Porosity inspection in directed energy deposition additive manufacturing based on transient thermoreflectance measurement

In additive manufacturing, material melting-solidification often introduces defects like porosity, lack-of-fusion, delamination, and crack. Such defects are detrimental to the mechanical properties and quality of the manufactured component. In this study, a transient thermoreflectance (TTR) technique using a femtosecond laser is used for porosity inspection in directed energy deposition (DED) additive manufacturing. First, a femtosecond laser TTR measurement system is developed for noncontact measurement of thermoreflectance from a deposited layer. Subsequently, porosity is inspected by comparing the thermoreflectance measured at different pump excitation modulation frequencies. Due to this variation in modulation frequency, the developed porosity inspection technique is rendered sensitive to porosity rather than thermal property variation. Owing to the noncontact nature and scanning capability of the proposed TTR technique, it can be readily applied to in-situ porosity monitoring during DED additive manufacturing. Validation tests were performed on Ti-6Al-4V samples additively manufactured with different printing parameters. The results highlight the feasibility of the proposed technique for in-situ monitoring of porosity in DED additive manufacturing.