Magnesium alloy preparation process achieved through low-temperature rapid solidification

Abstract

The invention belongs to the technical field of metal material processing and preparation, and particularly relates to a magnesium alloy preparation process achieved through low-temperature rapid solidification. The technology comprises a special preparation system, and the special preparation system comprises a melt purification and supercooling preparation unit, a high-cleanliness high-speed injection unit, an integrated surface induced nucleation mold unit and a full-process closed-loop precision control unit. The preparation method specifically comprises the preparation steps of charging and smelting, refining and purifying, filtering and transferring, supercooled-state preparation, high-speed injection and induced solidification, and original heat treatment and demolding. By means of the method, the contradiction between slow solidification and rapid solidification requirements in traditional casting is solved, instant overall solidification of the magnesium alloy can be achieved while the macroscopic forming precision is guaranteed, the superfine structure with the average grain size smaller than 5 micrometers is obtained, component segregation is remarkably inhibited, and the comprehensive mechanical property is improved.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing and preparation technology, specifically relating to a magnesium alloy preparation process achieved through low-temperature rapid solidification. Background Technology

[0002] In the existing magnesium alloy preparation technology system, the forming process based on melt casting has become dominant. Specifically, the traditional process route usually involves heating a pre-prepared magnesium alloy ingot to a temperature far above its liquidus line under a protective atmosphere, so that it is completely melted and a high-temperature melt with good fluidity is obtained. Then, the melt is injected into a pre-set mold cavity by means of die casting, gravity casting or sand casting, and finally solidified through a relatively slow cooling process to obtain a casting of the desired shape.

[0003] However, traditional casting processes require high pouring temperatures to ensure sufficient fluidity of the melt to fill complex mold cavities. Furthermore, to avoid macroscopic defects such as shrinkage cavities and cracks caused by rapid cooling, the solidification rate is typically strictly controlled, resulting in a generally slow cooling process. While this thermodynamic and kinetic approach of high-temperature melting and slow solidification ensures the macroscopic quality of the casting, it inevitably negatively impacts the material's microstructure. During slow solidification, alloying elements have ample time for long-range diffusion, leading to grain engulfment during growth and ultimately the formation of coarse dendritic or equiaxed crystal structures.

[0004] Therefore, how to effectively overcome the inherent constraints between solidification rate and microstructure control in traditional casting processes, and achieve significant grain refinement, suppress macroscopic component segregation, and optimize the distribution morphology of the second phase through precise control of the solidification process while ensuring good molding quality, thereby comprehensively improving the overall mechanical properties of magnesium alloy materials, has become a key challenge and an urgent technical problem for those skilled in the art. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a magnesium alloy preparation process achieved by low-temperature rapid solidification. This process aims to resolve the fundamental technical contradiction between the slow solidification used in traditional casting methods to ensure macroscopic forming quality and the rapid solidification required to obtain excellent microstructure and mechanical properties.

[0006] To achieve the above-mentioned objectives, the process of this invention includes a dedicated preparation system. This dedicated preparation system comprises a melt purification and subcooling preparation unit, a high-purity high-speed injection unit, an integrated surface-induced nucleation mold unit, and a closed-loop precision control unit. The operation steps in the dedicated preparation system are as follows:

[0007] In the melt purification and undercooling preparation unit, a metastable magnesium alloy liquid phase without non-spontaneous nucleation cores and with a preset undercooling degree is prepared under the coordinated scheduling of the whole process closed-loop precision control unit.

[0008] The metastable magnesium alloy liquid phase is stably filled into the mold cavity of the integrated surface-induced nucleation mold unit through a high-cleanliness high-speed injection unit. At the moment the metastable magnesium alloy liquid phase is filled, the nucleation particles pre-fixed on the surface of the mold cavity synchronously and on a large scale trigger heterogeneous nucleation events, guiding the metastable magnesium alloy liquid phase to undergo explosive volume solidification. Through the precise cooperation of the whole-process closed-loop precision control unit, magnesium alloy parts with ultra-fine microstructure are obtained.

[0009] Furthermore, the melt purification and undercooling preparation unit is configured as a dual-chamber vacuum induction melting furnace, including a melting and refining chamber, a filtration channel, and an undercooling and constant temperature chamber. The steps for preparing the metastable magnesium alloy liquid phase include:

[0010] In the melting and refining chamber, the magnesium alloy raw materials are melted and refined to obtain a clean magnesium alloy melt.

[0011] The clean magnesium alloy melt is transferred through a filtration channel to a supercooling and constant temperature room;

[0012] Inside the supercooling and constant temperature chamber, the clean magnesium alloy melt is subjected to controlled rapid cooling through a precise temperature control system until the preset supercooling target temperature is reached, and this temperature is precisely maintained constant, thereby preparing the metastable magnesium alloy liquid phase.

[0013] Furthermore, within the melting and refining chamber, the magnesium alloy raw materials undergo melting and refining processes to obtain a clean magnesium alloy melt, specifically including:

[0014] Magnesium alloy ingots are loaded into the first crucible, and the smelting and refining chamber is evacuated to 10°C. -3 After the vacuum state of Pa is reached, the medium frequency induction heating coil is started to heat the magnesium alloy ingot to 710°C and hold it at that temperature.

[0015] Subsequently, a protective atmosphere consisting of 0.4-0.6% SF6 and the balance N2 is introduced into the melting and refining chamber to a positive pressure of 0.05 MPa, and a fluorine-free refining agent accounting for 0.7-0.9% of the melt weight is added to the melt.

[0016] Start the mechanical agitator and stir at a speed of 70-90 r / min for 10-20 min, then let it stand for 15-25 min to obtain a clean magnesium alloy melt.

[0017] Furthermore, the specific steps for transferring the clean magnesium alloy melt through a filtration channel to the subcooling and constant temperature chamber are as follows:

[0018] Argon gas is added to the melting and refining chamber to create a pressure difference of 0.01 to 0.03 MPa between it and the supercooling and constant temperature chamber. The clean magnesium alloy melt is then slowly and smoothly transferred through the filtration channel to the second crucible in the supercooling and constant temperature chamber.

[0019] Furthermore, the precise temperature control process performed within the supercooled and constant temperature chamber specifically includes:

[0020] The clean magnesium alloy melt is contained in a second crucible;

[0021] The high-frequency induction heating coils and liquid nitrogen cryogenic circulation system surrounding the second crucible are coordinated and scheduled by the full-process closed-loop precision control unit to rapidly cool the melt from 650-750°C to the preset supercooling target temperature of 550-600°C at a cooling rate of more than 15°C / min.

[0022] Once the target temperature is reached, the temperature fluctuation range is precisely maintained within ±0.5℃ and kept stable for 30 seconds.

[0023] Furthermore, the specific steps for stable filling using the high-purity, high-speed injection unit are as follows:

[0024] A vertical injection structure is adopted to allow the metastable magnesium alloy liquid phase to flow from the second crucible into the injection chamber of the injection unit;

[0025] The inner wall of the injection chamber is coated with a 5μm thick amorphous diamond-like carbon film, and its inner wall temperature is preheated by a heater and precisely maintained within a range that does not differ from the supercooled target temperature by more than 1°C.

[0026] Furthermore, the filling step is performed by an all-electric servo drive system driving the injection plunger; the filling process follows a preset three-stage speed curve, including:

[0027] In the first stage, the melt is smoothly pushed to the inner gate of the mold at a slow injection speed of 0.5 m / s.

[0028] In the second stage, the supercooled melt is completely filled into the mold cavity within 15 to 20 ms at a high injection speed of 10 m / s.

[0029] In the third stage, the pressure boosting stage is switched at the moment the mold cavity is filled, and a final specific pressure of 150 MPa is applied for compaction and feeding.

[0030] Furthermore, the inner surface of the mold cavity is a multi-layered composite surface that has undergone special treatment. Its structure, from the inside out, consists of a functional induced nucleation layer, a titanium nitride bonding bottom layer, and an H13 hot work die steel substrate.

[0031] Furthermore, the functional induced nucleation layer is an amorphous-nanocrystalline composite structure layer with a thickness of 300 nm, specifically composed of:

[0032] Hexagonal boron nitride nanocrystals with an average particle size of 40 nm are used as nucleation particles, and the crystal planes of the hexagonal boron nitride nanocrystals have a lattice mismatch of less than 2% with the basal planes of α-Mg.

[0033] An amorphous diamond-like carbon matrix is ​​used as a carrier to uniformly and diffusely immobilize the hexagonal boron nitride nanocrystals therein;

[0034] The volume percentage of the hexagonal boron nitride nanocrystals in the composite structure layer is precisely controlled at 25%.

[0035] Furthermore, after the explosive volume solidification is completed and the final feeding is performed, an in-situ heat treatment step is also included, which specifically includes:

[0036] Before mold opening, the induction heating coil, which is embedded in the mold unit and close to the surface of the cavity, is activated by the full-process closed-loop precision control unit.

[0037] Within 50 to 70 seconds, the temperature of the solidified magnesium alloy part is rapidly and uniformly raised to 180°C and held at this temperature for 120 seconds. After the heat treatment is completed, the high-pressure cooling water circuit is turned on, and the mold temperature is rapidly reduced to 120°C within 45 seconds. Then the mold is opened, and the final magnesium alloy part is ejected by the ejection mechanism.

[0038] The beneficial effects of this invention are as follows:

[0039] The solidification process of this invention is completed instantaneously under the combined effects of extreme supercooling and a massive number of pre-set nucleation sites. Its solidification kinetics are completely different from the traditional layer-by-layer solidification mode that relies on temperature gradients. This explosive volumetric solidification causes grains to collide with each other before they can grow, resulting in an ultrafine equiaxed grain structure with an average grain size of less than 5 μm. At the same time, the extremely fast solidification rate does not allow solute atoms enough time for long-range diffusion and macroscopic segregation. Most alloying elements are uniformly dissolved in the magnesium matrix, forming a highly supersaturated solid solution. Even if a small amount of second phase precipitates, it is as fine, diffusely distributed particles with a size of less than 100 nm due to the extremely short solidification time, rather than the coarse, continuously distributed, brittle phase in traditional processes. Therefore, the magnesium alloy components prepared by this invention have a fundamentally optimized microstructure, resulting in a simultaneous and significant improvement in yield strength, tensile strength, and elongation, completely breaking through the inherent contradiction between mechanical properties and forming quality in traditional casting processes. Attached Figure Description

[0040] 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.

[0041] Figure 1 This is an overall functional block diagram of the magnesium alloy preparation system described in this invention.

[0042] Figure 2 This is the control logic block diagram of the full-process closed-loop precision control unit described in this invention.

[0043] Figure 3 This is a schematic flowchart of the magnesium alloy preparation process described in this invention. Detailed Implementation

[0044] 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.

[0045] This invention provides a detailed implementation of a magnesium alloy preparation process achieved through low-temperature rapid solidification. The core of this process lies in creating and maintaining a deeply supercooled metastable state for the magnesium alloy melt, free of external nucleation sites, using a specially designed and precisely coordinated preparation system. This metastable melt then triggers explosive interfacial nucleation upon contact with a specially designed mold cavity surface, achieving instantaneous and integral solidification of the entire casting. This novel solidification path fundamentally avoids the inherent contradiction between cooling rate and macroscopic forming quality in traditional casting processes, thereby enabling the preparation of complex precision components while simultaneously achieving ultra-fine microstructure and excellent comprehensive mechanical properties.

[0046] Reference Figure 1 The preparation system upon which this invention is based is mainly divided into four interconnected units: a melt purification and supercooling preparation unit, a high-purity high-speed injection unit, an integrated surface-induced nucleation mold unit, and a full-process closed-loop precision control unit. These four units are closely connected in physical space and are precisely scheduled at the millisecond level by the control unit in terms of process timing, together forming a complete and efficient technical implementation platform.

[0047] Furthermore, the structure and function of the melt purification and undercooling preparation unit are designed to achieve two key objectives: first, to purify the magnesium alloy melt to an extremely high degree of cleanliness through chemical and physical methods, eliminating all potential non-spontaneous nucleation particles; second, to controllably and rapidly cool the purified melt in an absolutely clean environment to achieve and maintain a preset degree of undercooling. To achieve this, the melt purification and undercooling preparation unit is constructed as a dual-chamber vacuum induction melting furnace. The furnace body is welded from stainless steel, with polished inner and outer walls, and is equipped with a double-layer water-cooling jacket. Internally, it is divided into a first chamber and a second chamber by an isolation valve plate, namely the melting and refining chamber and the undercooling and constant-temperature chamber, respectively. The entire furnace body is connected by a composite vacuum system consisting of molecular pumps and mechanical pumps, capable of stably maintaining the internal vacuum at 10... -3 The Pa level provides a high level of environmental isolation for the smelting process.

[0048] Specifically, the melting and refining chamber is the starting point of the process. Its core component is the first crucible, made of high-purity, high-density isostatic graphite. The selection of the first crucible aims to minimize carbon contamination of the melt during melting. Surrounding the first crucible is a medium-frequency induction heating coil wound with copper tubing, operating at a frequency of 2.5 kHz and a rated power of 50 kW, providing rapid and uniform volumetric heating for the alloy ingots within the crucible. The top flange of the melting and refining chamber integrates multi-functional ports, connecting to the vacuum system, the inert gas protection system, and the automatic feeding device. The inert gas protection system can precisely refill the chamber with high-purity argon, or mix and fill it with a pre-defined protective atmosphere of SF6 and N2, and stabilize the chamber pressure at a positive pressure of 0.05 MPa using a precision mass flow controller. The interior also features a mechanical stirring paddle made of high-temperature resistant molybdenum alloy, which is designed as a three-bladed propeller and driven by a variable frequency motor located outside the furnace body through a magnetohydrodynamic sealed bearing. Its speed can be continuously adjusted within the range of 0 to 150 r / min.

[0049] After the melt undergoes preliminary melting and refining in the melting and refining chambers, it is transferred to the supercooling and constant temperature chamber through a filtration channel located between the two chambers. The filtration channel is filled with a foam ceramic filter made of sintered zirconia ceramic material, with a porosity of 85% and a pore size density of 30 ppi. This three-dimensional mesh filter can effectively intercept residual oxide inclusions larger than 50 μm, unreacted refining agent particles, and scum in the melt, ensuring that the melt entering the second chamber has extremely high physical purity.

[0050] The supercooling and constant temperature chamber is the key area for realizing the core technology of this invention, namely, the preparation of metastable supercooled melt. Inside the chamber is a second crucible made of pyrolytic boron nitride. The pyrolytic boron nitride is a high-purity ceramic material prepared by chemical vapor deposition, achieving a purity of up to 99.999%, and its deposited inner surface has atomic-level smoothness, with a measured surface roughness Ra < 0.1 μm. This extremely smooth and chemically inert inner wall surface, from both physical and chemical perspectives, minimizes the possibility of the crucible wall acting as a heterogeneous nucleation site, creating the prerequisite for deep supercooling of the melt. Outside the second crucible, a more precise temperature control system is arranged, comprising a set of high-frequency induction heating coils for rapid heating and a liquid nitrogen deep-cooling circulation system for rapid cooling. Liquid nitrogen circulates through a closed, serpentine stainless steel pipe wound around the outer wall of the crucible, with its flow rate precisely controlled by a high-speed electromagnetic proportional valve. The output power of the high-frequency heating coil and the opening of the liquid nitrogen circulation valve are both coordinated and adjusted by the full-process closed-loop precision control unit based on real-time temperature feedback. This enables the melt to be cooled at a rate greater than 20℃ / min, and after reaching the target supercooling temperature, the temperature fluctuation is precisely controlled within an extremely narrow range of ±0.2℃. To accurately monitor the temperature field distribution of the melt inside the crucible, a multi-point temperature measuring array consisting of three independent K-type sheathed thermocouples is also arranged inside the crucible to measure the temperature of the upper, middle, and lower regions of the melt.

[0051] The high-purity, high-speed injection unit functions to inject and fill the mold cavity with metastable supercooled melt prepared in a supercooled and constant-temperature chamber, without introducing any contamination or disrupting its supercooled state, at extremely high speed, pressure, and stability. The high-purity, high-speed injection unit employs a vertical injection structure, utilizing gravity to assist the melt filling the injection chamber, effectively reducing the air entrapment defects common in traditional horizontal structures. Its core component, the injection chamber, is made of H13 hot-work die steel that has undergone quenching and three high-temperature tempering treatments to ensure its dimensional stability and fatigue resistance under high temperature and pressure. Crucially, the inner wall of the injection chamber is uniformly coated with a 5μm thick amorphous diamond-like carbon film in a vacuum environment using physical vapor deposition technology. This amorphous diamond-like carbon film has an extremely low coefficient of friction and excellent chemical inertness, reducing the movement resistance of the injection plunger and effectively preventing the high-temperature magnesium alloy melt from reacting with the steel substrate, thus avoiding the introduction of new nucleation sites. To prevent the supercooled melt from solidifying prematurely due to temperature difference when it comes into contact with the injection chamber wall, an independent cylindrical resistance heater is wrapped around the outer wall of the injection chamber and equipped with a high-precision temperature sensor. This heater can preheat the inner wall temperature and precisely maintain it within a range that does not differ from the target supercooled melt temperature by more than 1°C.

[0052] The head of the injection plunger is made of alumina-reinforced copper. This material maintains the high thermal conductivity of copper while significantly improving its high-temperature strength and wear resistance through dispersed nano-sized alumina particles, enabling it to withstand the severe thermal shock and mechanical wear during high-pressure injection. The injection plunger is driven by a fully electric servo drive system, consisting of a high-power permanent magnet synchronous servo motor, a high-precision ball screw mechanism, and a high-performance servo driver. Compared to traditional hydraulic systems, the fully electric servo drive system enables fully digital closed-loop control of the injection process, ensuring accurate reproduction of complex injection curves and instantaneous establishment of end-fill pressure. The nozzle of the injection unit, serving as the channel connecting the injection unit and the mold unit, is also made of H13 steel, with a silicon nitride ceramic bushing embedded in its inner bore to enhance erosion and corrosion resistance. An independent micro-induction heating device is installed on the outside of the nozzle to ensure that the final stage of the melt flow is maintained at a preset supercooled temperature.

[0053] The integrated surface-induced nucleation mold unit is the final execution site for realizing the explosive solidification concept of this invention. It does not passively dissipate heat, but actively and on a large scale triggers heterogeneous nucleation the instant the supercooled melt contacts the cavity surface. To achieve this function, the inner surface of the mold unit's cavity undergoes a special multi-layer composite surface treatment. Specifically, on the H13 mold steel substrate, a 100nm thick titanium nitride layer is first deposited as a bonding underlayer using multi-arc ion plating technology at a deposition temperature of 350℃ and a bias voltage of -100V. The titanium nitride layer possesses excellent hardness and good adhesion to the steel substrate, providing a foundation for the firm adhesion of subsequent functional coatings. Subsequently, on top of the titanium nitride substrate, a 300 nm thick functional induced nucleation layer was deposited using unbalanced magnetron sputtering in a mixed atmosphere of Ar and N2 by co-sputtering graphite and hexagonal boron nitride targets. This functional layer is a carefully designed amorphous-nanocrystalline composite structure, consisting of hexagonal boron nitride nanocrystals with an average grain size of 40 nm, uniformly and diffusely distributed within an amorphous diamond-like carbon matrix. The (002) crystal plane of the hexagonal boron nitride nanocrystals and the (0001) basal plane of α-Mg are both hexagonal close-packed structures with minimal difference in lattice constants; the calculated lattice mismatch is less than 2%. According to classical heteronucleation theory, extremely low lattice mismatch implies extremely low solid / liquid interface energy, making it a highly efficient heteronucleation substrate for magnesium alloys. By precisely controlling the power ratio of the sputtering targets, the volume fraction of boron nitride nanocrystals in the amorphous carbon matrix was precisely controlled at 25%. This design, which pre- and uniformly immobilizes billions of highly efficient nucleation particles on the entire mold cavity surface at the nanoscale, is the key technology for achieving instantaneous integral solidification. When a metastable melt carrying enormous supercooling rushes across this surface at high speed, almost every hexagonal boron nitride nanocrystal in contact with the melt becomes an independent nucleation event trigger point, thereby initiating an avalanche-like nucleation reaction.

[0054] In addition, to facilitate the rapid removal of the large amount of latent heat of crystallization released during instantaneous solidification, the mold unit is also equipped with densely arranged high-pressure cooling water channels based on the geometry of the part, using conformal cooling technology. Simultaneously, within approximately 5mm of the cavity surface, multiple sets of flat, spiral induction heating coils are pre-embedded inside the mold. These coils are powered by independent medium-frequency power supplies and are used for rapid in-situ solid-state heat treatment of the part after solidification and before mold opening.

[0055] Finally, as Figure 2As shown, the closed-loop precision control unit is the central nervous system of the entire process system. Its hardware platform is built on a high-performance industrial programmable logic controller and equipped with high-speed I / O and communication modules. Its core software is a real-time control program developed using a sequential function chart language, supporting multi-task parallel processing. During process execution, the control unit collects and processes massive amounts of sensor signals from various subsystems in real time, including thermocouple temperature signals from the melting and refining chambers, thermocouple array signals from the subcooling and constant temperature chambers, grating ruler signals from the injection plunger position and current feedback signals from the drive motor, multi-point heat flow sensor signals from inside the mold, and environmental status signals from the vacuum gauge and mass spectrometer.

[0056] Based on this real-time feedback data, the control unit dynamically and precisely adjusts each actuator using advanced control algorithms. During the supercooled preparation stage, the control unit employs a composite algorithm combining model predictive control and PID feedback correction. Based on feedback from the thermocouple array, it precisely coordinates the output power of the high-frequency induction coil and the opening of the liquid nitrogen circulation valve, ensuring that the melt temperature strictly follows a preset temperature curve for rapid cooling, precise supercooling, and stable maintenance. During the high-speed injection stage, the control unit executes a preset multi-segment injection speed curve with sub-millisecond resolution based on the real-time position of the injection plunger fed back by the grating ruler. Furthermore, it can instantly switch to a high-pressure boosting stage at the end of cavity filling based on a sharp increase in the pressure sensor signal, ensuring sufficient compaction and feeding of the part. Every step of the process transition and every parameter execution is seamlessly integrated under the coordination of this control unit, ensuring high process stability and repeatability.

[0057] Example

[0058] This embodiment aims to illustrate the specific process of preparing a complex thin-walled magnesium alloy structural component using the process described in this invention, such as... Figure 3 As shown.

[0059] The alloy used is commercial AZ91D magnesium alloy, with the following chemical composition: Al 8.5-9.5%, Zn 0.45-0.9%, Mn 0.17-0.4%, Si ≤ 0.05%, Cu ≤ 0.025%, Ni ≤ 0.001%, Fe ≤ 0.004%, other impurities ≤ 0.02%, and the balance being Mg. Its liquidus temperature is approximately 600℃.

[0060] Step 1, Charging and Melting: 10 kg of dry, clean AZ91D magnesium alloy ingots are charged into the first crucible of the melting and refining chamber in the melt purification and supercooling preparation unit. The furnace door is closed, the vacuum system is started, and the vacuum level in the chamber is evacuated to 1.2 × 10⁻⁶ within 25 minutes. -3Pa, start the medium frequency induction heating coil, heat the ingot to 710℃ at a heating rate of 10℃ / min, and hold at this temperature for 30min to ensure that the alloy is completely melted and the composition is homogenized.

[0061] The second step, refining and purification: While maintaining a temperature of 710℃, turn off the vacuum pump and fill the melting and refining chamber with a mixed protective atmosphere consisting of 0.5% SF6 and 99.5% N2 until the chamber pressure reaches 0.05 MPa. Evenly sprinkle 0.8% (80g) of a special fluorine-free refining agent (by weight of the melt) into the melt through the automatic feeding port. Start the mechanical agitator and mechanically stir the melt at 80 rpm for 15 minutes to allow the refining agent to fully react with impurities such as magnesium oxide and hydrogen in the melt and aggregate into slag. After stirring, stop the agitator and let it stand for 20 minutes to allow the slag to rise fully and separate from the lower layer of clean melt.

[0062] The third step, filtration and transfer, involves adding argon gas to the first chamber to create a pressure difference of 0.02 MPa between it and the second chamber. The clean magnesium alloy melt located below the scum layer is then slowly and steadily pressed through a zirconia ceramic foam filter with a pore size of 30 ppi into the second crucible made of PBN material, which is in a supercooled and constant temperature chamber. The transfer process is carried out under an inert atmosphere to prevent secondary oxidation of the melt.

[0063] The fourth step is the supercooled state preparation. After the melt is completely transferred to the second crucible 1, the closed-loop precision control unit immediately initiates the precise temperature control program. By coordinating the power output of the high-frequency induction heating coil and the valve opening of the liquid nitrogen cryogenic circulation system, the melt is rapidly cooled from 700℃ to the preset supercooling target temperature of 575℃ at a linear cooling rate of 15℃ / min. This target temperature corresponds to a supercooling of 25℃. After reaching 575℃, the control system switches to isothermal mode, and by fine-tuning the heating power and cooling flow rate, the melt temperature is precisely maintained within the range of 575℃ ± 0.3℃ and held stably for 30 seconds to ensure that the melt temperature field within the entire crucible is uniform and consistent, and that it is in a metastable state awaiting solidification.

[0064] The fifth step is high-speed injection and induced solidification. At the moment the 30-second supercooling isothermal time ends, the control unit issues a synchronization command, and the stopper valve at the bottom of the second crucible opens. Under gravity and slight positive pressure, the supercooled melt flows into the injection chamber of the high-cleanliness high-speed injection unit, preheated to 575°C, within 800ms. The injection plunger immediately starts, injecting according to a preset three-stage speed curve: the first stage is a slow injection at 0.5 m / s with a stroke of 150 mm, smoothly pushing the melt to the ingate; the second stage, at a high speed of 10 m / s, completely fills the mold cavity with the supercooled melt within 18ms; the third stage is the pressurization stage. At the instant the cavity is filled, the servo system instantly switches to pressure control mode, increasing the final specific pressure to 150 MPa within 5ms. When a metastable liquid flow carrying a supercooling of 25°C impacts and spreads at extremely high speed on the surface of a mold cavity covered with a hexagonal boron nitride nanocrystal functional layer, the melt comes into contact with billions of highly efficient nucleation particles. The nucleation energy barrier is greatly reduced, resulting in a massive, avalanche-like heterogeneous nucleation event that erupts instantaneously across the entire contact interface. The enormous supercooling provides a powerful driving force for crystallization, and coupled with the mold's efficient thermal conductivity, the entire casting completes its transformation from liquid to solid state in approximately 35 ms.

[0065] Step 6: In-situ heat treatment and demolding. After maintaining a pressurized pressure of 150 MPa for 5 seconds to complete the final shrinkage compensation, the control unit activates the induction heating coil embedded in the mold, rapidly and uniformly raising the temperature of the part to 180°C within 60 seconds, and holding it at this temperature for 120 seconds to complete a rapid artificial aging treatment to promote β-Mg 17 Al 12 The strengthening phase is dispersed and precipitated. After heat treatment, the high-pressure cooling water circuit is fully opened, and the mold temperature is rapidly reduced to 120°C within 45 seconds. Then the mold is opened, and the final magnesium alloy part is ejected by the ejection mechanism.

[0066] The metallographic structure and mechanical properties of the parts obtained in the examples were observed and tested. Metallographic analysis showed that the microstructure of the parts consisted of extremely fine equiaxed crystals, with an average grain size of 4.2 μm measured by the truncation method. Scanning electron microscopy revealed β-Mg... 17 Al 12 The phase is dispersed in the grains and grain boundaries as fine particles with a size of less than 100 nm, completely avoiding the coarse, brittle phases that are distributed in a continuous network in traditional castings.

[0067] Comparative Example

[0068] For comparison, identical structural parts were manufactured using the traditional cold chamber high-pressure die casting process.

[0069] Using the same batch of AZ91D magnesium alloy ingots as in the example, the alloy was melted in a conventional crucible resistance furnace at a controlled melting temperature of 680°C. After slag removal, the melt was manually scooped and poured into the injection chamber of the die-casting machine. The injection chamber was not specially coated, the injection speed was set to 4 m / s, the pressurization pressure was 80 MPa, and the mold was a standard H13 steel mold with no special surface treatment. The mold temperature was controlled at 200°C. After die casting, the part underwent standard T6 heat treatment: solution treatment at 415°C in a protective atmosphere for 16 hours, followed by water quenching, and finally artificial aging at 170°C for 10 hours.

[0070] Metallographic observation and mechanical property testing were performed on the parts obtained in the comparative model. Metallographic analysis showed that the microstructure was typical of die-casting, with uneven grain size and an average grain size of approximately 25 μm. Numerous coarse, semi-continuous network-distributed eutectic β-Mg atoms were observed at the grain boundaries. 17 Al 12 Mutually.

[0071] Performance Comparison

[0072] The microstructure parameters and room temperature tensile mechanical properties of the parts obtained in the examples and comparative examples are compared, and the results are shown in Table 1:

[0073] Table 1

[0074]

[0075] As shown in Table 1, the magnesium alloy preparation process achieved through low-temperature rapid solidification provided by this invention significantly refines the microstructure of the resulting parts, with an average grain size only about one-sixth that of traditional die-casting processes. This ultra-fine microstructure and dispersed reinforcing phases result in a comprehensive and substantial improvement in the material's mechanical properties. Compared to traditional die-cast parts treated with standard T6 heat treatment, the parts produced in this embodiment exhibit a 30.0% increase in yield strength, a 29.2% increase in tensile strength, and a 261.3% increase in elongation, achieving simultaneous growth in strength and plasticity. This fully demonstrates the advanced nature and superiority of the technical solution of this invention.

[0076] 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 magnesium alloy preparation process achieved through low-temperature rapid solidification, characterized in that, This includes a dedicated preparation system, which comprises a melt purification and supercooling preparation unit, a high-purity high-speed injection unit, an integrated surface-induced nucleation mold unit, and a closed-loop precision control unit for the entire process. The operation steps in the dedicated preparation system are as follows: In the melt purification and undercooling preparation unit, a metastable magnesium alloy liquid phase without non-spontaneous nucleation cores and with a preset undercooling degree is prepared under the coordinated scheduling of the whole process closed-loop precision control unit. The metastable magnesium alloy liquid phase is stably filled into the mold cavity of the integrated surface-induced nucleation mold unit through a high-cleanliness high-speed injection unit. At the moment the metastable magnesium alloy liquid phase is filled, the nucleation particles pre-fixed on the surface of the mold cavity synchronously and on a large scale trigger heterogeneous nucleation events, guiding the metastable magnesium alloy liquid phase to undergo explosive volume solidification. Through the precise cooperation of the whole-process closed-loop precision control unit, magnesium alloy parts with ultra-fine microstructure are obtained.

2. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 1, characterized in that, The melt purification and undercooling preparation unit is configured as a dual-chamber vacuum induction melting furnace, including a melting and refining chamber, a filtration channel, and an undercooling and constant temperature chamber. The steps for preparing the metastable magnesium alloy liquid phase include: In the melting and refining chamber, the magnesium alloy raw materials are melted and refined to obtain a clean magnesium alloy melt. The clean magnesium alloy melt is transferred through a filtration channel to a supercooling and constant temperature room; Inside the supercooling and constant temperature chamber, the clean magnesium alloy melt is subjected to controlled rapid cooling through a precise temperature control system until the preset supercooling target temperature is reached, and this temperature is precisely maintained constant, thereby preparing the metastable magnesium alloy liquid phase.

3. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 2, characterized in that, In the melting and refining chamber, the magnesium alloy raw materials are melted and refined to obtain a clean magnesium alloy melt, specifically including: Magnesium alloy ingots are loaded into the first crucible, and the smelting and refining chamber is evacuated to 10°C. -3 After the vacuum state of Pa is reached, the medium frequency induction heating coil is started to heat the magnesium alloy ingot to 710°C and hold it at that temperature. Subsequently, a protective atmosphere consisting of 0.4-0.6% SF6 and the balance N2 is introduced into the melting and refining chamber to a positive pressure of 0.05 MPa, and a fluorine-free refining agent accounting for 0.7-0.9% of the melt weight is added to the melt. Start the mechanical agitator and stir at a speed of 70-90 r / min for 10-20 min, then let it stand for 15-25 min to obtain a clean magnesium alloy melt.

4. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 2, characterized in that, The specific steps for transferring the clean magnesium alloy melt through the filtration channel to the supercooling and constant temperature chamber are as follows: Argon gas is added to the melting and refining chamber to create a pressure difference of 0.01 to 0.03 MPa between it and the supercooling and constant temperature chamber. The clean magnesium alloy melt is then slowly and smoothly transferred through the filtration channel to the second crucible in the supercooling and constant temperature chamber.

5. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 2, characterized in that, The precise temperature control process performed within the supercooling and constant temperature chamber specifically includes: The clean magnesium alloy melt is contained in a second crucible; The high-frequency induction heating coils and liquid nitrogen cryogenic circulation system surrounding the second crucible are coordinated and scheduled by the full-process closed-loop precision control unit to rapidly cool the melt from 650-750°C to the preset supercooling target temperature of 550-600°C at a cooling rate of more than 15°C / min. Once the target temperature is reached, the temperature fluctuation range is precisely maintained within ±0.5℃ and kept stable for 30 seconds.

6. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 1, characterized in that, The specific steps for stable filling using the high-purity, high-speed injection unit are as follows: A vertical injection structure is adopted to allow the metastable magnesium alloy liquid phase to flow from the second crucible into the injection chamber of the injection unit; The inner wall of the injection chamber is coated with a 5μm thick amorphous diamond-like carbon film, and its inner wall temperature is preheated by a heater and precisely maintained within a range that does not differ from the supercooled target temperature by more than 1°C.

7. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 6, characterized in that, The filling step is performed by an injection plunger driven by a fully electric servo drive system; the filling process follows a preset three-segment speed curve, including: In the first stage, the melt is smoothly pushed to the inner gate of the mold at a slow injection speed of 0.5 m / s. In the second stage, the supercooled melt is completely filled into the mold cavity within 15 to 20 ms at a high injection speed of 10 m / s. In the third stage, the pressure boosting stage is switched at the moment the mold cavity is filled, and a final specific pressure of 150 MPa is applied for compaction and shrinkage.

8. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 7, characterized in that, The inner surface of the mold cavity is a multi-layered composite surface that has undergone special treatment. Its structure, from the inside out, consists of a functional induced nucleation layer, a titanium nitride bonding bottom layer, and an H13 hot work die steel substrate.

9. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 8, characterized in that, The functional induced nucleation layer is an amorphous-nanocrystalline composite structure layer with a thickness of 300 nm, and its specific composition is as follows: Hexagonal boron nitride nanocrystals with an average particle size of 40 nm are used as nucleation particles, and the crystal planes of the hexagonal boron nitride nanocrystals have a lattice mismatch of less than 2% with the basal planes of α-Mg. An amorphous diamond-like carbon matrix is ​​used as a carrier to uniformly and diffusely immobilize the hexagonal boron nitride nanocrystals therein; The volume percentage of the hexagonal boron nitride nanocrystals in the composite structure layer is precisely controlled at 25%.

10. The magnesium alloy preparation process achieved by low-temperature rapid solidification according to claim 1, characterized in that, After the explosive volume solidification is completed and the final feeding is performed, an in-situ heat treatment step is also included, which specifically includes: Before mold opening, the induction heating coil, which is embedded in the mold unit and close to the surface of the cavity, is activated by the full-process closed-loop precision control unit. Within 50 to 70 seconds, the temperature of the solidified magnesium alloy part is rapidly and uniformly raised to 180°C and held at this temperature for 120 seconds. After the heat treatment is completed, the high-pressure cooling water circuit is turned on, and the mold temperature is rapidly reduced to 120°C within 45 seconds. Then the mold is opened, and the final magnesium alloy part is ejected by the ejection mechanism.

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