Additive manufacturing of ultra-high strength steels: A review

Ultra-high strength steels (UHSSs) have excellently comprehensive mechanical properties, which has attracted significant interest in their advanced manufacturing. Additively manufactured UHSSs have a unique microstructure that provides significant potential to achieve good mechanical properties. This paper reviews the latest research progress in the additive manufacturing (AM) of UHSSs with a special focus on defect optimization, microstructure-property regulation and the application of advanced assisted techniques for additively manufactured UHSSs. The property origin of UHSSs is firstly summarized in detail, including the influence of different alloying elements and phase composition in the microstructure on the mechanical properties of UHSSs. The AM techniques used to fabricate UHSSs and the characteristics of their forming microstructure are further illustrated, which involve laser powder bed fusion (L-PBF), laser direct energy deposition (L-DED), and wire arc additive manufacturing (WAAM). The research progress on the three typical techniques for UHSSs is also reviewed. Moreover, the property regulation methodology of UHSSs is reviewed, mainly in the optimization of defects and the adjustment of the whole forming process. Significantly, the shortcomings of the present researches on AMed UHSSs and future challenges for development are discussed. This work is important for a better scientific understanding of AMed UHSSs and provides vital meaningful guidelines for the future work in UHSSs manufacturing.