In-situ monitoring and control of laser-directed energy deposition with wire – part 1: parameter-signature-quality analysis of duplex stainless steel

Laser-directed energy deposition with wire (L-DED/W) has gained attention due to its high deposition and utilization rates. However, components fabricated using this method exhibit inhomogeneous microstructure and anisotropic mechanical properties, primarily stemming from the complex thermomechanical phenomena inherent to the process. To indirectly monitor and control final qualities, such as microstructure and hardness—which cannot be directly measured during deposition—this study employs a parameter–signature–quality (PSQ) framework specifically tailored to duplex stainless steel (DSS) 2209. This paper, the first in a two-part series, systematically investigates the correlations within the PSQ framework with microstructure and hardness as the target qualities. A DSS 2209 ring is fabricated using the L-DED/W process with continuous in-situ melt pool monitoring using a coaxial camera. Advanced image processing techniques are developed and applied to extract critical melt pool signatures, including melt pool width, length, and area, from over 100,000 captured images. Post-deposition characterization involves detailed microhardness testing and microstructural analysis across 288 measurement locations, each replicated at least three times, to quantify microstructural features such as grain size, grain shape, and phase content. The results highlight significant correlations within the PSQ framework, emphasizing that melt pool signatures provide stronger and more sensitive indications of microstructural evolution compared to direct process parameters. It is established that hardness predominantly depends on phase composition (austenite content), followed by grain size, whereas laser power—despite its critical role in controlling melt pool width and thus geometry—has minimal influence on hardness and microstructure. Finite element analysis simulations further support experimental observations by analyzing how the cooling rate varies with the bead positioning and travel speed, influencing the grain size and hardness distributions. This study provides foundational understanding essential for implementing real-time monitoring and closed-loop control strategies for microstructure and hardness in DSS components produced by L-DED/W, which is discussed in Part 2 of this series. Ultimately, the insights gained advance the potential for optimized and consistent mechanical performance in metal additive manufacturing.