Analysis on the microstructure regulation based on the pulsed laser oscillating molten pool in Laser-PTA additive manufacturing

A technological method of pulse laser assisted plasma transferred arc (Laser-PTA) additive manufacturing was proposed. The characteristics of microstructure such as austenite morphology, grain size and grain orientation were investigated through Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD) test. In particular, the mechanisms of pulsed laser affecting the molten pool and regulating the microstructure were revealed by the analysis on the temperature and dynamic response of molten pool. The experimental results show that an appropriate intensity of pulsed laser promotes the distribution uniformity of austenite, grain refinement and porosity reduction. The microstructure is transformed from lath-shaped austenite into skeleton-shaped austenite in deposition layer manufactured by Laser-PTA additive manufacturing. Furthermore, distribution of grain orientations in deposition layer became more random compared with those manufactured by PTA additive manufacturing. Under the excitation of pulsed laser, the molten pool oscillates in the boiling phase and oscillating phase, which strengthens the molten fluid flow. The enhanced fluid flow could reduce the temperature gradient and change the direction of the maximum temperature gradient, furthermore break the dendrites to form new heterogeneous nucleation particles. Therefore, the main factor causing the microstructure change is the oscillation of molten pool induced by pulsed laser.