The evolution of surface topography of 304L stainless steel, induced by long pulse laser with a wavelength of 1070 nm and a pulse width of 10 μs and a pulse number of 400 in air, was investigated in the fluence range between around 3 J/cm2 and 210 J/cm2. Three-dimensional (3D) morphologies and two-dimensional (2D) cross-sectional profiles of microstructures at different laser fluences were compared. Furthermore, surface profiles of the microstructures were quantified by using the inner diameter, the outer diameter, the elevation of central area and the rim height thereof. The trends of the above four geometrical parameters and surface profiles suggest five laser fluence ranges, where the corresponding topographies change from bump to crater. Energy dispersive spectrometer (EDS) was used to analyze surface elements of different microstructures, and the results showed that the content of oxygen had a great influence on surface tension and surface topography. In this study, a complex picture was drawn where melting, vaporization, solidification, melt displacement, melt expulsion and plasma shielding take place at different laser fluences.

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