Design and validation of a dynamic optical system for beam monitoring and automatic focus compensation

Owing to the increase in laser-based metal additive manufacturing technologies, it becomes necessary to understand the main process parameters leading to flaw formation, so that proper strategies can be adopted in order to tackle them. Laser powder interaction at the melt pool level is one of the most critical aspects to deal with, since the characteristics of the molten region, such as its size and shape, directly impact micro- and macro-structural properties of the built part. Hence, achieving precise focus control, in presence of mechanical and thermal effects causing unintended shifts during operation, is essential to ensure process stability and consistent product quality. The objective of the thesis is the design and experimental validation of a dynamic focusing unit to be integrated in a processing head, capable of automatically compensating for focus variations. For the control scheme to operate in a closed-loop fashion, a feedback signal is generated through the acquisition of the laser beam by a CMOS camera positioned at the end of the optical sensing branch, designed to replicate the power optical path. A dedicated image processing algorithm analyzes the captured beam profile, estimates its dimensions, and adjusts the position of a lens within the optical chain to achieve optimal spot properties. In view of the compactness of the proposed system, a motorized linear stage has been added to the camera to allow for a complete characterization of the laser beam by acquiring cross-section images at various axial positions, which is particularly useful for auto-diagnostic purposes. To this end, efforts have been made to collect measurements for beam caustic and quality factor evaluation. Experimental tests have demonstrated the effectiveness of the system to cope with undesired disturbances, promising for its integration into commercial processing heads as a cost-effective and compact solution offering automatic real-time focus correction.