Electron beam melting TC4 alloy isotropy optimization method based on staged temperature control HIP

Abstract

The invention belongs to the technical field of TC4 alloy additive manufacturing, and discloses an electron beam melting TC4 alloy isotropic optimization method based on staged temperature control HIP, which comprises the following steps of: depositing TC4 alloy powder along the normal direction of a substrate, and printing a part through an electron beam powder bed melting additive manufacturing technology; and then, staged temperature control hot isostatic pressing treatment is carried out on the printed part, and defects generated by printing are closed. According to the method, the hot isostatic pressing (HIP) technology is adopted, under the synergistic effect of the high temperature of 920 DEG C and the high pressure (Ar protection) of 150 MPa, the defects of air holes, incomplete fusion and the like in the TC4 alloy formed through EBM are eliminated through a plastic deformation and diffusion creep mechanism, and the density is remarkably improved. Meanwhile, a metastable martensite alpha'phase is promoted to be decomposed into an equilibrium-state alpha + beta double-phase structure in a high-temperature environment, and alpha-phase grains are moderately coarsened (the average size is increased to 3.12 microns from 1.71 microns).

Description

Technical Field

[0001] The present invention relates to the field of TC4 alloy additive technology, and in particular to an isotropic optimization method for electron beam melting TC4 alloy based on staged temperature-controlled HIP. Background Art

[0002] Electron beam powder bed fusion additive manufacturing (EBM) is an advanced manufacturing technology that uses a high-energy electron beam as a heat source to melt metal powder layer by layer and solidify it into a shape. It is widely recognized by the industry for its advantages such as optimized geometric design freedom, functional integration, and integrated component forming. As a duplex titanium alloy, Ti-6Al-4V (TC4) alloy is widely used in the biomedical and aerospace fields due to its high specific strength, excellent thermal stability and corrosion resistance. Ti-6Al-4V alloy is usually required to serve in a cyclic load environment and experience complex stress states, which puts a great test on the quality of the material itself.

[0003] Currently, the production of TC4 alloy parts using electron beam powder bed fusion additive manufacturing (EBM-TC4) suffers from two major drawbacks: first, the generation of internal porosity and unfused defects during the printing process. These defects are difficult to eliminate through conventional heat treatment and remain in the final part, acting as stress concentration sources and significantly degrading high-cycle fatigue performance. Second, the rapid melting and solidification of the material during EBM can induce the formation of a metastable martensitic α' phase, severely degrading plasticity. Furthermore, the significant temperature gradient in the deposition direction induces nonequilibrium solidification epitaxial growth, resulting in a columnar structure extending along the deposition direction. The specific preferred orientation of these grains is highly consistent with the direction of the maximum temperature gradient. When loaded along the major and minor axes, the columnar grains exhibit different deformation resistance, ultimately leading to anisotropy in the material's macroscopic properties. This texture also contributes to anisotropy in mechanical properties, significantly impacting the practical use of the part. Currently, few solutions have been reported to address the porosity and anisotropy of mechanical properties in TC4 samples. To address these issues, near-net-shape parts are usually post-processed with hot isostatic pressing. Summary of the Invention

[0004] The purpose of the present invention is to provide an isotropic optimization method for electron beam melted TC4 alloy based on staged temperature-controlled HIP. This method uses a staged temperature-controlled HIP process to simultaneously achieve defect repair, grain size regulation and texture optimization within a specific temperature-pressure-time window, thereby solving the three major problems of EBM-TC4 alloy, namely strength-plasticity inversion, anisotropy and poor fatigue performance.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for isotropic optimization of TC4 alloy by electron beam melting (EBM) based on staged temperature-controlled HIP (HIP) processing comprises: first, depositing TC4 alloy powder along the normal direction of a substrate to print a part using electron beam powder bed fusion additive manufacturing technology; then, performing staged temperature-controlled hot isostatic pressing (HIP) on the printed part to seal defects generated by printing;

[0007] The staged temperature-controlled hot isostatic pressing treatment refers to first heating the temperature to 880°C at 10°C / min, then heating the temperature to the target temperature of 920°C at 5°C / min; keeping the temperature at 920°C for 3 hours, applying an isotropic pressure of 150 MPa, and protecting with argon gas; cooling the furnace to below 100°C and then air cooling.

[0008] The TC4 alloy powder adopts TC4 alloy powder with a purity of more than 99.9% and a particle size of 53 to 106 μm, and its alloy components by mass percentage are: C-0.007%, Al-6.20%, V-4.13%, Mn-<0.005%, Fe-0.15%, Cu-<0.005%, Zr-<0.005%, Mo<0.005%, Sn<0.005%, Ti-balance.

[0009] The printing parameters for electron beam powder bed fusion additive manufacturing (EBPF) are: scan line length of 15 mm, scan pitch of 0.1 mm, beam current of 11 mA, scan speed of 1.78 m / s, and areal energy density of 3.7 J / mm². Deposition is performed along the substrate normal direction according to the set printing parameters.

[0010] Defect Elimination Principle: Hot isostatic pressing (HIP) simultaneously applies high temperature and isotropic high-pressure gas (argon) to induce plastic deformation and creep in the material at high temperature. Simultaneously, through the atomic diffusion effect, the pores within the material shrink or close under uniform pressure. High temperature reduces the material's yield strength and accelerates atomic migration, while high pressure provides a continuous driving force for pore collapse. Ultimately, material flow fills the pores, significantly improving density and eliminating defects.

[0011] Compared with directly heating the temperature to 920°C, the present invention adopts a staged HIP (first heating to 880°C at 10°C / min, and then heating to 920°C at 5°C / min) to optimize the synergy of the thermal-mechanical coupling process. While ensuring densification, it also takes into account the microstructure regulation and defect repair quality, ultimately improving the comprehensive performance of the EBM-TC4 component.

[0012] Principle of Improved Plasticity: Due to the high temperatures encountered during the hot isostatic pressing (HIP) process, while simultaneously eliminating defects, it also promotes grain coarsening. According to the classic Hall-Petch relationship, the contribution of α-phase size to yield strength can be expressed as: σy = σ0 + kd-1 / 2, where σy is the yield strength, σ0 and k are constants, and d is the average α-phase grain size. This coarsening of the α-phase (from 1.71 μm in the as-deposited state to 3.12 μm after HIP at 920°C) directly leads to a decrease in material strength. The as-deposited sample primarily exhibits cylindrical slip, with the majority of regions (white areas) having SF values below 0.4. This slip behavior is one of the key reasons for its highest strength but lowest plasticity. HIPing shifts the material's dominant slip mode from cylindrical slip in the as-deposited state to basal slip, indicating that the high temperature and high pressure environment induces a preferential shift in crystal orientation, shifting the grains toward an orientation more favorable for basal slip. Since basal slip occurs along the direction of the closest atomic packing, the lattice resistance that dislocation movement needs to overcome is smaller, so it can promote more uniform plastic deformation and significantly improve the ductility of the material.

[0013] Principle of reducing anisotropy: Due to the high temperature conditions during the hot isostatic pressing process, while defects are eliminated, variant selection is also weakened. High-temperature treatment has a dual impact on phase composition and defect structure: High-temperature HIP treatment near the phase transformation point promotes the transformation of more α phase to β phase. Because the β phase with a body-centered cubic (bcc) structure has a high plastic deformation capacity, it is easier to achieve structural homogenization under high pressure, significantly reducing the density of crystal defects such as grain boundaries and dislocations. These structural defects usually serve as preferential locations for the nucleation of α variants. Therefore, the defect density is reduced, the degree of variant selection is correspondingly weakened, and its macroscopic anisotropy is basically eliminated.

[0014] This invention utilizes hot isostatic pressing (HIP) technology, utilizing a synergistic effect of 920°C and 150 MPa (under Ar protection) pressure. This technology eliminates defects such as porosity and lack of fusion within the EBM-molded TC4 alloy through plastic deformation and diffusion creep mechanisms, significantly improving density. Simultaneously, the high temperature environment promotes the decomposition of the metastable martensitic α' phase into an equilibrium α+β dual-phase structure, and induces moderate coarsening of the α phase grains (the average size increases from 1.71 μm to 3.12 μm). BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Microstructure in the deposition direction.

[0016] Figure 2 This is the microstructure of the fracture after tensile test.

[0017] Figure 3 is the Schmidt factor distribution diagram. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0019] The present embodiment discloses an isotropic optimization method for electron beam melting of TC4 alloy based on staged temperature-controlled HIP, which overcomes the problems of defect cavities and poor comprehensive mechanical properties of the material caused by near-net shaping of electron beam melting.

[0020] The isotropic optimization method of electron beam melting TC4 alloy based on staged temperature-controlled HIP provided in this embodiment includes the following steps:

[0021] (1) Material selection;

[0022] Commercial Ti-6Al-4V alloy powder was selected with a purity of >99.9% and a particle size of 53 to 106 μm. The alloy components by mass percentage were: C-0.007%, Al-6.20%, V-4.13%, Mn-<0.005%, Fe-0.15%, Cu-<0.005%, Zr-<0.005%, Mo<0.005%, Sn<0.005%, and Ti-balance.

[0023] (2) Electron beam powder bed fusion additive manufacturing (EBM)

[0024] The printing parameters of the electron beam powder bed fusion additive manufacturing technology are: scanning line length of 15 mm, scanning pitch of 0.1 mm, beam current of 11 mA, scanning speed of 1.78 m / s, and surface energy density of 3.7 joules / square millimeter.

[0025] Through the electron beam powder bed process, the printing parameters are set to deposit and form along the normal direction of the substrate.

[0026] (3) Hot isostatic pressing (HIP) treatment;

[0027] First, the temperature is raised to 880°C at a rate of 10°C / min to avoid excessive grain growth caused by long-term high-temperature exposure; then the temperature is raised to the target temperature of 920°C at a rate of 5°C / min to accurately control the phase transition temperature and promote partial dissolution and recrystallization of the β phase.

[0028] Hold at 920°C for 3 hours, apply 150 MPa isotropic pressure, and use Ar gas as a shielding agent. Furnace cooling to below 100°C is followed by air cooling. 920°C approaches the β transition point of TC4, but does not fully enter the β region, balancing densification and structural stability. This ample time ensures sufficient diffusion and pore closure while avoiding abnormal grain coarsening. 150 MPa isotropic pressure closes internal pores through plastic deformation and creep. The Ar gas environment prevents high-temperature oxidation and maintains material composition stability.

[0029] Obtain deposition direction microstructure images of the part printed in step (2) and the part after HIP in step (3), and perform a tensile test along the deposition direction.

[0030] Figure 1 The microstructure in the deposition direction is shown in Figures (a) and (b) of the printed part in step (2). The low plasticity caused by the presence of the martensite α' phase, which affects the plasticity of the part, is shown in the figure. The columnar crystal structure is formed along the deposition direction. (c) and (d) are the microstructures in the deposition direction of the part after HIP in step (3). The martensite α' phase disappears and forms equiaxed crystals, with random grain orientation and weakened texture. The average size of the deposited α phase is 1.71 μm, which increases to 3.12 μm after HIP. Grain coarsening directly reduces strength but increases plasticity. Figure 1 In the as-deposited state, martensitic α' phase can be observed within the columnar grains. High-magnification SEM images reveal interlaced needle-like α / α' phases with short rod-like or dot-like β phases. After HIP treatment, the α' phase completely decomposes into α+β phases, forming a basketweave structure. The α phase morphology evolves into short rod-like and partially spheroidized α phases. The α phase undergoes significant coarsening, with the as-deposited size reaching 1.71±0.13μm and the HIP treatment reaching 3.12±0.34μm (a 67.74% increase compared to HIP).

[0031] During the EBM process, the molten pool formed by localized electron beam melting of the metal powder undergoes rapid solidification. This high cooling rate inhibits the diffusional transformation of the β phase into an equilibrium α+β dual-phase structure. To reduce strain energy, the β phase transforms into the metastable α' martensite via a diffusionless shear mechanism. This transformation mechanism results in the formation of a high density of crystal defects such as dislocations and twins within the β phase. The abundant presence of acicular martensitic α' phase in the as-deposited specimen significantly increases the alloy's strength but simultaneously reduces its ductility. When the isothermal annealing temperature exceeds 850°C, the α' phase completely decomposes into the α+β phase. According to the classic Hall-Petch relationship, the contribution of the α phase size to the yield strength can be expressed as: σy = σ0 + kd-1 / 2, where σy is the yield strength, σ0 and k are constants, and d is the average α phase grain size. It can be seen that the α phase coarsens (from 1.71 μm in the as-deposited state to 3.12 μm after HIP at 920°C), directly contributing to the reduction in material strength.

[0032] Under tensile stress, the lamellar α structure maintains moderate strength but limited ductility. However, the spherical α phase enhances the initiation of multiple slip systems and dislocation reorganization, effectively alleviating local stress concentration and significantly improving the material's plastic deformation capacity. This hybrid structure of lamellar and spherical structures exhibits a stronger ability to accommodate dislocation motion, and its coordinated deformation performance is significantly superior to that of a fully lamellar structure.

[0033] Figure 2 Figures 1 and 2 show the microstructures of the fracture site after the tensile test. (a) and (b) show the microstructures of the fracture site of the part printed in step (2), showing the presence of some holes, cracks, and unfused defects. (c) and (d) show the microstructures of the fracture site of the part after HIP treatment in step (3). After HIP treatment, the internal pores and unfused defects are significantly reduced. Under high temperature and pressure, the material fills the pores through plastic deformation and creep, increasing its density.

[0034] Figure 3 Figure 2 is the Schmidt factor distribution diagram, where (a) and (b) are the Schmidt factor distribution diagrams of the part printed in step (2); (c) and (d) are the Schmidt factor distribution diagrams of the part after HIP in step (3). The deposition state is mainly cylindrical slip (mostly in the region of Schmid factor SF < 0.4), resulting in low plasticity. After HIP, the dominant slip mode changes to basal slip (higher SF value), the resistance to dislocation movement is reduced, and the plasticity is significantly improved.

[0035] Table 1 Tensile properties of as-deposited and hot isostatically pressed steel.

[0036]

[0037] Table 1 shows the tensile properties of the as-deposited (part from step 2) and hot isostatically pressed (hot isostatically pressed) parts (part from step 3). The difference in tensile strength between the X, Y, and Z directions in the as-deposited state is 34.5 MPa, and the difference in elongation is 6.9%. After HIP treatment, the difference in tensile strength between the X, Y, and Z directions is reduced to 3.3 MPa, and the difference in elongation is reduced to 2.2%. Anisotropy is essentially eliminated. After HIP treatment, the elongation in the X, Y, and Z directions increases to 15.7% and 17.9%, respectively.

[0038] Table 2 Fatigue properties after hot isostatic pressing

[0039]

[0040] Table 2 shows the fatigue properties of the as-deposited and HIPed samples, respectively. Under maximum stresses of 575-600 MPa, the HIPed samples achieved over 6.8 × 10⁶ cycles, with some reaching 1.0 × 10⁷ cycles without fracture. The fatigue strength of the as-deposited sample was significantly lower than that of the HIPed sample due to the presence of pores. Defect elimination effectively suppressed crack initiation, significantly improving fatigue life.

[0041] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for isotropic optimization of TC4 alloy by electron beam melting based on staged temperature control HIP, characterized in that: include: First, TC4 alloy powder is deposited along the normal direction of the substrate to print the part using electron beam powder bed fusion additive manufacturing technology; The printed parts are then subjected to a staged temperature-controlled hot isostatic pressing process to seal defects caused by printing; The staged temperature-controlled hot isostatic pressing treatment refers to first heating the temperature to 880°C at 10°C / min, then heating the temperature to the target temperature of 920°C at 5°C / min; keeping the temperature at 920°C for 3 hours, applying an isotropic pressure of 150 MPa, and protecting with argon gas; cooling the furnace to below 100°C and then air cooling.

2. The isotropic optimization method of TC4 alloy by electron beam melting based on staged temperature control HIP according to claim 1, characterized in that: The TC4 alloy powder adopts TC4 alloy powder with a purity of >99.9%, and its alloy components are as follows by mass percentage: C-0.007%, Al-6.20%, V-4.13%, Mn-<0.005%, Fe-0.15%, Cu-<0.005%, Zr-<0.005%, Mo<0.005%, Sn<0.005%, Ti-balance.

3. The isotropic optimization method of TC4 alloy by electron beam melting based on staged temperature control HIP according to claim 2, characterized in that: The particle size of the TC4 alloy powder is 53-106 μm.

4. The isotropic optimization method of TC4 alloy by electron beam melting based on staged temperature control HIP according to claim 1, characterized in that: The printing parameters of the electron beam powder bed fusion additive manufacturing technology are: scanning line length of 15 mm, scanning pitch of 0.1 mm, beam current of 11 mA, scanning speed of 1.78 m / s, and surface energy density of 3.7 joules / square millimeter.

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