Grain Formation Model And Experimental Research On Laser Cladding Metal Layer

Material properties depend on microstructure, such as phase types and grain morphology. Grain morphology is decided by grain growth orientation and the growth velocity in each direction during solidification of melted metal. Laser cladding technology has the characteristics of rapid melt and rapid solidification and can produce some advanced materials which have good properties. In this thesis, grain morphology, mechanism and model of grain formation are investigated based on the theory of metal solidification and the method of grain simulation.To begin with, the powders transportation system was reformed. The electromagnetic speed regulation motor was equipped with electromagnetic speed regulation control apparatus. Even stepless speed regulation apparatus consists of motor, electromagnetic speed regulation control apparatus and redactor and has the advantage to control powder feed rate. We obtained some samples by changing the laser processing parameters such as laser power, scanning speed and powder feed rate. The microstructural analysis of samples showed that microstructure of laser cladding layers was made up of equiaxed crystal under the present experiment condition.Phase field model for pure material and control equation of laser cladding temperature field were set up on the basis of metal solidification theory, analyzing microstructure simulation methods and phase field model. Firstly, we calculated the temperature field equation of laser cladding, and then a macrocell chosen from macro temperature field calculation domain was acted as the micro calculation domain of microstructure simulation. By coupling microstructure simulation model namely phase field model with macro temperature field equation, this set of equations offer the prospect of being able to simulate grain morphology. Microstructural pictures indicated that grain morphology of laser cladding layer was equiaxed crystals. Simulation result was consistent with the experiment result. It was showed that microstructure simulation model and the method coupling phase-field model and macro temperature field model were right and feasible. Grain morphology can be predicted through this method. At the same time, calculation time was decreased by adopting this new method and using different size grid in different calculation domains.In this paper, grain simulation method of solidification was successfully used to simulate grain morphology of laser cladding metal layer through applying this macro- and microscale coupling method. The coupling method can predict grain morphology of laser cladding layer. By analyzing the influence of laser cladding parameters on even grain size, it showed that grain size can be decreased through increasing scanning speed and powder feed rate and decreasing laser power. The mechanism of equiaxed crystals formation was analyzed according to dynamics and thermodynamics conditions of solidification and the experimental condition of laser cladding.