High-Throughput Phase Screening and Laser-Directed Energy Deposition of Ti-Ni-Nb Gradient Alloys

Abstract

This work presents an integrated directed energy deposition (DED) approach utilizing a multi-powder feeder with real-time continuously variable composition functionality, a multi-powder mixer and a multi-powder nozzle to fabricate Ti-Ni-Nb gradient alloys with controlled compositional variations. The high-throughput methodology enables rapid alloy design and optimization by allowing precise manipulation of chemical composition and phase structures within a single deposited track. The EDS analysis confirms a gradual increase in titanium content, a nearly constant nickel content and a decrease in niobium along the scanning path, aligning with the expected powder-feeding trends. X-ray diffraction (XRD) analysis further reveals a phase transition from niobium-rich intermetallic compounds (NbNi₄, Nb₈Ni) at the beginning of the deposition to titanium-rich phases (Ti, Ti₂Ni) at the end, demonstrating the ability to tailor phase distributions through real-time composition control. This high-throughput methodology enables rapid alloy design and optimization by integrating theoretical predictions with experimental phase screening. This study establishes a novel framework for the rapid discovery and optimization of functionally graded materials, paving the way for advanced applications in aerospace, biomedical implants and high-performance structural components.

1. Introduction

Multi-material additive manufacturing is the process of continuously joining multiple materials for numerous tiny melting pools to make heterogeneous parts from three-dimensional model data, usually layer upon layer, as opposed to traditional subtractive manufacturing and formative manufacturing methodologies. It provides significant advantages in fabricating continuously real-time variable composition and structural gradients for metallic materials [1,2,3]. Specifically, directed energy deposition (DED) using a laser beam, where powder-feeding ratios and other parameters can be manipulated in real-time during processing, is more suitable for fabricating the heterogeneous alloy [4,5,6]. However, the real-time variable composition deposition has a huge challenge: achieving the continuous uniformly and variably feeding of multiple powders.

In the last decade, most researchers focused on graded metals and noncontinuous variable metals. Hofmann et al. [7,8,9] provided an overview of multi-material additive manufacturing, with emphasis on compositionally graded metals designed with the help of phase diagrams. W. Li et al. [10,11] controlled the metallurgical quality of interfaces between dissimilar materials by DED. According to these results, the different critical metallurgical incompatibilities between metals were identified as preventing their sound combination by additive manufacturing. To circumvent such incompatibilities, various studies proved the use of appropriate intermediate material layers to be an effective solution. For instance, Eliseeva et al. [12,13] proposed an algorithm for automatic optimization of the multi-metallic gradient transition between incompatible metals based on equilibrium phase calculations, followed by manufacturing via DED.

Recent experimental advancements have focused on improving real-time powder feeding and mixing strategies to enhance composition control. Studies have demonstrated the effectiveness of multi-powder mixers in stabilizing and homogenizing powder flow, thus improving deposition uniformity [14,15]. From a theoretical perspective, computational methods have played a crucial role in optimizing multi-material deposition processes. Computational fluid dynamics (CFD) simulations have been employed to study powder flow characteristics, assisting in the design of efficient powder feeders and nozzles [16,17].

Current works [18,19,20] have presented the multi-powder mixer details and verified the mixer’s stability and uniformity utilizing directed energy deposition. However, these studies do not provide a real-time variable composition and structural gradients for metallic materials. A dynamic composition adjusting approach is still urgent for multi-material directed energy deposition.

This work presents a novel additive manufacturing approach utilizing a multi-powder feeder with real-time continuously variable composition function, a multi-powder mixer and a multi-powder nozzle to fabricate the Ti-Ni-Nb variable composition single-track deposited layer. This approach offers a significant advantage in terms of material properties and practical applicability. Enabling precise control over the composition of Ti-Ni-Nb during deposition facilitates the creation of functionally graded materials (FGMs) with tailored mechanical and thermal properties. This approach also accelerates alloy development by eliminating the need for conventional, lengthy and costly casting and thermomechanical processing. Furthermore, its capability to fabricate complex multi-material structures with precise compositional control enhances its industrial adaptability, particularly in aerospace, biomedical and energy applications where customized material performance is critical. By bridging the gap between experimental alloy development and industrial-scale production, this technology paves the way for the next generation of multi-material additive manufacturing.

2. Materials and Methods

2.1. Materials

Previous research results have proved that these mixed powders perform excellent in situ alloying characteristics among pure titanium powder, pure nickel powder and pure niobium powder under laser irradiation. Therefore, pure titanium powder, pure nickel powder and pure niobium powder with a particle size of titanium of 15~53 μm, nickel of 53~105 μm and niobium of 15~53 μm and a purity of 99.99 wt.% are selected as raw powder materials. Figure 1 shows the SEM (scanning electron microscopy) morphology and laser particle size distribution of titanium, nickel and niobium powders. These figures provide a detailed comparison of the physical characteristics of the three metal powders. All powder particles exhibit an excellent spherical shape. In Figure 1a, the D10, D50 and D90 of titanium powder are 18.02 μm, 34.26 μm and 55.40 μm, respectively. In Figure 1b, the D10, D50 and D90 of nickel powder are 9.69 μm, 28.67 μm and 53.59 μm, respectively. In Figure 1c, the D10, D50 and D90 of niobium powder are 25.12 μm, 38.26 μm and 56.09 μm, respectively. A pure titanium plate with a size of 150.0 × 150.0 × 4.0 mm as the build platform was machined in this work. The argon with a purity of 99.99 wt.% was used for powder feeding carried gas and preventing the melting pool from being oxidated. These optimal depositing process parameters have a laser power of 600 W, a laser scanning speed of 10 mm/s and a laser spot diameter of about 1.0 mm with a uniform energy distribution.

2.2. Manufacturing

The powder feeding system of the self-designed novel multi-material directed energy deposition additive manufacturing equipment consists of three key components: a three-channel powder feeder with a real-time variable composition function, a three-channel powder mixer and a circular coaxial multi-powder nozzle.

The three-channel powder feeder enables independent and precise control over the feeding rates of different powders, allowing for real-time adjustments to the material composition during the deposition process. This dynamic control capability ensures the seamless transition of material properties along the deposited track, making it possible to fabricate functionally graded structures with tailored characteristics.

The three-channel powder mixer is designed to ensure thorough and uniform blending of the different powder materials before they reach the nozzle. This minimizes composition fluctuations and enhances the consistency of the deposited layer, contributing to improved material homogeneity and mechanical properties.

The circular coaxial multi-powder nozzle is engineered for efficient and uniform powder delivery into the laser-melted pool. Its optimized structure ensures precise powder convergence, reducing material loss and improving deposition efficiency. The coaxial design also enhances the interaction between the powder stream and the laser beam, promoting stable and high-quality material deposition.

During the powder feeding process, a dual argon shielding strategy is employed to protect the molten pool from oxidation, particularly to prevent titanium (Ti) from reacting with oxygen. The first layer of protection comes from the argon gas used in the powder feeding system, which carries the powder particles and forms an inert atmosphere around the deposition area. The second layer of protection is provided by the argon gas blown directly from the center of the nozzle, creating a concentrated shielding effect over the molten pool. This dual shielding mechanism effectively minimizes oxygen exposure, ensuring a high-purity deposition environment and preventing oxidation-related defects in the fabricated material.

Overall, this advanced powder feeding system provides a robust and adaptable solution for multi-material directed energy deposition, enabling real-time composition control, precise material distribution, and improved process stability. It plays a crucial role in enhancing the flexibility and precision of additive manufacturing for high-throughput phase screening and functionally graded materials. Figure 2 shows the schematic diagram of the novel real-time continuously variable composition-directed energy deposition.

In Figure 3, the titanium powder feeding volume is increased linearly from 0 to 0.52 cm³, the nickel powder feeding volume is constant at 0.26 cm³, and the niobium powder feeding volume is decreased linearly from 0.52 cm³ to 0. This means that the powder feeding mass of titanium powder is increased linearly from zero to a maximum value, nickel powder as a constant value, and niobium powder is decreased linearly from a certain value to zero. The powder feeder can fix the total amount of powder feeding in the three channels in unit time. In the building process, the three-channel powder feeders are open at the same time, and the duration time for mixing is all set for 10 s, respectively. Thus, a length of 100 mm deposited layer in the build platform was acquired at the laser scanning speed of 10 mm/s. The deposited layer sample was divided into 10 units every 10.00 mm along the direction of the laser scanning speed. The energy dispersing spectrum (EDS) element analysis based on a 45 sampling points random strategy and the X-ray diffraction (XRD) phase constitution based on a representative sampling strategy was implemented to verify the reliability of this unique approach presented in this work.

2.3. Characterization

To analyze the real-time variable composition directed energy deposition (DED) layer, energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were performed to investigate elemental distribution and phase evolution, respectively.

For EDS analysis, the deposited layer was first sectioned using wire electrical discharge machining (WEDM) to ensure precise and damage-free cutting. The cross-sectional samples were then mounted in epoxy resin and polished to a smooth surface. Scanning electron microscopy (SEM) equipped with an EDS detector was used to conduct elemental mapping in the direction of laser scanning. This analysis revealed the mass norm percentage variations in titanium (Ti), nickel (Ni) and niobium (Nb) along the deposited layer. The observed trends were correlated with the real-time variable powder feeding rates to validate the control strategy.

For XRD analysis, three key positions—the beginning, middle and end of the deposited layer—were selected for phase characterization. Thin sections were extracted and polished for accurate diffraction measurements. The X-ray diffractometer was used to scan the samples over a 2θ range of 20–90° with fine step sizes to identify phase compositions. Micro-XRD measurements were also performed at ten regularly spaced points along the scanning direction to capture the continuous phase evolution.

By combining EDS and XRD analyses, the study effectively demonstrated the relationship between real-time powder-feeding variations and material composition evolution in the DED process.

3. Results and Discussions

Figure 4 presents the EDS analysis results of a single-track deposited layer under the real-time continuously variable powder feeding rate. From Figure 4a, the mass norm percent of titanium element gradually continuously increases along the scanning speed direction. In Figure 4b, the mass norm percent of nickel element does not change. In Figure 4c, the mass norm percent of niobium element gradually continuously decreases. In Figure 4d, the relationship between the rotation speed (Revolutions Per Minute, RPM) and time of the powder disk of titanium, nickel and niobium powder. Figure 4e shows the relationship between powder feeding volume per second (cm³/s) and powder feeding time of titanium, nickel and niobium. Figure 4f shows the diagram of the single-track deposited layer with a mark. These results reveal that the mass norm percent of titanium, nickel and niobium is a real-time variable along the laser scanning direction. It is interesting that the trends are consistent with the experimental expectation due to the three powders in accordance with the linear powder feeding volume per second (cm³/s) change trends. We can also find that the absolute value of the slope between the mass norm percent curve of the titanium element and the mass norm percent curve of the niobium element results from the loose packing density when the volume of the three elements is equal. The slope of the mass norm percent curve of the nickel element is approximately zero. These phenomena indicate that a real-time, continuously variable composition-directed energy deposition strategy is reliable.

The deposited layer illustrated in Figure 4f indicates that a thermal reaction occurred in the laser-melted pool among three different kinds of powders. The products of thermal relevant reaction generated by the laser irradiation are probably different along the laser scanning direction. To preliminarily explore the law of phase constitution changing with the distance from the laser scanning beginning position, we selected three key points of the deposited layer as the sampling points. Three samples were cut using the wire electrical discharge machining method and then polished for X-ray diffraction phase analysis at the beginning, middle and end of the deposited layer (Figure 5). Figure 5a–c shows the phase constitution of the above three samples. The data in Figure 5a shows that NiTi, Ni₈Nb and Ni₄Nb intermetallic compounds are generated at the beginning of the layer; Figure 5b shows that NiTi, Ti₂Ni and Ni₄Nb intermetallic compounds are generated at the middle of the layer; and Figure 5c shows that NiTi and Ti₄Nb intermetallic compounds are generated at the end of the layer. It is obvious that the NiTi always exists in the layer regardless of the ratio of the three powders. This indicates that the nickel element has a good affinity with the titanium element in the laser-melted pool. It is also apparent from these figures that Ni-Nb phases are more likely to generate than Ti-Nb phases. This means that the Ni-Nb phases are priority phases when the niobium element content is sufficient. According to these data, we can infer that the chemical formulas of these three alloys are Ti_11.6Ni_31.4Nb_57.0, Ti_14.8Ni_47.7Nb_37.5 and Ti_38.2Ni_44.3Nb_17.5, respectively.

To further study the phase evolution in the deposited layer along the laser scanning speed, ten sampling points are selected for micro-XRD analysis to show the benefit of continuously variable powder composition. Regular micro-XRD point measurements along the laser scanning speed could show the key point of this paper. Figure 6 presents the phase evolution of the deposited layer during the directed energy deposition process with real-time variation in the powder feeding ratio of titanium, nickel and niobium powder. The powder feeding ratio of titanium powder gradually increases, nickel powder remains constant, and niobium powder gradually decreases. Based on this information, the phase evolution follows these trends: for titanium element dominant phase formation, as the titanium powder feeding rate increases, the titanium-rich phases such as Ti and Ti₂Ni appear more frequently. For nickel element-related phases, since the nickel powder feeding rate remains constant, the compounds formed by any combination of two elements among titanium, nickel and niobium, including Ti₂Ni, NiTi and NbNi₄, consistently appear in the middle regions. The presence of Ni₃Nb and Ni phases suggests that nickel interacts with both niobium and titanium, but as the niobium powder feeding rate decreases, niobium-associated nickel compounds diminish. For niobium element-related phase reduction, at the initial stage, niobium-rich compounds such as NbNi₄ and Nb₈Ni are present, indicating a higher niobium element concentration. As the niobium powder feeding rate decreases, the presence of niobium-containing phases such as NbNi₄ and Ni₃Nb becomes less frequent. Towards the final regions, NbTi₄ appears, showing a stronger interaction between titanium and niobium as titanium content becomes dominant. From Figure 6, we can also observe an obvious overall trend that the transition from niobium-rich intermetallic compounds (NbNi₄, Nb₈Ni) to titanium-rich phases (Ti, Ti₂Ni) indicates a progressive shift in composition. The stable presence of nickel phases (NiTi, Ni₃Ti, Ni) suggests that nickel remains a key component in the phase distribution, but its interactions shift from niobium to titanium due to the changing powder ratio. The gradual disappearance of niobium dominant phases and the emergence of titanium dominant phases (e.g., Ti, Ti₂Ni) align with the increasing titanium powder feeding rate in the feedstock. These results indicate that the phase evolution in the deposited layer follows the expected trend based on the real-time powder feeding rate variation: niobium-related phases diminish, titanium-rich phases increase, and nickel-related phases maintain a presence but shift interactions from niobium to titanium. This demonstrates how composition control in directed energy deposition can effectively tailor the resulting phase distribution [21,22].

4. Conclusions

This study demonstrates the effectiveness of a real-time, continuously variable powder feeding strategy in directed energy deposition (DED) for achieving controlled composition variations along the laser scanning direction. The EDS analysis confirms that the mass norm percentage of titanium increases, nickel remains constant, and niobium decreases along the scanning path, aligning with the expected trends dictated by the controlled powder feeding rates. The observed changes in elemental composition directly impact the phase evolution in the deposited layer.

X-ray diffraction (XRD) analysis at different positions along the deposited layer reveals a progressive transition in phase constitution. Initially, niobium-rich phases such as NbNi₄ and Nb₈Ni dominate, but as the titanium powder feeding rate increases and the niobium rate decreases, titanium-rich phases (e.g., Ti and Ti₂Ni) become more prevalent. The nickel element consistently interacts with both titanium and niobium, but its associated phases shift from niobium-rich intermetallic to titanium-dominant compounds as deposition progresses.

The observed phase evolution supports the hypothesis that real-time control over powder composition enables precise tailoring of the resulting material properties. This approach provides a reliable method for high-throughput phase screening and gradient alloy production with site-specific compositions and microstructures. The findings highlight the potential of real-time composition control in DED to optimize material characteristics for advanced manufacturing applications, offering a pathway toward customizable alloy systems with tailored mechanical and chemical properties.