Cooling rate measurement in directed energy deposition using photodiode-based planck thermometry (PDPT)

A novel method to measure cooling rates during solidification in Directed Energy Deposition (DED) using on-axis photodiode-based Planck thermometry (PDPT) was developed. The proposed method can monitor the average in-situ melt pool temperature at a 100 kHz acquisition rate. This is much faster than other thermal imaging methods, such as an IR (50∼200 Hz) or melt pool camera (25∼100 Hz), due to the use of photodiode single-pixel measurements. PDPT measures the average melt pool temperature by detecting two visible wavelengths from a coaxial, top-down field of view (FOV) of the melt pool through the cladding nozzle. To investigate the viability of the proposed method, single clads of Inconel 718 were manufactured with various energy densities (20∼90 J/mm²) and characterized in terms of cooling rates and dimensions. Cooling rates measured from an off-axis IR camera as well as FEM simulations were compared with the cooling rates from the proposed method to verify the method. The measured solidification cooling rate values from PDPT were consistent with the reported values in the literature (800 °C/s∼5000 °C/s), which validated the proposed approach. Furthermore, the developed method can measure rapid solidification cooling rates over 5000 °C/s, beyond the current limit of IR cameras in industrial applications and at a fraction of the cost. The findings indicate that solidification cooling rates can be monitored with respect to processing conditions, ultimately enabling a connection between process parameters and the resulting local microstructure. This opens possibilities for effective certification or for tailoring desired mechanical properties through closed-loop control.