Control of melt pool temperature and deposition height during direct metal deposition process

This paper presents a hybrid control system that is able to improve dimensional accuracy of geometrically complex parts manufactured by direct metal deposition process. The melt pool height is monitored by three high-speed charged couple device cameras in a triangulation setup. The melt pool temperature is monitored by a dual-color pyrometer. A two-input single-output hybrid control system including a master height controller and a slave temperature controller is used to control both height growth and melt pool temperature at each deposition layer. The height controller is a rule-based controller and the temperature controller uses a generalized predictive control algorithm with input constraints. When the melt pool height is above a prescribed layer thickness, the master height controller blocks control actions from the temperature controller and decreases laser power to avoid over-building. When the melt pool height is below the prescribed layer thickness, the temperature controller bypasses the height controller and dynamically adjusts laser power to control the melt pool temperature. This hybrid controller is able to achieve stable layer growth by avoiding both over-building and under-building through heat input control. A complex 3-D turbine blade with improved geometrical accuracy is demonstrated using the hybrid control system.