Low temperature brazing method of NiTi shape memory alloy and 316L stainless steel

Abstract

The application relates to a brazing method, in particular to a low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel; in order to solve the problem of poor joint precision prepared by adopting the existing connection method of the NiTi shape memory alloy and the 316L stainless steel, the low-temperature brazing method comprises the following steps: (1) preparation of the NiTi alloy and the 316L stainless steel sample; (2) brazing of the NiTi alloy and the 316L stainless steel; the obtained joint still maintains good integrity after being soaked in an SBF simulated body fluid for 72 hours, no cracks and holes appear in the joint, and the corrosion resistance is good; the maximum strength of the brazed joint is 34.3 MPa, and the use strength of the medical instrument can be met.

Description

Technical Field

[0001] This invention relates to a brazing method, and more particularly to a low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel. Background Technology

[0002] Shape memory alloys (MMEs) have found wide application in aerospace, medical, electronics, construction, and automotive fields due to their shape memory effect and superelasticity. With increasing application demands, NiTi MMEs alone are too expensive and lack toughness to meet product performance requirements. Therefore, it is necessary to combine NiTi MMEs with other materials, particularly with 316 stainless steel, to achieve complementary functions. For example, guidewires combining NiTi MMEs and 316 stainless steel offer good support and ease of insertion at the proximal end (stainless steel) and good compliance at the distal end (NiTi MME), facilitating introduction into tortuous blood vessels.

[0003] In the current welding process between NiTi alloy and 316L stainless steel: traditional fusion welding, such as laser welding and arc welding, tends to result in coarse grains in the fusion zone and brittle joints due to the welding temperature being higher than the melting point of the base material. It also tends to absorb N, O, H, etc., leading to large post-weld strain and poor precision in the joint. Solid-state welding, such as explosive welding and friction welding, has the disadvantages of limited flexibility and large post-weld deformation, making it unsuitable for precision instrument connections. It also affects the shape memory effect of NiTi alloy. Therefore, neither of these methods can meet the requirements of medical device manufacturing. Summary of the Invention

[0004] To address the issue of poor welding performance between existing NiTi shape memory alloys and 316L stainless steel, this invention provides a low-temperature brazing method for NiTi shape memory alloys and 316L stainless steel. Joints brazed using this method maintain good integrity after being immersed in simulated body fluid for 72 hours, with no cracks or holes appearing in the joints, demonstrating good corrosion resistance.

[0005] A low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel includes the following steps:

[0006] Step 1: Grind and clean the NiTi alloy and 316L stainless steel surfaces to be welded for 10-20 minutes.

[0007] Step 2: After cleaning the AuSi eutectic solder for 10-20 minutes, place it between the NiTi alloy surface to be soldered and the 316L stainless steel surface to be soldered, forming a workpiece to be soldered in the order of NiTi alloy / AuSi solder / 316L stainless steel.

[0008] Step 3: Place the workpiece to be welded into a vacuum brazing furnace, heat it under vacuum to 550-625℃, and then cool it to room temperature to obtain a NiTi alloy / 316L stainless steel brazed joint.

[0009] In step one of this invention, the NiTi alloy and 316L stainless steel surfaces to be soldered are ground to a surface roughness Ra≤2.0μm. The smooth surfaces allow for a more complete reaction between the NiTi alloy, 316L stainless steel, and AuSi eutectic solder.

[0010] In steps one and two of this invention, the NiTi alloy surface to be soldered, the 316L stainless steel surface to be soldered, and the AuSi eutectic solder are first ultrasonically cleaned with acetone for 5-15 minutes, and then ultrasonically cleaned with alcohol for 5 minutes to obtain dust-free and oil-free soldering materials.

[0011] The AuSi eutectic solder used in step two of this invention has a composition of AuSi3.15, a thickness of 50μm, and an area size that is the same as that of the NiTi alloy or 316L stainless steel surface to be soldered.

[0012] Step three of this invention, which involves vacuum heating to 550-625°C followed by cooling to room temperature, includes 5x10... - 3 Heating is performed under vacuum conditions below Pa, with the temperature increased to 550-625℃ at a rate of 5℃ / min and brazed and held at that temperature for 10-40 minutes. After brazing, the temperature is reduced to 200℃ at a rate of 10℃ / min, and finally cooled to room temperature with the furnace.

[0013] Before the brazing heating in step three of this invention, the workpiece to be brazed is preheated: the temperature is raised to 400°C at a rate of 5°C / min and held for 10 minutes. This makes the composition of the AuSi eutectic brazing filler metal more uniform and accelerates the reaction between the NiTi alloy, 316L stainless steel and AuSi eutectic brazing filler metal.

[0014] The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel of the present invention first prepares samples of NiTi alloy and 316L stainless steel, then clamps them in the order of NiTi alloy / AuSi brazing filler metal / 316L stainless steel, and places them in a vacuum brazing furnace. Welding is carried out at a certain brazing temperature and holding time to obtain a NiTi alloy / 316L stainless steel brazed joint. After electrochemical corrosion, no obvious corrosion phenomenon was found in the obtained joint, which has wide applicability.

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

[0016] 1. The present invention uses a preheating method to ensure close contact between the base material and the solder, which is conducive to the diffusion of interface elements and facilitates the formation of subsequent compounds.

[0017] 2. The AuSi eutectic solder used in this invention has a low melting point and good biocompatibility, making it very suitable for connecting medical devices. The NiTiSi layer and Ni4Si7Ti4 whiskers generated at the NiTi shape memory alloy help improve the joint strength. At the same time, the NiSiTi layer can also hinder the diffusion of elements into the NiTi alloy, thus preserving the shape memory properties of the NiTi alloy.

[0018] 3. This invention can achieve a reliable connection between NiTi shape memory alloy and stainless steel at a lower temperature, which can largely preserve the shape memory effect of NiTi alloy and reduce stress and deformation caused by high temperature.

[0019] 4. The method of the present invention is simple and efficient, with fewer requirements on the shape and size of the workpiece, and can be widely used in the connection between NiTi shape memory alloy and stainless steel. Attached Figure Description

[0020] Figure 1 The microstructure is that of AuSi eutectic solder;

[0021] Figure 2 This is a schematic diagram of the brazing assembly.

[0022] Figure 3 The microstructure of the NiTi shape memory alloy / 316L stainless steel brazed joints obtained in Examples 1-4 (brazing and holding for 30 min);

[0023] Figure 4 Microstructure diagrams of the NiTi shape memory alloy / 316L stainless steel brazed joints obtained in Examples 3 and 5-7 (brazing temperature 600℃);

[0024] Figure 5 The shear strength of the NiTi shape memory alloy / 316L stainless steel brazed joints obtained in Examples 1-4 (brazing and holding for 30 min);

[0025] Figure 6 The shear strength of the NiTi shape memory alloy / 316L stainless steel brazed joints obtained in Examples 3 and 5-7 (brazing temperature 600℃);

[0026] Figure 7 The image shows the microstructure of the brazed joint obtained in Example 3 after immersion in SBF solution for 72 hours.

[0027] Figure 8This is a microstructure diagram of a joint obtained by laser welding NiTi shape memory alloy / 316L stainless steel using silver-based brazing filler metal as an intermediate layer in the prior art. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0029] A low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel includes the following steps:

[0030] Step 1: Grind and clean the NiTi alloy and 316L stainless steel surfaces to be welded for 10-20 minutes.

[0031] Step 2: After cleaning the AuSi eutectic solder for 10-20 minutes, place it between the NiTi alloy surface to be soldered and the 316L stainless steel surface to be soldered, forming a workpiece to be soldered in the order of NiTi alloy / AuSi solder / 316L stainless steel.

[0032] Step 3: Place the workpiece to be welded into a vacuum brazing furnace, heat it under vacuum to 550-625℃, and then cool it to room temperature to obtain a NiTi alloy / 316L stainless steel brazed joint.

[0033] Preferably, in step one, the NiTi alloy and 316L stainless steel surfaces to be soldered are ground to a surface roughness Ra≤2.0μm. A smooth surface allows the NiTi alloy, 316L stainless steel and AuSi eutectic solder to react more fully.

[0034] Preferably, in steps one and two, the NiTi alloy surface to be soldered, the 316L stainless steel surface to be soldered, and the AuSi eutectic solder are first ultrasonically cleaned with acetone for 5-15 minutes, and then ultrasonically cleaned with alcohol for 5 minutes to obtain dust-free and oil-free soldering materials.

[0035] Preferably, the AuSi eutectic solder used in step two has a composition of AuSi3.15, a thickness of 50μm, and an area size that is the same as that of the NiTi alloy or 316L stainless steel solder surface to be soldered.

[0036] Preferably, the vacuum heating to 550-625°C followed by cooling to room temperature in step three includes 5x10 -3Heating is performed under vacuum conditions below Pa, with the temperature increased to 550-625℃ at a rate of 5℃ / min and brazed and held at that temperature for 10-40 minutes. After brazing, the temperature is reduced to 200℃ at a rate of 10℃ / min, and finally cooled to room temperature with the furnace.

[0037] Preferably, before the brazing heating in step three, the workpiece to be brazed is preheated: the temperature is raised to 400°C at a rate of 5°C / min and preheated and held for 10 minutes. This can make the composition of AuSi eutectic brazing filler metal more uniform and accelerate the reaction of NiTi alloy, 316L stainless steel and AuSi eutectic brazing filler metal.

[0038] The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel of the present invention first prepares samples of NiTi alloy and 316L stainless steel, then clamps them in the order of NiTi alloy / AuSi brazing filler metal / 316L stainless steel, and places them in a vacuum brazing furnace. Welding is carried out at a certain brazing temperature and holding time to obtain a NiTi alloy / 316L stainless steel brazed joint. After electrochemical corrosion, no obvious corrosion phenomenon was found in the obtained joint, which has wide applicability.

[0039] Example 1:

[0040] Step 1: To facilitate subsequent shear tests, 316L stainless steel and NiTi shape memory alloy were processed into welding samples with dimensions of 20mm×10mm×5mm and 5mm×5mm×5mm, respectively. Then, the welding surfaces of the 316L stainless steel and NiTi alloy were polished with silicon carbide sandpaper of 180#, 400#, 800#, 1200# and 2000# in sequence to make their surface roughness Ra≤2.0μm. First, ultrasonic cleaning with acetone was performed for 5-15 minutes, and then ultrasonic cleaning with alcohol was performed for 5 minutes to obtain dust-free and oil-free welding materials.

[0041] Step 2: The 50μm AuSi eutectic solder with a composition of AuSi3.15 is processed into a 5×5mm thin sheet. First, it is ultrasonically cleaned with acetone for 5-15 minutes, then ultrasonically cleaned with alcohol for 5 minutes to obtain a dust-free and oil-free soldering material. Its microstructure is as follows... Figure 1 As shown.

[0042] The components to be welded, clamped in the order of NiTi alloy / AuSi brazing filler metal / 316L stainless steel, are placed on a graphite plate coated with yttrium oxide solder resist. The assembled sample, along with the graphite plate, is then placed into a vacuum brazing furnace. An assembly diagram is shown below. Figure 2 As shown, in 5×10 -3Under vacuum conditions below Pa, the temperature was first increased to 400℃ at a rate of 5℃ / min and preheated for 10 min. Then, the temperature was increased to 550℃ at a rate of 5℃ / min and brazed for 30 min. After brazing, the temperature was decreased to 200℃ at a rate of 10℃ / min, and finally cooled to room temperature with the furnace to obtain a NiTi alloy / 316L stainless steel brazed joint. The microstructure of the resulting joint is as follows: Figure 3 As shown in (a).

[0043] Example 2:

[0044] The difference between this embodiment and Embodiment 1 is that the brazing temperature in step three is 575℃, while the other steps are the same as in Embodiment 1. The microstructure of the resulting joint is shown in the figure below. Figure 3 As shown in (b).

[0045] Example 3:

[0046] The difference between this embodiment and Embodiment 1 is that the brazing temperature in step three is 600℃, while the other steps are the same as in Embodiment 1. The microstructure of the resulting joint is shown in the figure below. Figure 3 As shown in (c).

[0047] Example 4:

[0048] The difference between this embodiment and Embodiment 1 is that the brazing temperature in step three is 625℃, while the other steps are the same as in Embodiment 1. The microstructure of the resulting joint is shown in the figure below. Figure 3 As shown in (d).

[0049] Example 5:

[0050] The difference between this embodiment and embodiment 3 is that the brazing holding time in step three is 10 minutes. The other steps are the same as in embodiment 3. The microstructure of the resulting joint is shown in the figure below. Figure 4 As shown in (a)

[0051] Example 6:

[0052] The difference between this embodiment and embodiment 3 is that the brazing holding time in step three is 20 minutes. The other steps are the same as in embodiment 3. The microstructure of the resulting joint is shown in the figure below. Figure 4 (b) shows

[0053] Example 7:

[0054] The difference between this embodiment and embodiment 3 is that the brazing holding time in step three is 40 minutes. The other steps are the same as in embodiment 3. The microstructure of the resulting joint is shown in the figure below. Figure 4 (d) shows

[0055] Figure 5The shear strength of the joints obtained in Examples 1-4 shows that as the brazing temperature increases, the shear strength of the joints first increases and then decreases, reaching a maximum of 34.3 MPa at 600℃.

[0056] Figure 6 The shear strength of the joints obtained in Examples 3 and 5-7 shows that as the heat preservation time increases, the shear strength of the joints first increases and then decreases, reaching a maximum of 34.3 MPa at 30 min.

[0057] Figure 7 The image shows the microstructure of the brazed joint obtained in Example 3 after immersion in SBF simulated body fluid for 72 hours. After being corroded by the body fluid, the joint remained intact, with no cracks or voids, demonstrating good corrosion resistance.

[0058] Figure 8 The image shows the microstructure of a joint obtained by laser welding NiTi shape memory alloy / 316L stainless steel using silver-based brazing filler metal as an intermediate layer in the prior art. As can be seen from the image, there are large areas of voids and cracks in the interface.

[0059] This invention employs AuSi eutectic brazing filler metal to achieve low-temperature brazing (550-625℃) connection between NiTi alloy and 316L stainless steel. The NiSiTi layer and Ni4Si7Ti4 whiskers formed on the NiTi alloy side effectively enhance the bonding strength of the NiTi interface. Simultaneously, the NiSiTi layer also hinders element diffusion into the NiTi alloy, preserving the shape memory properties of the NiTi alloy. The (Fe,Cr)5Si3 intermetallic compound layer ensures the bonding strength of the 316L stainless steel interface. Its influence on the joint strength first increases and then decreases with increasing intermetallic compound thickness. When the compound layer is thin, the bond between 316L and the brazed joint is weak, resulting in lower joint strength. When the (Fe,Cr)5Si3 intermetallic compound layer is too thick, the interfacial stress is high, and the joint strength is also low. When brazing at 600℃ and holding for 30 minutes, the highest shear strength of the brazed joint can reach 34.3MPa, meeting the strength requirements of medical devices. Moreover, the joint maintains good integrity after electrochemical corrosion testing, with no cracks or voids appearing in the joint. This method can solve the problems of large post-weld strain and poor precision caused by traditional welding of NiTi alloys and 316L stainless steel, and can retain the shape memory effect of NiTi alloys, meeting the needs of long-term service of medical devices.

Claims

1. A low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel, characterized in that, The brazing method includes the following steps: Step 1: Grind and clean the NiTi alloy and 316L stainless steel surfaces to be welded for 10-20 minutes. Step 2: After cleaning the AuSi eutectic solder for 10-20 minutes, place it between the NiTi alloy surface to be soldered and the 316L stainless steel surface to be soldered, forming a workpiece to be soldered in the order of NiTi alloy / AuSi solder / 316L stainless steel. Step 3: Place the parts to be soldered into a vacuum brazing furnace, at a temperature of 5x10... -3 Heating is performed under vacuum conditions below Pa, with the temperature increased to 550-625℃ at a rate of 5℃ / min and brazed and held at that temperature for 10-40 minutes. After brazing, the temperature is reduced to 200℃ at a rate of 10℃ / min, and finally cooled to room temperature with the furnace to obtain a NiTi alloy / 316L stainless steel brazed joint.

2. The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel according to claim 1, characterized in that: In step one, the NiTi alloy and 316L stainless steel surfaces to be welded are ground until the surface roughness Ra ≤ 2.0 μm.

3. The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel according to claim 1, characterized in that: In steps one and two, the NiTi alloy surface to be soldered, the 316L stainless steel surface to be soldered, and the AuSi eutectic solder are first ultrasonically cleaned with acetone for 5-15 minutes, and then ultrasonically cleaned with alcohol for 5 minutes.

4. The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel according to claim 1, characterized in that: The AuSi eutectic solder used in step two has an AuSi3.15 composition, a thickness of 50µm, and an area size that is the same as that of the NiTi alloy or 316L stainless steel surface to be soldered.

5. The low-temperature brazing method for NiTi shape memory alloy and 316L stainless steel according to claim 1, characterized in that: Step 3: Before brazing, preheat the workpiece to be brazed: heat to 400℃ at a rate of 5℃ / min and hold for 10 minutes.

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