The laser fabrication of Ti6Al4V (TC4) alloy in an atmospheric environment is susceptible to nitrogen (N) and oxygen (O); consequently, defects such as cracks can occur because of the induced embrittled nitride and oxide phases. Therefore, the industrial application of LDED (laser direct energy deposition) -treated titanium alloy components has been severely hindered by the limited space and high cost in the closed environment. In this study, a novel nozzle with a protective hood was designed for the laser additive manufacturing of TC4 alloys to alleviate the adverse effects of N and O in an atmospheric environment. The microstructures and mechanical properties of the as-deposited TC4 specimens with and without hoods (named TC4-Y and TC4-N, respectively) were evaluated. A functional prototype of the multiflow-path nozzle was developed using computational fluid dynamics (CFD) simulations with species transport and the k?ε gas model. This study significantly benefits the laser fabrication of low-cost and high-performance Ti components in various industrial fields.

Gas-atomized TC4 powder with an average size range of 75 μm was employed to fabricate LDED-treated specimens using an FL-1500 1.5 kW fiber laser. The processing parameters were set as follows: laser power, 500 W; scanning speed, 600 mm/min; and powder delivery rate, 4.85 g/min. Both the central and side gases are high-purity Ar (99.99%) and were flowed at a rate of 10 L/min. CFD simulations of the gas flow adjacent to the substrate surface, which was located 2.5 mm away from the nozzle of the air hood, were performed to evaluate the effectiveness of the hood. To investigate the microstructural evolution of the LDED-treated TC4 alloy, the samples were polished and then etched with Kroll's reagent. The phase compositions were determined using a Miniflex600 X-ray diffractometer (XRD). The microstructure was investigated using a MERLIN scanning electron microscope (SEM) operated at an accelerating voltage of 20 kV and a JEOL-2100 transmission electron microscope (TEM) operated at 200 kV. The mechanical properties of the samples were evaluated using an HVS-1000 microhardness tester and a PWS-E100 universal testing machine.

The simulation results indicate that the facet average mass fractions of N₂ and O₂ reduced significantly from 1.628×10^-3 to 2×10^-4 and from 4.37×10^-4 to 5.4×10^-5, respectively (Fig. 3), which agree well with the experimental results. The TC4-N specimen is composed of needle-like α′ martensite, Widmanst?tten α-laths, β-phase, and nitrides (Figs. 6?8). By applying the protective hood, the TC4-Y specimen exhibits a decrease in α/α′ martensite content, an increase in the β-phase fraction, and the precipitation of Ti₃AlC₂ phase (Figs. 6?8). The average microhardness values of the TC4-N and TC4-Y specimens are 410 HV_{and 365 HV(Fig. 4), respectively. The higher microhardness of the TC4-N specimen is primarily due to the in-situ formation of hard nitride TiN (2900 HV) during LDED. In comparison, the TC4-Y samples indicate a slightly lower value (365 HV) that is equivalent to those fabricated in a chamber filled with an inert gas (316?369 HV). Under the protection of the hood, the TC4-Y samples exhibit an average UTS of 1037 MPa, a YS of 952 MPa, and an EL of 10.2% (Fig. 5), which are comparable to those of TC4 counterparts achieved in a closed environment. This demonstrates the effectiveness and feasibility of the protective hood.}

The newly designed protective hood effectively eliminates the adverse effects of N and O. A CFD simulation was conducted, which demonstrated that the hood successfully prevented contamination by impurities, including N and O. The N and O mass fraction adjacent to the sample surface decreased by 1.38×10^-3 and 5.7×10^-4, respectively. The TC4-N specimen is composed of needle-like α′ martensite, Widmanstatten α-laths, β-phase, and nitrides. The TC4-Y specimen primarily comprises coarsened α′ martensite, Widmanstatten α-laths, Ti₃AlC₂ nanoprecipitates, and β-phase. Under the synergistic effect of refinement strengthening, solid-solution strengthening, and second-phase strengthening, the TC4-N specimen exhibits higher levels of strength (UTS of 1249 MPa, YS of 1028 MPa) and microhardness (410 HV). By contrast, an exceptional combination of high strength (UTS of 1037 MPa, YS of 952 MPa) and high ductility (10.2%) is achieved owing to the presence of α/α′ with a low aspect ratio, a high fraction of β-phase, and Ti₃AlC₂ nanoprecipitates in the TC4-Y specimen. This study reports a simple yet effective approach for producing LDEDed TC4 alloys with outstanding mechanical properties in an atmosphere, which significantly benefits industrial applications.