## 2 Materials and Methods  
Titanium components printed for this study were deposited in a custom, large-scale laser hot-wire DED workcell. The workcell contained a 6-axis industrial robot, wirefeeder, hot-wire system, and was supplied by two fiber-delivered diode lasers, the power from which was combined into a single fiber for 20 kW of total laser power. In addition to the laser optics, the print head contained in-axis process and thermal cameras, as well as a laser line scanner. The in-axis cameras received emissions from the melt pool that were transmitted through a dichroic mirror in the laser optics. The process camera was primarily used to monitor process stability and identify process interruptions, such as an improper wire input location or unstable wire feed behavior. The thermal camera was used to image the melt pool and generate a thermal field, which was processed to generate a melt pool definition in real time. The generated definition of melt pool size was low noise, which is desirable from a control perspective, and consistently tracked with the thermal properties of the build, behavior that has been realized through an extensive research and development effort. Additional detail regarding the thermal camera, including calibration and several other melt pool measurement considerations, are available in prior work [10]. Example false-color images from the thermal camera are shown in Figure 1, along with the corresponding thresholded images that contain only that which is defined as the melt pool. The melt pool size signal was provided to a closed-loop controller that manipulated laser power in real time to control melt pool size. The controller was of a typical linear, feedback control architecture. While the specific control law and gain values utilized are considered protected IP, it is noteworthy that the controller was tuned for setpoint tracking performance. The controller architecture is diagramed in Figure 2. The desired melt pool size setpoint was provided to the controller in pixels. While this parameter would typically be maintained at a constant value throughout the printing of a component, in this research it was changed at specified trigger points, the locations of which were determined by a secondary geometry. Figure 1: Thermal camera images of the melt pool: As-collected nominal size (**top left**), thresholded nominal size (**top right**), as-collected increased size (**middle left**), thresholded increased size (**middle right**), and as-collected comparison (**bottom left**). In addition to laser power being continuously modulated, other process variables were also subjected to manipulation through additional process control algorithms. The laser line scanner on the print head was used to measure layer height, relative to the nominal ideal layer height, on an interlayer basis. Deviations then drove parameter modifications on subsequent layers to correct errors, a process described in [11]. Given that the process variables were modulated over a range of values, Table 1 displays only the nominal deposition values used for the test geometry. The feedstock used was Ti-6 Al-4V wire in 1.6 mm diameter. The build plate material was also Ti-6 Al-4V, with a 6.35 mm (0.25 inch) thickness. Deposition rate was a function of wire feed rate, wire diameter, and feedstock density (4.43 g/cm\({}^{3}\) for Ti-6 Al-4V). The delivered amount of laser power was determined through a calibration process that used a power measurement device. Depositions were completed in an argon environment maintained by a tent-like enclosure that surrounded the build plate and was purged prior to printing. A steady flow of Argon throughout the printing process maintained oxygen levels below 300 ppm. The concept of trigger points generated by a secondary geometry is diagramed in Figure 3. The primary geometry is the traditional geometry that would be input to a slicer, i.e., it is the three-dimensional (3D) object to be printed.