In-situ subsurface defect detection in directed energy deposition using pulse phase thermography and thermal motion magnification

This study presents a novel thermal motion magnification technique for in-situ subsurface defect detection in directed energy deposition (DED) processes. The proposed technique integrates pulse phase thermography (PPT) with phase-based motion magnification to enhance thermal signal sensitivity and mitigate noise interference in the challenging DED environment. An in-situ monitoring system, comprising a coaxial infrared (IR) camera and a laser line scanner, is developed to detect subsurface defects in real time with micron-scale precision. Motion magnification is applied to amplify thermal variations at a specific target frequency band in the IR camera's thermal images, while phase differences in PPT are extracted from the same frequency band for defect detection. The target frequency band is determined according to the current deposition layer height measured by the laser line scanner. This technique was validated with simulation signals under different signal-to-noise ratios and experimentally applied to Ti-6Al-4V samples with subsurface defects, demonstrating its potential as a robust solution for real-time defect detection in DED.