In-situ monitoring techniques for additive manufacturing are in high demand to help produce reliable parts. The structural integrity of these parts depends on both the presence of flaws and their microstructure. Ultrasonic Rayleigh waves have the potential to identify flaws and assess the local microstructure during directed energy deposition (DED) additive manufacturing processes, but the scattering associated with the surface roughness degrades the ultrasonic signal and must be understood to extract useful information. Herein, the microstructures and surface profiles of DED and wrought Ti–6Al–4V are compared to provide context for measured Rayleigh wave speeds and second harmonic generation. The Rayleigh wave speed and second harmonic generation for DED and wrought Ti–6Al–4V materials having comparable surface roughness are significantly different. The wave speed measured in DED material is 3% slower than in wrought material, and the relative nonlinearity parameter, commonly used to characterize second harmonic generation, is 3.5–6.0 times higher for polished surfaces. Wave speed and second harmonic generation measurements are also made along the hatch and across the hatch for both as-built and glazed DED surfaces. Based on our results, we conclude that in-situ Rayleigh wave linear and nonlinear measurements are possible; although we acknowledge that in-situ angle-beam transducer generation would be challenging, and thus we will investigate pulsed laser generation in future work.