Opportunities and challenges in additive manufacturing of functionally graded metallic materials via powder-fed laser directed energy deposition: A review

Since the inception, additive manufacturing (AM) continues to grow and change the paradigm of manufacturing for developing advanced materials. One such case is the functionally graded materials (FGM). FGM, in general, is a class of engineered materials, which is identified by a gradual change in composition and/or microstructure of one material into another over a defined volume, causing a variation in properties based on the position. Laser directed energy deposition (LDED) through powder feeding, as one of the AM classifications, is capable of fabricating functionally graded metallic materials. Over the past decade, the number of studies reporting the feasibility of this process in AM of metallic FGMs (AM-FGM) for high-performance applications has dramatically increased. For this reason, the present paper tries to provide a comprehensive review of the state-of-the-art systems in AM-FGM through the LDED process. The reported studies in the field are categorized according to their material system to highlight the opportunities and the challenges associated with each metallic system. Recent findings in processing, phase formation, and properties of different metallic FGMs are summarized and discussed. Finally, the prospects for AM-FGM are presented in an attempt to shed some light on the future research direction.