## 1 Introduction  
Monitoring and control of metal additive manufacturing (AM) processes is the subject of intense research interest, from powder bed fusion to wire- or powder-based directed energy deposition (DED). In particular, for large-scale wire-based DED, real-time sensing and control are part of the array of techniques being employed to address the primary technical challenge, which is simultaneous management of geometry, material properties, and residual stress and distortion [1]. In terms of thermal control, closed-loop melt pool size control via power modulation has proven to be an important capability; this mode of control has been demonstrated by [PERSON] et al. in laser cladding, [PERSON] et al. and [PERSON] et al. in laser powder based DED, [PERSON] et al. in electron beam freeform fabrication, and the present authors in laser wire-based DED [2; 3; 4; 5; 6]. This mode of control effectally enables control of local bead geometry and management of interlayer energy density as heat accumulates in the component during construction [2]. The degree to which thermal gradients and heating and cooling rates are modified by this mode of control, and the subsequent impacts on solidification dynamics, microstructure, and material properties, is the subject of continuing investigation, largely on a machine- and material-specific basis.  
The literature indicates that, in the research demonstrated to date, the common goal of efforts to control the melt pool size in DED has been to maintain a consistent melt pool size with a constant controller setpoint. This type of control has the effect of maintaining nominal bead geometry both on an intralayer basis and throughout the printing of components, yielding global geometry control. However, the concept of site-specific modifications to deposition parameters, which are driven by additional information attached to the toolpath, is of great interest to AM researchers; after all, one of the unique benefits of AM is that it affords manufacturing flexibility in the form of shape, hierarchical, material, and functional complexity [7]. Site-specific parameter modifications can enable local bead geometry control, vary the magnitude of energy input, and induce changes to thermal gradients and rates of solidification for functionally grading the material properties of components [8]. This capability was demonstrated in electron beam melting, a powder-bed AM process, with site-specific control of crystallographic grain orientation in a nickel-based superalloy, Inconel 718, which was achieved through open-loop changes to scan strategy [9].  
For large-scale DED, pre-programmed, open-loop changes to primary process parameters, such as laser power, print speed, and deposition rate, are also possible, however, the mode of operation proposed here is to command site-specific modifications (e.g., melt pool size) at desired setpoints under closed-loop control. In this way, a system would be capable of automatically achieving consistent property modifications in the presence of the ever-changing thermal conditions of the component under construction. Demonstrating this technology served as the motivation for this research. In this work, a proof-of-principle experiment was completed in which a laser hot-wire DED process was used to print Ti-6 Al-4V components with closed-loop control of melt pool size through laser power modulation, and site-specific changes were commanded to the melt pool size at locations determined by a secondary, extra-toolpath (\"extra\" denoting specific, printed geometry that occurs beyond the toolpath) geometry. The performance of the control system, melt pool size response, and resulting geometry are presented. Because this research was conducted through a public-private partnership, some details are considered protected intellectual property (IP), and therefore have not been included in the article.