Owing to the excellent strength, plasticity, and corrosion resistance, 316L stainless steel is widely used in nuclear and chemical industries. The efficient cutting of thick plates is realized using lasers, which are high-energy-density heat sources. During the laser cutting process, the plate material melts and is blown off vertically under the action of a coaxial compressed gas. Therefore, a kerf is formed. During a rapid thermal cycle, an extremely thin recast layer (the order of microns) is formed on the surface of the kerf. During the solidification of the recast layer, a particular temperature gradient and fluid motion significantly influence the morphology and the texture of the structure at room temperature. In previous studies, researchers have mainly focused on the influence of parameters, such as laser power, cutting speed, and pressure of compressed gas, on the cut formation and its quality. Few studies have focused on the microstructural morphology and formation mechanism of the recast layer. The differences between the as-solidified microstructure and the substrate may lead to non-negligible changes in the properties of the edge, which in turn affects the overall characteristics. To study the morphology and microstructural growth of the recast layer, an 18 mm thick 316L austenitic stainless steel plate is taken as the object of laser cutting for this study. The solidification mechanism of the recast layer at different kerf sites during the laser cutting process is revealed.

An 18 mm thick 316L austenitic stainless steel plate was employed as the base metal for this study. A pulsed laser was used to cut the base metal to form a kerf. N₂ was chosen as the compressed gas, and its flow direction was coaxial with the laser. Representative specimens were then sampled to analyze their surfaces. Transverse and surficial microstructural morphologies of the recast layer, under the laser action, were analyzed using scanning electron microscopy and electron back scattering diffraction (EBSD). In addition, the recast surfaces were cleaned using anhydrous ethanol. The transverse surfaces were treated using coarse grinding, fine grinding, and polishing techniques. The polished surface was then etched with diluted aqua regia (volume ratio of HCl, HNO₃ and H₂O is 3∶1∶4).

The distribution of the main elements on the surface of the recast layer is analyzed using energy dispersive spectroscopy. The results indicate that no significant element change occurred along the thickness, except for a slight loss of Fe (Table 5). The grain growth mode of the recast layer is further analyzed using EBSD at the 1/3 site from kerf top and the kerf bottom site . The results indicate that epitaxial growth is the primary growth mode. However, the proportion of non-epitaxial growth at the 1/3 site from kerf top (Fig. 5) is observed to be higher than that at the bottom site (Fig. 7). A comparison between the IPF orientation distribution and pole figures in Figs. 9 and 10 also shows that the grain growth at the 1/3 site from kerf top exhibits some fluctuations with unmixed and unperturbed features.

The results show that a small amount of Fe evaporates from the recast layer surface. A variation in flow state from turbulent at the top to laminar at the bottom surface is observed, with an increase in thickness and needle-like grains. For crystal orientation, the ratio of the epitaxial growth at the top surface of the recast layer is lower than that at the bottom surface. Such a random distribution of epitaxial growth is caused by the turbulent flow at the former, whereas the dominant epitaxial growth is induced by the laminar flow at the latter. Considering the grain profiles, the γ phase in the base metal is equiaxed, whereas the δ phase is arranged in a banded form. The morphology of the γ phase grains in the recast layer is irregular and coarsen by approximately 2 times compared to those of the base metal. However, the δ phase is dispersed and refined from 1/6 to 1/2 of the base metal. Under the conditions of an extremely high-temperature gradient and a disordered disturbance owing to melting, a substantially reduced duration of δ phase formation with considerable dispersion is produced.