Abstract
Sensing and closed-loop control are critical attributes of a robust 3D printing process, such as Directed Energy Deposition (DED), in which it is necessary to manage geometry, material properties, and residual stress and distortion. The present research demonstrates multiple modes of closed-loop melt pool size control in laser-wire based DED, a form of large-scale metal additive manufacturing. First, real-time closed-loop melt pool size control through laser power modulation was demonstrated for intralayer control of bead geometry. Aspects such as controller tuning, response time, interaction with primary process variables, and disturbance rejection are presented. Next, an interlayer trend in laser power during the printing of layered components was documented, which inspired the development of novel modes of control. A controller that modulates print speed and deposition rate on a per-layer basis was developed and demonstrated, enabling the control of either average melt pool size alone or average laser power in coordination with real-time melt pool size control. This work demonstrates that accumulated heat in components under construction can be exploited to maintain process stability as print speed and deposition rate are automatically increased under closed-loop control. This has major implications for overall production efficiency. Control modes are characterized in terms of their effect on local bead geometry, global part geometry, and interlayer effect on energy density, among other factors.
