## 4 Discussion  
In this work, a closed-loop, site-specific control of melt pool size was successfully utilized to impart local process changes and print an extra-toolpath geometry, i.e., a geometry that occurs beyond the toolpath. The embossing resolution of the extra-toolpath geometry was dependent upon the melt pool size response time and the print speed. The ability of this technique to control local bead geometry has interesting implications, particularly for a large-scale AM process such as laser wire-based DED, which requires a different set of design rules than the majority of AM processes and has traditionally been limited to lower resolution of component details [13]. It is anticipated that the technique can be used in the near future for volumetric defect mitigation in toolpaths where local overlap of adjacent beads is inadequate. The capability to emboss specific, secondary geometry means that part identification features, such as a serial number or quick response (QR) code, could be permanently added to components during the printing process. The prospect of printing single toolpath walls with varying wall widths is also attractive from a post-print machining perspective, in that, when machining thin-walled structures, preforms ideally contain integrated structural support such as thicker sections that buttress and support adjacent thinner sections during the machining process. An important aspect of the process demonstrated here is the closed-loop nature of the process. While it would certainly be possible to pre-program open-loop, site-specific modifications to process parameters in laser wire-based DED, a closed-loop system ensures that variables, like melt pool size, are controlled regardless of the thermal properties of the component under construction. This was evident in the laser power magnitude variance between beads one and two of the walls in this study. A model-referenced feed-forward approach would be an improvement over feed-forward alone, but there are certain things that are difficult to model, like unexpected print interruptions, which can happen at any time and have varying durations. Another method that could be used to emboss secondary geometry would be to actually make the secondary geometry part of the primary toolpath, in that the sliced 3D object would contain the geometry to be embossed, and the toolpath would joggle laterally while nominal process parameters are maintained to create the embossing. The issue associated with this method is that toolpath turns generally induce print head velocity reductions, which can be a challenge to handle from a process stability perspective, particularly when they are sharp turns, such as those likely associated with  
Figure 10: Photographs of Wall 2, side two; as-printed (**left**) and after heat treatment (**right**). secondary geometry embossing. Additionally, the small, localized nature of the toolpath joggles may induce print system vibrations that are undesirable. The technique presented here is a fundamentally unique way of 3D printing a geometry, and accordingly, an area in which there is much room for advancement is process planning and automation, i.e., slicing and generation of what may otherwise be known as support code. The challenge in slicing is using the projection of the secondary geometry onto the primary toolpath to generate additional information that is attached to the toolpath and used to make site-specific adjustments to process parameters, e.g., melt pool size. The embossing resolution and the ability to deal with irregularities such as singularities (locations at which the layer intersects the secondary geometry at only a single point) must be built into the capabilities of the slicer. An additional challenge is in realizing the format by which additional information is attached to the toolpath.