Towards Smart Directed Energy Deposition: An Industry 4.0 Maturity-Based Review of Monitoring, Prediction, and Adaptive Control

Directed energy deposition (DED) is a key metal additive manufacturing process for large, high-value components. Nevertheless, its industrial adoption is hampered by complex process dynamics and limited process robustness. To address this complexity, Industry 4.0 methods are increasingly being explored to improve process understanding, monitoring, and control in DED. This paper investigates the Industry 4.0 maturity of DED with a particular focus on data-driven monitoring and control. A tailored adaptation of the acatech Industry 4.0 Maturity Index is applied at the process level, covering the stages of Visibility, Transparency, Predictive Capacity, and Adaptability. A structured Scopus- and Web of Science-based literature search, using stage-specific search strings, is conducted to identify and compare literature associated with these maturity stages. The resulting record counts are used as search-strategy-based indicators of research activity and are complemented by a content-based assessment of selected studies. The results show a strong concentration of work in powder-based DED with advanced melt pool monitoring, multimodal sensing, and machine-learning-based prediction of defects, geometry, and microstructure. However, the analysis reveals a pronounced gap at the Adaptability stage. Within the literature identified and analyzed in this review, process information is rarely embedded in closed feedback loops, and no fully self-improving DED control architecture was found. To address this gap, the paper proposes a two-level, data-driven control concept that combines a Process Quality Index (PQI) for real-time feedback with a Control Quality Index (CQI) for meta-level evaluation and optimization of control strategies. Powder-based DED is identified as a promising domain for implementing and validating this concept. The study thereby outlines development paths for advancing DED toward higher Industry 4.0 maturity and more reliable industrial deployment.