Low-hysteresis NiTiCu elastic heat refrigeration small-diameter thin-wall pipe and preparation method thereof

Abstract

The invention discloses a low-hysteresis NiTiCu elastic heat refrigeration small-diameter thin-wall pipe and a preparation method thereof, and the preparation method comprises the following steps: carrying out hot forging on a cast ingot, and then carrying out hot rolling to obtain a wire rod; carrying out hot drawing on the hot-rolled wire rod to obtain a thick wire; the thick wire is subjected to hot straightening, and electric spark drilling is conducted along the diameter center; inserting the perforated thick wire into a core rod and then carrying out hot drawing; nicking the two ends of the pipe with the core, stretching a core rod to uniformly thin the pipe, and then drawing out the pipe to obtain a coreless hot drawing pipe; the coreless pipe is subjected to high-temperature solution treatment; carrying out cold drawing on the solid-solution-state coreless pipe at room temperature; carrying out cold straightening and stress relief annealing on the cold-drawn thin-wall pipe; and then acid pickling and polishing are conducted, and finally the low-hysteresis NiTiCu elastic heat refrigeration small-diameter thin-wall pipe is obtained. Compared with an existing room-temperature elastic heat refrigerating alloy pipe, the NiTiCu alloy pipe prepared through the method has the high adiabatic temperature change, the low stress lag and the large specific surface area at the same time, and the requirement of an actual elastic heat refrigerating system is better met.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material processing, and particularly relates to a low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube and a preparation method thereof. Background Art

[0002] Based on the advantages of zero greenhouse gas emissions, high energy efficiency potential (theoretical Carnot efficiency up to 84%), and large refrigeration power density, the elastocaloric refrigeration technology based on solid-state phase change is listed as one of the most promising new refrigeration technologies. Shape memory alloys are currently the main type of refrigeration elastocaloric materials. The root cause of their excellent refrigeration ability lies in the stress-induced reversible martensitic phase change that occurs in the alloy. Near-equiatomic NiTi alloys, as a typical type of shape memory alloy, have a phase change strain of up to 7% from the B2 parent phase to the B19' martensite phase. This relatively large spontaneous strain makes the alloy exhibit a relatively high isothermal entropy change and adiabatic temperature change. Therefore, such alloys have been used as model materials for the application verification of solid-state elastocaloric refrigeration prototypes.

[0003] However, the relatively large lattice difference between the martensite phase and the parent phase will lead to a relatively high phase change energy barrier, resulting in a relatively large stress hysteresis during loading and unloading, and causing the functional characteristics of the alloy to decline significantly with the increase in the number of loading and unloading cycles, seriously restricting the practical engineering applications of such materials. In addition, the elastocaloric refrigeration system is more dependent on thin-wall tubes with a large specific surface area. On the one hand, it can accelerate the heat exchange process and improve the refrigeration power. On the other hand, the compression of the tube has a higher fatigue life compared to the stretching of plates and wires. Therefore, how to further regulate the NiTi alloy composition and preparation process to control the martensitic phase change behavior, so as to prepare a new type of elastocaloric refrigeration alloy tube with both large adiabatic temperature change and low stress hysteresis, has become one of the keys to promoting the development of the next generation of green refrigeration technology. Summary of the Invention

[0004] To solve the above performance defects existing in the application of existing NiTi alloys in elastocaloric refrigeration, the purpose of the present invention is to provide a low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube and a preparation method thereof. Specifically, the phase change path of the NiTi alloy is regulated by doping with Cu element, and a high-performance seamless tube with a nanocrystalline structure is prepared by using a coreless equal-wall drawing technology.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube, the alloy composition of which is calculated by mass percentage, includes 48.8%-49.3% of Ni, 6.5%-7.2% of Cu, and the balance is Ti and inevitable impurity elements.

[0007] Preferably, the adiabatic unloading temperature change of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube is 14.5 - 17.0 K, the isothermal compression stress hysteresis is 150 - 174 MPa, and the yield strength is 1.5 - 1.7 GPa.

[0008] The present invention also provides a method for preparing the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube as described above, including the following processes:

[0009] Hot forging and rolling: hot forging the alloy ingot, and then hot rolling it into a wire rod;

[0010] Hot drawing: hot drawing the wire rod, and hot drawing it into a thick wire with a preset diameter;

[0011] Hot straightening and hole opening: hot straightening the thick wire with the preset diameter, and then processing a through hole along the center of the diameter;

[0012] Core-pulling hot drawing: inserting a mandrel into the through hole of the thick wire with a processed through hole, and then hot drawing the thick wire with a processed through hole to a preset outer diameter to obtain a core-bearing tube with a preset outer diameter;

[0013] Core removal: scoring both ends of the core-bearing tube, stretching the mandrel to make it thinner, and then pulling the mandrel out of the tube to obtain a coreless hot-drawn tube;

[0014] Solution treatment: performing solution treatment on the coreless hot-drawn tube to dissolve the second phase formed during the hot drawing process to obtain a solution-treated coreless tube;

[0015] Coreless cold drawing: performing cold drawing on the solution-treated coreless tube, and cold drawing the solution-treated coreless tube to a preset size to obtain a cold-drawn thin-wall tube with a preset size;

[0016] Stress relief straightening annealing: placing the cold-drawn thin-wall tube in a straightening die and performing stress relief annealing to obtain a low-temperature straightening annealed tube;

[0017] Surface treatment: performing pickling and polishing on the low-temperature straightening annealed tube to obtain the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube.

[0018] Preferably, when hot forging the alloy ingot and then hot rolling it into a wire rod:

[0019] The forging start temperature is 900 °C, and the final forging temperature is not lower than 700 °C:

[0020] The rolling start temperature is 900 °C, and the final rolling temperature is not lower than 700 °C.

[0021] Preferably, when hot drawing the wire rod, the die is gradually replaced during drawing to reduce the hole size, and the wire rod is hot drawn into a thick wire with a preset diameter, wherein the diameter reduction amount per pass is controlled not to be greater than 0.4 mm.

[0022] Preferably, when hot straightening the thick wire with the preset diameter, the thick wire with the preset diameter is set in a straightening die, straightened at 600 - 650 °C for 20 - 60 min, and then cooled to complete the hot straightening;

[0023] The thick wire with the preset diameter after hot straightening is processed with a through hole along the diameter center by means of electric discharge drilling.

[0024] Preferably, during the hot drawing with a mandrel, the thick wire with the processed through hole is hot drawn to the preset outer diameter by gradually replacing the die to reduce the hole size;

[0025] During hot drawing, the thick wire with the processed through hole is heated to 650 - 750 °C for hot drawing, and the diameter reduction per pass is controlled to be no more than 0.4 mm;

[0026] The mandrel is made of NiTi binary wire.

[0027] Preferably, during solution treatment, the solution temperature is 850 - 950 °C and the solution time is 0.15 - 0.5 h.

[0028] Preferably, during cold drawing without a mandrel, drawing is carried out at room temperature (generally 20 - 25 °C), the diameter reduction per pass is controlled to be no more than 0.2 mm, and the total cross-sectional shrinkage rate is 20% - 25%.

[0029] Preferably, during stress relief straightening annealing, the annealing temperature is 350 - 400 °C and the annealing time is 0.25 - 2 h.

[0030] The present invention has the following beneficial effects:

[0031] In terms of composition, the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube of the present invention regulates the phase transformation path of the NiTi alloy by doping a small amount of Cu atoms to replace part of the Ni atoms, realizing the B2 - B19 structure transformation with better crystallographic compatibility, which is beneficial to reducing the phase transformation stress hysteresis of the alloy.

[0032] At the process level, the preparation of traditional NiTi tubes generally involves cold drawing with a mandrel, followed by high-temperature straightening annealing, and then removing the mandrel to obtain the tubes. However, this high-temperature annealing process often leads to coarsening of the alloy grains and plastic deformation during the loading process. In the preparation method of the present invention, the yield strength of the alloy phase transformation matrix can be effectively improved through coreless cold drawing and low-temperature straightening annealing methods, avoiding dislocation slip during the stress-induced phase transformation, thereby enhancing the reversibility of the phase transformation and increasing the adiabatic temperature change of the alloy. The NiTiCu thin-diameter and thin-wall tubes prepared by the process method of the present invention have a large specific surface area, which is beneficial to accelerating the heat exchange process and improving the refrigeration power of the refrigeration system. At the same time, the large adiabatic temperature change and low stress hysteresis characteristics are also beneficial to improving the refrigeration efficiency of the refrigeration system and reducing energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and do not constitute an improper limitation of the present invention. In the drawings:

[0034] Figure 1 It is a process technical route diagram for the preparation of low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes of the present invention.

[0035] Figure 2 It is an external view of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention.

[0036] Figure 3 It is a TEM microstructural characterization diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention. Among them, Figure (a) is a bright-field image of the transmission electron microscope, and Figure (b) is the corresponding electron diffraction pattern.

[0037] Figure 4 It is a heat flow curve diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention.

[0038] Figure 5 It is an isothermal loading curve diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention.

[0039] Figure 6 It is an adiabatic temperature change test diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention.

[0040] Figure 7 It is an elastocaloric fatigue test diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 1 of the present invention.

[0041] Figure 8 It is an isothermal loading curve diagram of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared in Example 2 of the present invention.

[0042] Figure 9 Adiabatic temperature change test chart of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube prepared in Example 2 of the present invention.

[0043] Figure 10 Isothermal loading curve graph of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube prepared in Example 3 of the present invention.

[0044] Figure 11 Adiabatic temperature change test chart of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube prepared in Example 3 of the present invention. Detailed implementation manners

[0045] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. Here, the schematic embodiments of the present invention and the descriptions are used to explain the present invention, but do not limit the present invention.

[0046] As Figure 1 shown, the preparation method of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-wall tube of the present invention includes the following steps:

[0047] Step 1: Mix high-purity metal Ti, Ni, and Cu particles to obtain a metal raw material. By mass percentage, the metal raw material contains 48.8%-49.3% Ni particles, 6.5%-7.2% Cu particles, and the rest are Ti particles;

[0048] Step 2: Use the vacuum induction melting method to melt and mix evenly the metal raw material obtained in Step 1, and then cast to obtain an initial polycrystalline ingot;

[0049] Step 3: Heat the initial polycrystalline ingot obtained in Step 2 to 900 °C for forging, and the final forging temperature is not lower than 700 °C. Then, perform hot rolling. When hot rolling, heat the forged bar to 900 °C for hot rolling, and the final rolling temperature of hot rolling is not lower than 700 °C to obtain a hot-rolled wire rod;

[0050] Step 4: Heat the hot-rolled wire rod obtained in Step 3 to 650-750 °C for hot drawing, and control the diameter reduction per pass to be no more than 0.4 mm, and finally obtain a thick wire with a diameter of 8 mm;

[0051] Step 5: Set the thick wire obtained in Step 4 in a straightening die for hot straightening. The hot straightening temperature is 600-650 °C, and the hot straightening time is 20-60 min. After cooling, perform electrical discharge drilling along the center of the thick wire diameter.

[0052] Step 6: Insert a commercial NiTi binary wire into the central hole of the thick wire obtained in Step 5 as a mandrel, and perform hot drawing at 600 - 700 °C. When performing hot drawing, gradually replace the die to reduce the hole size, and obtain a core-containing pipe with a preset outer diameter. Control the diameter reduction amount per pass to be no more than 0.4 mm.

[0053] Step 7: Notch both ends of the core-containing pipe obtained after processing in Step 6. After stretching the mandrel to make it uniformly thinner, pull out the mandrel from the pipe to obtain a coreless hot-drawn pipe.

[0054] Step 8: Perform high-temperature solution treatment on the coreless pipe obtained in Step 7. The solution temperature is 850 - 950 °C, and the solution time is 0.15 - 0.5 h, so as to dissolve the second phase formed during the hot drawing process.

[0055] Step 9: Perform cold drawing on the solution-treated coreless pipe obtained in Step 8 at room temperature. Control the diameter reduction amount per pass to be no more than 0.2 mm, and the total cross-sectional shrinkage rate is 20% - 25%.

[0056] Step 10: Perform cold straightening stress relief annealing on the cold-drawn thin-walled pipe obtained in Step 9 in a straightening die. The annealing temperature is 350 - 400 °C, and the annealing time is 0.25 - 2 h.

[0057] Step 11: Pickle the pipe in the low-temperature straightening and annealing state obtained in Step 10, and then polish the outer surface through a centerless grinder to remove the oxides and impurities on the pipe surface, and obtain the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe.

[0058] In the above solution, the hot forging (or hot forging), hot drawing (or hot drawing), and hot straightening mentioned refer to the corresponding processing carried out above the recrystallization temperature of the alloy material, while cold drawing is the drawing process carried out below the recrystallization temperature of the alloy material.

[0059] The following further illustrates the present invention through specific embodiments.

[0060] Example 1:

[0061] The composition of the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe in this example includes 49.0% Ni, 7.1% Cu by mass ratio, and the rest is Ti and unavoidable impurities. Its preparation process includes the following steps:

[0062] First step: Weigh the Ti, Ni, and Cu metal particle raw materials with a purity of 99.9% by mass percentage, and the total weight of the raw materials is 10 kg.

[0063] Second step: Mix the metal raw materials in the first step evenly, place them in a crucible, and use the vacuum induction melting method to melt and mix them evenly, and then cast to obtain an initial polycrystalline ingot.

[0064] In the third step, the polycrystalline ingot obtained in the second step is forged at 900 °C, and the final forging temperature is not lower than 700 °C. Then hot rolling is carried out. The starting rolling temperature of hot rolling is 900 °C, and the final rolling temperature is not lower than 700 °C to obtain hot-rolled wire rods.

[0065] In the fourth step, the hot-rolled wire rods obtained in the third step are hot drawn at 750 °C, and the diameter reduction per pass is controlled to be 0.3 mm to obtain thick wires with a diameter of 8 mm.

[0066] In the fifth step, the thick wires obtained in the fourth step are placed in a straightening die for hot straightening. The hot straightening temperature is 600 °C, and the hot straightening time is 60 min. Then it is cooled, and electric discharge drilling is carried out along the center of the thick wire diameter (i.e., the central hole).

[0067] In the sixth step, commercial NiTi binary wire materials are inserted into the central holes of the perforated thick wires obtained in the fifth step as mandrels, and hot drawing is carried out at 650 °C. The diameter reduction per pass during hot drawing is controlled to be 0.3 mm to obtain core-containing tubes with an outer diameter of 3.8 mm.

[0068] In the seventh step, the two ends of the core-containing tubes obtained after the sixth step are scored. After stretching the mandrel to make it uniformly thinner, the tubes are pulled out from the tubes to obtain core-free hot-drawn tubes.

[0069] In the eighth step, the core-free hot-drawn tubes obtained in the seventh step are subjected to solution treatment. The solution temperature is 900 °C, and the solution time is 0.2 h.

[0070] In the ninth step, the solution-state core-free tubes obtained after the eighth step are cold drawn at room temperature. The diameter reduction per pass is controlled to be 0.2 mm, and the total cross-sectional shrinkage rate is 22% to obtain core-free cold-drawn tubes with an outer diameter of 3 mm and a wall thickness of 0.3 mm.

[0071] In the tenth step, the cold-drawn thin-walled tubes obtained in the ninth step are simultaneously cold straightened and stress-relieved annealed in a straightening die. The annealing temperature is 400 °C, and the annealing time is 0.5 h.

[0072] In the eleventh step, the low-temperature straightened and annealed tubes obtained in the tenth step are pickled, and then the outer surface is polished by a centerless grinder to obtain the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled tubes, as Figure 2 shown.

[0073] Figure 3 is the transmission electron microscope micrograph of the NiTiCu thin-diameter thin-walled tubes prepared in Example 1. The alloy exhibits a typical nanocrystalline microstructure.

[0074] Figure 4 is the DSC heat flow curve of the NiTiCu thin-diameter thin-walled tubes prepared in Example 1 during the heating and cooling process,Figure 4 The two obvious exothermic and endothermic peaks indicate that the alloy has undergone a reversible temperature-induced martensitic transformation. The reverse martensitic transformation end temperature of the alloy is 2.1 °C, the transformation enthalpy is 10.8 J / g, and the thermal hysteresis is 19.7 °C. The alloy is in a fully austenitic state at room temperature, so it is expected to exhibit reversible stress-induced martensitic transformation and good elastocaloric effect near room temperature.

[0075] Figure 5 The isothermal compression curve of the NiTiCu fine-diameter thin-wall tube prepared in Example 1 at room temperature is shown. It can be seen that the alloy exhibits good superelastic recovery, and the stress hysteresis is 158 MPa. Further crushing experiments show that the yield strength of the alloy reaches 1.55 GPa.

[0076] Figure 6 The adiabatic temperature change results of the NiTiCu fine-diameter thin-wall tube prepared in Example 1 under different applied stress conditions are shown. During the test, the unloading rate was 0.06 s -1 to approximately meet the adiabatic condition; it can be seen that the adiabatic temperature change of the alloy increases with the increase of the applied stress, the maximum temperature rise can reach 19.1 K, and the maximum temperature drop can reach 16.6 K.

[0077] Figure 7 The elastocaloric fatigue test results of the low-hysteresis NiTiCu elastocaloric refrigeration fine-diameter thin-wall tube prepared in Example 1 are shown. After the alloy was cyclically loaded 1000 times at 950 MPa, the adiabatic temperature drop only decayed from 16.59 K to 16.35 K, and the decay value was less than 0.3 K. In addition, after 100,000 cycles, the alloy can still maintain an adiabatic temperature rise higher than 17 K and an adiabatic temperature drop higher than 15 K, indicating that the alloy has excellent functional cycle stability.

[0078] Example 2:

[0079] The composition of the low-hysteresis NiTiCu elastocaloric refrigeration fine-diameter thin-wall tube in this example includes 49.3% Ni, 6.5% Cu by mass ratio, and the rest is Ti and unavoidable impurities. Its preparation process includes the following steps:

[0080] First step, weigh the metal particle raw materials of Ti, Ni, and Cu with a purity of 99.9% by mass percentage, and the total weight of the raw materials is 10 kg;

[0081] Second step, mix the metal raw materials in the first step evenly, place them in a crucible, and use the vacuum induction melting method to melt and mix them evenly, and then cast to obtain an initial polycrystalline ingot;

[0082] Third step, forge the polycrystalline ingot obtained in the second step at 900 °C, and the final forging temperature is not lower than 700 °C. Then carry out hot rolling. The hot rolling starting temperature is 900 °C, and the final rolling temperature is not lower than 700 °C to obtain a hot-rolled wire rod;

[0083] In the fourth step, the hot-rolled wire rod obtained in the third step is hot-drawn at 700 °C, and the diameter reduction per pass is controlled to be 0.2 mm to obtain a thick wire with a diameter of 8 mm.

[0084] In the fifth step, the thick wire obtained in the fourth step is set in a straightening die for hot straightening. The hot straightening temperature is 630 °C and the hot straightening time is 40 min. Then, electric discharge drilling is carried out along the diameter center.

[0085] In the sixth step, a commercial NiTi binary wire is inserted into the central hole of the perforated thick wire obtained in the fifth step as a mandrel, and hot drawing is carried out at 600 °C. The diameter reduction per pass is controlled to be 0.2 mm during drawing to obtain a core-containing pipe with an outer diameter of 4.0 mm.

[0086] In the seventh step, the two ends of the core-containing pipe obtained after the sixth step are scored. After stretching the mandrel to make it uniformly thinner, the pipe is pulled out from the pipe to obtain a coreless hot-drawn pipe.

[0087] In the eighth step, the coreless hot-drawn pipe obtained in the seventh step is subjected to solution treatment. The solution temperature is 850 °C and the solution time is 0.5 h.

[0088] In the ninth step, the solution-treated coreless pipe obtained in the eighth step is cold-drawn at room temperature. The diameter reduction per pass is controlled to be 0.2 mm and the total cross-sectional shrinkage rate is 25% to obtain a coreless cold-drawn pipe with an outer diameter of 3.3 mm and a wall thickness of 0.32 mm.

[0089] In the tenth step, the cold-drawn thin-walled pipe obtained in the ninth step is simultaneously cold-straightened and stress-relieved annealed in a straightening die. The annealing temperature is 400 °C and the annealing time is 2 h.

[0090] In the eleventh step, the low-temperature straightened and annealed pipe obtained in the tenth step is pickled, and then the outer surface is polished by a centerless grinder to obtain the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe.

[0091] Figure 8 The isothermal compression curve of the NiTiCu thin-diameter thin-walled pipe prepared in Example 2 at room temperature is shown. The alloy exhibits good superelastic recovery, and the stress hysteresis is 171 MPa. Further crushing experiments show that the yield strength of the alloy reaches 1.59 GPa.

[0092] Figure 9 The adiabatic temperature change results measured for the NiTiCu thin-diameter thin-walled pipe prepared in Example 2 under different applied stress conditions are shown. During the test, the unloading rate was 0.06 s -1 to approximately meet the adiabatic condition. It can be seen that the adiabatic temperature change of this alloy increases with the increase of the applied stress, and the maximum temperature rise can reach 20.1 K, and the maximum temperature drop can reach 15.7 K.

[0093] Example 3:

[0094] In this example, the composition of the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe includes 48.8% Ni, 7.2% Cu by mass ratio, and the rest is Ti and inevitable impurities. The preparation process includes the following steps:

[0095] First step, weigh Ti, Ni, and Cu metal particle raw materials with a purity of 99.9% by mass percentage, and the total weight of the raw materials is 10 kg;

[0096] Second step, mix the metal raw materials in the first step evenly, place them in a crucible, and use the vacuum induction melting method to melt and mix evenly, and then cast to obtain an initial polycrystalline ingot;

[0097] Third step, forge the ingot obtained in the second step at 900 °C, and the final forging temperature is not lower than 700 °C. Then, perform hot rolling. The starting rolling temperature of the hot rolling is 900 °C, and the final rolling temperature is not lower than 700 °C to obtain a hot-rolled wire rod;

[0098] Fourth step, perform hot drawing on the hot-rolled wire rod obtained in the third step at 650 °C, and control the diameter reduction amount per pass to be 0.2 mm to obtain a thick wire with a diameter of 8 mm;

[0099] Fifth step, place the thick wire obtained in the fourth step in a straightening die for hot straightening. The hot straightening temperature is 650 °C, and the hot straightening time is 20 min. Then, cool it, and perform electric discharge drilling along the center of the thick wire diameter.

[0100] Sixth step, insert a commercial NiTi binary wire into the center hole of the perforated thick wire obtained in the fifth step as a mandrel, and perform hot drawing at 700 °C. Control the diameter reduction amount per pass to be 0.2 mm during hot drawing to obtain a core-containing pipe with an outer diameter of 3.7 mm;

[0101] Seventh step, mark the two ends of the core-containing pipe obtained after the sixth step is processed. After stretching the mandrel to make it uniformly thinner, pull out the pipe from the pipe to obtain a coreless hot-drawn pipe;

[0102] Eighth step, perform solution treatment on the coreless hot-drawn pipe obtained in the seventh step. The solution temperature is 950 °C, and the solution time is 0.3 h.

[0103] Ninth step, perform room-temperature cold drawing on the solution-state coreless pipe obtained after the eighth step is processed. Control the diameter reduction amount per pass to be 0.2 mm, and the total cross-sectional shrinkage rate is 20% to obtain a coreless cold-drawn pipe with an outer diameter of 3.1 mm and a wall thickness of 0.32 mm.

[0104] Step 10: Synchronously perform cold straightening and stress relief annealing on the cold-drawn thin-walled pipe obtained in Step 9 in a straightening die. The annealing temperature is 350 °C, and the annealing time is 1 h.

[0105] Step 11: Pickle the low-temperature straightening and annealing state pipe obtained in Step 10, and then grind the outer surface through a centerless grinder to obtain the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe.

[0106] Figure 10 Figure 1 shows the isothermal compression curve of the NiTiCu thin-diameter thin-walled pipe prepared in Example 2 at room temperature. The alloy exhibits good superelastic recovery, and the stress hysteresis is 174 MPa. Further crushing experiments show that the yield strength of the alloy reaches 1.68 GPa.

[0107] Figure 11 Figure 2 shows the adiabatic temperature change results measured for the NiTiCu thin-diameter thin-walled pipe prepared in Example 2 under different applied stress conditions. During the test, the unloading rate was 0.06 s -1 to approximately meet the adiabatic condition. It can be seen that the adiabatic temperature change of this alloy increases with the increase of the applied stress, the maximum temperature rise can reach 19.4 K, and the maximum temperature drop can reach 15.0 K.

[0108] In summary, the low-hysteresis NiTiCu elastocaloric refrigeration thin-diameter thin-walled pipe prepared by the method of the present invention can simultaneously exhibit a large adiabatic temperature change and a low stress hysteresis.

[0109] Table 1 gives a comparison of the elastocaloric refrigeration performance between the low-hysteresis NiTiCu thin-diameter thin-walled pipe prepared by the method of the embodiments of the present invention and the existing NiTi alloy pipes. The performance data of the coarse-grained NiTi alloy pipe 1 are from the literature Applied Materials Today 20(2020)100712; the performance data of the coarse-grained NiTi alloy pipe 2 are from the literature AIP Advances 14,055123(2024); the fine-grained NiTi alloy pipe is purchased from Jiangsu Pearl Technology Co., Ltd., and its performance data are from self-testing in the laboratory.

[0110] Table 1

[0111] Typical room-temperature elastocaloric alloy tubes Maximum adiabatic temperature rise Maximum adiabatic temperature drop Stress hysteresis Refrigeration efficiency COP Example 1 19.1K 16.6K 158 MPa 10.2 Example 2 20.1K 15.7K 171 MPa 9.0 Example 3 19.4K 15.0K 174 MPa 8.6 Coarse-grained NiTi alloy 1 21.0K 14.8K 350 MPa 5.7 Coarse-grained NiTi alloy 2 20.2K 11.0K 420 MPa 3.2 Fine-grained NiTi alloy 13.8K 12.2K 280 MPa 7.5

[0112] As can be seen from Table 1, for the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared by the above preparation method of the present invention, the stress hysteresis is lower than 200 MPa, and the maximum adiabatic temperature change is higher than 19 K. From the above comparison results, it can be seen that compared with the existing NiTi alloy tubes, the NiTiCu thin-diameter and thin-wall tubes prepared by the method of the present invention have a significantly improved coefficient of performance (COP) of the refrigeration efficiency on the basis of maintaining a large adiabatic temperature change at room temperature. At the same time, the obtained thin-diameter and thin-wall tubes have a large specific surface area, which is beneficial to accelerating the heat exchange process of the refrigeration system, thereby increasing the refrigeration power. In addition, the alloy tube preparation process of this method is relatively simple, can be industrially prepared, and has a low cost. Generally speaking, the low hysteresis NiTiCu elastocaloric refrigeration thin-diameter and thin-wall tubes prepared by the method of the present invention have high commercial application potential.

[0113] The present invention is not limited to the above embodiments. Based on the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and deformations to some of the technical features without creative labor according to the disclosed technical content, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A low hysteresis NiTiCu elastic caloric cooling thin-diameter thin-walled tube, characterized in that: The alloy composition thereof includes, by mass percentage, 48.8%-49.3% Ni, 6.5%-7.2% Cu, and the remainder Ti and inevitable impurity elements.

2. A low hysteresis NiTiCu elastic caloric cooling thin-diameter thin-walled tube according to claim 1, characterized in that: The low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube has an adiabatic unloading temperature change of 14.5-17.0K, an isothermal compressive stress hysteresis of 150-174MPa, and a yield strength of 1.5-1.7GPa.

3. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 1 or 2, characterized in that: The process includes the following: Hot forging and rolling: hot forging the alloy ingots and then hot rolling them into wire rods; Hot drawing: hot drawing the wire rod to a thick wire with a preset diameter; Thermal straightening and hole opening: thermally straightening the thick wire of the preset diameter, and then processing a through hole along the center of the diameter; Core hot drawing: insert a core rod into the through hole of the thick wire with through holes, then hot draw the thick wire with through holes to a preset outer diameter to obtain a cored tube with a preset outer diameter; Coring: Score both ends of the cored tube, stretch the mandrel to make it thinner, and then pull the mandrel out of the tube to obtain a coreless hot-drawn tube; Solution treatment: The coreless hot-drawn tube is subjected to solution treatment to dissolve the second phase formed during the hot-drawing process to obtain a solution-treated coreless tube; Coreless cold drawing: cold drawing the solution treated coreless tube to a preset size to obtain a cold drawn thin-walled tube of preset size; Stress relief straightening annealing: The cold drawn thin-walled tube is placed in a straightening die and subjected to stress relief annealing to obtain a low-temperature straightening annealed tube; Surface treatment: pickling and polishing the low-temperature straightening annealed tube to obtain the low-hysteresis NiTiCu elastic thermal cooling thin-diameter thin-wall tube.

4. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: When the alloy ingot is hot forged and then hot rolled into wire rod: The starting forging temperature is 900℃ and the final forging temperature is not less than 700℃: The starting rolling temperature is 900℃ and the final rolling temperature is not less than 700℃.

5. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: The wire rod is hot-drawn, and the mold is replaced successively during drawing to reduce the hole size, and the wire rod is hot-drawn to a thick wire with a preset diameter, wherein the diameter reduction in each pass is controlled to be no more than 0.4 mm.

6. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: When the thick wire of the preset diameter is subjected to thermal straightening, the thick wire of the preset diameter is placed in a straightening mold, straightened at 600-650° C. for 20-60 minutes, and then cooled to complete the thermal straightening; The thick wire of preset diameter after heat straightening is processed into a through hole along the center of the diameter by electric spark drilling.

7. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: When hot drawing with a core, the thick wire with through holes is hot drawn to a preset outer diameter by replacing the die successively to reduce the hole size; When hot drawing with core, heat the thick wire with through holes to 650-750℃ for hot drawing, and control the diameter shrinkage of each pass to be no more than 0.4mm; The core rod is made of NiTi binary wire.

8. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: During solution treatment, the solution temperature is 850-950°C and the solution time is 0.15-0.5h.

9. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: During coreless cold drawing, drawing is carried out at 20-25°C, and the diameter reduction of each pass is controlled to be no more than 0.2mm, and the total cross-sectional shrinkage rate is 20%-25%.

10. The method for preparing the low hysteresis NiTiCu elastic caloric refrigeration thin-diameter thin-walled tube according to claim 3, characterized in that: During stress relief straightening annealing, the annealing temperature is 350-400°C and the annealing time is 0.25-2h.

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