Temporally Gated Active Thermography for Defect Detection in Laser-Based Powder Bed Fusion of Metals

Abstract Laser Powder Bed Fusion of Metal is a key additive manufacturing technology, but its industrial adoption is limited by inherent process imperfections. While in-situ non-destructive testing is essential, current monitoring methods face limitations. In-line active thermography, as a promising testing approach, faces significant challenges with overpowering reflections from the high-energy process laser, which can saturate sensors and mask faint, defect-related thermal emissions. This paper introduces a novel stroboscopic active thermography methodology designed to overcome this limitation. The approach utilizes the Synchronized Path Infrared Thermography dual scanhead setup, which employs the processing laser as a modulated thermal excitation source. This system is based on precise temporal gating of the acquisition of the IR camera: it isolates the desired thermal emission signal from interfering laser reflections, and it ensures that the same, specific segment of the material cooling curve is consistently sampled for every measurement point. The stroboscopic approach was validated on specimens with artificial subsurface defects. It successfully eliminates sensor saturation, providing a high-fidelity thermal signal for analysis. The system identifies voids with actual mean diameters as small as 111 $$\upmu $$ μ m at depths of down to 40 $$\upmu $$ μ m. The study also quantifies the technical boundaries of the method, where signal attenuation from thermal diffusion limits detection at 120 $$\upmu $$ μ m depth to the largest defect measuring 977 $$\upmu $$ μ m.