# Interferometry of Gas-Phase Flows during Selective Laser Melting
## 3 Results  
Information contained in the interference patterns can be decrypted locally, at individual points of interest in the observation area, or globally for all pixels of the interference pattern. Powder particles emitted during laser processing in separate frames are the cause of artifacts, but in general do not have a critical effect on the quality of the interference pattern. Information about the object observed lies in the change in the optical length of the probing laser beam passing through the zone of interest of the object field. In gas-phase flows, temperature and gas concentration are functions of spatial coordinates. As a result of this, the optical path length of the probe beam of the interferometer was modulated by the studied object, and a phase increment \(\Delta\phi\) (\(x\),\(y\)) arises, associated with the average change in the refractive index \(\langle\Delta n\rangle\):  
\[\Delta\varphi(x,y)=2\pi L\langle\Delta n\rangle/\lambda, \tag{4}\] \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline
**Material** & **Fe** & **C** & **Si** & **Cu** & **Mn** & **Ni** & **Cr** & **Ti** \\ \hline
12X18H10T & bal. & \textless{}0.12 & \textless{}0.8 & \textless{}0.3 & \textless{}2.0 & 9–11 & 17–19 & 0.4–1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the 12X18H10T steel, %wt. Figure 2: Scanning electron microscopy image of 12X18H10T steel powder (**a**), particle size distribution bar graph (**b**). where \(L\) is the geometric length of the optical path of the probing laser beam in the affected area and \(\lambda\) the wavelength of the probing laser. Previous works on the study of gas-phase flows in SLM indicate several aspects that accompany their formation: changes in pressure, temperature and concentration of metal vapor [5; 25; 26; 27; 28]. These three components affect the refractive index. Therefore, unlike ordinary high-speed recording, registration of the spatial change in the refractive index gives more information about the plume. The interferograms obtained during processing made it possible to visualize the gas-vapor flow (Figure 3). The differential interferogram (Figure 3b) shows the presence of a bright spot near the substrate, the width of which correlates with the diameter of the plume at its base in Figure 3c,d and probably corresponds to the evaporation spot. The interferograms show that the optical path length of the probe beam increases in the plume compared to the length in the unaffected zone outside. For instance, in the gas-phase flow shown in Figure 3, the phase increment \(\Delta\phi\) increases up to 45 radians due to a change in the optical path. Varying working laser parameters made it possible to reveal their effect on the geometry of the jet and a change in the optical path length of the probe laser (Figures 4 and 5). To make sure that the steady-state process was analyzed, the first 0.1-0.15 s of laser spot movement were ignored. Figure 3: Gas-phase flow induced by the working laser at a power of 170 W: (**a**) initial interferogram before processing; (**b**) differential interferogram—the difference between the interference patterns of the unperturbed layer of air and the investigated gas-phase flow; (**c**) interferogram after spatial Fourier filtering; (**d**) pseudo-colored interferogram. At laser power of 50 W, stable plume formation and particle emission were not observed (Figure 4a). In this case, parameter \(\Delta\phi\) attains the maximum directly at the zone of interaction of the laser beam with the powder layer. Figure 4b-d shows that the region of the greatest phase change is formed at a certain distance from the laser processing zone. With increasing power, the distance from the substrate to the \(\Delta\phi\) maximum increases, as does the value of the maximum itself, and the gas jet becomes more stable. To evaluate the jet velocity, 10 consecutive frames of the processed interferograms of the steady-state jet were analyzed (Figure 6).