Research On Modeling And Simulating Of Microstructure During Laser Melting Depositon For Invar Alloy

Invar alloy has been widely used to manufacture the precise structure and the mould of aeronautical composites owing to the low coefficient of thermal expansion.Laser melting deposition technology with the unique characteristics has a significant advantage in the forming of precise Invar alloy structure and repairing of Invar alloy mould.The properties of deposition structure are closely related to the characteristics of microstructure in deposition layer,while the traditional experiment method is difficult to develop the effective research on the evolution of microstructure in molten pool.This paper simulates the evolution of microstructure during laser melting deposition for Invar alloy by creating and coupling the finite element model and cellular automaton model.First of all,the characteristic geometry models of deposition structure for different parameters are created.The finite element models for temperature field during laser melting deposition of Invar alloy with different process parameters are created taking account of the interaction between the deposition powder and the laser beam.In addition,the cellular automaton model for the evolution of microstructure in molten pool is created based on the relevant theories of cellular automaton method.Moreover,the coupling model by the interpolation between the finite element model and cellular automaton model is created.Secondly,the temperature field during the laser melting deposition for Invar alloy with different process parameters is solved and the simulated results are checked.The simulated results show that the front shape of molten pool is ellipse with the short size before the laser beam and the long size behind the laser beam.In addition.The gradient of temperature before the molten pool is obviously larger than that behind the molten pool.The highest temperature in molten pool and the heat affected zone gradually increase with the increasing of laser power.However,the effect of scanning velocity on the temperature field is opposite of the laser power.Also,based on the interpolation calculation for the results of temperature field,the evolution of microstructure with different scale in molten pool is simulated.The simulated results show that the competitive advantage of mesoscopic columnar grains at the bottom of the molten pool is not only related to its own growth angle but also affected by the growth angle of surrounding grains.The essence of columnar to equiaxed transition at the top of deposition layer is the competitive growth between columnar grains and equiaxed grains.In addition,the cellular subgrains with the same growth angle at the bottom of the molten pool grow in a mutually parallel way and there is no obvious competition phenomenon.Finally,the confirmatory experiment on the microstructure during laser melting deposition for Invar alloy is developed.The macrostructure and microstructure of deposition layer are analyzed.The experimental results show that the different process parameters have an important influence on the size of deposition layer.The microstructure of deposition layer consists of the epitaxial columnar grains located at the bottom of molten pool and equiaxed grains located at the top of molten pool.In addition,at the top of the single-layer and multi-pass deposition layer,the equiaxed grains and columnar grains appear alternately.The rationality of simulated results are verified by comparing the experimental results and simulated results of different microstructure in deposition layers.Moreover,it can be concluded that the simulation model in this paper can effectively simulate the evolution process of microstructure in the molten pool during laser melting deposition for Invar alloy.