A cyclic heat treatment process based on NiTi shape memory alloys

Abstract

The application provides a cyclic heat treatment process based on NiTi shape memory alloy, which comprises vacuum treatment, hot forging, hot rolling, cold drawing, annealing, cyclic heat treatment and aging treatment processes, and obtains a NiTi shape memory alloy wire with a diameter of 2-3 mm. In the process, the addition of Co and Mn elements in the NiTi alloy can significantly improve the phase transition temperature, secondary recrystallization occurs in the process of heat treatment, the obtained bamboo joint crystal has a great relative grain size, and good superelasticity is exhibited. The cyclic heat treatment process can improve the grain boundary migration rate, and the subgrain boundary energy induced by the precipitation and dissolution process of the alpha phase during the cyclic heat treatment is the driving force for the abnormal grain growth phenomenon; the process can overcome the problems of the existing technology, such as the large internal stress of the directly formed NiTi part, the non-compliance of the phase transition temperature and the like, and the cyclic heat treatment can effectively eliminate or weaken the negative effects caused by the problems.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of alloy materials, and particularly relates to a cyclic heat treatment process based on NiTi shape memory alloy. BACKGROUND

[0002] The mechanical property of shape memory alloy is excellent, and the recoverable deformation can be up to 10%, while the general metal material is only below 0.1%, which is almost two orders of magnitude higher than the elastic strain of ordinary metal material, and can be used to improve the impact toughness of the material. The shape memory alloy is made into a certain shape at a higher temperature, and is arbitrarily deformed at a low temperature, and can recover the shape before deformation after heating.

[0003] The NiTi shape memory alloy has great application potential in the fields of aerospace, biological medicine, automobile and mechanical manufacturing due to its unique shape memory effect and super-elasticity, good biocompatibility, corrosion resistance and damping performance. However, the shape structure of the NiTi alloy parts prepared by the traditional method is relatively simple, which greatly limits the application and promotion of the NiTi alloy.

[0004] The traditional preparation method of the NiTi alloy includes a melting and casting method and a powder metallurgy method. The melting and casting method is generally carried out under the atmosphere, and the C and O elements in the air are easily combined with the relatively active Ti element in the NiTi alloy to generate compounds such as Ti4Ni2O and TiC, so that the Ni / Ti ratio in the NiTi alloy is increased, the phase transition temperature is increased, the brittleness of the alloy is increased, and the mechanical property is decreased. The powder metallurgy method realizes the near-net forming of some simple structure parts, avoids the hot mechanical processing treatment required after traditional casting, but still has the problems of simple shape and structure of the parts, high cost of the complex mold, difficult to avoid the introduction of impurities in high-temperature sintering, mechanical property is not as good as the melting and casting method, and the process flow is complex, so it is necessary to explore a more suitable preparation method for the characteristics of the NiTi alloy. SUMMARY

[0005] In order to solve the above technical problems, the application provides a cyclic heat treatment process based on NiTi shape memory alloy, which comprises the following steps:

[0006] S1, vacuum treatment: the metal raw materials Ti, Ni, Mn and Co are put into a vacuum arc furnace, and after the vacuum arc furnace is vacuumized, the temperature is increased to 830-850 DEG C for heating and melting;

[0007] S2, hot forging: the cast ingot obtained in the step S1 is removed from the surface impurities, and after being kept at 780-800 DEG C, hot forging is carried out to form a rod-shaped material;

[0008] S3, hot rolling: the material in the step S2 is taken out and hot rolled into a thin rod-shaped material at 780-800 DEG C;

[0009] S4, cold drawing: the material in S3 is cold drawn;

[0010] S5, annealing: after cold drawing, the material is annealed at 550-580℃ and then air-cooled to obtain 2mm alloy wire;

[0011] S6, cyclic heat treatment: the wire obtained in S5 is heated to 800℃ at a rate of 5℃ / min and held for 10min, then cooled to 400-420℃ and held, and then slowly heated to 800-820℃, and this process is repeated 1-5 times, and after the last heating, the material is held at 800-820℃ and then water quenched, and the cycle is completed;

[0012] S7, aging treatment: the material after the cycle is aged at 180-190℃.

[0013] As a preferred, the mass ratio of Ti, Ni, Mn, Co is: 52-58:43-46:10-13:1-2.

[0014] As a preferred, the vacuum degree in S1 is 0.1-0.3Pa, and the melting time is 20-35min.

[0015] As a preferred, the holding time in S2 is 12-20h, and the diameter of the rod-shaped material is 22-25mm.

[0016] As a preferred, the diameter of the thin rod-shaped material in S3 is 10-12mm.

[0017] As a preferred, the deformation in S4 is 30%-35%.

[0018] As a preferred, the holding time in S5 is 1.2-1.5h, and the diameter of the alloy wire is 2-3mm.

[0019] As a preferred, the heating rate in S6 is 5℃ / min, and the holding time after heating is 10-15min; the heating rate is 3℃ / min, and the holding time after cooling is 10-15min.

[0020] As a preferred, the aging treatment time in S7 is 12-48h.

[0021] Mechanism of the cyclic heat treatment in the process:

[0022] With the precipitation and dissolution of the alpha phase in the repeated heating, holding and cooling process in the high temperature beta phase region and alpha + beta phase region, the grain will abnormally grow with the cycle of heat treatment. The reason is that this heat treatment process will improve the grain boundary mobility, and the subgrain boundary energy induced by the precipitation and dissolution process of the alpha phase during the cycle heat treatment is the driving force of the abnormal grain growth phenomenon.

[0023] Compared with the prior art, the present application has the following beneficial effects:

[0024] (1) The present process can significantly improve the phase transition temperature by adding Co and Mn elements in the NiTi alloy, and the bamboo joint crystal obtained by secondary recrystallization during heat treatment has a great relative grain size and exhibits good superelasticity.

[0025] (2) The cycle heat treatment process can improve the grain boundary mobility, and the subgrain boundary energy induced by the precipitation and dissolution process of the alpha phase during the cycle heat treatment is the driving force of the abnormal grain growth phenomenon; the present process can overcome the problems of large internal stress, unsatisfactory phase transition temperature and poor superelasticity of the NiTi parts directly formed in the prior art, and the cycle heat treatment can effectively eliminate or weaken the negative effects of these problems.

[0026] (3) The grain of the sample after cycle heat treatment abnormally grows with the increase of the cycle number, which significantly improves the mechanical properties and damping properties of the alloy. DETAILED DESCRIPTION

[0027] In order to make the technical problems, technical solutions and advantages of the present application clearer, specific embodiments will be described in detail below.

[0028] The following examples are used to illustrate the present application, but are not used to limit the scope of the present application. Modifications or replacements of the methods, steps or conditions of the present application without departing from the spirit and essence of the present application all belong to the scope of the present application.

[0029] If not specifically indicated, the technical means used in the examples is the conventional means known to those skilled in the art; if not specifically indicated, the reagents used in the examples are commercially available.

[0030] Example 1

[0031] Preparation of shape memory alloy by cycle heat treatment

[0032] S1, vacuum treatment: put the metal raw materials Ti, Ni, Mn and Co with a mass ratio of 52:43:10:1 into a vacuum arc furnace, vacuumize the furnace to 0.1 Pa, then heat to 830℃ for heating and smelting for 20 min;

[0033] S2, hot forging: the ingot obtained in step S1 is removed of surface impurities, and then hot forging is performed at 780 DEG C after 12 h of heat preservation, to form a 22 mm rod-shaped material;

[0034] S3, hot rolling: the material in step S2 is hot rolled at 780 DEG C to form a 10 mm thin rod-shaped material;

[0035] S4, cold drawing: the material in step S3 is cold drawn, and the deformation amount of cold drawing is 30%;

[0036] S5, annealing: after the cold drawing is completed, air cooling is performed after 1.2 h of heat preservation at 580 DEG C, to obtain an alloy wire;

[0037] S6, cyclic heat treatment: the wire obtained in step S5 is heated to 800 DEG C and heat preserved, and then cooled to 400 DEG C at a rate of 3 DEG C / min and heat preserved for 10 min, and then slowly heated to 800 DEG C, and the heating is repeated for 5 times, and after the last heating, the material is heat preserved at 800 DEG C and then water quenched, and the cycle is completed;

[0038] S7, aging treatment: the material after the cycle is completed is aged at 180 DEG C for 12 h.

[0039] Example 2

[0040] Cyclic heat treatment for preparing shape memory alloy

[0041] S1, vacuum treatment: metal raw materials Ti, Ni, Mn and Co are put into a vacuum arc furnace in a mass ratio of 58:46:13:2, the arc furnace is vacuumized to 0.3 Pa, and then heated and melted at 850 DEG C for 20 min;

[0042] S2, hot forging: the ingot obtained in step S1 is removed of surface impurities, and then hot forging is performed at 780 DEG C after 12 h of heat preservation, to form a 22 mm rod-shaped material;

[0043] S3, hot rolling: the material in step S2 is hot rolled at 800 DEG C to form a 10 mm thin rod-shaped material;

[0044] S4, cold drawing: the material in step S3 is cold drawn, and the deformation amount of cold drawing is 30%;

[0045] S5, annealing: after the cold drawing is completed, air cooling is performed after 1.2 h of heat preservation at 550 DEG C, to obtain an alloy wire;

[0046] S6, cyclic heat treatment: the wire obtained in step S5 is heated to 820 DEG C and heat preserved, and then cooled to 420 DEG C at a rate of 3 DEG C / min and heat preserved for 10 min, and then slowly heated to 820 DEG C, and the heating is repeated for 5 times, and after the last heating, the material is heat preserved at 820 DEG C and then water quenched, and the cycle is completed;

[0047] S7, aging treatment: the material after the cycle is aged at 180℃ for 12h.

[0048] Comparative Example 1

[0049] General heat treatment for preparing shape memory alloy

[0050] S1, vacuum treatment: the metal raw materials Ti, Ni, Mn and Co are put into a vacuum arc furnace at a mass ratio of 58:46:13:2, the arc furnace is vacuumed to 0.3 Pa, and then heated and melted at 850℃ for 20 min;

[0051] S2, hot forging: the ingot obtained in step S1 is removed from the surface impurities, and then hot forged at 780℃ for 12h to form a 22mm rod-shaped material;

[0052] S3, hot rolling: the material in step S2 is hot rolled at 800℃ to form a 10mm thin rod-shaped material;

[0053] S4, cold drawing: the material in step S3 is cold drawn, and the deformation amount of cold drawing is 30%;

[0054] S5, annealing: after cold drawing, air cooling is carried out after 550℃ heat preservation for 1.2h to obtain an alloy wire;

[0055] S6, general heat treatment: the wire prepared in step S5 is water quenched after heat preservation at 850℃ for 10 min, and then aged at 190℃ for 15 min.

[0056] Comparative Example 2

[0057] General NiTi shape memory alloy prepared by cyclic heat treatment

[0058] S1, vacuum treatment: the metal raw materials Ti and Ni are put into a vacuum arc furnace at a mass ratio of 55:45, the arc furnace is vacuumed to 0.1 Pa, and then heated and melted at 830℃ for 20 min;

[0059] S2, hot forging: the ingot obtained in step S1 is removed from the surface impurities, and then hot forged at 780℃ for 12h to form a 22mm rod-shaped material;

[0060] S3, hot rolling: the material in step S2 is hot rolled at 780℃ to form a 10mm thin rod-shaped material;

[0061] S4, cold drawing: the material in step S3 is cold drawn, and the deformation amount of cold drawing is 30%;

[0062] S5, annealing: after cold drawing, air cooling is carried out after 580℃ heat preservation for 1.2h to obtain an alloy wire;

[0063] S6, cyclic heat treatment: the wire obtained in S5 is heated to 800℃ and kept, then cooled to 400℃ at a rate of 3℃ / min and kept for 10min, then slowly heated to 800℃, and the heating is repeated for 3 times, after the last heating, the material is kept at 800℃ and then water quenched, and the cycle is ended;

[0064] S7, aging treatment: the material after the cycle is aged at 180℃ for 12h.

[0065] Comparative Example 3

[0066] Cyclic heat treatment for preparing shape memory alloy with less cycles

[0067] The preparation method is the same as that in Example 1, except that the cycle number in S6 is changed from 5 times to 1 time.

[0068] Experimental Example 1

[0069] Investigation of the mechanical properties and damping properties of the alloys prepared in Examples 1-2 and Comparative Examples 1-2

[0070] The yield strength and elongation are tested according to the provisions of GB / T 228.1-2010 standard

[0071] The breaking strength is tested according to GB 3851-1983 standard

[0072] The damping coefficient is detected by a damping tester

[0073] The results are shown in Table 1. The shape memory alloy after the process of the cyclic heat treatment of the application has better mechanical properties and damping coefficient, and the damping coefficient value is improved by one order of magnitude compared with the conventional heat treatment titanium-nickel wire. Therefore, by using the processing technology in the application, a nickel-titanium shape memory alloy wire with super-high damping performance can be obtained.

[0074] Table 1: Mechanical properties and damping coefficient of the alloys prepared in Examples 1-2 and Comparative Examples 1-2

[0075] Material Yield strength Elongation Breaking strength Damping coefficient Example 1 300 Pa 28％ 800 Pa 0.35 Example 2 310 Pa 26％ 780 Pa 0.28 Comparative Example 1 280 Pa 23％ 730 Pa 0.08 Comparative Example 2 290 Pa 20％ 700 Pa 0.12 Comparative Example 3 300 Pa 25％ 760 Pa 0.26

Claims

1. A cyclic heat treatment process based on NiTi shape memory alloy, characterized in that: The following steps are involved: S1, vacuum treatment: The metal raw materials Ti, Ni, Mn, Co into a vacuum arc furnace, the arc furnace was evacuated and heated to 830 ℃ ~ 850 ℃ for heating and melting; the mass ratio of Ti, Ni, Mn, Co is: 52-58:43-46:10-13:1-2; S2, hot forging: removing surface impurities from the ingot obtained in step S1, keeping it at 780°C-800°C and then hot forging it into a rod-shaped material; S3, hot rolling: hot rolling the material in step S2 at 780°C-800°C into thin rod-shaped material; S4, cold drawing: cold drawing the material in step S3; S5, annealing: After the cold drawing, keep at 550-580℃ and then air cool to obtain alloy wire; S6, cyclic heat treatment: heating the wire obtained in step S5 to 800°C-820°C and keeping the temperature, then cooling it to 400°C-420°C and keeping the temperature, then slowly heating it to 800°C-820°C, repeating this heating process 1-5 times, and after the last heating, keeping the temperature at 800°C-820°C and then water quenching, and the cycle ends; S7, aging treatment: the material after the cycle is aged at 180-190℃.

2. The cyclic heat treatment process according to claim 1, characterized in that: The vacuum degree of the vacuum pumping in the step S1 is 0.1-0.3 Pa, and the smelting time is 20-35 min.

3. The cyclic heat treatment process according to claim 1, characterized in that: The holding time in step S2 is 12-20 hours, and the diameter of the rod-shaped material is 22-25 mm.

4. The cyclic heat treatment process according to claim 1, characterized in that: In the step S3, the diameter of the thin rod-shaped material is 10-12 mm.

5. The cyclic heat treatment process according to claim 1, characterized in that: The deformation amount of the cold drawing in the S4 step is 30%-35%.

6. The cyclic heat treatment process according to claim 1, characterized in that: The holding time in step S5 is 1.2-1.5 hours, and the diameter of the alloy wire is 2-3 mm.

7. The cyclic heat treatment process according to claim 1, characterized in that: In the step S6, the heating rate is 5°C / min, and the holding time after heating is 10-15 minutes; the cooling rate is 3°C / min, and the holding time after cooling is 10-15 minutes.

8. The cyclic heat treatment process according to claim 1, characterized in that: The aging treatment time in the S7 step is 12-48 hours.