Three-dimensional (3D) printing of metallic materials involves the layerwise consolidation of feedstock materials in the form of powder, wire, or sheet using various energy sources to form complex shapes. The past two decades have witnessed significant advances in the field, in terms of both technologies and materials for metal 3D printing. This has led to widespread exploration and adoption of the technologies across industry, academia, and R&D organizations. This article presents an overview of the field of metal 3D printing. A brief history of metal 3D printing is followed by an overview of metal 3D printing methods and metallic material systems used in these methods. Microstructure and properties, and their relationship to process parameters are discussed next, followed by current challenges and qualification issues. The article concludes with future trends and a brief description of the invited articles included in this special issue.

Aluminum alloys are in high demand for additive manufacturing (AM) processing. However, the physical properties of Al alloys are less favorable for the production of repeatable and reliable parts, with factors such as surface oxide scales, high thermal conductivity, and large solidification shrinkage. Despite these characteristics, processing strategies have been developed to overcome these hurdles. The objective of this article is to highlight the different microstructure–processing–properties characteristics for the three main families of aluminum alloys: pure, casting, and wrought chemistries. The article focuses on AM processes involving solidification, including powder bed and direct energy deposition for both powder and wire feedstock.

This article overviews electron-beam melting (EBM), including process optimization issues. Examples of EBM-fabricated components described include hexagonal close-packed Ti-6Al-4V, face-centered-cubic René 142 (a Ni-based superalloy), and body-centered-cubic pure iron, corresponding to a melt temperature range from 1375°C to 1630°C. Residual microstructures observed for these fabricated components by optical microscopy, scanning electron microscopy, and transmission electron microscopy include equilibrium as well as nonequilibrium features, which illustrate prospects for novel structure–property manipulation in the EBM process. The EBM process relies on available pre-alloyed, precursor powders that are selectively melted layer by layer by a computer-aided design scanned electron beam to form relatively small, but often complex, products or components. Direct metal deposition innovations capable of truly three-dimensional metal printing are described, especially high-temperature metals and alloys for future additive manufacturing technologies.

There is increasing interest in the use of additive manufacturing (AM) for Ni-based superalloys due to their various applications in the aerospace and power-generation sectors. Ni-based superalloys are known to have a complex chemistry, with over a dozen alloying elements in most alloys, enabling them to achieve outstanding high-temperature mechanical performance as well as oxidation resistance when processed using conventional routes (e.g., casting and forging). Nonetheless, this complex chemistry results in the formation of various phases that could affect their processability using AM, resulting in cracking. Furthermore, due to the directional solidification and rapid cooling associated with AM processes, the alloys experience significant anisotropy due to the epitaxially grown microstructure, as well as the residual stresses that can sometimes be difficult to mitigate using thermal postprocessing techniques. This article highlights the outstanding issues in Ni-based superalloys AM processing, with special emphasis on defect formation mechanisms, process optimization, and residual stress development.

NiTi alloys are well known not only due to their exceptional shape-memory ability to recover their primary shape, but also because they show high ductility, excellent corrosion and wear resistance, and good biological compatibility. They have received significant attention especially in the field of laser additive manufacturing (AM). Among laser AM techniques, selective laser melting and laser metal deposition are utilized to exploit the unique properties of NiTi for fabricating complex shapes. This article reviews the properties of bulk and porous laser-made NiTi alloys as influenced by both process and material parameters. The effects of processing parameters on density, shape-memory response, microstructure, mechanical properties, surface corrosion, and biological properties are discussed. The article also describes potential opportunities where laser AM processes can be applied to fabricate dedicated NiTi components for medical applications.

The capabilities of metal additive manufacturing (AM) are evolving rapidly thanks to both increasing industry demand and improved scientific understanding of the process. This article provides an overview of AM of the Ti-6Al-4V alloy, which has essentially been used as a yardstick to gauge the capability of each metal AM process developed to date. It begins by summarizing the metal AM processes existing today. This is followed by a discussion of the macro- and microstructural characteristics, defects, and tensile and fatigue properties of AM Ti-6Al-4V by selective laser melting, laser metal deposition (both powder and wire), and selective electron-beam melting compared to non-AM Ti-6Al-4V. The tensile and fatigue properties of as-built AM Ti-6Al-4V (with machined or polished surfaces) can be made comparable, or even superior, to those of Ti-6Al-4V in the most commonly used mill-annealed condition. However, these properties can exhibit a large degree of scatter and are often anisotropic, affected by AM build orientations. Post-AM surface treatments or both the post-AM surface and heat treatments are necessary to ensure the minimum required properties and performance consistency. Future directions to further unlock the potential of AM of Ti-6Al-4V for superior and consistent mechanical properties are also discussed.

Conducting polymers are difficult to process, since unlike conventional polymers, they generally do not dissolve in common solvents or melt. By synthesizing nanostructured forms of the conjugated polymer polyaniline, simple methods for making conducting polymer inks become possible. By using either interfacial polymerization or a rapid-mixing technique, nanostructured polyaniline has been synthesized in a readily scalable process. Polyaniline nanofibers make excellent sensors for acids and bases. When decorated with metal nanoparticles, they can be used for molecular memory devices and catalysis. Using a flash from a camera, polyaniline nanofibers can be melted and patterned to make sensors, actuators, and asymmetric membranes. Single crystals of tetraaniline can be grown that exhibit conductivities approaching that of the bulk polymer.

Transmission electron microscopy is a powerful technique for the analysis of solid samples, but it can also be used to image in liquid environments, gaining a unique view of processes and structures in liquids. Here, we describe recent developments in electron microscopy of liquids and discuss applications in several areas. We first describe closed-liquid-cell microscopy with its opportunities for visualizing electrochemical processes. We then discuss imaging of low-vapor-pressure liquids relevant to the operation of rechargeable batteries. Finally, we describe imaging of thick biological materials to obtain information on membrane proteins in intact mammalian cells that cannot be observed classically under dry or frozen conditions. Electron microscopy in liquid environments is developing rapidly and has the potential to solve key problems in materials science, physics, chemistry, and biology.