Abstract
A quasi-continuous-wave laser direct energy deposition (QCW-DED) process was employed to regulate the deposit characteristics, molten pool morphology and microstructure of single-track nickel-based alloy. The variation of the deposit characteristics with laser pulse frequency was investigated through experiment and numerical simulation. The molten pool flow pattern and the dendrite growth mode were described based on the observations of the deposition morphology and microstructure in the cross- and longitudinal-sections. It was concluded that the QCW-generated single-track deposits decreased in width and depth, yet increased solely in height as the pulse frequency increased. A “U” shaped structure composed of substrate materials was observed in the molten pool at the low pulse frequency. The flow in the molten pool of QCW specimens was alternately dominated by the Marangoni force and impact force, whereas the fluid flow in the molten pool formed by continuous wave (CW) was entirely dominated by the Marangoni force. The disappearance of the “U” shaped structure and the rapid remelting effect of the molten pool underlie the epitaxial growth of dendrites at the high pulse frequency. In the CW specimen, disorganized dendrites were produced in the heat flow direction along the molten pool boundary.
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