A review on in situ monitoring technology for directed energy deposition of metals

Directed energy deposition (DED) is an important additive manufacturing method for producing or repairing high-end and high-value equipment. Meanwhile, the lack of reliable and uniform qualities is a key problem in DED applications. With the development of sensing devices and control systems, in situ monitoring (IM) and adaptive control (IMAC) technology is an effective method to enhance the reliability and repeatability of DED. In this paper, we review current IM technologies in IMAC for metal DED. First, this paper describes the important sensing signals and equipment to exhibit the research status in detail. Meanwhile, common problems that arise when gathering these signals and resolvent methods are presented. Second, process signatures obtained from sensing signals and transfer approaches from sensing signals for processing signatures are shown. Third, this work reviews the developments of the IM of product qualities and illustrates ways to realize quality monitoring. Lastly, this paper specifies the main existing problems and future research of IM in metal DED.