A method for additive manufacturing of strong and tough NiTi alloy based on interlayer process regulation

Abstract

The application discloses a kind of strong and tough NiTi alloy additive manufacturing methods based on interlayer process regulation;Belong to NiTi alloy additive manufacturing technical field.The present application solves the technical problems that the existing LPBF NiTi usually adopts uniform process parameters, and it is difficult to consider strength and plasticity.The application selects NiTi spherical powder as raw material;It is loaded into LPBF equipment, and NiTi alloy substrate is used as forming substrate, the substrate is preheated and leveled, inert gas is introduced into the forming cabin, and the oxygen content in the cabin is controlled to be less than 10 ppm;Set the interlayer rotation scanning strategy in the slicing software;Set two, three or more laser scanning speeds, calculate the total number of sample printing layers according to the powder layer thickness, generate the printing process file corresponding to the laser scanning speed and import the equipment;Then, along the building direction, switch different laser scanning speeds alternately to scan and form.The application is applied to thin-walled components, lattice structures, intelligent driving components and buffer energy-absorbing structures and other complex NiTi parts.

Description

Technical Field

[0001] This invention belongs to the field of NiTi alloy additive manufacturing technology, specifically, it relates to a method for additive manufacturing of strong and tough NiTi alloys based on interlayer process control. Background Technology

[0002] NiTi shape memory alloys, with their shape memory effect, superelasticity, and excellent damping properties, have broad application prospects in aerospace, biomedicine, and intelligent drive fields. Laser powder bed melting (LPBF) provides an effective way to integrally form complex NiTi components. However, existing LPBF NiTi typically employs uniform process parameters, making it difficult to balance strength and plasticity. Lower scanning speeds, corresponding to higher energy inputs, can promote full fusion but also enhance Ni volatilization and alter the alloy composition, phase transformation behavior, and room-temperature phase composition, resulting in lower structural strength but higher plasticity. Higher scanning speeds, on the other hand, can suppress Ni volatilization, increase the critical stress of phase transformation, and load-bearing capacity, resulting in higher structural strength but lower plasticity. Therefore, a single process parameter cannot fully leverage the complementary advantages of NiTi microstructure and mechanical response under different process conditions. Based on this, it is necessary to develop a method for synergistic control of strength and plasticity in NiTi alloys that does not require changing the powder composition or introducing a second reinforcing phase and is compatible with existing LPBF equipment. Summary of the Invention

[0003] This invention addresses the problem that existing LPBF NiTi processes typically employ uniform process parameters, making it difficult to balance strength and plasticity. Instead, it provides an additive manufacturing method for strong and tough NiTi alloys based on interlayer process control.

[0004] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for additive manufacturing of strong and tough NiTi alloys based on interlayer process control, comprising the following steps: Step 1: Select NiTi spherical powder as raw material; Step 2: Then load it into the LPBF equipment, using a NiTi alloy substrate as the forming substrate. Preheat the substrate and level it. Inert gas is introduced into the forming chamber to control the oxygen content in the chamber to be below 10 ppm. Step 3: Set the interlayer rotation scanning strategy in the slicing software. The strategy shall include at least the following process parameters: rotation angle between adjacent layer scanning directions, powder layer thickness, scanning spacing, and laser power. Step 4: Set two, three or more laser scanning speeds, calculate the total number of printing layers for the sample based on the powder layer thickness, generate the printing process file corresponding to the laser scanning speed, and import it into the equipment; Step 5: Then, alternately switch different laser scanning speeds along the construction direction to scan and shape the object.

[0005] Further specifying, in step 1, the Ni content in the raw material is 55 wt.%-60 wt.%.

[0006] Further specifying, in step 1, the particle size of the raw material is 15 μm to 55 μm.

[0007] Further specifying, in step 2, the preheating temperature is 90 ℃-110 ℃; the inert gas is argon.

[0008] Further specifying, in step 3, the rotation angle between adjacent layer scans is 60°-80°, the powder layer thickness is 30 μm-60 μm, the scanning interval is 40 μm-100 μm, and the laser power is 150 W-180 W.

[0009] Further specifying, in step 4, two laser scanning speeds are set: 400 mm / s and 1200 mm / s, or three or more of the following laser scanning speeds: 400 mm / s, 600 mm / s, 800 mm / s, 1000 mm / s, and 1200 mm / s.

[0010] To further specify, in step 5, printing is performed alternately in an independent process zone of 20-40 printing layers.

[0011] Further, two laser scanning speeds were set: 400 mm / s and 1200 mm / s. The first 20 layers were printed using a scanning speed of 400 mm / s. After completion, the process file was switched to 1200 mm / s to print the 21st to 40th layers. Subsequently, the process was carried out alternately using 400 mm / s and 1200 mm / s.

[0012] Another objective of this invention is to provide a NiTi alloy gradient functional component, which is prepared by any of the methods described above, and the component has a periodic alternation along the construction direction.

[0013] Furthermore, the component forms alternating bands of microstructure along the construction direction with a period of 20-40 printed layers.

[0014] Regarding tensile strength, the interlayer scanning speed control method described in this invention is not limited to the process of alternating between the two scanning speeds of 400 mm / s and 1200 mm / s in equal proportion. It can also be designed by coding the number, thickness, spatial proportion, and arrangement of process zones corresponding to different scanning speeds, based on the specific service environment and target mechanical properties of the NiTi alloy component. Specifically, under the condition that the laser power, powder layer thickness, scanning spacing, and scanning strategy remain unchanged, the volume fraction of various process zones along the construction direction can be adjusted by changing the number of printing layers covered by different scanning speeds.

[0015] For example, such as Figure 6 As shown, for a NiTi sample with a total of 120 layers, a single layer thickness of 30 μm, and a total forming height of 3.6 mm, when it is necessary to improve the overall tensile strength and load-bearing capacity of the component, the proportion of the high-scanning-speed process zone can be appropriately increased. For example, the 120 printed layers can be divided into 6 process zones, each containing 20 layers. Four process zones use a scanning speed of 1200 mm / s, and the other two process zones use a scanning speed of 400 mm / s. This results in the thicknesses of the high-scanning-speed process zone and the low-scanning-speed process zone being 2.4 mm and 1.2 mm, respectively, corresponding to a volume percentage of approximately 66.7% and 33.3%. Accordingly, the sample can be formed sequentially along the construction direction in the order of "1200 mm / s—400 mm / s—1200 mm / s—1200 mm / s—400 mm / s—1200 mm / s", thereby increasing the proportion of the microstructure corresponding to the high scanning speed in the component. When a component prioritizes ductility, deformation tolerance, or damage tolerance under large deformation conditions, the proportion of low-scanning-speed processing zones can be increased. For example, for a 120-layer sample, four processing zones using a 400 mm / s scan rate and two using a 1200 mm / s scan rate can be configured, increasing the thickness proportion of the low-scanning-speed region to approximately 66.7%. These processing zones can be arranged along the construction direction in a sequence of "400 mm / s—1200 mm / s—400 mm / s—400 mm / s—1200 mm / s—400 mm / s," allowing the microstructure and mechanical response corresponding to low scan rates to occupy a larger proportion of the component's internal space, thereby adjusting the overall ductility and deformation capacity.

[0016] Furthermore, the present invention can also adjust the spatial control scale by changing the number of printing layers contained in a single process zone. For example, as Figure 7As shown, while keeping the powder layer thickness constant at 30 μm, each process zone can be adjusted from 20 layers to 10, 30, or 40 layers, corresponding to thicknesses of 0.30 mm, 0.90 mm, and 1.20 mm along the construction direction for individual process zones, respectively. Decreasing the process zone thickness increases the alternation frequency of regions at different scanning speeds along the construction direction, resulting in a finer periodic tissue distribution; increasing the process zone thickness leads to larger continuous tissue regions. Therefore, by changing the number of process zone layers, the spatial scale and the number of interfaces for different tissue states can be controlled.

[0017] In addition to using two scanning speeds, this invention can further set three or more scanning speeds to form a multi-level process coding. For example, as Figure 8 As shown, under the conditions of constant laser power of 160 W, powder layer thickness of 30 μm, and scanning spacing of 80 μm, three scanning speeds of 400 mm / s, 800 mm / s, and 1200 mm / s can be set respectively, and the 120 printing layers are divided into 6 process zones, each containing 20 layers. The process zones can be arranged periodically in the pattern "400 mm / s—800 mm / s—1200 mm / s—400 mm / s—800 mm / s—1200 mm / s" to form periodic microstructure regions corresponding to three different heat input conditions; alternatively, they can be arranged in the pattern "400 mm / s—400 mm / s—800 mm / s—800 mm / s—1200 mm / s—1200 mm / s" to gradually increase the scanning speed along the construction direction, thereby constructing NiTi alloy components with multi-level microstructure and mechanical property variations. Therefore, this invention does not require changing the initial composition of the pre-alloyed NiTi powder or the external geometry of the component. By pre-coding and partitioning the laser scanning speed of different building layers during the LPBF forming process, it can change the heat input and thermal cycling history experienced by different regions. Furthermore, it can realize the spatial configuration of different microstructures, phase transformation behaviors, and mechanical response regions within the component, enabling different performance regions to work together. This allows for customized matching of strength, ductility, phase transformation characteristics, and functional response according to service requirements.

[0018] Furthermore, since the above-mentioned control process only involves changes in the scanning speed and arrangement of different building layers, it does not affect the layer-by-layer forming characteristics of the LPBF process itself, thus maintaining the advantages of LPBF in the integrated manufacturing of complex NiTi components. This method is not only applicable to simple bulk or tensile samples, but can also be further applied to complex NiTi parts such as thin-walled components, lattice structures, smart drive components, and buffer energy absorption structures.

[0019] Taking a sample with a height of 3.6 mm as an example, the additive manufacturing method for strong and tough NiTi alloys based on interlayer process control is carried out according to the following steps: Step 1: Select pre-alloyed NiTi spherical powder with a Ni content of 56.3 wt.% as raw material, with a powder particle size range of 15-55 μm; Step 2: Load NiTi alloy powder into the LPBF equipment and use NiTi alloy substrate as the forming substrate. First, preheat the substrate to 100 ℃ and complete the leveling. Then, continuously introduce high-purity Ar into the forming chamber as a protective gas to reduce the oxygen content in the chamber to below 10 ppm, so as to reduce the oxidation of NiTi alloy during the forming process. Step 3: Set the interlayer rotation scanning strategy in the slicing software, rotate the scanning direction of adjacent layers by 66.7°, set the powder layer thickness to 30 μm, set the scanning interval to 80 μm, and set the laser power to 160 W. The above parameters remain unchanged throughout the forming process, and only the laser scanning speed of different building layers is adjusted. Step 4: Set two laser scanning speeds of 400 mm / s and 1200 mm / s respectively, and generate the corresponding printing process files. Based on the powder layer thickness of 30 μm, the sample contains a total of 120 printing layers along the construction direction. Then, import the process files corresponding to the two scanning speeds into the LPBF device.

[0020] Step 5: Print 20 printing layers as an independent process zone, with each process zone having a thickness of 0.6 mm along the build direction, forming a total of 6 consecutive process zones. Specifically, layers 1 to 20 are printed at a scanning speed of 400 mm / s; after completion, switch to a process file of 1200 mm / s to print layers 21 to 40; then alternate between 400 mm / s and 1200 mm / s for forming, i.e., layers 41 to 60 are printed at 400 mm / s, layers 61 to 80 at 1200 mm / s, layers 81 to 100 at 400 mm / s, and layers 101 to 120 at 1200 mm / s.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention can achieve synergistic enhancement of strength and ductility by adjusting the scanning speed of different layers.

[0022] This invention can form different mechanical response regions using the same batch of pre-made NiTi alloy powder, without the need to add additional reinforcing particles or a second type of alloy powder.

[0023] This invention can be achieved simply by modifying the scanning speed parameters corresponding to different building layers in the slice file, without the need to add new forming equipment.

[0024] This invention allows for customized adjustment of the strength-plasticity matching relationship of materials by changing the process coding method of the number of layers, thickness ratio, and arrangement period of high and low scanning speed layers.

[0025] The method of this invention inherits the integrated manufacturing capability of LPBF for complex structures and can be applied to thin-walled parts, lattice structures, driving components and buffer energy absorption structures.

[0026] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0027] Figure 1 DSC phase transformation curves of NiTi alloy samples prepared at different scanning speeds; Figure 2 Tensile stress-strain curves of NiTi alloy samples prepared at different scanning speeds; Figure 3 X-ray diffraction (XRD) patterns of NiTi alloy samples prepared at different scanning speeds; Figure 4 This is a schematic diagram of the NiTi alloy interlayer process coding and additive manufacturing based on alternating scanning speed control. Figure 5 This is a comparison chart of the tensile properties of samples prepared at different scanning speeds.

[0028] Figure 6 This is a schematic diagram showing the percentage of process bands at different scanning speeds.

[0029] Figure 7 These are schematic diagrams of 10-layer, 30-layer, and 40-layer process strips.

[0030] Figure 8 This is a schematic diagram of the process belt using scanning speeds of 400mm / s, 800mm / s, and 1200mm / s. Detailed Implementation

[0031] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0032] Example 1: The NiTi alloy additive manufacturing method based on interlayer process control in this example is carried out according to the following steps: Step 1: Raw material preparation: Pre-alloyed NiTi spherical powder with a Ni content of 56.3 wt.% was selected as the raw material, and the powder particle size range was 15–55 μm.

[0033] Step 2, LPBF equipment preparation: NiTi alloy powder is loaded into an LPBF device, and a NiTi alloy substrate is used as the forming substrate.

[0034] First, the substrate is preheated to 100 ℃ and leveled. Then, high-purity Ar is continuously introduced into the forming chamber as a protective gas to reduce the oxygen content in the chamber to below 10 ppm, thereby reducing the oxidation of the NiTi alloy during the forming process.

[0035] Step 3, setting basic process parameters: In the slicing software, an interlayer rotation scanning strategy was set, with adjacent layers rotated 66.7° in the scanning direction. The powder layer thickness was set to 30 μm, the scanning interval to 80 μm, and the laser power to 160 W. These parameters remained constant throughout the forming process; only the laser scanning speed of different build-up layers was adjusted.

[0036] Step 4: Creating process files for different scanning speeds: For a sample to be formed with a height of 3.6 mm, two laser scanning speeds of 400 mm / s and 1200 mm / s were set respectively, and corresponding printing process files were generated.

[0037] Based on a powder layer thickness of 30 μm, the sample contains a total of 120 printing layers along the construction direction.

[0038] Subsequently, the process files corresponding to the two scanning speeds were imported into the LPBF equipment.

[0039] Step 5: Interlayer alternation process control: like Figure 4 As shown, each 20-layer printing area is used as an independent process zone for alternating printing. The thickness of each process zone along the build direction is 0.6 mm, forming a total of 6 consecutively arranged process zones. Specifically, layers 1 to 20 are printed at a scanning speed of 400 mm / s; after completion, the process file is switched to 1200 mm / s for printing layers 21 to 40; subsequently, the process is alternately printed at 400 mm / s and 1200 mm / s, i.e., layers 41 to 60 are printed at 400 mm / s, layers 61 to 80 at 1200 mm / s, layers 81 to 100 at 400 mm / s, and layers 101 to 120 at 1200 mm / s.

[0040] Using the above method, without changing parameters such as powder composition, laser power, scanning spacing, and powder layer thickness, a layered region with alternating LPBF process states can be formed inside the same NiTi component by periodically and alternately adjusting the scanning speed along the construction direction. The thickness of each process region is 0.6 mm.

[0041] Laser bed fusion (LPBF) is an additive manufacturing process that involves uniformly depositing metal powder onto the surface of a substrate, using a laser beam to rapidly melt and solidify selected areas according to a preset scanning path, and then accumulating the powder layer by layer through layer-by-layer deposition and melting to create three-dimensional components. The forming process can be controlled by process parameters such as laser power, scanning speed, scanning spacing, powder layer thickness, and scanning strategy. Phase transition temperatures, such as... Figure 1 As shown, NiTi alloys undergo a martensitic-to-austenitic transformation during heating, which mainly includes the onset, peak, and end temperatures of the austenitic transformation. Similarly, NiTi alloys undergo a martensitic-to-martenitic transformation during cooling, also mainly including the onset, peak, and end temperatures of the martensitic transformation. NiTi alloys exhibit unique mechanical properties such as shape memory effect and superelasticity through their reversible martensitic-austenitic transformation.

[0042] like Figure 2 As shown, under the condition that other process parameters remain unchanged, increasing the scanning speed can significantly improve the tensile strength of NiTi alloy, but its elongation at break is relatively reduced; decreasing the scanning speed shows the opposite trend. This difference mainly stems from the effect of scanning speed on the volatilization of Ni and the control of alloy composition during LPBF. Since the boiling point of Ni (3186 K) is lower than that of Ti (3560 K), higher energy input will exacerbate the preferential volatilization of Ni, thereby reducing the Ni content in the alloy. Previous studies have shown that for every 0.1 at.% decrease in Ni content in NiTi alloy, its phase transformation temperature can increase by about 10–20 K. Therefore, as Figure 1 and Figure 3 As shown, lower scan rates correspond to higher energy input and more significant Ni volatilization, resulting in NiTi alloys with higher phase transformation temperatures and higher martensite content at room temperature. Conversely, at higher scan rates, Ni volatilization is suppressed, the phase transformation temperature decreases, and the room temperature microstructure is dominated by austenite. Due to the lower tensile strength and higher deformability of martensite, NiTi alloys prepared at lower scan rates exhibit lower tensile strength and higher elongation at fracture, while NiTi alloys prepared at higher scan rates exhibit higher tensile strength and lower elongation at fracture.

[0043] Depend on Figure 5It can be seen that the tensile strength of the sample printed by using the interlayer process control method is increased by 69.11% compared with the single process parameter of scanning speed of 400 mm / s. Compared with the scanning speed of 1200 mm / s, the elongation at break is increased by 32.65%.

[0044] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for additive manufacturing of strong and tough NiTi alloys based on interlayer process control, characterized in that, Includes the following steps: Step 1: Select NiTi spherical powder as raw material; Step 2: Then load it into the LPBF equipment, using a NiTi alloy substrate as the forming substrate. Preheat the substrate and level it. Inert gas is introduced into the forming chamber to control the oxygen content in the chamber to be below 10 ppm. Step 3: Set the interlayer rotation scanning strategy in the slicing software. The strategy shall include at least the following process parameters: rotation angle between adjacent layer scanning directions, powder layer thickness, scanning spacing, and laser power. Step 4: Set two, three or more laser scanning speeds, calculate the total number of printing layers for the sample based on the powder layer thickness, generate the printing process file corresponding to the laser scanning speed, and import it into the equipment; Step 5: Then, alternately switch different laser scanning speeds along the construction direction to scan and shape the object.

2. The method according to claim 1, characterized in that, In step 1, the Ni content in the raw material is 55 wt.%-60 wt.%.

3. The method according to claim 1, characterized in that, In step 1, the particle size of the raw material is 15 μm to 55 μm.

4. The method according to claim 1, characterized in that, In step 2, the preheating temperature is 90 ℃-110 ℃; the inert gas is argon.

5. The method according to claim 1, characterized in that, In step 3, the rotation angle between adjacent layer scans is 60°-80°, the powder layer thickness is 30 μm-60 μm, the scanning interval is 40 μm-80 μm, and the laser power is 150 W-180 W.

6. The method according to claim 1, characterized in that, In step 4, set two laser scanning speeds: 400 mm / s and 1200 mm / s; or set three or more of the following laser scanning speeds: 400 mm / s, 600 mm / s, 800 mm / s, 1000 mm / s, and 1200 mm / s.

7. The method according to claim 6, characterized in that, In step 5, printing is performed alternately in an independent process zone, with each 20-40 printing layers forming a separate zone.

8. The method according to claim 1, characterized in that, Set two laser scanning speeds: 400 mm / s and 1200 mm / s. Print layers 1 to 20 using a scanning speed of 400 mm / s. After completion, switch to the 1200 mm / s process file to print layers 21 to 40. Then, alternate between 400 mm / s and 1200 mm / s for forming.

9. A NiTi alloy gradient functional component, characterized in that, The component is prepared by any one of claims 1 to 8, and the component has a periodic alternation along the construction direction.

10. The NiTi alloy graded functional component according to claim 9, characterized in that, The component forms alternating bands of microstructure along the construction direction, with a cycle of 20-40 printed layers.

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