High-diameter-uniformity electrochemical machining thinning method for NiTi alloy wire

Abstract

The invention relates to a NiTi alloy wire high-diameter uniformity electrochemical machining thinning method which comprises the following steps: S1, removing a surface oxide layer of a NiTi alloy wire; s2, the NiTi alloy wire treated in the step S1 is heated to the austenite transformation temperature or above; s3, the NiTi alloy wire treated in the S2 is fixed in an electrolytic tank, the NiTi alloy wire is kept parallel to a tool cathode, power parameters are set, first electrochemical machining treatment is carried out, and the NiTi alloy wire is thinned in the axial direction in a gradient mode to form an inverted cone shape; s4, the NiTi alloy wire treated in the S3 is taken out and placed in the electrolytic tank in an inverted mode, secondary electrochemical machining treatment is carried out with the same parameters, the NiTi alloy wire is thinned in the axial direction in a gradient mode, the thinning amount of the primary electrochemical machining treatment is equal to the thinning amount of the secondary electrochemical machining treatment, and therefore the inverted cone shape is changed into the cylindrical shape; and S5, the S3 and the S4 are repeated till the NiTi alloy wire is thinned to the required diameter. According to the method, NiTi alloy wires with different diameters and the same phase change behavior and mechanical property can be prepared.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of intelligent drive materials, and particularly to a method for electrochemically machining and thinning NiTi alloy wires with high diameter uniformity. Background Art

[0002] NiTi-based shape memory alloy (SMA), as a typical intelligent drive material, shows irreplaceable application value in the fields of precision actuation, adaptive structures, and flexible robots. During the design process of SMA actuators, the relatively long response time is a frequently discussed issue. Reducing the wire diameter is an effective way to increase the response frequency without additionally increasing the volume of the actuator. The driving performance of NiTi SMA is strongly correlated with its geometric dimensions. The response behaviors of NiTi SMA actuators, such as actuation strain, energy density, and energy conversion efficiency, are closely related to the phase transformation behavior and mechanical properties of the SMA wire.

[0003] Currently, the preparation process of NiTi alloy wires is generally as follows: raw materials → melting alloy ingots → forging → repeated cold drawing / hot drawing → annealing. Among them, the thinning of NiTi alloy wires is achieved through cold drawing / hot drawing. During the drawing process, a large amount of plastic deformation will bring about work hardening of the NiTi alloy wire. Considering the increase in strength caused by work hardening, an intermediate annealing process needs to be introduced to eliminate work hardening during multiple drawing and thinning processes. During the drawing process, large-strain plastic deformation is carried out on the alloy, the dislocation density inside the grains is high and the grains are coarse, and it is easy to generate a wire texture along the drawing direction; different drawing reduction ratios and intermediate heat treatment processes will also affect the phase transformation behavior and mechanical properties of NiTi alloy wires, etc. Therefore, it is difficult to achieve the goal of NiTi alloy wires with different diameters but the same phase transformation behavior and mechanical properties by using the drawing method for thinning treatment.

[0004] Electrochemical machining is a non-contact machining process, and complex structure machining of materials can be quickly carried out by dissolving materials through electrolysis. During the electrochemical machining process, the surface removal rate of NiTi alloy wires is related to the potential on the material surface. When the power electrode is connected to one end of the NiTi alloy wire, due to the small diameter and high resistance of the NiTi alloy wire, the electric potential drops significantly along the length direction, which results in non-uniformity of the NiTi alloy wire diameter along the length direction. The electric potential of each position of the NiTi alloy wire drops uniformly along the length direction, and its surface removal rate also drops uniformly along the length direction, resulting in a uniform gradient of the NiTi alloy wire diameter along the axis. The shape of the NiTi alloy wire in the electrolyte is an inverted cone.

[0005] Therefore, there is an urgent need to develop a simple and efficient processing method that can prepare NiTi alloy wires with different diameters but the same phase transformation behavior and mechanical properties. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the object of the present invention is to provide a method for electrochemically processing and thinning NiTi alloy wires with high diameter uniformity, which can prepare NiTi alloy wires with different diameters but the same phase transformation behavior and mechanical properties.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: A method for electrochemically processing and thinning NiTi alloy wires with high diameter uniformity includes the following steps. S1. Cut the NiTi alloy wire from the raw material, remove the surface oxide layer and clean and dry it. S2. Heat the NiTi alloy wire processed in S1 to a temperature above the austenite transformation temperature, and then restore it to the initial straight state. S3. Fix the NiTi alloy wire processed in S2 in the electrolytic cell, keep the NiTi alloy wire parallel to the tool cathode, set the power supply parameters, and perform the first electrochemical processing to make the NiTi alloy wire gradually thinner along the axial direction to form an inverted cone; take out the NiTi alloy wire after processing, and then soak it in absolute ethanol and ultrasonically clean it.

[0008] S4. Invert the NiTi alloy wire processed in S3 and put it into the electrolytic cell, and perform the second electrochemical processing with the same parameters to make the NiTi alloy wire gradually thinner along the axial direction. The thinning amount of the first electrochemical processing is equal to that of the second electrochemical processing, so that the inverted cone becomes a cylinder; the two electrochemical processings are a complete uniform thinning process.

[0009] S5. Repeat S3 and S4 until the NiTi alloy wire is thinned to the required diameter.

[0010] S6. Mechanically polish the finally formed NiTi alloy wire to remove the surface oxide layer and ultrasonically clean and dry it.

[0011] Further, the method for removing the surface oxide layer in S1 is mechanical polishing, and the surface of the NiTi alloy wire after mechanical polishing treatment shows a metallic luster.

[0012] Further, a nitric acid-methanol electrolyte is placed in the electrolytic cell, and the nitric acid-methanol electrolyte is prepared according to a volume ratio of concentrated nitric acid: methanol = 1.0 ± 0.5: 3 ± 0.5.

[0013] Further, when preparing the electrolyte, concentrated nitric acid is added to methanol, and ice-water bath cooling is carried out throughout the process.

[0014] Further, the distance between the cathode and the anode in the electrolytic cell is controlled at 45 mm to 50 mm.

[0015] Further, the current density in both the first electrochemical machining and the second electrochemical machining is 1.1 A / cm 2 ~1.5 A / cm 2 .

[0016] Further, the time for both the first electrochemical machining and the second electrochemical machining is 2 min to 15 s.

[0017] Further, during the first electrochemical machining and the second electrochemical machining, the NiTi alloy wire is placed in the electrolytic cell in opposite directions and with the same length.

[0018] Further, by controlling the electrolysis time or the number of repetitions of the inversion thinning method, stepless adjustment and precise control of the diameter of the NiTi alloy wire can be achieved.

[0019] Further, when the cross-sectional reduction rate of the NiTi alloy wire reaches 80%, the relative extreme difference in diameter along the length direction is less than 3%.

[0020] Generally speaking, the present invention has the following advantages: 1. High diameter uniformity. Compared with the NiTi alloy wire prepared by the method of two-time electrochemical machining without using the inversion method, the electrochemical machining method using the inversion method can achieve that when the cross-sectional reduction rate of the NiTi alloy wire reaches 80%, the relative extreme difference in diameter along the length direction is no more than 3%.

[0021] 2. Simple process and rapid preparation. Compared with the NiTi alloy wire thinning method combining cold drawing and intermediate heat treatment, the electrochemical machining method using the inversion method does not require large-scale drawing machines, complex drawing processes, and intermediate annealing heat treatment. Only by adjusting the electrolysis parameters can the diameter be thinned. The total duration of electrochemical machining can achieve an 80% cross-sectional reduction rate of the NiTi alloy wire within 20 min.

[0022] 3. Does not change the phase transformation behavior and mechanical properties of the NiTi alloy wire. Since the electrochemical machining method has the characteristics of no stress generation, no heat generation, and no need for masks during the machining process, there is no need for subsequent deburring, residual stress, and work hardening processes. The electrochemical machining method is based on Faraday's law and processes by dissolving the surface of the anode workpiece, which has no impact on the internal microstructure of the NiTi alloy wire, and there is no significant change in its mechanical properties and phase transformation performance.

[0023] 4. Stepless thinning. In the electrochemical machining thinning method, the amount of material removed from the surface of the NiTi alloy wire is controlled by the electrolysis time. During electrolysis, stepless adjustment and precise control of the diameter of the NiTi alloy wire can be achieved by controlling the electrolysis time or the number of repetitions of the inversion thinning method. Description of the Drawings

[0024] Figure 1 Schematic diagram of the electrochemical machining device used in the present invention.

[0025] Figure 2 Schematic diagram of the preparation process of the NiTi alloy wire with high diameter uniformity in the present invention.

[0026] Figure 3 Specific dimension diagrams of the NiTi alloy wires prepared in Examples 1, 2, and 3 of the present invention and the original wire.

[0027] Figure 4 Mechanical property curves of the NiTi alloy wires prepared in Examples 1, 2, and 3 of the present invention and the original wire.

[0028] Figure 5 DSC curves of the NiTi alloy wires prepared in Examples 1, 2, and 3 of the present invention and the original wire.

[0029] In the figure: 1. NiTi alloy wire; 2. Cathode; 3. Electrolytic cell; 4. Nitric acid - methanol electrolyte. Specific implementation mode

[0030] The following will further elaborate on the present invention in detail.

[0031] Example 1 As Figure 1 , Figure 2 shown, a commercial NiTi alloy wire 1 with an original diameter of 0.45 mm is selected. Use diagonal pliers to cut a 100 - mm alloy wire from the raw material, and use 3000# fine sandpaper to polish and remove the surface oxide layer. After polishing, select 0.3 - μm alumina polishing liquid to polish the sample on the polishing cloth. The surface of the NiTi alloy wire 1 after mechanical polishing shows a metallic luster, and then it is soaked in absolute ethanol and ultrasonically cleaned for 5 min. Use a water bath to raise the temperature of the SMA to 20 °C above the A f temperature. The A f of the NiTi alloy wire 1 used in this example is 65 °C, the water - bath temperature is 85 °C, and it is maintained for 60 s to make it return to a straight state, and then it is dried with the cold - air mode of a hair dryer for standby.

[0032] Take 500 ml of nitric acid - methanol electrolyte 4 prepared according to a volume ratio of 1:3 and place it in the electrolytic cell 3. Use a sample clamp to fix the NiTi alloy wire 1 to be processed in the electrolytic cell 3, ensure that the NiTi alloy wire 1 is parallel to the stainless - steel tool cathode 2, the distance between the two electrodes is 50 mm, the length of the clamped end of the NiTi alloy wire 1 is 25 mm, and the immersion depth in the electrolyte is 75 mm. Measure the surface area of the NiTi alloy wire 1 immersed in the electrolyte to be 106.03 mm2 Taking 1.3 A / cm2 of current density as an index, the current during electrolytic polishing should be set to 1.37 A. Connect the positive pole of the DC power supply to the NiTi alloy wire 1 to be processed, and the negative pole to the tool electrode. Turn on the switch of the DC power supply, adjust the current to 1.37 A, and the first electrochemical machining time is 2 min. After the electrochemical machining is completed, take out the NiTi alloy wire 1. After the first electrochemical machining treatment, the NiTi alloy wire 1 is thinned along the axial gradient to form an inverted cone; Invert the processed NiTi alloy wire 1 and place it into the electrolyte in the opposite direction of the first electrochemical machining, and perform the second electrochemical machining with the same parameters and time, so that the NiTi alloy wire 1 is thinned along the axial gradient. The amount of thinning in the first electrochemical machining treatment is equal to that in the second electrochemical machining treatment, so that the inverted cone becomes a cylinder. Two electrochemical machining treatments with the same parameters are a complete uniform thinning treatment. After the machining is completed, quickly take it out, immerse it in absolute ethanol, ultrasonically clean it for 3 min, and then dry it with a hair dryer. The diameters of the clamping ends at both ends of the finally formed NiTi alloy wire 1 are larger than the gauge part in the middle, forming a dog-bone-shaped specimen.

[0033] Measure the specific dimensions of the NiTi alloy wire 1 with a vernier caliper. Starting from 0.5 mm at one end of the NiTi alloy wire 1, take points every 1 mm. After measurement, the gauge length of the NiTi alloy wire 1 is 50 mm, the diameter is 0.4 mm, the reduction of area rate is 20%, and the relative extreme difference of the diameter along the length direction is less than 3%. Clamp both ends of the thinned NiTi alloy wire 1 on the Shimadzu EZ-graph to test the mechanical properties of the NiTi alloy wire 1; take samples from the middle of the thinned NiTi alloy wire 1 for DSC testing. After testing, its mechanical properties are the same as those of the original NiTi alloy wire 1, and the phase transformation characteristic temperature fluctuates within 3 °C.

[0034] Example 2 The difference between this example and Example 1 is that after completing a complete uniform thinning treatment (including two electrochemical machining treatments), repeat this process 2 more times to obtain a smaller diameter of the NiTi alloy wire 1.

[0035] After measurement, the gauge part of the NiTi alloy wire 1 obtained in this example is 50 mm long, the diameter is 0.30 mm, the reduction of area rate is 55%, and the relative extreme difference of the diameter along the length direction is less than 3%. After testing the mechanical properties and DSC curve, its mechanical properties are the same as those of the original NiTi alloy wire 1, and the phase transformation characteristic temperature fluctuates within 3 °C.

[0036] Example 3 The difference between this embodiment and Embodiment 1 is that after a complete and uniform thinning process is completed, this process is repeated 4 more times to obtain a smaller diameter of the NiTi alloy wire 1.

[0037] After measurement, the gage length of the NiTi alloy wire 1 obtained in this embodiment is 50 mm, the diameter is 0.20 mm, the reduction of area is 80%, and the relative extreme difference of the diameter along the length direction is less than 3%. After mechanical property and DSC curve tests, its mechanical properties are the same as those of the original NiTi alloy wire 1, and the fluctuation of its phase transformation characteristic temperature is within 3 °C.

[0038] Comparative Example 1 This comparative example provides an electrochemical machining method for NiTi alloy wire without using the inversion method. The main difference is that during the electrochemical machining process, the NiTi alloy wire 1 is not taken out and inverted and then put back into the electrolyte, but the electrochemical machining process is continuously carried out.

[0039] The electrochemical machining time is 12 min, which is the same as the total electrochemical machining time in Embodiment 2. After the electrochemical machining is completed, it is quickly taken out, immersed in absolute ethanol for ultrasonic cleaning for 3 min, and then dried with a hair dryer. The finally formed NiTi alloy wire 1 has an inverted cone shape in the electrolyte.

[0040] Use a vernier caliper to measure the specific dimensions of the NiTi alloy wire 1. Starting from 0.5 mm from one end of the NiTi alloy wire 1, a point is taken every 1 mm. Starting from 0.5 mm from one end of the NiTi alloy wire 1, a point is taken every 1 mm. After measurement, the NiTi alloy wire 1 has an obvious diameter gradient along the length direction, the average diameter is 0.30 mm, and the relative extreme difference of the diameter along the length direction exceeds 30%.

[0041] According to the above embodiments and comparative examples, for the NiTi alloy wire 1 obtained in the embodiments, there is no obvious diameter gradient along the length direction, the relative extreme difference of the diameter along the length direction is less than 3%, and its diameter uniformity is greatly improved.

[0042] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for thinning NiTi alloy wire with high diameter uniformity by electrochemical machining, characterized in that: The following steps are included: S1. Remove the surface oxide layer of the NiTi alloy wire and clean and dry it; S2, heating the NiTi alloy wire treated in S1 to above the austenite transformation temperature, and then restoring the wire to its initial straight state; S3, fixing the NiTi alloy wire treated in S2 in an electrolytic cell, keeping the NiTi alloy wire parallel to the tool cathode, setting power supply parameters, and performing the first electrochemical processing to make the NiTi alloy wire thinned along the axial gradient to form an inverted cone; S4, taking out the NiTi alloy wire treated in S3 and placing it upside down in an electrolytic cell, and performing a second electrochemical processing with the same parameters to make the NiTi alloy wire thinner along the axial gradient, and the thinning amount of the first electrochemical processing is equal to the thinning amount of the second electrochemical processing, so that the inverted cone is changed into a cylindrical shape; S5. Repeat S3 and S4 until the NiTi alloy wire is thinned to a desired diameter.

2. The thinning method according to claim 1, characterized in that: In S1, the surface oxide layer is removed by mechanical polishing, and the surface of the NiTi alloy wire after mechanical polishing presents a metallic luster.

3. The thinning method according to claim 1, characterized in that: A nitric acid-methanol electrolyte is placed in the electrolytic cell, and the nitric acid-methanol electrolyte is configured according to a volume ratio of concentrated nitric acid: methanol = 1.0±0.5:3±0.

5.

4. The thinning method according to claim 3, characterized in that: The electrolyte was prepared by adding concentrated nitric acid to methanol, and an ice water bath was used for cooling throughout the process.

5. The thinning method according to claim 1, characterized in that: The distance between the cathode and anode of the electrolytic cell is controlled at 45mm to 50mm.

6. The thinning method according to claim 1, characterized in that: The current density of the first electrochemical machining and the second electrochemical machining is 1.1A / cm 2 ~1.5A / cm 2 .

7. The thinning method according to claim 1, characterized in that: The time for the first electrochemical machining and the second electrochemical machining is 2 minutes to 15 seconds.

8. The thinning method according to claim 1, characterized in that: In the first electrochemical machining and the second electrochemical machining, the NiTi alloy wires are placed in the electrolytic cell in opposite directions and have the same length.

9. The thinning method according to claim 1, characterized in that: By controlling the electrolysis time or the number of repetitions of the inverted thinning method, the diameter of the NiTi alloy wire can be adjusted steplessly and precisely controlled.

10. The thinning method according to claim 1, characterized in that: When the area reduction rate of NiTi alloy wire reaches 80%, the relative range of its diameter along the length direction is less than 3%.

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