Physics-based computational modeling and time-resolved imaging of plasma plume generated by nanosecond laser interaction with a bed of micro metallic powder

Abstract

Although continuous lasers see frequent applications in selective laser melting or sintering, lasers with short pulse durations (such as those with nanosecond scale durations) possess potential advantages that may include good spatial resolutions and small heat-affected zones. Previous work on selective laser sintering with nanosecond lasers has been reported, particularly for laser micro sintering. At a sufficiently high intensity, a nanosecond (ns) laser pulse can generate a plasma plume from its irradiated metallic powder bed surface. The plasma plume evolution, such as the pressure it generates on the surface of the powder bed, may significantly influence the sintering process in the powder bed. However, physics-based modeling work for plasma generated by nanosecond-pulsed laser interaction with a metallic micro powder bed has been seldomly seen in literatures according to the knowledge of the authors’. Such modeling work has been reported in this paper for a ~4-ns laser pulse interaction with a cobalt micro powder bed, integrated with time-resolved plasma imaging using an intensified CCD (ICCD) camera with nanosecond scale gate widths for the model validation. For the conditions investigated, the model-predicted plasma plume evolutions agree reasonably well with the ICCD imaging results for the given period of comparison. This has reasonably supported the hypothesis posed in this paper (which has been rarely tested according to the knowledge of the authors’) that a short ns laser pulse-induced plasma from a metallic micro powder bed below the critical temperature can be reasonably well described by solving gas dynamic equations in the gaseous phase together with solving the heat transfer equation in the powder bed condensed phase, where the coupling is via the Knudsen layer (KL) relations for vaporization at the interface between the two phases. The model calculations show that in comparison with the bulk cobalt situation, the ns laser pulse can induce more significant surface vaporization from the cobalt powder bed, leading to a plasma plume with typically higher peak temperatures and densities in the simulated period. The plume can generate a short total pressure pulse with a ~491-MPa peak magnitude on the surface of the powder bed.