## 1 Introduction  
Additive manufacturing (AM) is a production process used to form three dimensional structures in which material is deposited on top of an already set structure--commonly this is implemented by adding shaped layers. The method enables manufacturers to create parts that cannot be made using traditional fabrication methods such as machining, casting or forging. It also allows for rapid prototyping and complex near-net shape manufacture. For precision metal alloy manufacture, the powdered bed category of AM techniques is often identified as being able to produce parts that have intricate features.  
Selective laser melting (SLM) is a technique where metal powder material is fused under a high power laser source [1]. Once a layer shape is formed, the bed is lowered and additional powder is applied using a re-coater. To prevent oxidisation and contamination, the build is performed inside an inert gas chamber (e.g., Argon). SLM machines give manufacturers the ability to finely control processes and environmental variables--production is controlled through laser scanning powers and pattern regimes, the gas flow in the enclosed build chamber environment, and the powder application rates. Since the additive manufacturing process is in its infancy, parts currently produced suffer from non-conformities which include incorrect part dimensions and inappropriate microstructure. Moreover, structures created suffer from manufacturing defects, which could include cracks and pores [2].  
To find these problems, material inspection is required--however, the complex designs made possible through using AM present a challenge for standard contact-based measurement techniques. Simple visual inspection methods also do not provide sufficient information about the quality of the built part, as the key-holing phenomenon observed during manufacture (where a subsurface pore is created) would not be identified [3]. It is possible to characterise parts through destructive testing, which would provide statistical information based on AM build parameters [4], however, this indirect part information is undesirable to high performance part manufacturers. Post-production inspection is also possible--X-ray computer tomography (XCT) is a widely adopted technique for this purpose [5]. However, due to the method of manufacture, it is possible to use a surface-based non-destructive evaluation (NDE) techniques on each top layer as a structure is built--an in-situ inspection technique could be used to collect volumetric data of a part being built. Rapid measurements could be fed back into the system for reworking or in severe cases, scrapping builds to save material and time. Current areas of focus for in-situ AM inspection includes using purely optical measurement techniques to observe the surface topography and to find large surface breaking defects [6], and observing the melt-pool created when the feed material is locally heated to create the build structure [7].  
In this paper, we present the laser ultrasonic inspection technique known as spatially resolved acoustic spectroscopy, or spatially resolved acoustic spectroscopy (SRAS) [8], for imaging texture and defects on additively manufactured components. The technique is non-contact, all optical and non-destructive. In previous work, we showed that it was possible to use SRAS to image the surface texture [9], and identify both surface and subsurface defects [10] on prepared samples made using SLM. In these cases, prepared samples were polished so that the surface roughness was \(R_{a}<100\) nm. Results in this paper show our images, obtained using a prototype SRAS setup, of an as-deposited SLM Ti-6 Al-4V (Ti46) sample with a surface roughness of \(S_{a}\approx 6\)\(\upmu\)m (the \(R_{a}\) and \(S_{a}\) are measurements of the mean surface profile deviation along a line and area, respectively). To illustrate that a SRAS system could be implemented to provide in-situ AM measurements, we also report on our efforts to integrate a SRAS system into an SLM build chamber shell.