## 4 Conclusions and Discussion  
The coaxial LMD-w process is considered a promising method for future additive manufacturing processes. Depending on the wire size, the LMD-w process can be used in a variety of applications, from the production of 3D micro components and complex 3D structures to the use of LMD-w for the repair of existing tools or components of any dimension. Especially for the use in safety-relevant environments, such as aircraft construction, the tolerance limits for shape deviations are tight. In order to meet those high-quality standards while forming a 3D structure from a single weld bead, inline topography monitoring is required to react to deviations of the additively manufactured structure. The quality of the individual layers is particularly decisive for the strength and has a considerable influence on the function of the manufactured component. To ensure sufficient quality control and to continuously monitor the process, OCT was successfully coaxially integrated into an LMD-w process for the first time, as part of this work. It could be shown that the weld bead topography can be captured within the running LMD-w process with high measurement frequency and high spatial resolution. In the inline OCT measurements of individual weld beads, the occurrence of waviness, shape deviations, or other defects could be resolved. One core aspect of the developed system is its flexibility. The flexibility of the system is achieved by the combination of two key concepts: the coaxial LMD-w process head itself with a centrally guided wire, which allows for movement of the process head in any direction and adaptive circular scanning of the process lasers, and, in particular, the measurement laser, which allows for in-process topography control. In combination, these two systems allow for what is called in the context of this work multidirectional process control of the LMD-w process. In A-Scan mode, the distance of the optics to the substrate surface can be quantified and set with an accuracy of \(\sim\)1 \(\upmu\)m before the process and monitored over  
Figure 10: (**A**) Picture and OCT Volume and B-scans of a weld bead showing high uniformities with up to 1.8 mm differences in z-direction (**B**) Picture and volume scan of a weld bead showing form deviations. (**C**) OCT-B-Scan showing changes in z-position (distance between processing head and weld bead). the duration of the process. In 3D mode, the weld bead topography including defects can be captured. In offline mode (without active process laser), the substrate surface can be scanned and qualitatively evaluated. This is particularly helpful when the LMD-w process is to be used to repair existing workpieces. The workpiece is scanned enface in 3D mode and its defects are detected with the OCT. The repair process planned on the basis of the measurement data can then be monitored in inline measuring mode. The described attributes distinguish the method presented in this work from other coaxial monitoring systems of laser welding processes. It has been described that coaxial camera monitoring can be used to assess melt pool expansion within the process [23]. However, a topography of the final weld bead cannot be extracted using this method, which means that defects such as pores or similar phenomena can remain hidden from the measurements, which makes the coaxial integration of a camera for melt pool control another indirect process monitoring solution. Laser triangulation has been used coaxially to assess the height in a powder-based LMD process [24]. The big advantage of the approach of this work to integrate OCT in the process is the ability of OCT to measure geometries with high aspect ratios and challenging surfaces (high or low reflectivity, high surface roughness), in cases where laser triangulation would reach its limits. Additionally, OCT can reach much higher spatial resolutions in comparison to laser triangulation, that makes an LMD-w process monitored with OCT also suitable for very demanding production scenarios of small scale products.