| Pulsed laser has the advantages of higher cooling rate,lower thermal accumulation and better mechanical properties compared with continuous laser melting deposition.However,the effects of pulsed laser on molten pool flow behavior,liquid-gas interface fluctuation,temperature distribution and surface quality are still unclear,and there are few reports on liquidgas interface fluctuations.Therefore,an in-depth understanding of the flow behavior of molten pool under different process parameters is of great significance studying the evolution of molten pool liquid-gas interface,improving surface quality and selecting reasonable pulse process parameters.In this paper,a three-dimensional symmetric numerical model is established based on Fluent for transient motion and heat and mass transfer of molten pool.The evolution process and flow behavior of molten pool liquid-gas interface under different process parameters are analyzed,and the relationship between flow,temperature and molten pool size and surface quality is established.The specific research contents and results are as follows:The flow behavior of molten pool and the evolution of liquid-gas interface of molten pool with different laser modes were studied,and the differences of molten pool size,maximum surface velocity and maximum temperature were compared.It was found that the pulsed mode increased the fluctuation of liquid-gas interface,and effectively enhanced the mixing and melting of powder particles in the molten pool.The maximum fluctuation difference of pulsed mode after stabilization was 1.8 times that of continuous mode.At the same time,the pulse mode can avoid the over-melting phenomenon in the deposition process.By changing the frequency and duty cycle,the formation process of molten pool,the fluctuation law of liquid-gas interface and the trend of surface velocity of pulsed laser melting deposition were studied.The fluctuation of liquid-gas interface first increases and then decreases with the increase of frequency,and first increases and then decreases with the increase of duty ratio.In the pulse mode,two annular convection rings are formed in the molten pool at the end of each cycle.The maximum flow velocity is located at the junction of the liquid-gas interface and the edge of the molten pool.The maximum flow velocity on the surface is inversely proportional to the frequency and directly proportional to the duty ratio.The effects of molten pool temperature and surface tension on the size and dilution rate were studied.The temperature change of the fixed point on the surface of the deposition layer and the change trend of the dynamic temperature of the molten pool under different frequencies and duty cycles were analyzed.The frequency and duty cycle range of the reasonable dilution rate were determined.It is found that the variation trend of the height of the deposition layer corresponds to the thermal accumulation trend of the molten pool.The width and penetration depth of the deposition layer are determined by the size of the heat affected zone.More than50 Hz will lead to poor metallurgical combination between the sedimentary layer and the matrix,and duty ratio greater than 4:1 will increase the probability of over-melting phenomenon and affect the performance of the matrix.The relationship between surface quality and liquid-gas interface fluctuation and surface flow was studied.The slag movement was analyzed,and the melt flow characteristics on the surface of the deposition layer and the interface morphology of pulse laser melting deposition were clarified.The liquid metal and slag particles on the molten pool surface flow radially to the surrounding under the drive of surface tension,and the fluctuation of liquid-gas interface can better balance the unmelted slag particles.The changing trend of the average value of Ra on the surface of the workpiece is the same as that of the liquid-gas interface,and the surface quality is relatively better when the liquid-gas interface fluctuates more. |
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