Research On The Mechanism Of Molten Pool Solidification And Grain Growth During Laser Rapid Forming

Laser rapid forming manufacturing technology has been developed rapidly today, but currently the research and theory of the imbalanced temperature and the rapid solidification process in the process of laser rapid forming of molten pool is not yet clear. The microstructure of molten pool of laser rapid forming parts has a significant impact on the performance of the mechanical properties and the comprehensive utilization of the workpiece. Laser process parameters such as laser power, scanning speed and so on determine the macro temperature field and flow field etc. The degree of undercooling drives the liquid metal solidification and the flow of liquid metal influences the growth of grain, under their influence the micro morphology of the molten pool is formed. Laser rapid forming is a non-balanced and hightemperature transient. The real-time observation detection through the experiment to research the growth of the microstructure evolution in the molten pool is difficult to achieve at present. Therefore, the simulation of the growth and evolution of the microstructure of molten pool in the process of laser rapid forming has become a frontier subject. At present, the simulation of temperature field and flow field in the solidification process of the molten pool has achieved fruitful results. But the growth and evolution of TC4 titanium alloy laser rapid forming molten pool is in the initial stage, and there is no mature simulation outcomes published.In this paper, we use the technology that combined with Cellular automata method and Finite Element method to build a macro-micro coupling CA- FE(Cellular Automaton- Finite Element) model to simulate the growth and evolution of the microstructure of the molten pool solidification of TC4 titanium alloy laser rapid forming, combining with the theoretical simulation and experiment to analysis the mechanism of microstructure solidification of molten pool of laser rapid forming. FE method is mainly used to simulate the complex macroscopic physical fields, such as the temperature field and flow field in the solidification process of molten pool. Based on the macro finite element simulation results, then the CA method is used to simulate the growth and evolution of microstructure during solidification process of molten pool.Temperature field of the molten pool at the beginning of laser rapid forming process is a symmetrical shape, after then shows trailing phenomenon; liquid metal in the molten pool is the annular flow that from the center of the molten pool to the edge and the bottom of the molten pool. Combined with laser rapid forming experiment, temperature test and metallurgical microstructure segregation experiments, measuring and recording the microstructure and the temperature of the molten pool to obtain the degree of undercooling, cooling rate, the distribution of content elements etc under different process parameters. As a result, build the mathematical relationship between the laser process parameters and the degree of undercooling, cooling rate, temperature gradient to analysis the influence of process parameters on the laser molten pool solidification process. Compared with the experiments, The results indicated that microstructure of molten pool is a structure of ? columnar crystal which is compose of '? acicular martensite. Success to explore the mechanism of the molten pool solidification and grain growth.