A method of welding a nickel-titanium alloy wire to a stainless steel wire

Abstract

This invention provides a method for welding nickel-titanium alloy wire and stainless steel wire, specifically including the following steps: clamping the nickel-titanium alloy wire and stainless steel wire onto two electrodes of a resistance welding device; controlling the weldable end faces of the nickel-titanium alloy wire and stainless steel wire to extend beyond the electrodes and adhere to each other; setting welding parameters such that the energy provided by the resistance welding device is initially greater than the melting temperature of the nickel-titanium alloy wire and stainless steel wire, and then less than the melting temperature of the nickel-titanium alloy wire and stainless steel wire; continuously applying pressure to the two electrodes of the resistance welding device along the welding direction, so that after the resistance welding device is turned on, the compounds generated on the weldable end faces of the nickel-titanium alloy wire and stainless steel wire are first squeezed out, and then the nickel-titanium alloy wire and stainless steel wire are welded together; and subjecting the welded sample to annealing heat treatment. This invention solves the problem of decreased mechanical properties caused by excessive intermetallic compounds and facilitates control of the pressure welding process at the critical melting temperature.

Description

Technical Field

[0001] This invention belongs to the field of welding technology, specifically a welding method for nickel-titanium alloy wire and stainless steel wire. Background Technology

[0002] In the automotive manufacturing field, nickel-titanium alloys are commonly used in automotive engine heat shield fan clutches, automatic exhaust nozzle adjustment systems, diesel engine radiator vent automatic switches, and shape memory springs for jet engine oil filters. In 2013, Leary et al. verified the resistivity of Ni-Ti SMA linear actuators through theoretical calculations and experiments, calculated the resistance of various actuator arrangements, and proved the feasibility of directly powering SMA actuators from automotive batteries. In 2017, NASA developed a pneumatic tire made of nickel-titanium alloy, suitable for complex terrain. In the same year, Wu Zhipeng used Ni-Ti shape memory alloy wire as a primary energy-absorbing material to design a shape memory alloy automotive secondary energy-absorbing device, and conducted experimental and simulation studies on the energy absorption of each stage of the device. Shen et al. experimentally measured that martensitic nickel-titanium wire has good impact protection performance, laying the foundation for the application of novel shape memory damping elements in automotive safety systems.

[0003] Stainless steel is one of the most widely used structural materials in various industrial fields due to its excellent heat and corrosion resistance, superior weldability, and thermal stability. Because of its unparalleled performance and cost-effectiveness, it is widely considered an important component in the development of advanced technologies such as aerospace, automotive, pressure vessels, and corrosion-resistant piping. Joining these two alloys can combine their advantages; for example, after joining, stainless steel can be used as the structural material, while the nickel-titanium alloy can utilize its shape memory and energy absorption functions, thus expanding their application range. However, due to the different physical and chemical properties of these two alloys, direct welding of NiTi alloy and stainless steel faces two main challenges. The first is the formation of brittle intermetallic compounds (IMCs) in the joint, such as the brittle TiFe2 and TiCr2 phases. The second is the generation of cracks and residual stresses: the coefficients of linear expansion of the NiTi alloy and the stainless steel base material are 8.6 μm·(m K)⁻¹ and 16.6 μm·(m K)⁻¹, respectively. During the welding process, the two base materials deform differently. After the welding process is completed, assuming that the two materials are initially at the same temperature, the stainless steel will shrink significantly during the cooling process, and the residual stress generated will be much higher than that of the NiTi alloy. Without considering any phases formed in the weld, based on the formation of IMC in the joint, and the stress mismatch between the NiTi alloy and the stainless steel base material at the joint, cracks are very easy to form.

[0004] Based on current research, the key challenge in welding nickel-titanium alloys and stainless steel remains suppressing the formation of intermetallic compounds. Current methods primarily involve adding an intermediate layer and adjusting the welding process. However, the performance of welded joints using existing technologies needs improvement. Furthermore, there is limited research on welding nickel-titanium alloy wires and stainless steel wires, and no related research has been found in Chinese patents. This significantly limits the development of fields such as pneumatic tires, secondary energy-absorbing devices for automobiles, and automotive safety systems. Summary of the Invention

[0005] The purpose of this invention is to improve and innovate upon the shortcomings and problems existing in the prior art, and to provide a welding method for nickel-titanium alloy wire and stainless steel wire.

[0006] This invention provides a method for welding nickel-titanium alloy wire and stainless steel wire, specifically including the following steps: Step S1: Clamp the nickel-titanium alloy wire and the stainless steel wire onto the two electrodes of the resistance welding equipment respectively; control the welding end faces of the nickel-titanium alloy wire and the stainless steel wire to extend out of the electrodes and stick together with each other; Step S2: Set the welding parameters so that the energy provided by the resistance welding equipment is first sufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire, and then the energy provided by the resistance welding equipment is reduced to a level that is insufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire. Step S3: Apply pressure continuously to the two electrodes of the resistance welding device along the welding direction so that after the resistance welding device is turned on, the compound generated on the welding end face of the nickel-titanium alloy wire and the stainless steel wire is squeezed off first, and then the nickel-titanium alloy wire and the stainless steel wire are welded together. Step S4: Perform annealing heat treatment on the welded sample.

[0007] As can be seen from the above technical solution, after the resistance welding equipment is turned on, the energy provided by the equipment first melts the welded ends of the nickel-titanium alloy wire and the stainless steel wire. Although intermetallic compounds are generated, under continuous pressure, these compounds are squeezed out. Then, the energy provided by the resistance welding equipment decreases, and the welded ends of the nickel-titanium alloy wire and the stainless steel wire are no longer melted. Because the temperature of the welded ends of the nickel-titanium alloy wire and the stainless steel wire drops slowly in a short time, pressure welding is achieved at the critical melting temperature, thereby enabling the atomic diffusion rate to reach the theoretical optimal rate, improving the mechanical properties of the welded joint, and greatly enhancing the tensile strength of the welded sample. Therefore, this application organically combines fusion welding and pressure welding. Compared with fusion welding, it can greatly suppress the generation of intermetallic compounds, while compared with pressure welding, it is easier to control the pressure welding process at the critical melting temperature, and easier to control the atomic diffusion rate to reach the theoretical optimal rate during the welding process. In addition, this application performs annealing heat treatment on the welded samples to optimize the samples. While maintaining almost no loss of tensile strength, it greatly improves the plasticity of the welded joint and reduces the risk of fatigue fracture when the material is used in automotive energy absorption devices, tires and suspension systems in the future.

[0008] A further embodiment is that step S2 specifically includes: The resistance welding equipment first heats the nickel-titanium alloy wire and the stainless steel wire with a first preset current, and then heats the nickel-titanium alloy wire and the stainless steel wire with a second preset current. The energy provided by the first preset current is sufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire, while the energy provided by the second preset current is insufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire.

[0009] A further embodiment is that step S1 specifically includes: The ratio of the squares of the lengths of the welded end faces of the nickel-titanium alloy wire and the stainless steel wire extending beyond the electrode is equal to the ratio of the Young's modulus of the nickel-titanium alloy wire and the stainless steel wire. Right now ,in The length of the nickel-titanium alloy wire extending beyond the electrode from the welded end face, where... The length of the stainless steel wire extending beyond the electrode from the welded end face, where... The Young's modulus of the nickel-titanium alloy wire is given by: This refers to the Young's modulus of stainless steel wire.

[0010] As can be seen from the above technical solution, this application controls the ratio of the squares of the lengths of the welded end faces of the nickel-titanium alloy wire and the stainless steel wire extending beyond the electrode to be equal to the Young's modulus ratio of the nickel-titanium alloy wire and the stainless steel wire. Thus, under pressure, the nickel-titanium alloy wire and the stainless steel wire have compatible resistance to instability, which is conducive to applying sufficient pressure to the welded end faces of the nickel-titanium alloy wire and the stainless steel wire. Applying as much pressure as possible is conducive to promoting diffusion between atoms, making the welding quality more uniform and stable, and further improving the mechanical properties of the welded joint.

[0011] A further embodiment is that, in step S3, continuously applying pressure along the welding direction to the two electrodes of the resistance welding device specifically includes: The magnitude of the continuously applied pressure is controlled to prevent the nickel-titanium alloy wire and / or stainless steel wire from bending. It is understood that bending of the nickel-titanium alloy wire and / or stainless steel wire makes material alignment difficult, especially for thin wire welding, which severely affects weld quality. Therefore, this application controls the magnitude of the continuously applied pressure, which can both promote atomic diffusion through pressure and avoid material alignment difficulties.

[0012] A further option is that the nickel-titanium alloy wire and the stainless steel wire have the same diameter.

[0013] A further option is that the continuously applied pressure is no greater than [amount missing]. ; in, n is a constant. The Young's modulus of the nickel-titanium alloy wire is given by: This refers to the Young's modulus of stainless steel wire. The length of the nickel-titanium alloy wire extending beyond the electrode from the welded end face, where... The length of the stainless steel wire extending beyond the electrode from the welded end face. It is the polar moment of inertia.

[0014] A further option is that the diameter of both the nickel-titanium alloy wire and the stainless steel wire is 0.3 mm.

[0015] A further option is that, in step S4, the annealing heat treatment of the welded sample is replaced by vacuum tube furnace heat treatment of the welded sample.

[0016] A further approach is that step S4, which involves heat-treating the welded sample in a vacuum tube furnace, specifically includes: First, the temperature is increased at 5℃ / min until it reaches above the recrystallization temperature of the sample. Then, it is held at this temperature for 30 minutes and finally allowed to cool naturally in the vacuum tube furnace.

[0017] As can be seen from the above technical solution, the present application can heat the sample evenly by heating at 5℃ / min, and the holding time of 30min can ensure that the internal stress of the material is completely released. The sample plasticity is greatly improved when the temperature is raised above the recrystallization temperature of the sample. Moreover, compared with the traditional heat treatment process, the sample obtained by the present application has almost no difference in strength from the untreated sample.

[0018] A further option is that the device that continuously applies pressure to the two electrodes of the resistance welding equipment along the welding direction is a hydraulic system, a pneumatic system, or a servo motor.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: After the resistance welding equipment is turned on, the energy provided by the resistance welding equipment first melts the welded end faces of the nickel-titanium alloy wire and the stainless steel wire. Although intermetallic compounds are generated, under the continuous pressure, the compounds generated on the welded end faces of the nickel-titanium alloy wire and the stainless steel wire will be squeezed out. Then, the energy provided by the resistance welding equipment decreases, and the welded end faces of the nickel-titanium alloy wire and the stainless steel wire will no longer melt. Because the temperature of the welded end faces of the nickel-titanium alloy wire and the stainless steel wire drops slowly in a short time, pressure welding is achieved at the critical melting temperature of the welded end faces of the nickel-titanium alloy wire and the stainless steel wire. This allows the atomic diffusion rate to reach the theoretical optimal rate, improves the mechanical properties of the welded joint, and greatly enhances the tensile strength of the welded sample. Therefore, the present application organically combines fusion welding and pressure welding. Compared with fusion welding, it can greatly suppress the generation of intermetallic compounds. Compared with pressure welding, it is convenient to control the pressure welding process at the critical melting temperature and to control the atomic diffusion rate to reach the theoretical optimal rate during the pressure welding process. In addition, the samples obtained by welding in this application are subjected to annealing heat treatment, which optimizes the samples. Compared with the traditional annealing heat treatment process, the test data shows that the samples of this application have greatly improved the plasticity of the welded joint while almost without losing tensile strength, which reduces the risk of fatigue fracture when the material is used in automotive energy absorption devices, tires and suspension systems in the future. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of a welding method between nickel-titanium alloy wire and stainless steel wire provided in an embodiment of the present invention. Figure 2Tensile strength diagrams of samples obtained under different welding currents, provided in embodiments of the present invention; Figure 3 This is a light microscope image of a sample obtained by welding at a welding current of 40, as provided in an embodiment of the present invention. Figure 4 Electron micrograph of a sample obtained by annealing heat treatment of a sample with a welding current of 40 according to an embodiment of the present invention; Figure 5 The tensile curve of the sample after annealing heat treatment when the welding current is 40, as provided in the embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, features, and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0023] It should be noted that when a component is said to be "fixed to" another component, it can be directly attached to the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0025] Please see Figure 1 This invention provides a method for welding nickel-titanium alloy wire and stainless steel wire, specifically including the following steps: Step S1: Clamp the nickel-titanium alloy wire and the stainless steel wire onto the two electrodes of the resistance welding equipment respectively; control the welding end faces of the nickel-titanium alloy wire and the stainless steel wire to extend out of the electrodes and stick together with each other; Specifically, the two electrodes of the resistance welding equipment are equipped with clamps for clamping nickel-titanium alloy wires and stainless steel wires. The clamps are provided with several fixing slots for placing nickel-titanium alloy wires or stainless steel wires. The nickel-titanium alloy wires and stainless steel wires are placed into the corresponding fixing slots and then clamped using the clamps provided with the resistance welding equipment.

[0026] During the clamping process of nickel-titanium alloy wire and stainless steel wire, it is necessary to control the welding end faces of the nickel-titanium alloy wire and stainless steel wire to extend out of the electrode and stick together with each other so that the nickel-titanium alloy wire and stainless steel wire can be welded together after the resistance welding equipment is turned on.

[0027] Step S2: Set the welding parameters so that the energy provided by the resistance welding equipment is first sufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire, and then the energy provided by the resistance welding equipment is reduced to a level that is insufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire. Specifically, the resistance welding equipment can be controlled to first heat the nickel-titanium alloy wire and the stainless steel wire with a first preset current, and then heat the nickel-titanium alloy wire and the stainless steel wire with a second preset current; wherein, the energy provided by the first preset current is sufficient to melt the welded end faces of the nickel-titanium alloy wire and the stainless steel wire, while the energy provided by the second preset current is insufficient to melt the welded end faces of the nickel-titanium alloy wire and the stainless steel wire.

[0028] For example, in this embodiment, the diameter of the nickel-titanium alloy wire is set to 0.3 mm, and the diameter of the stainless steel wire 20 is also set to 0.3 mm. The welding parameters are specifically set as follows: the first preset current is a welding current of 38 ohms, and the welding time is 2 seconds; the second preset current is an annealing current of 10 ohms, and the annealing time is 5 seconds. It should be noted that the welding current and annealing current have no units; these are definitions of current magnitude by the equipment itself, and the current parameters vary depending on the resistance welding equipment. Under the heating action of the first preset current, the weld joint will melt and produce intermetallic compounds. However, under the action of the second preset current, the weld joint will not melt and produce intermetallic compounds. Because the second preset current is used immediately after the first preset current heating action, the temperature at the weld joint will be maintained close to the melting temperature critical point, making the welding process closer to pressure welding. The metal atoms on the surfaces of the nickel-titanium alloy wire and the stainless steel wire approach to a distance where atoms can interact (approximately 0.3~0.5 nm) to form metallic bonds, thereby connecting the two metals into one, achieving the welding purpose. Compared to fusion welding, this greatly suppresses the formation of intermetallic compounds.

[0029] It is important to note that when starting the resistance welding equipment, simply move the lever on the equipment to initiate welding. After welding is complete, one of the two clamps must be opened first, and then the lever must be moved back to open the other clamp. If the lever is moved directly, the two clamps will move away in opposite directions. Since the clamps are not yet open, this will inevitably cause tension on the welded joint, affecting its performance.

[0030] Step S3: Apply pressure continuously to the two electrodes of the resistance welding device along the welding direction so that after the resistance welding device is turned on, the compound generated on the welding end face of the nickel-titanium alloy wire and the stainless steel wire is squeezed off first, and then the nickel-titanium alloy wire and the stainless steel wire are welded together. Specifically, after setting the welding parameters in step S2, pressure is continuously applied to the two electrodes of the resistance welding device along the welding direction. After the resistance welding device is turned on, as mentioned above, the energy provided by the welding current can melt the welded end faces of the nickel-titanium alloy wire and the stainless steel wire, and produce intermetallic compounds. However, because this application continuously applies pressure to the two electrodes along the welding direction, it will squeeze out the intermetallic compounds and part of the base material generated on the welded end faces of the nickel-titanium alloy wire and the stainless steel wire. Then, the nickel-titanium alloy wire and the stainless steel wire are heated with annealing current. At this time, the welded end faces of the nickel-titanium alloy wire and the stainless steel wire will no longer melt. The weld ends melt, and the temperature of the welded ends of the nickel-titanium alloy wire and stainless steel wire drops slowly in a short time, allowing pressure welding to be achieved at the critical melting temperature of the welded ends of the nickel-titanium alloy wire and stainless steel wire. Compared with traditional pressure welding, this application makes it easier to control the pressure welding process at the critical melting temperature. It can be seen that this application organically combines fusion welding and pressure welding, which enables the atomic diffusion rate corresponding to pressure welding to reach the theoretical optimal rate, improves the mechanical properties of the welded joint, greatly enhances the tensile strength of the welded sample, and removes the intermetallic compounds generated by the welded joint.

[0031] Furthermore, applying pressure during welding promotes metal diffusion and chemical reactions, which is beneficial for weld fusion and formation. This results in more uniform and stable weld quality. Without pressure, weld quality may be affected, leading to defects such as incomplete welds, weld beads, and slag inclusions. Therefore, in this embodiment, pressure is continuously applied to the weld joint along the welding direction using the two electrodes. To controllably apply pressure to the two electrodes, they are connected to a servo motor and a pressure sensor. The pressure of the two electrodes is provided by the servo motor, and the pressure sensor monitors the pressure value in real time and feeds the result back to the servo motor. Of course, the pressure of the two electrodes can also be provided by a pneumatic or hydraulic system; this application does not impose specific limitations, and all such limitations are within the scope of protection of this application.

[0032] However, it is important to note that in order to maximize atomic diffusion, achieve more uniform and stable weld quality, and improve the mechanical properties of the weld joint, sufficient pressure should be applied to the welded ends of the nickel-titanium alloy wire and the stainless steel wire. However, excessive pressure can lead to instability in the nickel-titanium alloy wire and the stainless steel wire, making it difficult to align the materials when they bend, especially for welding thin wires, which will severely affect the weld quality. Therefore, in this embodiment, the ratio of the squares of the lengths of the welded ends of the nickel-titanium alloy wire and the stainless steel wire extending beyond the electrode is controlled to be equal to the ratio of their Young's moduli. This ensures that the resistance to instability of the nickel-titanium alloy wire and the stainless steel wire is compatible, thereby applying maximum pressure to the welded ends of the nickel-titanium alloy wire and the stainless steel wire. This ensures that the nickel-titanium alloy wire and the stainless steel wire are applied in a commensurate manner, thus applying sufficient pressure to the weld joint without bending the wires, thereby improving the mechanical properties of the weld joint. Specifically, the ratio of the squares of the lengths of the welded ends of the nickel-titanium alloy wire and the stainless steel wire extending beyond the electrode is equal to the ratio of their Young's moduli. ,in The length of the nickel-titanium alloy wire extending beyond the electrode from the welded end face, where... The length of the stainless steel wire extending beyond the electrode from the welded end face, where... The Young's modulus of the nickel-titanium alloy wire is given by: This refers to the Young's modulus of stainless steel wire.

[0033] Furthermore, the continuously applied pressure is no greater than ; in, n is a constant, a length factor, and is related to the clamping method. The Young's modulus of the nickel-titanium alloy wire is given by: This refers to the Young's modulus of stainless steel wire. The length of the nickel-titanium alloy wire extending beyond the electrode from the welded end face, where... The length of the stainless steel wire extending beyond the electrode from the welded end face. The polar moment of inertia is related to the cross-sectional shape. In this embodiment, the diameters of both the nickel-titanium alloy wire and the stainless steel wire are set to 0.3 mm.

[0034] It should be noted that because the Young's modulus of stainless steel wire is greater than that of nickel-titanium alloy wire, when the length of the welded end face of the stainless steel wire extending beyond the electrode is the same as that of the nickel-titanium alloy wire, F CR不锈钢 >F CR镍钛 Therefore, when the extension length is the same, nickel-titanium alloy wire is more prone to instability than stainless steel wire; that is, when the applied pressure is greater than F... CR镍钛If the nickel-titanium alloy wire is not properly aligned, it will become unstable, leading to difficulties in material alignment, especially in welding thin wires, which severely affects the weld quality. Therefore, stainless steel wire is more resistant to instability. In order to apply greater pressure to promote atomic diffusion, the stainless steel wire should extend beyond the length of the nickel-titanium alloy wire.

[0035] Furthermore, in order to illustrate the welding effect of the method of the present invention in more detail and to make the present invention more convincing, the welding current was changed multiple times while keeping other parameters unchanged. Here, welding effect data of nickel-titanium alloy wire and stainless steel wire overall samples obtained with welding currents of 40 and 42 are provided.

[0036] Figure 2 The graph compares the tensile strength under three welding currents. It shows that the tensile mechanical properties are best at a current of 40 N / A, reaching 53.7 N, which is higher than 44.66 N at a current of 38 N / A and 43.01 N at a current of 42 N / A. Since the diameter of the base material is 0.3 mm, its area is approximately 0.07 x 10⁻⁶ mm. -6 m 2 According to calculations, when the current is 40, its tensile strength is 759.69 MPa.

[0037] Furthermore, Figure 3 The image shows a light micrograph of the welded joint of the sample when the welding current is 40 ohms. Figure 3 It can be seen that the fusion line morphology is good, and no obvious welding defects or intermetallic compound layers were found.

[0038] Step S4: Perform annealing heat treatment on the welded sample; Specifically, in this embodiment, the sample with a welding current of 40 is subjected to vacuum tube furnace heat treatment. The heat treatment heating stage is 5℃ / min, the heat treatment temperature is 700℃, and it is held at 700℃ for 30min. Finally, the sample is allowed to cool naturally in the vacuum tube furnace.

[0039] It should be noted that, firstly, the theoretical minimum recrystallization temperature of the material is 0.4Tm. To ensure that the sample of this invention can reach its recrystallization temperature, the temperature is set above the melting point of stainless steel wire (0.5Tm) (because the melting point of stainless steel wire is higher, at 1538℃). Therefore, according to the formula, 0.5Tm = (1538 + 273) x 0.5 - 273 ≈ 620℃. Therefore, during the experiment, the temperature is rounded up, i.e., the heat treatment temperature is set to 700℃, to ensure that the heat treatment temperature rises above the sample's recrystallization temperature. To ensure uniform heating of the sample, a conventional heating rate of 5℃ / min is used. Holding at this temperature for 30 minutes ensures complete release of internal stress in the material. Figure 4The image shows an electron microscope image of the sample obtained after heat treatment. As can be seen from the image, the internal structure of the sample is uniform, and the grains are equiaxed, undistorted grains commonly found after heat treatment, which verifies the rationality of the heat treatment parameters.

[0040] Figure 5 The figure shows the tensile curve of the sample after heat treatment. It can be seen that the sample was subjected to a force of 52.33 N and a tensile strength of 740.32 MPa, almost identical to the tensile strength before treatment; however, the sample's plasticity was greatly improved. Before heat treatment, the sample had almost no plasticity, such as... Figure 2 As shown, after heat treatment, the plastic deformation stage of the sample reached nearly 0.25 mm, which demonstrates the rationality of the heat treatment parameter settings and the necessity of the heat treatment process.

[0041] In summary, after the resistance welding equipment is turned on, the energy provided by the equipment first melts the welded ends of the nickel-titanium alloy wire and the stainless steel wire. Although intermetallic compounds are generated, under continuous pressure, these compounds and some of the base material are squeezed out. Then, the energy provided by the resistance welding equipment decreases, and the welded ends of the nickel-titanium alloy wire and the stainless steel wire are no longer melted. Because the temperature of the welded ends of the nickel-titanium alloy wire and the stainless steel wire drops slowly in a short time, pressure welding is achieved at the critical melting temperature of the welded ends of the nickel-titanium alloy wire and the stainless steel wire. This allows the atomic diffusion rate to reach the theoretical optimal rate, improving the mechanical properties of the welded joint and greatly enhancing the tensile strength of the welded sample. Therefore, this application organically combines fusion welding and pressure welding. Compared with fusion welding, it can greatly suppress the generation of intermetallic compounds, while compared with pressure welding, it is easier to control the pressure welding process at the critical melting temperature and to control the atomic diffusion rate to reach the theoretical optimal rate during the pressure welding process. In addition, the samples obtained by welding in this application are subjected to annealing heat treatment, which optimizes the samples. Compared with the traditional annealing heat treatment process, the test data shows that the samples of this application have greatly improved the plasticity of the welded joint while almost without losing tensile strength, which reduces the risk of fatigue fracture when the material is used in automotive energy absorption devices, tires and suspension systems in the future.

[0042] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0043] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method of welding a nickel-titanium alloy wire to a stainless steel wire, characterized by: Specifically, the following steps are included: Step S1: Clamp the nickel-titanium alloy wire and the stainless steel wire onto the two electrodes of the resistance welding equipment respectively; control the welding end faces of the nickel-titanium alloy wire and the stainless steel wire to extend out of the electrodes and stick together with each other; Wherein, the ratio of the squares of the lengths of the welded end faces of the nickel-titanium alloy wire and the stainless steel wire extending out of the electrode is equal to the ratio of the Young's modulus of the nickel-titanium alloy wire and the stainless steel wire. That is wherein is the length of the electrode protruding from the welded end face of the nickel-titanium alloy wire, wherein is the length of the electrode protruding from the welded end face of the stainless steel wire, wherein is the Young's modulus of the nickel-titanium alloy wire; and is the Young's modulus of the stainless steel wire. Step S2: Set the welding parameters so that the energy provided by the resistance welding equipment is first sufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire, and then the energy provided by the resistance welding equipment is reduced to a level that is insufficient to melt the welded ends of the nickel-titanium alloy wire and the stainless steel wire. Step S2 specifically includes: controlling the resistance welding equipment to first heat the nickel-titanium alloy wire and the stainless steel wire with a first preset current, and then heat the nickel-titanium alloy wire and the stainless steel wire with a second preset current; wherein, the energy provided by the first preset current is sufficient to melt the welded end faces of the nickel-titanium alloy wire and the stainless steel wire, while the energy provided by the second preset current is insufficient to melt the welded end faces of the nickel-titanium alloy wire and the stainless steel wire. Step S3: Apply pressure continuously to the two electrodes of the resistance welding device along the welding direction so that after the resistance welding device is turned on, the compound generated on the welded end face of the nickel-titanium alloy wire and the stainless steel wire is squeezed out during the welding energy application process, and then the nickel-titanium alloy wire and the stainless steel wire are welded together. The size of the continuously applied pressure is controlled so that the nickel-titanium alloy wire and / or the stainless steel wire is not bent; the diameters of the nickel-titanium alloy wire and the stainless steel wire are equal; and the continuously applied pressure is not greater than ; wherein n is a constant, E is the Young's modulus of the nickel-titanium alloy wire, wherein E is the Young's modulus of the stainless steel wire, L is the length of the nickel-titanium alloy wire protruding from the electrode at the welded end face, wherein L is the length of the stainless steel wire protruding from the electrode at the welded end face, I is the polar moment of inertia; Step S4: Perform vacuum tube furnace heat treatment on the welded sample. The vacuum tube furnace heat treatment specifically includes: first, heating at 5℃ / min until the temperature reaches above the sample recrystallization temperature, then holding at that temperature for 30 minutes, and finally allowing natural cooling in the vacuum tube furnace.

2. The welding method for nickel-titanium alloy wire and stainless steel wire according to claim 1, characterized in that: The diameter of both the nickel-titanium alloy wire and the stainless steel wire is 0.3 mm.

3. The welding method for nickel-titanium alloy wire and stainless steel wire according to claim 1, characterized in that: The device that continuously applies pressure to the two electrodes of the resistance welding equipment along the welding direction is a hydraulic system, a pneumatic system, or a servo motor.

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