High-toughness light-weight aluminum alloy die casting and preparation method thereof

Abstract

This invention discloses a high-strength, high-toughness, and lightweight aluminum alloy die-casting part and its preparation method, belonging to the technical field of pressure casting. The method includes: preparing a metastable aluminum alloy melt containing immiscible components such as bismuth and lead through compositional design; subsequently, injecting the melt into a mold, using high-frequency impedance and infrared sensing technology to monitor the solid fraction in real time and accurately determine the semi-solid critical window; within this window period, simultaneously applying ultrasonic mechanical oscillation and pulsed thermal extraction treatment through an intelligent hollow extrusion pin, utilizing the cavitation effect to break coarse bismuth-lead droplets to the nanoscale and freeze them in situ at the grain boundaries. The resulting die-cast part has a rigid framework phase that ensures static yield strength; the nanoscale energy-absorbing phase pinned to the grain boundaries undergoes instantaneous melting and energy absorption under the adiabatic heating generated by high-speed collisions, inducing grain boundary slip and achieving a nonlinear leap in impact toughness. This achieves a fundamental decoupling of strength and toughness in large, thin-walled structural parts under the premise of extreme lightweighting.

Description

Technical Field

[0001] This invention relates to the technical field of pressure casting, and more particularly to a high-strength, high-toughness, lightweight aluminum alloy die casting and its preparation method. Background Technology

[0002] With the rapid development of the new energy vehicle industry, lightweighting has become a core technological path to improve driving range. Aluminum alloys, due to their low density and excellent overall performance, are widely used in vehicle body structural components. Among them, integrated die-casting technology can integrate multiple parts into a complex thin-walled component, achieving significant weight reduction and cost advantages by reducing welding and riveting processes. The battery pack tray, as a key structural component of the new energy vehicle chassis system, bears the dual function of supporting the battery module and transferring the load to the vehicle body; its manufacturing precision and mechanical properties directly affect the safety of the entire vehicle.

[0003] Traditional die-cast aluminum alloys, such as A380 and ADC12, possess excellent casting fluidity and demolding properties, and are widely used in various die-cast parts. However, these alloys generally have low elongation and are prone to brittle fracture under impact loads. To meet the higher toughness requirements of structural components, heat-treatment-free high-strength and high-toughness aluminum alloys have emerged in recent years. By optimizing the proportions of elements such as silicon, magnesium, and manganese, and adding grain-refining elements such as titanium and zirconium, the material can achieve high strength and toughness in the die-cast state, avoiding the dimensional deformation problems caused by heat treatment.

[0004] In existing technologies, the fabrication of large, thin-walled die-cast parts such as battery trays typically employs a vacuum-assisted high-pressure die-casting process, combined with conformal cooling water channels and local extrusion pins within the mold to reduce porosity and shrinkage defects. However, these techniques still have shortcomings in practical applications. While conformal cooling water channels can improve the uniformity of the overall temperature field, significant differences in cooling rates still exist in geometrically abrupt regions such as thickness transition zones and stiffener intersections. This leads to localized thermal stress concentration, resulting in micro-deformation and residual stress, which affects the dimensional accuracy and long-term fatigue performance of the parts. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a high-strength, high-toughness, lightweight aluminum alloy die-casting part and its preparation method.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a high-strength, high-toughness, lightweight aluminum alloy die-casting part and its preparation method, comprising:

[0007] S1. Prepare a metastable aluminum alloy melt, which contains silicon, magnesium, manganese, iron, bismuth, lead, titanium, zirconium and the balance aluminum, wherein bismuth and lead are in a liquid-liquid immiscible state in the metastable aluminum alloy melt.

[0008] S2. The metastable aluminum alloy melt is injected into the preheated die-casting mold cavity. Multi-dimensional physical field characteristic data of a specific area in the die-casting mold cavity are collected in real time. Based on the multi-dimensional physical field characteristic data, the solid fraction of the alloy slurry in the specific area is determined, and the semi-solid critical window where the solid fraction is within the preset range is identified.

[0009] S3. During the duration of the semi-solid critical window, mechanical oscillation disturbance of ultrasonic frequency band is simultaneously applied to the alloy slurry in a specific area, and pulsed thermal extraction treatment with a preset phase difference from the mechanical oscillation disturbance is simultaneously applied to the specific area to induce cavitation effect to break up the coarse bismuth-lead droplets in the alloy slurry and freeze the bismuth-lead droplets at the grain boundary.

[0010] S4. Stop mechanical vibration and pulsed heat extraction treatment, hold pressure until the alloy slurry is completely solidified, and open the mold to obtain high-strength, tough, and lightweight aluminum alloy die castings.

[0011] In a preferred embodiment of the present invention, the composition of the metastable aluminum alloy melt by mass percentage is: silicon 7.5%~9.5%, magnesium 0.3%~0.6%, manganese 0.4%~0.8%, iron 0.6%~1.0%, bismuth 0.1%~0.5%, lead 0.05%~0.2%, titanium 0.05%~0.15%, zirconium 0.05%~0.12%, with the balance being aluminum and unavoidable impurities, and the melting temperature is controlled at 720℃-760℃.

[0012] In a preferred embodiment of the present invention, the specific area includes at least one of the longitudinal beam transition area, the root of the mounting lug, and the intersection of the reinforcing ribs.

[0013] In a preferred embodiment of the present invention, the step of determining the solid fraction in step S2 specifically includes: constructing a multi-dimensional feature fusion model, fusing the characteristic derivative of the high-frequency impedance signal acquired in real time with the theoretical solid fraction based on the non-equilibrium solidification equation, using the Kalman filter algorithm to correct the hysteresis effect of temperature measurement, and outputting the accurate solid fraction of the alloy slurry in a specific region in real time.

[0014] In a preferred embodiment of the present invention, in step S3, the mechanical oscillation disturbance is applied through an intelligent hollow extrusion pin system, wherein the intelligent hollow extrusion pin system integrates a micro-channel with a diameter of 0.1 mm to 1 mm, and the frequency of the mechanical oscillation disturbance is 100 Hz to 500 Hz, and the amplitude is 0.05 mm to 0.2 mm.

[0015] In a preferred embodiment of the present invention, the cooling medium used in the pulsed thermal extraction process is a nanofluid, which is composed of high thermal conductivity nanoparticles, low viscosity base liquid and surfactant; the high thermal conductivity nanoparticles are graphene, added at an amount of 0.5~2.0 vol%, dispersed in a 1:1 volume ratio of ethylene glycol and water mixed base liquid, and sodium dodecylbenzenesulfonate is added as a surfactant to achieve high-density thermal extraction under microchannel laminar flow conditions.

[0016] In a preferred embodiment of the present invention, the injection parameters in S2 are set as follows: injection speed 2 m / s~5 m / s, injection specific pressure 80 MPa~120 MPa; mold preheating temperature is set to 200~250℃; the specific area is the high stress area or energy absorption key node of the die casting, and a high-frequency impedance sensor and an infrared temperature sensor for collecting multi-dimensional physical field characteristic data are pre-embedded in the area.

[0017] In a preferred embodiment of the present invention, the mechanical oscillation disturbance and pulsed thermal extraction treatment applied in the ultrasonic frequency band in step S3 are performed by an intelligent hollow extrusion pin system. The intelligent hollow extrusion pin system integrates a micro-flow channel and is connected to a high-frequency servo drive motor and a piezoelectric ceramic actuator at its tail. During the semi-solid critical window period, the control system drives the extrusion pin to output a mechanical oscillation frequency of 100 Hz to 500 Hz and an amplitude of 0.05 mm to 0.2 mm, and injects a pulsed cooling medium through the micro-flow channel.

[0018] In a preferred embodiment of the present invention, the timing for stopping the mechanical oscillation disturbance in step S4 is as follows: when the multidimensional feature fusion model detects that the solid fraction exceeds 75%, the system switches to static pressure mode and applies a holding pressure of 80 MPa to 120 MPa until complete solidification; wherein the holding pressure is higher than the conventional feeding pressure to adapt to the local stress field generated by the dispersed nanodroplets at the grain boundaries and prevent micro-shrinkage.

[0019] This invention provides a high-strength, high-toughness, and lightweight aluminum alloy die-casting part. The die-casting part has a dual-modal microstructure, including a continuous rigid framework phase composed of an α-Al solid solution matrix and a magnesium silicide reinforcing phase, and a non-equilibrium nanoscale flexible energy-absorbing phase formed by the transformation of bismuth and lead. The nanoscale flexible energy-absorbing phase is gradient-distributed, with bismuth-lead droplets of characteristic size of 50 nm to 200 nm pinned at the grain boundaries. Under static conditions, it does not constitute a crack source, but under dynamic impact, it undergoes a phase transformation and absorbs energy.

[0020] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0021] (1) This invention introduces immiscible elements such as bismuth and lead into an aluminum-silicon-magnesium-manganese substrate and uses ultrasonic mechanical oscillation and pulsed thermal extraction for multi-field synergistic intervention, thereby realizing the structural reconstruction of immiscible components from macroscopic segregation to nanoscale dispersed phase. This transforms elements that were originally considered harmful impurities into flexible energy-absorbing phases with energy dissipation function. Under the premise of ensuring the basic strength of the material, it endows the casting with excellent dynamic impact resistance. Compared with traditional aluminum alloys that rely on single alloying or heat treatment for strengthening, it solves the intrinsic contradiction between strength and toughness of large thin-walled parts under complex stress conditions, and further improves the safety of new energy vehicle chassis parts in collision scenarios.

[0022] (2) This invention identifies the semi-solid critical window with solid fraction in a specific range by multi-dimensional data fusion of high-frequency impedance and infrared temperature sensor, and applies mechanical disturbance and chilling pulse with preset phase difference within this extremely narrow time window. It uses micro-jets generated by cavitation effect and instantaneous high pressure to overcome the interfacial tension of droplets, and realizes in-situ freezing and gradient distribution of low melting point phase at grain boundary. Compared with the open-loop control method of the prior art that only relies on fixed delay to start the extrusion pin, it ensures the precise matching of energy field intervention timing and slurry physical state, further eliminates micro-shrinkage in uneven thickness areas, and improves the structural compactness and fatigue life of the geometric change part of battery tray.

[0023] (3) By applying the intelligent hollow extrusion pin system to the collision energy absorption zone of the battery tray, the present invention utilizes the synergistic logic of the relaxation period of mechanical oscillation and the pulse cooling peak to construct a dual-modal structure composed of a rigid aluminum-based skeleton and nano-scale low-melting-point particles at the microscale. This enables the material to absorb the latent heat of phase change through instantaneous melting of particles and induce grain boundary slip under the adiabatic heating environment generated by high-speed impact. Compared with the brittle fracture mechanism caused by impurity accumulation in conventional die castings, this triggers a mode transformation from cleavage fracture to extremely tough fracture, further realizing the nonlinear leap of dynamic impact toughness under the extreme lightweight design.

[0024] (4) By switching mechanical oscillation to static pressure feeding at a specific pressure during the holding stage, and in conjunction with the nanoscale particle gradient topology structure formed in the early stage, the invention utilizes the microscopic adjustment effect of nanodroplets on the grain boundary stress field and the silicon-rich liquid phase penetration effect under high pressure to achieve a deep integration of macroscopic feeding and microstructure stability, ensuring the morphological stability of the nanoscale energy-absorbing phase during the complete solidification process, and further strengthening the bonding strength between the surface chilling and vibration layer of the casting and the high-rigidity structure in the core. Attached Figure Description

[0025] 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 recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a flowchart of a preferred embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of the principle of the multi-dimensional physical field collaborative control die casting molding system according to a preferred embodiment of the present invention. Detailed Implementation

[0028] 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.

[0029] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0030] Application Overview:

[0031] This invention targets key structural components of new energy vehicle chassis, particularly large, thin-walled, complex die-cast parts such as battery trays. As a critical chassis structural component that supports the battery module and connects to the vehicle body, the battery tray requires high yield strength under static load conditions to resist plastic deformation and ensure the structural stability of the battery module. Under collision energy absorption conditions, the material needs excellent impact toughness to absorb energy through plastic deformation and prevent battery pack intrusion. Existing aluminum alloy die-casting materials generally suffer from an inherent contradiction between strength and toughness. While traditional heat treatment processes can improve individual properties to some extent, for large, thin-walled structural parts, heat treatment easily induces severe deformation and surface blistering, failing to meet the requirements of mass production.

[0032] The fundamental problem that existing technologies cannot solve lies in the disconnect between process control and microstructure evolution, as well as the limited understanding of the mechanisms of action of specific alloying elements. In current die-casting processes, mold design and material design are typically optimized as two independent dimensions. Traditional extrusion pins serve only as mechanical feeding tools to eliminate local shrinkage porosity, and their motion parameters are unrelated to the evolution of the material's microstructure. Furthermore, material design aims for uniform microstructure across the entire casting, neglecting the differences in performance requirements across different regions. More critically, in the field of aluminum alloy die casting, bismuth and lead have long been considered harmful impurities. Due to their immiscibility in molten aluminum, they easily form micron-sized coarse spherical droplets or segregation bands under conventional solidification conditions, severely disrupting the continuity of the matrix and leading to a simultaneous decrease in strength and toughness. Using only conventional methods such as macroscopic conformal cooling channels or static extrusion pins cannot achieve precise and coordinated control of the thermal field, nor can it suppress the coarsening and aggregation behavior of elements such as bismuth and lead. This results in brittle fracture characteristics of the material under dynamic impact conditions, hindering its energy absorption capabilities.

[0033] This invention breaks with convention by coupling ultrasonic mechanical oscillation with pulsed thermal extraction to achieve multi-field synergistic control of the solidification process of semi-solid slurry. Bismuth and lead, traditionally considered harmful impurities, are transformed into functional alloying elements. The cavitation effect generated by ultrasonic oscillation in the semi-solid slurry breaks bismuth-lead droplets down to nanoscale size, and pulsed cooling achieves rapid solidification and dispersion at grain boundaries. Under static load conditions, the nanoscale bismuth-lead particles do not constitute crack initiation sites, ensuring high yield strength of the material. Under high-speed impact conditions, the particles, due to their low melting point, melt instantaneously under adiabatic heating conditions, absorbing impact energy and inducing micro-slip at grain boundaries, significantly improving the material's impact toughness.

[0034] Exemplary method:

[0035] like Figure 1 , Figure 2 As shown, a method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part includes:

[0036] S1. Prepare a metastable aluminum alloy melt, which contains silicon, magnesium, manganese, iron, bismuth, lead, titanium, zirconium and the balance aluminum, wherein bismuth and lead are in a liquid-liquid immiscible state in the metastable aluminum alloy melt.

[0037] S2. The metastable aluminum alloy melt is injected into the preheated die-casting mold cavity. Multi-dimensional physical field characteristic data of a specific area in the die-casting mold cavity are collected in real time. Based on the multi-dimensional physical field characteristic data, the solid fraction of the alloy slurry in the specific area is determined, and the semi-solid critical window where the solid fraction is within the preset range is identified.

[0038] S3. During the duration of the semi-solid critical window, mechanical oscillation disturbance of ultrasonic frequency band is simultaneously applied to the alloy slurry in a specific area, and pulsed thermal extraction treatment with a preset phase difference from the mechanical oscillation disturbance is simultaneously applied to the specific area to induce cavitation effect to break up the coarse bismuth-lead droplets in the alloy slurry and freeze the bismuth-lead droplets at the grain boundary.

[0039] S4. Stop mechanical vibration and pulsed heat extraction treatment, hold pressure until the alloy slurry is completely solidified, and open the mold to obtain high-strength, tough, and lightweight aluminum alloy die castings.

[0040] To achieve a metastable and uniform distribution of immiscible alloy components in an aluminum matrix, it is necessary to first control the composition ratio and melting process to construct an initial melt with specific thermodynamic properties, thereby providing a material basis for subsequent microstructure refinement.

[0041] Step S1 aims to prepare a metastable aluminum alloy melt, which forms the material basis for subsequent processing. During die casting, the final properties of the material are influenced not only by process parameters but also by the melt's composition and thermodynamic state. For large, thin-walled structural components such as battery trays in new energy vehicles, service conditions require materials to possess both high strength and high toughness; however, traditional aluminum alloy compositions have inherent limitations in this regard. Therefore, step S1 uses a specific alloy composition and constructs a non-equilibrium melt state with unique thermodynamic characteristics during the melting process, thus laying the material foundation for subsequent microstructure control.

[0042] The alloy system selected in this invention is based on the aluminum-silicon-magnesium-manganese system.

[0043] The silicon mass fraction is 7.5% to 9.5%, a range closely related to the structural characteristics of the battery tray as a large, thin-walled component. Due to the large size and thin walls of the component, the flow path of the molten aluminum in the mold cavity is relatively long, making it prone to casting defects such as cold shuts and incomplete filling during the filling process. Silicon can reduce the viscosity of the molten aluminum, improve its fluidity, and at the same time narrow the solidification temperature range, reducing the tendency for hot cracking. Controlling the silicon content between 7.5% and 9.5% preserves the excellent fluidity of the hypoeutectic composition while avoiding the coarsening of primary silicon due to excessive silicon content, which would adversely affect the toughness of the material.

[0044] The mass fraction of magnesium is 0.3% to 0.6%. Magnesium and silicon form a strengthening phase during solidification. These phases precipitate during subsequent cooling, providing the basic yield strength of the material. When the magnesium content is too low, the strengthening effect is insufficient. When the magnesium content is too high, it will increase the susceptibility to hot cracking. For large thin-walled parts, hot cracking is a type of defect that needs to be avoided.

[0045] Manganese and iron act as synergistic elements in alloy systems, with iron having a mass fraction of 0.6% to 1.0% and manganese a mass fraction of 0.4% to 0.8%. In aluminum alloy die casting, iron is an unavoidable impurity element, but an appropriate amount of iron helps prevent sticking to the mold. However, the morphology of the iron-rich phase formed by iron in the aluminum matrix has a direct impact on the toughness of the material. When the iron content is high and the manganese content is insufficient, the iron-rich phase takes the form of needles. This phase is prone to becoming a crack source under stress, disrupting the continuity of the matrix. By controlling the iron-manganese ratio, manganese and iron preferentially form a skeletal iron-rich phase, thereby reducing the adverse effect of iron on plasticity. At the same time, this iron-manganese ratio range matches the demolding performance of the extrusion pin in subsequent processes, avoiding the sticking problem caused by excessively low iron content.

[0046] Titanium and zirconium are added grain-refining elements, with titanium having a mass fraction of 0.05%~0.15% and zirconium a mass fraction of 0.05%~0.12%. Both form fine dispersed phases in the aluminum matrix and act as heterogeneous nucleation sites during solidification, refining the grain size. For large, thin-walled die castings, the fine grain structure can not only improve the yield strength but also enhance the material's plastic deformation capacity. In addition, the addition of zirconium has the effect of inhibiting recrystallization, and in areas that may undergo local thermal cycling, zirconium helps maintain the stability of the grain structure.

[0047] Using a reverse approach, bismuth and lead are introduced. The theoretical basis for this approach lies in the matching relationship between the physical properties of bismuth and lead and the service conditions of the battery tray. Bismuth has a melting point of 271℃, and lead has a melting point of 327℃, significantly lower than the melting temperature of the aluminum alloy matrix. Under normal static load-bearing conditions, bismuth-lead droplets exist in solid form at the grain boundaries. Due to the small size and dispersed distribution of the droplets, their impact on mechanical properties is within a controllable range. However, under high-speed collision conditions, adiabatic heating occurs in localized areas of the material, with temperatures rapidly rising to 200-300℃. Under these temperature conditions, bismuth and lead melt, and the phase transition process absorbs a large amount of... Heat and liquefied bismuth-lead simultaneously form a slip layer at the grain boundaries, inducing micro-region slip at the grain boundaries, thereby improving the impact toughness of the material. This mechanism allows elements that are originally considered harmful impurities to be transformed into functional reinforcing phases under specific conditions. However, to effectively utilize this mechanism, the coarsening problem of bismuth-lead droplets during conventional solidification must be solved to ensure that they do not damage the matrix properties under normal conditions, while playing an energy-absorbing role under dynamic impact conditions. This is the problem that subsequent process steps need to solve. The goal of step S1 is to construct an initial melt with metastable characteristics during the smelting stage, providing a compositional basis for subsequent nano-fragmentation.

[0048] The smelting process is carried out within a temperature range of 720~760℃. The selection of this temperature range is based on multiple considerations. The setting of the lower temperature limit is related to the macroscopic dispersion of bismuth and lead: when the smelting temperature is too low, the atomic diffusion kinetics of bismuth and lead are insufficient, making it difficult to uniformly disperse in the aluminum melt on a macroscopic scale. Gravity segregation is likely to form at the bottom of the furnace, resulting in uneven composition. The setting of the upper temperature limit is related to the gas absorption behavior of the aluminum melt: when the temperature exceeds 760℃, the solubility of hydrogen in the aluminum melt increases, making subsequent degassing more difficult and increasing the probability of pinhole defects in the die casting. The temperature range of 720~760℃ can control the gas absorption reaction while ensuring the macroscopic dispersion of immiscible elements, so that the melt is in a thermodynamic state suitable for subsequent processing.

[0049] The smelting process requires refining and degassing. The specific operation involves introducing an inert gas into the melt and using a sodium-free refining agent for rotary degassing. The inert gas is usually high-purity argon or nitrogen. Its mechanism is that the bubbles adsorb hydrogen atoms and oxide inclusions as they float in the melt and carry them to the surface of the melt. The sodium-free refining agent is used to avoid the adverse effects of sodium on the subsequent demolding performance. The refining time is usually controlled at 10-20 minutes to ensure that the hydrogen content in the melt is reduced to below 0.15 mL / 100g Al and to remove oxide inclusions.

[0050] After the above smelting process, the resulting melt is defined as a metastable aluminum alloy melt. Metastable means that the distribution of bismuth and lead in the melt deviates from the completely separated state predicted by the equilibrium phase diagram. Under thermodynamic equilibrium conditions, bismuth and lead should completely separate from the aluminum matrix, forming independent liquid phase layers or coarse droplets. However, through temperature control during the smelting process and subsequent rapid solidification, bismuth and lead are forced to remain in a highly dispersed non-equilibrium state. This metastable state is not a stable thermodynamic state, but a high-energy state temporarily maintained due to kinetic obstacles. Its significance lies in providing operational space for subsequent intervention during the semi-solid critical window period.

[0051] In metastable aluminum alloy melts, bismuth and lead exist in a liquid-liquid immiscible state. This state is specifically manifested in the microstructure of the melt after quenching, which shows that bismuth and lead are distributed in the aluminum matrix in the form of micron-sized droplets, but have not yet undergone significant coarsening and aggregation. The liquid-liquid immiscible state is a condition for the effectiveness of this composition design. It ensures that there are dispersed phase droplets that can be intervened in subsequent process steps, rather than a separated structure that has formed coarse aggregates. Without this state, subsequent nano-fragmentation will lack a target, and the functions of bismuth and lead cannot be realized.

[0052] The metastable aluminum alloy melt obtained after step S1 has a composition and microstructure that provides the necessary foundation for subsequent process steps. In this melt, bismuth and lead exist as immiscible droplets, but have not yet coarsened to the point of affecting material properties; silicon, magnesium, manganese, and iron form the basis of the alloy's casting properties and mechanical strength; titanium and zirconium contribute potential for grain refinement. The state of each element collectively provides the material basis for subsequent process interventions.

[0053] The metastable aluminum alloy melt prepared in step S1 already possesses the characteristics of a bismuth-lead immiscible system in terms of composition. However, after the melt is filled by injection and enters the solidification process, the morphological evolution and final distribution of the bismuth-lead droplets inside still depend on the timing of process intervention and energy input during the solidification stage. Since the die-casting solidification process is completed within 100 ms to several seconds, the time window for bismuth-lead droplets from nucleation, growth to coarsening is extremely limited. If the intervention is too early, the droplets will not have fully formed; if the intervention is too late, the droplets will have already coarsened to a size that is difficult to break.

[0054] Therefore, in step S2, after the melt is injected into the mold, the solid fraction in a specific region of the alloy slurry is monitored in real time to accurately identify the semi-solid critical window where the solid fraction is between 55% and 75%. This window corresponds to the stage where bismuth-lead droplets have precipitated but have not yet undergone significant coarsening, and is the optimal time to apply mechanical oscillation and pulse cooling to achieve nanoscale fragmentation.

[0055] In step S2, the injection parameters and mold preheating are set first;

[0056] Metastable aluminum alloy melt is injected into the die-casting mold cavity via an injection mechanism. The selection of the injection mechanism and its parameter settings directly affect the filling quality and the temperature field distribution in the early stage of solidification. This invention adopts a high-pressure die-casting process, with the injection speed typically set to 2 m / s to 5 m / s and the injection specific pressure set to 80 MPa to 120 MPa. The determination of this parameter range is based on the characteristics of the battery tray as a large, thin-walled structural component. If the injection speed is too low, cold shut defects are likely to occur in the aluminum melt during the long-flow filling process. If the injection speed is too high, air entrapment is likely to occur, thereby increasing the risk of porosity defects. The setting of the injection specific pressure is closely related to the density requirements of the casting. A higher specific pressure helps to eliminate shrinkage defects. However, for melts containing bismuth-lead immiscible systems, an excessively high injection specific pressure may lead to increased liquid phase separation during the filling process. Therefore, the specific pressure needs to be controlled within the above-mentioned range.

[0057] The mold needs to be preheated to 200-250°C before injection. The determination of this preheating temperature range is based on a comprehensive consideration of multiple factors. In the prior art, in order to improve the production cycle, the mold preheating temperature is often set in the lower range of 150-180°C. However, for the bismuth-lead alloy system involved in this invention, an excessively low mold temperature will cause the melt to form a thick chilling layer after contacting the mold wall. This chilling layer will hinder the transfer of subsequent mechanical oscillation energy to the interior of the slurry, causing energy to be lost in the chilling layer and unable to effectively act on the bismuth-lead droplets. On the other hand, if the mold temperature is too high, for example, exceeding 280°C, the solidification time will be prolonged, the residence time of the bismuth-lead droplets in the liquid phase will increase, and the probability of Oswald curing will increase, resulting in the droplets coarsening to the micron-scale size before receiving mechanical oscillation treatment. Therefore, a preheating range of 200-250°C can ensure the integrity of the filling while providing suitable initial conditions for subsequent energy field intervention.

[0058] Within a specific area of ​​the mold, this invention pre-embeds a high-frequency impedance sensor and an infrared temperature sensor. This specific area refers to the local geometric region of the battery tray that experiences high stress or requires high energy absorption characteristics during service, specifically including the transition connection area between longitudinal and transverse beams, the root area of ​​the mounting lugs, and the intersection of reinforcing ribs. These areas face significant temperature gradient differences during die-casting solidification due to the transition between thick and thin sections, becoming areas of concentrated thermal stress and key nodes in the force transmission path under collision conditions. Precise control of the microstructure in these areas is crucial for improving the overall structural safety in collisions.

[0059] The selection and arrangement of the high-frequency impedance sensor are customized according to the specific requirements of this invention. The sensor achieves its monitoring function by emitting a high-frequency AC signal with a frequency of 100 kHz to 1 MHz into the alloy slurry and measuring the impedance amplitude and phase angle of the feedback signal. Its working principle is based on the fact that during the transformation of aluminum alloy from liquid to solid, the formation of solid-phase dendritic overlapping network causes nonlinear changes in the material's conductivity and dielectric constant. In the liquid stage, aluminum liquid has good conductivity and exhibits low impedance characteristics. As the solid fraction increases, the establishment of dendritic network gradually restricts the current channel, and the impedance amplitude shows an upward trend. When the solid fraction exceeds a certain threshold, the impedance signal will show a significant characteristic inflection point. This electrical characteristic has a significantly higher sensitivity to solid fraction than traditional thermocouple temperature measurement, and the response speed reaches the microsecond level, which can effectively overcome the hysteresis effect of temperature measurement.

[0060] Infrared temperature sensors are used for non-contact measurement of temperature distribution at the solidification front, providing temperature gradient data as an auxiliary feature. The sensor typically uses an infrared detection element with a wavelength of 8 μm to 14 μm, which is aligned with the surface or near-surface area of ​​the slurry through a light-transmitting window opened on the mold. The advantage of infrared temperature measurement is its non-contact nature, which avoids interference with the solidification process caused by embedding the sensor, and at the same time provides a faster response speed than thermocouples.

[0061] The data collected by the high-frequency impedance sensor and the infrared temperature sensor constitute multi-dimensional physical field characteristic data, specifically including the time series of impedance amplitude, the time series of impedance phase angle, and the spatial distribution data of temperature gradient. These multi-source heterogeneous data reflect the comprehensive thermodynamic and kinetic state of the alloy slurry during the solidification process. Data from a single type of sensor is difficult to accurately reflect the complex microstructure evolution, while the fusion of multi-dimensional data provides a basis for the accurate determination of the solid fraction.

[0062] Multidimensional physical field feature data are input into a multidimensional feature fusion model, which is the key algorithm architecture for achieving accurate window recognition in this invention. The multidimensional feature fusion model is a data processing algorithm based on machine learning and metallurgical kinetic equations. Its function is to map the apparent sensor signals to the underlying material solid phase fraction. This invention has made targeted improvements to the model. Its calculation logic is not a simple linear regression or threshold judgment, but introduces non-equilibrium solidification equations as prior knowledge.

[0063] Specifically, the model first uses the modified Scheil nonequilibrium solidification equation to theoretically estimate the solid fraction; for the multi-component aluminum alloy system of this invention, the basic theoretical estimation of the solid fraction adopts the modified Scheil nonequilibrium solidification equation; the equation is expressed as follows: ,in, The solid fraction of the alloy slurry represents the percentage of the volume of solid dendrites that have crystallized into solid dendrites in the total volume, and its value ranges from 0 to 1. This is the melting point of pure aluminum. The temperature of the alloy slurry is collected in real time by the sensor. The liquidus temperature of the aluminum alloy composition used is determined by thermodynamic phase diagram calculation software based on the mass percentage of elements such as silicon, magnesium, manganese, iron, bismuth, lead, titanium, and zirconium in the alloy. For the composition of this invention, the range is between 590 and 620°C. Let be the solute partition coefficient. This equation describes the nonlinear change in solid fraction with decreasing temperature under non-equilibrium solidification conditions. However, due to the inherent hysteresis of thermocouples or infrared thermometry, and the microscopic segregation of local components during actual solidification, the solid fraction calculated solely based on temperature data contains systematic errors.

[0064] The model further extracts the characteristic derivatives of the high-frequency impedance signal, specifically including the first and second derivatives of the impedance amplitude with respect to time. When the solid fraction of the alloy slurry reaches a certain critical value, the derivative of the impedance signal will exhibit an extreme point, the occurrence of which is closely related to the penetration threshold of the dendritic network. The model uses the occurrence time of the extreme point of the impedance derivative to perform real-time Kalman filtering correction on the aforementioned temperature-based solid fraction estimate. The Kalman filtering algorithm can combine the model prediction value and the measured characteristic value to provide the optimal solid fraction estimate under the premise of considering measurement noise. Through this fusion of temperature data and impedance data, the model can achieve real-time determination of solid fraction at the millisecond level, and its accuracy and response speed are superior to the measurement results of a single sensor.

[0065] When the multi-dimensional feature fusion model calculates that the real-time solid fraction of the alloy slurry in a specific region reaches the range of 55% to 75%, the system identifies and outputs a trigger signal for the semi-solid critical window. The determination of the 55% to 75% window range is based on a systematic study of the evolution behavior of bismuth-lead droplets. When the solid fraction is below 55%, although bismuth-lead droplets have precipitated from the aluminum matrix, the number of droplets is small, their size has not grown sufficiently, and the proportion of liquid phase in the slurry is high, with an overall low viscosity. Under these conditions, applying mechanical oscillation will cause energy to dissipate rapidly in the liquid phase, making it difficult to generate sufficient shear stress to act on the dispersed droplets. When the solid fraction is above 75%, the solid dendrites have formed interconnections. The rigid framework of the slurry causes a sharp increase in viscosity and exhibits solid-like mechanical behavior. At this point, applying mechanical oscillation not only makes it difficult to transfer energy to the bismuth-lead droplets, but may also cause the dendrite network to break due to vibration stress, leading to macroscopic thermal cracks. When the solid phase fraction is between 55% and 75%, the slurry is in a special thixotropic state. The solid dendrites have not yet fully formed a rigid network, and the liquid phase still has the ability to flow. At this point, applying mechanical oscillation can generate effective shear stress. At the same time, the cavitation effect caused by the oscillation can form a local high-pressure zone in the liquid phase, which acts on the surface of the bismuth-lead droplets to overcome the interfacial tension and achieve breakage. Accurate identification of this window is a prerequisite for the successful implementation of the subsequent step S3.

[0066] Compared to existing technologies that rely solely on thermocouple temperature measurement combined with a fixed time delay for feeding operations, the window recognition method of this invention has a fundamental advantage. Existing technologies in die casting of large thin-walled parts typically set the extrusion pin or other intervention measures based on experience, setting a few seconds after injection to be activated. This open-loop control method cannot adapt to the fluctuations in the actual solidification process, resulting in a time misalignment between the intervention timing and the actual state of the slurry. This invention, through multi-sensor fusion and model-driven real-time closed-loop control, achieves precise capture of the solidification process, enabling subsequent mechanical oscillation and pulse cooling to be implemented within the optimal physical window period, thereby creating the process conditions for the nano-fragmentation of bismuth-lead droplets.

[0067] After step S2 identifies the semi-solid critical window with a solid fraction of 55% to 75%, the core of step S3 lies in applying multi-field synergistic intervention to the alloy slurry in a specific region using this extremely narrow time window. This breaks the coarse bismuth-lead droplets down to the nanoscale and disperses them at the grain boundaries. This window period typically lasts only 150ms to 400ms during the forming process of large thin-walled die castings. Completing the breakup and freezing of droplets in this extremely short time places stringent requirements on the mode, timing, and intensity of energy input. Step S3 combines ultrasonic mechanical oscillation with pulsed thermal extraction treatment, utilizing the synergistic effect of the two to overcome the surface tension of the bismuth-lead droplets at the microscale, achieving nanoscale breakup and preventing them from coarsening again. This transforms what is traditionally considered a harmful impurity into a functional reinforcing phase.

[0068] The execution of mechanical oscillation disturbance relies on an intelligent hollow extrusion pin system. This system is the core hardware carrier that distinguishes this invention from existing die-casting molds. Unlike traditional solid extrusion pins used in existing technologies to apply unidirectional static pressure at the end of solidification to eliminate shrinkage cavities, the intelligent hollow extrusion pin system integrates micro-channels with a diameter of 0.1 mm to 1 mm. A high-frequency servo drive motor and a piezoelectric ceramic actuator are connected at the tail, enabling simultaneous output of high-frequency mechanical displacement and high-frequency fluid pulses. After identifying the semi-solid critical window, the control system drives the extrusion pin to apply mechanical oscillation disturbance to the alloy slurry in a specific area. The frequency of this disturbance is set to 100 Hz to 500 Hz, and the amplitude is set to 0.05 mm to 0.2 mm. This parameter range is determined based on a systematic study of the balance between the cavitation effect and the maintenance of the slurry structure. Existing dynamic extrusion pins typically operate at 10 Hz to 50 Hz. In the low-frequency range of 100 Hz, the effect is limited to macroscopic mechanical stirring and feeding, and cannot generate sufficient shear stress at the microscale to break immiscible droplets. This invention finds that when the frequency is below 100 Hz, the mechanical energy density of the input slurry is insufficient to overcome the interfacial tension between liquid bismuth and lead, and cannot induce cavitation; the bismuth-lead droplets only undergo elastic deformation without breaking. When the frequency is above 500 Hz or the amplitude is greater than 0.2 mm, the excessive mechanical energy input will generate significant shear heat, causing local secondary melting of the formed solid dendrites, destroying the basic strength framework of the material. At the same time, the excessive amplitude may cause wear on the fit between the extrusion pin and the die, affecting the service life of the system. Therefore, the frequency range of 100 Hz to 500 Hz and the amplitude range of 0.05 mm to 0.2 mm can precisely target mechanical energy to the immiscible liquid phase without destroying the main dendrite structure, providing the energy basis for subsequent cavitation breakage.

[0069] During the application of mechanical oscillation disturbance, step S3 simultaneously performs pulsed thermal extraction treatment. This treatment injects nanofluid cooling medium into a specific area through microchannels inside the extrusion pin. The nanofluid is a ternary suspension system composed of highly thermally conductive nanoparticles, a low-viscosity base liquid, and a surfactant. Its specific ratio is determined by this invention based on the optimization of microchannel heat transfer characteristics and process compatibility. The base liquid is a 1:1 volume ratio of ethylene glycol and water, which ensures fluidity while providing good low-temperature stability, preventing the cooling medium from freezing at low temperatures of 5-10°C. The nanoparticles are graphene, with an addition amount of 0.5-2.0 vol%. Graphene's own thermal conductivity can reach 3000 W / (m·K)~5000 W / (m·K). After being dispersed in the base liquid, it can form a highly thermally conductive network, making the overall effective thermal conductivity of the fluid exceed 3 W / (m·K)~5 W / (m·K), which is much higher than that of traditional cooling media by about 0.6. The thermal conductivity is W / (m·K). The surfactant used is sodium dodecylbenzenesulfonate, with an addition amount of 0.1 wt%~0.3 wt%. Its function is to reduce the surface energy of nanoparticles, prevent graphene from agglomerating and clogging in the microchannels, and ensure stable flow of the cooling medium in the microchannels with a diameter of 0.1 mm~1 mm. Existing technologies usually use pure water or thermal oil as the cooling medium for die-casting molds. The heat transfer performance of these media can meet the requirements in macroscopic water channels. However, at the microchannel scale of this invention, the flow is in a laminar state, and heat transfer mainly relies on conduction rather than convection. The low thermal conductivity of traditional media becomes the bottleneck of heat transfer. After introducing nanofluids, the heat flux density of the microchannels is greatly improved, and a large amount of heat can be extracted in milliseconds to provide a sufficient cooling rate for subsequent droplet freezing.

[0070] The key feature of pulsed thermal extraction processing lies in the preset phase difference between the cooling medium supply method and the mechanical oscillation disturbance. This invention sets this phase difference to 90 degrees. Pulsed supply means that the cooling medium is not a continuous, constant flow, but rather controlled by a high-frequency solenoid valve, entering the extrusion pin in discrete pulse wave packets for heat exchange. The pulse frequency is consistent with the mechanical oscillation frequency, i.e., 100 Hz to 500 Hz. The 90-degree phase difference setting is based on an in-depth analysis of the cavitation effect mechanism and the timing of thermal extraction. When the extrusion pin retracts, the mechanical oscillation is in the relaxation phase, and the end of the extrusion pin exerts a stretching effect on the alloy slurry, resulting in a local pressure reduction and the formation of a negative pressure region. This stage is a favorable time for cavitation bubble nucleation and growth. Setting the peak of the pulse cooling wave during this relaxation phase ensures that the system inputs the maximum cooling capacity at the moment when the negative pressure is strongest and cavitation bubbles are most likely to be generated. When the extrusion pin moves forward, the mechanical oscillation is in the compression phase, and the cavitation bubbles collapse during this stage, releasing enormous energy. Meanwhile, the cooling pulse is in the trough or off state to avoid the cooling capacity interfering with the energy release from cavitation collapse. This timing coordination allows mechanical energy and thermal energy to work synergistically on the time axis, ensuring the full development of the cavitation effect and achieving rapid solidification of the broken droplets.

[0071] Under the synergistic effect of mechanical oscillation disturbance and pulsed thermal extraction treatment, cavitation occurs inside the semi-solid slurry. The physical process of cavitation is as follows: During the relaxation phase of mechanical oscillation, the local pressure of the alloy slurry is lower than the saturated vapor pressure of the liquid phase surrounding the bismuth-lead droplets, causing the liquid to tear and form micron-sized cavitation bubbles. During the subsequent compression phase, these bubbles collapse instantaneously under external pressure. When the cavitation bubbles collapse, the gas inside the bubbles is adiabatically compressed, generating local extreme high temperatures, reaching thousands of Kelvin, and simultaneously generating local extreme high pressures, reaching gigapascal levels. Microjets with velocities up to 100 m / s also accompany the bubble collapse. This series of extreme physical conditions acts on coarse bismuth-lead droplets with a size of 5–20 μm. Although the interfacial tension between bismuth and lead in the liquid state hinders the deformation and breakup of the droplets, under the impact of the microjets and shock waves generated by cavitation collapse, the interfacial tension is overcome by enormous kinetic energy. The coarse droplets are forcibly torn apart and dispersed, forming droplets with a size of 50–200 μm. Nanoscale droplets of nm; it is worth noting that this fragmentation process is not a simple mechanical cutting, but rather utilizes the energy focusing characteristics of cavitation effect to amplify the macroscopic mechanical oscillation energy at the microscopic scale, thereby overcoming the energy density bottleneck that conventional mechanical stirring cannot break through.

[0072] After bismuth-lead droplets are broken down into nanoscale particles, the main threat they face is Oswald ripening, where smaller droplets, due to their higher surface energy, tend to dissolve, and solute atoms redeposit onto larger droplets via liquid-phase diffusion, leading to further coarsening of the droplets. To prevent this, step S3 utilizes the quenching effect of pulsed thermal extraction to rapidly lower the local temperature below the solidus of the alloy at the instant the nanodroplets have just formed and before they collide and merge. The 90-degree phase difference design plays a crucial role here again: the cooling pulse is precisely at its peak when cavitation collapse is complete and the nanodroplets form. The input cooling force causes the aluminum matrix surrounding the nanodroplets to solidify rapidly, freezing the nanodroplets in situ at the growing grain boundaries. Due to the temperature gradient at the front end of the extrusion pin, the cooling rate is higher in areas closer to the extrusion pin surface, and the distribution of nanodroplets exhibits a gradient characteristic that gradually decreases from the extrusion pin surface to the interior. This gradient distribution matches the performance requirements of the key stress areas of the battery tray: the area near the extrusion pin receives stronger mechanical intervention and cooling, resulting in a higher nanodroplet density and a more significant energy absorption effect during high-speed impacts; the area far from the extrusion pin maintains relatively high matrix strength, meeting the static load-bearing requirements.

[0073] Throughout step S3, the various components of the alloy system participate in different physical metallurgical processes. During cooling, the aluminum matrix forms primary α-Al dendrites, and the reinforcing phases formed by silicon and magnesium precipitate between the dendrites, providing basic strength for the material. The iron-rich phases formed by iron and manganese are distributed at the grain boundaries, improving demolding performance and reducing damage to plasticity. The dispersed phases formed by titanium and zirconium act as heterogeneous nucleation cores, refining grain size. The bismuth-lead droplets undergo a morphological transformation from coarse spherical droplets to nanoscale droplets, and their distribution changes from random distribution to gradient distribution along grain boundaries. This series of microstructural evolutions enables the material to maintain high strength under conventional static conditions and achieve a leap in toughness under high-speed impact conditions by utilizing the melting and energy absorption mechanism of bismuth-lead nanodroplets. Compared with the existing technology that only eliminates shrinkage porosity by extrusion pins or controls the temperature field through macroscopic cooling, step S3 of this invention achieves deep coupling of mechanical energy and thermal energy on a spatiotemporal scale, extending process intervention from macroscopic defect control to microstructural regulation, providing a new technical path for decoupling the static strength and dynamic toughness of materials.

[0074] Step S3 completes the synergistic effect of mechanical oscillation disturbance and pulsed thermal extraction treatment within the semi-solid critical window period, breaking the coarse bismuth-lead droplets into nanoscale and freezing them at the grain boundaries; this energy field intervention process continues until the solid fraction of the alloy slurry reaches a certain critical value, after which the external energy input needs to be terminated in time to avoid damage to the formed microstructure; the task of step S4 is to accurately determine the timing of intervention termination and ensure the density of the casting through static pressure feeding during the subsequent solidification process, and finally obtain an aluminum alloy die casting with specific microstructure characteristics and macroscopic mechanical properties.

[0075] When the multidimensional feature fusion model detects that the real-time solid fraction of the alloy slurry in a specific region exceeds 75%, the control system issues a command to stop the mechanical oscillation disturbance and pulsed thermal extraction treatment. This termination timing is based on a systematic analysis of the rheological behavior of the alloy slurry. When the solid fraction is below 75%, the slurry is in a thixotropic state; the solid dendrites have not yet formed a continuous network structure, and the liquid phase still has flowability. At this time, applying mechanical oscillation can transfer energy to the bismuth-lead droplets. When the solid fraction exceeds 75%, the inter-dendritic bonding gradually forms a rigid framework, and the slurry changes from a liquid-like state to a solid-like state, with a sharp increase in viscosity. Continuing to apply mechanical oscillation in this state... Mechanical oscillation causes the shear stress generated by the high-frequency motion of the extrusion pin to no longer act on the bismuth-lead droplets in the liquid phase, but instead directly on the already formed dendritic network, which can easily lead to dendrite fracture and the formation of macroscopic thermal cracks. At the same time, the cavitation effect depends on the local pressure fluctuations in the liquid phase. When the solid fraction exceeds 75%, the liquid phase is divided in the closed dendritic gaps and cannot form continuous cavitation bubbles. Continued oscillation not only fails to further refine the droplets, but may also destroy the already formed nanoscale bismuth-lead distribution. Therefore, using 75% of the solid fraction as the threshold for intervention termination can avoid secondary damage to the solidified structure while ensuring the stability of the nanodroplet morphology.

[0076] After stopping mechanical oscillation and pulse cooling, the intelligent hollow extrusion pin system switches to static pressure mode, applying a holding pressure of 80 MPa to 120 MPa to a specific area. Static pressure mode refers to the extrusion pin stopping its high-frequency reciprocating motion and switching to a continuous forward output of constant thrust. This mode is similar to the extrusion compensation function in traditional die casting processes, but it has a more complex physical effect in this invention. The holding pressure is set to 80 MPa to 120 MPa, which is higher than the approximately 50 MPa compensation pressure commonly used in traditional die casting processes. The selection of a higher holding pressure is based on the following considerations: After the ultrasonic oscillation treatment in step S3, a large number of nano-sized bismuth-lead droplets with a size of 50 nm to 200 nm are distributed at the grain boundaries. The presence of these droplets changes the local stress field and solidification shrinkage characteristics at the grain boundaries. On the one hand, the nano-droplets occupy a certain grain boundary volume, changing the compensation requirement for solidification shrinkage. On the other hand, the nano-droplets dispersed at the grain boundaries may undergo slight plastic deformation under high pressure, assisting in filling the dendrite gaps. 80 MPa to 120 MPa The holding pressure of MPa can force the residual silicon-rich eutectic liquid phase to penetrate more fully into the terminal region of the dendritic network, eliminating micro-shrinkage, while not causing compression deformation or coagulation of the formed nanodroplets.

[0077] The holding pressure process continues until the temperature of the alloy slurry drops below the solidus temperature, achieving complete solidification. Complete solidification refers to the thermodynamic endpoint where all liquid phases inside the alloy transform into solid phases. For the aluminum alloy composition system of this invention, the solidus temperature is typically between 500 and 540°C. During the holding pressure stage, the extrusion pin maintains a constant thrust to compensate for the volume shrinkage of the alloy from liquid to solid, preventing the formation of shrinkage cavities and porosity defects. Since this invention introduces bismuth and lead elements in the early stage, the solidification shrinkage behavior of these two elements differs from that of the aluminum matrix. A higher holding pressure is crucial for ensuring the quality of the interface bonding.

[0078] After complete solidification, the die-casting machine is opened, and the casting is ejected from the mold cavity through the ejection mechanism to obtain a high-strength, high-toughness, lightweight aluminum alloy die-casting with a gradient distribution of nanoscale bismuth-lead droplets. The casting exhibits characteristics that distinguish it from traditional die-castings in terms of microstructure: a continuous aluminum matrix forms the main framework of the material, and the reinforcing phase formed by silicon and magnesium is uniformly distributed in the matrix, providing basic stiffness and strength; the iron-rich phase formed by iron and manganese is distributed at the grain boundaries in a Chinese character shape, improving demolding performance and reducing damage to plasticity; the dispersed phase formed by titanium and zirconium plays a role in grain refinement. The key feature is that bismuth and lead, which are traditionally considered harmful impurities, are transformed into nanoscale droplets with a size of 50 nm to 200 nm in this invention, which are located at the grain boundaries in a gradient distribution. The density is higher in a specific area near the extrusion pin and gradually decreases away from the area.

[0079] This microstructure endows the casting with unique macroscopic mechanical properties. Under conventional static tensile conditions, because the size of the bismuth-lead particles is much lower than the critical threshold for traditional crack initiation (usually above 1 μm), the nanoscale bismuth-lead particles do not constitute stress concentration sources, and the yield strength of the material can reach 145 MPa~155 MPa, while the elongation remains at 10%~12%. Under high-speed impact conditions, the material undergoes adiabatic heating in local areas. When the strain rate exceeds 1000 / s, the local temperature rise can reach 100~250℃, reaching the melting point of bismuth and lead. At this time, the nanoscale bismuth-lead particles at the grain boundaries melt instantaneously. This phase transformation process absorbs a large amount of impact kinetic energy. At the same time, the molten bismuth and lead form a micro-lubricating layer at the grain boundaries, triggering micro-slip at the grain boundaries, forcing the crack propagation path to deflect in a Z-shape, and improving the impact toughness of the material.

[0080] Compared to existing technologies, step S4 of this invention demonstrates progress in the precise control of intervention termination and the optimized selection of holding pressure. In existing technologies, the initiation and termination of the extrusion pin are usually controlled based on a fixed time delay, which cannot adapt to the differences in solidification processes in different casting regions, often resulting in intervention that is too early or too late. This invention monitors the solid fraction in real time through a multi-dimensional feature fusion model, and precisely terminates the energy field intervention when the critical value of 75% is reached. This avoids damage to the dendritic network caused by excessive oscillation and ensures that the nanodroplets are no longer disturbed after freezing. In the holding pressure stage, existing technologies typically use a feeding pressure of around 50 MPa. This invention increases the holding pressure to 80 MPa~120 MPa based on the change in the grain boundary stress field after the distribution of nanodroplets. Combined with the gradient nanostructure formed in the early stage, it achieves the elimination of micro-shrinkage and further improvement of density. This series of optimized adjustments to process parameters are all based on a deep understanding of the behavioral evolution of bismuth-lead droplets during solidification, rather than simple empirical selection.

[0081] Exemplary casting:

[0082] A high-strength, high-toughness, lightweight aluminum alloy die-casting part, comprising:

[0083] In terms of macroscopic physical structure, the die-cast part is divided into a basic load-bearing area and an impact energy-absorbing area; wherein the basic load-bearing area is a conventional area without intervention from the intelligent hollow extrusion pin system, and the impact energy-absorbing area is a geometrically abrupt area subjected to mechanical oscillation disturbance and pulsed thermal extraction treatment by the intelligent hollow extrusion pin system.

[0084] The overall chemical composition of the die-cast parts, by mass percentage, is as follows: silicon 7.5%~9.5%, magnesium 0.3%~0.6%, manganese 0.4%~0.8%, iron 0.6%~1.0%, bismuth 0.1%~0.5%, lead 0.05%~0.2%, titanium 0.05%~0.15%, zirconium 0.05%~0.12%, with the balance being aluminum and unavoidable impurities.

[0085] Within the collision energy-absorbing region, the die-cast part possesses a dual-modal microstructure. This dual-modal microstructure comprises a continuous rigid framework phase and a non-equilibrium nanoscale flexible energy-absorbing phase. The continuous rigid framework phase consists of an α-Al solid solution matrix and a magnesium silicide reinforcing phase uniformly dispersed therein. The magnesium silicide reinforcing phase has a characteristic size of 0.5 μm to 2 μm and maintains a coherent or semi-coherent interface relationship with the α-Al solid solution matrix. The non-equilibrium nanoscale flexible energy-absorbing phase is formed by the conversion of bismuth and lead, and its morphology consists of nanoscale bismuth-lead droplets with a characteristic size of 50 nm to 200 nm. These nanoscale bismuth-lead droplets are pinned in an agglomerated manner at the α-Al grain boundaries and the eutectic silicon phase interface within the collision energy-absorbing region.

[0086] Along the surface of the die-cast part toward the center, the number density of the nano-sized bismuth-lead droplets exhibits an exponentially decreasing gradient distribution, reaching a peak volume fraction within the excitation layer 0.1 mm to 2 mm from the surface, and transitioning to a metastable solid solution structure in the central region with a depth exceeding 5 mm.

[0087] Example 1:

[0088] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part includes:

[0089] S1. Weigh the raw materials by mass percentage: silicon 8.5%, magnesium 0.45%, manganese 0.6%, iron 0.8%, bismuth 0.3%, lead 0.1%, titanium 0.1%, zirconium 0.08%, with the balance being aluminum; put the above raw materials into a melting furnace and melt them at 740℃. High-purity argon gas is introduced in combination with a sodium-free refining agent for rotary degassing for 15 minutes to obtain a metastable aluminum alloy melt in which bismuth and lead are in a liquid-liquid immiscible state;

[0090] S2. The metastable aluminum alloy melt is injected into the die-casting mold cavity preheated to 220°C at an injection speed of 3.5 m / s and an injection pressure of 100 MPa. Multidimensional physical field characteristic data of a specific area are collected in real time by an embedded high-frequency impedance sensor and an infrared temperature sensor. The data are input into the multidimensional feature fusion model. When it is determined that the real-time solid fraction of the alloy slurry in the area reaches 65% (within the semi-solid critical window of 55%~75%), a trigger signal is output.

[0091] S3. After the trigger signal is issued, the control system drives the intelligent hollow extrusion pin system to apply ultrasonic mechanical oscillation disturbance with a frequency of 300Hz and an amplitude of 0.12mm to the area; simultaneously, the micro-flow channel is opened and a nano-fluid cooling medium (containing 1.0 vol% graphene, ethylene glycol / water volume ratio of 1:1, and 0.2 wt% sodium dodecylbenzenesulfonate) at a temperature of 8℃ is injected to perform pulsed thermal extraction treatment with a preset phase difference of 90 degrees from the mechanical oscillation disturbance;

[0092] S4. When the model detects that the real-time solid fraction reaches 75%, stop the mechanical oscillation and pulse cooling, switch the extrusion pin to static pressure mode and apply a holding pressure of 100MPa until the alloy slurry temperature drops to 510℃ and completely solidifies. Then open the mold and eject the die casting to obtain the die casting.

[0093] Example 2:

[0094] The preparation method of a high-strength and lightweight aluminum alloy die casting is the same as that in Example 1, and will not be repeated here. The difference is that in step S2, the real-time solid fraction that triggers the intervention procedure in step S3 is 56%.

[0095] Example 3:

[0096] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting is similar to that in Example 1 and will not be repeated here. The difference is that in step S2, the real-time solid fraction that triggers the intervention procedure in step S3 is 74%.

[0097] Comparative Example 1:

[0098] The preparation method of a high-strength and lightweight aluminum alloy die casting is the same as that in Example 1, and will not be repeated here. The difference is that in step S2, the real-time solid fraction that triggers the intervention procedure in step S3 is 53%.

[0099] Comparative Example 2:

[0100] The preparation method of a high-strength and lightweight aluminum alloy die casting is the same as that in Example 1, and will not be repeated here. The difference is that in step S2, the real-time solid fraction that triggers the intervention procedure in step S3 is 77%.

[0101] Example 4:

[0102] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S3, the frequency of the mechanical oscillation disturbance is 110Hz.

[0103] Example 5:

[0104] A method for preparing a high-strength, tough, and lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S3, the frequency of the mechanical oscillation disturbance is 480Hz.

[0105] Comparative Example 3:

[0106] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S3, the frequency of the mechanical oscillation disturbance is 90Hz.

[0107] Comparative Example 4:

[0108] A method for preparing a high-strength, tough, and lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S3, the frequency of the mechanical oscillation disturbance is 520Hz.

[0109] Example 6:

[0110] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting is similar to that in Example 1, and will not be repeated here. The difference is that in step S3, the preset phase difference between the pulsed thermal extraction treatment and the mechanical oscillation disturbance is 85 degrees.

[0111] Comparative Example 5:

[0112] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S3, the preset phase difference between the pulsed thermal extraction treatment and the mechanical oscillation disturbance is 75 degrees.

[0113] Example 7:

[0114] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.1% bismuth, 0.1% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0115] Example 8:

[0116] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.5% bismuth, 0.1% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0117] Comparative Example 6:

[0118] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.08% bismuth, 0.1% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0119] Comparative Example 7:

[0120] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.55% bismuth, 0.1% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0121] Example 9:

[0122] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.3% bismuth, 0.05% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0123] Example 10:

[0124] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.3% bismuth, 0.2% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0125] Comparative Example 8:

[0126] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.3% bismuth, 0.03% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0127] Comparative Example 9:

[0128] A method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part is similar to that in Example 1 and will not be repeated here. The difference is that in step S1, the mass percentage composition of the raw materials is: 0.3% bismuth, 0.25% lead, and the proportions of the remaining components and the balance aluminum are adjusted accordingly.

[0129] Experimental Example 1:

[0130] Examples 1, 2, and 3, as well as Comparative Examples 1 and 2, were selected to sample and test the collision energy absorption zone of the obtained aluminum alloy die castings.

[0131] The parameter testing method is as follows:

[0132] The average equivalent diameter of bismuth-lead droplets is measured in nm. Thin film samples were prepared using focused ion beam (FIB). Micro-area scanning was performed using high-resolution transmission electron microscopy (HR-TEM) combined with energy dispersive spectroscopy (EDS). The equivalent circle diameters of at least 200 bismuth-lead enriched regions at grain boundaries were statistically analyzed using ImageJ software, and the average value was taken.

[0133] Static yield strength, in MPa; measured on a universal testing machine at 10... -3 The quasi-static strain rate was obtained by room temperature tensile testing.

[0134] Dynamic impact absorbed energy, in J; dynamic compression test was conducted using a split Hopkinson bar (SHPB) system under high-speed impact conditions with a strain rate of 1500 / s; the total energy absorbed by the specimen before fracture was calculated by integrating the stress-strain curve.

[0135] Table 1

[0136] Group Variable: Solid fraction that triggers intervention Average equivalent diameter of bismuth-lead droplets (nm) Static yield strength (MPa) Dynamic impact absorption energy (J) Example 1 65% 105 152 48.5 Example 2 56% 185 148 38.2 Example 3 74% 192 146 36.7 Comparative Example 1 53% 1850 115 12.4 Comparative Example 2 77% 2100 108 9.8

[0137] As can be seen from the data in Table 1, the 55%~75% solid fraction window defined by the present invention is not a conventional choice in the field, but rather exhibits a significant nonlinear abrupt change effect; within this range, bismuth-lead droplets can be broken down to the nanoscale, the static yield strength of the material remains at a high level, and the dynamic impact absorption energy is significantly improved.

[0138] However, even slight deviations beyond this range can lead to a precipitous drop in performance. When the solid fraction is too low, insufficient slurry viscosity results in energy dissipation from mechanical oscillations, failing to generate sufficient cavitation shear force and causing a significant increase in the size of the bismuth-lead droplets. When the solid fraction is too high, solid dendrites form a rigid framework, and oscillations fail to break the encased coarse droplets, instead severing the main dendrites. Both of these situations cause the coarse bismuth-lead droplets to become stress concentration sources under static tension, triggering brittle cleavage fracture and losing the phase transition energy absorption mechanism during dynamic impacts, demonstrating the harshness and unpredictability of the scope of protection of this application.

[0139] Experimental Example 2:

[0140] This experiment selected Examples 1, 4, 6, Comparative Example 3, and Comparative Example 5 for cross-validation.

[0141] The parameter testing method is as follows:

[0142] The adiabatic temperature rise in the impact micro-region is expressed in °C. In the SHPB high-speed impact test, the strain rate is 1500 / s, and a high-speed infrared thermal imager with a response time of 10μs is used simultaneously. The highest transient temperature in the deformation and shear bands of the specimen is captured, and the adiabatic temperature rise is obtained by subtracting the room temperature. This parameter reflects the material's ability to convert mechanical energy into latent heat of phase change.

[0143] The crack deflection Z-shaped angle is measured in degrees. After impact fracture, the longitudinal section of the fracture surface is scanned using electron backscatter diffraction (EBSD) technology. The average deflection angle when the main crack propagation path passes through the bismuth-lead enriched region at the grain boundary is measured. The larger the angle, the more significant the micro-lubricating and crack-arresting effect of liquid bismuth-lead.

[0144] Table 2

[0145] Group Variable 1: Oscillation frequency Variable 2: Cooling phase difference Impact micro-region adiabatic temperature rise (°C) Crack deflection Z-shaped angle (°) Dynamic impact absorption energy (J) Example 1 300Hz 90° 215 68 48.5 Example 4 110Hz 90° 165 45 35.6 Example 6 300Hz 85° 172 48 37.1 Comparative Example 3 90Hz 90° 45 12 14.2 Comparative Example 5 300Hz 75° 52 15 15.8

[0146] The data in Table 2 reveals the deep coupling mechanism of the process, structure and performance of the present invention; to achieve the instantaneous melting and energy absorption of bismuth-lead particles under high-speed impact, it is necessary to meet the conditions of bismuth melting point of 271°C and adiabatic temperature rise of about 200°C or more, while relying on the coordinated control of energy input and freezing timing.

[0147] In the embodiment, under the synergistic effect of two variables, the adiabatic temperature rise was significantly improved, the bismuth-lead phase transition was successfully triggered, the liquid metal caused the crack to deflect significantly, and the absorbed work showed an optimization trend.

[0148] When a single variable exceeds the protection range, the synergistic effect is significantly weakened; when the frequency is insufficient, cavitation effect cannot be triggered, and bismuth-lead droplets are in a coarse state, which leads to brittle fracture during impact and low adiabatic temperature rise, with a small crack propagation angle; when the frequency is appropriate but the phase difference is inappropriate, the cooling timing will be deviated, and the nanodroplets will coarsen before solidification, which also causes the phase transition mechanism to fail.

[0149] This indicates that the technical effect of the present invention is not a simple superposition of oscillation and cooling techniques, but rather a deep nonlinear physicochemical synergy between the two within a specific parameter domain. Exceeding this parameter domain not only leads to performance degradation but also causes the underlying physical mechanism of phase change energy absorption to fail, thus supporting the inventiveness of the present invention.

[0150] Experimental Example 3:

[0151] This experiment aims to select Examples 1, 7-8, 9-10, and Comparative Examples 6-9 for comparative testing.

[0152] The parameter testing method is as follows:

[0153] The grain boundary micro-region slip displacement, in nm; using in-situ transmission electron microscopy (In-situ TEM) combined with nanoindentation, under simulated high-speed impact with a high strain rate >1000 / s loading conditions, the maximum relative slip displacement of adjacent grains along the grain boundary was captured by high-frequency imaging; this parameter can be used to reveal the mechanism of action of bismuth-lead particles as a micro-liquid lubricant after melting.

[0154] Elongation at break under static tension, in percentages (%). -3 Tested under quasi-static strain rate conditions; this parameter is used to evaluate the degree of harmlessness of impurity elements under normal conditions. If harmlessness is not achieved, the elongation will decrease significantly due to brittle fracture.

[0155] Dynamic impact absorbed energy, in J; obtained by testing using the SHPB system under a strain rate of 1500 / s.

[0156] Table 3

[0157] Group Variable 1: Bi content Variable 2: Pb content Grain boundary micro-region slip displacement (nm) Elongation at break (%) Dynamic impact absorption energy (J) Example 1 0.3% 0.1% 125 11.5 48.5 Example 7 0.1% 0.1% 85 12.2 36.4 Example 8 0.5% 0.1% 142 10.1 45.2 Comparative Example 6 0.08% 0.1% 12 12.5 14.8 Comparative Example 7 0.55% 0.1% 25 4.2 11.5 Example 9 0.3% 0.05% 92 11.8 38.6 Example 10 0.3% 0.2% 138 10.5 46.8 Comparative Example 8 0.3% 0.03% 15 12 15.2 Comparative Example 9 0.3% 0.25% 18 3.8 9.6

[0158] The data in Table 3 reveals the critical abrupt change characteristics in the material composition design of this invention, providing experimental evidence for refuting the review opinions on conventional impurity content adjustments; the addition of bismuth (Bi) and lead (Pb) does not show a linear relationship of energy absorption increasing with content or safety decreasing with content, but rather there is a strict optimization range.

[0159] When the Bi or Pb content is below the lower limit of the protection range, the static elongation of the material remains good, but the dynamic impact absorption energy decreases significantly. This phenomenon is due to the insufficient density of nano-sized bismuth-lead particles at the grain boundaries, which cannot form a continuous micro-liquid lubricating film during high-speed impact adiabatic heating.

[0160] The key parameter, grain boundary micro-region slip displacement, shows that when the content is too low, the grain boundary slip is significantly reduced, causing the crack to be unable to undergo Z-shaped deflection and the phase transformation energy absorption mechanism to fail. This indicates that the lower limit value is the physical critical point for triggering the continuous liquid film lubrication effect.

[0161] When the Bi or Pb content exceeds the upper limit, the dynamic absorbed work and static tensile elongation after fracture of the material both decrease significantly, exhibiting obvious brittle characteristics. This is related to the physical limit of the ultrasonic cavitation crushing process, that is, when the total amount of immiscible Bi / Pb exceeds the critical upper limit, the ultrasonic energy density cannot break all the coarse droplets to below the nanoscale.

[0162] Unbroken micron-sized brittle Bi / Pb droplets form stress concentration sources in the matrix, causing brittle cleavage fracture of the material under static stress; the reduced grain boundary slip indicates that the material fractures in the early stages of impact and cannot play an energy absorption role.

[0163] The above comparative experiments show that the Bi 0.1%~0.5% and Pb 0.05%~0.2% ranges specified in this application are deeply coupled with the ultrasonic frequency oscillation + pulse quenching process; if the content exceeds the upper limit, it will break through the process crushing limit and lead to static brittle fracture, while if it is below the lower limit, it will be impossible to form a continuous liquid film and lead to dynamic energy absorption failure.

[0164] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for producing a high-toughness lightweight aluminum alloy die casting, characterized by, include: S1. Prepare a metastable aluminum alloy melt, which contains silicon, magnesium, manganese, iron, bismuth, lead, titanium, zirconium and the balance aluminum, wherein bismuth and lead are in a liquid-liquid immiscible state in the metastable aluminum alloy melt. S2. The metastable aluminum alloy melt is injected into the preheated die-casting mold cavity. Multi-dimensional physical field characteristic data of a specific area in the die-casting mold cavity are collected in real time. Based on the multi-dimensional physical field characteristic data, the solid fraction of the alloy slurry in the specific area is determined, and the semi-solid critical window where the solid fraction is within the preset range is identified. S3. During the duration of the semi-solid critical window, mechanical oscillation disturbance of ultrasonic frequency band is simultaneously applied to the alloy slurry in a specific area, and pulsed thermal extraction treatment with a preset phase difference from the mechanical oscillation disturbance is simultaneously applied to the specific area to induce cavitation effect to break up the coarse bismuth-lead droplets in the alloy slurry and freeze the bismuth-lead droplets at the grain boundary. S4. Stop mechanical vibration and pulsed heat extraction treatment, hold pressure until the alloy slurry is completely solidified, and open the mold to obtain high-strength, tough, and lightweight aluminum alloy die castings.

2. The method according to claim 1, wherein the method is characterized by: The composition of the metastable aluminum alloy melt, by mass percentage, is: silicon 7.5%~9.5%, magnesium 0.3%~0.6%, manganese 0.4%~0.8%, iron 0.6%~1.0%, bismuth 0.1%~0.5%, lead 0.05%~0.2%, titanium 0.05%~0.15%, zirconium 0.05%~0.12%, with the balance being aluminum and unavoidable impurities. The melting temperature is controlled at 720℃-760℃.

3. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: The specific area includes at least one of the following: the longitudinal beam transition area, the base of the mounting lugs, and the intersection of the reinforcing bars.

4. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: The steps for determining the solid fraction in step S2 specifically include: constructing a multi-dimensional feature fusion model, fusing the characteristic derivatives of the high-frequency impedance signal acquired in real time with the theoretical solid fraction based on the non-equilibrium solidification equation, using the Kalman filter algorithm to correct the hysteresis effect of temperature measurement, and outputting the accurate solid fraction of the alloy slurry in a specific region in real time.

5. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: In step S3, the mechanical oscillation disturbance is applied through an intelligent hollow extrusion pin system. The intelligent hollow extrusion pin system integrates a micro-channel with a diameter of 0.1 mm to 1 mm. The frequency of the mechanical oscillation disturbance is 100 Hz to 500 Hz, and the amplitude is 0.05 mm to 0.2 mm.

6. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: The cooling medium used in the pulsed thermal extraction process is a nanofluid, which is composed of high thermal conductivity nanoparticles, low viscosity base liquid, and surfactant. The high thermal conductivity nanoparticles are graphene, added at an amount of 0.5~2.0 vol%, dispersed in a 1:1 volume ratio of ethylene glycol and water in the base liquid, and sodium dodecylbenzenesulfonate is added as a surfactant to achieve high-density thermal extraction under microchannel laminar flow conditions.

7. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: The injection parameters in S2 are set as follows: injection speed 2 m / s~5 m / s, injection specific pressure 80 MPa~120 MPa; mold preheating temperature is set to 200~250℃; the specific area is the high stress area or energy absorption key node of the die casting, and a high-frequency impedance sensor and an infrared temperature sensor for collecting multi-dimensional physical field characteristic data are pre-embedded in this area.

8. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: In step S3, the mechanical oscillation disturbance and pulsed thermal extraction treatment applied in the ultrasonic frequency band are performed by an intelligent hollow extrusion pin system. The intelligent hollow extrusion pin system integrates a micro-flow channel and is connected to a high-frequency servo drive motor and a piezoelectric ceramic actuator at the tail. During the semi-solid critical window period, the control system drives the extrusion pin to output a mechanical oscillation frequency of 100 Hz to 500 Hz and an amplitude of 0.05 mm to 0.2 mm, and injects a pulsed cooling medium through the micro-flow channel.

9. The method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claim 1, characterized in that: The timing for stopping mechanical oscillation disturbance in step S4 is as follows: when the multidimensional feature fusion model detects that the solid fraction exceeds 75%, switch to static pressure mode and apply a holding pressure of 80 MPa to 120 MPa until complete solidification; the holding pressure is higher than the conventional feeding pressure to adapt to the local stress field generated by the dispersed nanodroplets at the grain boundaries and prevent micro-shrinkage.

10. A high-strength, high-toughness, lightweight aluminum alloy die-casting part, comprising the method for preparing a high-strength, high-toughness, lightweight aluminum alloy die-casting part according to claims 1-9, characterized in that: The die-cast part has a dual-modal microstructure, including a continuous rigid framework phase composed of an α-Al solid solution matrix and a magnesium silicide reinforcing phase, and a non-equilibrium nanoscale flexible energy-absorbing phase formed by the conversion of bismuth and lead. The nanoscale flexible energy-absorbing phase is distributed in a gradient, with bismuth-lead droplets of characteristic size of 50 nm to 200 nm pinned at the grain boundaries. Under static conditions, it does not constitute a crack source, but under dynamic impact, it undergoes a phase transformation and absorbs energy.

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