A round wire preparation method and rolling die for REBCO stacked conductor

Abstract

The present invention belongs to the technical field of high-temperature superconductivity, and specifically relates to a round wire preparation method and a rolling die for a REBCO stacked conductor. The strip is stacked to form a square conductor, the coating material is pre-rolled, and the initial rolling forming is followed by rolling again. The main body of the rolling die is a cylinder, and a rolling structure is processed in the middle of the outer side. The rolling structure is three parallel grooves processed along the circumference of the middle of the cylinder. The rolling die is suitable for the preparation of round wires of REBCO stacked conductors, and the rectangular REBCO multi-layer stacked conductors are coated and packaged into round conductors, so as to be used for winding various types of high-temperature superconducting magnets, such as fusion reactor magnets, accelerator magnets, detector magnets, mineral processing magnets, motors, transformers, etc. The mechanical strength of the coating material is reduced by heating, which is more suitable for the temperature-sensitive characteristics of the second-generation high-temperature superconducting tapes.

Description

Technical Field

[0001] The invention belongs to the technical field of high-temperature superconductors, and in particular relates to a round wire preparation method and a rolling die for a REBCO (second generation high-temperature superconductor) stacked conductor. Background Art

[0002] High-temperature REBCO tapes have an operating temperature and critical magnetic field far higher than low-temperature superconductors, providing a better choice for building high-field magnets, such as fusion reactor magnets, accelerator magnets, and detector magnets. REBCO high-temperature superconductors are multi-layer composite materials in the form of ribbons. The current-carrying capacity of a single ribbon is limited. To meet the requirements of high-current applications, multiple REBCO tapes need to be used in parallel. The ribbon structure makes it impossible for REBCO tapes to be twisted like traditional cables. Therefore, they need to be wound around a round core rod or stacked for rounding. The round REBCO conductor is then twisted to make a high-current conductor. Stacked conductors are generally stacked flat using one or more specifications of REBCO tapes. When stacking multiple specifications of tapes, the narrower tapes are stacked on the outside, making the stacked conductors closer to a circle.

[0003] REBCO is composed of multiple layers of materials such as a base layer, a superconducting layer, a stabilizing layer, and a protective layer. Although it has high axial tensile strength and vertical compressive strength, its superconducting layer is a brittle ceramic material, and the connection with the base material that provides mechanical strength is fragile, resulting in poor ability of the conductor to withstand shear and peel forces after stacking and welding. If the stacked conductors are directly used to twist cables, on the one hand, the corners of the conductors will be severely squeezed, and on the other hand, the non-uniform force may cause a large shear force along the plane of the strip. The combined effect of these two aspects will destroy the structure of the superconducting layer, causing the current carrying capacity of the REBCO stacked conductor to be severely reduced or even lose its superconductivity. Therefore, the stacked conductors must be prepared into round wires before they can be further twisted into high-current conductors.

[0004] The existing methods for preparing REBCO conductors into round shapes are mainly: wrapping with thin wires or thin strips (copper wire, copper foil, aluminum foil, solder wire, etc.), which has low preparation efficiency and low mechanical strength; wrapping with solder and then pressing, wrapping with a semicircular shell and then welding, and filling with arc materials to seal, which requires thicker coating materials and has low engineering current density. Summary of the invention

[0005] The purpose of the present invention is to provide a round wire preparation method and a rolling die for a REBCO stacked conductor, which can cover and package a rectangular REBCO multi-layer stacked conductor into a round conductor, so as to be applied to winding various high-temperature superconducting magnets, such as fusion reactor magnets, accelerator magnets, detector magnets, mineral processing magnets, motors, transformers, etc.

[0006] The technical solution of the present invention is as follows:

[0007] A method for preparing a round wire of a REBCO stacked conductor comprises the following steps:

[0008] Step 1, select a REBCO strip, clean it, apply flux on the upper and lower surfaces of the REBCO strip, immerse it in liquid solder, and take it out after the surface of the REBCO strip is evenly covered with solder;

[0009] REBCO strip width is 1-5mm, thickness is 0.05-0.2mm;

[0010] The liquid solder is selected from tin, lead, silver and their alloys, with a melting point not exceeding 250°C;

[0011] Step 2: Stack the strips to form a square conductor

[0012] The REBCO strips coated with liquid solder are stacked into a square conductor with a multi-layer structure;

[0013] Step 3: Pre-rolling of the cladding material

[0014] The flat belt specifications of the coating material are selected to be 3-12mm wide and 0.4-1.6mm thick;

[0015] The two flat strips of the coating material are preformed by rolling using a rolling die, so that the cross section of one side of the preformed flat strip of the coating material is a convex semicircle, and the other side is a concave trapezoid;

[0016] Step 4: Preliminary roll forming

[0017] Two preformed flat strips of covering material cover the square conductor so that the strip plane of the square conductor is parallel to the bottom surface of the concave trapezoid on one side of the corresponding flat strip of covering material;

[0018] Heat the flat strip of coating material to 150-200℃;

[0019] The flat strip of coating material and the square conductor are rolled multiple times by using a die, so that the flat strip of coating material wraps the square conductor to form a conductor with a circular cross section;

[0020] Step 5: Rotate the conductor, which now has a circular cross section, 90 degrees along the axis and roll it again.

[0021] The rolling dies are the first rolling dies and the second rolling dies; both of them are cylinders with a through hole processed on the central axis; three parallel grooves are processed circumferentially in the middle of the outer surface of the first rolling die, namely the central groove and two discharge grooves located on both sides of the central groove; the cross-sections of the three grooves are all semicircular; a W-shaped protrusion is formed in the middle of the cylindrical side of the first rolling die, and the two side surfaces of the W-shaped protrusion are the side surfaces of the cylinder itself; the highest points on both sides of the central groove are lower than the heights on both sides; a boss is processed circumferentially in the middle of the outer surface of the second rolling die; the cross-section of the boss is trapezoidal, and the top width of the boss is smaller than the bottom width.

[0022] The step 3 of pre-forming and rolling the two flat strips of the coating material by using a rolling die refers to rolling the flat strips of the coating material into a shape by using a first rolling die and a second rolling die, with one side of the flat strip of the coating material close to the outside of the boss of the second rolling die, and the other side of the flat strip of the coating material facing the central groove of the first rolling die.

[0023] The step 4 of using a mold to roll the flat strip of coating material and the square conductor for multiple passes refers to using two first rolling dies to roll the two flat strips of coating material coated with the square conductor for multiple passes until the corresponding two side surfaces of the two first rolling dies are tightly fitted, the two first rolling dies are placed side by side, a gap is reserved between them, and the axes are parallel; the two flat strips of coating material coated with the square conductor are placed in the gap between the two first rolling dies, so that the outer raised semicircles on the two flat strips of coating material are respectively located in the central grooves of the corresponding first rolling dies.

[0024] The cross section of the boss is a trapezoid, the upper width of the trapezoid is 1.05 times the width of the strip, the lower width is 1.1 times the width of the strip, and the height of the trapezoid is 0.35-0.45 times the width of the strip.

[0025] The diameter of the central groove is 1.5-1.8 times the width of the strip, and the diameter of the discharge grooves on both sides is 2-3 times the width of the strip; the height difference between the highest point on both sides of the central groove and the two sides is 0.05-0.2 times the width of the strip.

[0026] In the step 1, the REBCO strip is cleaned with a cleaning agent, and then cleaned after soaking and blown dry with air at room temperature. The cleaning agent is 916 copper cleaning brightener, and the soaking time is 20-120s.

[0027] The soldering flux in step 1 is NC-559.

[0028] The liquid solder in step 1 is Sn 63 Pb 37 Liquid, temperature is 185-250℃.

[0029] After the square conductors are stacked in step 2, the temperature is maintained at 185° C. to 250° C.

[0030] In the step 2, the square conductor is vigorously blown to clean the excess solder, the blowing speed is 50-120m / s, the temperature of the square conductor is maintained at 185-250°C during the blowing process, and it is naturally cooled after the blowing is completed.

[0031] The material of the flat strip of coating material in step 3 is selected from copper, aluminum, tin, lead, silver or an alloy containing one or more of the above metal elements.

[0032] In the step 4, the two first rolling dies are heated and kept warm at 150-170°C.

[0033] In step 4 and step 5, the two side surfaces of the first rolling die are parallel to the plane of the strip.

[0034] In step 4 and step 5, the rolling pressure is 100-300 MPa.

[0035] Before proceeding to step 5, the excess webbing that enters the discharge chute is cut off.

[0036] A rolling die for preparing REBCO stacked conductor round wires, wherein the rolling structure comprises three parallel grooves processed along the circumference of the middle of a cylinder, namely a central groove and two discharge grooves located on both sides of the central groove, the cross sections of the three grooves are all semicircular, a W-shaped protrusion is formed in the middle of the cylindrical side surface of the first rolling die, and the two side surfaces of the W-shaped protrusion are the side surfaces of the cylinder itself;

[0037] The highest points on both sides of the central groove are lower than the heights of the two sides.

[0038] The rolling structure is a boss processed along the circumferential direction in the middle of the outer surface of the cylinder, the cross section of the boss is trapezoidal, and the top width of the boss is smaller than the bottom width.

[0039] The remarkable effects of the present invention are as follows:

[0040] This method uses a rolling process to complete the preparation. Rolling is a commonly used profile production process, but the mechanical and thermal properties of ordinary profiles (aluminum, copper, steel, alloys, etc.) are very different from those of the second-generation high-temperature superconducting tape. Ordinary profiles are mechanically isotropic and non-thermally sensitive materials, while the second-generation high-temperature superconducting tape is a multi-layer composite material. When stress or temperature control is improper, the second-generation high-temperature superconducting tape will lose its superconductivity.

[0041] The mold designed by this method, the selection of materials, and the determination of temperature are conducive to the stress control during the rolling process of the second-generation high-temperature superconducting tape. The second-generation high-temperature superconducting tape is a multi-layer composite material, and its superconducting layer is a brittle ceramic material. The connection with the matrix material that provides mechanical strength is fragile, resulting in the poor ability of the conductor to withstand shear and peel forces after stacking and welding. The vertical pressure that high-temperature superconducting tape can withstand exceeds hundreds of megapascals, and when the shear or peeling stress exceeds tens of megapascals, the superconducting layer structure of the high-temperature superconducting tape will be destroyed, thereby losing superconductivity. Therefore, it is necessary to strictly control the magnitude and direction of the rolling stress during rolling, and comprehensively consider the influencing factors such as the design of the mold, the mechanical properties of the coating material, the rolling temperature, the rolling passes, and the installation direction of the second-generation high-temperature superconducting tape stacked conductor round wire during rolling.

[0042] This method uses temperature increase to reduce the mechanical strength of the coating material, which is more suitable for the temperature-sensitive characteristics of the second-generation high-temperature superconducting tape. The second-generation high-temperature superconducting tape is a temperature-sensitive material. When the temperature exceeds a certain value for a long time (typically 200°C), the high-temperature superconducting tape may lose its superconductivity. Therefore, the mechanical strength of the coating material is reduced by temperature increase. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a cross-sectional view of a square conductor;

[0044] Figure 2a is a three-dimensional structural diagram of the first rolling die;

[0045] Figure 2b is a side view of the first rolling die;

[0046] Figure 3a is a three-dimensional structural diagram of the second rolling die;

[0047] Figure 3b is a side view of the second rolling die;

[0048] Figure 4 A schematic diagram of pre-rolling a flat strip of a coating material using a rolling die;

[0049] Figure 5 It is a schematic diagram of the shape of the flat strip of the coated material after pre-rolling;

[0050] Figure 6 Schematic diagram of a square conductor wrapped with a flat ribbon of coating material;

[0051] Figure 7 A schematic diagram of multi-pass rolling using a rolling die;

[0052] Figure 8 It is a schematic diagram of a circular conductor with extra flat ribbon;

[0053] Fig. 9This is the final schematic diagram of the circular conductor;

[0054] In the figure: 1. Square conductor; 1-1 Strip plane; 2. First rolling die; 2-1. Center groove; 2-2. Discharge groove; 2-3. W-shaped protrusion; 2-4. Two side surfaces; 3. Second rolling die; 3-1. Boss; 4. Shape; 4-1. Concave trapezoid; 4-2. Raised semicircle; 5. Conductor; 5-1. Excess flat strip; 6. Round conductor. DETAILED DESCRIPTION

[0055] The present invention will be further described below through the accompanying drawings and specific implementation methods.

[0056] Step 1: Material preparation

[0057] 1.1. Select REBCO strip

[0058] The width and thickness of the strip need to be selected within a certain range so that the stacked conductor is closer to a square.

[0059] The preferred REBCO strip has a width of 1-5 mm and a thickness of 0.05-0.2 mm. In this embodiment, 20 strips are selected.

[0060] 1.2. Use a cleaning agent to clean the REBCO strip, soak it, clean it, and blow it dry with air at room temperature

[0061] Select 916 copper cleaning and brightening agent as the cleaning agent, and soak for 20-120s.

[0062] 1.3. Coat the upper and lower surfaces of the REBCO strip with flux and immerse it in liquid solder. Take it out after the surface of the REBCO strip is evenly covered with solder.

[0063] The temperature of the liquid solder does not exceed 250° C. to avoid excessively high temperature from damaging the superconductivity of the REBCO strip.

[0064] The liquid solder is selected from tin, lead, silver and their alloys, and the melting point does not exceed 250°C.

[0065] In this embodiment, Sn is selected 63 Pb 37 As liquid solder, the upper and lower surfaces of the REBCO strip are coated with NC-559 flux and then immersed in Sn 63 Pb 37 Liquid, the surface of REBCO strip is evenly covered with Sn 63 Pb 37 Remove after liquid; select Sn 63 Pb 37 The liquid temperature is 185-250°C to avoid excessive temperature that would destroy the superconductivity of the REBCO tape.

[0066] Step 2: Stacking the strips to form a square conductor 1

[0067] 2.1 After the REBCO strips coated with liquid solder are neatly laid and stacked, they are pressed into a square conductor 1 and kept at a temperature above 185°C so that the solder of the strips is kept in a liquid state during stacking. Ensure that the square conductor 1 after pressing is flat and dense, as shown in the figure. Figure 1 shown.

[0068] When stacking, the REBCO strips in the center and near the center can be replaced with copper strips of the same specification to increase the heat transfer efficiency of the square conductor and improve its thermal stability.

[0069] The Sn 63 Pb 37 , REBCO strips with a temperature of 185-200 degrees Celsius are neatly laid and stacked, and compacted to a cross-sectional area of ​​2*2mm 2 Square conductor 1; Keep the strip temperature above 185℃ when stacking, which can effectively ensure the Sn of the strip 63 Pb 37 The conductor is in a liquid state, ensuring that the square conductor 1 is flat and dense after being compressed.

[0070] 2.2 Use a hot air knife to forcefully blow clean the four sides of the square conductor 1 to remove excess solder. The blowing speed is 50-120m / s. After blowing, the surface of the square conductor is flat;

[0071] The flat surface of the square conductor 1 is a necessary prerequisite for ensuring that the REBCO strip is subjected to uniform force during rolling.

[0072] During the purging process, the temperature of the square conductor 1 is maintained at 185-250° C., and it is cooled naturally after the purging is completed.

[0073] Step 3: Pre-rolling of the cladding material

[0074] 3.1 Select the flat belt of covering material, clean it and let it dry naturally.

[0075] The coating material is selected from metals and their alloys with low hardness and high ductility, and can be pure copper, aluminum, tin, lead, silver and their alloys.

[0076] The specifications of the flat belt of the covering material are 3-12mm in width and 0.4-1.6mm in thickness;

[0077] In this embodiment, a Sn with a width of 6 mm and a thickness of 0.8 mm is selected. 63 Pb 37 Flat belt: Use alcohol to clean the upper and lower surfaces of the belt and let it air dry naturally after cleaning.

[0078] 3.2 Determine the rolling die.

[0079] The method requires two rolling dies, namely a first rolling die 2 and a second rolling die 3 .

[0080] like Figure 2a and Figure 2b As shown, the main body of the first rolling die 2 is a cylinder with a through hole processed along the central axis, and three parallel grooves are processed along the circumferential direction in the middle of the outer surface of the cylinder, namely the central groove 2-1 and two discharge grooves 2-2 located on both sides of the central groove 2-1, and the cross-sections of the three grooves are all semicircular. The central groove 2-1 is located in the middle of the cylinder as a roll forming groove, and its diameter is 1.5-1.8 times the width of the strip, and the diameter of the discharge grooves 2-2 on both sides is 2-3 times the width of the strip. Since the cross-sections of the three grooves are all semicircular, a W-shaped protrusion 2-3 is formed in the middle of the cylindrical side of the first rolling die 2, and the two side surfaces 2-4 of the W-shaped protrusion 2-3 are the side surfaces of the cylinder itself. The middle top height of the W-shaped protrusion 2-3 (i.e., the highest point on both sides of the central groove 2-1) is lower than the two sides 2-4, and the height difference is 0.05-0.2 times the width of the strip.

[0081] like Figure 3a and Figure 3b As shown, the main body of the second rolling die 3 is a cylinder with a through hole processed along the central axis, a boss 3-1 is processed circumferentially in the middle of the cylindrical outer surface, the cross-section of the boss 3-1 is a trapezoid, the upper width of the trapezoid (the top width of the boss 3-1) is 1.05 times the strip width, the lower width (the bottom width of the boss 3-1) is 1.1 times the strip width, and the height of the trapezoid (the height of the boss 3-1) is 0.35-0.45 times the strip width.

[0082] In this embodiment, the diameter of the central groove 2-1 is selected to be 3.2mm, the diameter of the discharge grooves 2-2 on both sides is selected to be 5mm, and the W-shaped protrusion 2-3 is 0.2mm lower than the planes of the two sides 2-4; the upper width of the boss 3-1 is selected to be 2.1mm, the lower width of the boss 3-1 is selected to be 2.2mm, and the height of the boss 3-1 is selected to be 0.8mm.

[0083] 3.3 Use a rolling die to pre-roll the flat strip of the coating material into shape 4

[0084] like Figure 4 and Figure 5 As shown, the flat strip of the coating material is rolled into a shape 4 by using a first rolling die 2 and a second rolling die 3, that is, the cross-section of one side of the preformed flat strip of the coating material is a convex semicircle 4-2, and the other side is a concave trapezoid 4-1.

[0085] During processing, one side of the flat strip of the coated material is close to the outside of the boss 3-1 of the second rolling die 3, and the other side of the flat strip of the coated material faces the central groove 2-1 of the first rolling die 2. The central mounting shaft of the die rotates, and the two dies roll the coated material, so that under the rolling of the dies on both sides, the cross section of one side of the flat strip of the coated material forms an inwardly concave trapezoid 4-1, and the cross section of the other side forms a convex semicircle 4-2.

[0086] Pre-press and form two flat strips of covering material according to the above method;

[0087] Step 4: Preliminary roll forming

[0088] 4.1 Two flat strips of covering material pre-pressed into shape 4 are wrapped around the square conductor 1, as shown in FIG. Figure 6 During coating, the strip plane 1-1 of the square conductor 1 (the upper surface or the lower surface of the square conductor 1) is parallel to the bottom surface of the concave trapezoid 4-1 on one side of the corresponding coating material flat strip, so that the rolling pressure is perpendicular to the strip plane 1-1.

[0089] The two Sn 63 Pb 37 Flat tape wrapped square conductor 1.

[0090] 4.2 The coated flat strip in step 4.1 is heated to 150-200°C to reduce the yield strength of the coated flat strip and improve its ductility.

[0091] 4.3 Select two first rolling dies 2 and keep them at 150-170°C;

[0092] 4.4 The two flat strips of coating material coated square conductor 1 structure formed in step 4.1 are subjected to multiple rolling passes using the two first rolling dies 2 after heat preservation, until the corresponding two side surfaces 2-4 of the two first rolling dies 2 are tightly fitted.

[0093] like Figure 7 As shown, two first rolling dies 2 are placed side by side with a gap between them and their axes are parallel.

[0094] The two flat strips of coated material wrapped square conductor 1 structure formed in step 4.1 are placed in the gap between the two first rolling dies 2, so that the outer raised semicircular shapes 4-2 on the two flat strips of coated material are respectively located in the central grooves 2-1 of the corresponding first rolling dies 2.

[0095] The two first rolling dies 2 are rotated to perform multiple rolling passes until the corresponding two side surfaces 2 - 4 of the two first rolling dies 2 are tightly fitted.

[0096] During rolling, the two side surfaces 2-4 of the first rolling die 2 are parallel to the strip plane 1-1 to prevent the REBCO strip from being subjected to excessive shear stress. The rolling pressure is 100-300 MPa. By heating, multiple rolling passes, rolling stress direction and rolling pressure control, the superconductivity of the REBCO strip is effectively ensured not to be destroyed during the rolling forming process.

[0097] Step 5: Sizing and forming rollers

[0098] 5.1 Cut off the excess flat tape 5-1 in the discharge trough 2-2 to form a REBCO round conductor 6.

[0099] like Figure 8 and Fig. 9 As shown, since the two first rolling dies 2 roll the flat strip of the coating material in both the upper and lower directions at the same time, the excess coating material will be squeezed into the discharge grooves 2-2 on both sides to form an excess flat strip 5-1, and at the same time, the flat strip of the coating material forms a spherical shape outside the square conductor 1. The excess flat strip 5-1 is cut off to form a REBCO round conductor 6.

[0100] 5.2 Rotate the circular conductor 6 by 90° along the axis so that the two side surfaces 2-4 of the two first rolling dies 2 are perpendicular to the strip plane 1-1.

[0101] 5.3 The round conductor rotated 90° is rolled again using the two first rolling dies 2;

[0102] The rolling pressure is 30-100 MPa, and the excess coating material after rolling enters the discharge grooves 2-2 on both sides and is cut off.

[0103] Adjusting the angle and rolling again can reduce the deviation of the outer diameter of the round conductor 6 after forming.

Claims

1. A method for preparing a round wire of a REBCO stacked conductor, characterized in that: The following steps are involved: Step 1, select a REBCO strip, clean it, apply flux on the upper and lower surfaces of the REBCO strip, immerse it in liquid solder, and take it out after the surface of the REBCO strip is evenly covered with solder; REBCO strip width is 1-5mm, thickness is 0.05-0.2mm; The liquid solder is selected from tin, lead, silver and their alloys, with a melting point not exceeding 250°C; Step 2: Stacking the strips to form a square conductor (1) The REBCO strips coated with liquid solder are stacked into a square conductor (1) with a multi-layer structure; Step 3: Pre-rolling of the cladding material The flat belt specifications of the coating material are selected to be 3-12mm wide and 0.4-1.6mm thick; The two flat strips of the coating material are preformed by rolling using a rolling die, so that one side of the preformed flat strip of the coating material has a convex semicircular cross-section (4-2) and the other side has a concave trapezoidal cross-section (4-1); Step 4: Preliminary roll forming Two preformed flat strips of coating material coat the square conductor (1), so that the strip plane (1-1) of the square conductor (1) is parallel to the bottom surface of the concave trapezoid (4-1) on one side of the corresponding flat strip of coating material; Heat the flat strip of coating material to 150-200℃; Using a die to roll the flat strip of coating material and the square conductor (1) multiple times, so that the flat strip of coating material wraps the square conductor (1) to form a conductor with a circular cross section; Step 5: Rotate the conductor, which now has a circular cross section, 90 degrees along the axis and roll it again.

2. A method for preparing a round wire of a REBCO stacked conductor according to claim 1, characterized in that: The rolling dies are a first rolling die (2) and a second rolling die (3); both of the main bodies are cylinders with a through hole processed on the central axis; the first rolling die (2) has three parallel grooves processed along the circumferential direction in the middle of the outer surface, namely, a central groove (2-1) and two discharge grooves (2-2) respectively located on both sides of the central groove (2-1); the cross-sections of the three grooves are all semicircular; a W-shaped protrusion (2-3) is formed in the middle of the cylindrical side surface of the first rolling die (2); the two side surfaces (2-4) of the W-shaped protrusion (2-3) are the side surfaces of the cylinder itself; the highest points on both sides of the central groove (2-1) are lower than the heights of the two side surfaces (2-4); the second rolling die (3) has a boss (3-1) processed along the circumferential direction in the middle of the outer surface; the boss (3-1) has a trapezoidal cross-section; the top width of the boss (3-1) is smaller than the bottom width.

3. A method for preparing a round wire of a REBCO stacked conductor as claimed in claim 2, characterized in that: The step 3 of pre-forming and rolling the two flat strips of the coating material using a rolling die refers to rolling the flat strips of the coating material into a shape (4) using a first rolling die (2) and a second rolling die (3), with one side of the flat strip of the coating material being in close contact with the outside of the boss (3-1) of the second rolling die (3), and the other side of the flat strip of the coating material facing the central groove (2-1) of the first rolling die (2).

4. A method for preparing a round wire of a REBCO stacked conductor as claimed in claim 3, characterized in that: The step 4 of using a mold to perform multiple passes of rolling on the flat strip of coating material and the square conductor (1) refers to using two first rolling molds (2) to perform multiple passes of rolling on the two flat strips of coating material coated with the square conductor (1) until the two corresponding side surfaces (2-4) of the two first rolling molds (2) are tightly fitted, the two first rolling molds (2) are placed side by side, a gap is reserved between them, and the axes are parallel; the two flat strips of coating material coated with the square conductor (1) are placed in the gap between the two first rolling molds (2), so that the outer raised semicircular shapes (4-2) on the two flat strips of coating material are respectively located in the central grooves (2-1) of the corresponding first rolling molds (2).

5. A method for preparing a round wire of a REBCO stacked conductor as claimed in claim 4, characterized in that: The cross section of the boss (3-1) is a trapezoid, the upper width of the trapezoid is 1.05 times the width of the strip, the lower width is 1.1 times the width of the strip, and the height of the trapezoid is 0.35-0.45 times the width of the strip.

6. A method for preparing a round wire of a REBCO stacked conductor as claimed in claim 4, characterized in that: The diameter of the central groove (2-1) is 1.5-1.8 times the width of the strip, and the diameter of the discharge grooves (2-2) on both sides is (2-3) times the width of the strip; the height difference between the highest point on both sides of the central groove (2-1) and the two side surfaces (2-4) is 0.05-0.2 times the width of the strip.

7. A method for preparing a round wire of a REBCO stacked conductor as claimed in claim 4, characterized in that: In the step 1, the REBCO strip is cleaned with a cleaning agent, and then cleaned after soaking and blown dry with air at room temperature. The cleaning agent is 916 copper cleaning brightener, and the soaking time is 20-120s.

8. The method for preparing a round wire of a REBCO stacked conductor according to claim 4, characterized in that: The soldering flux in step 1 is NC-559.

9. A method for preparing a round wire of a REBCO stacked conductor according to claim 4, characterized in that: The liquid solder in step 1 is Sn 63 Pb 37 Liquid, temperature is 185-250℃.

10. A method for preparing a round wire of a REBCO stacked conductor according to claim 9, characterized in that: After the square conductor (1) is stacked in step 2, the temperature is maintained at 185°C to 250°C.

11. A method for preparing a round wire of a REBCO stacked conductor according to claim 10, characterized in that: In the step 2, the square conductor (1) is vigorously blown to clean the excess solder, the blowing speed is 50-120m / s, the temperature of the square conductor (1) is maintained at 185-250°C during the blowing process, and it is naturally cooled after the blowing is completed.

12. A method for preparing a round wire of a REBCO stacked conductor according to claim 11, characterized in that: In step 3, the material of the flat strip of coating material is selected from one of copper, aluminum, tin, lead and silver, or an alloy composed of two or more of these metals.

13. A method for preparing a round wire of a REBCO stacked conductor according to claim 12, characterized in that: The material of the flat strip of the coating material in step 3 is Sn 63 Pb 37 .

14. A method for preparing a round wire of a REBCO stacked conductor according to claim 13, characterized in that: In the step 4, the two first rolling dies (2) are heated and kept warm at 150-170°C.

15. The method for preparing a round wire of a REBCO stacked conductor according to claim 4, characterized in that: In the steps 4 and 5, the two side surfaces (2-4) of the first rolling die (2) are parallel to the strip plane (1-1).

16. A method for preparing a round wire of a REBCO stacked conductor according to claim 4, characterized in that: In step 4 and step 5, the rolling pressure is 100-300 MPa.

17. A method for preparing a round wire of a REBCO stacked conductor according to claim 4, characterized in that: Before carrying out the step 5, the redundant flat belt (5-1) entering the discharge chute (2-2) is cut off.

18. A rolling die for preparing a round wire of a REBCO stacked conductor used in the method for preparing a round wire of a REBCO stacked conductor according to any one of claims 1 to 17, characterized in that: The main body is a cylinder with a through hole processed on the central axis, and a rolling structure is processed in the middle of the outer side of the cylinder; the rolling structure is three parallel grooves processed along the circumference of the middle of the cylinder, namely a central groove (2-1) and two discharge grooves (2-2) respectively located on both sides of the central groove (2-1), and the cross-sections of the three grooves are all semicircular, and a W-shaped protrusion (2-3) is formed in the middle of the cylindrical side surface of the first rolling mold (2), and the two side surfaces (2-4) of the W-shaped protrusion (2-3) are the side surfaces of the cylinder itself.

19. A rolling die for preparing a REBCO stacked conductor round wire according to claim 18, characterized in that: The highest points on both sides of the central groove (2-1) are lower than the heights of the two side surfaces (2-4).

20. A rolling die for preparing a REBCO stacked conductor round wire according to claim 18, characterized in that: The rolling structure is a boss (3-1) processed circumferentially in the middle of the outer surface of the cylinder, the cross section of the boss (3-1) is trapezoidal, and the top width of the boss (3-1) is smaller than the bottom width.

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