Nondestructive Inspection of Directed Energy Deposited Components Using Scanning Acoustic Microscopy with Metalworking Fluids

Directed energy deposition (DED) is a metal additive manufacturing (AM) technique that can create new products or repair damaged ones by depositing molten metal powders with a high-power source. However, defects such as spherical gas pores or irregularly shaped lack-of-fusion pores can occur during the DED process, and it is necessary to evaluate those defects to ensure the stability of the DED process and to improve the quality of DED products. This work studied scanning acoustic microscopy (SAM) using metalworking fluids—cutting oil and anti-rust oil—as coupling media to characterize the internal and interfacial defects in DED components. Water as generally used in SAM was also tested as a reference and its results were compared with those of the fluids. First, the physical properties of each coupling medium were measured, and then its acoustic attenuation coefficient was calculated based on Stokes’ law of sound attenuation. Second, the capability for defect detection was evaluated using drilled holes in an aluminum specimen, using each coupling medium. Finally, the internal and interfacial porosities of the DED specimens were characterized using SAM. The experimental results showed that the coupling performance of the cutting oil was similar to that of water, while that of the anti-rust oil was clearly degraded because that oil caused high attenuation of ultrasonic waves. These comparison results suggest that it would be better to inspect metal DED components using SAM with cutting oil.