Ultrasonic and Eddy Current Non-Destructive Evaluation of Near-Surface Flaws in Additively Manufactured Metal

Abstract

Additive manufacturing (AM) is a rapidly evolving technology that promises to revolutionize the aerospace industry. The purported benefits of AM metal—reduced cost, environmental impact, and lead times; and increased light-weighting, design complexity, and autonomy—are outweighed by inconsistency in material properties and mechanical performance. For AM metal to become a viable alternative to conventionally manufactured metal, coordinated efforts by industry, academia, and regulators are required to standardize AM metal components. Non-destructive evaluation (NDE) plays a pivotal role in this journey toward standardization. NDE methods are not yet optimized for the unique challenges presented by AM metal, particularly with respect to surface roughness, representative flaws, and anisotropic material properties. The development of reference blocks that produce consistent results when inspected with specific NDE methods, such as ultrasonic testing (UT) and eddy current testing (ET), is critical. UT and ET studies were conducted on AM metal reference blocks manufactured using laser powder bed fusion (L-PBF) technology. The advanced UT techniques of phased array ultrasonic testing (PAUT) and full matrix capture with the total focusing method (FMC/TFM) were evaluated for their abilities to detect, localize, and size near-surface side-drilled holes (SDHs) in AM Ti–5Al–5V–5Mo–3Cr. Both techniques proved effective for inspection. FMC/TFM demonstrated superior detection capability, imaging a SDH with a diameter of 0.67 ± 0.05 mm and a depth of 0.40 ± 0.05 mm, and provided more accurate amplitude-based flaw sizing than PAUT. ET was performed on AM alloy blocks of Al–10Si–Mg, Ti–6Al–4V, and 17–4 precipitate-hardened stainless steel, each containing flaws filled with trapped feedstock powder. Analysis of ET signal amplitude and phase revealed that trapped powder did not obscure signal amplitude and had only minor effects on phase. Several ET probe configurations—absolute pencil, absolute-reflection, and split-D differential—were assessed for detecting 1-mm-diameter, 1-mm-deep through-channels at test frequencies of 10–50 kHz. Probes with separate transmitting and receiving coils achieved high signal-to-noise ratios while maintaining a localized signal response, guiding future ET probe design for AM metal.