This special feature is edited in accordance with JVSS symposium held on December 2024 of the frontier of technology and application of metal additive manufacturing. One comprehensive report along with seven technical reviews will enhance the knowledge of users and manufactures of the advanced technology and application of metal additive manufacturing.

Additive Manufacturing (AM), popularly known as 3D printing, has been dramatically attracting attention as a breakthrough technology in advanced manufacturing due to capability of producing 3D complex structure and integrating many parts. ISO/ASTM standard categorized AM processes into seven categories. In this paper, the recent trend and perspective of metal AM, especially powder bed fusion (PBF), directed energy deposition (DED), binder jetting (BJT), and material extrusion (MEX), are described. However, AM technology is prone to lead to the generation of defects due to its intrinsic properties. Therefore, an in-process monitoring and feedback system was developed to assure the final product quality and process repeatability in the New Energy and Industrial Technology Development Organization (NEDO) project (FY2019-FY2023) conducted by TRAFAM. The results of the project on the in-process monitoring and feedback system are introduced.

This paper introduces AM application cases to aerospace products and AM technology development in MHI.

Single-crystals exhibit excellent creep resistance at high temperatures and are used as high-temperature components in jet engines. Recently, the number of additive-manufactured components used in jet engines has increased. However, single-crystal structure fabrication has been difficult via laser powder bed fusion (LPBF) which is the most widely adopted process for the additive manufacturing of metallic components. In this paper, grain growth and single-crystal structure formation in LPBF using a flat-top laser with a uniform intensity distribution were described for pure Ni. In addition, a single-crystal beta-grain structure was obtained for a near-beta Ti alloy using a flat-top laser. A unique variant of the alpha phase with ⟨2-1-10⟩ along the build direction was formed during heat treatment and air cooling, which can enhance the anisotropy of the mechanical properties. Furthermore, the selection of the alpha variant was discussed.

The vacuum performance of the Ti-6Al-4V vacuum chamber, which was produced by additive manufacturing (AM), was measured. It was shown that a 10^－6 Pa ultra-high vacuum was achieved by exhausting with a turbomolecular pump. The residual gases in the AM chamber were H₂, H₂O, CO, CO₂, etc. as in a general machined stainless steel vacuum chamber. A vacuum with the order of 10^－9 Pa was achieved by exhausting with a turbomolecular pump after baking. It was also shown that outgassing can be significantly reduced by vacuum firing, which is a heat treatment at high temperature in a vacuum furnace. We also developed a prototype vacuum transfer case with NEG coating by utilizing the roughness of the AM surface. The NEG-coated AM chamber achieved an order of 10^－8 Pa without a vacuum pump. The results of this study show that metal AM can be utilized in the field of UHV.

Electron beam powder bed fusion (EB-PBF), one of additive manufacturing, utilizes an electron beam as a heat source and has advantages such as a scanning strategy with high-speed operation, high thermal conversion efficiency, and residual stress relief by preheating process. On the other hand, it has inferior surface roughness, a powder scattering phenomenon known as smoke, and the need to remove pre-sintered powder. In addition to these general features of EB-PBF, other features of EB-PBF have been developed in recent years : high-quality electron beam, and defect detection through backscattered electron (BSE) image monitoring. EB-PBF is suitable for Ti-6Al-4V alloys, pure copper, Ni718 alloys and pure tungsten, and meets AMS standard certification, crack-free and microstructure control.

Metal additive manufacturing (AM) is a technology that can fabricate shapes that are difficult to produce by machining or casting and is excellent for weight reduction and the fabrication of parts with complex designs. However, its application to actual parts is still limited in Japan due to many quality and cost issues. In this paper, we combine AM technology with conventional technologies such as machining and welding to fabricate high quality and low-cost high vacuum chambers.

Metal additive manufacturing is a technology that enables the production of mechanical parts with complex geometries, which were difficult to create using conventional manufacturing methods. It is expected to be utilized in a wide range of fields, from aerospace to general industrial machinery, as it contributes to greater design freedom and shorter development periods.

While this technology has strengths in creating complex shapes and reducing lead times, it also has its challenges and is not a replacement for all parts manufacturing. Therefore, it is important to compare it with traditional manufacturing techniques and select the optimal method according to the specific characteristics and requirements of the product.

Although this technology is still considered to be in its infancy, its strengths and the challenges to its practical application are already clear. In this paper, we discuss the strengths of metal additive manufacturing technology, its applicability through comparison with conventional technologies, and the challenges to its practical application.