Directed energy deposition additive manufacturing: Microstructure and composition engineering for high-performing metallic materials

Modern industries often desire metals, alloys and their composites with both improved load bearing capabilities and multi-functionalities (i.e., high-performing metallic materials). The development of such materials poses unprecedented challenges to conventional manufacturing, demanding fine control over material microstructures and compositions. Additive manufacturing (AM), particularly directed energy deposition (DED), offers significant advantages to this end due to its flexibility in multi-material feedstocks and easy auxiliary hardware setup. Here, we categorise four microstructure and composition engineering approaches that uniquely benefit from DED in producing new high-performing metallic materials that are not possible, or not easy, to be achieved using conventional manufacturing processes: (i) discontinuous functionally graded materials (DFGMs), (ii) continuous functionally graded materials (CFGMs), (iii) high-throughput in-situ alloying for new alloy design, and (iv) the control of microstructure and composition using external field assisted DED. Under each category, we first discuss the mechanical and functional performances, focusing on the underlying mechanisms through the lens of process-structure-property (P-S-P) relationship. Then, we summarise design and modelling strategies, spanning traditional approaches and emerging approaches, including high-throughput experimentation and machine learning. Lastly, we discuss the challenges towards achieving desired mechanical and multi-functional (e.g., thermal, magnetic, and data encoding) properties, from both process- and property-related perspectives. This review serves as resource for researchers and industrial stakeholders, inspiring innovative solutions using DED for next-generation high-performing metallic materials for a wide range of engineering applications.