Additive manufacturing (AM) technology plays a vital role as a new process capable of manufacturing complex-shaped products with functionalities that are difficult to fabricate via conventional processes. In addition, AM technology has been widely recognized one of the essential processes for “smart factory′ in near future due to digital manufacturing technology. In this paper, the trends of the development of powder bed fusion (PBF) , directed energy deposition (DED) , binder jetting (BJT) and material extrusion (MEX) machines were reviewed. Moreover, the trends of the development of metallic materials for AM were introduced including the research results of the author′s group. Finally, a new development on AM technology was stated.

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3D Additive Manufacturing is the heaven-sent child for Internet of Things (IoT) in the digital age, and is a process that can be used for customized design and production. In particular, metal Additive Manufacturing enables not only the fabrication of complex shapes but also the control of crystal orientation at the atomic level by locally melting and solidifying the metal materials, thereby realizing high functionality of the product through simultaneous design of shape and materials properties. Therefore, it is expected to be applied in various social infrastructure fields including medical, energy-related,aerospace, and automotive, and also as a means of adding high value. In this review article, the new manufacturing concept that can be realized by metal Additive Manufacturing is introduced.

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Casting is a process characterized by free shape creation by pouring molten metal into a mold, and is useful for manufacturing parts that have a function in the internal shape. The technology for directly molding by 3D printer using inkjet technology is useful for molding of complex-shaped molds and cores with undercuts that cannot be transferred from a model, and also useful for improving mold cavity accuracy through integral molding. Since this technology is essentially capable of modeling at a higher speed than other 3D printers, it is possible to mass-produce thousands to tens of thousands of products per month. As a result of development aimed at speeding up these sand-molded 3D printers, effects have been recognized for speeding up rapid trial production, small-quantity production, weight reduction, integration of multiple functions, integration of multiple components and mass production application.

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Metal additive manufacturing (AM) is attracting focus as the new manufacturing technology. Metal powder is used in two major metal AM methods, powder bed fusion (PBF) and directed energy deposition (DED) . The powder specifications including preferred powder diameter distribution depend on the AM methods, and proper selection of powder feedstock is important to realize the build quality improvement. The unique heat history characterized by the local fusion and rapid solidification in PBF and DED affords the characteristic microstructures in the build products. Some novel materials are proposed to utilize the benefits of AM process features. Multi-principal-element alloys or high entropy alloys are successfully obtained by selective laser melting (SLM) and they exhibit excellent mechanical properties. Ni-Cr-Mo corrosion-resistant alloy, MAT21^® is applied to SLM and the product realizes superior tensile strength and hardness keeping its excellent corrosionresistant properties. Selective laser melting of metal magnets is proposed for the low-volume productions of cast magnets. Optimization of the process parameters provides the good magnetic properties of SLM magnets relevant or superior to those of corresponding cast ones.

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Additive Manufacturing (AM) is the process that can produce parts with the complex shapes that are difficult to be fabricated by conventional techniques. AM is classified into seven categories, of which Directed Energy Deposition (DED) method and Powder Bed Fusion (PBF) method are the processes for directly melting and solidifying metal powders. In PBF method, it is important to form a uniform powder bed. Firstly, we explain the properties required for metal powders suitable for PBF.
In PBF, electron beam (EB) or laser beam (LB) is utilized as the heat source. In the case of using LB, pure Cu is especially difficult to be built because of the high reflectance for laser light with a wavelength near 1060 nm generally used in PBF and high thermal conductivity. Therefore, alloying has been attempted to facilitate the building of Cu-base metal. Secondly, as an example of the Cu alloy building, we report that the Cu-Cr model after aging treatment showed Vickers hardness superior to that of the arc-melted material because Cr could be dissolved in supersaturation due to the high cooling rate of PBF process

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