Magnesium alloy large component semi-solid injection molding microstructure control method and application

Abstract

The application belongs to the technical field of metal material forming, and particularly relates to a microstructure control method for magnesium alloy large components by semi-solid injection molding and application. The core of the preparation method is accurate preparation of semi-solid slurry, gradient temperature control of mold partition, three-stage injection and synergistic effect of ultrasonic vibration, combined with subsequent heat treatment, to realize global fine crystallization, equiaxialization and homogenization of the microstructure of the magnesium alloy large component, refine the average size of the primary alpha-Mg solid phase particles in the semi-solid slurry to less than or equal to 60 microns, and the shape tends to be spherical and the shape factor is greater than or equal to 0.7, the solid phase rate of the semi-solid slurry is 40-60%, and the solid phase rate accuracy deviation is less than or equal to plus or minus 3%. The porosity of the magnesium alloy component prepared by the method is less than or equal to 1.5%, the tensile strength is greater than or equal to 260 MPa, the elongation rate is greater than or equal to 5%, and the equiaxed crystal rate is greater than or equal to 85%. Meanwhile, the preparation method is suitable for the intelligent manufacturing demand of high-strength lightweight large structural parts in the fields of aerospace, rail transportation and high-end equipment manufacturing.

Description

Technical Field

[0001] This invention belongs to the field of metal material forming technology, specifically relating to a method and application for controlling the microstructure of semi-solid injection molding of large magnesium alloy components. Background Technology

[0002] Magnesium alloys are considered ideal lightweight structural materials due to their low density, high specific strength, good damping properties, and electromagnetic shielding performance. However, traditional liquid die casting is prone to defects such as shrinkage porosity, hot cracking, and gas pores, and also results in coarse grains and unstable mechanical properties; while solid-state molding is difficult and energy-intensive. Semi-solid injection molding technology combines the advantages of liquid and solid-state molding, enabling the manufacture of complex components with high density and low defects at lower molding temperatures, and represents an important development direction for magnesium alloy molding.

[0003] Currently, research on semi-solid forming of magnesium alloys mainly focuses on small-sized specimens or simple structural parts, while research on parts with a projected area ≥1500 cm² is less common. 2 For large components with a wall thickness ≥3mm, significant challenges remain in semi-solid slurry delivery, filling stability, temperature field uniformity, and control of grain nucleation and growth. Especially within large mold cavities, large temperature gradients and uneven cooling rates lead to significant regional differences in microstructure, resulting in problems such as columnar crystal coarsening, low equiaxed crystal ratio, and second-phase segregation, which seriously affect the consistency and reliability of the overall component performance.

[0004] In existing technologies, although there have been attempts to improve the microstructure by optimizing parameters such as pouring temperature and injection speed, there is a lack of a coordinated control mechanism for the semi-solid slurry state, mold thermal field distribution and dynamic recrystallization process, making it difficult to achieve full-domain fine, uniform and equiaxed microstructure control of large components.

[0005] Therefore, developing a microstructure control method suitable for semi-solid injection molding of large magnesium alloy components has significant engineering value and industrialization implications. Summary of the Invention

[0006] In view of this, and in response to the above problems, the first objective of this invention is to provide a method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components. This method optimizes the grain size, morphology, and distribution of magnesium alloy by adjusting the preparation process of the semi-solid slurry, the zoned gradient temperature control of the injection molding mold, the synergistic effect of three-stage injection and ultrasonic vibration, combined with subsequent heat treatment and cooling processes, thereby improving the mechanical properties of the material and achieving full-domain grain refinement, equiaxed and homogenized microstructure of large magnesium alloy components.

[0007] The second objective of this invention is to provide an application of magnesium alloy components prepared based on the above method in the fields of aerospace, rail transportation, and high-end equipment manufacturing.

[0008] To achieve the objectives of the invention described above, the present invention adopts the following technical solution:

[0009] In a first aspect, the present invention provides a method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components, comprising the following steps:

[0010] S1. Material pretreatment: Large magnesium alloy ingots are crushed and dried to obtain magnesium alloy particles;

[0011] S2. Preparation of semi-solid slurry: The magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device to prepare a semi-solid slurry; the solid fraction of the semi-solid slurry is 40-60%, and the accuracy deviation of the solid fraction is ≤±3%;

[0012] S3. Mold system optimization and thermal field control: The semi-solid slurry is placed in a zoned temperature-controlled mold, and the zoned temperature-controlled mold is preheated to 220-260°C, and dynamic gradient temperature control is used for filling.

[0013] S4. Injection molding: The semi-solid slurry is injected into a zoned temperature-controlled mold in three stages. An ultrasonic vibration auxiliary device is introduced in the third stage of injection to finally obtain a magnesium alloy component.

[0014] S5. Post-treatment: The magnesium alloy component is subjected to T4 solution treatment and T6 aging treatment in sequence.

[0015] Preferably, the large magnesium alloy ingot has a projected area ≥1500 cm². 2 Large components with a wall thickness ≥ 3mm; the specific process of crushing and drying large magnesium alloy ingots to obtain magnesium alloy particles in the material pretreatment of step S1 includes:

[0016] Large magnesium alloy ingots of high purity AZ series or ZK series are selected, and the large magnesium alloy ingots are crushed into magnesium alloy particles with a particle size of 3-8 mm. The magnesium alloy particles are placed in a vacuum drying oven at 120°C and dried for 2 hours. The content of Fe, Ni and Cu impurity elements in the large magnesium alloy ingots of high purity AZ series or ZK series is less than 0.005 wt%. The large magnesium alloys of AZ series or ZK series include any one of AZ91D, ZK60, AM60, WE43, AZ31 and ZK61 magnesium alloys.

[0017] Preferably, the specific process of preparing the semi-solid slurry in step S2, which involves feeding the magnesium alloy particles into an electromagnetic stirring-cooling composite slurry preparation device, includes:

[0018] The magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The device is heated and electromagnetic stirring is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are then cooled to the target solid fraction range and stirred continuously for 120–180 s to obtain a semi-solid slurry with an average size of ≤60 μm for the primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0019] More preferably, the temperature of the preparation apparatus is set to 550–650°C, the frequency of the electromagnetic stirring is 20–40 Hz, the intensity of the electromagnetic stirring is 0.1–0.3 T, and the cooling rate is 1–3°C / s.

[0020] Preferably, in step S3, mold system optimization and thermal field control, the semi-solid slurry is placed in a zoned temperature-controlled mold, and the zoned temperature-controlled mold is preheated to 220-260°C. The specific operation of dynamic gradient temperature control filling is as follows:

[0021] The semi-solid slurry is placed in a zoned temperature-controlled mold with multiple independent heating and cooling channels. The zoned temperature-controlled mold is divided into a gate zone, a runner cone zone, a cavity zone, and an ejector zone. The temperature of the gate zone is 250°C; the temperature of the runner cone zone is 240-245°C; the temperature of the cavity zone is 230°C, and a temperature difference gradient of 1.5-2.5°C / cm is provided; the temperature of the ejector zone is 220-225°C.

[0022] Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the partitioned temperature control mold to provide real-time feedback of the temperature field. Based on the feedback of the temperature field and combined with the preset temperature gradient curve, the heating power of each temperature zone of the partitioned temperature control mold is dynamically adjusted by a PID controller to control the local cooling rate at 5 to 15°C / s.

[0023] The control logic of the PID controller is as follows:

[0024] A three-dimensional temperature field model of the cavity of the temperature-controlled mold along the filling path is established. The deviation between the actual temperature and the target temperature in each region is identified. Based on the deviation, the heating power or cooling medium flow rate of each temperature zone is dynamically adjusted to ensure that the gate area is maintained at 250°C and the temperature at the far end of the cavity area is gradually reduced to 230°C, forming a directional solidification temperature gradient from the inside to the outside.

[0025] More preferably, the partitioned temperature control mold is either H13 or DH360, and the working surface of the partitioned temperature control mold is treated with a nano-ceramic coating, wherein the thickness of the nano-ceramic coating is 20-30 μm.

[0026] Preferably, in step S4 injection molding, the semi-solid slurry is injected into a zoned temperature-controlled mold in three stages, and an ultrasonic vibration auxiliary device is introduced in the third stage of injection to finally obtain the magnesium alloy component. The specific operation steps are as follows:

[0027] The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling. The holding time in the zoned temperature-controlled mold is ≤90s, and finally, magnesium alloy components are obtained by injection molding.

[0028] The three-stage injection process takes ≤12 seconds, and the specific operation of the three-stage injection process is as follows:

[0029] The first stage is a slow filling process: the injection speed is 1.5 to 2.5 m / s, filling to 40 to 50% of the cavity volume;

[0030] The second stage is a medium-speed transition process: the injection speed is increased to 3.0-4.0 m / s, and the cavity volume is filled to 90%;

[0031] The third stage is the low-pressure compaction process: switch to the pressurization mode, increase the pressure to 80-120MPa, hold the pressure for 15-25s, and fill to 100% of the cavity volume;

[0032] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 15-25μm are applied for a duration of 3-5s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0033] Preferably, the specific steps for performing T4 solution treatment and T6 aging treatment on the magnesium alloy component in the post-processing of step S5 are as follows:

[0034] After the magnesium alloy component is slowly cooled to below 150°C in a temperature-controlled mold, the mold is opened and immediately after demolding. The T4 solution treatment is performed, which involves heating the magnesium alloy component to a solution temperature of 400±10°C and holding it for 8–12 hours. After that, the magnesium alloy component after the T4 solution treatment is quickly immersed in cold water to cool. Then, the T6 aging treatment is performed, which involves placing the magnesium alloy component after the T4 solution treatment in an aging furnace preheated to 200±10°C and holding it for 16–24 hours. After that, the magnesium alloy component after the T6 aging treatment is allowed to cool naturally in air.

[0035] More preferably, the magnesium alloy component has a porosity of 0.3-0.45%, a tensile strength of 270-310 MPa, an equiaxed crystallinity of 84-90%, and an elongation of 9.8-12%.

[0036] The second aspect of the present invention provides the application of the microstructure control method for semi-solid injection molding of large magnesium alloy components described in the first aspect of the present invention in the fields of aerospace, rail transportation, and high-end equipment manufacturing.

[0037] The beneficial effects of this invention are:

[0038] 1. This invention utilizes electromagnetic stirring-cooling composite slurry preparation and precise control of the solid fraction to cause the primary α-Mg solid particles to undergo dendritic arm breakage and rounding in the solid-liquid two-phase region under electromagnetic shearing and convection, resulting in uniform solid particles that are nearly spherical with a shape factor ≥0.7 and an average size ≤60μm. Simultaneously, combined with zoned temperature control and dynamic gradient temperature control, it suppresses localized supercooling and shelling and localized coarsening within large mold cavities, significantly reduces the proportion of columnar crystals and increases the proportion of equiaxed crystals, thereby achieving consistency and uniformity of the microstructure in various regions of large components.

[0039] 2. This invention employs a three-stage injection strategy during the semi-solid slurry molding process: the first stage involves slow filling to avoid air entrapment and rapid cooling; the second stage involves medium-speed transition to ensure filling at the far end; and the third stage involves compaction to promote shrinkage compensation and densification. Simultaneously, dynamic gradient temperature control prevents premature freezing of the gate area, which could interrupt the "shrinkage compensation channel," allowing pressure to be effectively transmitted to the final solidification area. Furthermore, an ultrasonic vibration auxiliary device is introduced in the third stage to promote gas discharge and further enhance densification through synergistic pressurization.

[0040] 3. This invention improves load-bearing capacity through fine grain strengthening and uniform distribution of equiaxed grains, and significantly reduces the number of crack initiation points by reducing defects, thereby increasing elongation; T4 solution treatment can dissolve the as-cast non-equilibrium second phase, weaken segregation and brittle network and achieve microstructure homogenization, followed by T6 aging treatment to precipitate strengthening phases and achieve enhanced strengthening effect, ultimately achieving a synergistic improvement in strength and plasticity.

[0041] 4. This invention is applicable to projected areas ≥1500cm². 2 For large components with a wall thickness of ≥3mm, the good fluidity and low shrinkage characteristics of the semi-solid slurry are ensured by precise control of the solid fraction. By using zoned temperature control and gradient temperature field preheating of the mold, coupled with three-stage injection and final pressurization and ultrasonic coupling, the filling and solidification process of key areas with thin-thickness differences, large cross sections, and corners can be controlled, thereby expanding the manufacturability of large and complex magnesium alloy components and improving the consistency of molding quality.

[0042] 5. This invention uses H13 or DH360 hot-working zoned temperature control mold material, and sets a nano-ceramic coating with a thickness of 20-30μm on the working surface of the zoned temperature control mold. This can reduce the instantaneous heat extraction and thermal shock on the surface of the zoned temperature control mold, reduce the risk of thermal fatigue cracking, and improve the interfacial friction and anti-adhesion properties. Combined with zoned temperature control, it reduces drastic local temperature fluctuations, thereby improving the stability of the molding process and extending the service life of the zoned temperature control mold. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a schematic flowchart of a method for controlling the microstructure of a large magnesium alloy component through semi-solid injection molding, provided in an embodiment of the present invention. Detailed Implementation

[0045] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] like Figure 1 As shown in the figure, this invention discloses a method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components, including the following steps:

[0047] S1. Material pretreatment: Large magnesium alloy ingots are crushed and dried to obtain magnesium alloy particles.

[0048] S2. Preparation of semi-solid slurry: Magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device to prepare semi-solid slurry; the solid fraction of the semi-solid slurry is 40-60%, and the solid fraction accuracy deviation is ≤±3%.

[0049] S3. Mold system optimization and thermal field control: Place the semi-solid slurry in a zoned temperature-controlled mold and preheat the zoned temperature-controlled mold to 220-260℃, and dynamically gradient temperature control for filling.

[0050] S4. Injection Molding: The semi-solid slurry is injected into a temperature-controlled mold in three stages. An ultrasonic vibration auxiliary device is introduced in the third stage of injection to finally obtain a magnesium alloy component.

[0051] S5. Post-treatment: The magnesium alloy components are subjected to T4 solution treatment and T6 aging treatment in sequence.

[0052] In step S1, the large magnesium alloy ingot has a projected area ≥1500 cm². 2 Large components with a wall thickness ≥ 5mm; the specific process of crushing and drying large magnesium alloy ingots to obtain magnesium alloy particles in step S1 material pretreatment includes:

[0053] High-purity AZ or ZK series large magnesium alloy ingots are selected and crushed into magnesium alloy particles with a particle size of 3-8 mm. Crushing allows for more uniform and faster heating into the solid-liquid two-phase region, reducing localized overheating or undercooling caused by temperature differences between the inside and outside of the large magnesium alloy ingot. It also facilitates the formation of a uniform semi-solid slurry under electromagnetic stirring. Magnesium alloy particles larger than 8 mm result in slower heating and difficulty in the particle center synchronously entering the solid-liquid two-phase region; particles smaller than 3 mm lead to more severe oxidation and agglomeration. The magnesium alloy particles are then treated in a vacuum drying oven at 120℃ for 2 hours to remove adsorbed water and grease from their surface. Magnesium alloys are extremely sensitive to water and grease, decomposing upon heating to produce gas and increase porosity. Spectroscopic analysis shows that the content of Fe, Ni, and Cu impurities in the high-purity AZ or ZK series magnesium alloy ingots is less than 0.005 wt%. For example, the AZ series or ZK series magnesium alloys include any one of AZ91D, ZK60, WE43, AZ31, and ZK61 magnesium alloys. Of course, other magnesium alloys can also be selected, and this invention does not limit them.

[0054] In some embodiments of the present invention, the magnesium alloy particle size is any value of 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, or a range formed by any two of these values.

[0055] The specific process of preparing semi-solid slurry in step S2, where magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device, includes:

[0056] Magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 550–650℃, and electromagnetic stirring is performed at a frequency of 20–40Hz and an intensity of 0.1–0.3T to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are then cooled at a cooling rate of 1–3℃ / s to the target range of 40–60% solid phase content. The stirring is continued for 120–180s to obtain a semi-solid slurry with an average size of ≤60μm of primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0057] In some embodiments of the present invention, the frequency is any value of 20Hz, 25Hz, 30Hz, 35Hz, 40Hz, or a range formed by any two of them. For example, the frequency is preferably 20Hz.

[0058] It should be noted that when the solid fraction is below 40%, more liquid phase is generated, making solidification shrinkage more difficult and easily increasing porosity; when the solid fraction is above 60%, insufficient filling is likely to occur. The purpose of setting the temperature of the preparation device is to place AZ series or ZK series magnesium alloy particles in the solid-liquid two-phase region. The lower limit of the temperature ensures that magnesium alloy particles of different series can form a semi-solid slurry, while the upper limit of the temperature avoids excessive liquid phase leading to uncontrolled solid fraction, grain coarsening, and accelerated oxidation. Electromagnetic stirring is used to generate sufficient electromagnetic force to drive convection and shearing of the semi-solid slurry, causing dendrite arms to break, solid phase to become rounded, and uniformly dispersed. Too low a stirring frequency and intensity will lead to insufficient stirring, solid phase agglomeration, and dendrite residue; too high a stirring frequency and intensity will exacerbate temperature fluctuations. A cooling rate that is too slow will cause the primary α-Mg solid phase particles to grow and become insufficiently spherical; a cooling rate that is too fast will result in an excessive temperature gradient, easily leading to uneven solid fraction formation. Stirring needs sufficient time to complete to ensure uniform solid fraction.

[0059] In step S3, mold system optimization and thermal field control, the semi-solid slurry is placed in a zoned temperature-controlled mold, and the zoned temperature-controlled mold is preheated to 220-260℃. The specific operation of dynamic gradient temperature control filling is as follows:

[0060] The semi-solid slurry is placed in a zoned temperature-controlled mold with multiple independent heating and cooling channels. The mold is divided into a gate zone, a runner cone zone, a cavity zone, and an ejector zone. The gate zone temperature is 250℃ to prevent premature solidification of the slurry at the inlet and ensure continuous filling. The runner cone zone temperature is 240–245℃ to avoid cold flow or eddies during slurry splitting. The cavity zone temperature is 230℃, with a temperature gradient of 1.5–2.5℃ / cm to guide the orderly advancement of the solidification front. The ejector zone temperature is 220–225℃ to facilitate demolding and reduce thermal stress. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on this feedback and a preset temperature gradient curve, a PID controller dynamically adjusts the heating power of each temperature zone of the mold.

[0061] The control logic of the PID controller is as follows:

[0062] Temperature field modeling and deviation identification: Establish a three-dimensional temperature field model of the cavity of the temperature-controlled mold along the filling path, identify the deviation between the actual temperature and the target temperature in each region, and focus on monitoring the temperature fluctuations in the gate area, the runner cone area, the far end of the cavity and the thick cross-section area to control the local cooling rate at 5-15℃ / s.

[0063] Independent temperature control in each zone: Based on the temperature deviation, the heating power or cooling medium flow rate of each zone is dynamically adjusted to ensure that the gate zone is maintained at 250℃ and the temperature at the far end of the cavity is gradually reduced to 230℃, forming a directional solidification temperature gradient from the inside to the outside.

[0064] Solidification front tracking and response: Combine the filling progress and the position of the solidification front to adjust the temperature control strategy in real time, avoid local overcooling that could lead to "cold shut-off" or "interruption of the feeding channel", and ensure that the pressure is effectively transmitted to the final solidification area.

[0065] Cooling rate closed-loop control: With a target cooling rate range of 5 to 15℃ / s, the local cooling rate is precisely controlled by adjusting the flow rate and temperature of the cooling medium in the cooling channel, thereby suppressing the formation of coarse grains.

[0066] It should be noted that gradient temperature control is a global strategy covering the entire temperature-controlled mold cavity and runner system. By actively constructing a temperature field that decreases sequentially from the gate area to the ejector area, it fundamentally solves the problem of thermal imbalance caused by the coexistence of local overcooling and overheating in large components due to their complex structure. It ensures that the feeding channel remains unobstructed during the pressure holding stage, guiding the semi-solid slurry to achieve directional solidification from the inside out, thereby effectively eliminating internal defects such as shrinkage porosity and thermal cracking. Furthermore, by precisely controlling the local cooling rate at 5-15℃ / s, it inhibits the growth of columnar crystals and promotes the uniform nucleation and refinement of equiaxed crystals. Ultimately, it achieves full-domain grain refinement, equiaxing, and homogenization of the microstructure of large magnesium alloy components, serving as a key bridge connecting the excellent properties of the semi-solid slurry with the high performance of the final component.

[0067] The setting of a temperature gradient of 1.5~2.5℃ / cm is based on a comprehensive consideration of the properties of magnesium alloy materials, the requirements of semi-solid forming processes, and the difficulty of controlling the thermal field of large components.

[0068] Magnesium alloys have a high thermal conductivity, resulting in rapid heat dissipation in temperature-controlled molds. If the temperature gradient is less than 1.5℃ / cm, the temperature field is too flat, and the solidification driving force is insufficient, leading to slow overall solidification, coarse grains, and an inability to form effective directional solidification, which can easily cause shrinkage porosity. If the temperature gradient is greater than 2.5℃ / cm, the local temperature difference will be too large, and the temperature difference between the melt and the far end will be significant. The far end will solidify prematurely, the feeding channel will be interrupted, the thermal stress will increase, and it will easily cause thermal cracking or deformation.

[0069] Semi-solid slurries have a higher viscosity than liquid slurries, requiring a certain temperature gradient to maintain their fluidity. A gradient of 1.5–2.5℃ / cm ensures smooth filling of the mold cavity without causing premature solidification at the far end due to excessive temperature difference, resulting in incomplete filling. In semi-solid molding processes, the total injection time across three stages and the residence time of the semi-solid slurry in the temperature-controlled mold are relatively short. Within this short time, the temperature gradient must be sufficiently large to establish and maintain directional solidification conditions quickly. However, the gradient cannot be too large, otherwise the solidification rate will be too fast, making it impossible to complete filling and compaction.

[0070] For large components, a gradient of 1.5 to 2.5℃ / cm means that the temperature difference from the gate to the far end can reach 15 to 40℃. This temperature difference range is within the precisely controllable range of existing heating or cooling systems. This range will not cause control difficulties due to the temperature difference being too small, nor will it cause excessive thermal stress due to the temperature difference being too large.

[0071] Furthermore, the partition temperature control mold is either H13 or DH360. The working surface of the partition temperature control mold is treated with a nano-ceramic coating with a thickness of 20-30μm, which can reduce the instantaneous heat extraction and thermal shock on the surface of the partition temperature control mold, reduce the risk of thermal fatigue cracking, and improve the interfacial friction and anti-adhesion properties, which is conducive to improving the service life of the partition temperature control mold.

[0072] In step S4 injection molding, the semi-solid slurry is injected into a temperature-controlled mold in three stages. An ultrasonic vibration auxiliary device is introduced in the third stage of injection to finally obtain the magnesium alloy component. The specific operation steps are as follows:

[0073] The semi-solid slurry was injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold was ≤90s. Finally, magnesium alloy components were obtained by injection molding.

[0074] Because semi-solid slurries are very sensitive to temperature, prolonged residence time can lead to an increase in solid fraction and viscosity, resulting in unstable filling or even blockage. Therefore, it is essential to control the injection time throughout the entire injection process. The three-stage injection process time is controlled to ≤12 seconds. The specific operation of the three-stage injection process is as follows:

[0075] The first stage is a slow filling process: the injection speed is 1.5 to 2.5 m / s, and the cavity volume is filled to 40 to 50% to avoid air entrapment and rapid cooling.

[0076] The second stage is a medium-speed transition process: the injection speed is increased to 3.0-4.0 m / s, and the cavity volume is filled to 90%.

[0077] The third stage of low-pressure compaction process: switch to pressurization mode, increase the pressure to 80-120MPa, hold pressure for 15-25s, fill to 100% of the cavity volume, promote shrinkage and densification.

[0078] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 15-25μm are applied for 3-5s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity to avoid direct contact with the melt.

[0079] It should be noted that the introduction of an ultrasonic vibration-assisted device in the third stage serves two main purposes. First, it improves the rheological behavior of the semi-solid slurry, enhancing its filling capacity. The high-frequency mechanical vibration generated by ultrasonic waves propagating in the semi-solid slurry causes relative sliding and rearrangement between primary α-Mg solid particles, disrupting their overlapping structure and significantly reducing the apparent viscosity of the semi-solid slurry, thus enhancing its fluidity. Simultaneously, ultrasonic vibration promotes the thickening of the liquid film on the surface of the primary α-Mg solid particles, strengthening the thixotropic properties of the semi-solid slurry and facilitating remote filling of complex cavities and complete molding of thin-walled regions. Second, it promotes grain refinement and equiaxed formation. Ultrasonic vibration generates cavitation and acoustic flow effects in the semi-solid slurry: the collapse of cavitation bubbles releases high-energy shock waves, breaking up primary dendrites and increasing the number of crystal nuclei; the acoustic flow effect enhances the uniformity of the solute and temperature fields, inhibiting directional dendrite growth and promoting the transformation of grains towards equiaxed formation. Furthermore, ultrasonic vibration introduces high-density dislocations on the surface of primary α-Mg solid particles, which can serve as recrystallization nucleation sites during subsequent heat treatment, further refining the grains. Finally, it also promotes gas expulsion and reduces porosity. Ultrasonic vibration promotes the precipitation of dissolved gases in the liquid phase, forming microbubbles, which collide and merge with the vibration disturbance through the acoustic flow effect, forming larger bubbles that are more easily floated and expelled from the cavity. At the same time, ultrasonic vibration can destroy the gas adsorption layer on the surface of the zoned temperature-controlled mold or at the interface of primary α-Mg solid particles, improving the wettability between the semi-solid slurry and the zoned temperature-controlled mold, and reducing interfacial gas residue.

[0080] The specific steps for performing T4 solution treatment and T6 aging treatment on the magnesium alloy component in step S5 post-processing are as follows:

[0081] After the magnesium alloy component is slowly cooled to below 150℃ in a zoned temperature-controlled mold, the mold is opened and immediately after demolding, a T4 solution treatment is performed. Specifically, the magnesium alloy component is heated to a solution temperature of 400±10℃ and held for 8 to 12 hours. Then, the magnesium alloy component after the T4 solution treatment is quickly immersed in cold water to cool. Subsequently, a T6 aging treatment is performed. Specifically, the magnesium alloy component after the T4 solution treatment is placed in an aging furnace preheated to 200±10℃ and held for 16 to 24 hours. Then, the magnesium alloy component after the T6 aging treatment is allowed to cool naturally in air.

[0082] It should be noted that cooling the temperature of the zoned temperature-controlled mold before demolding reduces the risk of thermal shock and deformation or cracking of magnesium alloy components. Additionally, the residual stress in large components is more sensitive, so cooling to a safer temperature before demolding ensures greater stability. A solution temperature of 400±10℃ is close to the effective solution temperature range of many magnesium alloys, allowing for rapid diffusion. This not only dissolves a significant amount of the second phase but also avoids the risk of abnormal growth or localized melting of primary α-Mg solid phase particles due to overheating. A holding time of 8–12 hours provides sufficient diffusion time for the subsequent cross-sections of large magnesium alloys, ensuring adequate solution even in thicker areas. Water quenching is used to obtain a supersaturated solid solution, providing a driving force for subsequent aging precipitation. An aging temperature of 200±10℃ is a commonly used artificial aging range for many magnesium alloys, effectively forming precipitation strengthening. A holding time of 16–25 hours covers different magnesium alloy components, ensuring strength improvement and batch stability. Air cooling avoids introducing excessive quenching stress.

[0083] As a preferred embodiment of the present invention, the magnesium alloy component prepared by the present invention has a porosity of 0.3-0.45%, a tensile strength of 270-310 MPa, an equiaxed crystal ratio of 84-90%, and an elongation of 9.8-12%.

[0084] Meanwhile, the present invention also provides an application of large magnesium alloy components prepared based on the above method in the fields of aerospace, rail transportation, and high-end equipment manufacturing.

[0085] Next, this invention provides several specific embodiments and comparative examples to compare the mechanical properties under different post-processing techniques and verify the effect of process optimization.

[0086] Example 1

[0087] A 1600 cm² projected area was prepared using AZ91D magnesium alloy. 2 Take, for example, an aircraft cabin connector with a maximum wall thickness of 8mm.

[0088] S1: Crush AZ91D magnesium alloy into magnesium alloy particles with a particle size of 3-8mm, and place the AZ91D magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0089] S2: AZ91D magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 550℃, and electromagnetic stirring at a frequency of 20Hz and an intensity of 0.1T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 40% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 120s to obtain a semi-solid slurry with an average size of 55μm for the primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0090] S3: Preheat the zoned temperature-controlled mold to 240℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating-cooling channels. The zoned temperature-controlled mold is divided into a gate zone, a runner cone zone, a cavity zone, and an ejector zone. The gate zone temperature is 250℃, the runner cone zone temperature is 243℃, and the cavity zone temperature is 230℃, with a temperature gradient of 2.2℃ / cm. The ejector zone temperature is 223℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is H13, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 25μm.

[0091] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0092] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0093] The first stage of slow filling process: the injection speed is 2.0 m / s, filling to 45% of the cavity volume;

[0094] The second stage is a medium-speed transition process: the injection speed is increased to 3.5 m / s, and the cavity volume is filled to 90%;

[0095] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 100MPa, hold the pressure for 20s, and fill to 100% of the cavity volume;

[0096] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 20μm are applied for 4s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0097] S5. After slowly cooling the magnesium alloy component to below 150°C in the temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 400°C and hold for 10 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 200°C and hold for 20 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0098] Comparative Example 1

[0099] The magnesium alloy used in Example 1 is the same, except that:

[0100] This comparative example uses the traditional liquid high-pressure die casting method, without ultrasonic and gradient temperature control treatment, with a pouring temperature of 680℃ and a mold temperature of 200℃.

[0101] Example 2

[0102] A projected area of ​​1700 cm² was prepared using ZK60 magnesium alloy. 2 Take, for example, the inner panel of a car door with a maximum wall thickness of 10mm.

[0103] S1: Crush ZK60 magnesium alloy into magnesium alloy particles with a particle size of 3-8mm, and place the ZK60 magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0104] S2: ZK60 magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 580℃, and electromagnetic stirring at a frequency of 20Hz and an intensity of 0.2T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 45% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 120s to obtain a semi-solid slurry with an average size of 58μm for the primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0105] S3: Preheat the zoned temperature-controlled mold to 250℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating and cooling channels. The zoned temperature-controlled mold is divided into a gate area, a runner cone area, a cavity area, and an ejector area. The gate area temperature is 250℃, the runner cone area temperature is 244℃, and the cavity area temperature is 230℃, with a temperature gradient of 2.0℃ / cm. The ejector area temperature is 224℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is a DH360, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 22μm.

[0106] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0107] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0108] The first stage of slow filling process: the injection speed is 2.2m / s, filling to 50% of the cavity volume;

[0109] The second stage is a medium-speed transition process: the injection speed is increased to 3.8 m / s, and the cavity volume is filled to 92%;

[0110] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 110MPa, hold the pressure for 22s, and fill the cavity to 100% of its volume;

[0111] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 22μm are applied for 4s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0112] S5. After slowly cooling the magnesium alloy component to below 150°C in the temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 400°C and hold for 11 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 200°C and hold for 22 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0113] Comparative Example 2

[0114] The magnesium alloy used in Example 2 is the same, except that:

[0115] This comparative example uses traditional semi-solid injection molding, in which ultrasonic and gradient temperature control treatments are not performed, and the mold is uniformly preheated to 240°C.

[0116] Example 3

[0117] A projected area of ​​1800 cm² is prepared using AM60 magnesium alloy. 2 Take, for example, an aerospace support with a maximum wall thickness of 12mm.

[0118] S1: Crush AM60 magnesium alloy into magnesium alloy particles with a particle size of 3-8mm, and place the AM60 magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0119] S2: AM60 magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 600℃, and electromagnetic stirring at a frequency of 20Hz and an intensity of 0.1T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 50% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 120s to obtain a semi-solid slurry with an average size of ≤50μm of primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0120] S3: Preheat the zoned temperature-controlled mold to 260℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating-cooling channels. The zoned temperature-controlled mold is divided into a gate zone, a runner cone zone, a cavity zone, and an ejector zone. The gate zone temperature is 250℃, the runner cone zone temperature is 244℃, and the cavity zone temperature is 230℃, with a temperature gradient of 1.5℃ / cm. The ejector zone temperature is 224℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is H13, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 28μm.

[0121] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0122] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0123] The first stage of slow filling process: the injection speed is 2.8m / s, filling to 48% of the cavity volume;

[0124] The second stage is a medium-speed transition process: the injection speed is increased to 3.2 m / s, and the cavity volume is filled to 88%;

[0125] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 90MPa, hold the pressure for 18s, and fill to 100% of the cavity volume;

[0126] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 18μm are applied for 4.5s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0127] S5. After slowly cooling the magnesium alloy component to below 150°C in the temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 395°C and hold for 9 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 195°C and hold for 18 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0128] Comparative Example 3

[0129] The magnesium alloy used in Example 3 is the same, except that:

[0130] This comparative example uses traditional die casting and conventional heat treatment technology. The pouring temperature is 670℃ and the mold temperature is 180℃. No ultrasonic or gradient temperature control treatment is performed during this process. The post-treatment is only T6 aging treatment, in which the furnace is placed in an aging furnace preheated to 200℃ and held for 20 hours.

[0131] Example 4

[0132] A 1550 cm² projected area was prepared using WE43 magnesium alloy. 2 Take, for example, an aerospace engine bracket with a maximum wall thickness of 6mm.

[0133] S1: Crush WE43 magnesium alloy into magnesium alloy particles with a particle size of 4-6 mm, and place the WE43 magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0134] S2: WE43 magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 580℃, and electromagnetic stirring at a frequency of 30Hz and an intensity of 0.1T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 40% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 150s to obtain a semi-solid slurry with an average size of ≤50μm of primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0135] S3: Preheat the zoned temperature-controlled mold to 230℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating-cooling channels. The zoned temperature-controlled mold is divided into a gate area, a runner cone area, a cavity area, and an ejector area. The gate area temperature is 250℃, the runner cone area temperature is 244℃, and the cavity area temperature is 230℃, with a temperature gradient of 2.2℃ / cm. The ejector area temperature is 224℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is a DH360, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 20μm.

[0136] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0137] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0138] The first stage of slow filling process: the injection speed is 2.0 m / s, filling to 48% of the cavity volume;

[0139] The second stage is a medium-speed transition process: the injection speed is increased to 3.6 m / s, and the cavity volume is filled to 92%;

[0140] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 95MPa, hold the pressure for 18s, and fill to 100% of the cavity volume;

[0141] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 18μm are applied for 3.8s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0142] S5. After slowly cooling the magnesium alloy component to below 150°C in the temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 410°C and hold for 10 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 190°C and hold for 20 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0143] Comparative Example 4

[0144] The magnesium alloy used in Example 4 is the same, except that:

[0145] This comparative example uses traditional semi-solid injection molding, without ultrasonic and gradient temperature control treatment. The mold is uniformly preheated to 230°C, and other process parameters are the same as in Example 4.

[0146] Example 5

[0147] A 1900 cm² projected area was prepared using AZ31 magnesium alloy. 2 Take, for example, an aerospace engine bracket with a maximum wall thickness of 8mm.

[0148] S1: Crush AZ31 magnesium alloy into magnesium alloy particles with a particle size of 3-7mm, and place the AZ31 magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0149] S2: AZ31 magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 600℃, and electromagnetic stirring at a frequency of 20Hz and an intensity of 0.1T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 42% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 160s to obtain a semi-solid slurry with an average size of ≤55μm of primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0150] S3: Preheat the zoned temperature-controlled mold to 250℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating-cooling channels. The zoned temperature-controlled mold is divided into a gate zone, a runner cone zone, a cavity zone, and an ejector zone. The gate zone temperature is 250℃, the runner cone zone temperature is 244℃, and the cavity zone temperature is 230℃, with a temperature gradient of 1.8℃ / cm. The ejector zone temperature is 224℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is H13, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 30μm.

[0151] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0152] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0153] The first stage of slow filling process: the injection speed is 1.8m / s, filling to 45% of the cavity volume;

[0154] The second stage is a medium-speed transition process: the injection speed is increased to 3.4 m / s, and the cavity volume is filled to 90%;

[0155] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 105MPa, hold the pressure for 22s, and fill the cavity to 100% of its volume;

[0156] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 20μm are applied for 4s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0157] S5. After slowly cooling the magnesium alloy component to below 150°C in the temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 395°C and hold for 11 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 195°C and hold for 21 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0158] Comparative Example 5

[0159] The magnesium alloy used in Example 5 is the same, except that:

[0160] This comparative example uses the traditional liquid die casting process, in which the pouring temperature is 690℃, the mold temperature is 190℃, and no ultrasonic or gradient temperature control treatment is performed. The post-treatment is only T6 aging treatment, in which the aging treatment temperature is placed in an aging furnace preheated to 200℃ and held for 20 hours.

[0161] Example 6

[0162] A projected area of ​​1750 cm² was prepared using ZK61 magnesium alloy. 2 Take, for example, a rail transit seat frame with a maximum wall thickness of 7mm.

[0163] S1: Crush ZK61 magnesium alloy into magnesium alloy particles with a particle size of 5-8mm, and place the ZK61 magnesium alloy particles in a vacuum drying oven at 120℃ for 2 hours.

[0164] S2: ZK61 magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The temperature of the preparation device is set to 620℃, and electromagnetic stirring at a frequency of 20Hz and an intensity of 0.2T is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are cooled to the target range of 50% solid phase at a cooling rate of 2℃ / s. Stirring is continued for 140s to obtain a semi-solid slurry with an average size of ≤58μm of primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.7.

[0165] S3: Preheat the zoned temperature-controlled mold to 245℃. Place the semi-solid slurry into the zoned temperature-controlled mold, which has multiple independent heating and cooling channels. The zoned temperature-controlled mold is divided into a gate area, a runner cone area, a cavity area, and an ejector area. The gate area temperature is 250℃, the runner cone area temperature is 244℃, and the cavity area temperature is 230℃, with a temperature gradient of 2.5℃ / cm. The ejector area temperature is 224℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the zoned temperature-controlled mold to provide real-time temperature field feedback. Based on the temperature field feedback and combined with the preset temperature gradient curve, the heating power of each temperature zone of the zoned temperature-controlled mold is dynamically adjusted by a PID controller to control the local cooling rate at 10℃ / s. The zoned temperature-controlled mold is a DH360, and the thickness of the nano-ceramic coating on the working surface of the zoned temperature-controlled mold is 24μm.

[0166] S4: The semi-solid slurry is injected and transferred to a zoned temperature-controlled mold in three stages for filling, and the holding time in the zoned temperature-controlled mold is ≤90s. Finally, magnesium alloy components are obtained by injection molding.

[0167] The three-stage injection process takes ≤12 seconds. The specific steps for the three-stage injection process are as follows:

[0168] The first stage of slow filling process: the injection speed is 2.3m / s, filling to 50% of the cavity volume;

[0169] The second stage is a medium-speed transition process: the injection speed is increased to 3.9 m / s, and the cavity volume is filled to 92%;

[0170] The third stage is the low-pressure compaction process: switch to pressurization mode, increase the pressure to 115MPa, hold the pressure for 24s, and fill to 100% of the cavity volume;

[0171] In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 24μm are applied for 3.2s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity.

[0172] S5. After slowly cooling the magnesium alloy component to below 150°C in the zoned temperature-controlled mold, open the mold and immediately perform T4 solution treatment after demolding. Specifically, heat the magnesium alloy component to a solution temperature of 405°C and hold for 9 hours. Then, quickly immerse the magnesium alloy component after T4 solution treatment in cold water and then perform T6 aging treatment. Specifically, place the magnesium alloy component after T4 solution treatment in an aging furnace preheated to 205°C and hold for 19 hours. Then, allow the magnesium alloy component after T6 aging treatment to cool naturally in air.

[0173] Comparative Example 6

[0174] The magnesium alloy used in Example 6 is the same, except that:

[0175] This comparative example uses traditional semi-solid injection molding, only electromagnetic stirring is used, and ultrasonic and gradient temperature control treatments are not performed. The mold is uniformly preheated to 240°C, and other parameters are the same as in Example 6.

[0176] The magnesium alloy components prepared in Examples 1-6 and Comparative Examples 1-6 were measured using a metallographic microscope combined with image analysis software and tested for mechanical properties using a universal testing machine. The results are shown in Table 1.

[0177] Table 1. Test results of mechanical properties of magnesium alloy components prepared in Examples 1-6 and Comparative Examples 1-6

[0178]

[0179] As can be seen from Table 1, the data of all embodiments meet the requirements of porosity ≤1.5%, tensile strength ≥260MPa, elongation ≥5%, average particle size ≤60μm, and equiaxed crystal ratio ≥85% for the formed workpiece, indicating that each magnesium alloy component synthesized under the preparation process of the present invention has excellent mechanical properties.

[0180] The results from Example 1 and Comparative Example 1 show that in Comparative Example 1, AZ91D magnesium alloy was liquefied at high temperature, and the liquefied semi-solid slurry was directly injected into a mold. This caused turbulence in the AZ91D magnesium alloy within the mold, resulting in the outer layer solidifying first while the inner layer could not be fed back in time. This led to coarse internal dendrites, resulting in coarse grains, numerous defects, and poor performance. In Example 1, electromagnetic stirring caused the primary α-Mg phase particles to become spherical and refined the grains. The semi-solid slurry effectively suppressed dendrites. Setting a temperature gradient promoted sequential solidification, and the final pressurization and ultrasonic synergistic venting and compaction caused the refined grains to recrystallize, transforming the microstructure from coarse columnar crystals to fine equiaxed crystals, thereby further improving the performance of the AZ91D magnesium alloy.

[0181] The results from Example 2 and Comparative Example 2 show that although Comparative Example 2 was a semi-solid, the mold had only one temperature and was not subjected to ultrasonic treatment. As a result, some areas formed a crust that blocked the "shrinkage compensation channel," trapping gas inside and forming pores and localized coarse grains. Example 2, by using gradient temperature control to avoid early crust formation at the leading edge and maintain the shrinkage compensation channel, combined with ultrasonic treatment to promote dynamic recrystallization and gas escape, ultimately significantly improved strength and elongation, and eliminated shrinkage cracks.

[0182] The results from Example 3 and Comparative Example 3 show that Comparative Example 3 experienced severe air entrapment and shrinkage during liquid filling, coupled with a low mold temperature. This resulted in the outside freezing first, while the inside remained stagnant. Consequently, the coarse columnar crystals and high porosity generated by liquid solidification could not be improved by a single aging treatment. Example 3 used a semi-solid slurry to first suppress the "shrinkage and turbulence" at the source. Then, gradient temperature control was used to directionalize the solidification, and final pressure combined with ultrasound compacted or dislodged the pores and promoted recrystallization and refinement. Finally, a T4 solution treatment was used to dissolve the segregation and coarse phases, followed by a T6 aging treatment for strengthening. This achieved a synergistic improvement in strength and toughness, and resulted in a uniform microstructure without agglomeration.

[0183] The results from Example 4 and Comparative Example 4 show that the process parameters of Comparative Example 4 and Example 4 are almost the same. However, Comparative Example 4 lacks gradient temperature control and ultrasonic treatment, resulting in uneven internal temperature. This causes localized cooling areas to agglomerate first, resulting in coarser solidification with segregation, and making it easier for gas to be trapped. Example 4 uses zoned temperature control to achieve temperature uniformity and creates a gradient that decreases from the gate to the far end, allowing it to solidify sequentially. In the final stage, ultrasonic treatment is used to remove air bubbles and refine the grains, combined with pressurization to further refine the grains and homogenize the composition, resulting in a microstructure with ultra-high strength and low defects.

[0184] The results from Example 5 and Comparative Example 5 show that Comparative Example 5 uses liquid die casting without gradient temperature control or ultrasonic treatment, and only uses T6 aging treatment. High-temperature semi-solid slurry is poured at high speed into a temperature-controlled mold, causing severe shrinkage and air entrapment, resulting in numerous pores and coarse grains. Example 5 first prepares a semi-solid slurry, which has more stable flow and is less prone to air entrapment. Gradient temperature control is then used to compensate for shrinkage. Final pressurization and ultrasonic treatment compact the pores and refine the microstructure. Finally, a combination of T4 solid-liquid treatment and T6 aging treatment is used to adjust the phase and strength, resulting in a magnesium alloy component with fine grains, uniform microstructure, low porosity, and high elongation.

[0185] The results from Example 6 and Comparative Example 6 show that Comparative Example 6 only uses electromagnetic stirring to prepare a semi-solid slurry, but the mold lacks gradient temperature control and ultrasonic assistance. This results in uneven local cooling, leading to insufficient control over grain size and porosity. Example 6, based on the same semi-solid material, adds stronger gradient temperature control to ensure the solidification sequence and pressure are transmitted to the end. Then, during the final pressure holding stage, ultrasound is used to remove air bubbles and promote grain refinement, thus improving both strength and elongation.

[0186] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components, characterized in that, Includes the following steps: S1. Material pretreatment: Large magnesium alloy ingots are crushed and dried to obtain magnesium alloy particles; the large magnesium alloy ingots have a projected area ≥1500 cm². 2 Large components with a wall thickness of ≥3mm; S2. Preparation of semi-solid slurry: The magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device to prepare a semi-solid slurry; the solid fraction of the semi-solid slurry is 40-60%, and the accuracy deviation of the solid fraction is ≤±3%; S3. Mold System Optimization and Thermal Field Control: The semi-solid slurry is placed in a zoned temperature-controlled mold, and the zoned temperature-controlled mold is preheated to 220-260℃, with dynamic gradient temperature control during filling; specifically, the semi-solid slurry is placed in a zoned temperature-controlled mold with multiple independent heating-cooling channels. The zoned temperature-controlled mold is divided into a gate area, a runner cone area, a cavity area, and an ejector area. The gate area temperature is 250℃; the runner cone area temperature is 240-245℃; the cavity area temperature is 230℃, with a temperature difference gradient of 1.5-2.5℃ / cm; and the ejector area temperature is 220-225℃. Infrared temperature probes and multi-point thermocouple arrays are embedded in key areas such as the thick cross-section and corners of the partitioned temperature control mold to provide real-time feedback of the temperature field. Based on the feedback of the temperature field and combined with the preset temperature gradient curve, the heating power of each temperature zone of the partitioned temperature control mold is dynamically adjusted by a PID controller to control the local cooling rate at 5 to 15°C / s. The control logic of the PID controller is as follows: A three-dimensional temperature field model of the cavity of the temperature-controlled mold along the filling path is established. The deviation between the actual temperature and the target temperature in each region is identified. Based on the deviation, the heating power or cooling medium flow rate of each temperature zone is dynamically adjusted to ensure that the gate area is maintained at 250°C and the temperature of the far end of the cavity area is gradually reduced to 230°C, forming a directional solidification temperature gradient from the inside to the outside. S4. Injection Molding: The semi-solid slurry is injected into a zoned temperature-controlled mold in three stages. An ultrasonic vibration auxiliary device is introduced in the third stage of injection to finally obtain a magnesium alloy component. Specifically, the semi-solid slurry is injected into a zoned temperature-controlled mold in three stages for filling. The holding time in the zoned temperature-controlled mold is ≤90s, and the magnesium alloy component is finally obtained by injection molding. The three-stage injection process takes ≤12 seconds, and the specific operation of the three-stage injection process is as follows: The first stage is a slow filling process: the injection speed is 1.5 to 2.5 m / s, filling to 40 to 50% of the cavity volume; The second stage is a medium-speed transition process: the injection speed is increased to 3.0-4.0 m / s, and the cavity volume is filled to 90%; The third stage is the low-pressure compaction process: switch to the pressurization mode, increase the pressure to 80-120MPa, hold the pressure for 15-25s, and fill the cavity to 100% of its volume; In the third stage of low-pressure compaction, an ultrasonic vibration auxiliary device is introduced, and ultrasonic waves with a frequency of 20kHz and an amplitude of 15-25μm are applied for a duration of 3-5s. The ultrasonic probe in the ultrasonic vibration auxiliary device is placed at the bottom and side wall of the temperature-controlled mold cavity. S5. Post-treatment: The magnesium alloy component is subjected to T4 solution treatment and T6 aging treatment in sequence. Specifically, the magnesium alloy component is slowly cooled to below 150°C in a temperature-controlled mold, and then the mold is opened. Immediately after demolding, the T4 solution treatment is performed. Specifically, the magnesium alloy component is heated to a solution temperature of 400±10°C and held for 8-12 hours. Then, the magnesium alloy component after the T4 solution treatment is quickly immersed in cold water to cool. Subsequently, the T6 aging treatment is performed. Specifically, the magnesium alloy component after the T4 solution treatment is placed in an aging furnace preheated to 200±10°C and held for 16-24 hours. Then, the magnesium alloy component after the T6 aging treatment is naturally cooled in air.

2. The method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components according to claim 1, characterized in that, The specific process of crushing and drying large magnesium alloy ingots to obtain magnesium alloy particles in step S1 material pretreatment includes: Large magnesium alloy ingots of high purity AZ series or ZK series are selected, and the large magnesium alloy ingots are crushed into magnesium alloy particles with a particle size of 3-8 mm. The magnesium alloy particles are placed in a vacuum drying oven at 120°C and dried for 2 hours. The content of Fe, Ni and Cu impurity elements in the large magnesium alloy ingots of high purity AZ series or ZK series is less than 0.005 wt%. The large magnesium alloys of AZ series or ZK series include any one of AZ91D, ZK60, AM60, WE43, AZ31 and ZK61 magnesium alloys.

3. The method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components according to claim 1, characterized in that, The specific process of preparing the semi-solid slurry in step S2, which involves feeding the magnesium alloy particles into an electromagnetic stirring-cooling composite slurry preparation device, includes: The magnesium alloy particles are fed into an electromagnetic stirring-cooling composite slurry preparation device. The device is heated and electromagnetic stirring is performed to bring the magnesium alloy particles to the solid-liquid two-phase region. The particles are then cooled to the target solid fraction range and stirred continuously for 120–180 s to obtain a semi-solid slurry with an average size of ≤60 μm for the primary α-Mg solid phase particles. The primary α-Mg solid phase particles are close to spherical and have a shape factor ≥0.

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4. The method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components according to claim 3, characterized in that, The temperature of the preparation apparatus was set to 550–650℃, the frequency of the electromagnetic stirring was 20–40Hz, the intensity of the electromagnetic stirring was 0.1–0.3T, and the cooling rate was 1–3℃ / s.

5. The method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components according to claim 1, characterized in that, The partition temperature control mold is either H13 or DH360, and the working surface of the partition temperature control mold is treated with a nano-ceramic coating, wherein the thickness of the nano-ceramic coating is 20-30μm.

6. A method for controlling the microstructure of semi-solid injection molding of large magnesium alloy components according to any one of claims 1 to 5, characterized in that, The magnesium alloy component has a porosity of 0.3–0.45%, a tensile strength of 270–310 MPa, an equiaxed crystal ratio of 84–90%, and an elongation of 9.8–12%.

7. The application of the microstructure control method for semi-solid injection molding of large magnesium alloy components according to any one of claims 1 to 6 in the fields of aerospace, rail transportation, and high-end equipment manufacturing.

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