Closed-loop control of microstructure and mechanical properties in additive manufacturing by directed energy deposition

A closed-loop directed energy deposition (CDED) system is developed to control the microstructure and mechanical properties of laser additive manufactured material in real-time. An austenitic stainless-steel grade of S316-L, which avoids eutectoid phase transformation, is examined to evaluate this unique control technique's performance. A set of walls are constructed in an open-loop manner by the laser additive manufacturing process to study the thermal dynamics and microstructure variations of a multi-layer structure under fixed processing conditions. The microstructure and subsequent mechanical properties of different thicknesses are affected mainly by the cooling rate change, resulting in a graded microstructure profile and mechanical properties in the deposition cross-section. The CDED system is implemented to eliminate cooling rate variations and locally control the deposition cooling rate to the desired setpoint value in similar five-layer depositions. This control is achieved by adaptively adjusting the traveling speed in real-time rather than keeping it constant. The closed-loop control of the cooling rate results in a uniform microstructure and hardness throughout the deposition cross-section and different layers. Although improved uniformity is observed in closed-loop samples, some microstructural and hardness deviations are still present, caused by post-solidification heating cycles of subsequent layers.