Non-contact quantification of subsurface defects in additive manufacturing via laser ultrasonic Rayleigh wave polarity reversal

Selective laser melting (SLM), as an efficient additive manufacturing technique, demonstrates exceptional forming capabilities. However, minor variations in process parameters may induce microstructural alterations that subsequently generate manufacturing defects. This technological limitation underscores the urgent requirement for reliable in-situ monitoring methods to ensure component integrity and optimize processing parameters. In this study, a fully non-contact and efficient inspection method based on all-optical laser ultrasound (LU) is proposed for identifying and quantifying subsurface defects in SLM components. Through finite element simulations, the interaction between Rayleigh wave (R-wave) and subsurface defects was systematically investigated, revealing distinct polarity reversal phenomena during wave-defect interactions. Experimental studies further elucidated the positional dependence of laser excitation relative to defect geometry. As it turned out, when the laser was excited above the edge of the defect, the amplitude of the resulting R-wave was larger than when it was excited in a smaller surrounding area. Based on this amplitude anomaly feature and further combined with the B-scan images, quantitative characterisation of defect location and size is achieved by unilateral and bilateral reception methods, respectively. These findings demonstrate that the method can effectively detect subsurface defects in SLM components and improve the efficiency and resolution of LU detection. The feasibility of LU technology in additive manufacturing quality inspection is further validated and its application in online monitoring of metal additive manufacturing components is promoted.