A magnesium alloy super-refining agent based on melt suspension and multi-physical field coupling, and a preparation method and application thereof

Abstract

The application discloses a magnesium alloy super-refining agent based on melt suspension and multi-physical field coupling and a preparation method and application thereof. Graphite powder, titanium-coated graphene nanosheet, magnesium powder and zirconium-containing salt are premixed, and cold-crucible suspension melting is carried out under a protective atmosphere, so that the zirconium salt is decomposed to generate nano Zr particles, and the titanium-coated graphene is in-situ generated to nano TiC particles, and a uniformly dispersed magnesium-based precursor melt is obtained. An ultrasonic field is applied in the heat preservation stage, and the nano particles are uniformly dispersed by relying on cavitation and acoustic streaming effect, and finally, the alloy rod refining agent is formed by on-line extrusion. The application initiates a suspension melting preparation process, relies on multi-phase nucleation and multi-physical field coupling for synergistic effect, the Zr element yield can reach more than 41%, which is about 50% higher than that of a conventional process, and the magnesium alloy grain refinement efficiency is more than 91%, and the application is suitable for high-end magnesium alloy component manufacturing in the fields of aviation, automobile, 3C electronics and the like.

Description

Technical Field

[0001] This invention relates to the fields of metallic materials and metallurgical technology, specifically to a magnesium alloy superfiner based on melt suspension and multi-physics field coupling, its preparation method, and its application. Background Technology

[0002] Magnesium alloys are the lightest metallic structural materials used in engineering applications, with significant value in aerospace, rail transportation, and new energy electronics. Conventional cast magnesium alloys have coarse grains, and are prone to defects such as shrinkage porosity and hot cracking during solidification, resulting in weaker mechanical and corrosion resistance properties, limiting their application in high-end engineering. Grain refinement is the core means to improve the microstructure of magnesium alloys and enhance their overall performance.

[0003] Traditional single-chemical refining processes suffer from limited refining effects, easy particle agglomeration, low yield of active ingredients, and poor tissue stability. Ultrasonic, electromagnetic, and other physical field-assisted refining technologies have gradually become research hotspots due to their green and controllable nature and excellent refining effects. However, existing technologies are mostly limited to local improvements of single-chemical refining or single-physical field assistance, and have not yet achieved deep coupling of multi-system, multi-physical field, and shaping refining processes, resulting in significant overall shortcomings in the technology.

[0004] In the area of ​​carbon-based and carbon-titanium composite grain refinement, existing patented technologies are generally limited in scope and have limited refining capabilities. CN121700209A uses a composite of micron-sized titanium powder and graphene nanosheets, coupled with electromagnetic pulse and ultrasonic dual-physical fields, to refine magnesium alloy grains to below 50μm. However, this system uses only graphene as a single carbon source and micron-sized titanium powder as a single titanium source, resulting in low Ti-C atom contact efficiency, insufficient in-situ reaction, a single nucleation core, and only yields conventional ingots without a sizing agent product, thus limiting its adaptability and stability. CN107904428B relies solely on graphene surface defects to achieve heterogeneous nucleation, without introducing zirconium-based and TiC high-strength nucleation systems, resulting in a single refining mechanism and a low upper limit for performance improvement. CN120230942A generates graphene / magnesium oxide biphase particles through in-situ gas-liquid reaction, achieving a refinement efficiency of over 80% for Mg-Al alloys. However, this technology does not involve a TiC particle strengthening system and does not combine external field synergistic enhancement, making it difficult to further improve the refinement depth and efficiency.

[0005] In zirconium-based particle refinement, traditional commercial Mg-Zr refiners often contain Zr as coarse micron-sized particles with severe agglomeration. These particles dissolve slowly and settle quickly in magnesium melts, resulting in an effective yield of less than 30%, severely limiting the refinement effect. Existing research has developed a highly efficient nano-Zr refinement system by controlling the dissolution-precipitation behavior of Zr through non-equilibrium solidification and solid-state phase transitions, improving refinement efficiency by over 70%. Simultaneously, it has been confirmed that the effective nucleation Zr particles are mainly at the 200–400 nm submicron scale, correcting the misconception of traditional micron-sized particles and providing theoretical support for the design of efficient zirconium-based particle refinement. CN121161075A employs composite zirconium salt synergistic ultrasonic refinement, alleviating Zr sedimentation problems and improving element utilization. However, this technology only optimizes a single Zr system, without incorporating a multi-carbon source in-situ composite mechanism or a styling agent preparation process, resulting in insufficient technological integration.

[0006] In terms of physical field coupling regulation, existing acousto-magnetic coupling technology can improve the dispersion of enhanced particles and suppress agglomeration. ZL200910302856.3 discloses a method for preparing in-situ particle-reinforced magnesium-based composite materials by electromagnetic / ultrasonic coupling. However, this method focuses on the overall preparation of composite materials and does not specifically optimize for the refining agent. It does not construct a multi-carbon source-Zr composite system, nor does it combine key processes such as melt suspension melting and alloy rod forming. Therefore, it cannot prepare highly stable and highly adaptable shaping refining agents, and its engineering practicality is limited.

[0007] In summary, existing technologies can only achieve single-dimensional improvements and generally suffer from defects such as a single nucleation system, low elemental reaction efficiency, poor particle dispersion, low yield of effective components, lack of stylistically refined products, and insufficient stability. Currently, there is no magnesium alloy super-refining agent technology that deeply integrates melt suspension melting technology, multi-carbon source-Zr composite raw material system, high-pressure-ultrasonic external field coupling and alloy rod forming process. Summary of the Invention

[0008] This invention aims to address the technical shortcomings of existing magnesium alloy grain refinement technologies, such as limited refinement efficiency of single carbon sources, easy agglomeration and sedimentation of multiphase particles, low Zr yield in zirconium-containing systems, insufficient external field synergistic effect, and unfavorable product form of refiners for industrial applications. It provides a magnesium alloy super refiner with multiphase synergy, external field coupling, and convenient use, as well as its preparation method and application.

[0009] To achieve the above objectives, this invention provides a method for preparing a magnesium alloy super grain refiner based on melt suspension and multi-physics field coupling. This method is the first to systematically apply cold crucible suspension melting technology to the field of magnesium alloy grain refiner preparation, and innovatively integrates a multi-carbon source-Zr composite raw material system, high-pressure environment, ultrasonic field synergistic treatment, and alloy rod forming process. Specifically, it includes the following steps: Step 1: Premixing raw materials Graphite powder, graphene nanosheets with a nano-metal Ti layer deposited on the surface, magnesium powder, and zirconium salt are premixed at a preset mass ratio to obtain a uniformly mixed powder.

[0010] The mass percentages of each raw material are as follows: graphite powder 0.5–5.0 wt.%, graphene nanosheets 0.05–1.0 wt.%, magnesium powder 70–95 wt.%, and zirconium salt 1.0–10.0 wt.%, wherein the effective zirconium content based on Zr element is 0.3–2.0 wt.%. The zirconium salt is selected from one or more of zirconium chloride, potassium fluorozirconate, or zirconium carbonate.

[0011] The graphene nanosheets are functionalized graphene with a nano-metallic Ti layer deposited on their surface via non-equilibrium magnetron sputtering or chemical plating, with a Ti layer thickness of 1–10 nm. This design allows graphene itself to serve as both a carbon source for TiC generation and a carbon precursor in the in-situ reaction, achieving atomic-level contact between the carbon and titanium sources. This fundamentally avoids the problems of excessive diffusion distance and incomplete reaction associated with the addition of micron-sized titanium powder in existing technologies. Combined with graphite powder, a dual-carbon source system is formed, which can generate abundant heterogeneous carbide nucleation sites in the melt, further expanding the number of nucleation sites.

[0012] Step Two: Suspension Melt Melting and Encapsulation The mixed powder obtained in step one is placed in a cold crucible suspension melting device. The cold crucible adopts a segmented structure, with each segment made of copper with an independent water-cooling system. A high-frequency electromagnetic field is generated by coupling a multi-layer conical ultra-strong suspension coil with an ultra-high frequency electromagnetic induction heating coil. The electromagnetic field frequency is 30–50 kHz, and the power is 30–60 kW, so that the material is in a completely suspended or quasi-suspended state in the electromagnetic field. A protective atmosphere, which is a mixture of argon and sulfur hexafluoride, is introduced simultaneously, with the SF6 volume concentration being 0.2–1.0%. The melting temperature is controlled at 720–820℃, the pressure inside the reactor is 1.0–1.5 MPa, and the holding time is 20–60 minutes.

[0013] Under these process conditions, magnesium powder is fully melted; zirconium salts undergo thermal decomposition to generate nano-Zr particles with a size of 30–150 nm; simultaneously, the Ti layer on the graphene surface reacts in situ with the C element in the graphene substrate to generate nano-TiC particles with a size of 50–200 nm; graphite powder reacts in the melt to generate magnesium carbide nucleation particles. These various particle types are dispersed together in the molten magnesium, forming a multiphase composite precursor melt.

[0014] The core advantages of suspension melting are: the material remains suspended throughout the process without contacting the crucible wall, completely eliminating crucible contamination and achieving high-purity melting; high-intensity electromagnetic stirring promotes homogenization of the melt composition and removes volatile impurities; electromagnetic levitation force effectively counteracts the density difference between Zr particles and magnesium melt, fundamentally inhibiting the sedimentation of high-density Zr particles and significantly improving Zr yield. Simultaneously, the 1.0–1.5 MPa high-pressure sealed environment significantly inhibits the volatilization and burn-off of high-temperature magnesium melt, improving melting stability and reaction sufficiency.

[0015] This invention strictly limits the melting process window: below 720℃, magnesium powder melts insufficiently and the in-situ reaction of TiC is difficult to proceed; above 820℃, nano-Zr particles coarsen rapidly and magnesium burn-off intensifies. A holding time shorter than 20 min results in incomplete reaction, while a time longer than 60 min induces particle agglomeration, coarsening, and sedimentation. This precise parameter range ensures the controllable generation and stable dispersion of nano-multiphase particles.

[0016] Step 3: Co-processing of the ultrasonic field During the melt holding stage, the ultrasonic transducer is activated to apply an ultrasonic field to the melt, with an ultrasonic frequency of 15–30 kHz, a power of 500–2000 W, and a treatment time of 8–12 min.

[0017] The cavitation and acoustic flow effects generated by ultrasound in the melt are the core mechanisms for grain refinement: the collapse of cavitation bubbles releases local high temperature, high pressure and impact, breaking dendrites and particle agglomerates, and greatly increasing the number of crystal nuclei; the acoustic flow effect drives the convection of the melt throughout the entire process, achieving uniform dispersion of nano-Zr and TiC particles and stably controlling the size of nanoparticles.

[0018] This invention constructs a triple physical field coupling system of "electromagnetic levitation – ultrasonic cavitation – acoustic flow macroscopic flow": electromagnetic levitation achieves macroscopic homogeneity and anti-settling, ultrasonic cavitation achieves micro-nano scale fragmentation and activation, and acoustic flow effect achieves uniform diffusion throughout the entire domain, forming a multi-dimensional synergistic enhancement mechanism from macroscopic melt regulation to nanoparticle modification.

[0019] The ultrasonic field can be single-frequency or dual-frequency ultrasound: single-frequency ultrasound is preferably 20 kHz, 1000 W, and processed for 10 min; dual-frequency ultrasound applies 15–20 kHz low-frequency and 25–30 kHz high-frequency ultrasound simultaneously, with low frequency enhancing macroscopic mixing and high frequency enhancing microscopic cavitation dispersion, resulting in a better refinement effect.

[0020] Step 4: Alloy Rod Forming The homogeneous composite melt, after ultrasonic coupling treatment, is cast into a rod-shaped mold via a coaxial online extrusion device. Magnesium alloy rods with diameters of 5–20 mm are then formed using extrusion casting or semi-continuous casting processes. After cooling and solidification, the shaped magnesium alloy superfine agent is obtained. The rod-shaped structure facilitates storage, transportation, and automated industrial feeding.

[0021] This invention also provides a magnesium alloy superfiner prepared by the above method, with magnesium as the matrix, in which in-situ generated nano-TiC particles, nano-Zr particles and exfoliated and dispersed graphene nanosheets are uniformly dispersed. The multiphase particles are non-agglomerated and uniformly distributed, forming a high-density multi-heterogeneous nucleation system; wherein the size of TiC particles is 50–200 nm and the size of Zr particles is 30–150 nm.

[0022] The rod-shaped alloy refining agent of this invention is added to the target magnesium alloy melt by mechanical feeding or melt feeding, with an addition amount of 0.2–1.5 wt.% of the total mass of the target magnesium alloy melt; the melt temperature is controlled at 700–750℃; after adding the agent, the mixture is stirred for 3–5 min, and then allowed to stand for 10–20 min before casting. The refining agent rapidly melts and releases multiphase nano-nucleation particles, synergistically constructing multiple heterogeneous nucleation cores, efficiently refining the solidification structure and suppressing casting defects.

[0023] Compared with the prior art, the present invention has the following substantial features and significant progress: Compared with existing magnesium alloy grain refinement technologies, this invention has the following significant innovations and technological advancements: (i) Pioneering cold crucible suspension melting process to achieve ultra-high purity melt. This invention is the first to apply cold crucible suspension melting technology to the preparation of magnesium alloy refining agents. It utilizes an electromagnetic field to achieve crucible-free suspension melting of materials, completely avoiding the problems of high-temperature leaching of impurities and interfacial reaction contamination associated with traditional graphite and ceramic crucibles, resulting in a high-purity magnesium-based composite melt. Simultaneously, the electromagnetic suspension enhances the surface activity of the melt, effectively promoting zirconium salt decomposition and in-situ Ti-C reactions, significantly improving reaction efficiency and product yield.

[0024] (ii) Overcoming industry pain points and significantly improving the effective yield of Zr element Traditional Mg-Zr master alloy refining techniques suffer from drawbacks such as Zr yield below 30%, easy refinement degradation, and failure in Al / Mn-containing magnesium alloys. This invention significantly improves Zr utilization through a triple synergistic mechanism of high-pressure in-situ nano-sizing, electromagnetic levitation to prevent sedimentation, and ultrasonic dispersion activation. In-situ decomposition of zirconium salt generates ultrafine nano-Zr particles; levitation force overcomes density differences to inhibit sedimentation; and the ultrasonic field effectively improves particle agglomeration and elemental segregation.

[0025] Experimental data show that the Zr element recovery rate of this invention can reach more than 41%, which is more than 50% higher than that of conventional processes, significantly reducing production costs and solving the industry bottleneck of low Zr utilization and poor stability of traditional refining agents.

[0026] (III) Constructing a multiphase composite heterogeneous nucleation system and refining the multidimensional synergistic mechanism. This invention constructs a quadruple heterogeneous nucleation network of "nano Zr + nano TiC + Al4C3 carbide + graphene". Compared with existing single TiC, single graphene, biphase particles or pure Zr refinement systems, the nucleation sites are more dense and the refinement mechanism is more complete.

[0027] The hierarchical nano / submicron Zr particles can simultaneously provide compositional supercooling and efficient nucleation sites; titanium-plated graphene achieves atomic-level contact between Ti and C, generating clean nano-TiC particles in situ, thus simultaneously improving the refining effect and corrosion resistance; graphite-derived Al4C3 particles supplement the nucleation sites; graphene generates a pinning effect at grain boundaries to inhibit grain growth, and multiple mechanisms work together to achieve the ultra-refining effect.

[0028] (iv) Deep coupling of three physical fields, breaking through traditional technical understanding and industry prejudices This invention constructs a triple physical field coupling system of "electromagnetic levitation – high-pressure environment – ​​ultrasonic cavitation" to achieve full-dimensional melt control from macroscopic homogeneity and mesoscopic dispersion to nanoscale activation. Non-contact high-purity melt reduces ultrasonic attenuation, high pressure enhances cavitation and inhibits magnesium volatilization, and the electromagnetic-ultrasonic synergy forms a dynamic equilibrium of "anti-settling and anti-agglomeration," resulting in a particle dispersion effect significantly superior to traditional electromagnetic stirring and atmospheric pressure dual-field processes.

[0029] This invention breaks through the inherent technical prejudices in the industry: it overcomes the common perception that suspension melting is not suitable for mass production of magnesium alloys and easily aggravates magnesium oxidation and volatilization. Through high-pressure synergistic protection, it reduces magnesium volatilization by about 40%, and utilizes the unique characteristics of suspension melting to solve the problem of Zr particle sedimentation. The technology is innovative and creative.

[0030] (v) Standardized alloy rod shape, suitable for industrial mass production applications. This invention prepares a shaped alloy rod refining agent with a diameter of 5–20 mm, overcoming the shortcomings of traditional powder refining agents such as easy oxidation, moisture absorption, dust generation, and unstable feeding. The alloy rod can be mechanically and automatically fed, seamlessly adapting to existing casting production lines. Furthermore, the internal particles are uniformly pre-dispersed, exhibiting excellent interfacial bonding. After feeding, the nucleated particles are released rapidly and have strong anti-fading capabilities, significantly improving its engineering practicality. Currently, there are no publicly reported similar forming processes applied to magnesium alloy refining agents.

[0031] (vi) Synergistic enhancement through multiple mechanisms to comprehensively improve the mechanical properties of magnesium alloys The synergistic refinement and strengthening of multiple nanoparticles and graphene can significantly optimize the solidification microstructure and mechanical properties of magnesium alloys. Application tests show that adding 0.5 wt.% of the refining agent of this invention to AZ91 magnesium alloy refines the as-cast grains from 400 μm to 35 μm, with a refining efficiency exceeding 91%; tensile strength increases by 31%, yield strength by 35%, elongation by 77%, and hardness by 17%, achieving a simultaneous improvement in both strength and plasticity of the magnesium alloy, resulting in significant overall performance optimization.

[0032] In summary, this invention innovatively integrates a multi-carbon source-Zr composite system, high-pressure suspension high-purity smelting, multi-physical field coupling control, and rod-shaped forming process. It solves the core problems of existing refining technologies, such as single nucleation, low element yield, easy particle agglomeration, unstable refining, and difficulty in mass production. It has high technical barriers, excellent refining effect, and strong industrial adaptability, and has broad application prospects in the field of high-end magnesium alloy component manufacturing. Attached Figure Description

[0033] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 is a schematic flowchart of the preparation method of magnesium alloy superfiner based on melt suspension and multi-physics field coupling. Figure 2 shows an electron microscope image of the microstructure of the refining agent alloy rod.

[0034] Figure 3 shows a comparison of the metallographic structure of AZ91 magnesium alloy before and after refining with a refining agent; In the figure: (a) is the as-cast microstructure of the unrefined AZ91 magnesium alloy (average grain size of about 400 μm), and (b) is the as-cast microstructure after adding 0.5 wt.% of the refining agent described in this invention (average grain size of about 35 μm). Detailed Implementation

[0035] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0036] Example 1: Basic Scheme (Single-frequency ultrasonic coupling suspension melting) This embodiment provides a method for preparing a magnesium alloy superfiner based on melt suspension and multi-physics field coupling. The specific steps are as follows: (1) Raw material ratio: 2.0 wt.% graphite powder, 0.3 wt.% graphene nanosheets with Ti layer deposited on the surface, 92.7 wt.% magnesium powder, and 5.0 wt.% zirconium salt (the effective zirconium content is about 1.3 wt.% based on Zr element). The graphene nanosheets are pre-deposited with a 1–5 nm thick nano-metal Ti layer by non-equilibrium magnetron sputtering process.

[0037] (2) Premixing: Place the above raw materials in a V-type mixer and mechanically mix for 2 hours to ensure that the components are evenly dispersed.

[0038] (3) Melt suspension melting and encapsulation under gas protection: The mixed powder is placed in a cold crucible suspension melting device, and after vacuuming, a mixed protective atmosphere of argon and 0.5 vol.% SF6 is introduced to start the suspension melting system. The cold crucible adopts a segmented structure, with each segment made of copper with an independent water-cooling system. A high-frequency electromagnetic field is generated by coupling a multi-layer conical ultra-strong suspension coil with an ultra-high frequency electromagnetic induction heating coil. The electromagnetic field frequency is 30–50 kHz and the power is 30–60 kW, so that the material is completely suspended in the electromagnetic field. The pressure inside the reactor is maintained at 1.2 MPa, and the temperature is raised to 780 ℃ at a heating rate of 15 ℃ / min and held for 30 minutes. The material is completely melted in the suspension state. Zirconium salt decomposes to generate nano-Zr particles, and the Ti layer on the surface of graphene reacts in situ with the carbon elements in graphene to generate nano-TiC particles. Graphite powder forms dispersed carbon particles in the melt. Electromagnetic levitation force inhibits Zr particle sedimentation, providing a key process guarantee for high Zr yield.

[0039] (4) Ultrasonic Co-processing: During the heat preservation stage, the ultrasonic transducer is activated to apply single-frequency ultrasonic waves with a frequency of 20 kHz and a power of 1000 W for 10 minutes. The ultrasonic waves generate strong cavitation and acoustic flow effects in the melt. The high temperature and impact force released by the contraction and collapse of cavitation bubbles cause the dendrites to break. The acoustic flow effect disperses the crystal nuclei to various parts of the melt. The Zr particles and TiC particles are broken up and uniformly dispersed in the melt. The Zr particle size is controlled at 50–120 nm, and the TiC particle size is controlled at 80–180 nm.

[0040] (5) Alloy rod forming: The treated melt is cast into a rod mold through a coaxial online extrusion device, and a magnesium alloy rod with a diameter of 10 mm is formed by semi-continuous casting. After cooling, the magnesium alloy superfine agent product is obtained.

[0041] Example 2: Dual-frequency ultrasound optimization scheme The difference between this embodiment and Embodiment 1 lies in the raw material ratio and the optimization of ultrasonic parameters. Details are as follows: Raw material ratio: 3.0 wt.% graphite powder, 0.5 wt.% graphene nanosheets, 89.5 wt.% magnesium powder, and 7.0 wt.% zirconium salt. The melting temperature was 800 ℃, the pressure inside the vessel was 1.5 MPa, the electromagnetic field frequency for suspension melting was 40 kHz, and the power was 45 kW. The ultrasonic parameters were set to a dual-frequency ultrasonic field of 18 kHz (low frequency) + 28 kHz (high frequency), with a total power of 1500 W and a processing time of 12 minutes. The dual-frequency ultrasonic field device has two piezoelectric transducers, capable of simultaneously applying two different frequencies of ultrasound to the alloy melt. Low-frequency ultrasound enhances macroscopic convection, while high-frequency ultrasound enhances microscopic cavitation, achieving simultaneous macroscopic homogenization of the melt composition and microscopic dispersion of nanoparticles. Other steps were the same as in Example 1.

[0042] In the alloy rods prepared using the above process, the Zr particle size is 40–100 nm, and the TiC particle size is 60–150 nm. The particle uniformity and dispersion are superior to those obtained by single-frequency ultrasonic treatment. ICP elemental analysis showed that the Zr yield reached 41.2%, which is more than 50% higher than that of conventional smelting processes (yield less than 30%), significantly reducing production costs and improving the stability of the refining effect.

[0043] Example 3: Low-content refinement verification This embodiment verifies the application effect of the grain refiner at low concentrations. The alloy rod prepared in Example 1 was added to the AZ91 magnesium alloy melt at a concentration of 0.2 wt.%, with other process conditions the same as in Example 2. The test results show that the average grain size of the as-cast AZ91 magnesium alloy was refined to approximately 58 μm, with a refinement efficiency exceeding 85%. This result verifies the effectiveness of the alloy rod refiner of the present invention at low concentrations, indicating its significant competitive advantage in cost-sensitive applications.

[0044] Comparative Example 1: Non-suspended smelting This comparative example provides a method for preparing a magnesium alloy grain refiner. The difference from Example 1 is that it does not employ suspension melting, but instead uses conventional resistance furnace melting (ceramic crucible). Other raw material ratios and process parameters are basically the same as in Example 1. The obtained grain refiner alloy rod was applied to AZ91 magnesium alloy at an addition amount of 0.5 wt.%, resulting in an average grain size of approximately 78 μm after refinement. The Zr element recovery rate was measured to be approximately 28%. Compared to Example 2 (grain size approximately 35 μm, Zr recovery rate 41.2%), both the refinement efficiency and Zr recovery rate decreased significantly. The grain size increased from approximately 35 μm to 78 μm, and the Zr recovery rate decreased from 41.2% to 28%, a reduction of nearly 50%. This demonstrates that suspension melting technology plays a crucial role in achieving efficient grain refinement.

[0045] Comparative Example 2: Graphene-free nanosheets This comparative example provides a method for preparing a magnesium alloy grain refiner, which differs from Example 1 in that graphene nanosheets are not added, and the amount of graphite powder is increased to 2.3 wt.%, while other raw material ratios and process parameters are basically the same as in Example 1. The obtained grain refiner alloy rod was applied to AZ91 magnesium alloy at an addition amount of 0.5 wt.%, resulting in an average grain size of approximately 102 μm after refinement. Compared with Example 2 (grain size approximately 35 μm), the grain refinement efficiency decreased significantly, and the grain size increased from approximately 35 μm to 102 μm. This demonstrates the originality of this invention in using graphene nanosheets with a surface-deposited Ti layer as the core carbon source, and the introduction of graphene nanosheets plays a key role in improving grain refinement efficiency.

[0046] Comparative Example 3: No ultrasonic field treatment This comparative example provides a method for preparing a magnesium alloy grain refiner. The difference from Example 1 is that ultrasonic field treatment is not performed during the heat preservation stage; instead, mixing is achieved solely through electromagnetic stirring during suspension melting. Other raw material ratios and process parameters are essentially the same as in Example 1. The obtained grain refiner alloy rod was applied to AZ91 magnesium alloy at an addition amount of 0.5 wt.%, resulting in an average grain size of approximately 96 μm and a Zr element recovery rate of approximately 35%. Compared to Example 2 (grain size approximately 35 μm), the grain size is more than doubled. These results indicate that ultrasonic field treatment plays a crucial role in achieving efficient dispersion of nanoscale particles and dissolution and activation of Zr particles; the combined effects of ultrasonic cavitation and acoustic flow are indispensable.

[0047] Comparative Example 4: No High-Pressure Environment The difference between this comparative example and Example 1 is that no high pressure was applied (atmospheric pressure melting, pressure 0.1 MPa), while other raw material ratios and process parameters were basically the same as in Example 1. The obtained grain refiner alloy rod was added to AZ91 magnesium alloy at a dosage of 0.5 wt.%, resulting in an average grain size of approximately 52 μm and a Zr yield of approximately 36%. Compared to Example 2 (grain size 35 μm, Zr yield 41.2%), the grain size increased from 35 μm to 52 μm, and the Zr yield decreased from 41.2% to 36%. These results indicate that a high-pressure environment plays a crucial role in suppressing magnesium volatilization and promoting Zr decomposition and nano-sizing. Simultaneously, the high-pressure environment increased the melt density and enhanced the intensity of the ultrasonic cavitation effect, further demonstrating the synergistic coupling effect of "high pressure-ultrasound".

[0048] Application example: The refining effect of the refining agent of this invention in AZ91 magnesium alloy. AZ91 magnesium alloy ingots were melted in an electric resistance furnace under a protective atmosphere of SF5 / CO2. After the melt temperature stabilized at 720 °C, alloy rods prepared in Example 1 were added to the melt using a mechanical feeding method at an alloy mass fraction of 0.5 wt.%, while simultaneously mechanically stirring at 100 rpm for 3 minutes. The melt was then heated to 740 °C, held at that temperature for 10 minutes, and then poured into a metal mold at approximately 700 °C to cool and solidify.

[0049] The test results show that: The average grain size of AZ91 magnesium alloy in the as-cast state without the addition of a grain refiner is about 400 μm; after adding the magnesium alloy super grain refiner of this embodiment, the average grain size of AZ91 magnesium alloy in the as-cast state is refined to about 35 μm, with a grain refinement efficiency of over 91%.

[0050] The refined AZ91 magnesium alloy has increased tensile strength from 210 MPa to approximately 275 MPa (an increase of approximately 31%), yield strength from 95 MPa to approximately 128 MPa (an increase of approximately 35%), elongation from 3.5% to approximately 6.2% (an increase of approximately 77%), and hardness from 65 HV to approximately 76 HV (an increase of approximately 17%).

[0051] In summary, this invention successfully prepared a magnesium alloy super-refining agent with high stability, high utilization rate, and high refining efficiency by integrating four core technologies: a multi-carbon source-Zr composite raw material system, cold crucible melt suspension melting, high-pressure-ultrasonic multi-physical field coupling, and alloy rod forming. Multiple comparative examples verified the synergistic effect between suspension melting, the titanium-plated graphene dual-carbon source system, ultrasonic coupling treatment, and the high-pressure environment. This invention's refining agent requires low dosage and has high refining efficiency, simultaneously improving the strength, plasticity, and hardness of magnesium alloys. It is suitable for the large-scale preparation of high-end lightweight magnesium alloy components in aerospace, rail transportation, new energy, and 3C electronics industries.

[0052] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a magnesium alloy superfiner based on melt suspension and multi-physics field coupling, characterized in that, Includes the following steps: S1. Raw material premixing: Graphite powder, graphene nanosheets with a nano-metal Ti layer deposited on the surface, magnesium powder and zirconium salt are premixed to obtain a uniformly mixed powder. S2, Suspension melt smelting: The mixed powder obtained in step S1 is placed in a cold crucible suspension smelting equipment. The material is smelted in a suspended or quasi-suspended state under a protective atmosphere and pressure environment, so that the magnesium powder is fully melted, and the zirconium salt is thermally decomposed to generate nano Zr particles. At the same time, the graphene nanosheets with nano Ti layers deposited on the surface react in situ with the carbon source to generate nano TiC particles, forming a composite precursor melt in which nano Zr particles and nano TiC particles are uniformly dispersed in molten magnesium. S3. Ultrasonic external field coupling treatment: Apply ultrasonic field-assisted treatment to the composite melt during the melting and heat preservation stage to break up particle agglomeration and improve the dispersion uniformity of multiphase particles. S4. Alloy rod forming: The composite melt processed in step S3 is sized and formed, and after cooling and solidification, the shaped rod-shaped magnesium alloy superfine agent product is obtained.

2. The preparation method of the magnesium alloy superfiner based on melt suspension and multi-physics field coupling as described in claim 1, characterized in that, The mass percentages of each raw material in step S1 are as follows: graphite powder 0.5–5.0 wt.%, graphene nanosheets 0.05–1.0 wt.%, magnesium powder 70–95 wt.%, and zirconium salt 1.0–10.0 wt.%; the zirconium salt is selected from one or more of zirconium chloride, potassium fluorozirconate, or zirconium carbonate, and the amount of zirconium salt added satisfies that the effective zirconium addition in the final product, calculated as Zr, is 0.3–2.0 wt.%.

3. The preparation method of the magnesium alloy superfiner based on melt suspension and multi-physics field coupling as described in claim 1, characterized in that, The thickness of the nano-metallic Ti layer deposited on the surface of the graphene nanosheets is 1–10 nm.

4. The preparation method of the magnesium alloy superfiner based on melt suspension and multi-physics field coupling as described in claim 1, characterized in that, In step S2, the melting temperature is 720–820℃, the pressure inside the melting chamber is 1–1.5 MPa, and the holding time is 20–60 min; the protective atmosphere is a mixture of argon and sulfur hexafluoride, wherein the volume concentration of sulfur hexafluoride is 0.2–1.0%.

5. The preparation method of the magnesium alloy superfiner based on melt suspension and multi-physics field coupling as described in claim 1, characterized in that, The ultrasonic field processing parameters in step S3 are: frequency 15–30 kHz, power 500–2000 W, and processing time 8–12 min.

6. The preparation method of the magnesium alloy superfiner based on melt suspension and multi-physics field coupling as described in claim 1, characterized in that, The diameter of the alloy rod prepared in step S4 is 5–20 mm.

7. A magnesium alloy superfiner prepared by the method according to any one of claims 1-6, characterized in that, The microstructure of the refining agent is based on magnesium, in which in-situ generated nanoscale TiC particles, nanoscale Zr particles and exfoliated graphene nanosheets are dispersed. The multiphase particles are uniformly distributed without obvious agglomeration, forming a multi-heterogeneous nucleation system. The size of the TiC particles is 50–200 nm, and the size of the Zr particles is 30–150 nm.

8. The application of the magnesium alloy super-refining agent as described in claim 7 in the grain refinement of magnesium and magnesium alloys, characterized in that: Adding rod-shaped alloy refining agent to the target magnesium alloy melt allows for the synergistic construction of multiple heterogeneous nucleation cores through the release of nano-Zr particles, nano-TiC particles, and graphene nanosheets from the melt dissolution. This efficiently refines the solidification structure of the magnesium alloy and inhibits the formation of coarse grains and casting defects.

9. The application according to claim 8, characterized in that, The alloy rod is added to the target magnesium alloy melt by mechanical feeding or melt feeding, and the amount added is 0.2–1.5 wt.% of the total mass of the target magnesium alloy melt.

10. The application according to claim 8, characterized in that, The target magnesium alloy melt temperature is controlled at 700–750℃. After adding the alloy rod, stir for 3–5 minutes, let stand and keep warm for 10–20 minutes, and then cast into shape.

Images