# Imaging Microstructure on Optically Rough Surfaces Using Spatially Resolved Acoustic Spectroscopy
###### Abstract
For example, small sections of material are prepared and polished, and the microstructure is imaged with electron backscatter diffraction (EBSD) [12]. Clearly, this raises two key issues; firstly, additive manufacturing has many benefits that make it especially attractive when low production runs are required, even producing one-off components [13], making established qualification regimes an unsuitable approach [14]. Turning this problem on its head also presents a significant opportunity; if the material that sees service, be it machined or additively produced, can be characterised, there is an opportunity to realise a new paradigm in material efficiency, removing the need for material design tolerances to ensure safety. Spatially resolved acoustic spectroscopy (SRAS) [15; 16] is a laser ultrasound technique using surface acoustic waves (SAWs) to measure material microstructure. SAWs are generated with a fixed wavelength, meaning the waves that propagate have a frequency determined by the acoustic velocity of the material at the generation location. As this does not rely on the time of flight, this produces a simple and robust measurement. Measuring the velocity allows spatial maps of the material microstructure to be obtained as the SAW phase velocity varies with the crystallographic orientation for most materials. Beyond grain maps, if more measurements for a range of SAW propagation directions are taken, then the crystal orientation [17; 18], and more recently, the single crystal elasticity matrix [19], can also be obtained. SRAS has been applied to a range of engineering materials and their alloys, e.g., titanium, nickel, steel, and silicon [18; 20; 21]. Samples are typically prepared before scanning, for example, sliced, ground flat, or lightly polished. The smooth surface is currently required as the knife edge detector (KED) that is usually used only works with specular reflections (see Sections 2.1 and 2.2). While this preparation is acceptable for research specimens, the preparation of real parts is undesirable as it affects their function. Translating SRAS measurements from prepared and polished surfaces to those of real parts introduces a number of new challenges. These can impact the form of the instrument (curved surfaces require complex sample handling), while others have a more fundamental impact on how the measurement can be performed. This includes the impact of the optically rough surface found on most real parts. The roughness is determined by how the material was made (if additively manufactured) or how the part was machined and the treatments it might have experienced. The scale of the roughness determines the challenge it imposes for both the generation and propagation of ultrasound, as well as the impact on the detection laser beam. This will be discussed in more detail in Section 2.1, but briefly, roughness of the surface can modify the ultrasound generation process by changing the effective wavelength and thereby reduce the efficiency of the generation process. Extremely rough surfaces can have additional effects on the measurement; they may increase the attenuation, change the apparent velocity, or interfere with the ultrasound generation process [22]. For the detection process, the main effect is the scattering of the detection laser beam, not only reducing the light return to the detector but also producing optical speckles, which not all detection schemes can tolerate. This paper presents progress on using SRAS on optically rough surfaces for machined, etched, and additively manufactured parts (via WAM, WLAN, and L-PBF). The challenges and limitations imposed by the surface on the spatial and velocity resolution, as well as the impact on scan speed, are also discussed.