A method of additive manufacturing of a bimodal hetero-grain structure titanium alloy

Abstract

The application discloses an additive manufacturing method of a bimodal heterogeneous grain structure titanium alloy, which is formed by layer-by-layer deposition on a substrate under an inert gas protection environment by using a powder feeding type laser directional energy deposition device, wherein titanium alloy fine powder with a particle size of 20-63 mu m and titanium alloy coarse powder with a particle size of 90-150 mu m are alternately fed, and after 1-10 layers of the same particle size powder are deposited, the other particle size powder is switched to, and the bimodal heterogeneous grain structure titanium alloy is obtained after the deposition is completed. The application realizes the layered difference distribution of the grain size by utilizing the difference of the powder particle size to regulate the melting and solidification behavior under the condition that the chemical composition of the titanium alloy is not changed, no multi-material interface is introduced, and no external field auxiliary means is needed, and further, the titanium alloy component with the synergistic promotion of the strength and plasticity is obtained, so that a technical scheme with simple process and strong engineering applicability is provided for the preparation of the high-performance additive manufacturing titanium alloy component.

Description

An additive manufacturing method for a bimodal heterogeneous titanium alloy Technical Field

[0001] This invention relates to the field of titanium alloy additive manufacturing technology, and specifically to an additive manufacturing method for a titanium alloy with a bimodal heterogeneous grain structure. Background Technology

[0002] Heterogeneous metallic materials, by introducing spatially non-uniform distributions of microstructures or mechanical properties, and utilizing the synergistic effect of heterogeneous deformation, have overcome the bottleneck of balancing strength and plasticity in traditional metallic materials, and have become an important research direction for high-performance metallic materials. Laser additive manufacturing technology, with its advantages of high forming freedom and high material utilization, is widely used in the manufacture of complex titanium alloy components. However, due to its repeated thermal cycling and directional solidification characteristics, the microstructure grows epitaxially along adjacent layers, easily forming coarse columnar crystal structures within the component. This makes it difficult to prepare heterogeneous grain structures with significant performance synergistic effects, limiting the practical application of heterogeneous structure design concepts in titanium alloy additive manufacturing.

[0003] To improve the overall mechanical properties of additively manufactured metallic materials, existing technologies have proposed various methods for microstructure control and heterogeneous structure construction: First, different thermal histories are introduced by adjusting process parameters such as laser power and scanning speed to create differences in grain morphology or size; second, compositional gradient design or multi-material synergistic forming is employed to achieve spatial non-uniform distribution of chemical composition or phase composition; and third, external field assistance methods such as ultrasound and electromagnetic fields are used to intervene in the flow or solidification process of the molten pool. However, these technologies all have significant shortcomings: process parameter control methods are highly dependent on equipment stability and process window, have limited grain control range, and are difficult to guarantee microstructure consistency and repeatability in complex or large-sized components; compositional gradient or multi-material synergistic forming methods are complex, and their actual service performance after adjusting the chemical composition of the material still needs to be verified; external field assistance methods have high requirements for equipment integration and process control, are costly, and are difficult to promote and apply.

[0004] Especially without altering the chemical composition of the material, existing technologies lack simple, stable, and versatile solutions, failing to achieve stable and controllable grain size distribution over a wide range. This severely restricts the construction of bimodal heterogeneous grain structures in additively manufactured titanium alloys and their engineering application potential. Therefore, a new microstructure control method is urgently needed to effectively regulate the grain size within components and construct heterogeneous structures while maintaining consistent alloy composition and avoiding excessive reliance on complex process parameters or external auxiliary conditions, in order to meet the practical application requirements of high-performance additively manufactured titanium alloy components. Summary of the Invention

[0005] The purpose of this invention is to solve the technical problems of complex processes, limited grain size control range, poor stability and repeatability in the construction of heterogeneous titanium alloy structures by existing laser additive manufacturing, and difficulty in achieving a stable and controllable distribution of grain size over a wide range without changing the chemical composition. The invention provides an additive manufacturing method for titanium alloys with bimodal heterogeneous grain structure, which uses titanium alloy powders of different particle sizes to feed in layers, and uses the difference in powder particle size to control the melting and solidification behavior, thereby achieving a layered differential distribution of grain size, and thus obtaining titanium alloy components with synergistic improvement in strength and plasticity.

[0006] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0007] An additive manufacturing method for a bimodal heterogeneous grain structure titanium alloy includes the following steps:

[0008] In an inert gas protected environment, a powder-feeding laser directional energy deposition (EDA) device is used to deposit layers on a substrate. Fine titanium alloy powder with a particle size of 20-63 μm and coarse titanium alloy powder with a particle size of 90-150 μm are alternately fed in. After every 1-10 layers of powder with the same particle size, the powder is switched to another particle size. After deposition, the bimodal heterogeneous grain structure titanium alloy is obtained. The process parameters for the forming process include: laser power 800-1200 W, scanning speed 4-8 mm / s, single-layer deposition thickness 0.6-0.8 mm, powder feed rate 6-10 g / min, and laser defocusing amount 2-4 mm.

[0009] This invention provides a method for constructing a bimodal heterogeneous grain structure in laser additive manufacturing based on layered feeding of titanium alloy powders with different particle sizes. The core of this method is to layer titanium alloy powders with varying particle sizes during the additive manufacturing process. Utilizing the inherent differences in melting behavior (such as melting rate and melting uniformity) and solidification conditions (such as cooling rate and thermal conductivity) caused by the different particle sizes, a structure with differentiated grain size distribution along the layered direction is formed within the shaped titanium alloy component without altering the original chemical composition. This ultimately yields a bimodal heterogeneous grain structure with both high strength and high plasticity. This method overcomes the limitations of existing technologies that rely solely on single process parameters such as laser power and scanning speed to control grain size. It also solves the problem that traditional single-parameter control results in a narrow process window, easily leading to forming defects and limiting the adjustable range of grain size. By using powder particle size as the core control variable, the adjustable range of grain size is significantly broadened. Meanwhile, this method does not require complex process parameter optimization or the introduction of external auxiliary equipment, which can effectively ensure the stability of the additive manufacturing process and the repeatability of product performance, and successfully prepare additively manufactured titanium alloy components with synergistic improvement in strength and plasticity.

[0010] This invention limits the switching to a different particle size powder after every 1-10 layers of the same particle size. This is a precise process design based on the synergistic effect of bimodal heterogeneous grain structure construction, the metallurgical bonding strength of the heterogeneous interface, and the mechanical properties of the component. This layer design allows the deposition area of ​​a single particle size powder to fully develop the corresponding grain characteristics, forming continuous and uniform fine and coarse grain layers, giving full play to the heterogeneous deformation coordination effect to achieve optimal complementarity of strength and plasticity. At the same time, the laser heat input of the next layer allows for sufficient remelting between heterogeneous layers, ensuring the quality of interlayer metallurgical bonding. It is also compatible with layer-by-layer deposition processes, avoiding problems such as uneven powder feeding and molten pool fluctuations caused by frequent powder switching, and taking into account both process stability and structural design flexibility.

[0011] Furthermore, it is preferable to switch to a different particle size powder after depositing 2-4 layers of powder of the same particle size.

[0012] This invention limits the laser power to 800-1200 W, the scanning speed to 4-8 mm / s, the single-layer deposition thickness to 0.6-0.8 mm, the powder feed rate to 6-10 g / min, and the laser defocusing amount to 2-4 mm. This set of process parameters is the optimal conventional range for the layered deposition of 20-63 μm titanium alloy fine powder and 90-150 μm titanium alloy coarse powder, and for the stable construction of a bimodal heterogeneous grain structure. It can ensure that the two types of powders are fully and uniformly melted to form a high-density deposition layer, and can also match the different melting-solidification behaviors of the two to ensure the stable development of the grain characteristics of the fine and coarse grain layers. At the same time, it maintains the continuity and consistency of the layer-by-layer deposition, and takes into account the synergy of process stability, forming quality and component mechanical properties.

[0013] Furthermore, the inert gas is argon, preferably high-purity argon (99.999% purity). As an inert protective gas, argon has stable chemical properties and does not react chemically with titanium alloys. It can effectively isolate air from the high-temperature molten pool, reduce the risk of oxidation and nitridation of the molten pool metal, avoid the performance degradation of components caused by oxide and nitride inclusions, and ensure forming stability.

[0014] Furthermore, the flow rate of the inert gas is 15-20 L / min.

[0015] Furthermore, the laser in the powder-feeding laser directional energy deposition equipment adopts a ring laser output mode. Compared with traditional circular spot lasers, the ring laser output mode has a more uniform energy distribution, forming a ring-shaped energy barrier around the molten pool, effectively constraining the flow of the molten pool and improving its stability. At the same time, the ring laser allows the powder to be more evenly distributed within the laser beam, achieving synchronous and uniform melting of the powder, avoiding insufficient melting or localized excessive burning caused by powder aggregation, further improving the density and uniformity of the deposited layer, and ensuring the stable construction of the bimodal heterogeneous grain structure.

[0016] Furthermore, the substrate is a titanium alloy sheet, preferably a TC4 titanium alloy sheet, but TA15, TA2, or other titanium alloy sheets with good compatibility with the deposited titanium alloy composition can also be used. Using a titanium alloy substrate ensures good matching of the thermal expansion coefficients between the substrate and the deposited layer, reducing problems such as warping and cracking caused by differences in thermal stress during deposition, while also improving the bonding strength between the substrate and the deposited layer and preventing delamination.

[0017] Furthermore, the substrate is pretreated before forming, and the pretreatment includes mechanical polishing, ultrasonic cleaning and drying.

[0018] Furthermore, the ultrasonic cleaning medium is anhydrous ethanol or acetone.

[0019] Furthermore, the titanium alloy is TC4 titanium alloy (composed of Ti-6Al-4V), TA15 titanium alloy (composed of Ti-6.5Al-2Zr-1Mo-1V), TC18 titanium alloy (composed of Ti-5Al-5Mo-5V-1Cr-1Fe), Ti-6Al-7Nb titanium alloy, or Ti-22Al-25Nb titanium alloy.

[0020] Furthermore, the titanium alloy fine powder and titanium alloy coarse powder were obtained by sieving and classifying titanium alloy powder prepared by gas atomization from the same batch using a mechanical vibrating screen. No compositional adjustments were made to the powders during the sieving process, and the chemical composition of the two types of powders remained completely consistent, with only a significant difference in particle size distribution.

[0021] This invention uses powders prepared by gas atomization from the same batch, ensuring consistency in the basic properties such as chemical composition, sphericity, and flowability of fine and coarse powders. This avoids differences in thermal processing behavior due to batch variations and guarantees the uniformity of the bimodal heterogeneous grain structure. The titanium alloy powder prepared by gas atomization has high sphericity and good flowability, which is suitable for the powder feeding requirements of powder-distributed laser directional energy deposition, ensuring the continuity and uniformity of powder feeding. Mechanical vibrating screen is used for sieving and grading, which is a mature industrial-grade process that is simple to operate, low in cost, and easy to achieve industrial mass production. No composition adjustment is made during the sieving process, ensuring the consistency of composition between fine and coarse powders from the source, avoiding microstructure segregation caused by compositional differences, and ensuring that the component is a single titanium alloy system without multiple material interfaces, thus improving the structural integrity of the component.

[0022] This invention limits the particle size of the titanium alloy fine powder to 20-63 μm and the particle size of the titanium alloy coarse powder to 90-150 μm. These particle size ranges fall within the conventional effective powder particle size range for powder-fed laser directional energy deposition (EDG), exhibiting good compatibility with existing powder feeding systems (screw feeding or pneumatic feeding), ensuring uniform powder feeding and sufficient melting. Furthermore, both can be obtained through conventional mechanical vibrating screen grading, balancing industrial feasibility and economic efficiency.

[0023] This "particle size gap" design is key to achieving a good match between strength and plasticity in this invention. By deliberately avoiding the conventional intermediate particle size range of 63-90 μm, this invention artificially creates a "step-like" difference in specific surface area and thermal inertia between fine and coarse powders. Under the same laser energy input, fine powder, due to its large specific surface area and high laser absorption rate, can quickly reach its melting point and fully melt, tending to form larger grain structures during the subsequent rapid solidification process. Coarse powder, due to its larger volumetric heat capacity and thermal inertia, has a relatively delayed thermal history of melting and solidification, and a relatively slower cooling rate. It can partially melt and act as a nucleation point for grains, generating more grains and resulting in smaller grain structures. This differentiation of structures induced by the difference in powder physical properties ensures that a clear heterogeneous interface can be formed between the fine and coarse grain layers during layered deposition, thereby fully utilizing the heterogeneous deformation-induced strengthening mechanism.

[0024] Furthermore, the difference in median diameter (D50) between the coarse titanium alloy powder and the fine titanium alloy powder is 60-120 μm. If the difference in median diameter (D50) between the two types of powder is too large (for example, D50 of coarse powder exceeds 150 μm or D50 of fine powder is less than 20 μm), it is very easy to cause process defects: fine powder that is too fine is prone to agglomeration and overheating, resulting in splashing and porosity; coarse powder that is too coarse is prone to powder separation due to gravity and there is a risk of incomplete fusion, thereby reducing the density and structural integrity of the component.

[0025] Preferably, the difference in median diameter (D50) between the coarse and fine titanium alloy powders is 70-100 μm. Within this particle size range, the melting-solidification behavior of the fine and coarse powders differs significantly, which can stably form a bimodal heterogeneous structure with obvious grain size differentiation, fully leveraging the heterogeneous deformation coordination effect to achieve optimal synergy between strength and plasticity. At the same time, both can achieve full and uniform melting under the conventional process parameters of this invention, with high stability in powder feeding and forming processes and no obvious process compatibility conflicts, which can effectively ensure the density and microstructure uniformity of titanium alloy components and improve the consistency of product performance.

[0026] Furthermore, during the forming process, one of the following scanning paths is used: parallel linear scanning, circular scanning, or polygonal scanning. Different scanning paths can be flexibly adjusted according to the shape, size, and performance requirements of the titanium alloy components. For example, parallel linear scanning is suitable for simple flat components, while circular scanning is suitable for cylindrical and irregularly shaped components. Both can achieve the alternating deposition of fine and coarse powder layers and the stable construction of bimodal heterogeneous grain structures.

[0027] Furthermore, the process parameters remain constant throughout the forming process. This invention eliminates the need to adjust process parameters for fine and coarse powder deposition layers, significantly reducing the difficulty of process control and avoiding problems such as molten pool instability and poor overlap of deposition layers that are easily caused during parameter switching. At the same time, constant process parameters ensure consistent forming conditions for each deposition layer, improving the uniformity of component structure and properties, further guaranteeing process stability and product repeatability, making it more suitable for industrial mass production.

[0028] Furthermore, after deposition, the process includes cooling the bimodal heterogeneous grain structure titanium alloy to room temperature (15-25 °C) in the inert gas protective environment. The deposited titanium alloy component is still at a high temperature. If it is directly exposed to air for cooling, surface oxidation is likely to occur. Rapid cooling will also generate significant thermal stress, which can easily lead to warping and cracking. Cooling the titanium alloy component in an inert gas protective environment allows for a gradual cooling rate, effectively isolating it from air and preventing surface oxidation. It also reduces thermal stress, avoiding forming defects caused by thermal stress, and further improving the structural integrity and dimensional accuracy of the component.

[0029] The above-described technical solution of the present invention has the following beneficial effects:

[0030] 1. This invention enables stable regional control of grain size within a range of hundreds of micrometers, successfully constructing a bimodal heterogeneous grain structure. By performing sieve grading on the same titanium alloy powder and implementing zonal or layered powder feeding during additive manufacturing, regions with significantly different grain sizes can be simultaneously formed within the same component. The powder particle size affects the melting-solidification behavior, ultimately determining the scale of grain growth. Microstructural characterization verifies that the characteristic grain size of the fine-grained layer region formed using coarse-grained titanium alloy powder can be stably controlled within the range of 300-500 μm, while the characteristic grain size of the coarse-grained layer region formed using fine-grained titanium alloy powder can reach 800-1000 μm. This results in a bimodal heterogeneous grain structure with a significant span in grain size distribution within the component.

[0031] 2. The grain size control range of the present invention is significantly wider than that of existing methods based on process parameters. Compared with existing methods that only adjust process parameters such as laser power, scanning speed or scanning strategy to achieve grain size control, the present invention uses the intrinsic physical parameter of powder particle size as the basis for control. This transforms the grain size from the state of limited scale variation (1.2-1.5 times) and distribution around a single scale in the prior art to a bimodal distribution feature in which the grain size difference reaches 2-3 times and the spatial position is controllable within the same component. This effectively overcomes the technical problem of limited grain size control range in the prior art.

[0032] 3. Since the different particle sizes of the powders used in this invention are all derived from the same alloy powder, they can be obtained simply by sieving. There is no need to switch between multiple materials or design complex compositions. The process is simple and clear. Under the condition of keeping the conventional additive manufacturing process parameters unchanged, it can stably obtain a bimodal heterogeneous grain structure with similar grain size distribution characteristics and has good process repeatability.

[0033] 4. The additive manufacturing method for bimodal heterogeneous grain structure titanium alloys provided by this invention has the advantage of strong versatility. It does not depend on specific titanium alloy composition design, is compatible with a variety of titanium alloy material systems, is suitable for various additive manufacturing titanium alloy components, and can meet the forming requirements of components of different shapes and sizes. It has good technical versatility and engineering application potential. Attached Figure Description

[0034] Figure 1 is a microstructure of the bimodal heterogeneous grain structure TC4 titanium alloy of Example 1.

[0035] Figure 2 is a microstructure diagram of the bimodal heterogeneous grain structure TC18 titanium alloy of Example 3.

[0036] Figure 3 shows the microstructure of the coarse-grained TC4 titanium alloy of Comparative Example 1.

[0037] Figure 4 shows the microstructure of the fine-grained TC4 titanium alloy of Comparative Example 2.

[0038] Figure 5 is a comparison of the average grain size of the fine-grained and coarse-grained layers of the bimodal heterostructure TC4 titanium alloy in Example 1.

[0039] Figure 6 is a comparison of the average microhardness of the fine-grained and coarse-grained layers of the bimodal heterostructure TC4 titanium alloy in Example 1.

[0040] Figure 7 shows the tensile strength and elongation at break data of the TC4 titanium alloy components of Examples 1-2 and Comparative Examples 1-2. Detailed Implementation

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] This invention provides an additive manufacturing method for a bimodal heterogeneous grain structure titanium alloy, comprising the following steps:

[0043] In an inert gas protected environment, a powder-feeding laser directional energy deposition (EDA) device is used for layer-by-layer deposition. Fine titanium alloy powder with a particle size of 20-63 μm and coarse titanium alloy powder with a particle size of 90-150 μm are alternately fed in. After every 2-4 layers of powder with the same particle size, the powder is switched to another particle size. After deposition, the bimodal heterogeneous grain structure titanium alloy is obtained. The process parameters for the forming process include: laser power 800-1200 W, scanning speed 4-8 mm / s, single-layer deposition thickness 0.6-0.8 mm, powder feed rate 6-10 g / min, and laser defocusing amount 2-4 mm.

[0044] To address the technical challenges of existing additive manufacturing titanium alloys, such as narrow grain size control range and poor stability of heterogeneous grain structure construction, this invention proposes a method for preparing additive manufacturing titanium alloys by controlling the physical properties of powder materials and making targeted improvements to the powder feeding strategy in the additive manufacturing process. This method achieves in-situ construction of bimodal heterogeneous grain structures through powder particle size classification and precise control.

[0045] The basic technical idea of ​​this invention is as follows: under the premise of ensuring that the chemical composition of the titanium alloy is completely consistent, the titanium alloy powder is subjected to particle size classification treatment, and powders of different particle sizes are introduced into the forming process as core microstructure control variables; by utilizing the inherent differences in melting behavior, heat input characteristics and solidification conditions of powders of different particle sizes in laser additive manufacturing, the grain size can be stably and controllably adjusted within a wide range of scales, thereby forming a bimodal heterogeneous grain structure in situ with a synergistic distribution of coarse and fine grain layers inside the component.

[0046] This invention employs a powder-feeding laser additive manufacturing process to deposit titanium alloy components layer by layer. During the forming process, fine or coarse titanium alloy powder is selectively fed into the molten pool according to a preset forming height range, layer number range, or spatial region, achieving partitioned or layered deposition of powders of different particle sizes within the component. Furthermore, throughout the forming process, process parameters such as laser power, scanning speed, and scanning path remain within the conventional forming range, eliminating the need for complex partitioned control of process parameters for different powder deposition areas. This effectively ensures the stability of the forming process and the repeatability of product performance.

[0047] This invention selects titanium alloy raw powder with consistent chemical composition as raw material, and sieves it with screens of different aperture sizes to obtain titanium alloy fine powder and titanium alloy coarse powder with significantly different particle size distributions. This significant difference in particle size range ensures that the two types of powder exhibit distinctly different melting behaviors and solidification characteristics during additive manufacturing, laying the foundation for subsequent differential control of grain size. Specifically, due to the inherent differences in absorption characteristics, thermal inertia, and molten pool formation behavior between fine-particle-size and coarse-particle-size powders under laser irradiation, the coarse-particle-size powder forming region is more likely to form a microstructure with small grain size and high degree of microstructure refinement during solidification, while the fine-particle-size powder forming region, due to the difference in molten pool size and thermal gradient, is more likely to form a microstructure with larger grain size and predominantly columnar crystals. By rationally designing the spatial distribution of the two types of powder with different particle sizes within the component, a bimodal heterogeneous grain structure with a synergistic distribution of fine and coarse grain layers can be constructed in situ without subsequent heat treatment.

[0048] This invention achieves stable and controllable adjustment of grain size over a wide range of scales without changing the chemical composition of titanium alloys, introducing multi-material interfaces, or requiring external field assistance. It significantly expands the design space of heterogeneous grain structures in additive manufacturing of titanium alloys and provides a simple and highly engineering-applicable technical solution for the preparation of high-performance additive manufacturing titanium alloy components.

[0049] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0050] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, and the materials and reagents used are commercially available.

[0051] Example 1

[0052] An additive manufacturing method for a bimodal heterogeneous grain structure TC4 titanium alloy includes the following steps:

[0053] (1) Substrate preparation: Cast TC4 titanium alloy plate is selected as the substrate for additive manufacturing. The substrate size is 100 mm × 100 mm × 15 mm. Before forming, the substrate surface is mechanically polished to remove the surface oxide layer. Then, the polished substrate is ultrasonically cleaned with anhydrous ethanol and dried before being fixed on the forming platform of the additive manufacturing equipment.

[0054] (2) Sieving and classifying the raw powder: TC4 titanium alloy powder prepared by gas atomization in the same batch was selected as the raw powder. The raw powder was sieved and classified by mechanical vibrating screen. Two types of TC4 titanium alloy powder with different particle size ranges were obtained after sieving. The particle size of the fine TC4 titanium alloy powder was 20-63 μm, and the median diameter D50 was 38 μm. The particle size of the coarse TC4 titanium alloy powder was 105-150 μm, and the median diameter D50 was 125 μm.

[0055] (3) Additive manufacturing equipment and atmosphere conditions: Layer-by-layer deposition modeling experiment was carried out using a powder-feeding laser directional energy deposition equipment, and the fiber laser was adjusted to the ring laser output mode. The entire modeling process was carried out in an inert gas protection environment, with high-purity argon as the protective gas and the argon flow rate controlled at 18 L / min.

[0056] (4) Setting of forming process parameters: During the additive manufacturing process, the laser power is set to 900 W, the scanning speed is 6 mm / s, the single-layer deposition thickness is 0.6 mm, the powder feed rate is 8 g / min, the laser defocusing amount is 3 mm, and the outer diameter of the annular spot is 3 mm. All the above process parameters are kept constant throughout the forming process. The scanning path adopts the parallel straight line scanning method to prepare a single-pass thin wall sample with a length of 80 mm and a height of 36 mm.

[0057] (5) TC4 titanium alloy fine powder deposition and forming: In the initial stage of forming, TC4 titanium alloy fine powder is fed into the molten pool area and deposited layer by layer according to the process parameters set above. After two layers are deposited, the powder feeding system is paused to prepare for switching the powder particle size.

[0058] (6) TC4 titanium alloy coarse powder deposition: After completing the deposition of two layers of TC4 titanium alloy fine powder, under the premise that the laser power, scanning speed, single-layer deposition thickness, powder feeding amount and laser defocusing amount remain unchanged, switch the powder feeding system and feed TC4 titanium alloy coarse powder into the molten pool area, and continue to deposit two layers according to the above process parameters.

[0059] (7) Periodic alternating particle size deposition: According to the rule of switching the powder particle size every two layers, the fine powder of TC4 titanium alloy and the coarse powder of TC4 titanium alloy are deposited alternately layer by layer until the preset single-pass thin wall forming process is completed; a total of 60 layers are deposited in the whole sample, forming fine and coarse particle size powder deposition layers that are periodically and alternately distributed along the deposition direction inside the TC4 titanium alloy component.

[0060] (8) TC4 titanium alloy component forming and cooling: After all layers have been deposited, the fiber laser and powder feeding system are turned off, and the inert gas protective atmosphere is kept unchanged. The formed TC4 titanium alloy component is cooled to room temperature (20 ℃). No heat treatment is performed after forming throughout the process, and finally a bimodal heterogeneous grain structure TC4 titanium alloy component is obtained.

[0061] Example 2

[0062] An additive manufacturing method for a bimodal heterogeneous grain structure TC4 titanium alloy is basically the same as that in Example 1, except that in steps (5), (6), and (7), the powder feed particle size is switched every four layers deposited.

[0063] Example 3

[0064] An additive manufacturing method for a bimodal heterogeneous grain structure TC18 titanium alloy includes the following steps:

[0065] (1) Substrate preparation: Cast TC18 titanium alloy plate is selected as the substrate for additive manufacturing. The substrate size is 100 mm × 100 mm × 15 mm. Before forming, the substrate surface is mechanically polished to remove the surface oxide layer. Then, the polished substrate is ultrasonically cleaned with anhydrous ethanol and dried before being fixed on the forming platform of the additive manufacturing equipment.

[0066] (2) Sieving and classifying the raw powder: TC18 titanium alloy powder prepared by gas atomization in the same batch was selected as the raw powder. The raw powder was sieved and classified by mechanical vibrating screen. Two types of TC18 titanium alloy powder with different particle size ranges were obtained after sieving. The particle size of the fine TC18 titanium alloy powder was 20-63 μm, and the median diameter D50 was 35 μm. The particle size of the coarse TC18 titanium alloy powder was 90-130 μm, and the median diameter D50 was about 110 μm.

[0067] (3) Additive manufacturing equipment and atmosphere conditions: Layer-by-layer deposition modeling experiment was carried out using a powder-feeding laser directional energy deposition equipment, and the fiber laser was adjusted to the ring laser output mode. The entire modeling process was carried out in an inert gas protection environment, with high-purity argon as the protective gas and the argon flow rate controlled at 18 L / min.

[0068] (4) Setting of forming process parameters: During the additive manufacturing process, the laser power is set to 800 W, the scanning speed is 4 mm / s, the single-layer deposition thickness is 0.6 mm, the powder feed rate is 6 g / min, the laser defocusing amount is 2 mm, and the outer diameter of the annular spot is 3 mm. All the above process parameters are kept constant throughout the forming process. The scanning path adopts the parallel straight line scanning method to prepare a single-pass thin wall sample with a length of 80 mm and a height of 36 mm.

[0069] (5) TC18 titanium alloy fine powder deposition and forming: In the initial stage of forming, TC18 titanium alloy fine powder is fed into the molten pool area and deposited layer by layer according to the above-set process parameters. After two layers are deposited, the powder feeding system is paused to prepare for switching the powder particle size.

[0070] (6) TC18 titanium alloy coarse powder deposition: After completing the deposition of two layers of TC18 titanium alloy fine powder, under the premise that the laser power, scanning speed, single-layer deposition thickness, powder feeding amount and laser defocusing amount remain unchanged, switch the powder feeding system and feed TC18 titanium alloy coarse powder into the molten pool area, and continue to deposit two layers according to the above process parameters.

[0071] (7) Periodic alternating particle size deposition: According to the rule of switching the powder particle size every two layers, the fine powder of TC18 titanium alloy and the coarse powder of TC18 titanium alloy are alternately deposited layer by layer until the preset single-pass thin wall forming process is completed; a total of 60 layers are deposited in the whole sample, forming fine and coarse particle size powder deposition layers that are periodically and alternately distributed along the deposition direction inside the TC18 titanium alloy component.

[0072] (8) TC18 titanium alloy component forming and cooling: After all layers have been deposited, the fiber laser and powder feeding system are turned off, and the inert gas protective atmosphere is kept unchanged. The formed TC18 titanium alloy component is cooled to room temperature (20 ℃). No heat treatment is performed after forming throughout the process, and finally a bimodal heterogeneous grain structure TC18 titanium alloy component is obtained.

[0073] Comparative Example 1

[0074] An additive manufacturing method for a coarse-grained TC4 titanium alloy includes the following steps:

[0075] (1) Substrate preparation: Cast TC4 titanium alloy plate is selected as the substrate for additive manufacturing. The substrate size is 100 mm × 100 mm × 15 mm. Before forming, the substrate surface is mechanically polished to remove the surface oxide layer. Then, the polished substrate is ultrasonically cleaned with anhydrous ethanol and dried before being fixed on the forming platform of the additive manufacturing equipment.

[0076] (2) Sieving and classifying the original powder: TC4 titanium alloy powder prepared by gas atomization in the same batch was selected as the original powder. The original powder was sieved and classified by mechanical vibrating screen. After sieving, TC4 titanium alloy fine powder with a particle size of 20-63 μm was obtained, with a median diameter D50 of 38 μm.

[0077] (3) Additive manufacturing equipment and atmosphere conditions: Layer-by-layer deposition modeling experiment was carried out using a powder-feeding laser directional energy deposition equipment, and the fiber laser was adjusted to the ring laser output mode. The entire modeling process was carried out in an inert gas protection environment, with high-purity argon as the protective gas and the argon flow rate controlled at 18 L / min.

[0078] (4) Setting of forming process parameters: During the additive manufacturing process, the laser power is set to 900 W, the scanning speed is 6 mm / s, the single-layer deposition thickness is 0.6 mm, the powder feed rate is 8 g / min, the laser defocusing amount is 3 mm, and the outer diameter of the annular spot is 3 mm. All the above process parameters are kept constant throughout the forming process. The scanning path adopts the parallel straight line scanning method to prepare a single-pass thin wall sample with a length of 80 mm and a height of 36 mm.

[0079] (5) TC4 titanium alloy fine powder deposition forming: In the initial stage of forming, TC4 titanium alloy fine powder is fed into the molten pool area and deposited layer by layer according to the above-set process parameters. 60 layers are continuously deposited to complete the preset single-pass thin wall forming process.

[0080] (6) TC4 titanium alloy component forming and cooling: After all layers have been deposited, the fiber laser and powder feeding system are turned off, and the inert gas protective atmosphere is kept unchanged. The formed TC4 titanium alloy component is cooled to room temperature (20 °C). No heat treatment is performed after forming throughout the process, and finally a coarse-grained TC4 titanium alloy component is obtained.

[0081] Comparative Example 2

[0082] An additive manufacturing method for a fine-grained TC4 titanium alloy includes the following steps:

[0083] (1) Substrate preparation: Cast TC4 titanium alloy plate is selected as the substrate for additive manufacturing. The substrate size is 100 mm × 100 mm × 15 mm. Before forming, the substrate surface is mechanically polished to remove the surface oxide layer. Then, the polished substrate is ultrasonically cleaned with anhydrous ethanol and dried before being fixed on the forming platform of the additive manufacturing equipment.

[0084] (2) Sieving and classifying the original powder: TC4 titanium alloy powder prepared by gas atomization in the same batch was selected as the original powder. The original powder was sieved and classified by mechanical vibrating screen. After sieving, TC4 titanium alloy coarse powder with a particle size of 105-150 μm was obtained, with a midpoint diameter D50 of 125 μm.

[0085] (3) Additive manufacturing equipment and atmosphere conditions: Layer-by-layer deposition modeling experiment was carried out using a powder-feeding laser directional energy deposition equipment, and the fiber laser was adjusted to the ring laser output mode. The entire modeling process was carried out in an inert gas protection environment, with high-purity argon as the protective gas and the argon flow rate controlled at 18 L / min.

[0086] (4) Setting of forming process parameters: During the additive manufacturing process, the laser power is set to 900 W, the scanning speed is 6 mm / s, the single-layer deposition thickness is 0.6 mm, the powder feed rate is 8 g / min, the laser defocusing amount is 3 mm, and the outer diameter of the annular spot is 3 mm. All the above process parameters are kept constant throughout the forming process. The scanning path adopts the parallel straight line scanning method to prepare a single-pass thin wall sample with a length of 80 mm and a height of 36 mm.

[0087] (5) TC4 titanium alloy coarse powder deposition forming: In the initial stage of forming, TC4 titanium alloy coarse powder is fed into the molten pool area and deposited layer by layer according to the above-set process parameters. 60 layers are continuously deposited to complete the preset single-pass thin wall forming process.

[0088] (6) TC4 titanium alloy component forming and cooling: After all layers have been deposited, the fiber laser and powder feeding system are turned off, and the inert gas protective atmosphere is kept unchanged. The formed TC4 titanium alloy component is cooled to room temperature (20 °C). No heat treatment is performed after forming throughout the process, and finally a fine-grained TC4 titanium alloy component is obtained.

[0089] Test Example 1

[0090] The microstructure of the titanium alloy components from Examples 1, 3, and Comparative Examples 1-2 was characterized. Metallographic samples were prepared by wire cutting along the deposition direction of the titanium alloy components. After mounting, the samples were successively ground with sandpaper of different grits and then mechanically polished to obtain a mirror-like surface without obvious scratches. The polished metallographic samples were etched with Kroll reagent (prepared from 1-3% hydrofluoric acid, 6% nitric acid, and deionized water) to fully reveal the microstructure characteristics of the titanium alloy. Subsequently, the overall microstructure of the samples was observed using an optical microscope, and the grain morphology characteristics of different forming regions were analyzed using ImageJ software to complete the quantitative characterization of grain size.

[0091] The test results are shown in Figures 1-5. Figure 1 shows the microstructure of the bimodal heterogeneous grain structure TC4 titanium alloy of Example 1; Figure 2 shows the microstructure of the bimodal heterogeneous grain structure TC18 titanium alloy of Example 3; Figure 3 shows the microstructure of the coarse-grained structure TC4 titanium alloy of Comparative Example 1; Figure 4 shows the microstructure of the fine-grained structure TC4 titanium alloy of Comparative Example 2; and Figure 5 compares the average grain size of the fine-grained and coarse-grained layers of the bimodal heterogeneous grain structure TC4 titanium alloy of Example 1. It can be clearly seen from Figures 1 and 2 that both the TC4 titanium alloy of Example 1 and the TC18 titanium alloy of Example 3 exhibit a periodically alternating distribution of fine-grained and coarse-grained layers along the deposition direction, successfully constructing a bimodal heterogeneous grain structure. As shown in Figures 3 and 4, the TC4 titanium alloy of Comparative Example 1 exhibits only a single coarse-grained structure, while the TC4 titanium alloy of Comparative Example 2 exhibits only a single fine-grained structure. This verifies that the method of the present invention can stably achieve the construction of a bimodal heterogeneous grain structure in different titanium alloy systems. The quantitative analysis in Figure 5 further shows that the average grain size of the coarse-grained layer in Example 1 can reach 870.1 μm, while the average grain size of the fine-grained layer is 324.5 μm. The difference between the two is significant, which fully demonstrates that a wide range of stable control over grain size can be achieved by adjusting the powder particle size, thereby providing a reliable microstructure basis for the synergistic improvement of the strength and plasticity of titanium alloy components.

[0092] Test Example 2

[0093] Microhardness tests were conducted on different regions along the deposition direction of the TC4 titanium alloy component from Example 1. A Vickers microhardness tester was used, with a load of 500 g and a holding time of 10 s. Multiple test points were selected in both fine and coarse powder deposition areas for measurement, and the average value was taken.

[0094] As shown in Figure 6, the microhardness of the fine-grained layer formed by coarse-particle-size powder deposition is approximately 387.6 HV, while the microhardness of the coarse-grained layer formed by fine-particle-size powder deposition is approximately 338.2 HV, showing a significant difference. This result corroborates the microstructure characterization, further confirming that by controlling the powder particle size, regions with significantly differentiated microstructure and properties can be formed within the component, providing direct performance evidence for achieving a synergistic improvement in the strength and plasticity of titanium alloy components.

[0095] Test Example 3

[0096] Mechanical properties of the TC4 titanium alloy components from Examples 1-2 and Comparative Examples 1-2 were tested using a room-temperature uniaxial tensile test. The tensile specimens (gauge length dimensions: 10 mm long × 2.5 mm wide × 1.5 mm thick) were fabricated from additively formed titanium alloy components along the deposition direction. The specimen surfaces were mechanically polished to remove machining marks. The tensile tests were performed on an electronic universal testing machine, with a loading rate controlled at 1 × 10⁻⁶.-3 s -1 The strain is accurately measured by an extensometer to obtain mechanical property parameters such as tensile strength and elongation at break of the material; three parallel specimens are tested under each test condition, and the final result is the average value.

[0097] As shown in Figure 7, the test results show that although the single fine-grained TC4 titanium alloy component has the highest tensile strength (1094 MPa), its elongation at break is only 6.3%, exhibiting a significant defect of high strength but poor plasticity. The single coarse-grained TC4 titanium alloy component has a relatively high elongation at break (12.3%), but its tensile strength is only 976 MPa, exhibiting good plasticity but insufficient strength. In contrast, the bimodal heterogeneous grain structure TC4 titanium alloy component prepared in this invention achieves a synergistic improvement in both strength and plasticity. The bimodal grain structure TC4 titanium alloy component with alternating layers every two layers has a tensile strength of 1056 MPa and an elongation at break of 10.8%, while the bimodal grain structure TC4 titanium alloy component with alternating layers every four layers has a tensile strength of 1021 MPa and an elongation at break of 11.1%. This result fully demonstrates that the layered alternating bimodal heterogeneous grain design can effectively integrate the high strength of the fine-grained layer and the high plasticity of the coarse-grained layer, avoiding the performance imbalance of the single grain structure and significantly improving the comprehensive mechanical properties of the titanium alloy component.

[0098] In summary, this invention uses powder particle size as the core technology for grain structure control, breaking through the traditional approach of additive manufacturing that relies on process parameters such as laser power and scanning speed to adjust grain size. It introduces powder particle size, an intrinsic physical parameter of the material, as the core control variable, achieving effective control of grain structure through the differences in melting and solidification behavior of powders with different particle sizes. Its core technical feature lies in using titanium alloy powder with the same alloy composition, obtaining powders with different particle size ranges through sieving, fundamentally avoiding the instability in microstructure and properties caused by compositional gradients or multi-material interfaces. By partitioning, layering, or periodically alternating the deposition of powders with different particle sizes during additive manufacturing, spatially controllable regions of microstructure difference are constructed within the component. Furthermore, while maintaining constant key process parameters such as laser power and scanning speed, grain structure control is achieved solely through changes in powder particle size, significantly improving process stability and repeatability. The bimodal heterogeneous grain structure of this invention is formed in situ during the forming process, without relying on post-forming heat treatment or external field assistance. The particle size switching method can be flexibly adjusted according to preset rules, and the technical solution is applicable to powder-feed laser additive manufacturing processes, possessing good equipment adaptability and engineering versatility. The technical essence of this invention is to utilize the differences in absorption characteristics, thermal inertia, and molten pool formation behavior of powders with different particle sizes under laser irradiation to induce the formation of microstructures with different grain sizes in different regions, providing a completely new path for the preparation of high-performance titanium alloy components.

[0099] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art should understand that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. An additive manufacturing method for a titanium alloy with a bimodal heterogeneous grain structure, characterized in that, Includes the following steps: In an inert gas protected environment, a powder-feeding laser directional energy deposition device is used to deposit layers on a substrate. Fine titanium alloy powder with a particle size of 20-63 μm and coarse titanium alloy powder with a particle size of 90-150 μm are alternately fed in. After every 1-10 layers of powder with the same particle size are deposited, the powder with the other particle size is switched. After the deposition is completed, the bimodal heterostructure titanium alloy is obtained. The process parameters for the forming process include: laser power 800-1200 W, scanning speed 4-8 mm / s, single-layer deposition thickness 0.6-0.8 mm, powder feed rate 6-10 g / min, and laser defocusing amount 2-4 mm; the difference in median diameter between the coarse titanium alloy powder and the fine titanium alloy powder is 70-100 μm.

2. The additive manufacturing method according to claim 1, characterized in that, The flow rate of the inert gas is 15-20 L / min.

3. The additive manufacturing method according to claim 1, characterized in that, The laser in the powder-feeding laser directional energy deposition equipment adopts a ring laser output mode.

4. The additive manufacturing method according to claim 1, characterized in that, The substrate is a titanium alloy plate.

5. The additive manufacturing method according to claim 1, characterized in that, Before forming, the substrate is pretreated, including mechanical polishing, ultrasonic cleaning and drying.

6. The additive manufacturing method according to claim 1, characterized in that, The titanium alloy is TC4 titanium alloy, TA15 titanium alloy, TC18 titanium alloy, Ti-6Al-7Nb titanium alloy, or Ti-22Al-25Nb titanium alloy.

7. The additive manufacturing method according to claim 1, characterized in that, The titanium alloy fine powder and titanium alloy coarse powder are obtained by sieving and classifying titanium alloy powder prepared by gas atomization in the same batch using a mechanical vibrating screen.

8. The additive manufacturing method according to claim 1, characterized in that, During the forming process, one of the following methods is used as the scanning path: parallel straight line scanning, circular scanning, or polygonal scanning.

9. The additive manufacturing method according to claim 1, characterized in that, The process parameters for forming remain constant throughout the entire process.

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