A post-processing method for eliminating anisotropy of a surface layer structure of 3D printed titanium and titanium alloy

Abstract

This invention belongs to the field of post-processing of 3D printed titanium alloy materials, and discloses a post-processing method for eliminating the anisotropy of the surface microstructure of 3D printed titanium and titanium alloys. The method includes the following steps: using titanium or titanium alloy powder to prepare titanium or titanium alloy parts using laser selective melting 3D printing technology as the original parts, the original parts are sequentially subjected to surface shot peening and vacuum annealing to obtain 3D printed titanium or titanium alloy parts with uniform surface microstructure and consistent corrosion resistance. After shot peening and annealing, the xoy and xoz surfaces are transformed into a "depression" morphology, the surface roughness of the parts is reduced to about 2μm, and a shot-peening reinforcement layer with a thickness of about 70μm is obtained. The microstructure of both the xoy and xoz surfaces is equiaxed α phase + network β phase. The corrosion current density of both the xoy and xoz surfaces is reduced to less than half of that of the original parts, exhibiting excellent corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of post-processing of metal 3D printing materials, and particularly relates to a post-processing method for eliminating the anisotropy of the surface microstructure of 3D printed titanium and titanium alloys. Background Technology

[0002] Selective Laser Melting (SLM) is a commonly used metal 3D printing technology. Ti6Al4V alloy is a hot material in the field of metal 3D printing. However, due to the unique deposition modeling method of 3D printing, Ti6Al4V alloy prepared using SLM technology exhibits macro- and micro-structural differences between the end face (xoy plane) and the side face (xoz plane) of the printed part, i.e., structural anisotropy. This structural anisotropy will inevitably lead to a bottleneck effect during the service life of the part. Structural anisotropy is an inherent characteristic of 3D printing manufacturing technology and cannot be avoided by optimizing process parameters. Therefore, appropriate post-processing methods should be sought. CN117620900A discloses a post-processing method to improve the biofouling resistance of titanium alloy surface. The method involves shot peening the original part, incorporating copper powder during the shot peening process. The incorporation of copper impurities may adversely affect the corrosion resistance of the titanium alloy surface. CN109175376A discloses a post-processing method to improve the comprehensive mechanical properties of 3D-printed titanium and titanium alloys. This method employs a combination of heat treatment, surface treatment, and magnetic pulse treatment of the original part, which is complex and costly. The aforementioned two titanium alloy post-processing methods primarily address the improvement of the material's tensile mechanical properties. However, they lack solutions for improving the surface corrosion resistance of titanium alloys, especially addressing the unique anisotropy of properties inherent in 3D-printed titanium alloys.

[0003] Material failure often begins at the surface. Homogenizing the surface structure can prevent the short-board effect and extend the service life of parts. Therefore, exploring a post-processing method to eliminate the anisotropy of the surface structure and properties of Ti6Al4V alloy prepared by SLM is of great significance for improving the service life and reliability of 3D printed titanium alloys. Summary of the Invention

[0004] In order to overcome the shortcomings and deficiencies of the prior art, the primary objective of this invention is to provide a post-processing method for eliminating the anisotropy of the surface microstructure of 3D printed titanium and titanium alloys.

[0005] Another object of the present invention is to provide 3D printed titanium and titanium alloys with uniform surface structure and uniform corrosion resistance.

[0006] Another objective of this invention is to provide an application of 3D printed titanium and titanium alloys with uniform surface structure and consistent corrosion resistance.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A post-processing method for eliminating anisotropy in the surface microstructure of 3D-printed titanium and titanium alloys includes the following steps:

[0009] Titanium or titanium alloy powder is used to prepare titanium or titanium alloy parts using selective laser melting 3D printing technology. The original parts are then subjected to surface shot peening and vacuum annealing treatments to obtain 3D printed titanium or titanium alloy parts with uniform surface structure and consistent corrosion resistance.

[0010] Preferably, the surface shot peening treatment utilizes a closed mechanical shot peening machine to treat the original parts, with appropriate shot peening pressure, shot peening time, and shot gun movement strategy. The shot peening pressure is 0.3-0.5 MPa, the shot peening time is 10-20 min, and the shot gun nozzle must be directly facing the part and in a reciprocating motion state.

[0011] Preferably, the vacuum annealing conditions are: a holding temperature of 780℃~1020℃, a holding time of 30~120min, and a heating rate of 5~15℃ / min.

[0012] Preferably, the vacuum degree of the vacuum annealing process is ≤2×10⁻⁶. -5 Pa.

[0013] Preferably, the laser selective melting process parameters are: laser power of 150-210W, scanning speed of 800-1000mm / s, scanning spacing of 0.05-0.08mm, powder layer thickness of 0.02-0.06mm, and rotation angle of (θ, θ+(kπ / 2)), where θ∈(0°, 360°) and k∈Z.

[0014] More preferably, the laser selective melting process parameters are: laser power of 180W, scanning speed of 900mm / s, scanning interval of 0.065mm, powder layer thickness of 0.03mm, and rotation angle of (135°, 225°).

[0015] Preferably, the titanium is industrially pure titanium, and the titanium alloy is at least one of α titanium alloy, β titanium alloy, and α+β titanium alloy.

[0016] Preferably, the α titanium alloy is Ti4Al2V, the β titanium alloy is Ti15Mo2.7Nb3Al0.2Si, and the α+β titanium alloy is Ti6Al4V, and the composition meets the requirements of standard GB / T 3620.2-2007.

[0017] Preferably, the Ti6Al4V alloy powder is prepared by gas atomization and has the following chemical composition: C≤0.08%, H≤0.015%, O≤0.12%, N≤0.05%, Fe≤0.3%, Al: 5%~7%, V: 3.2%~4.8%, with the balance being Ti.

[0018] Preferably, in the titanium alloy powder, 10% of the powder diameter is less than or equal to 21.4 μm, 50% of the powder diameter is less than or equal to 32.2 μm, 90% of the powder diameter is less than or equal to 55.6 μm, and the loose powder density is 2.42 g / cm³. 3 Tap density 2.85 g / cm³ 3 , fluidity 1.39g / s.

[0019] Preferably, the laser selective melting 3D printing includes slicing the part model into layers using slicing software, importing the slices into the 3D printing equipment, and selecting, for example... Figure 1 (a) shows the orthogonal layered staggered laser scanning strategy and appropriate printing parameters for printing.

[0020] Preferably, before laser selective melting and forming, argon gas is used to remove the oxygen content in the chamber to below 0.02%. After forming, the part is cooled to room temperature and then separated from the substrate by wire cutting.

[0021] Preferably, during the shot peening process, the part to be shot peened needs to be fixed in a suitable position in the working chamber, so that the spray direction of the shot is perpendicular to the shot-peened surface of the part, and the distance between the nozzle and the shot-peened surface is kept at 80-100mm. A diameter of [missing information] is selected. 304 stainless steel shot.

[0022] Preferably, the nozzle diameter of the spray gun is less than 20mm. In order to prevent large macroscopic deformation of the material surface caused by fixed shot peening, the spray gun is always in reciprocating motion in this experiment, with a movement speed of 30-50mm / s.

[0023] Preferably, before vacuum annealing, the parts to be heated are arranged separately and placed in an alumina crucible. After annealing, the parts are taken out and put into use after the furnace temperature drops to room temperature.

[0024] A type of 3D-printed titanium and titanium alloy with uniform surface structure and uniform corrosion resistance is prepared by the above method.

[0025] Preferably, the corrosion current density of the 3D-printed titanium and titanium alloy with uniform surface structure and consistent corrosion resistance is 1.5 × 10⁻⁶ on both the xoy and xoz planes. -6 A / cm 2 ~2.0×10 -6 A / cm 2 .

[0026] The above-mentioned 3D-printed titanium and titanium alloys with uniform surface structure and consistent corrosion resistance are used in aerospace, shipbuilding, and biomedical fields.

[0027] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0028] This invention uses a 90° interlayer rotation scanning strategy to prepare a Ti6Al4V alloy with a density of over 97.5% as the original part. Appropriate surface shot peening parameters are selected to treat the original part, followed by vacuum annealing with suitable parameters to obtain a 3D-printed titanium alloy part with uniform surface microstructure and consistent corrosion resistance.

[0029] In terms of macroscopic structure: the original part's xoy surface has a final layer scan melt channel morphology, and the xoz surface has an unmelted powder adhesion morphology. After shot peening and annealing, the xoy surface and xoz surface are transformed into a "depression" morphology. The surface roughness of the part is reduced to about 2μm when the shot peening parameters are selected as 0.5MPa-15min, and a shot peening strengthening layer with a thickness of about 70μm is obtained.

[0030] In terms of microstructure: the original part's xoy surface has a "checkerboard" microstructure composed of blocky primary β crystals, while the xoz surface has a "grid" microstructure composed of columnar primary β crystals. Both primary β crystals contain needle-like α' grains. The α' grains on the xoy surface are smaller in length and grow randomly, while those on the xoz surface are larger and form an angle with the sample's orientation. After shot peening and annealing, the microstructure of both the xoy and xoz surfaces is determined to be equiaxed α phase + network β phase.

[0031] Regarding corrosion resistance: In a 3.5% wt NaCl solution, using Ag / AgCl as the reference electrode, at the same potential (1.2V), the corrosion current density on the xoy surface of the original part is 4.27 × 10⁻⁶. -6 A / cm 2 The corrosion current density is lower than that of the xoz surface (5.73 × 10⁻⁶). -6 A / cm 2 Electrochemical impedance spectroscopy (Nyquist plot) showed that the passivation film formed on the xoy surface performed better than that on the xoz surface. After shot peening and annealing, the difference in corrosion resistance between the xoy and xoz surfaces was greatly reduced. Specifically, after SP+780℃ annealing, the corrosion current density on the xoy surface was 1.65 × 10⁻⁶. -6 A / cm 2 ) and xoz surface corrosion current density (1.73×10 -6 A / cm 2 The corrosion resistance is reduced to less than half that of the original parts, exhibiting excellent corrosion resistance. Attached Figure Description

[0032] Figure 1 A schematic diagram of the scanning strategy and model used for the titanium alloys in the comparative examples and embodiments;

[0033] Figure 2 This is a macroscopic organizational diagram for Comparative Example 1;

[0034] Figure 3 This is a macroscopic organizational diagram of Example 1;

[0035] Figure 4 This is a macroscopic organizational diagram of Example 2;

[0036] Figure 5 This is a metallographic diagram of Comparative Example 1;

[0037] Figure 6 This is a cross-sectional metallographic diagram of Example 1;

[0038] Figure 7 Metallographic diagrams of cross sections for Examples 2 and 3: (a) SP+780℃ xoy, (b) SP+780℃ xoz, (c) SP+850℃ xoy, (d) SP+850℃ xoz;

[0039] Figure 8 The image shown is a scanning electron microscope image of Comparative Example 1.

[0040] Figure 9 This is a scanning electron microscope image of Example 1;

[0041] Figure 10 Scanning electron microscope images of Examples 2 and 3;

[0042] Figure 11 The potentiodynamic polarization curves of the titanium alloys used in Comparative Example 1 and the Examples are shown.

[0043] Figure 12 Electrochemical impedance spectroscopy (EIS) spectra of the titanium alloys used in Comparative Example 1 and the Examples;

[0044] Figure 13 Metallographic images of the original parts after vacuum annealing: end face (xoy): (a) 780℃, (c) 850℃, (e) 955℃, (g) 1020℃; side face (xoz): (b) 780℃, (d) 850℃, (f) 955℃, (h) 1020℃.

[0045] Figure 14 SEM images of the original part for Comparative Example 3 + vacuum annealing (955℃) + shot peening: (a) end face (xoy), (b) side face (xoz);

[0046] Figure 15The surface morphology of the xoy plane of the Ti6Al4V alloy sample formed by SLM in Example 5 after shot peening with different parameters;

[0047] Figure 16 Metallographic images of the cross-section of the xoy surface of the sample in Example 5 under different shot peening parameters: (a) 0.3MPa-10min, (b) 0.4MPa-10min, (c) 0.5MPa-10min, (d) 0.3MPa-15min, (e) 0.4MPa-15min, (f) 0.5MPa-15min, (g) 0.3MPa-20min, (h) 0.4MPa-20min, (i) 0.5MPa-20min. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0049] Shot peening machine model: BA600D standard enclosed shot peening machine manufactured by Dongguan Boai Sandblasting and Polishing Equipment Co., Ltd.

[0050] Vacuum tube furnace model: GCVD-1600 vacuum tube furnace manufactured by Hefei Gaoge Advanced Electric Furnace Equipment Co., Ltd.

[0051] Comparative Example 1

[0052] Step 1: Fabrication of 3D Printed Titanium Alloy Parts

[0053] Ti6Al4V alloy parts were fabricated using selective laser melting (SLM) 3D printing technology, including the use of slicing software. Figure 1 (b) The part model created in this step is sliced ​​into layers and imported into a 3D printing device; select, for example... Figure 1 (a) shows an orthogonal layered laser scanning strategy with an interlayer rotation angle of 90°. The selected laser selective melting process parameters are: laser power (P) 180W, scanning speed (v) 900mm / s, scanning spacing (h) 0.065mm, powder layer thickness (d) 0.03mm, and rotation angle (θ) (135°, 225°). Commercially available Ti6Al4V alloy powder was used, with a chemical composition conforming to standard GB / T 3620.2-2007: C≤0.08%, H≤0.015%, O≤0.12%, N≤0.05%, Fe≤0.3%, Al: 5%~7%, V: 3.2%~4.8%, with the balance being Ti. Powder particle size distribution: 10% of the powder has a diameter less than or equal to 21.4 μm, 50% has a diameter less than or equal to 32.2 μm, and 90% has a diameter less than or equal to 55.6 μm. The loose bulk density of the powder is 2.42 g / cm³. 3Tap density 2.85 g / cm³ 3 The fluidity is 1.39 g / s. Before laser selective melting and forming, argon gas is used to remove the oxygen content in the chamber to below 0.02%. After forming, the parts are cooled to room temperature and then separated from the Ti6Al4V alloy substrate by wire cutting.

[0054] Step Two:

[0055] Using the Ti6Al4V alloy prepared by 3D printing technology in step one as the original part, we obtain Comparative Example 1.

[0056] Example 1

[0057] Step 1: Fabrication of 3D Printed Titanium Alloy Parts

[0058] Ti6Al4V alloy parts were fabricated using selective laser melting (SLM) 3D printing technology, including the use of slicing software. Figure 1 (b) The part model created in this step is sliced ​​into layers and imported into a 3D printing device; select, for example... Figure 1 (a) shows an orthogonal layered laser scanning strategy with an interlayer rotation angle of 90°. The selected laser selective melting process parameters are: laser power (P) 180W, scanning speed (v) 900mm / s, scanning spacing (h) 0.065mm, powder layer thickness (d) 0.03mm, and rotation angle (θ) (135°, 225°). Commercially available Ti6Al4V alloy powder was used, with a chemical composition conforming to standard GB / T 3620.2-2007: C≤0.08%, H≤0.015%, O≤0.12%, N≤0.05%, Fe≤0.3%, Al: 5%~7%, V: 3.2%~4.8%, with the balance being Ti. Powder particle size distribution: 10% of the powder has a diameter less than or equal to 21.4 μm, 50% has a diameter less than or equal to 32.2 μm, and 90% has a diameter less than or equal to 55.6 μm. The loose bulk density of the powder is 2.42 g / cm³. 3 Tap density 2.85 g / cm³ 3 The fluidity is 1.39 g / s. Before laser selective melting and forming, argon gas is used to remove the oxygen content in the chamber to below 0.02%. After forming, the parts are cooled to room temperature and then separated from the Ti6Al4V alloy substrate by wire cutting.

[0059] Step 2: Select appropriate surface shot peening parameters

[0060] The Ti6Al4V alloy parts prepared in step one were treated using a closed-type mechanical shot peening machine. The surface shot peening parameters included appropriate shot peening pressure, shot peening time, and a shot gun movement strategy. Specifically, the shot peening pressure was 0.5 MPa, the shot peening time was 15 minutes, and the parts to be shot peened needed to be fixed in a suitable position within the working chamber, ensuring the shot spray direction was perpendicular to the shot-peened surface of the part, and maintaining a nozzle-to-shot-peened-surface distance of 80-100 mm. A diameter of [missing information - likely a specific diameter] was selected. The shot is made of 304 stainless steel. The spray gun is always in a reciprocating motion at a speed of 40 mm / s.

[0061] Step 3:

[0062] Using the Ti6Al4V alloy prepared by 3D printing technology in step one as the original part, and performing post-processing according to step two, a 3D printed titanium alloy with uniform surface structure was obtained, namely Example 1.

[0063] Example 2

[0064] Step 1: Fabrication of 3D Printed Titanium Alloy Parts

[0065] Ti6Al4V alloy parts were fabricated using selective laser melting (SLM) 3D printing technology, including the use of slicing software. Figure 1 (b) The part model created in this step is sliced ​​into layers and imported into a 3D printing device; select, for example... Figure 1 (a) shows an orthogonal layered laser scanning strategy with an interlayer rotation angle of 90°. The selected laser selective melting process parameters are: laser power (P) 180W, scanning speed (v) 900mm / s, scanning spacing (h) 0.065mm, powder layer thickness (d) 0.03mm, and rotation angle (θ) (135°, 225°). Commercially available Ti6Al4V alloy powder was used, with a chemical composition conforming to standard GB / T 3620.2-2007: C≤0.08%, H≤0.015%, O≤0.12%, N≤0.05%, Fe≤0.3%, Al: 5%~7%, V: 3.2%~4.8%, with the balance being Ti. Powder particle size distribution: 10% of the powder has a diameter less than or equal to 21.4 μm, 50% has a diameter less than or equal to 32.2 μm, and 90% has a diameter less than or equal to 55.6 μm. The loose bulk density of the powder is 2.42 g / cm³. 3 Tap density 2.85 g / cm³ 3 The fluidity is 1.39 g / s. Before laser selective melting and forming, argon gas is used to remove the oxygen content in the chamber to below 0.02%. After forming, the parts are cooled to room temperature and then separated from the Ti6Al4V alloy substrate by wire cutting.

[0066] Step 2: Select appropriate surface shot peening parameters

[0067] The Ti6Al4V alloy parts prepared in step one were treated using a closed-type mechanical shot peening machine. The surface shot peening parameters included appropriate shot peening pressure, shot peening time, and a shot gun movement strategy. Specifically, the shot peening pressure was 0.5 MPa, the shot peening time was 15 minutes, and the parts to be shot peened needed to be fixed in a suitable position within the working chamber, ensuring the shot spray direction was perpendicular to the shot-peened surface of the part, and maintaining a nozzle-to-shot-peened-surface distance of 80-100 mm. A diameter of [missing information - likely a specific diameter] was selected. The shot is made of 304 stainless steel. The spray gun is always in a reciprocating motion at a speed of 40 mm / s.

[0068] Step 3: Select appropriate vacuum annealing parameters

[0069] The shot-peened parts from step two were annealed using a vacuum tube furnace. The temperature control program was set as follows: heating rate 10℃ / min, holding temperature 780℃, holding time 60min, followed by furnace cooling after holding. Before vacuum annealing, the parts to be heated were dispersed and placed in an alumina crucible. The vacuum level inside the furnace tube was reduced to 2×10⁻⁶ using a mechanical pump. -5 For parts heated to below Pa, after annealing, wait for the furnace temperature to drop to room temperature before removing them for use.

[0070] Step Four:

[0071] Using the Ti6Al4V alloy prepared by 3D printing technology in step one as the original part, post-processing was carried out according to steps two to three to obtain a 3D printed titanium alloy with uniform surface structure and corrosion resistance, namely Example 2.

[0072] Example 3

[0073] Unlike Example 2, the annealing temperature in step three of the preparation process in Example 3 is 850°C.

[0074] Figure 1 (a) shows the laser scanning strategy used in the preparation process of Comparative Example 1 and Examples 1 and 2, namely orthogonal stacking faults, with rotation angles of (135°, 225°). The original parts used in Comparative Example 1 and Examples 1 and 2 were all... Figure 1 (b) The block sample was obtained by wire cutting, and the size of the block sample is 20mm×20mm×20mm.

[0075] Figure 2 The macroscopic morphology of Comparative Example 1 is shown, revealing a significant difference between the macroscopic morphology of the end face (xoy) and the side face (xoz) of Comparative Example 1. Figure 2 (a) It can be seen that the xoy surface is greatly affected by the current scanning path. The interlayer rotation angle of (135°, 225°) causes it to exhibit a melt channel morphology at an oblique 45°. Figure 2(b) It can be seen that a large number of incompletely fused powder particles adhere to the surface of xoz.

[0076] Figure 3 The macroscopic morphology of Example 1 is shown. It can be seen that the macroscopic morphology of both the end face (xoy) and the side face (xoz) of Example 1 has changed. The original morphology of the final melt channel on the xoy face and the unmelted powder adhesion morphology on the xoz face have both transformed into a "depression" morphology. The surface roughness has decreased from 7.45 μm on the xoy face and 15.87 μm on the xoz face to approximately 2 μm, respectively, and the difference in macroscopic morphology between the two has disappeared. Example 2 involves an additional step, vacuum annealing, compared to Example 1. However, step three has almost no effect on the macroscopic morphology of the part; therefore, the difference in macroscopic morphology between the end face (xoy) and the side face (xoz) of Example 2 has also disappeared.

[0077] Figure 5 The metallographic structure of Comparative Example 1 is shown. Comparative Example 1 exhibits differentiated corrosion morphologies between its xoy and xoz planes. Figure 5 (a) It can be seen that the xoy surface exhibits a "chessboard" shaped corrosion morphology under an optical microscope, which is due to... Figure 5 (b) It can be seen that the xoz surface exhibits a "grid" corrosion morphology under an optical microscope.

[0078] Figure 6 The cross-sectional metallographic structure of Example 1 is shown, consisting of... Figure 6 (a) It can be seen that in Example 1, the upper part of the red line on the xoz surface is the substrate area unaffected by shot peening, and the corrosion morphology presents a "grid" pattern. The lower part is the shot-peened area, and the corrosion morphology is not yet clear under an optical microscope. Figure 6 (b) It can be seen that the upper part of the red line on the xoy surface of Example 1 is the substrate area that is not affected by shot peening, and the corrosion morphology presents a "chessboard" pattern. The lower part is the shot peening reinforcement area, and the corrosion morphology is also not clear under an optical microscope. However, it can be distinguished that the thickness of the shot peening reinforcement layer in both is similar, about 70 μm.

[0079] Figure 7 (a) and 7(b) show the cross-sectional metallographic structure of Example 2, and 7(c) and 7(d) show the cross-sectional metallographic structure of Example 3. Figure 7 It can be seen that in Examples 2 and 3, the upper part of the red lines on the xoy and xoz surfaces, i.e., the matrix region unaffected by shot peening, inherited the needle-like structure morphology of the original parts. In Examples 2 and 3, the lower part of the red lines on the xoy and xoz surfaces, i.e., the shot-peening reinforced region, has a uniform equiaxed structure. The equiaxed structure in Example 3 is larger in size than that in Example 2. However, regardless of Example 2 or Example 3, the thickness of the shot-peening reinforced layer below the red lines of all parts is similar to that of Example 1, approximately 70 μm.

[0080] Figure 8The scanning electron microscope (SEM) microstructure of Comparative Example 1 is shown. The microstructure of both the xoy and xoz planes of Comparative Example 1 consists of acicular martensite α', but with different sizes. Figure 8 (a) It can be seen that in Comparative Example 1, the α' grains on the xoy plane are relatively fine, and their growth direction is random (e.g., Figure 8 (a) marked with a red dotted circle), by Figure 8 (b) It can be seen that in Comparative Example 1, the α' grains on the xoz plane are larger in size along the length direction. Figure 8 (b) indicated by the red dot circle), and the grain growth direction is at a certain angle to the sample construction direction (the sample construction direction is parallel to the z-axis direction in the figure).

[0081] Figure 9 The scanning electron microscope microstructure of Example 1 is shown. The "chessboard" corrosion morphology in the xoy plane and the "grid" corrosion morphology in the xoz plane of Example 1 have disappeared. The surface grain size of Example 1 has been refined to less than 1 μm. The microstructure of the xoy plane and the xoz plane is relatively consistent. The grain morphology has changed from needle-like to cellular, but it is still martensite α'.

[0082] Figure 10 (a) and (b) show the scanning electron microscope microstructure of Example 2. Figure 10 Images (c) and (d) show the scanning electron microscope (SEM) microstructure of Example 3. In Examples 2 and 3, the microstructure of both the xoy and xoz planes changed from martensite α' to equiaxed α phase + network β phase, with the β phase embedded in the vicinity of the grain boundaries of the equiaxed α grains in a "rice grain" shape. It is evident that Examples 2 and 3 show no difference in the surface microstructure at the ends and sides, and revert to the typical dual-phase microstructure of Ti6Al4V alloy. As is well known, α+β dual-phase titanium alloys possess the advantages of both α and β single-phase titanium alloys: high strength, good toughness, and the ability to be strengthened by heat treatment. These are the outstanding advantages of Ti6Al4V alloy.

[0083] Example 4

[0084] Unlike Example 2, in the preparation step of Example 4, industrial pure titanium powder was used instead of Ti6Al4V alloy powder.

[0085] Example 5

[0086] Different shot peening pressures and times were selected to perform shot peening on the original parts. The shot peening surface was selected as the original xoy surface of the printed part, which had not been processed by wire cutting.

[0087] After shot peening with different combinations of pressure and time, the macroscopic morphology of the xoy surface is as follows: Figure 15The first, second, and third columns represent samples with shot peening pressures of 0.3 MPa, 0.4 MPa, and 0.5 MPa, respectively, while the first, second, and third rows represent samples with shot peening times of 10 min, 15 min, and 20 min, respectively. The average roughness of the xoy surface of each sample under different shot peening parameters is shown in Table 1.

[0088] Table 1. Average roughness of the xoy surface of each sample under different shot peening parameters.

[0089]

[0090] As can be seen from Table 1:

[0091] (1) When the shot peening time is short (10 min and 15 min), if the shot peening time is controlled, the surface roughness of the SLM part gradually decreases with the increase of the shot peening pressure. However, when the shot peening time is extended to 20 min, if the shot peening pressure does not exceed 0.4 MPa, the surface roughness decreases with the increase of the shot peening pressure. However, when the shot peening pressure is further increased to 0.5 MPa, the surface roughness increases instead. This is related to the deep pit caused by excessively high shot peening pressure.

[0092] (2) When the shot peening pressure is low (0-0.4MPa), the surface roughness of the SLM part gradually decreases with the increase of shot peening time when the pressure is constant. However, when the shot peening pressure is as high as 0.5MPa, the surface roughness continuously decreases with the increase of shot peening time in the first 15 minutes. When the shot peening time is extended to 20 minutes, the surface roughness rises again.

[0093] (3) Overall, the surface roughness of all shot-peened samples was lower than that of the original surface. However, excessive shot-peening pressure and excessive shot-peening time are not conducive to improving surface quality. From the perspective of obtaining good surface quality, the better shot-peening parameters should be 0.5 MPa-15 min. Of course, this is not necessarily the final optimal solution, and it needs to be evaluated in conjunction with other performance characteristics.

[0094] Metallographic sections of the shot-peened layers of each sample were obtained under different shot peening parameters. Figure 16 As shown in the table below, the first, second, and third columns represent shot peening pressures of 0.3 MPa, 0.4 MPa, and 0.5 MPa, respectively, and the first, second, and third rows represent shot peening times of 10 min, 15 min, and 20 min, respectively. The thickness parameters of the deformed layer on the xoy surface under different shot peening parameters are shown in the table below. From the table, it can be seen that when the shot peening time is constant, the thickness of the deformed layer gradually increases with increasing shot peening pressure; when the shot peening pressure is constant, the thickness of the deformed layer also gradually increases with increasing shot peening time. Specifically, the shot peening parameters of 0.3 MPa-10 min correspond to the minimum refined layer thickness of 17.51 ​​μm, while 0.5 MPa-20 min yields the thickest refined layer of 80.95 μm.

[0095] Table 2 shows the thickness of the deformed layer of the xoy samples under different shot peening parameters.

[0096]

[0097] Test Example 1

[0098] Potentiodynamic polarization (PDP) curves were tested in a 3.5 wt% NaCl solution at 25 °C using a Gamryreference 3000 electrochemical workstation. A three-electrode system was employed: an Ag / AgCl reference electrode, a graphite rod as the counter electrode, and the sample as the working electrode. Open-circuit potential measurements were performed before PDP curve measurements for 30 min. The initial potential for PDP curve measurements was -0.7 V, the termination potential was 1.5 V, and the scan step was 1 mV / s. All electrochemical experiments were preceded by cathodic polarization at -1.7 V for 10 min to remove the passivation film formed on the sample in air. Each measurement was taken three times, and the average value was calculated.

[0099] Figure 11 The potentiodynamic polarization curves of Comparative Example 1, Example 1, and Examples 2 and 3 in 3.5% wt NaCl solution at room temperature are shown. At the same potential, a higher corrosion current density indicates a higher corrosion rate. In this example, the corrosion current density at the same potential of 1.2V in the passivation region is compared. The corrosion current density of Example 2 is lower than that of Example 1, and the corrosion current density of Example 1 is lower than that of Comparative Example 1, indicating that Example 2 exhibits better corrosion resistance. Furthermore, at the same potential, a larger difference in corrosion current density between the xoy and xoz surfaces indicates a greater difference in corrosion resistance between the two surfaces. The difference in corrosion current density between the xoy and xoz surfaces in Example 2 is comparable to that of Example 1, and both are lower than that of Comparative Example 1. In summary, Example 2 exhibits excellent corrosion resistance while maintaining a low level of difference in corrosion resistance between its ends and sides.

[0100] In Example 3, the difference in corrosion current density between the xoy and xoz surfaces is comparable to that in Example 2, but the corrosion current density is slightly higher than that in Example 2. This shows that the difference in corrosion resistance between the ends and sides of Example 3 can be kept at a low level, but the corrosion resistance is slightly worse than that in Example 2.

[0101] Test Example 2

[0102] Electrochemical impedance spectroscopy (EIS) measurements were performed in a 3.5 wt% NaCl solution at 25 °C using a Gamryreference 3000 electrochemical workstation. A three-electrode system was employed: an Ag / AgCl reference electrode, a graphite rod as the counter electrode, and the sample as the working electrode. The initial frequency for the EIS measurements was 10⁻⁶. 5 Hz, the end frequency is 10 -2The frequency was Hz, the AC voltage was 10mV, and the DC voltage was 0V relative to the open circuit potential. Open circuit potential measurement should be performed before electrochemical impedance spectroscopy (EIS) testing, with a measurement time of 30 minutes. Before starting the open circuit potential measurement, the sample was cathodically polarized at -1.7V for 10 minutes to remove the passivation film formed on the sample in air. Cathodometric polarization will not be performed before EIS testing to preserve the passivation film formed on the sample in solution during open circuit potential measurement. Each test value was measured three times, and the average value was taken.

[0103] Figure 12 The electrochemical impedance spectroscopy (EIS) spectra of Comparative Example 1, Example 1, Example 2, and Example 3 in 3.5% wt NaCl solution at room temperature are shown. In the Nyquist plot, the radius of the capacitive arc is proportional to Rct (charge transfer resistance), meaning that a larger capacitive arc radius indicates greater resistance to charge transfer and makes corrosion more difficult. Comparative Example 1, Example 1, and Example 2 all exhibit semi-circular capacitive impedance arcs in solution. Both molded surfaces show capacitive behavior in the high-frequency range (near the origin), while exhibiting resistivity in the low-frequency range (far from the origin). In Comparative Example 1, the radius of the capacitive impedance arc on the xoy surface is higher than that on the xoz surface, indicating a difference in passivation film performance. This may be related to differences in microstructure; on the one hand, the original β-crystal morphology on the xoy surface is more uniform than that on the xoz surface; on the other hand, the martensite size within the original β-crystals on the xoy surface is also finer than that on the xoz surface. In Example 1, the capacitive impedance arcs on the xoy and xoz surfaces are almost identical, and the arc radii are both higher than those in Comparative Example 1. This indicates that the resistive properties of the passivation film on the end and side surfaces of Example 1 are similar and superior to those of Comparative Example 1. This is related to the uniform equiaxed structure of the shot-peened area on the surface of Example 1. In Example 2, the capacitive impedance arcs on the xoy and xoz surfaces are also very similar, and the impedance arc radii are much larger than those in Example 1. This indicates that the surface passivation film performance on the xoy and xoz surfaces of Example 2 is similar, and it exhibits the best corrosion resistance. This is because the restoration of equiaxed α and network β grains in Example 2 leads to a more uniform distribution of V element in the surface structure of the titanium alloy, and V element can improve the stability of the surface passivation film.

[0104] In Example 3, the capacitive impedance arcs of the xoy and xoz surfaces are very similar, but the radius of the impedance arc is slightly lower than that of Example 2. This indicates that the surface passivation film performance of the xoy and xoz surfaces in Example 3 is similar, but the corrosion resistance is slightly worse than that of Example 2.

[0105] Comparative Example 2

[0106] Vacuum annealing was performed on the original parts at 780℃, 850℃, 955℃, and 1020℃ respectively, without shot peening.

[0107] like Figure 13The metallographic structure of the original part after vacuum annealing is shown. After annealing at 780℃, the low-magnification metallographic structure of the SLM sample remains essentially unchanged, exhibiting a checkerboard-like morphology on the end faces and a grid-like morphology on the sides. For the xoy plane, the original β crystal length and width are approximately 50 μm (see...). Figure 13 (a) The red dashed line block), for the xoz plane, has a primitive β crystal width of approximately 100 μm and a length of several hundred micrometers (see Figure 13 (b) Yellow dashed stripe).

[0108] After annealing at 850℃, the typical features of the ends and sides were not eliminated, but there were signs of fusion at the boundaries of the original β crystals, and the size of the original β crystals also increased. For the xoy plane, the length and width of the original β crystals were approximately 100 μm (see...). Figure 13 (c) Red dashed line block), for the xoz facet, its original β crystal width is about 150 μm, and its length is still several hundred micrometers (see Figure 13 (d) Yellow dashed stripe).

[0109] After annealing at 955℃, some larger α grains grew on the xoy surface along the interface of the original β grains. Figure 13 (e) near the red dashed line), while on the xoz surface, some elongated α grains grow along the interface of the original β grains. Figure 13 (f) near the yellow dashed line). Through these abnormally grown α grains, typical end and side morphological features can still be faintly seen. The length and width of the β grains on the xoy face reach about 130 μm, while the original β grains on the xoz face are about 200 μm wide and still several hundred micrometers long. However, apart from the α grains at these boundaries, the difference in microstructure between the xoy and xoz faces has decreased to a considerable extent.

[0110] After annealing at 1020℃, the typical morphological features of the ends and sides disappeared, but new microstructure differences appeared. This was because the α grains that grew faster at the original β crystal interface showed shape differences, with the α grains on the xoy plane mostly being equiaxed, with both length and width less than 50 μm. Figure 13 (g) marked by the red solid line, equiaxed α grains and “bundled” Widmanstätten phases appear alternately; the original long strip-shaped α grains on the xoz surface are connected to form clustered regions, forming coarse Widmanstätten phases.

[0111] In summary, after annealing at different temperatures, except for the sample annealed at 1020℃, the typical characteristics of the end and side corrosion morphology of the other annealed samples were not completely eliminated. The "checkerboard" structure on the end face and the "grid" structure on the side face could still be observed. However, as the heat treatment temperature increased, these characteristics gradually weakened, and there were signs of fusion at the boundaries of the original β grains, with the size of the original β grains continuously increasing. Furthermore, some larger α grains gradually grew along the interface of the original β grains on the xoy and xoz surfaces. When the temperature increased to 1020℃, these rapidly growing α grains exhibited shape differences. The α grains on the xoy surface were mostly equiaxed, with alternating equiaxed α grains and "bundled" Widmanstätten phases; the original elongated α grains on the xoz surface connected, forming a Widmanstätten phase region.

[0112] Comparative Example 3

[0113] The original parts were first vacuum annealed (holding temperature of 955℃, holding time of 60min), and then shot peening was performed under the same conditions as in Example 2.

[0114] As shown in Comparative Example 2, the end and side morphological features of the original parts cannot be completely eliminated after simple annealing. However, at 955℃, the boundaries of the original β grains are significantly reduced, and the microstructure difference between the xoy and xoz planes has decreased to some extent. Here, parts annealed at 955℃ are subjected to re-shot peening. Even after this re-treatment, significant differences in the microstructure of the ends and sides still exist. Figure 14 (a) It can be seen that in Comparative Example 3, the α grains on the xoy plane are finer and the growth direction is more random. Figure 14 (b) Comparative Example 3 shows that the α grains on the xoz plane are larger in length and their growth direction is at a certain angle to the sample's construction direction. The sample's construction direction is parallel to the z-axis in the figure. It can be considered that after the original part was annealed at 955℃, the needle-like α' structure was transformed into a needle-like α+β structure with increased width, and the plasticity was improved. Shot peening (a method of large plastic deformation of the surface) could not break the needle-like structure into an equiaxed structure, especially the grain growth direction, which was more obviously inherited from the 955℃ annealed sample.

[0115] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A post-processing method for eliminating anisotropy in the surface microstructure of 3D-printed titanium and titanium alloys, characterized in that, Includes the following steps: Titanium or titanium alloy parts prepared by selective laser melting 3D printing technology using titanium or titanium alloy powder are used as the original parts. The original parts are then subjected to surface shot peening and vacuum annealing treatment in sequence to obtain 3D printed titanium or titanium alloy parts with uniform surface structure and uniform corrosion resistance. The surface shot peening conditions are as follows: shot peening pressure is 0.3~0.5MPa, and shot peening time is 10~20min; The vacuum annealing conditions are as follows: holding temperature is 780℃~1020℃, holding time is 60~120min; the vacuum degree of the vacuum annealing process is ≤2×10 -5 Pa.

2. The post-processing method for eliminating anisotropy in the surface microstructure of 3D printed titanium and titanium alloys according to claim 1, characterized in that, The laser selective melting process parameters are as follows: laser power is 150~210W, scanning speed is 800~1000mm / s, scanning interval is 0.05~0.08mm, and powder layer thickness is 0.02~0.06mm.

3. The post-processing method for eliminating anisotropy in the surface microstructure of 3D printed titanium and titanium alloys according to claim 2, characterized in that, The titanium alloy is at least one of α titanium alloy, β titanium alloy, or α+β titanium alloy.

4. The post-processing method for eliminating anisotropy in the surface microstructure of 3D printed titanium and titanium alloys according to claim 3, characterized in that, The α titanium alloy is Ti4Al2V, the β titanium alloy is Ti15Mo2.7Nb3Al0.2Si, and the α+β titanium alloy is Ti6Al4V.

5. The post-processing method for eliminating anisotropy in the surface microstructure of 3D printed titanium and titanium alloys according to claim 1, characterized in that, The laser selective melting 3D printing process involves using slicing software to slice the part model into layers, importing the slices into a 3D printing device, selecting an orthogonal layer-by-layer laser scanning strategy and appropriate printing parameters for printing.

6. A 3D-printed titanium or titanium alloy with uniform surface structure and consistent corrosion resistance, characterized in that, It is prepared by the method described in any one of claims 1 to 5.

7. The application of the 3D-printed titanium or titanium alloy with uniform surface structure and uniform corrosion resistance as described in claim 6 in the fields of aerospace, shipbuilding, and biomedicine.

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