Method for optimizing structure and performance of high-pressure casting rare earth microalloyed magnesium alloy

Abstract

The invention discloses a method for optimizing the structure and performance of a high-pressure cast rare earth microalloyed magnesium alloy, and relates to the technical field of metal materials and casting processes, in particular to the method for optimizing the structure and performance of the high-pressure cast rare earth microalloyed magnesium alloy. The technical problems that in the high-pressure casting process of an existing commercial magnesium alloy, the obdurability is difficult to improve at the same time, and a large component manufacturing process for improving the performance through heat treatment is inconvenient to adopt are solved. The structure regulation and control are realized through a strength and plasticity synergistic improvement mechanism, the nucleation rate is increased by reducing the pouring temperature to refine grains, and meanwhile, the strength and plasticity are improved. In the solidification process, Pr atoms can be enriched at the front edge of a solid-liquid interface, and component supercooling is generated, so that grain growth is inhibited. Pr also has very strong chemical activity, and can be combined with harmful impurities in the melt and precipitated; the crack propagation speed of the magnesium alloy is slowed down when the magnesium alloy is impacted due to refined grains and inhibited coarse eutectic phases, and the impact toughness is improved.

Description

Technical Field

[0001] This invention relates to the fields of metallic materials and casting processes, specifically to a method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloys. Background Technology

[0002] As the global automotive industry accelerates its transformation towards electrification and lightweighting, integrated die-casting technology has become a focus of the industry due to its significant cost reduction and efficiency improvement advantages. Magnesium alloys, as the lightest engineering metal material, have a density only two-thirds that of aluminum alloys, demonstrating great potential to replace aluminum alloys in the manufacture of large body structural components such as shock absorber towers and rear floors. However, existing commercially available die-casting magnesium alloy systems are insufficient to meet the increasingly stringent strength and toughness requirements of key safety components in new energy vehicles. Taking the most widely used Mg-Al-Zn system, such as AZ91D, as an example, although it achieves good casting fluidity and a high tensile strength of 230MPa due to its high aluminum content of approximately 9%, its elongation is less than 7%. While the low-aluminum-content Mg-Al-Mn system, such as AM60B, shows some improvement in toughness, its yield strength is low, making it difficult to resist deformation under impact loads.

[0003] The synergistic strengthening of strength and plasticity is the biggest bottleneck faced by traditional magnesium alloys. Generally, increasing strength leads to an increase in brittle phases, significantly reducing plasticity; conversely, increasing plasticity results in a loss of strength. Furthermore, traditional high-pressure casting processes are prone to gas entrapment during high-speed filling, making heat treatment for strengthening impossible. Therefore, developing a magnesium alloy material and process that can break the inverse relationship between strength and plasticity in the as-cast state, achieving a synergistic improvement in both, is key to overcoming current application bottlenecks. Summary of the Invention

[0004] The present invention aims to address the technical problem that existing commercial magnesium alloys are difficult to simultaneously improve in terms of strength and toughness during high-pressure casting, and that it is inconvenient to use heat treatment to improve the performance of large components. Instead, it provides a method for optimizing the microstructure and properties of high-pressure cast rare earth microalloyed magnesium alloys.

[0005] The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to the present invention is carried out according to the following steps:

[0006] I. Preparing Magnesium Alloy Raw Materials: Weigh the raw materials according to the mass percentage of each element in the magnesium alloy;

[0007] The magnesium alloy comprises the following elements by mass percentage: Al 2.1%~3.6%, La 2.1%~3.1%, Pr 0.3%~0.8%, Mn 0.2%~0.5%, Si 0.05%~0.2%, with the balance being Mg;

[0008] II. Melting and Refining: The raw materials weighed in step one are melted at high temperature under a protective atmosphere, and then refined.

[0009] III. Vacuum-Extrusion Composite Structure Controlled Die Casting: The pouring temperature is controlled at 690℃~700℃, and the mold preheating temperature is 180℃~205℃; a staged injection process is adopted in the pressure chamber, starting slowly and then increasing the speed. The slow injection speed is 0.3m / s~0.5m / s, and the fast injection speed is controlled at 4m / s~6m / s. The vacuum valve is opened within 10ms~30ms of the filling time to ensure that more than 90% of the gas in the cavity is discharged; and an overall pressure of 30MPa~40MPa is applied at the moment of completion of filling, and the pressure is held for 70s~75s before opening the mold and removing the part.

[0010] This invention achieves microstructure regulation through a synergistic enhancement mechanism of strength and plasticity. It refines grains by increasing the nucleation rate through lowering the casting temperature, thereby improving both strength and plasticity. Polymer (Pr) has a certain solid solubility in the magnesium matrix. During solidification, Pr atoms accumulate at the solid-liquid interface front, resulting in compositional supercooling and inhibiting grain growth. Pr also exhibits strong chemical reactivity, combining and precipitating with harmful impurities in the melt (such as Fe, Ni, and Cu). La can form complex intermetallic compounds with Al and impurity elements, preferentially abstracting Al to form the thermally stable Al3La. Al3La narrows the solidification temperature range, resulting in a denser internal microstructure in the casting. Mn can eliminate the harmful effects of Fe, improving the alloy's corrosion resistance. The refined grains and suppressed coarse eutectic phases slow down crack propagation in the magnesium alloy under impact, enhancing its impact toughness.

[0011] The magnesium alloy component obtained by this invention has a yield strength of at least 130 MPa, a tensile strength of at least 240 MPa, and an elongation of at least 10% in the as-cast (F-state) condition. The microstructure of the prepared component has significant grain refinement characteristics, with an average grain size of less than 8 μm in the surface fine-grained region and an average grain size of less than 20 μm in the core grained region. The prepared component exhibits ductile fracture mode under high-speed impact load, with the fracture morphology consisting of a large number of dimples and no brittle cleavage steps extending along the grain boundaries. Attached Figure Description

[0012] Figure 1 Model diagram of the magnesium alloy rear floor casting for automobile prepared for Experiment 1, and mechanical property diagram of each sampling area;

[0013] Figure 2 for Figure 1 (a) OM diagram at position 5;

[0014] Figure 3 for Figure 1 (a) SEM image of position 5 in the middle;

[0015] Figure 4 for Figure 1 (a) SEM image of the fracture at position 5 in the middle;

[0016] Figure 5 for Figure 1 (a) TEM image of position 5. Detailed Implementation

[0017] Specific Implementation Method 1: This implementation method is a method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloys, specifically carried out according to the following steps:

[0018] I. Preparing Magnesium Alloy Raw Materials: Weigh the raw materials according to the mass percentage of each element in the magnesium alloy;

[0019] The magnesium alloy comprises the following elements by mass percentage: Al 2.1%~3.6%, La 2.1%~3.1%, Pr 0.3%~0.8%, Mn 0.2%~0.5%, Si 0.05%~0.2%, with the balance being Mg;

[0020] II. Melting and Refining: The raw materials weighed in step one are melted at high temperature under a protective atmosphere, and then refined.

[0021] III. Vacuum-Extrusion Composite Structure Controlled Die Casting: The pouring temperature is controlled at 690℃~700℃, and the mold preheating temperature is 180℃~205℃; a staged injection process is adopted in the pressure chamber, starting slowly and then increasing the speed. The slow injection speed is 0.3m / s~0.5m / s, and the fast injection speed is controlled at 4m / s~6m / s. The vacuum valve is opened within 10ms~30ms of the filling time to ensure that more than 90% of the gas in the cavity is discharged; and an overall pressure of 30MPa~40MPa is applied at the moment of completion of filling, and the pressure is held for 70s~75s before opening the mold and removing the part.

[0022] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the mass percentage composition of each element in the magnesium alloy described in step one is: Al 2.8%, La 2.4%, Pr 0.5%, Mn 0.3%, Si 0.15%, with the balance being Mg. Everything else is the same as in Specific Implementation Method One.

[0023] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the raw materials mentioned in step one include pure magnesium ingots, pure aluminum ingots, Mg-30La master alloy, Mg-25Pr master alloy, Mg-10Mn master alloy, and Mg-10Si master alloy. Everything else is the same as in Specific Implementation Method One or Two.

[0024] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the protective atmosphere described in step two is a mixture of SF6 and CO2. Everything else is the same as in Specific Implementation Methods One to Three.

[0025] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Four in that the volume percentage of SF6 in the SF6 and CO2 mixture mentioned in step two is 0.5%. Everything else is the same as in Specific Implementation Method Four.

[0026] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the refining temperature in step two is 730℃. Everything else is the same as in Specific Implementation Method Five.

[0027] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the melting temperature in step two is 700℃. Everything else is the same as in Specific Implementation Method Six.

[0028] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that the acceleration from slow injection speed to fast injection speed in step three is 400 m / s². 2 Everything else is the same as in Specific Implementation Method Seven.

[0029] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that the pouring temperature in step three is 690℃. Everything else is the same as in Specific Implementation Method Eight.

[0030] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Nine in that: in step three, an overall pressure of 35 MPa is applied at the moment the filling process ends, and the pressure is maintained for 70 seconds. Everything else is the same as in Specific Implementation Method Nine.

[0031] The invention was verified using the following experiments:

[0032] Experiment 1: This experiment presents a method for optimizing the microstructure and properties of high-pressure casting rare-earth microalloyed magnesium alloys. The specific steps are as follows:

[0033] I. Preparation of Magnesium Alloy Raw Materials: Weigh the raw materials according to the mass percentage of each element in the magnesium alloy. The raw materials include pure magnesium ingots, pure aluminum ingots, Mg-30La master alloy, Mg-25Pr master alloy, Mg-10Mn master alloy and Mg-10Si master alloy.

[0034] The magnesium alloy is composed of the following elements by mass percentage: Al 2.8%, La 2.4%, Pr 0.5%, Mn 0.3%, Si 0.15%, with the balance being Mg;

[0035] II. Melting and Refining: The raw materials weighed in step one are melted at high temperature under a protective atmosphere, and then refined.

[0036] The protective atmosphere described in step two is a mixture of SF6 and CO2, with SF6 accounting for 0.5% of the volume of the mixture.

[0037] The melting temperature in step two is 700℃;

[0038] The refining temperature in step two is 730℃, and RJ-2 flux is added at a rate of 1% of the total weight of the charge.

[0039] III. Vacuum-Extrusion Composite Structure Controlled Die Casting: The pouring temperature is controlled at 690℃, and the mold preheating temperature is 180℃. A staged injection process is adopted in the pressure chamber, starting slowly and gradually increasing in speed. The length of the pressure chamber is 1910mm. From the 0 to 950mm position, the speed is uniformly accelerated from 0 to 0.4m / s. Starting from the 950mm position, the speed increases from 0.4m / s to 6m / s, with an acceleration of 400m / s². 2 Maintain a speed of 6 m / s until the extrusion ends, and open the vacuum valve within 10 ms to 30 ms of the filling time to ensure that more than 90% of the gas in the cavity is discharged; and apply an overall pressure of 35 MPa at the moment the filling ends, hold the pressure for 70 s, open the mold, and remove the part.

[0040] Figure 1 The model drawing of the magnesium alloy rear floor casting for automobile prepared for Experiment 1 is shown below. The sampling location and sampling direction are as follows: Figure 1 As shown in (a). Figure 1 As shown in (a), six representative locations were selected in the main body area of ​​the casting for body sampling, covering the area near the gating system (locations 4, 5 and 6), the end of the filling area (locations 1, 2 and 3), and the plate area and the intersection area of ​​the reinforcing ribs, in order to comprehensively evaluate the overall quality of the casting. Figure 1 (b) is the corresponding Figure 1 (a) Mechanical properties of each sampling area: The optimal area achieved a yield strength (YS) of 120 MPa, a tensile strength (UTS) of 240 MPa, and an elongation (EL) of 15%. The tensile strength of all locations remained at a high level, with an average exceeding 230 MPa. In particular, positions 2 and 5 achieved tensile strengths of 242 MPa and 237 MPa, respectively, while the yield strength (YS) remained stable within the 120–130 MPa range. The elongation exhibited excellent levels: 15% at position 5, 12% at position 4, and 11% at position 2. Although there were slight fluctuations in performance at different flow lengths due to the large, thin-walled nature of the component, the overall performance met the high-performance targets of yield strength ≥130 MPa, tensile strength ≥240 MPa, and elongation of 10–15%. The OM diagram of the optimal position 5 is shown below. Figure 2As shown, the metallographic structure reveals that the primary α-Mg grains are nearly spherical or rose-shaped, with an average grain size refined to 8–10 μm. This fine-grained structure significantly increases the total grain boundary area, which helps to hinder dislocation slip, while the rounded grain boundary morphology reduces stress concentration.

[0041] Figure 3 for Figure 1 (a) SEM image at position 5, showing α-Mg grains, Al3La at grain boundaries, and a small amount of blocky La(MnAl2)4. No continuous network brittle phase, common in traditional magnesium alloys, is formed. This microstructure allows for coordinated deformation under stress, avoiding intergranular brittle fracture.

[0042] Figure 4 for Figure 1 (a) The SEM image of the fracture surface at position 5 in the middle shows that the fracture mode is a mixed ductile fracture with many dimples, which is consistent with the macroscopic behavior of high elongation.

[0043] Figure 5 for Figure 1 (a) TEM image at position 5 shows a large number of nanoscale needle / rod precipitates. These dispersed precipitates and semi-coherent phases exert a strong pinning effect on dislocation movement, which significantly improves the yield strength of the material without sacrificing too much plasticity.

Claims

1. A method for optimizing the microstructure and properties of high-pressure cast rare-earth microalloyed magnesium alloys, characterized in that... The method is performed according to the following steps: I. Preparing Magnesium Alloy Raw Materials: Weigh the raw materials according to the mass percentage of each element in the magnesium alloy; The magnesium alloy comprises the following elements by mass percentage: Al 2.1%~3.6%, La 2.1%~3.1%, Pr 0.3%~0.8%, Mn 0.2%~0.5%, Si 0.05%~0.2%, with the balance being Mg; II. Melting and Refining: The raw materials weighed in step one are melted at high temperature under a protective atmosphere, and then refined. III. Vacuum-Extrusion Composite Structure Controlled Die Casting: Control the pouring temperature to 690℃~700℃ and the mold preheating temperature to 180℃~205℃; adopt a staged injection process in the pressure chamber, starting slowly and then increasing the speed. The slow injection speed is 0.3m / s~0.5m / s, and the fast injection speed is controlled at 4m / s~6m / s. The vacuum valve is opened within 10ms~30ms of the filling time to ensure that more than 90% of the gas in the cavity is discharged; and apply an overall pressure of 30MPa~40MPa at the moment of completion of filling, hold the pressure for 70s~75s, open the mold, and remove the part.

2. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The magnesium alloy described in step one has the following composition by mass percentage: Al 2.8%, La 2.4%, Pr 0.5%, Mn 0.3%, Si 0.15%, with the balance being Mg.

3. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The raw materials mentioned in step one include pure magnesium ingots, pure aluminum ingots, Mg-30La master alloy, Mg-25Pr master alloy, Mg-10Mn master alloy and Mg-10Si master alloy.

4. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The protective atmosphere described in step two is a mixture of SF6 and CO2.

5. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 4, characterized in that... In step two, the volume percentage of SF6 in the SF6 and CO2 mixture is 0.5%.

6. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The refining temperature in step two is 730℃.

7. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The melting temperature in step two is 700℃.

8. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... In step three, the acceleration from slow injection speed to fast injection speed is 400 m / s². 2 .

9. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... The pouring temperature in step three is 690℃.

10. The method for optimizing the microstructure and properties of high-pressure casting rare earth microalloyed magnesium alloy according to claim 1, characterized in that... In step three, apply an overall pressure of 35 MPa at the moment the filling is completed, and hold the pressure for 70 seconds.

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