Solidification behavior and microstructural evolution during laser beam-material interaction

A laser-assisted visualization technique has been used to monitor the solidification behavior at the tail of a molten pool created by scanning high energy density laser beam. A high speed digital camera with spatial resolution of 64 x 64 pixels and temporal resolution of 40,500 frames/s has been employed along with a novel concept of illuminating the interaction zone by a secondary visible probe laser. This technique enabled in situ monitoring of the solid/liquid interface due to the characteristic difference in the reflectivity between solid and liquid surfaces. It is observed that the solidification behavior is unstable and is highly influenced by the instabilities in the flow, which develop from the complex laser-material interaction process. Quite often the growth front remelted back due to the fluctuating thermal field driven by flow instability. The fluctuations in the growth front and the fluctuations in the laser-material interaction process have been monitored simultaneously, however, no correlation is apparent. The influence of flow instability on the resulting microstructure has been analyzed.