A low resistivity gradient crystal wire and its preparation method

Abstract

The present invention discloses a low resistivity gradient crystal wire and a preparation method thereof. The present invention uses a high-frequency induction heating device to perform continuous and controllable heat treatment on the wire. By setting parameters such as the diameter of the induction coil, the inter-turn spacing, the power supply frequency, the current, and the incoming wire rate, and utilizing the skin effect of induction heating, a non-uniformly distributed temperature field is obtained on the cross-section of the wire. If there are residual stresses in the wire, the surface temperature can be controlled to be higher than the recrystallization temperature to achieve short-time recrystallization growth of grains; if there are no residual stresses in the wire, the surface temperature can be controlled to be close to but lower than the recrystallization temperature, and the grain growth is achieved by accelerating the grain boundary interface migration rate. The core temperature is lower than the surface temperature, and the change in grain size is not obvious, forming a gradient structure distribution of grains from the surface to the core. During the alternating current transmission process, the large grains on the surface can effectively reduce the hindrance of grain boundaries to the current, reduce the resistivity of the wire, and at the same time retain the strong plastic properties of the wire.

Description

Technical Field

[0001] The present invention belongs to the technical field of electric wire materials, and particularly relates to a low resistivity gradient crystal wire and a preparation method thereof. Background Art

[0002] Utilizing AC power transmission technology to achieve cross-regional and long-distance power transmission can optimize resource allocation and improve energy utilization efficiency. Although AC power transmission has been widely applied, the skin effect during power transmission is still inevitable. The skin effect refers to the phenomenon that the current density on the wire during AC power transmission always shows a large value on the surface layer and a small value in the core, which will cause serious problems such as overheating of the wire surface layer, reduced transmission efficiency, and impact on electromagnetic compatibility. Currently, there are mainly two solutions to the skin effect. One is to divide the wire into many fine stranded or braided filaments, and each filament is insulated, thereby reducing the skin effect. However, this method is complex in processing and increases the cost. The other method is to use a hollow conductor, so that the current mainly flows on the outer shell of the conductor. However, the strength of this structure is worse than that of a solid conductor, and the cost may also increase. Summary of the Invention

[0003] The present invention provides a low resistivity gradient crystal wire and a preparation method thereof to solve the problems of heat loss and low efficiency caused by the skin effect of the wire during AC power transmission.

[0004] The technical solution of the present invention is as follows:

[0005] In the first aspect, the present invention provides a low resistivity gradient crystal wire. Starting from the center of the wire circle, along the cross-sectional direction and the axial direction of the wire, from the inside to the outside, the grain size of the wire changes in a gradient from fine grains to coarse grains.

[0006] Preferably, the wire is divided into a surface layer structure and a core structure along its cross-sectional direction. The interface between the surface layer structure and the core structure is defined by a circle with the center of the conductor as the center of the circle, and the radius of the circle where the core structure is located is 60-80% of the wire radius.

[0007] Preferably, the grain diameter of the surface layer structure is 400-800 μm, and the grain diameter of the core structure is 50-100 μm.

[0008] In the second aspect, the present invention provides a preparation method of a low resistivity gradient crystal wire, including the following steps:

[0009] (1) Determine the recrystallization temperature of the wire according to the wire material, adjust the power current and frequency of the induction heating device, and determine the wire feeding speed, coil diameter, and inter-turn spacing. If the wire has residual stress, make the surface temperature of the wire greater than the recrystallization temperature by 50-100 K. If the wire has no residual stress, make the surface temperature of the wire less than the recrystallization temperature by 0-50 K;

[0010] (2) Fix the inlet end and outlet end of the wire. The traction power is driven by a motor. After the stable induction heating device has been operating for 5 minutes, let the wire start to enter from the inlet reel. After passing through the induction coil, the wire is wound up by the take-up reel to complete the wire-winding operation;

[0011] (3) Repeat step (2) until the grain size of the surface layer structure of the wire is 400 - 800 μm and the grain size of the core structure of the wire is 50 - 100 μm.

[0012] Preferably, the coil diameter is 1.5 - 5 times the wire diameter, and the turn spacing should be as small as possible to improve the electromagnetic induction efficiency of the coil. To ensure sufficient heat treatment time, the turn spacing is 1 - 5 mm.

[0013] Preferably, the wire material is any one of copper and aluminum.

[0014] Preferably, the inlet rate is 1 - 20 cm / s, the induction current is 50 - 500 A, and the frequency is 10 kHz - 50 kHz.

[0015] Compared with the prior art, the remarkable advantages of the present invention are as follows:

[0016] (1) The present invention is universal for the initial state of the wire. Regardless of whether there is residual stress on the wire surface layer, a gradient structure can be prepared.

[0017] (2) The prepared large / small grain gradient structure not only reduces the resistivity of the wire (≤0.01690 Ω·mm 2 / m), but also can simultaneously improve the strength and plasticity of the wire, thereby reducing the working deformation of the wire, lowering the maintenance and replacement costs of the wire, and improving the safety performance of the wire.

[0018] (3) The preparation process of the present invention is simple, fast, efficient, and low-cost. It can continuously prepare low-resistivity gradient crystal wires, and has general applicability and popularization value.

[0019] The present invention will be described in detail below with reference to the accompanying drawings. Description of the Drawings

[0020] Figure 1 It is the current density distribution diagram during the continuous preparation of the low-resistivity gradient crystal wire described in Example 1.

[0021] Figure 2 It is the temperature distribution diagram during the continuous preparation of the low-resistivity gradient crystal wire described in Example 1.

[0022] Figure 3 It is the schematic diagram of the large / small grain gradient distribution of the cross-section of the low-resistivity gradient crystal wire described in Example 1.

[0023] Figure 4It is the temperature distribution diagram during the continuous preparation process of a low-resistivity gradient crystal wire at a frequency of 500 Hz in Example 2.

[0024] Figure 5 It is a scanning picture of the cross-section of the aluminum wire prepared in Example 3. Specific implementation manners

[0025] In the present invention, an induction heating device is used to heat the wire. After determining the coil diameter and the inter-turn spacing, the induction current, frequency, and the wire feeding rate are controlled. By utilizing the skin effect during the induction heating process, the preparation of the gradient structure wire described in the present invention is realized, and the resistivity of the wire is significantly reduced. The present invention selects appropriate process parameters according to the initial state and recrystallization temperature of the wire. If the wire has undergone drawing treatment but not annealing treatment, the surface residual stress is relatively large. The surface layer temperature can be controlled to be 50 - 100 K higher than the recrystallization temperature to realize the short-time recrystallization growth of grains. If the wire has undergone drawing treatment and has been fully annealed, there is no residual stress. The surface layer temperature can be controlled to be close to but lower than the recrystallization temperature (0 - 50 K). By accelerating the migration rate of the outer grain boundary interface, the growth of grains is realized, so that along the cross-sectional direction and the axial direction of the wire, from the inside to the outside, the grain size changes from fine grains to coarse grains in a gradient manner. The cross-sectional schematic diagram is as Figure 3 shown.

[0026] Example 1

[0027] (1) The wire is selected as a copper wire (10 mm, resistivity is 0.01851 Ω·mm 2 / m). The wire has undergone drawing treatment but not annealing, and a large amount of residual stress has accumulated on the surface layer. By referring to the data, it can be known that the recrystallization temperature of the copper wire is about 573 K. According to the Comsol simulation software to simulate the coupled electric field and thermal field during induction heating, the power supply current of the induction heating device is adjusted to 100 A, the frequency is 30 kHz, the wire feeding rate is determined to be 3 cm / s, the coil size is 20 mm, and the coil inter-turn spacing is 2 mm. The current density distribution in the wire is obtained as Figure 1 shown, and the temperature field in the wire is as Figure 2 shown, so that the surface layer temperature is about 620 K, which is greater than 573 K, while the internal temperature is about 400 K, which is significantly lower than 573 K;

[0028] (2) Fix the inlet end and the outlet end of the wire. The traction power is driven by a motor. After the induction heating device is stable for 5 min, the wire starts to be fed from the inlet reel at a rate of 3 cm / s. After the wire passes through the induction coil, the take-up reel completes the take-up operation;

[0029] (3) To further grow the surface grains, step (2) was repeated 3 times, thus completing the continuous preparation of the low resistivity gradient crystal wire. After testing, the resistivity of the obtained low resistivity gradient crystal wire was 0.01690 Ω·mm 2 / m.

[0030] Example 2

[0031] Other conditions not mentioned were the same as those in Example 1. The frequency was changed to 500 Hz, and the temperature field in the wire was as Figure 4 shown. The surface and internal temperatures were both greater than 573 K.

[0032] Due to the low frequency, the skin effect was not obvious enough, and the heating temperature field was very uniform, resulting in the recrystallization and grain growth processes occurring in the whole processed wire. Finally, the cross-section of the obtained wire was a uniform distribution of large equiaxed grains.

[0033] Example 3

[0034] Other conditions not mentioned were the same as those in Example 1. The wire was changed to an aluminum wire (which had undergone drawing treatment but was fully annealed without residual stress, and its resistivity was 0.02756 Ω·mm 2 / m). After consulting, its recrystallization temperature was 473 K. The power supply current was adjusted to 60 A and the frequency to 50 kHz, and a wire with a gradient structure of large grains (about 500 μm) on the surface and small grains (about 100 μm) in the core was prepared. The proportion of core grains was about 70%, as Figure 5 shown. After testing, the resistivity of the obtained low resistivity gradient crystal wire was 0.02645 Ω·mm 2 / m.

[0035] Example 4

[0036] Other conditions not mentioned were the same as those in Example 1. The traction power was driven by a motor, and the fixed incoming wire speed was 15 cm / s. Due to the too fast incoming wire speed, the surface temperature was much lower than the recrystallization temperature, and a gradient structure wire with a combination of large / small grains could not be obtained. The cross-section was all small equiaxed grains. After testing, the resistivity of the obtained wire was 0.01845 Ω·mm 2 / m.

Claims

1. A preparation method of a low resistivity gradient crystal wire, characterized in that It includes the following steps: (1) Determine the recrystallization temperature of the wire according to the wire material, adjust the power current and frequency of the induction heating device, and determine the wire feeding speed, coil diameter and inter-turn spacing. If there is residual stress in the wire, make the surface temperature of the wire 50-100K higher than the recrystallization temperature. If there is no residual stress in the wire, make the surface temperature of the wire 0-50K lower than the recrystallization temperature; (2) Fix the wire feeding end and the wire discharging end of the wire. The traction power is driven by a motor. After stabilizing the induction heating device for 5 minutes, make the wire start to feed from the wire feeding reel. After the wire passes through the induction coil, the wire winding operation is completed by the wire take-up reel. Among them, the wire feeding rate is 1-20 cm / s, the induction current is 50-500 A, and the frequency is 10 kHz-50 kHz; (3) Repeat step (2) until the grain size of the surface layer structure of the wire is 400-800 μm and the grain size of the core structure of the wire is 50-100 μm. Among them, the wire is divided into a surface layer structure and a core structure along its cross-sectional direction. The interface between the surface layer structure and the core structure is defined by a circle, the center of the circle is the center of the conductor, and the radius of the circle where the core structure is located is 60-80% of the wire radius.

2. The method according to claim 1, wherein The coil diameter is 1.5-5 times the wire diameter, and the inter-turn spacing is 1-5 mm.

3. The method according to claim 1, wherein The wire material is any one of copper and aluminum.

4. The low-resistivity gradient crystal wire prepared by the method according to any one of claims 1-3, characterized in that, Starting from the center of the wire, along the cross-sectional direction and the axial direction of the wire, from the inside to the outside, the grain size changes in a gradient from fine grains to coarse grains. The grain size of the surface layer structure of the wire is 400-800 μm, and the grain size of the core structure of the wire is 50-100 μm.

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