Additive Manufacturing of Metallic Materials: Structures, Properties and Methodologies

1. Introduction and Scope

Additive manufacturing (AM) is a crucial aspect of contemporary science and engineering, enabling the layer-by-layer production of components. The past two decades have witnessed a surge in the development of AM processes for metal materials, which offer numerous advantages over conventional manufacturing processes such as casting, forging, and machining. AM has proven to be particularly effective in reducing material wastage, optimizing material properties, and shortening lead production times to meet component requirements. As a result, it has been widely adopted across various industries, including in aerospace, biomedical, marine and offshore, energy, and automotive applications.

This Special Issue aims to present the latest advances in the AM of metallic materials, with a particular emphasis on novel structures, properties, methodologies, and applications. It compiles research on various metal AM processes, including wire arc additive manufacturing (WAAM), laser powder bed fusion (L-PBF), and directed energy deposition (DED) techniques. This Special Issue will also showcase research on metal production using both laser 3D printers and other laser machines, as well as investigations into the AM of high-density and porous materials, and into protective coatings obtained by atmospheric plasma spray.

2. Contributions

This Special Issue comprises ten articles that cover various metallic materials and related fields of AM, including four on L-PBF, four on WAAM, one on DED, and one on atmospheric plasma spray (APS).

Sales et al. [1] reported on the fatigue design of super-duplex stainless steel structures fabricated by WAAM. The results indicated a significant anisotropy in fatigue properties and crack initiation caused by internal defects rather than surface flaws. Surface treatments, such as post-process machining or polishing, can be effective when the dominant failure mechanisms are largely associated with surface defects and/or high levels of residual stresses near the surfaces.

Wang et al. [2] studied the effect of Cr, Mo, and V elements on the microstructure and thermal fatigue properties of the chromium hot-work steels fabricated by L-PBF. The findings indicated that an increase in medium-level V content was primarily responsible for grain refinement, resulting in enhanced hardness and thermal fatigue resistance compared to low-level V content.

Spalek et al. [3] investigated laminated metal composites consisting of multiple bilayers of alternating layers of ductile and high-strength steel that were processed by WAAM for the first time. The findings demonstrated that the mechanical properties of the laminated metal composites were significantly influenced by the dilution between unalloyed SG2 and X90 weld beads, leading to a similar range in hardness for both SG2 and X90 steel.

Kazantseva et al. [4] revealed the micromechanisms of deformation and fracture in porous L-PBF 316L stainless steel at different strain rates. Despite the presence of porosity, the printed specimens exhibited high strength which increased with the loading rate. With an increase in strain rate, the nucleation of new pores became less pronounced. At the highest strain rate of 8 × 10⁻³ s⁻¹, only pore coalescence was observed as the dominant microscopic mechanism of ductile fracture.

Qian et al. [5] focused on the pass design, optimal formation process, and boiling heat transfer performance of microchannel liquid-cooled plates produced via L-PBF. The findings indicated that the self-supporting microchannel structure significantly improved T/R module heat dissipation efficiency and resolved local overheating issues.

Fang et al. [6] reported on the preparation of a Cu-Cr-Zr alloy by L-PBF, including parameter optimization and an analysis of the microstructure, mechanical properties, and thermal properties for its potential use in microelectronic applications. The results illustrated that the surface morphology of the microstructure was affected by the laser energy density. Additionally, α-Cu was identified as the primary phase present in both the LPBF samples and virgin powder. Cr spherical precipitates with a size of about 1 μm were observed in certain areas. Finally, the tensile fracture mode was found to be a mixed ductile–brittle mode.

Tian et al. [7] discussed the microstructure, mechanical properties, and galvanic corrosion of 10CrNi3MoV fabricated by WAAM. The results revealed that the deposited layer exhibited a hardness range of 221–282 HV_0.2, a yield strength above 550 MPa, and a tensile strength above 760 MPa. Moreover, the matrix had a higher corrosion potential than the deposited layer by 0.08 V and a lower corrosion current density than the deposited layer by 1.99 × 10⁻⁶ A⋅cm⁻².

Ferreira et al. [8] investigated the optimization of laser-based DED of Metco 42C martensitic steel powder on 42CrMo4 steel. By utilizing experimental complex parameters, they identified conditions that prevent cracking and ensured a sound clad with a high deposition yield. The obtained values were 2000 and 5000 (kW)⁴·(mm/s)²/(g/s) for samples with and without preheating, respectively.

Lorenzo-Bañuelos et al. [9] studied the influence of APS on the mechanical properties of Ni-Al coatings deposited on an aluminum alloy substrate. Coatings with higher strength were obtained by employing low spraying speeds and increasing spraying distance while maintaining intermediate values of argon flow rates within the assessed range of variables.

Yao et al. [10] elucidated the effect of the duty ratio and current mode on the fabrication of 316L stainless steel through robot-based WAAM. The results showed that the low-frequency double-pulse mode induced stirring in the molten pool, resulting in parts with a larger width, smaller height, finer microstructure, and superior properties compared to those fabricated using the single-pulse mode.