Background orientated and shadowgraphy schlieren monitoring in laser-based additive manufacturing

Schlieren monitoring techniques were applied to Laser Directed Energy Deposition (L-DED) and Laser Cladding (LC) processes to evaluate their performance in capturing thermal dynamics and spatter formation under varying process parameters. Two schlieren setups were compared based on their ability to detect changes in temperature, pressure and material state within the process zone. The first, using a background-oriented design, exhibited higher resolution and sensitivity to local thermal gradients and plume dynamics, while the second, with a broader field of view, demonstrated enhanced stability in monitoring cumulative thermal buildup. Optical flow analysis revealed a strong correlation between line energy and the magnitude of optical turbulence, particularly in regions where vaporization occurred, with a clear plateau observed between 110.6 J/mm and 243.9 J/mm, corresponding to optimal melt pool conditions. Beyond 243.9 J/mm, a significant increase in optical flow was observed, indicating plasma formation and enhanced turbulence. A dome-like schlieren structure consistently formed above the melt pool, expanding with higher energy input, offering insights into the balance between thermal buoyancy and vapor pressure. Additionally, the quadratic relationship between line energy and the schlieren dome volume of enclosed optical flow provided a means to identify energy-efficient and stable process conditions. The findings underscore the potential of schlieren-based monitoring for precise control and optimization of additive manufacturing processes, with implications for improving process stability and minimizing defects like spatter and porosity. • Successfully calculated schlieren as optical flow in the process zone by integrating a speckle pattern, telecentric imaging and the Farneback method for image analysis. • Optical flow analysis reveals thermal turbulence and process stability, with higher line energy increasing turbulence, indicating vaporization and plasma formation, thus reflecting energy input's influence on melt pool dynamics. • A dome-like schlieren structure above the melt pool consistently emerges across all measurements, reflecting a complex interaction of thermal buoyancy and convection, influenced by increasing line energy and dynamic gas flows. • Quadratic dependence between line energy and the schlieren dome optical flow volume identifies optimal process parameters.