Method for constructing periodic directional solidification structure based on modulated ultrasound

Abstract

The invention discloses a method for constructing a periodic directional solidification structure based on modulated ultrasound, and relates to the technical field of metal material preparation. The method comprises the following steps: smelting a metal raw material to obtain a metal melt; the metal melt is subjected to directional solidification, and periodic modulation ultrasound is applied to the front edge of a liquid-solid interface of the metal melt in the directional solidification process; and after periodical modulation ultrasound is applied, a periodical directional solidification structure with the grain size controllable and columnar / equiaxed crystals alternately appearing is obtained. According to the method, the sound field characteristics at the front edge of the liquid-solid interface in the directional solidification process can be detected in real time, feedback adjustment is conducted, and it is guaranteed that multi-mode ultrasonic treatment is efficient, stable, accurate and controllable.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material preparation, and particularly to a method for constructing a periodic directional solidification structure based on modulated ultrasound. Background Art

[0002] A heterogeneous structure refers to a region within a material that has different compositional structures and significantly different mechanical properties, commonly referred to as soft and hard regions. During deformation, the mutual constraint and mechanical property incompatibility between the two result in significant heterogeneous deformation-induced strengthening and back stress hardening in the material during deformation, enabling metal materials to simultaneously possess high strength, high plasticity, and work hardening, thereby integrating the advantages of the two structures and overcoming the performance limitations of traditional homogeneous structure metals. Among them, a periodic heterogeneous structure can exhibit unique performance advantages macroscopically by periodically arranging different structural units, such as enhancing strength or toughness in a specific direction. However, there are still significant technical and process difficulties in precisely controlling the spatial distribution of the periodic heterogeneous structure.

[0003] Directional solidification is an advanced technology for preparing single-crystalline materials with orderly growth. By establishing a temperature gradient between the solidified solid phase and the non-solidified liquid phase, the liquid phase solidifies along a specific direction at a specific growth rate. In recent years, the columnar-equiaxed grain transition in directional solidification technology has attracted extensive attention. However, simply adjusting the heat flow parameters cannot quickly, efficiently, and real-time regulate the grain size and growth direction. The power ultrasonic field can generate various nonlinear effects such as cavitation, acoustic streaming, and vibration in the liquid alloy, and is an effective means for regulating the solidification structure and improving the application performance of alloys. Introducing modulated ultrasound at the solid / liquid interface of directional solidification can achieve the regulation of the directional solidification tissue structure, resulting in refined columnar grains and fine equiaxed grains and other tissue structures. It can be reasonably assumed that if directional solidification is combined with ultrasound, it is expected to achieve the controllable construction of a periodic columnar / equiaxed heterogeneous structure. Summary of the Invention

[0004] The technical problem to be solved by the present invention is how to provide a method for constructing a periodic directional solidification structure based on modulated ultrasound that can real-time detect the acoustic field characteristics at the front of the liquid-solid interface during the directional solidification process and perform feedback regulation to ensure the high efficiency, stability, accuracy, and controllability of multi-mode ultrasonic treatment.

[0005] To solve the above technical problem, the technical solution adopted by the present invention is: A method for constructing a periodic directional solidification structure based on modulated ultrasound, comprising the following steps:

[0006] Melting metal raw materials to obtain a metal melt;

[0007] Performing directional solidification on the metal melt, and applying periodic modulated ultrasound to the front of the liquid-solid interface of the metal melt during the directional solidification process;

[0008] After applying periodic modulated ultrasound, a periodic directional solidification structure with alternating columnar / equiaxed crystals is obtained.

[0009] A further technical solution lies in that the length period d of the periodic directional solidification structure is d = d1 + d2, where the length of the equiaxed crystal region is d1 and the length of the columnar crystal region is d2.

[0010] A further technical solution lies in that the periodic modulated ultrasound includes n cycles, where a single pulse period t = t1 + t2, t1 is the strong ultrasound action time, t1 = d1 / V, satisfying t1 > t1', t1' = L / V is the critical time required for equiaxed crystal growth; t2 is the weak ultrasound action time, t2 = d2 / V, satisfying τ is the time required to obtain columnar crystals through competitive growth.

[0011] A further technical solution lies in that within the single strong ultrasound action time t1, the transient cavitation sound intensity generated by the strong ultrasound at the front of the solid-liquid interface satisfies I c1 > I N I N is the sound intensity required for equiaxed crystal nucleation; within the weak ultrasound action time t2, the transient cavitation sound intensity satisfies 0 < I c2 < I N .

[0012] A further technical solution lies in that the method for measuring and feedback controlling the sound intensity of the strong and weak ultrasounds includes:

[0013] (1) When starting the pulsed ultrasound, detect the sound spectrum signal at the front of the liquid-solid interface, extract and analyze to obtain the transient cavitation sound intensity I, set and input the cavitation threshold I0, and adjust A0 so that I = I0 when starting, where A0 is the amplitude of the ultrasonic horn end face;

[0014] (2) During the directional solidification process, obtain the transient cavitation sound intensity I t at the front of the liquid-solid interface in real time, and adjust A0 through a computer so that under the strong ultrasound condition, I c1 > I N , and under the weak ultrasound condition, keep 0 < I c2 < I N .

[0015] The beneficial effects of adopting the above technical solution are as follows: In the method of the present invention, during the dynamic drawing process of directional solidification, pulsed ultrasonic vibration is applied to the crucible and acts on the front of the liquid-solid interface of the melt, so as to prepare a heterogeneous structure material covering any of the three structures of equiaxed / fine columnar / coarse columnar, realizing the active design of the expected structure, and significantly improving the mechanical properties of the prepared metal material. Aiming at the action effects of ultrasonic waves at different stages of the directional solidification process, the ultrasonic signals corresponding to different positions are detected in real time and feedback-regulated, ensuring the high efficiency, stability and controllability of ultrasonic treatment. Brief Description of the Drawings

[0016] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0017] Figure 1 is the main flow chart of the method described in the embodiment of the present invention;

[0018] Figure 2 is the comparison diagram of the microstructures of the alloys in Example 1 and Comparative Examples 1-2 of the present invention;

[0019] Figure 3 is the comparison diagram of the tensile properties of the alloys in Example 1 and Comparative Examples 1-2 of the present invention. Detailed Description of the Embodiments

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] Many specific details are set forth in the following description in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0022] Generally, as [[ID=I]] Figure 1 shown, the present invention discloses a method for constructing a periodic directional solidification structure based on modulated ultrasound, and the method includes the following steps:

[0023] Melting metal raw materials to obtain a metal melt;

[0024] Performing directional solidification on the metal melt, and applying periodic modulated ultrasound to the front of the liquid-solid interface of the metal melt during the directional solidification process;

[0025] After applying periodic modulated ultrasound, a periodic directionally solidified structure with alternating columnar / equiaxed grains is obtained.

[0026] The above steps will be described in detail below in conjunction with specific content.

[0027] The method of the present invention specifically includes the following steps:

[0028] 1) Prepare the master alloy raw material rod:

[0029] Melt the alloy raw materials into a master alloy ingot, and then cut out several raw material rods with diameters smaller than the inner diameter of the crucible from the master alloy ingot.

[0030] 2) Install the liquid metal replenishing device:

[0031] The specific steps are as follows:

[0032] i. Fix the main body of the replenishing device above the directional solidification platform through the bottom limit screw to ensure that the main body can move together with the directional solidification platform during the pulling process. The replenishing device includes five parts: a replaceable replenishing needle, a syringe, a stepper motor, a fixed base, and a program control terminal. Among them, the syringe, stepper motor, and fixed base are collectively referred to as the main body of the replenishing device; the directional solidification platform includes three parts: a heating coil, a heat insulation plate, and a liquid cooling tank; ii. Fix the injection needle on the injection head above the syringe; iii. Adjust the tip position of the injection needle so that it is completely immersed in the liquid metal, and the liquid metal is placed in the liquid cooling tank. iv. Connect the replenishing main body motor and the program control terminal through a data cable and set the corresponding parameters.

[0033] The material of the heat insulation plate can be quartz / boron nitride, and the thickness is about greater than 5 mm. The composition of the liquid metal can be Ga:In:Sn = 68.5%:21.5%:10% (wt.). It should be noted that this method is also applicable to other various alloy systems. The material of the replenishing needle can be a metal with a melting point above 1000 °C, stable physical and chemical properties, and does not react with the liquid metal, such as 304 stainless steel, etc. The international model of the needle is 10G, and the needle model is best when it can smoothly extrude the liquid metal and does not leak, and the shape is a flat-mouth tubular shape.

[0034] Furthermore, the replenishing speed v = k 比例系数 *v 实 , and the proportionality coefficient k is calculated by the formula

[0035] to ensure that the liquid metal liquid level is flush with the actual pulling position through program control, and to ensure the stability of the temperature gradient and the actual solidification speed; where R 变幅杆 is the radius of the end face of the horn, in mm; R 坩埚 is the outer radius of the crucible, in mm; and R 注射器内半径is the inner radius of the syringe, in mm; x is the number of steps per millimeter, in step / mm; y is the number of turns of the stepping motor per step, in n / mm; z is the pitch of the transmission stud in the stepping motor, in μm / n; v 实 is the actual drawing speed of directional solidification, in μm / s.

[0036] 3) Install the crucible in the ultrasonic directional solidification system.

[0037] The specific steps are as follows: Place the bottom of the crucible in the melting device on the cylindrical boss at the upper end face of the ultrasonic horn, then put the raw material rod into the crucible from above, and finally install the graphite heating sleeve, mullite insulation sleeve, quartz protective cover and high-frequency coil around the crucible in sequence.

[0038] Preferably, the crucible material can be high-purity corundum or graphite, with a shape of a round tube, a wall thickness of less than 2 mm, an inner diameter of the crucible > the radius of the cylindrical boss, and an outer diameter of the crucible < the outer diameter of the ultrasonic horn.

[0039] 4) Install the high-temperature resistant press head and adjust the pressing force.

[0040] The specific steps are as follows: Install the high-temperature resistant press head directly above the crucible, insert the boss at the lower end face of the press head into the crucible from above, connect the upper end face of the press head to the cylinder, and adjust the pressing force.

[0041] Preferably, the material of the high-temperature resistant press head should still maintain a strength higher than 50 GPa at (T L +200) K. It has a cylindrical appearance, with a through groove for placing the waveguide rod, and the boss at the lower end face can be inserted into the top of the crucible. A pressing device is equipped above to apply a pressing force F to fix the crucible.

[0042] Preferably, the pressing force F provided by the cylinder to press the crucible should satisfy 10 N < F < 20 N.

[0043] 5) Install the sound field detection device.

[0044] The specific steps are as follows: i. Insert the high-temperature resistant waveguide rod into the crucible through the through groove of the high-temperature resistant press head so that the test end face is on the central axis of the crucible; ii. Connect the other end of the high-temperature resistant waveguide rod to the acoustic sensing system; iii. Fix the overall connection system on the displacement stage to adjust the height of the test end face of the waveguide rod on the central axis of the crucible; iv. Fix the displacement stage on the directional solidification platform.

[0045] Preferably, the material of the high-temperature resistant waveguide rod is a metal with a melting point above 2000 °C and stable physical and chemical properties, such as tungsten, molybdenum, niobium, etc., and its diameter is less than 1.5 mm.

[0046] 6) Conduct atmosphere protection on the ultrasonic directional solidification system.

[0047] The atmosphere protection process is as follows: an inert gas is introduced for atmosphere protection.

[0048] Preferably, the inert gas can be argon, and the flow rate is 2 - 3 L / min.

[0049] 7) Turn on the high - frequency induction coil to heat up and reach the preset temperature for static heat preservation for a certain period of time.

[0050] Preferably, the preset temperature is a temperature 100 - 200 K higher than the liquidus line of the alloy, and the heat preservation and standing time is 20 - 40 minutes.

[0051] 8) Start the directional drawing and perform real - time feedback adjustment according to the sound field detection results. The specific steps are as follows:

[0052] i. After the heat preservation is completed, control the motor to make the heating platform move upward at a constant drawing speed V for static directional drawing, where V is 1 - 500 μm / s;

[0053] ii. After static directional drawing for a certain distance h, turn on the multi - mode ultrasound for ultrasonic directional drawing. The ultrasound is controlled by an ultrasonic modulation power supply and a resonance frequency real - time regulator.

[0054] Preferably, the ultrasonic intensity shows periodic strong - weak changes over time. The action time of the strong - ultrasound stage is t1, and the action time of the weak - ultrasound stage is t2, where t1 + t2 = T P is defined as the ultrasonic period, and t1 / (t1 + t2)*100% is defined as the ultrasonic duty cycle D [[ID=??]] c In the strong - ultrasound stage, the transient cavitation sound intensity I c1 > I N , I N is the sound intensity required for equiaxed crystal nucleation, and the length of the equiaxed crystal region is obtained as d1 = V*t1, t1 > t1', t1' = L / V is the critical time required for equiaxed crystal growth, and L is the average size of equiaxed crystal grains. In the weak - ultrasound stage, the transient cavitation sound intensity 0 < I c2 < I N , where I c = 0 means the ultrasound is turned off. The weak - ultrasound action time t2 should be greater than τ, where τ is the competition time required for the equiaxed crystal growth to stabilize and transform into a columnar crystal structure, and can be approximately calculated using the formula

[0055] ; where D L is the diffusion coefficient of solute atoms, k0 is the equilibrium segregation coefficient, v e is the sample moving speed, and δ is a dimensionless parameter representing the degree to which the system tends to a steady state. The length of the columnar crystal region is obtained as d2 = V*t2. It should be noted that there seems to be an unclear tag "??" in the original text which might be an error. If this is a crucial part for accurate understanding, it needs to be further clarified in the original source.

[0056] Furthermore, the specific working mode of the resonance frequency real-time regulator is as follows: i. The computer sends a working signal to the impedance analyzer every T0 time; ii. The impedance analyzer measures the resonance frequency of the ultrasonic directional solidification instrument as and feeds back the numerical signal to the computer. Generally, the resonance frequency ranges from 20 kHz to 100 kHz; iii. The computer then outputs the numerical signal to the ultrasonic drive power supply of the external AC signal source, and the ultrasonic drive power supply works with the new resonance frequency to replace the old resonance frequency ; where represents the resonance frequency measured by the impedance analyzer at time t; iv. The computer repeats the above steps, continuously searches for the optimal resonance frequency and makes the ultrasonic drive power supply vibrate according to the optimal frequency to achieve the most efficient ultrasonic application effect and prevent the power supply from overloading.

[0057] iii. Turn on the real-time detection of the sound field and regulate the ultrasonic parameters.

[0058] The real-time feedback adjustment process is as follows: When pulsed ultrasound is turned on, the acoustic spectrum signal at the front of the liquid-solid interface is detected by the high-temperature sound field detection device, the continuous spectrum signal is extracted and analyzed, and the transient cavitation intensity I c is obtained. Set and input the cavitation threshold I0, and adjust A0 so that I c = I0 when it is turned on. I0 can be determined according to the directional solidification experimental system; A0 is the amplitude of the end face of the horn; during the ultrasonic directional drawing process, the transient cavitation intensity I c -t corresponding to different drawing times is obtained, and the ultrasonic amplitude A0 is adjusted by the computer so that I c1 > I N during the strong ultrasonic action stage, and 0 < I c2 < I N during the weak ultrasonic action stage.

[0059] Furthermore, according to the preset different ultrasonic action section lengths and tissue morphologies, the transient cavitation intensity I c -t corresponding to different drawing times is obtained, and a heterogeneous structure material composed of different tissues such as coarse columnar crystals / fine columnar crystals / fine equiaxed crystals is prepared.

[0060] The above method will be described below with specific examples, comparative examples and test examples:

[0061] Example 1

[0062] This example is a method for ultrasonic preparation of Mg-1.5at.% Gd alloy with periodically directionally solidified structure, including the following steps:

[0063] Load 70 g of high-purity Mg blocks into a graphite crucible with an internal size of Φ30 mm × 120 mm, and conduct induction heating melting under the protection of argon with a gas flow rate of 2.5 L / min. After the melting temperature reaches 700 °C, hold for 5 min; then add 30 g of Mg-30Gd alloy blocks, increase the induction current, raise the melting temperature to 750 °C, and hold for 3 min to obtain a master alloy ingot. Cut a raw material rod with a diameter of 9.5 mm and a length of 100 mm from the master alloy ingot.

[0064] Place a graphite crucible with an inner diameter of 10 mm, a wall thickness of 2 mm, and a length of 140 mm on the first cylindrical boss at the upper end of the ultrasonic horn. The first cylindrical boss is inserted into the crucible from below, and the ultrasonic horn is made of 45# steel. Then place the two raw material rods into the crucible from above, and install a heating system around the crucible (graphite heating sleeve, mullite insulation sleeve, quartz protective cover, and high-frequency induction coil arranged in sequence on the periphery of the crucible). Install a high-temperature pressure head directly above the crucible. A second cylindrical boss is provided at the lower end of the high-temperature pressure head, and the second cylindrical boss is inserted into the crucible from above. The high-temperature pressure head is made of niobium. Adjust the pre-tightening force F through the cylinder provided on the upper end face of the high-temperature pressure head to make it satisfy 10 N < F < 20 N.

[0065] Put the high-temperature acoustic sensor probe into the crucible through the through groove of the high-temperature pressure head. The high-temperature acoustic sensor probe is made of molybdenum. The test end face of the high-temperature acoustic sensor probe is located on the central axis of the crucible, and ensure that it can move with the heat insulation plate during the pulling process through a displacement device. A cooling system is provided below the heat insulation plate. The cooling system consists of a liquid cooling tank and a liquid metal placed in the liquid cooling tank. The composition of the liquid metal is Ga:In:Sn = 68%:22%:10% (wt.).

[0066] Introduce argon into the quartz protective cover for atmosphere protection, and the argon flow rate is 2 L / min. Turn on the high-frequency induction, and heat up through the high-frequency induction coil. After the alloy melt in the crucible reaches 750 °C, hold and stand for 30 min. After the holding is over, move the displacement table under the high-temperature piezoelectric conversion device so that the test end face of the high-temperature acoustic sensor probe moves down until it touches the solid-liquid interface, and then move up 15 mm and lock the displacement table. Control the motor to make the positioning platform move upward at a constant pulling speed of V = 100 μm / s for static directional pulling. After static pulling for 3 cm, turn on the pulsed ultrasound, and the vibration treatment frequency of the ultrasound f0 = 20 kHz, and the initial amplitude A0 = 8.7 μm.

[0067] In this embodiment, the ultrasonic period t P = 15 s, where the time ratio of the strong ultrasound is 33%, that is, the strong ultrasound works for 5 s within a single cycle. Set the transient cavitation sound intensity I c1 = 1 kW / m2 ; In the weak ultrasound stage, the ultrasound works for 10 s, and the transient cavitation sound intensity I is set c2 = 5 W / m 2 . After the drawing is completed, turn off the power supply, wait for the sample to cool down and then take it out to obtain Mg 98.5 Gd 1.5 alloy.

[0068] Example 2

[0069] The preparation method is basically the same as that of Example 1, except that the ultrasound period is adjusted to t P = 45 s, in which the strong ultrasound works for 15 s and the weak ultrasound works for 30 s.

[0070] Comparative Example 1

[0071] The preparation method is basically the same as that of Example 1, except that no pulsed ultrasound is performed during the drawing process.

[0072] Test Example

[0073] As Figure 2 shown, the directional structures of Comparative Example 1 ( Figure 1 (a)) and Examples 1 and 2 ( Figure 1 (b), (c)) were compared. It was found that under the action of modulated ultrasound, a periodic columnar crystal / equiaxed crystal structure was prepared, and the effective lengths of both along the temperature gradient direction were consistent with the ultrasonic parameter settings.

[0074] The tensile properties of the alloys prepared in Examples 1 and 2 and Comparative Example 1 are as Figure 3 and Table 1 shown. The superelastic strain rate is the strain rate that continues to occur after the end of the yield stage of the tensile curve, that is, the strain rate value from the second sudden change of the stress / strain slope to the fracture of the specimen.

[0075] Table 1 Tensile properties of the alloys prepared in Examples 1 and 2 and Comparative Example 1

[0076] Condition (250 °C) Comparative Example 1 Example 1 Example 2 Tensile strength / MPa 59 73 78 Tensile strength / MPa 72 151 157 Elongation / % 35 29 16

[0077] From the comparison results, it was found that the yield and tensile strengths of the alloy at 250 °C in Example 1 were increased by 1.3 and 2.1 times respectively compared with Comparative Example 1, while maintaining good plasticity (that is, the stress / strain slope that continues to occur after the end of the yield stage is close to the deformation in the elastic deformation stage). The yield and tensile strengths of the alloy in Example 2 were further improved, but the plasticity decreased.

[0078] Based on the above comparison, in the method of the present invention, during the process of directional solidification with dynamic drawing, pulse ultrasonic vibration is applied to the crucible, acting on the front of the liquid-solid interface of the melt, to prepare a heterogeneous structure material that arbitrarily covers three types of structures: equiaxed / fine columnar / coarse columnar, realizing the active design of the expected structure, and significantly improving the mechanical properties of the prepared metal material. Aiming at the action effects of ultrasonic waves at different stages of the directional solidification process, the ultrasonic signals corresponding to different positions are detected in real time and feedback adjustment is carried out, ensuring the high efficiency, stability and controllability of the ultrasonic treatment.

[0079] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for constructing periodic directional solidification structures based on modulated ultrasound, characterized in that The method includes the following steps: Melting the metal raw materials to obtain a metal melt; Performing directional solidification on the metal melt, and applying periodic modulated ultrasound to the front of the liquid-solid interface of the metal melt during the directional solidification process; After applying the periodic modulated ultrasound, a periodic directional solidification structure with alternating columnar / equiaxed crystals is obtained.

2. The method for constructing a periodic directional solidification structure based on modulated ultrasound according to claim 1, wherein: The directional solidification is carried out under the condition of directional drawing, and the speed of directional drawing is 5-200 μm / s.

3. The method for constructing periodically directionally solidified structure based on modulated ultrasound according to claim 1, characterized in that: During the directional solidification process, the cold-end liquid metal alloy is replenished through a liquid replenishing device to keep the height of the melt surface constant.

4. The method for constructing a periodic directionally solidified structure based on modulated ultrasound according to claim 3, wherein: The fluid infusion rate v = k 比例系数 *v 实 , and the proportionality coefficient k is determined by the formula: Calculated; Where: R 变幅杆 is the end face radius of the horn in the liquid replenishing device, with the unit of mm; R 坩埚 is the outer radius of the crucible in the metal melting device, with the unit of mm; R 注射器内半径 is the inner radius of the syringe in the liquid replenishing device, with the unit of mm; x is the number of steps corresponding to the stepping motor per millimeter in the liquid replenishing device, with the unit of step / mm; y is the number of rotations of the stepping motor corresponding to each step, with the unit of n / mm; z is the pitch of the transmission stud in the stepping motor, with the unit of μm / n; v 实 is the actual pulling speed of directional solidification, with the unit of μm / s.

5. The method for constructing a periodic directionally solidified structure based on modulated ultrasound according to claim 1, characterized in that: The length period d of the periodic directional solidification structure = d1 + d2, where the length of the equiaxed crystal region is d1 and the length of the columnar crystal region is d2.

6. The method for constructing a periodic directional solidification structure based on modulated ultrasound according to claim 5, wherein: The periodic modulated ultrasound includes n cycles, where a single pulse period t = t1 + t2, t1 is the strong ultrasound action time, t1 = d1 / V, satisfying t1 > t1', t1' = L / V is the critical time required for equiaxed crystal growth; t2 is the weak ultrasound action time, t2 = d2 / V, satisfying τ is the time required for competitive growth to obtain columnar crystals.

7. The method for constructing periodic directionally solidified structure based on modulated ultrasound according to claim 6, wherein: During the action time t1 of a single high-intensity ultrasound, the transient cavitation sound intensity I generated by the high-intensity ultrasound at the front of the solid-liquid interface c1 > I N , I N is the sound intensity required for equiaxed crystal nucleation. The grain size D1 of the equiaxed crystal is negatively correlated with I c1 ; during the action time t2 of the weak ultrasound, the transient cavitation sound intensity 0 < I c2 < I N , and the columnar crystal spacing D2 is negatively correlated with I c2 .

8. The method for constructing a periodic directional solidification structure based on modulated ultrasound according to claim 7, wherein: The weak ultrasonic action time t2 is greater than τ, where τ is the competition time required for the equiaxed crystal growth to stabilize and transform into a columnar crystal structure, and can be approximately calculated using the formula ; where D L is the diffusion coefficient of solute atoms, k0 is the equilibrium segregation coefficient, v e is the specimen moving speed, and δ is a dimensionless parameter representing the degree to which the system tends to a steady state.

9. The method for constructing periodic directionally solidified structure based on modulated ultrasound according to claim 7, characterized in that, The periodic modulated ultrasound is applied through an ultrasonic directional solidification instrument, and its control method is as follows: The computer sends a working signal to the impedance analyzer every T0 time interval; The impedance analyzer measures the resonance frequency of the ultrasonic directional solidification instrument to be and feed the numerical signal back to the computer; The computer then outputs the numerical signal to the ultrasonic driving power supply of the external AC signal source, and the ultrasonic driving power supply operates at the new resonance frequency to replace the old resonance frequency ; where represents the resonance frequency measured by the impedance analyzer at time t; The computer repeats the above steps to continuously find the optimal resonance frequency and make the ultrasonic drive power vibrate at the optimal frequency.

10. The method for constructing periodic directionally solidified structure based on modulated ultrasound according to claim 7, characterized in that, The method for measuring and feedback controlling the sound intensity of the strong and weak ultrasound includes: (1) When the pulsed ultrasound is turned on, detect the sound spectrum signal at the front of the liquid-solid interface, extract and analyze to obtain the transient cavitation sound intensity I, set and input the cavitation threshold I0, and adjust A0 so that I = I0 when it is turned on, where A0 is the amplitude of the ultrasonic horn end face; (2) During the directional solidification process, the transient cavitation sound intensity I at the front of the liquid-solid interface is obtained in real time t , and by adjusting A0 through a computer, I is maintained under strong ultrasonic conditions c1 > I N , and 0 < I is maintained under weak ultrasonic conditions c2 < I N .

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