# The Key Role of Laser Ultrasonics in the Context of Sustainable Production in an I 4.0 Value Chain
## 4 The Key Role of LUS in a Smart Manufacturing Production Site  
Despite the multitude of established methods for in-line in-situ quality inspection and off-line in-situ material characterization methods mentioned, there are hardly any inspection methods that provide a reasonable application for both fields. One possibility to investigate in a contactless manner, in-situ, and at high temperatures is the LUS method. This measurement technique has been developed over the last decades for many fields of application and can be used in various different configurations. This method can be applied to a variety of analyses known from the conventional ultrasonic technique. In particular, defect detection is one of the most common applications, especially the detection of defects in terms of pores, voids, or adhesion defects, especially in castings or welds [92]. However, such defect detections are also of great advantage in additive manufacturing. In this context, this method can also be applied in-situ, during the build-up process [93; 94; 95]. Furthermore, the ultrasonic method can also be used for the detection of corrosion phenomena and the associated fatigue analysis [77; 96; 97]. Damping analyses can be utilized to determine hardening depths in surface-hardened components. Most of the analysis options mentioned are based on approaches that can already be investigated with conventional ultrasonic measurement and are applied to static components not exposed to high temperatures. Nevertheless, the important aspect here is the possibility for advanced in-situ microstructural characterizations at high temperatures or velocities, which is the core competence of LUS. Due to a high measurement frequency, this method can record time-resolved changes in the microstructure, even at temperatures above 1200 \({}^{\circ}\)C, thus providing a great contribution to an efficient characterization of metals, when implemented in-line for the in-spection of e.g., dimensions, phase constitution, or grain size [98; 99]. Unlike other systems, this method operates reliably in harsh environments as it is relatively insensitive to dirt and dust. The continuous development and improvement of the in-line system looks promising to extract progressively more microstructural information from, for example, hot-rolled steel strip and to contribute to a significant advance in QC. Currently tens of LUS systems are integrated in a production plant worldwide, the majority of them being oriented for wall thickness measurement in tube production plants, as described for in [100]. Recent LUS measurement devices are encountered in hot strip rolling mills. These are specialized for grain size measurement of austenite before the cooling section. The in-situ measurement of austenite grain size is an important factor in predicting the material properties of the steel strip. In [101], an example of a LUS system integrated in a hot strip mill is outlined. The excitation laser is a pulsed Q-switched laser operating at a frequency of 20 Hz, up to 200 mJ, and a pulse length of 6 ns, at a wavelength of 532 nm. The detection laser operates at a wavelength of 1064 nm and a pulse duration of 100 s, at a power of 600 W. A GaAs two-wave mixing interferometer was used as the interferometer. The hot strip mill rolls the pre-rolled strips in six slabs to a thickness of 2-15 mm. The LUS system is mounted behind the slabs and in front of the runout table on a linear displacement and can move between two strip guide rollers, at a distance of about half a meter from the strip. The detection laser is located at a distance of a few meters. This measurement method is very promising and will certainly be found in several rolling mills in the future. Further literature about LUS Systems implemented in industry can be found in [102; 103; 104; 105; 106]. Furthermore, such a LUS system can be coupled to a TMTS to study in-situ microstructural changes of samples during thermal and thermomechanical treatments.