy surface was observed. Before conducting the corrosion test, the sample surface usually needs to be ground and polished. The main purpose of this process is to reduce the surface roughness of the sample, improve surface uniformity, remove oxide layers and other contaminants, ensure the consistency of the sample under experimental conditions, and reduce errors caused by surface irregularities, thereby obtaining more accurate and reliable experimental data. The same applies below. The results are as follows. Figure 1 As shown.

[0034] from Figure 1 (a) It can be seen that when only Al (AW100) is added to the magnesium alloy, its hydrogen evolution corrosion rate within 15 days is 4.31 mL / cm. 2 / day (9.81 mm / a). While keeping the total mass percentage of Al and Zn constant (10%), the hydrogen evolution corrosion rate of the as-cast magnesium alloy showed a clear trend of first decreasing and then increasing with the gradient increase of Zn content (i.e., a corresponding decrease in Al content). Among them, Example 2 showed the lowest hydrogen evolution corrosion rate, as low as 0.04 mL / cm². 2 / day (0.10 mm / a). However, when the Zn content is as high as 10% (ZW100), its hydrogen evolution corrosion rate increases to 4.28 mL / cm. 2 / day (9.75 mm / a). This is because when only Al is added, there is a large amount of Mg in the metal matrix. 17 Al 12 The second phase, which can act as a cathode phase, exacerbates galvanic corrosion; conversely, adding a large amount of Zn to magnesium alloys also promotes the formation of a large amount of Mg-Zn phase, intensifying corrosion. Only when the Al and Zn contents are adjusted to an appropriate ratio can the corrosion of Mg be effectively controlled. 17 Al 12 The morphology of the phase changes it from a continuous network to a diffuse distribution, which in turn makes the second phase in the matrix more uniformly distributed, promotes the formation of a passivation film, greatly weakens the galvanic corrosion effect, and can reduce the corrosion rate of the alloy.

[0035] Furthermore, from Figure 1(b) It can be seen that when the total content of (Al+Zn) is 9% or 11%, the hydrogen evolution corrosion rate of the as-cast magnesium alloy is slightly higher than that of Example 2 (total content of 10%), but significantly better than that of Comparative Examples 1-4. This shows that fluctuations in the total (Al+Zn) content within the range of 9% to 11% still allow the alloy to achieve good corrosion resistance. However, when the total (Al+Zn) content is outside this range (e.g., 8% or 12%), the hydrogen evolution corrosion rate of the as-cast magnesium alloy increases significantly, and its corrosion resistance decreases.

[0036] 2. The cast magnesium alloys of Examples 1-4 and Comparative Examples 1-4 were immersed in 3.5 wt.% NaCl solution for 15 days, and their morphology was observed under a stereomicroscope. The results are as follows: Figure 2 As shown.

[0037] from Figure 2 As can be seen, after soaking in 3.5 wt.% NaCl solution for 15 days, Comparative Examples 1-4 exhibited obvious and severe localized corrosion characteristics, with numerous corrosion pits on the surface. Comparative Example 1 even showed signs of filamentous corrosion. In contrast, the surface corrosion of Example 2 was uniform, without obvious signs of filamentous corrosion or corrosion pits. Figure 1 The results were consistent.

[0038] 3. The cast magnesium alloys of Example 2 and Comparative Examples 1-4 were observed under a scanning electron microscope, and the results are as follows: Figure 3 As shown.

[0039] from Figure 3 As can be seen from A, a large amount of Mg is present in the microstructure of AW100. 17 Al 12 These phases tend to be distributed along grain boundaries, exhibiting a distinct coarse network characteristic. In addition, a small amount of Al₂Y phase can be observed; when the Zn content in ZW100 increases to 10%, a large number of coarse, blocky MgZn₂ phases appear in the microstructure. The precipitation of these coarse phases not only increases the crystallization temperature range, leading to casting defects, but also easily becomes a source of crack initiation. It is evident that the addition of either Al or Zn alone will lead to the formation of a brittle second phase (Mg). 17 Al 12 The presence of MgZn2 in a coarse, continuous form is both the main cause of decreased mechanical properties and the root cause of exacerbated galvanic corrosion. With the appropriate introduction of Zn, a crucial transformation occurs in the microstructure. The original coarse, network-like Mg... 17 Al 12 The phase transformation results in fine and dispersed particles or short strips. The grains are significantly refined, and the microstructure is more uniformly distributed. Specifically, when Al is 8% and Zn is 2%, the magnesium alloy grains exist in the form of dendritic crystals, while Mg... 17 Al12 The phases are distributed in a fine and dispersed manner, with a small amount of Al2Y and Al8Mn4Y phases present in the microstructure. The addition of Zn effectively breaks the network structure formed by Al segregation, enabling the second phase to transform from a "network" to a "dispersed" state. This dispersed distribution greatly weakens the galvanic corrosion effect and is conducive to the formation of a dense passivation protective film.

[0040] Furthermore, elemental analysis (EDS) was performed on the cast magnesium alloys of Examples 2 and Comparative Examples 1-4, such as... Figure 3 As shown in Figure B, the Al element in AW100 mainly exists as a network of Mg. 17 Al 12 The second phase is distributed in the form of magnesium matrix, and as the Zn content increases (i.e., the Al content decreases), the main second phase in this alloy system undergoes a process called "Mg" formation. 17 Al 12 →Mg 17 (Al,Zn) 12 →Mg x (Al,Zn) y The transformation of the MgZn phase also involves a change in phase morphology from coarse network to fine diffuse to coarse network. As for the Y element, it mainly combines with the Al element to form phases such as Al2Y and Al8Mn4Y. Therefore, the distribution of the Y element is roughly similar to that of the Mn element.

[0041] 4. The potentiodynamic polarization curves of the cast magnesium alloys prepared in Examples 2, 4, and Comparative Examples 1-2 were obtained after immersing them in a 3.5 wt.% NaCl solution for 1 hour. The scanning range was -2.2 V to 1.2 V (vs. Eoc), and the scanning rate was 1 mV / s. The results are as follows: Figure 4 As shown.

[0042] from Figure 4 It can be seen that with the increase of Al addition (i.e., the decrease of Zn content), the corrosion potential of magnesium alloy shows a trend of first increasing and then decreasing. Compared with Comparative Examples 1 and 2, Example 2 (AZW820) exhibits excellent intrinsic chemical stability and shows a "passivation-secondary passivation" phenomenon. This is due to the synergistic effect of the addition of a small amount of Y and a specific Al / Zn ratio. After immersion, the AZW820 sample induces the formation of a dense passivation film on the alloy surface, which increases the measured corrosion potential to -1.21 V and reduces the corrosion current density to 9.25 × 10⁻⁶ V. -7 A / cm 2 When the potential is increased to -1.06 V, the corrosion film breaks down and overpassivation occurs; when the potential is increased to -0.86 V, the polarization curve shows secondary passivation.

[0043] 5. Tensile tests were conducted on the cast magnesium alloys prepared in Examples 1-4 and Comparative Examples 1-4, respectively. The results are as follows: Figure 5 As shown.

[0044] from Figure 5 It can be seen that adding too much Al or too much Zn will result in poor mechanical properties of the alloy. Combined with... Figure 3 The SEM images show that excess Al forms a continuous network or coarse Mg. 17 Al 12 The brittle phases, distributed along grain boundaries, are prone to becoming crack initiation and propagation paths under stress, leading to a significant decrease in the mechanical properties of the alloy. Excessive Zn causes the MgZn eutectic phase to precipitate as coarse, massive forms, becoming crack initiators and increasing the crystallization temperature range, thus increasing defects such as porosity and hot cracking during casting and weakening mechanical properties. Only when Al and Zn are adjusted to an appropriate ratio can Mg... 17 Al 12 When the phase is uniformly dispersed in the magnesium matrix and Zn significantly refines the grains, the alloy's mechanical properties reach their optimal values. When the Al content is 8 wt.% and the Zn content is 2 wt.%, the alloy exhibits the best comprehensive mechanical properties, with a tensile strength of 250 MPa, a yield strength of 124 MPa, and an elongation of 7.1% in the as-cast state.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-cost cast magnesium alloy possessing both high corrosion resistance and high strength, characterized in that, The magnesium alloy comprises the following components by mass percentage: Zn 1%~4%, Al 6%~9%, Y 0.05%~2%, Mn 0.05%~2%, with the balance being magnesium and unavoidable impurities; wherein the sum of the mass percentages of Zn and Al is 9%~11%.

2. The low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 1, characterized in that, The magnesium alloy comprises the following components by mass percentage: Zn 1%~3%, Al 7%~9%, Y 0.1%~1%, Mn 0.1%~1%, with the balance being magnesium and unavoidable impurities.

3. The low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 1, characterized in that, The impurity content in the alloy is less than 0.02%, and the mass percentage of harmful impurities Fe, Cu and Ni is limited to: Fe≤0.005%, Cu≤0.005% and Ni≤0.001%, respectively.

4. The low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 1, characterized in that, In the microstructure of the magnesium alloy, the magnesium alloy grains exist in the form of dendritic crystals, and Mg 17 Al 12 The phases are distributed in a fine, diffuse pattern.

5. A method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength as described in claim 1, characterized in that, Includes the following steps: S1: Pure aluminum, pure zinc, pure magnesium, Mg-Y master alloy and Mg-Mn master alloy are used as raw materials for component mixing. Then, pure Mg is heated to melt under a protective atmosphere. Pure aluminum, pure zinc Mg-Y master alloy and Mg-Mn master alloy are added. After complete melting, an alloy melt is obtained. S2: The alloy melt obtained in step S1 is stirred and kept at a constant temperature, and then cooled to obtain the cast magnesium alloy.

6. The method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 5, characterized in that, The mass percentage of Y in the Mg-Y master alloy is 20-30%, and the mass percentage of Mn in the Mg-Mn master alloy is 10-30%.

7. The method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 5, characterized in that, The protective atmosphere in step S1 is a mixture of CO2 and SF6 in a volume ratio of 99:1; the heating temperature is 680~760℃.

8. The method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 5, characterized in that, The stirring method in step S2 is mechanical stirring, and the stirring time is 2-5 minutes; the heat preservation time is 30-50 minutes.

9. The method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 5, characterized in that, Slag removal is performed before stirring and during heat preservation.

10. The method for preparing a low-cost cast magnesium alloy with both high corrosion resistance and high strength according to claim 5, characterized in that, The cooling method is water cooling, with a cooling rate of 110~130℃ / min.

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