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In-process inspection of the additive manufacturing process requires a technique that can provide reliable measurements given the extreme operating environments, the small size of the defects and the cyclic melting and heating of the material, caused by subsequently deposited layers. A remote and couplant-free ultrasonic inspection technique using bulk waves that can image near-surface defects could address these in-process inspection requirements. Laser induced phased arrays (LIPA) generate and detect ultrasound based on laser ultrasonics principles, while the array is synthesised in post-processing. However, when using LIPAs for inspection, the surface acoustic waves (SAWs) interfere with the bulk wave modes giving rise to crosstalk and artefacts, which makes near-surface defect imaging difficult. This work experimentally validates and compares five techniques for SAW suppression: amplitude thresholding, mean waveform subtraction, principal component subtraction, frequency-wavenumber filtering, and phase coherence imaging. SAW suppression is demonstrated in ultrasonic images of transverse waves based on 71-element LIPA data synthesised on a Ti-6Al-4V directed energy deposition-arc (DED-Arc/Ti6Al4V) sample with a ∼1 mm diameter side drilled hole, located at ∼4 mm below the inspected surface. The reported results show that the principal component subtraction approach achieved the highest ‘signal-to-crosstalk ratio’ improvement of 16 dB, while successfully suppressing the SAW.

Automated in-process Non-Destructive Testing (NDT) systems are rapidly gaining traction within the manufacturing industry as they reduce manufacturing time and costs. When considering calibration and verification of such systems, creating defects of known geometry and nature during the deposition of a weld can: (I) help examine the capability of the automated system to detect and characterise defects, (II) be used to form a database of signals associated with different defect types to train intelligent defect classification algorithms, and (III) act as a basis for in-process gain calibration during weld inspection at high temperatures, where the ultrasound beam can be skewed as a result of velocity gradients. In view of this, this paper investigates two unique methodologies for introducing: (a) lack of fusion weld defects by embedding tungsten in the weld and (b) creating artificial weld cracks by quenching to imitate the real cracking scenarios. According to the results of Phased Array Ultrasound Testing (PAUT) inspections, the methodologies used for embedding the artificial defects were successful. The validity of inspections was also verified by extracting micrographs from the defective sections of the welds, and model-based simulations were carried out to gain a better understanding of the wave propagation path and interaction with the generated defects.

Additive manufacturing (AM) has the capability to build complex parts with internal features, which have many advantages over conventionally manufactured parts. This makes AM an alternative for advanced manufacturing sectors. AM components suffer from defects due to the lack of understanding in the build process. This makes the adaptation of AM in safety-critical industries, such as aerospace, problematic. The current AM work flow calls for costly off-line inspections to qualify components as defect-free. The layer by layer nature of the AM provides an opportunity for an on-line inspection to take place. This can provide early detection of defects as well as information for optimization and repair of the build. Laser Induced Phased Arrays (LIPA) present themselves as a viable remote, non-destructive, ultrasonic technique capable of being implemented as part of an on-line inspection of AM. Lasers are used to generate and detect ultrasound and a phased array is synthesized in post-processing. This paper demonstrates the capability of LIPA to successfully detect and locate features within AM components off-line. Cylindrical features as small as 0.2 mm in diameter and 26 mm above the inspection surface were detected using LIPA and verified using X-ray computed tomography (XCT). Keywords: Additive manufacturing, Laser Induced Phased Array, Non-destructive, Non-contact, Selective laser melting, Geometrical accuracy