Microstructure evolution of Ni-Sn eutectic alloy in laser rapid solidification

The microstructure evolution of Ni-Sn alloys (Ni-28%Sn, Ni-30%Sn, Ni-33%Sn and Ni-35%Sn) near eutectic during laser rapid solidification has been investigated. In low velocity laser scanning, the microstructures of Ni-28%Sn and Ni-35%Sn hypereutetic alloys consist of refined primary dendritic phase, which is α-Ni phase for the former and Ni3Sn phase for the latter, and (α-Ni+Ni₃Sn) eutectic phase. However, the microstructures of Ni-30%Sn and Ni-33%Sn near-eutectic alloys consist completely of (α-Ni+Ni₃Sn) eutectic, and they undergo a morphological transition from the columnar to equiaxed eutectic from bottom to the top of molten pool. There is a small amount of residual coarse primary dendritic phase of substrate in the bottom of molten pool for these four Ni-Sn alloys. With increasing laser scanning velocity, compared with the mixed lamella and rod eutectic microstructures in the substrates, for these alloys eutectic in molten pool is completely composed of lamella eutectic, and lamella eutectic spacing is reduced significantly after laser rapid solidification. Besides, lamella eutectic in molten pool grows epitaxially along the normal to the molten pool interface with the substrate. Moreover, the composition range and the critical laser scanning velocity are also obtained for coupled eutectic growth during laser rapid solidification. In the present work, further analysis of microstructure evolution is given by using KGT and TMK models, showing a good agreement between the simulated and the experimental results.