Numerical simulation and experimental study on laser micromachining of 304L stainless steel in ambient air

The evolution of surface topography of 304L stainless steel, induced by a fiber laser with a wavelength of 1070 nm and a pulse width of 10 mu s in ambient air, is studied experimentally in the fluence range between roughly 30 J/cm(2) and 118 J/cm(2). A two-dimensional (2D) transient model is developed to simulate the laser micromachining process of 304L stainless steel, which incorporates the heat and mass transfer as well as fluid flow. An oxidation kinetics model is established to describe the mass inflow caused by high temperature oxidation of stainless steel in both solid and liquid phases, and the oxygen concentration on the surface of the molten liquid is treated as a function of temperature and then applied to the expression of surface tension coefficient. A modified level-set method is developed to trace the free surface and consider the mass transfer caused by oxidation and evaporation, and the possible effects on the free surface boundary containing surface tension, recoil pressure, Marangoni effect and the shear stress caused by metallic vapor flow are coupled in the model. Based on this model, the impacts of laser fluence, ambient air and pulse duration on both laser micromachining processes and the evolution of surface topography from the bump to the crater are numerically investigated. Comparing the experimental results and the numerical results, the validity of the model is verified, and the formation mechanisms of both various topographies and phenomena during laser micromachining are analyzed. The results show that with the increase of laser fluence the irradiated zone presents the "flat-topped" bump, the "M-shaped" bump and the "Gaussian-like" crater successively. Furthermore, the mass inflow has a great influence on the topography of both the "flat-topped" bump and the "M-shaped" bump. Even though the surface tension considering oxygen concentration plays an important role in the formation of both the "M-shaped" bump and the "Gaussian-like" crater, evaporation largely determines the surface topography of the "Gaussian-like" crater at high laser fluences. (C) 2019 Elsevier Ltd. All rights reserved.