## 2 Materials and Methods  
Many details of the materials and methods used in this work are described in recent related publications [59; 60; 61] and are not repeated here. The laser ultrasound (LU) system was integrated into a LENS MR-7 Optomec DED-AM chamber as shown in Figure 4. Multiple layers of Ti-6 Al-4V and IN-718 were deposited onto baseplates of the same material. The deposition process was periodically interrupted to translate the 'build' into position to interrogate the deposition with laser generated Rayleigh waves. Figure 4: Laser ultrasound system integrated into DED-AM chamber: (**a**) photograph, (**b**) schematic. Rayleigh waves are generated using a Q-switched Nd:YAG pulsed laser (Inlite III-10, Continuum, Milpitas, CA, USA) with a 6 ns pulse duration and an 8 Hz repetition rate. The laser beam with an initial diameter of 7 mm is first expanded using a 3X beam expander and then reflected using a mirror (Parts 35-099 and 38-900, Edmond Optics Inc., Barrington, NJ, USA) onto a cylindrical lens (LJ1703 RM-B, Thorlabs Inc., Newton, NJ, USA) that forms a nominally 15 mm \(\times\) 0.36 mm linear footprint on the specimen surface. The stand-off distance for the cylindrical lens is approximately equal to its focal length (FL = 75 mm). The linear source actuates broadband Rayleigh waves that are received by a dynamic holographic interferometer based on two-wave mixing in a photorefractive crystal [62]. We reiterate that, unlike previous work [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20], absorbent liquid was not used on the surface to amplify the transduction, nor is the dual-probe-beam deflection method used for detection. The laser interferometer (AIR-1550-TWM, Intelligent Optical Systems Inc., Torrance, CA, USA) detects the out-of-plane displacement \(u_{3}\) at a point on the surface. Its adaptability to variable surface roughness and reflectivity makes it a critical component in the system. The reception laser head is mounted on an XY stage to enable scanning along the centerline of the nominally planar wavefield. In addition to the benefits of the two-wave mixing approach, the interferometer has two output signals - AC (alternating current) and DC (direct current). The AC signal contains the time-varying voltage proportional to the surface displacements, while the DC level provides a measure of the light reflected from the surface. Thus, normalizing the AC signal by the DC level provides a means to compare the signals obtained from surfaces with varying roughness and reflectivity. Additionally, the wave propagation distance is kept low to minimize the attenuation and diffraction effects and improve the signal-to-noise ratio (SNR). When scanning across the hatches, the scan increment is a multiple of the hatch spacing, such that the reception laser beam is focused on the top of each track, thereby maximizing the light collected by the reception laser head. As the build height increases the deposition head (part b in Figure 4) moves up, taking the reception laser head (part c) and the cylindrical lens (part d) with it. For the relatively small build heights used here, there is no need to change the angle of the mirror (part e). The reception laser head is scanned along the centerline of the Rayleigh wavefield. The AC and DC ports of the laser interferometer are connected to an oscilloscope (InfiniVision MSOX3024T, Keysight, Santa Rosa, CA, USA) to observe and record the wave signals. Synchronous averaging (128 times unless specified otherwise) increases the SNR. The AC signals are normalized with respect to the DC level. Matlab algorithms are used for further processing the recorded signals. In addition to in-situ experiments in the DED-AM chamber, experiments were also conducted on a LU test bed. The test bed was setup to mimic the in-situ experiments.