Method for regulating and controlling components of additive manufacturing NiTi shape memory alloy to enable components of NiTi shape memory alloy to approach target components

Abstract

The invention relates to the technical field of additive manufacturing, in particular to a method for regulating and controlling components of an additive manufacturing NiTi shape memory alloy to enable the components to approach target components. Aiming at the problem that components of the additive manufacturing NiTi shape alloy are unstable, the invention creatively puts forward to establish the relationship between the process parameter and the Ni atom increment, and correspondingly regulates and controls the components of the additive manufacturing NiTi shape alloy by increasing the content of Ni atoms in the second NiTi alloy powder by determining the evaporation capacity of the Ni atoms under the process parameter, so that the additive manufacturing NiTi shape alloy is more stable, and the performance of the additive manufacturing NiTi shape alloy is improved. And the components of the NiTi shape alloy prepared after regulation and control are close to target components.

Description

Technical Field

[0001] This invention relates to the field of additive manufacturing technology, and in particular to a method for controlling the composition of additively manufactured NiTi shape memory alloys to make them approach a target composition. Background Technology

[0002] Shape memory alloys (SMAs) have attracted much attention due to their shape memory effect (SME) and superelasticity (SE), with NiTi SMA being the most widely used. The energy change brought about by the latent heat of phase transformation during its phase transformation process gives rise to the elasto-thermal effect (eCE), making it important in the field of solid cooling.

[0003] The phase transformation temperature (TTs) of nickel-titanium alloys is highly sensitive to chemical composition. The TTs primarily depend on the nickel-to-titanium ratio and decrease with increasing nickel content. During laser high-temperature sintering, Ni evaporates, affecting the alloy composition and altering its phase transformation behavior. Currently, there is no definitive method to address this issue, posing a significant challenge to the application of additively manufactured NiTi shape memory alloys. Furthermore, different laser parameter configurations during additive manufacturing can lead to varying Ni evaporation behaviors. Therefore, controlling Ni evaporation during additive manufacturing to achieve a final NiTi shape memory alloy composition close to the target composition is a primary problem to be solved for the widespread application of additively manufactured NiTi shape memory alloys. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for controlling the composition of additively manufactured NiTi shape memory alloy to make it close to the target composition, which can ensure that the final NiTi shape memory alloy is close to the target composition.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition, comprising the following steps: Determine the additive manufacturing process parameters to be used in the proposed additive manufacturing process; According to the determined additive manufacturing process parameters, additive manufacturing was carried out using the first NiTi alloy powder to obtain the sample before adjustment; The actual composition of the sample before regulation is determined to obtain the amount of Ni atoms evaporated; or the amount of Ni atoms evaporated is obtained according to Equations 1-2. W'=0.01 E v ＇ Formula 1; Formula 2; In Equation 2 d 'Related to scanning speed:' When the scanning speed is <900mm / s, the calculation is based on Equation 3. d ', d '=-0.5 d +K L Formula 3; When the scanning speed is ≥900mm / s, it is calculated according to Equation 4. d ', d '=-0.5 d + L Equation 4; In equations 1-4, W' represents the amount of Ni atoms evaporated, at%; E v ＇ This is the corrected energy density; d ' represents the corrected scan spacing, in mm; P Laser power, W; V It is the scanning speed, mm / s; d The scanning spacing is in mm. H It is the interlayer thickness, in mm; L The maximum width of the molten pool of the sample before adjustment is in mm; K is the correction factor, taken as 0.78. A second NiTi alloy powder is provided; the second NiTi alloy powder has an increased target Ni atom percentage content compared to the additively manufactured NiTi shape memory alloy; in terms of atomic percentage, the increase in Ni atoms and the amount of Ni atoms evaporated satisfy: increase in Ni atoms = amount of Ni atoms evaporated + (0.1~0.2) at%; Using the same additive manufacturing process parameters, additive manufacturing was carried out with a second NiTi alloy powder to obtain a NiTi shape memory alloy with a composition close to the target composition.

[0006] Preferably, the Ni atom content in the first NiTi alloy powder is 0~0.5 at more than the target Ni content in the additively manufactured NiTi shape memory alloy.

[0007] Preferably, the actual composition of the sample before regulation is determined by using the energy dispersive spectroscopy (EDS) analyzer built into the scanning electron microscope, selecting at least 3 regions for each sample, and performing at least 3 EDS analyses for each region.

[0008] Preferably, the total size of the sampling area is not less than 5mm×5mm×5mm, and the size of each area is not less than 1mm×1mm×1mm.

[0009] Preferably, the additive manufacturing is laser powder bed melting.

[0010] Preferably, the additive manufacturing process parameters include: laser power of 175~200W, scanning speed of 700~1000mm / s, scanning spacing of 0.09~0.11mm, interlayer thickness of 0.025mm, scanning strategy of strip scanning, and rotation angle between adjacent layers of 67°.

[0011] Preferably, the additive manufacturing is carried out under argon protection, with an argon pressure of 0.4~0.6MPa and an oxygen concentration of less than 200ppm.

[0012] Preferably, the additive manufacturing process further includes preheating the substrate at a temperature of 180±3℃.

[0013] Preferably, the particle size of the first NiTi alloy powder and the second NiTi alloy powder is 15~53μm.

[0014] Preferably, the laser parameters for the outer ring of the additive manufacturing process include: a laser power of 100W and a scanning speed of 300mm / s.

[0015] This invention provides a method for controlling the composition of additively manufactured NiTi shape memory alloy to approach a target composition, comprising the following steps: determining the additive manufacturing process parameters to be used in the proposed additive manufacturing; performing additive manufacturing using a first NiTi alloy powder according to the determined additive manufacturing process parameters to obtain a sample before control; determining the actual composition of the sample before control to obtain the amount of Ni atoms evaporated; or obtaining the amount of Ni atoms evaporated according to formulas 1 to 3; providing a second NiTi alloy powder; the second NiTi alloy powder has an increased percentage of the target Ni atoms in the proposed additively manufactured NiTi shape memory alloy compared to the second NiTi alloy powder; the increase in Ni atoms and the amount of Ni atoms evaporated, in terms of atomic percentage, satisfy the following: increase in Ni atoms = amount of Ni atoms evaporated + (0.1~0.2) at%; performing additive manufacturing using the second NiTi alloy powder according to the same additive manufacturing process parameters to obtain the controlled NiTi shape memory alloy. This invention addresses the problem of unstable composition in additive manufacturing NiTi shape alloys by creatively proposing a relationship between process parameters and the amount of Ni atom increase. By measuring and determining the amount of Ni atom evaporation under the process parameters, the Ni atom content in the second NiTi alloy powder is increased accordingly to regulate the composition of the additive manufacturing NiTi shape alloy, so that the regulated NiTi shape alloy composition approaches the target composition. Attached Figure Description

[0016] Figure 1 The image shows the morphology of the NiTi shape memory alloy powder with the composition Ni50.9Ti49.1 used in Example 1. Figure 2 The morphology of the molten pool of the sample before conditioning in Example 1; Figure 3 The morphology of the molten pool of the sample before conditioning in Example 2; Figure 4 The morphology of the molten pool of the sample before conditioning in Example 3; Figure 5 The morphology of the molten pool of the sample before conditioning in Example 4; Figure 6 This is a schematic diagram of the selected area for energy spectrum analysis in Example 1. Detailed Implementation

[0017] This invention provides a method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition, comprising the following steps: Determine the additive manufacturing process parameters to be used in the proposed additive manufacturing process; According to the determined additive manufacturing process parameters, additive manufacturing was carried out using the first NiTi alloy powder to obtain the sample before adjustment; The actual composition of the sample before regulation is determined to obtain the amount of Ni atoms evaporated; or the amount of Ni atoms evaporated is obtained according to Equations 1-2. W'=0.01 E v ＇ Formula 1; Formula 2; In Equation 2 d 'Related to scanning speed:' When the scanning speed is <900mm / s, the calculation is based on Equation 3. d ', d '=-0.5 d +K L Formula 3; When the scanning speed is ≥900mm / s, it is calculated according to Equation 4. d ', d '=-0.5 d + L Equation 4; In equations 1-4, W' represents the amount of Ni atoms evaporated, at%; E v ＇ This is the corrected energy density; d ' represents the corrected scan spacing, in mm; P Laser power, W; V It is the scanning speed, mm / s; d The scanning spacing is in mm. H It is the interlayer thickness, in mm; L The maximum width of the molten pool of the sample before adjustment is in mm; K is the correction factor, taken as 0.78. A second NiTi alloy powder is provided; the second NiTi alloy powder has an increased target Ni atom percentage content compared to the additively manufactured NiTi shape memory alloy; in terms of atomic percentage, the increase in Ni atoms and the amount of Ni atoms evaporated satisfy: increase in Ni atoms = amount of Ni atoms evaporated + (0.1~0.2) at%; Using the same additive manufacturing process parameters, additive manufacturing was carried out with a second NiTi alloy powder to obtain a NiTi shape memory alloy with a composition close to the target composition.

[0018] This invention determines the additive manufacturing process parameters to be used in the proposed additive manufacturing process.

[0019] In this invention, the additive manufacturing is preferably laser powder bed fusion (L-PBF); the process parameters of the additive manufacturing include: laser power preferably 175~200W, scanning speed preferably 700~1000mm / s, scanning spacing preferably 0.09~0.11mm, interlayer thickness preferably 0.025mm, scanning strategy preferably strip scanning, and rotation angle between adjacent layers preferably 67°.

[0020] In embodiments of the present invention, the laser power of the additive manufacturing can be 175, 180, 185, 190, 195 or 200W; the scanning speed can be 700, 750, 800, 850, 900, 950 or 1000mm / s; and the scanning spacing can be 0.09, 0.10 or 0.11mm.

[0021] The present invention further specifies the outer ring parameters of the component. In the present invention, the laser parameters of the outer ring in the additive manufacturing process include: a laser power preferably of 100W and a scanning speed preferably of 300mm / s.

[0022] According to the determined additive manufacturing process parameters, the present invention uses first NiTi alloy powder for additive manufacturing to obtain a sample before adjustment.

[0023] In this invention, the Ni atom content in the first NiTi alloy powder is preferably 0-0.5 at% excess over the target Ni content in the additively manufactured NiTi shape memory alloy. By controlling the Ni atom content in the first NiTi alloy powder to be close to the target Ni content in the NiTi shape memory alloy, this invention helps to reflect the true evaporation behavior of Ni atoms in the target NiTi shape memory alloy, thereby facilitating the obtaining of a NiTi shape memory alloy with a composition close to the target composition.

[0024] In this invention, the particle size of the first NiTi alloy powder is preferably 15~53 μm. The first NiTi alloy powder can be purchased or prepared using a gas atomization method well-known in the art. When prepared using the gas atomization method, the specific process is as follows: A NiTi alloy ingot is placed in a water-cooled copper crucible and heated to a molten state under high vacuum using a medium-frequency induction coil. Electromagnetic stirring is used to promote homogenization of the melt composition. Subsequently, the molten alloy flows out through a guide tube at the bottom of the crucible, while high-pressure inert gas is ejected at high speed through an annular nozzle at a specific angle, breaking the molten metal flow into micron-sized droplets. The droplets are rapidly cooled and solidified into spherical powder in an inert atmosphere. Finally, the obtained powder is sieved using an ultrasonic vibrating screen and classified according to the target particle size range (e.g., 15~53 μm).

[0025] In this invention, prior to additive manufacturing, the substrate is preferably preheated at a temperature of 180±3℃. The substrate is not specifically limited; any substrate well-known in the art can be used, such as a NiTi shape memory alloy substrate. By preheating the substrate, this invention reduces the solidification temperature gradient and lowers the tendency for thermal cracking.

[0026] In this invention, the additive manufacturing is preferably carried out in an argon atmosphere, with the argon pressure preferably being 0.4~0.6 MPa and the oxygen concentration preferably being below 200 ppm.

[0027] In this invention, the process parameters for additive manufacturing have been discussed previously and will not be repeated here.

[0028] After obtaining the sample before regulation, the present invention determines the actual composition of the sample before regulation and obtains the amount of Ni atoms evaporated.

[0029] In this invention, the actual composition of the sample before regulation is preferably determined using the energy dispersive spectroscopy (EDS) instrument integrated into a scanning electron microscope (SEM). At least three regions are selected for each sample, and at least three EDS analyses are performed on each region. This invention involves at least nine EDS analyses per sample, which reduces measurement errors. In this invention, the composition measured by EDS is an atomic percentage. In this invention, the sampling location is preferably the middle of the sample before regulation (i.e., a non-edge region); the total size of the sampling area is preferably not less than 5 mm × 5 mm × 5 mm, and the size of each region is preferably not less than 1 mm × 1 mm × 1 mm.

[0030] In this invention, the amount of Ni atoms evaporated can also be obtained according to formulas 1-2.

[0031] W'=0.01 E v ＇ Formula 1; Formula 2; In Equation 2 d 'Related to scanning speed:' When the scanning speed is <900mm / s, the calculation is based on Equation 3. d ', d '=-0.5 d +K L Formula 3; When the scanning speed is ≥900mm / s, it is calculated according to Equation 4. d ', d '=-0.5 d + L Equation 4; In equations 1-4, W' represents the amount of Ni atoms evaporated, at%; E v ＇ This is the corrected energy density; d ' represents the corrected scan spacing, in mm; P Laser power, W; V It is the scanning speed, mm / s; d The scanning spacing is in mm. H It is the interlayer thickness, in mm; L is the maximum width of the molten pool of the sample before adjustment, in mm; K is the correction factor, taken as 0.78.

[0032] The present invention preferably obtains the Ni atom evaporation amount through Equations 1-2. In this case, EDS analysis is not required; the Ni atom evaporation amount can be calculated simply by measuring the molten pool width of the sample before control and then applying Equations 1-2. This saves time and reduces costs. Furthermore, the Ni atom evaporation amount calculation model provided by the present invention also has the advantage of high accuracy.

[0033] The derivation process of the above model is as follows: It is generally believed that: W Ni =-0.01 E v +W Formula 5; Among them, W Ni It refers to the Ni atom content in the sample after evaporation. E v W is the energy density, and W is the Ni atom content in the elemental powder.

[0034] However, this method can only predict the evaporation of Ni in a simple way and is not accurate. This is because the formula only simply fits the effect of energy density. However, in addition to the effect of temperature on evaporation caused by energy density, the interaction between different orbital molten pools also affects the amount of Ni atoms evaporated. Therefore, the influence relationship between various parameters cannot be simply summarized by energy density.

[0035] The energy density formula is shown in Equation 6: Formula 6; In Equation 6, P It's the laser power. V It's the scanning speed. d It is the scanning spacing. H It refers to the interlayer thickness.

[0036] The present invention relates to d Proposed revisions: d '=-0.5 d + L Equation 4; where L It is the maximum width of the molten pool.

[0037] Regarding the molten pool width, because low-scanning-speed lasers (<900mm / s) spend a longer time in the same spot, the molten pool is wider when parameters such as laser power and scanning spacing remain unchanged. Therefore, molten pool width correction is used... d The difference between low and high scan rates needs to be eliminated. Therefore, a correction factor K needs to be added when calculating the melt pool width at low scan rates.

[0038] Formula 7; in, V 1 indicates a relatively low scanning speed. V 2 refers to a relatively high scanning speed. In this invention, K=0.78.

[0039] Therefore, the relationship between the parameters and Ni evaporation proposed in this invention is as follows: W'=0.01E v Formula 1; Formula 2; In Equation 2 d 'Related to scanning speed:' When the scanning speed is <900mm / s, the calculation is based on Equation 3. d ', d '=-0.5 d +K L Formula 3; When the scanning speed is ≥900mm / s, it is calculated according to Equation 4. d ', d '=-0.5 d + L Formula 4.

[0040] In this invention, the maximum width of the molten pool is preferably measured by backscattering mode (BSE) of a scanning electron microscope.

[0041] After obtaining the amount of Ni atoms evaporated, the present invention provides a second NiTi alloy powder.

[0042] In this invention, the second NiTi alloy powder has an increased percentage of target Ni atoms compared to the additively manufactured NiTi shape memory alloy. In terms of atomic percentage, the increase in Ni atoms satisfies the following relationship with the amount of Ni atoms evaporated: Increase in Ni atoms = Amount of Ni atoms evaporated + (0.1~0.2) at%. In a specific embodiment, this could be: Increase in Ni atoms = Amount of Ni atoms evaporated + (0.1, 0.15, or 0.2) at%. In this invention, the particle size and method of obtaining the second NiTi alloy powder are the same as those of the first alloy powder, and will not be repeated here.

[0043] This invention adds an additional (0.1~0.2) at% to the amount of Ni atoms evaporated. The additional (0.1~0.2) at% is an empirical value obtained through a large number of experiments.

[0044] After obtaining the second alloy powder, the present invention uses the second NiTi alloy powder for additive manufacturing according to the same additive manufacturing process parameters to obtain a NiTi shape memory alloy with a composition close to the target composition.

[0045] This invention creatively addresses the problem of unstable composition in additive manufacturing NiTi shape alloys by establishing a relationship between process parameters and Ni atom evaporation rate. By measuring and determining the Ni atom evaporation rate under the process parameters, the Ni atom content in the second NiTi alloy powder is increased accordingly to regulate the composition of the additive manufacturing NiTi shape alloy, making the regulated NiTi shape alloy composition closer to the target composition.

[0046] The following detailed description, in conjunction with embodiments, illustrates the method for controlling the Ni content in additive manufacturing of NiTi shape memory alloys provided by this invention. However, these descriptions should not be construed as limiting the scope of protection of this invention.

[0047] In the following examples, the NiTi shape memory alloy powder used was prepared by gas atomization. The specific process is as follows: The NiTi alloy ingot was placed in a water-cooled copper crucible and heated to a molten state under high vacuum using a medium-frequency induction coil. Electromagnetic stirring was used to promote the homogenization of the melt composition. Subsequently, the molten alloy flowed out through the guide tube at the bottom of the crucible, while high-pressure inert gas was ejected at high speed at a specific angle through an annular nozzle, breaking the molten metal flow into micron-sized droplets. The droplets were rapidly cooled and solidified into spherical powder in an inert atmosphere. Finally, the obtained powder was sieved using an ultrasonic vibrating screen and classified according to the target particle size range (e.g., 15~53μm).

[0048] Example 1 A method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition of Ni50.6Ti49.4, comprising the following steps: (I) Thermal Drying: Near-spherical NiTi shape memory alloy powder, with a particle size range of 15-53 μm and a composition of Ni50.9Ti49.1, prepared by gas atomization, was placed in a vacuum drying oven for drying at 70℃ for 8 hours. The morphology of the NiTi shape memory alloy powder is as follows: Figure 1 As shown, by Figure 1 It can be seen that the obtained powder has good sphericity and fewer satellite powders.

[0049] (ii) Additive manufacturing: The NiTi shape memory alloy powder from step (1) is subjected to laser powder bed melting. The specific steps are as follows: (1) The additive manufacturing substrate is a NiTi shape memory alloy substrate. The substrate is preheated before printing at a temperature of 180±3℃ to reduce the solidification temperature gradient and reduce the tendency of thermal cracking. (2) High-purity argon (99.999%) is introduced into the printing chamber to replace oxygen. The argon pressure is 0.4~0.6MPa, and the oxygen concentration is controlled below 200ppm. (3) Setting parameters: laser power 175W, scanning speed 700mm / s, scanning spacing 0.09mm, interlayer thickness 0.025mm, scanning strategy is strip scanning, and the rotation angle between adjacent layers is 67°. An additive manufacturing NiTi shape memory alloy sample was obtained and is recorded as the sample before adjustment.

[0050] (III) Observation of the molten pool morphology: Using the laser parameters mentioned in step (II) above, the sample before adjustment was scanned, and then imaged and observed using a scanning electron microscope (SEM) with BSE. The molten pool morphology is as follows: Figure 2 As shown, the maximum width of the molten pool was measured to be 0.1784 mm.

[0051] The amount of Ni atoms evaporated is calculated according to Equation 1-3 as follows: d '=-0.5 d +K L =-0.5×0.09+0.78×0.1784=0.0942 106.2 W'=0.01 Ev = 0.01 × 10⁶.² = 1.062 at%.

[0052] (iv) Composition determination: The composition of the untreated sample obtained in step (ii) was determined using EDS in SEM. The selected EDS regions are as follows: Figure 6As shown in Table 1, the test results (before adjustment) are as follows. The measured Ni atom evaporation rate was 1.05 at%, which is basically consistent with the calculated value, proving the accuracy of the calculation model of this invention.

[0053] (v) Component regulation: Ni atom increase = Ni atom evaporation + (0.1~0.2) = 1.06 + (0.1~0.2) = 1.16~1.26 at%, the target Ni content is 50.6 at%, then the Ni content in the second nickel-titanium alloy powder required for printing is 50.6 + (1.16~1.26) = 51.76~51.86 at%. Therefore, the second NiTi alloy powder with composition Ni51.8Ti49.1 is used for printing. Steps (I) and (II) are repeated to obtain a NiTi shape memory alloy sample with composition close to the target composition, which is recorded as the adjusted sample.

[0054] (vi) Composition determination: The composition of the regulated sample obtained in step (v) was determined by EDS in SEM. The results are shown in Table 1. As shown in Table 1, the Ni content in the regulated sample is 50.63 at%, which is close to the target composition of 50.6 at%. The standard deviation of Ni evaporation between the two measurements is 0.06%, and the standard deviation between the measured Ni content and the target Ni content is 0.015%.

[0055] Table 1. Composition of the samples before and after conditioning in Example 1

[0056] Example 2 A method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition of Ni50.5Ti49.5, comprising the following steps: (i) Heat preservation and drying: NiTi shape memory alloy powder with a near-spherical shape, a particle size range of 15~53μm, and a composition of Ni50.9Ti49.1 prepared by gas atomization was placed in a vacuum drying oven for drying at a temperature of 70℃ for 8h.

[0057] (ii) Additive manufacturing: The NiTi shape memory alloy powder from step (1) is subjected to laser powder bed melting. The specific steps are as follows: (1) The additive manufacturing substrate is a NiTi shape memory alloy substrate. The substrate is preheated before printing at a temperature of 180±3℃ to reduce the solidification temperature gradient and reduce the tendency of thermal cracking. (2) High-purity argon (99.999%) is introduced into the printing chamber to replace oxygen. The argon pressure is 0.4~0.6MPa, and the oxygen concentration is controlled below 200ppm. (3) Setting parameters: laser power 175W, scanning speed 700mm / s, scanning spacing 0.11mm, interlayer thickness 0.025mm, scanning strategy is strip scanning, rotation angle between adjacent layers 67°, to obtain additive manufacturing NiTi shape memory alloy sample, which is recorded as the sample before adjustment.

[0058] (III) Observation of the molten pool morphology: Using the laser parameters mentioned in step (II) above, a scan was performed on the sample before adjustment, followed by imaging observation using the backscatter mode in a scanning electron microscope. The molten pool morphology is as follows: Figure 3 As shown, the measured width of the molten pool is 0.1822 mm.

[0059] The amount of Ni atoms evaporated is calculated according to Equation 1-3 as follows: d '=-0.5 d +K L =-0.5×0.11+0.78×0.1822=0.0871 W'=0.01 Ev =0.01 × 114.8 = 1.148 at%.

[0060] (iv) Composition determination: The composition of the sample obtained in step (ii) before regulation was determined by EDS in SEM. The results are shown in Table 2 (before regulation). The measured Ni atom evaporation amount was 1.14 at%, which is consistent with the calculated value, proving the accuracy of the calculation model of this invention.

[0061] (v) Component regulation: Ni atom increase = Ni atom evaporation + (0.1~0.2) = 1.15 + (0.1~0.2) = 1.25~1.35 at%, the target Ni content is 50.5 at%, then the Ni content in the second NiTi alloy powder required for printing is 50.5 + (1.25~1.35) = 51.75~51.85 at%. Therefore, NiTi shape memory alloy powder with composition Ni51.8Ti49.1 is used for printing. Steps (I) and (II) are repeated to obtain a NiTi shape memory alloy sample with composition close to the target composition, which is recorded as the adjusted sample.

[0062] (vi) Composition determination: The composition of the regulated sample obtained in step (v) was determined by EDS in SEM. The results are shown in Table 2. As shown in Table 2, the Ni content in the regulated sample is 50.52 at%, which is close to the target composition of 50.5 at%. The standard deviation of Ni evaporation between the two measurements is 0.07%, and the standard deviation between the measured Ni content and the target Ni content is 0.01%.

[0063] Table 2 Composition of the samples before and after conditioning in Example 2

[0064] Example 3 A method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition of Ni50.7Ti49.3, comprising the following steps: (i) Heat preservation and drying: NiTi shape memory alloy powder with a near-spherical shape, a particle size range of 15~53μm, and a composition of Ni50.9Ti49.1 prepared by gas atomization was placed in a vacuum drying oven for drying at a temperature of 70℃ for 8h.

[0065] (ii) Additive manufacturing: The NiTi shape memory alloy powder from step (1) is subjected to laser powder bed melting. The specific steps are as follows: (1) The additive manufacturing substrate is a NiTi shape memory alloy substrate. The substrate is preheated before printing at a temperature of 180±3℃ to reduce the solidification temperature gradient and reduce the tendency of thermal cracking. (2) High-purity argon (99.999%) is introduced into the printing chamber to replace oxygen. The argon pressure is 0.4~0.6MPa, and the oxygen concentration is controlled below 200ppm. (3) Setting parameters: laser power 200W, scanning speed 900mm / s, scanning spacing 0.09mm, interlayer thickness 0.025mm, scanning strategy is strip scanning, and the rotation angle between adjacent layers is 67°. An additive manufacturing NiTi shape memory alloy sample was obtained and recorded as the sample before adjustment.

[0066] (III) Observation of the molten pool morphology: Using the laser parameters mentioned in step (II) above, a scan was performed on the sample before adjustment, followed by imaging observation using the backscatter mode in a scanning electron microscope. The molten pool morphology is as follows: Figure 4 As shown, the measured width of the molten pool is 0.1351 mm.

[0067] The amount of Ni atoms evaporated is calculated according to Equation 1-3 as follows: d '=-0.5 d + L =-0.5×0.09+0.1351=0.0901 W'=0.01Ev'=0.01×98.6=0.986at%.

[0068] (iv) Composition determination: The composition of the sample obtained in step (ii) before regulation was determined by EDS in SEM. The results are shown in Table 3 (before regulation). The measured Ni atom evaporation amount was 0.98 at%, which is basically consistent with the calculated value, proving the accuracy of the calculation model of this invention.

[0069] (v) Component regulation: Ni atom increase = Ni atom evaporation + (0.1~0.2)at% = 0.99 + (0.1~0.2)at% = 1.09~1.19at%. The target Ni content is 50.7at. Therefore, the actual Ni content required for printing is 50.7 + (1.09~1.19) = 51.79~51.89at. Thus, the second NiTi alloy powder with composition Ni51.8Ti49.1 is used for printing. Steps (I) and (II) are repeated to obtain a NiTi shape memory alloy sample with a composition close to the target composition, which is recorded as the adjusted sample.

[0070] (vi) Composition determination: The composition of the regulated sample obtained in step (v) was determined by EDS in SEM. The results are shown in Table 3. As shown in Table 3, the Ni content in the regulated sample is 50.63 at%, which is close to the target composition of 50.7 at%. The standard deviation of Ni evaporation between the two measurements is 0.15%, and the standard deviation between the measured Ni content and the target Ni content is 0.035%.

[0071] Table 3 Composition of the samples before and after conditioning in Example 3

[0072] Example 4 A method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition of Ni50.7Ti49.3, comprising the following steps: (i) Heat preservation and drying: NiTi shape memory alloy powder with a near-spherical shape, a particle size range of 15~53μm, and a composition of Ni50.9Ti49.1 prepared by gas atomization was placed in a vacuum drying oven for drying at a temperature of 70℃ for 8h.

[0073] (ii) Additive manufacturing: The NiTi shape memory alloy powder from step (1) is subjected to laser powder bed melting. The specific steps are as follows: (1) The additive manufacturing substrate is a NiTi shape memory alloy substrate. The substrate is preheated before printing at a temperature of 180±3℃ to reduce the solidification temperature gradient and reduce the tendency of thermal cracking. (2) High-purity argon (99.999%) is introduced into the printing chamber to replace oxygen. The argon pressure is 0.4~0.6MPa, and the oxygen concentration is controlled below 200ppm. (3) Setting parameters: laser power 175W, scanning speed 900mm / s, scanning spacing 0.09mm, interlayer thickness 0.025mm, scanning strategy is strip scanning, rotation angle between adjacent layers 67°, to obtain additive manufacturing NiTi shape memory alloy sample, which is recorded as the sample before adjustment.

[0074] (III) Observation of the molten pool morphology: Using the laser parameters mentioned in step (II) above, a scan was performed on the sample before adjustment, followed by imaging observation using the backscatter mode in a scanning electron microscope. The molten pool morphology is as follows: Figure 5 As shown, the measured width of the molten pool is 0.1348 mm.

[0075] The amount of Ni atoms evaporated is calculated according to Equation 1-3 as follows: d '=-0.5 d + L =-0.5×0.11+0.1348=0.0798 W'=0.01Ev'=0.01×97.5=0.975at%.

[0076] (iv) Composition determination: The composition of the sample obtained in step (ii) before regulation was determined by EDS in SEM. The results are shown in Table 4 (before regulation). The measured Ni atom evaporation amount was 0.97 at%, which is basically consistent with the calculated value, proving the accuracy of the calculation model of this invention.

[0077] (v) Component regulation: Ni atom increase = Ni atom evaporation + (0.1~0.2)at% = 0.98at% + (0.1~0.2)at% = 1.08~1.18at%. The target Ni content is 50.7at. Therefore, the Ni content in the second NiTi alloy powder required for printing is 50.7 + 1.08~1.18 = 51.78~51.88at. Therefore, NiTi alloy powder with composition Ni51.8Ti49.1 is used for printing. Steps (I) and (II) are repeated to obtain NiTi shape memory alloy sample, which is recorded as the adjusted sample.

[0078] (vi) Composition determination: The composition of the regulated sample obtained in step (v) was determined by EDS in SEM. The results are shown in Table 4. As shown in Table 4, the Ni content in the regulated sample is 50.64 at%, which is close to the target composition of 50.7 at%. The standard deviation of Ni evaporation between the two measurements is 0.15%, and the standard deviation between the measured Ni content and the expected Ni content is 0.03%.

[0079] Table 4. Composition of the samples before and after conditioning in Example 4

[0080] Comparative Examples 1-4 The difference from Examples 1-4 is that the alloy powder of the target composition is directly used for laser powder bed melting, and the process parameters are the same as those of Examples 1-4. That is, Comparative Example 1 uses the printing parameters of Example 1, Comparative Example 2 uses the printing parameters of Example 2, and so on.

[0081] Table 5. Composition of the samples after comparative printing

[0082] from Figures 1 to 5 It can be seen that the wider and deeper the molten pool morphology, the more Ni atoms evaporate.

[0083] The results of the examples and comparative examples show that, after being adjusted by the method of the present invention, the Ni content in the NiTi shape memory alloy is closer to the target composition.

[0084] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for controlling the composition of additively manufactured NiTi shape memory alloys to approach a target composition, characterized in that, Includes the following steps: Determine the additive manufacturing process parameters to be used in the proposed additive manufacturing process; According to the determined additive manufacturing process parameters, additive manufacturing was carried out using the first NiTi alloy powder to obtain the sample before adjustment; The actual composition of the sample before regulation is determined to obtain the amount of Ni atoms evaporated; or the amount of Ni atoms evaporated is obtained according to Equations 1-2. W'=0.01 E v ＇ Formula 1; Formula 2; In Equation 2 d 'Related to scanning speed:' When the scanning speed is <900mm / s, the calculation is based on Equation 3. d ', d '=-0.5 d +K L Formula 3; When the scanning speed is ≥900mm / s, it is calculated according to Equation 4. d ', d '=-0.5 d + L Equation 4; In equations 1-4, W' represents the amount of Ni atoms evaporated, at%; E v ＇ This is the corrected energy density; d ' represents the corrected scan spacing, in mm; P Laser power, W; V It is the scanning speed, mm / s; d The scanning spacing is in mm. H It is the interlayer thickness, in mm; L The maximum width of the molten pool of the sample before adjustment is in mm; K is the correction factor, taken as 0.

78. A second NiTi alloy powder is provided; the second NiTi alloy powder has an increased target Ni atom percentage content compared to the additively manufactured NiTi shape memory alloy; in terms of atomic percentage, the increase in Ni atoms and the amount of Ni atoms evaporated satisfy: increase in Ni atoms = amount of Ni atoms evaporated + (0.1~0.2) at%; Using the same additive manufacturing process parameters, additive manufacturing was carried out with a second NiTi alloy powder to obtain a NiTi shape memory alloy with a composition close to the target composition.

2. The method according to claim 1, characterized in that, The Ni atom content in the first NiTi alloy powder is 0~0.5 at more than the target Ni content in the additively manufactured NiTi shape memory alloy.

3. The method according to claim 1, characterized in that, The actual composition of the sample before regulation was determined by using the energy dispersive spectroscopy (EDS) analyzer built into the scanning electron microscope. At least three regions were selected for each sample, and at least three EDS analyses were performed on each region.

4. The method according to claim 3, characterized in that, The total size of the sampling area shall not be less than 5mm×5mm×5mm, and the size of each area shall not be less than 1mm×1mm×1mm.

5. The method according to claim 1, characterized in that, The additive manufacturing process is laser powder bed melting.

6. The method according to claim 5, characterized in that, The additive manufacturing process parameters include: laser power 175~200W, scanning speed 700~1000mm / s, scanning spacing 0.09~0.11mm, interlayer thickness 0.025mm, scanning strategy is strip scanning, and the rotation angle between adjacent layers is 67°.

7. The method according to claim 5, characterized in that, The additive manufacturing is carried out under argon protection, with an argon pressure of 0.4~0.6MPa and an oxygen concentration below 200ppm.

8. The method according to claim 5, characterized in that, The additive manufacturing process also includes preheating the substrate at a temperature of 180±3℃.

9. The method according to claim 1, characterized in that, The particle size of the first NiTi alloy powder and the second NiTi alloy powder is 15~53μm.

10. The method according to claim 5, characterized in that, The laser parameters for the outer ring of the additive manufacturing process include: laser power of 100W and scanning speed of 300mm / s.

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