Fig. 1. Schematic of two mainstream categories of LAM technologies^[19]. (a) LMD; (b) LPBF

Fig. 2. Schematic of metalatmosphere reactions during LAM processing. (a) Direct-deposition LAM^[34]；(b) powder-bed-based LAM^[28]

Fig. 3. Influence of shielding atmospheres on the 2Cr13 melting tracks^[36]. (a) Melting track deposited in pure Ar; (b) melting track deposited in pure N₂

Fig. 4. Pure Ti LPBF-printed in Ar-N₂ hybrid reactive atmosphere^[28]. (a) 3D-reconstruction of defects in Ti samples printed by the same laser parameters but in different shielding atmospheres; (b) 3D-atom probe analysis results of one-dimensional profiles of N, O, and C and nearest neighbor distribution of N atoms (GB is acronym of grain boundary, and blue region represents the nearest neighbor analyzed region)

Fig. 5. Microstructural images of Ti-6Al-4V LPBF-printed in pure Ar and in Ar-N₂ hybrid reactive atmospheres^[41]. (a) Sample printed in pure Ar; (b) sample printed in low-N₂-containing atmosphere; (c) sample printed in medium-N₂-containing atmosphere; (d) sample printed in high-N₂-containing atmosphere; (e)－(h) magnifications of graphs Fig. 5(a)－(d)

Fig. 6. Microstructure and performance of Ti-6Al-4V＋TiC composite LPBF-printed in Ar-CH₄ hybrid reactive atmosphere^[32]. (a) TEM image and EDS analyzed region; (b)－(f) EDS mapping of C, Ti, Al, and V elements; (g)－(k) high-resolution TEM images of TiC nano-grains and corresponding FFT patterns; (l) HV hardness of Ti-6Al-4V and Ti-6Al-4V＋TiC composites; (m) room temperature uniaxial compressive curves of three materials

Fig. 7. 316L stainless steel LPBF-printed in Ar-O₂ and Ar-N₂ hybrid reactive atmospheres (TS, YS, Elong, and IE are acronyms of tensile strength, yield strength, elongation at fracture, and Charpy impact energy, respectively)^[44]. (a) Mechanical properties of 316L printed in Ar-O₂ atmospheres; (b) mechanical properties of 316L printed in Ar-N₂ atmospheres; (c) oxide particles in 316L printed in an Ar-O₂ atmosphere; (d) nitride particles in 316L printed in Ar-N₂ atmosphere

Fig. 8. CoCrFeMnNi high entropy alloy LPBF-printed in pure Ar (N-0) and Ar-50%N₂ (N-50) hybrid reactive atmospheres^[31]. (a)(b) Dislocation cell structure in samples printed in pure Ar and in Ar-50%N₂ atmospheres; (c)－(f) high-resolution TEM images and schematic of ordered nitrogen complex; (g) room-temperature uniaxial tensile curves of two types of samples

Fig. 9. TEM images of AlCu5MnCdVA alloy LPBF-printed in Ar-O₂ hybrid reactive atmospheres^[50]. (a)(b) Oxide particles in samples printed in high- and low-O₂-containing atmospheres; (c)(d) magnifications and SAED patterns of oxide nano particles

Fig. 10. Images of LMD-printed 316L sample using Ar-air mixture as shielding atmosphere^[34]. (a) SEM image; (b) TEM image with SAED pattern; (c) diameter distribution of oxide particles; (d) room-temperature uniaxial tensile curves of four types of samples

Fig. 11. Microstructural images of Ti-6Al-4V and TNZT alloys, LMD-printed in various shielding atmospheres^[56]. (a) Ti-6Al-4V printed in pure Ar; (b) Ti-6Al-4V printed in pure N₂; (c) TNZT printed in pure Ar; (d) TNZT printed in pure N₂; (e) nano-scale precipitates within the sample shown in Fig. 11(d)

Fig. 12. A compositional-modified 316L, atomized and LPBF-printed in pure N₂ and standard 316L LPBF-printed in pure Ar^[64].(a) Room temperature uniaxial tensile curves; (b) current/potential curves tested in the 0.5 mol/L H₂SO₄ solution

Fig. 13. Ti powder modified in pure N₂ reactive atmosphere^[67-69]. (a) Schematic of heat treatment modification; (b) effects of powder modification; (c) room temperature uniaxial tensile properties of LPBF-printed samples using various N₂-modified Ti feedstock powders

Fig. 14. Influence of ball milling in Ar-air hybrid reactive atmosphere on the feedstock Ti powder and the corresponding bulk samples^[23]. (a) Unmodified HDH Ti powder with no oxidized nanocrystalline shell and modified ball milling Ti powder with oxidized nanocrystalline shell; (b) microstructural comparison between LPBF-printed samples using two types of feedstock Ti powders

Fig. 15. Influence of dissolved oxygen on the deformation behaviors of pure Ti and TiZrHfNb high entropy alloy (OOC is acronym of ordered oxygen complex)^[45,82]. (a) Compression curves of Ti single crystals with various O mass fraction and corresponding TEM images before and after compressing; (b) TEM image demonstrating dislocation pinning effect of OOCs in TiZrHfNb sample; (c) schematic of effects of OOCs on dislocation behavior in TiZrHfNb sample

Fig. 16. Composites in metal matrix and their influence^[68,72,84]. (a) NbC precipitation particles in maraging steel; (b)(c) oxidized film of Cu powder after reactive atmospheric modification and oxides within film; (d) effect of N content on grain sizes of three LPBF-printed Ti samples

Table 1. Comparison of hardness and wear properties of Ti-6Al-4V and TNZT alloys, LMD-printed in different shielding atmospheres^[56]

Table 2. Representative metallic materials fabricated using reactive atmospheric LAM and their variations in properties