Metal additive manufacturing (AM) has matured from its infancy in the research stage to the fabrication of a wide range of commercial functional applications. In particular, at present, metal AM is now popular in the aerospace industry to build and repair various components for commercial and military aircraft, as well as outer space vehicles. Firstly, this review describes the categories of AM technologies that are commonly used to fabricate metallic parts. Then, the evolution of metal AM used in the aerospace industry from just prototyping to the manufacturing of propulsion systems and structural components is also highlighted. In addition, current outstanding issues that prevent metal AM from entering mass production in the aerospace industry are discussed, including the development of standards and qualifications, sustainability, and supply chain development.

This study investigates the porosity and microhardness of 316L stainless steel samples fabricated by selective laser melting (SLM). The porosity content was measured using the Archimedes method and the advanced X-ray computed tomography (XCT) scan. High densification level (≥99%) with a low average porosity content (~0.82%) were obtained from the Archimedes method. The highest porosity content in the XCT-scanned sample was ~0.61. However, the pores in the SLM samples for both cases (optical microscopy and XCT) were not uniformly distributed. The higher average microhardness values in the SLM samples compared to the wrought manufactured counterpart are attributed to the fine microstructures from the localised melting and rapid solidification rate of the SLM process.

Fabricating metallic components for highly specialised industries such as automotive and aerospace has become the main focus of additive manufacturing (AM) due to its many advantages over traditional processes. This review initially outlines current AM techniques for processing metallic components, particularly on ‘powder bed fusion’ and ‘directed energy deposition’ categories. Various solidification and metallurgical aspects, microstructure and properties of fabricated parts are described in subsequent sections. In addition, the influence of energy density on metallurgy, microstructure and mechanical properties is addressed. The need to establish processing maps for various materials and techniques, and the challenges currently faced in metal AM are then highlighted. The final section provides an outlook for the future of research in AM of metals.

For the first time, high-pressure torsion (HPT) was applied to additively manufactured AlSi10Mg built in two directions (vertical and horizontal) by selective laser melting (SLM), and the influence of extreme torsional strain on the porosity, microstructure and microhardness of the alloy was investigated. ImageJ analysis indicates that significant porosity reduction is achieved by 1/4 HPT revolution (low strain). Optical microscopy (OM) and scanning electron microscopy (SEM) observations reveal the steady distortion and elongation of the melt pools, the continuous elongation of the cellular-dendritic Al matrix and breakage of the eutectic Si phase network with increased HPT revolutions. Microhardness measurements indicate that despite the significant increase in hardness attained from HPT processing, hardness saturation and microstructural homogeneity are not achieved even after 10 HPT revolutions. X-ray diffraction (XRD) line broadening analysis demonstrates increased dislocation densities with increased HPT revolutions, which contributes to the considerably higher hardness values compared to as-received samples.

Differential scanning calorimetry (DSC) is a thermal analysis technique that measures the energy absorbed or released by a sample as a function of temperature or time. Analysis by DSC has wide applications for examining solid state reactions and solid–liquid reactions in many different materials. Analysis of the kinetics of reactions may be assessed by activation energy analysis methods. In recent years, DSC has been applied in the examination and analysis of bulk ultrafine grained materials processed through the application of severe plastic deformation. This overview examines these recent reports with reference to materials processed using the procedures of equal channel angular pressing, high pressure torsion and accumulative roll bonding. In addition, some critical issues related to DSC analysis are also discussed.