Ni-ti based shape memory alloy containing y and its forming process

Abstract

The present application relates to the technical field of powder injection molding material, and particularly relates to a Y-containing Ni-Ti based shape memory alloy and a molding process thereof, which comprises the following atomic mass percentage chemical components: Ti: 49.6-50.0%, Y: 0.2-0.6%; the balance is Ni and inevitable impurities; wherein, the chemical component Y is added in the form of Y powder in the shape memory alloy molding process, and is mixed with NiTi alloy powder by mechanical ball milling to form mixed powder, and then metal injection molding is performed. Compared with element Ti, Y has higher oxygen affinity, can reduce or eliminate the formation of TiO2, Ti4Ni2O x phases in the sintering process; the addition of Y reduces the consumption of Ti, ensures that the ratio of Ni and Ti is within a reasonable range, and the oxide formed by the Y element is within the micron range, which has little effect on the shape memory effect and mechanical properties of the NiTi alloy; the NiTi alloy component in the present application is suitable for powder metallurgy process, especially powder injection molding process (MIM).

Description

Technical Field

[0001] This invention relates to the field of powder injection molding materials technology, and in particular to a Y-containing Ni-Ti based shape memory alloy and its molding process. Background Technology

[0002] NiTi alloys were discovered in 1962 by William Buehler of the U.S. Naval Ordnance Laboratory. Superelasticity (SE) and shape memory (SME) are its two main properties, along with excellent biocompatibility, corrosion resistance, and mechanical properties. These characteristics make it suitable for important applications in medical, aerospace, and high-end manufacturing fields. However, due to the high reactivity and low thermal conductivity of NiTi alloys, machining them is difficult and tool wear is rapid. Mesh forming (MIM), with its near-dimensional forming characteristics, is a superior method for forming metal components with complex geometries, effectively solving the problems of difficult cold working and high processing costs associated with NiTi.

[0003] NiTi alloys contain nearly 50% Ti, an active metal that readily reacts with inclusions such as O, N, C, and H during powdering, debinding, and sintering. Compared to NiTi shape memory alloys, pure Ti can dissolve approximately 0.3% O by mass; however, current research on NiTi alloy MIM sintering and material development reveals that both high-vacuum sintering and argon atmosphere sintering result in the formation of TiO2 and Ti4Ni2O. X Secondary phases such as TiC are formed. Compared to conventional smelting, forging, and cold working processes, the formation of these phases alters the Ni / Ti ratio in NiTi alloys, reducing their superelasticity and shape memory effect, posing a significant challenge to the application of MIM (Metrological Injection Molding) technology in NiTi alloys. Current technologies add Y during smelting primarily to increase the thermoelastic martensitic phase transformation temperature of TiNi alloys. However, oxygen preferentially combines with Y, failing to address the issue of oxygen combining with titanium during sintering, leading to poor superelasticity and shape memory properties. Summary of the Invention

[0004] In view of this, the purpose of this invention is to propose a Ni-Ti based shape memory alloy containing Y and its forming process, so as to solve the problem that oxygen preferentially combines with titanium during sintering, resulting in poor superelasticity and shape memory properties.

[0005] To achieve the above objectives, the present invention provides a Ni-Ti based shape memory alloy containing Y, comprising the following chemical composition by atomic mass percentage: Ti: 49.6~50.0%, Y: 0.2~0.6%; the balance being Ni and unavoidable impurities; wherein, the chemical composition Y is added as Y powder during the shape memory alloy forming process and mixed with NiTi alloy powder by mechanical ball milling to obtain a mixed powder, which is then subjected to metal injection molding.

[0006] This invention also provides a forming process for the Y-containing Ni-Ti-based shape memory alloy, comprising the following steps:

[0007] S1. Prepare pre-alloyed spherical NiTi alloy powder and micron-sized pure metallic Y powder;

[0008] S2. Mix NiTi alloy powder with Y powder to form a mixed powder;

[0009] S3. The mixed powder and binder are used to prepare a feed;

[0010] S4. The feed material is shaped using a metal injection molding process to obtain a green blank;

[0011] S5. Catalytic and thermal degreasing of the green body;

[0012] S6. The degreased green body is sintered under an inert atmosphere to obtain the final product.

[0013] In S1, the pre-alloyed spherical NiTi alloy powder and Y powder are prepared by electrode induction melting gas atomization method.

[0014] The atomic percentage of oxygen atoms in the NiTi alloy powder is 0.2-0.3%; the laser particle size distribution of the NiTi alloy powder is D10: 4~7μm, D50: 8~12μm, and D90: 17~26μm. The impurity content in the prepared powder, especially O, is controlled within 0.20~0.30% (At%); the lower the content of other impurities, the better.

[0015] The mixing described in S2 is performed using a ball mill, with zirconium oxide abrasive balls at a ball-to-powder ratio of 5:1. Mixing is carried out under an argon protective gas atmosphere at a milling speed of 200-400 rpm. The mixing time is 5-10 hours, resulting in a uniformly mixed powder. Preferably, NiTi pre-alloyed powder and Y powder are mechanically mixed using a ball mill at a mass percentage of (99-99.5):(0.5-1).

[0016] The feedstock in S3 is prepared by first mixing and then granulating. The binder in the feedstock has the following composition: POM (polyoxymethylene): HDPE (high-density polyethylene): EVA (ethylene-vinyl acetate copolymer): SA (stearic acid): antioxidant = 86:8:3.5:2:0.5, and the mass-to-volume ratio of the mixed powder to the binder is 60:40. By optimizing the binder formulation, the hot degreasing temperature is controlled below 500℃, and the residual carbon content in the alloy after sintering is kept as low as possible.

[0017] The catalytic degreasing described in S5 is oxalic acid catalytic degreasing; the thermal degreasing is carried out under an argon atmosphere, with the temperature increased to 350℃ in 100 min and held for 60 min; then the temperature is increased from 350℃ to 450-500℃ in 100 min and held for 120 min.

[0018] The sintering described in S6 is carried out under an argon atmosphere, with a heating time of 4-8 hours to 1200-1280℃, and then held at this temperature for 2-4 hours. The holding pressure during the sintering process is 10-40 kPa.

[0019] The sintering process uses yttrium oxide plates as the sintering support plates, and the argon gas flow rate is 10~40L / min; the degreased green body is placed in a Mo box for sintering.

[0020] The following points should be noted during the sintering process: First, yttrium oxide plates should be used as the sintering plate to prevent the alloy from reacting with the substrate during sintering; second, the argon purity should be 5N or 6N or higher; third, the degreased blank should be sintered in a Mo box, and a two-layer nested Mo box is better, and titanium sponge should be placed around the inner Mo box. The purpose of the titanium sponge is to absorb residual oxygen or water vapor in the atmosphere; fourth, the gas flow rate should be 10~40L / min.

[0021] The beneficial effects of this invention are as follows: By adding titanium dioxide (Y), which exists as powder particles, this invention preferentially combines with oxygen during sintering, thus solving the problem of poor superelasticity and shape memory properties caused by the combination of oxygen and titanium during sintering. Compared with element Ti, Y has a higher affinity for oxygen. According to the Ellingham diagram of Y and Ti, the Gibbs free energy ΔG of the reaction between Y and oxygen is lower than that of Ti during sintering at 500℃, 1000℃, and 1500℃. Therefore, adding Y can reduce or eliminate the formation of TiO2 and Ti4Ni2O during powder preparation and sintering. X Phase formation; the addition of Y reduces Ti consumption, ensuring that Ni and Ti are within a reasonable ratio range. At the same time, the oxides formed by Y are all within the micrometer range, which has little impact on the shape memory effect and mechanical properties of NiTi alloy, and improves elongation. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 Here is a schematic diagram of the sample structure prepared in this invention:

[0024] Figure 2 Metallographic image of the sample prepared in Example 1 of this invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0026] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0027] Example 1:

[0028] The components and formulation of the mixed powder are shown in Table 1:

[0029] Table 1 At.%

[0030] Element Ti Y O C N Ni Content 49.7 0.42 0.22 0.030 0.011 Balance

[0031] The forming process of the Y-containing Ni-Ti-based shape memory alloy in this embodiment includes the following steps:

[0032] Powder preparation: Pre-alloyed spherical NiTi alloy powder was prepared by electrode induction melting gas atomization (EIGA). The specific particle sizes of the powder are as follows: D10: 5.80 μm, D50: 12.15 μm, D90: 21.60 μm and D50: 1.1 μm high-purity metallic Y powder.

[0033] Powder mixing: NiTi pre-alloyed powder and Y powder were mechanically mixed by ball milling at a ratio of 99:1, with a ball-to-powder ratio of 5:1; the rotation speed was 400 rpm, and the ball milling time was 6 hours; a uniformly mixed powder was obtained.

[0034] Intensive mixing and granulation: The above mixed powder and the binder system with a volume ratio of POM:HDPE:EVA:SA:antioxidant = 86:8:3.5:2:0.5 are mixed and weighed according to a volume ratio of 60:40, and then added to an intensive mixing tank for intensive mixing. The temperature of the intensive mixing tank is controlled at 190℃ and the rotation speed is 15r / min for 2 hours. After mixing evenly, the mixture is extruded and granulated.

[0035] Injection molding: The feed material is injected into the metal through the conventional MIM process.

[0036] Oxalic acid defatting: defatting temperature 110℃, time 15h;

[0037] Thermal degreasing: In an argon atmosphere, the temperature is raised to 350℃ in 100 min; then held for 60 min; then the temperature is raised from 350℃ to 450℃ in 100 min and held for 120 min.

[0038] Sintering: In a two-layer nested Mo box, the first stage is vacuum sintering at 200℃ for 30 min; the second stage is sintering in an argon atmosphere at 10 kPa, with the temperature increased from 200℃ to 1000℃ at a rate of 5℃ / min; then the temperature is increased to 1260℃ at a rate of 2℃ / min and held for 150 min.

[0039] Example 2:

[0040] The composition and formulation of the metal powder are shown in Table 1:

[0041] Table 2 At.%

[0042] Element Ti Y O C N Ni Content 49.8 0.48 0.25 0.035 0.017 Balance

[0043] The forming process of the Y-containing Ni-Ti-based shape memory alloy in this embodiment includes the following steps:

[0044] Powder preparation: Pre-alloyed spherical NiTi alloy powder was prepared by electrode induction melting gas atomization (EIGA). The specific particle sizes of the powder are as follows: D10: 5.80 μm, D50: 12.15 μm, D90: 21.60 μm and D50: 1.1 μm high-purity metallic Y powder.

[0045] Powder mixing: NiTi pre-alloyed powder and Y powder were mixed mechanically by ball milling at a ratio of 99.2:0.8, with a ball-to-powder ratio of 5:1; the rotation speed was 400 rpm, and the ball milling time was 6 hours; a uniformly mixed powder was obtained.

[0046] Intensive mixing and granulation: The above mixed powder and the binder system with the ratio of POM:HDPE:EVA:SA:antioxidant = 86:8:3.5:2:0.5 are mixed and weighed according to a volume ratio of 60:40, and then added to an intensive mixing tank for intensive mixing. The temperature of the intensive mixing tank is controlled at 190℃ and the speed is 15r / min for 2 hours. After mixing evenly, the mixture is extruded and granulated.

[0047] Injection molding: The feed material is injected into the metal through a conventional MIM process.

[0048] Oxalic acid defatting: defatting temperature 110℃, time 15h;

[0049] Thermal degreasing: In an argon atmosphere, the temperature is raised to 350℃ in 100 min; then held for 60 min; then the temperature is raised from 350℃ to 450℃ in 100 min and held for 120 min.

[0050] Sintering: In a two-layer nested Mo box, the first stage is vacuum sintering at 200℃ for 30 min; the second stage is sintering in an argon atmosphere at 10 kPa, with the temperature increased from 200℃ to 1000℃ at a rate of 5℃ / min; then the temperature is increased to 1260℃ at a rate of 2℃ / min and held for 150 min.

[0051] Comparative Example 1:

[0052] The composition of the metal powder is shown in Table 2. It should be noted that the oxygen content of the raw material powder in the comparative example is slightly higher; at the same time, its Ti element content is also slightly lower. This is the normal ratio of conventional NiTi alloy powder on the market, and the particle size is relatively fine.

[0053] Table 3 At.%

[0054] Element Ti O C N Ni Content 49.5 0.42 0.029 0.012 Balance

[0055] Powder preparation: Pre-alloyed spherical NiTi alloy powder was prepared by electrode induction melting gas atomization (EIGA). The specific particle sizes of the powder are as follows: D10: 4.31 μm, D50: 8.73 μm, D90: 17.11 μm;

[0056] Intensive mixing and granulation: The above-mentioned NiTi alloy powder and the binder system with a ratio of POM:HDPE:EVA:SA:antioxidant = 86:8:3.5:2:0.5 were mixed and weighed according to a volume ratio of 60:40, and then added to an intensive mixing tank for intensive mixing. The temperature of the intensive mixing tank was controlled at 190℃ and the rotation speed was 15r / min for 2 hours. After mixing evenly, the mixture was extruded and granulated.

[0057] Injection molding: Conventional MIM process;

[0058] Oxalic acid defatting: defatting temperature 110℃, time 15h;

[0059] Thermal degreasing: In an argon atmosphere, the temperature is raised to 350℃ in 100 min; then held for 60 min; then the temperature is raised from 350℃ to 550℃ in 200 min and held for 120 min.

[0060] Sintering: First stage: Vacuum sintering, temperature 200℃, holding for 30 min; Second stage: Argon atmosphere sintering, pressure 10 kPa, temperature increased from 200℃ to 1000℃ at a rate of 5℃ / min; then increased to 1260℃ at a rate of 2℃ / min, holding for 150 min.

[0061] Comparative Example 2:

[0062] The composition of the metal powder is shown in Table 2. It should be noted that the oxygen content of the raw material powder in the comparative example is slightly higher; at the same time, its Ti element content is also slightly lower. This is the normal ratio of conventional NiTi alloy powder on the market, and the particle size is relatively coarse.

[0063] Table 4 At.%

[0064] Element Ti O C N Ni Content 49.6 0.35 0.035 0.011 Balance

[0065] Intensive mixing and granulation: Pre-alloyed spherical NiTi alloy powder was prepared by electrode induction melting gas atomization (EIGA). The specific particle sizes of the powder are as follows: D10: 6.85μm, D50: 14.47μm, D90: 25.48μm;

[0066] Intensive mixing and granulation: The above-mentioned NiTi alloy powder and the binder system with a volume ratio of POM:HDPE:EVA:SA:antioxidant = 86:8:3.5:2:0.5 were mixed and weighed according to a volume ratio of 60:40, and then added to an intensive mixing tank for intensive mixing. The temperature of the intensive mixing tank was controlled at 190℃ and the rotation speed was 15r / min for 2 hours. After uniform mixing, the mixture was extruded and granulated.

[0067] Injection molding: Conventional MIM process;

[0068] Oxalic acid defatting: defatting temperature 110℃, time 15h;

[0069] Thermal degreasing: In an argon atmosphere, the temperature is raised to 350℃ in 100 min; then held for 60 min; then the temperature is raised from 350℃ to 550℃ in 200 min and held for 120 min.

[0070] Sintering: First stage: Vacuum sintering, temperature 200℃, holding for 30 min; Second stage: Argon atmosphere sintering, pressure 10 kPa, temperature increased from 200℃ to 1000℃ at a rate of 5℃ / min; then increased to 1260℃ at a rate of 2℃ / min, holding for 150 min.

[0071] Comparative Example 3

[0072] The difference between this comparative example and Example 1 is that the firing plate is replaced with an alumina plate, which causes the NiTi alloy to bond directly to the firing plate.

[0073] Comparative Example 4

[0074] The difference between this comparative example and Example 1 is that the sintering was not placed in a Mo box, resulting in severe oxidation and blackening.

[0075] Performance testing: Tensile properties were tested according to GB / T229.1. C and O content were tested using a NACK ON-3000 analyzer and a NACK CS-2800 analyzer.

[0076] The properties of the NiTi alloys in the examples and comparative examples after sintering are shown in Table 5.

[0077] Table 5

[0078] Tensile strength MPa Yield strength MPa Elongation % Oat. % Cat. % Sintered density g / cc Example 1 994 431 7.91 0.7252 0.2316 6.36 Example 2 1040 454 7.06 0.7920 0.2843 6.39 Comparative Example 1 857 357 3.64 0.9725 0.2890 6.32 Comparative Example 2 798 405 4.26 0.8633 0.2979 6.33

[0079] As can be seen from Table 5, the NiTi shape memory alloy prepared by the present invention has higher elongation and higher strength after sintering.

[0080] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in the details for the sake of brevity.

[0081] This invention is intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A Ni-Ti based shape memory alloy containing Y, characterized in that, The chemical composition includes the following atomic mass percentages: Ti: 49.6~50.0%, Y: 0.2~0.6%; the balance is Ni and unavoidable impurities; wherein, the chemical composition Y is added as Y powder during the shape memory alloy forming process and mixed with NiTi alloy powder by mechanical ball milling to form a mixed powder, which is then subjected to metal injection molding.

2. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 1, characterized in that, Includes the following steps: S1. Prepare pre-alloyed spherical NiTi alloy powder and micron-sized pure metallic Y powder; S2. NiTi alloy powder and Y powder are mixed by mechanical ball milling to form a mixed powder; S3. Mix the mixed powder with the binder to prepare a feed; S4. The feed material is shaped using a metal injection molding process to obtain a green blank; S5. Acid-catalyzed degreasing and thermal degreasing are performed on the green body; S6. The degreased green body is sintered in an inert atmosphere.

3. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 2, characterized in that, In S1, the pre-alloyed spherical NiTi alloy powder and Y powder are prepared by electrode induction melting gas atomization method.

4. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 2 or 3, characterized in that, The atomic percentage of oxygen atoms in the NiTi alloy powder is 0.2-0.3%; the laser particle size of the prepared NiTi alloy powder is D10: 4~7μm, D50: 8~12μm, and D90: 17~26μm.

5. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 2, characterized in that, The mixing described in S2 is performed using a ball mill, with zirconium oxide as the abrasive balls; the ball-to-powder ratio is 5:1; the mixing is carried out under an argon protective gas atmosphere, with a ball mill speed of 200-400 rpm; the mixing time is 5 to 10 hours, and finally a uniformly mixed powder is obtained.

6. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 2, characterized in that, The binder has the following composition: POM:HDPE:EVA:SA:antioxidant = 86:8:3.5:2:0.

5. The mass-volume ratio of the mixed powder to the binder is 60:

40. The feeding method in S3 according to the above formula is to first perform intensive mixing, then granulation, and finally obtain the green body through injection molding in S4.

7. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 2, characterized in that, The catalytic degreasing described in S5 is oxalic acid catalytic degreasing; the thermal degreasing is carried out under an argon atmosphere, with the temperature increased to 350°C in 100 min and held for 60 min; then the temperature is increased from 350°C to 500°C in 100 min and held for 120 min.

8. The forming process of the Y-containing Ni-Ti-based shape memory alloy according to claim 2, characterized in that, The sintering described in S6 involves heating to 1200-1280℃ in an argon atmosphere over a period of 4-8 hours, and holding at this temperature for 2-4 hours. The holding pressure during the sintering process is 10-40 kPa.

9. The forming process of the Y-containing Ni-Ti based shape memory alloy according to claim 8, characterized in that, The sintering process uses yttrium oxide plates as the sintering support plates, and the argon gas flow rate is 10~40L / min; the degreased green body is placed in a Mo box for sintering.

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