Effects of geometric accuracy and residual powder on ultrasonic reference reflectors produced by L-PBF and EDM

This paper evaluates the performance of flat-bottom holes (FBHs) fabricated using laser powder bed fusion (L-PBF) and electrical discharge machining (EDM) as ultrasonic reference reflectors. Specimens with closed and opened FBHs of variable diameters and depths were produced, and their geometry and surface condition were analyzed using X-ray computed tomography (CT). Equivalent reflector sizes (ERS) were determined using the distance-gain-size (DGS) method by multiple inspectors. Overall, L-PBF FBHs exhibited closer agreement with the nominal dimensions than the EDM FBHs despite rougher surfaces due to partially melted powder. Closed FBHs retained residual powder, reducing CT contrast and resulting in smaller DGS-measured FBH sizes compared with opened FBHs. The smallest FBHs, 0.6 mm in diameter, were detected inconsistently, indicating limited ultrasonic detectability at this reflector size. Pores near the reflector surface correlated with FBH size and were more prevalent in closed FBHs. However, their individual ultrasonic responses could not be separated from the FBH echoes because of their small size and close proximity to the reflector surfaces. These findings show the potential of L-PBF for producing geometrically accurate embedded reference reflectors, while highlighting the influence of residual powder on echo response and the limited detectability of pores close to significantly larger reflectors.