Effect of pulse duration on heat transfer and solidification development in laser-melt magnesium alloy

Abstract Compared to CW CO2 laser, Nd:YAG pulse laser is more suitable for some material processing due to higher absorptivity. However, surface ripples and microstructure inhomogeneities are main disadvantages during pulse laser melting on materials. Experiments were carried out to examine how laser melting influences surface topography and microstructure development of AZ91D Mg alloy. The laser-melt surface was characterized using Talysurf profiler, optical microscope and scanning electron microscope. Temperature distribution of molten pool was calculated from heat flow model to understand how heat transfer and fluid flow influence kinetics of rapid solidification under non-equilibrium conditions. The results implicate solidification behavior on microstructure evolution of laser-melt materials. At optimized parameters, thermally driven Marangoni flow became uniform, resulting in low height of ripples on top surface and less discrete bands in cross-sectional microstructure simultaneously, thereby improving homogeneity of microstructure in the molten pool.

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