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Page 4 of 21 Zhou et al. J Mater Inf 2022;2:18 https://dx.doi.org/10.20517/jmi.2022.27

[52]
Figure 2. Microstructures with concentration modulation and mechanical properties of as-printed Ti-6Al-4V+(4.5 wt.%) 316L alloy .

unfavorable properties. Compared with the FDM and BJ processes, metallic powders go through sufficient
melting and solidification during the PBF and DED processes, leading to near-fully dense parts with
excellent properties. PBF processes enable the fabrication of components with high precision by selectively
melting a pre-laid powder on an even substrate using a focused high-energy laser beam or electron beam.
Unlike PBF processes, DED processes enable the creation of parts on even and uneven substrates by melting
the powder or wire using a laser beam, electron beam or electric arc. Therefore, PBF processes are usually
used to fabricate the parts with complex structures, while DED processes are usually used to repair
engineering and aerospace components. Moreover, DED processes also have significant feasibility in
[54]
making functionally graded metallic materials .

Both PBF and DED processes undergo ultrafast heating and solidification due to the instantaneous
interaction between the high-energy beam and metal powder, which can be considered as nonequilibrium
[55]
processing . Therefore, the ultrafine microstructure of the as-printed samples can be obtained, leading to
[56]
excellent properties . More importantly, these AM processes provide a unique method to in situ design
high-performance alloys by changing the compositions and microstructures within the molten pool. For
example, Zhang et al. in situ designed an advanced Ti alloy by doping a small amount of 316L stainless steel
[52]
to Ti-6Al-4V using laser powder bed fusion (L-PBF), also known as selective laser melting (SLM) . The
micro-concentration modulation was obtained through the partial homogenization of the two alloy melts
within the molten pool during L-PBF. As illustrated in Figure 2, the as-printed sample showed a fine
scale-modulated β + α′ dual-phase microstructure, exhibiting a progressive transformation-induced
plasticity effect. Therefore, a high tensile strength of ~1.3 GPa with a uniform elongation of ~9% and an
excellent work-hardening capacity of > 300 MPa was successfully achieved in the as-printed samples.
Considering these unique features, PBF and DED processes show significant advantages in making
advanced metallic materials with designed microstructures and associated superior properties.

Key issues for HEA design and AM
[4,57]
In recent years, HEAs and AM, as two hot topics, have been extensively studied . However, there are still
some key issues that need to be resolved. As mentioned above, the development of single-phase HEAs may
have reached a bottleneck in terms of property optimization. Precipitation-strengthened HEAs were
therefore designed to break the strength-ductility trade-off of single-phase HEAs. Designing