Method for producing a metastable beta titanium alloy with uniform mechanical performance by additive manufacturing

Abstract

The application belongs to the technical field of metal additive manufacturing, and particularly relates to a method for preparing a metastable beta titanium alloy with uniform mechanical performance by additive manufacturing. The Ti-7Mo-3Nb-3Cr-3Al alloy is used in additive manufacturing for the first time, and by selecting a lower laser power and a higher scanning speed, heat accumulation during forming is controlled in a manner of canceling the substrate preheating treatment on the basis of a lower input energy density, the adverse effects brought by thermal cycles during forming are weakened, the microstructure and phase structure evolution of the material are affected, the precipitation and non-uniform distribution of the omega phase / alpha phase are effectively inhibited, and uniform tensile performance of the printed alloy is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal additive manufacturing, and specifically relates to a method for preparing a metastable β titanium alloy with uniform mechanical properties through additive manufacturing. Background Art

[0002] Selective Laser Melting (SLM), which utilizes a layer-by-layer powder deposition and melting process, enables efficient and dense formation of complex or extremely complex components. However, achieving precise microstructural control is more challenging than eliminating printing defects.

[0003] Titanium alloys are susceptible to the inhomogeneous phase distribution associated with thermal cycling during selective laser melting (SLM), posing a challenge to achieving uniform mechanical properties in additively manufactured titanium alloys. As an ideal high-strength titanium alloy, additive manufacturing (AM) of metastable β-titanium alloys is crucial for broadening their application. Based on the phase precipitation conditions of metastable β-titanium alloys, the thermal history of localized heating, cooling, and reheating cycles during SLM, acting as an intrinsic heat treatment, promotes phase decomposition and the precipitation of new phases within the alloy. This is exemplified by the precipitation of ω and α phases in Ti-5Al-5V-5Mo-3Cr and Ti-10V-2Fe-3Al metastable β-titanium alloys. Due to the spatially varying thermal cycling during the layer-by-layer deposition process, the corresponding intrinsic heat treatment effects can produce inhomogeneous phase distribution along the build direction, leading to spatial variations in mechanical properties, primarily manifested as significant dispersion in elongation, a phenomenon previously reported in similar metastable β-titanium alloys such as Ti-5Al-5V-5Mo-3Cr.

[0004] Ti-7333 alloy, with a nominal composition of Ti-7Mo-3Nb-3Cr-3Al, is a new metastable beta aerospace structural titanium alloy. Its design goal is to produce a new high-strength titanium alloy with an excellent balance of strength, toughness, and ductility. This alloy exhibits excellent heat treatment response and possesses comprehensive properties comparable to or superior to similar alloys. Currently, research on this alloy for additive manufacturing is lacking.

[0005] Metastable β titanium alloys have complex phase transformations. Complex thermal cycles during SLM can be used to manipulate the alloy's microstructure, but the spatially dependent microstructure and uneven mechanical properties caused by thermal cycling are undesirable. Therefore, it is crucial to mitigate the negative effects of thermal cycling through simple process control to achieve metastable β titanium alloys fabricated by selective laser melting with uniform mechanical properties. Summary of the Invention

[0006] In response to the above problems, the present invention provides a new metastable β titanium alloy prepared for the first time by selective laser melting. By controlling the process to weaken the adverse effects of thermal cycling, the problem of uneven mechanical properties of existing metastable β titanium alloys is solved, so as to obtain an SLM-formed metastable β titanium alloy with uniform and enhanced tensile plasticity.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for preparing a metastable β titanium alloy having uniform mechanical properties by additive manufacturing is provided, comprising the following steps:

[0009] S1, preparing alloyed Ti-7Mo-3Nb-3Cr-3Al alloy powder;

[0010] S2. Draw a three-dimensional model, perform layered slicing along the height direction, and design a scanning method within the slices;

[0011] S3, loading the alloy powder in step S1 into a powder bin of a selective laser melting forming device, and importing the slice data and slice scanning data obtained in step S2 into the selective laser melting forming device;

[0012] S4. Under the condition of removing the preheating treatment of the substrate, the powder spreading-melting process is continuously repeated according to the slice data and slice scanning data obtained in step S2 until the printing of the block sample is completed, and finally a printed metastable β titanium alloy material with uniform mechanical properties is obtained.

[0013] As a further illustration of the present invention, the Ti-7Mo-3Nb-3Cr-3Al alloy powder is a spherical powder, and the particle size of the spherical powder is 15-53 μm.

[0014] As a further illustration of the present invention, the nominal composition of the metastable β titanium alloy is Ti-7Mo-3Nb-3Cr-3Al, [Mo] eq :9.64.

[0015] As a further illustration of the present invention, in step 1, a plasma rotating electrode is used to prepare a highly alloyed Ti-7Mo-3Nb-3Cr-3Al alloy powder.

[0016] As a further illustration of the present invention, the alloy powder is vacuum dried to remove moisture before use, and the vacuum drying temperature is 70-90° C. and the time is not less than 4 hours.

[0017] As a further illustration of the present invention, in step 4, the substrate is a TC4 alloy substrate.

[0018] As a further illustration of the present invention, in step 4, the printing parameters are: spot diameter 60 μm, laser power 120-210 W, scanning speed 1000-1400 mm / s, scanning spacing 80 μm, powder layer thickness 40 μm; energy density range 27-66 J / mm 3 , a strip scanning strategy with 67° rotation between layers was adopted, and the entire deposition process was carried out in a high-purity argon chamber.

[0019] Among the above printing parameters, the laser power can be set to 120W, 150W, 180W, 210W, etc.; the scanning speed can be set to 1000mm / s, 1100mm / s, 1200mm / s, 1300mm / s, 1400mm / s, etc. The energy density is, for example, 26.79J / mm 3 、31.25J / mm 3 、33.48J / mm 3 、37.50J / mm 3 、39.06J / mm 3 , 46.88J / mm 3 、54.69J / mm 3 、56.25J / mm 3 、59.66J / mm 3 、65.63J / mm 3 wait.

[0020] The method provided by the present invention can influence the thermal history of the forming process by controlling heat accumulation during forming, based on its phase precipitation conditions, thereby suppressing the precipitation and uneven distribution of ω / α phases. Specifically, the present invention reduces heat accumulation by selecting lower laser power and higher scanning speed, controlling the input energy density within a low range to avoid promoting the nucleation and precipitation of small amounts of α phase. Simultaneously, the pre-printing substrate preheating treatment is eliminated to further reduce forming heat accumulation, thereby affecting the microstructure and phase composition, and achieving enhanced and uniform tensile ductility of the printed alloy.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] The metastable β-titanium alloy provided by this invention exhibits uniform mechanical properties, particularly elongation, after selective laser melting. Through simple process adjustments, including lower laser power and higher scanning speed, and by varying substrate preheating during the forming process at low input energy density, the microstructure and grain morphology of the alloy during additive manufacturing can be in situ controlled. While ensuring excellent forming quality, this process refines the alloy grains while effectively suppressing the precipitation and uneven distribution of ω / α phases. This process effectively reduces unnecessary time costs during printing, is simple to operate, energy-efficient, and highly stable.

[0023] 2. The large amount of β phase retained in the metastable β titanium alloy formed by laser selective melting of the present invention, without subsequent heat treatment, can help improve the strain hardening ability through the occurrence of phase transformation-induced plasticity effect during its tensile deformation, thereby facilitating the achievement of excellent strength-plasticity matching.

[0024] Other features and advantages of this technical solution will be described in the subsequent description, and in part will become apparent from the description, or understood by practicing this technical solution. The objectives and other advantages of this technical solution can be achieved and obtained through the structures specifically pointed out in the written description and the accompanying drawings.

[0025] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings are used to provide further understanding of the present technical solution and constitute a part of the specification. Together with the embodiments of the present technical solution, they are used to explain the present technical solution and do not constitute a limitation to the present technical solution.

[0027] Figure 1 This is an optical microscope image of the polished cross-section of the Ti-7Mo-3Nb-3Cr-3Al alloy in Example 1 along the construction direction;

[0028] Figure 2 Phase composition and characteristics of Ti-7Mo-3Nb-3Cr-3Al alloy at different deposition heights in Example 1: (ab) (cd) (ef) represent the transmission electron microscopy diffraction spots and dark field images of the upper, middle, and lower samples, respectively;

[0029] Figure 3 This is the microhardness distribution diagram of the cross section of the Ti-7Mo-3Nb-3Cr-3Al alloy along the construction direction in Example 1;

[0030] Figure 4 The engineering stress-strain curve of the tensile specimen of the Ti-7Mo-3Nb-3Cr-3Al alloy in Example 1 was taken along the construction direction;

[0031] Figure 5 Phase composition and characteristics of Ti-7Mo-3Nb-3Cr-3Al alloy at different deposition heights in Comparative Example 1: (ab) (cd) (ef) represent the transmission electron microscopy diffraction spots and dark field images of the upper, middle, and lower samples, respectively;

[0032] Figure 6 The engineering stress-strain curve of the tensile specimen of the Ti-7Mo-3Nb-3Cr-3Al alloy in Comparative Example 1 was taken along the construction direction. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present technical solution are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present technical solution and are not used to limit the present technical solution.

[0034] The Ti-7Mo-3Nb-3Cr-3Al alloy powder used in the following examples was obtained by ultra-high-speed plasma rotating electrode pulverization. The powder had good sphericity and a particle size range of 15-53 μm. Example 1

[0035] Step 1: Dry the Ti-7Mo-3Nb-3Cr-3Al alloy powder in a vacuum dryer at 80°C for 4 hours.

[0036] Step 2: Draw a 3D model of the target titanium alloy with dimensions of 10 × 10 × 10 mm and 44 × 10 × 10 mm (length × width × height). The model is sliced ​​and the scanning and filling paths for each layer are designed.

[0037] Step 3: Import the slice data and slice scan data from Step 2 into the laser selective melting (SLM) equipment. Secure the TC4 alloy substrate to the build platform. Next, load the dried alloy powder into the equipment's powder supply hopper. High-purity argon gas is introduced into the build chamber to reduce the oxygen content to below 50 ppm. No substrate preheating is performed.

[0038] Step 4: The alloy powder is laser melted according to the preferred process parameters, and the powder spreading-melting deposition process is repeated to obtain the printed Ti-7Mo-3Nb-3Cr-3Al alloy material. The printing parameters include: spot diameter 60μm, laser power 150W, scanning speed 1200mm / s, scanning spacing 80μm, powder layer thickness 40μm, and energy density 39.06J / mm 3 .

[0039] Step 5: Remove the prepared material from the forming equipment, clean the powder, separate the sample from the substrate using wire cutting, and directly analyze and test the printed sample without additional heat treatment.

[0040] Figure 1 This is an optical microscope image of the polished cross-section of the Ti-7Mo-3Nb-3Cr-3Al alloy sample obtained in this example along the construction direction. It can be seen that the printed alloy has good forming density and no obvious printing defects.

[0041] Figure 2 The phase composition and characteristics of Ti-7Mo-3Nb-3Cr-3Al alloy samples at different deposition heights in this embodiment are shown in Figure 2. Figure 2 It can be seen that small ω phase particles with low precipitation content are precipitated in the β matrix, and no obvious α phase precipitation is observed.

[0042] Figure 3 The microhardness distribution results of the tensile specimens of Ti-7Mo-3Nb-3Cr-3Al alloy in this embodiment are taken along the construction direction. Figure 3 It can be seen that the hardness distribution along the construction direction is uniform and there is no significant change.

[0043] Figure 4 This is the engineering stress-strain curve of the tensile test of the Ti-7Mo-3Nb-3Cr-3Al alloy in this embodiment. Sampling along the construction direction shows uniform mechanical properties. The yield strength is tested to be 672±23.76MPa, with a coefficient of variation CV=3.53%, the tensile strength is 858±2.13MPa, with a coefficient of variation CV=0.25%, and the elongation is 16.12±0.38%, with a coefficient of variation CV=2.36%. The coefficients of variation are all less than 5%, indicating that the tensile properties are uniform and the degree of dispersion is low.

[0044] Comparative Example 1

[0045] Step 1: Dry the Ti-7Mo-3Nb-3Cr-3Al alloy powder in a vacuum dryer at 80°C for 4 hours.

[0046] Step 2: Draw a 3D model of the target titanium alloy with dimensions of 10 × 10 × 10 mm and 44 × 10 × 10 mm (length × width × height). The model is sliced ​​and the scanning and filling paths for each layer are designed.

[0047] Step 3: Import the slice data and slice scan data from Step 2 into the laser selective melting (SLM) machine. Secure the TC4 alloy substrate to the build platform. Next, load the dried alloy powder into the machine's powder supply chamber. Set the substrate preheat temperature to 200°C, and introduce high-purity argon gas into the build chamber to reduce the oxygen content to below 50 ppm.

[0048] Step 4: The alloy powder is laser melted according to the preferred process parameters to obtain a printed Ti-7Mo-3Nb-3Cr-3Al alloy material. The printing parameters include: spot diameter 60 μm, laser power 150 W, scanning speed 1200 mm / s, scanning spacing 80 μm, powder layer thickness 40 μm, and energy density 39.06 J / mm 3 .

[0049] Step 5: Remove the prepared material from the forming equipment, clean the powder, separate the sample from the substrate using wire cutting, and directly analyze and test the printed sample without additional heat treatment.

[0050] Figure 5 The phase composition and characteristics of Ti-7Mo-3Nb-3Cr-3Al alloy samples at different deposition heights in this comparative example are as follows: Figure 5 It can be seen that the phase distribution is uneven along the construction direction. In addition to the dispersed ω phase particles in the β matrix, there are even fine α phase precipitations in the β matrix in the middle of the sample.

[0051] Figure 6 The engineering stress-strain curve of the tensile test of Ti-7Mo-3Nb-3Cr-3Al alloy in this comparative example is shown in Figure 2. Figure 6 It can be seen that the mechanical properties along the construction direction are significantly dispersed, especially its tensile plasticity. The tested yield strength is 900~1000MPa, the tensile strength is 940~1000MPa, and the elongation is 2~15%, with a large fluctuation range.

[0052] As can be seen from Example 1 and Comparative Example 1, in Comparative Example 1, printing was performed with the substrate preheated at 200°C. Due to the significant influence of thermal cycling, the ω / α phase precipitation and uneven distribution along the build direction resulted in significantly dispersed mechanical properties. However, the alloy prepared in Example 1 exhibited uniform mechanical properties along the build direction, especially tensile plasticity. This indicates that selecting lower laser power and higher scanning speeds, and eliminating substrate preheating at lower input energy density, can effectively weaken the adverse effects of thermal cycling and suppress uneven ω / α phase precipitation, resulting in uniform and enhanced tensile plasticity of the printed metastable β titanium alloy and uniform mechanical properties of the alloy.

[0053] Obviously, those skilled in the art may make various changes and modifications to this technical solution without departing from the spirit and scope of this technical solution. Thus, if these modifications and variations of this technical solution fall within the scope of the claims of this technical solution and their equivalents, this technical solution is intended to include these modifications and variations.

Claims

1. A method for preparing a metastable β titanium alloy with uniform mechanical properties by additive manufacturing, characterized in that: The following steps are involved: S1, preparing alloyed Ti-7Mo-3Nb-3Cr-3Al alloy powder; S2. Draw a three-dimensional model, perform layered slicing along the height direction, and design a scanning method within the slices; S3, loading the alloy powder in step S1 into a powder bin of a selective laser melting forming device, and importing the slice data and slice scanning data obtained in step S2 into the selective laser melting forming device; S4. Without the substrate preheating treatment, the powder spreading-melting process is continuously repeated according to the slice data and slice scanning data obtained in step S2 until the printing of the block sample is completed. The printing parameters are: spot diameter 60 μm, laser power 120-210 W, scanning speed 1000-1400 mm / s, scanning spacing 80 μm, powder layer thickness 40 μm; energy density range 27-66 J / mm 3 , a 67° interlayer rotation strip scanning strategy was adopted, and the entire deposition process was carried out in a high-purity argon chamber; finally, a printed metastable β titanium alloy material with uniform mechanical properties was obtained.

2. The method for preparing a metastable β titanium alloy having uniform mechanical properties by additive manufacturing according to claim 1, characterized in that: The Ti-7Mo-3Nb-3Cr-3Al alloy powder is a spherical powder, and the particle size of the spherical powder is 15-53 μm.

3. The method for preparing a metastable β titanium alloy having uniform mechanical properties by additive manufacturing according to claim 1, wherein: The alloy powder is vacuum dried to remove moisture before use. The vacuum drying temperature is 70-90° C. and the time is not less than 4 hours.

4. The method for preparing a metastable β titanium alloy having uniform mechanical properties by additive manufacturing according to claim 1, wherein: In step S4, the substrate is a TC4 alloy substrate.

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