Laser additive manufacturing (LAM) technology is not only a representative technology of intelligent manufacturing but also the most popular technology used for the additive processing of metallic materials at the moment. The metal products fabricated using LAM have many advantages, including high geometric freedom, good dimensional accuracy, and excellent performance and quality. They have been widely used in the aerospace, biomedical, and defense industries. Because LAM processing includes a high-temperature laser-heating process, the processing area is often shielded with inert atmospheres to avoid the air contamination of the metallic materials. Recently, LAM technologies utilizing reactive atmospheres, such as N2,Ar-O2,Ar-N2,and Ar-CH4,have rapidly emerged with remarkable achievements in improving the mechanical properties of various metals. It solves a long-lasting problem: the difficulty of modifying the properties of internal materials through atmospheric modification, which might be considered a historic breakthrough. Simultaneously, a new technological path has emerged: modifying feedstock materials before LAM processing utilizing active atmospheres. This study covers recent local and international research advances in the technology of reactive atmospheric LAM, which is a promising developing technology. The achievements are the result of two major technical directions: reactive atmospheric protected LAM processing and reactive atmospheric modified LAM feedstock materials. This review is concluded by analyzing the impacts of this new technology on representative metals such as steel, titanium alloy, aluminum alloy, and high-entropy alloy. In addition, it analyzes and compares material advancements in terms of mechanical properties, microstructural changes, and forming quality, as well as underlying mechanisms. At the same time, this paper discusses the current challenges and opportunities of the reactive atmospheric LAM technology.
In Section 2, this review critically examines the literature involving LAM processing shielded via reactive atmospheres. Figs. 2-11 summarizes and illustrates the effects of reactive shielding gases. Powder-bed-based LAM has received the most research interest due to its success in processing atmospheric-reinforced Ti, Ti alloys, and high-entropy alloys. The abundant and stable gas supply in the sealed processing chamber has been identified as a key advantage for achieving successful and homogeneous metal-gas in-situ reactions. The homogeneous distribution of solute atoms as well as compound precipitates can significantly improve the strength of metals and even boost. A number of attempts at direct-deposition LAM using reactive shielding atmospheres have also been made. However, the results are generally inferior to those achieved via powder-bed-based LAM, owing to insufficient gas-metal reactions during direct-deposition LAM processing. Section 3 introduces feedstock material modification using reactive atmospheres. This atmospheric feedstock modification technology can exert a significant influence before LAM processing, such as drastically increasing N concentration in austenitic stainless steels and improving Cu’s laser absorptivity via surface nitridation/oxidation of feedstock powders (Figs. 12 and 13). The method is particularly applicable to less-reactive metals and can avoid the unfavorable disturbance caused by metal-gas in-situ reactions during LAM processing. Finally, Table 2 summarizes representative achievements of reactive atmospheric LAM fabricated metals. Section 4 delves deeply into and elucidates the mechanisms of atmospheric modification, such as solute element diffusion, compound precipitate formation, influence on solid-state phase transformations, and strengthening mechanisms.
Recently, the application of reactive atmospheres in the LAM of metallic materials has emerged and developed rapidly. This technology has shown great potential in modifying the properties of metals during the fabrication of products. N, O, C, and other alloying elements from atmospheres have been successfully added to a variety of metals, including Ti alloys, steels, Al alloys, and high-entropy alloys. Remarkable strengthening (up to 40%-100% increment) has been achieved in Ti, Ti alloys, and austenitic stainless steels. Furthermore, the effects of reactive atmospheres may be accurately regulated by altering the contents of the atmosphere as well as laser parameters. This review has also proposed several prospects for further clarifying the mechanisms of atmospheric alteration and extending the applications of this technology. It is possible to learn more about the mechanisms at the subnano/atomic scale using in-situ microscopy and X-ray diffraction analysis. Additionally, the reactive atmospheric LAM has the ability to fabricate gradient materials by adjusting atmosphere or laser during processing. As a result, materials with different properties can be deposited at designated positions.
