Numerical Investigation On Temperature Field And Flow Field During Selective Laser Melting Of Ti-6Al-4V

Selective Laser Melting is a kind of metal additive manufacturing technology that melts and solidify metal powder under the thermal action of laser beam.It is very suitable for the production of small batch complex structures owing to the advantages of high molding precision,good mechanical properties,short production cycle and wide range of processing materials,and is widely used in the fields of aviation,aerospace,medical and industrial manufacturing.In the process of Selective Laser Melting,linear energy density directly determines the size of the molten pool and the forming quality.In addition,Selective laser melting(SLM)involves complex physics such as molten metal flow and rapidly changing gas-liquid interface in molten pool,which have a significant influence on microstructure and mechanical properties of forming parts.It has become an important means to deepen the understanding of the mechanism of SLM by numerical simulation.In this paper,the size of the molten pool,the molten metal flow and the gas-liquid interface evolution in molten pool based on Ti-6Al-4V titanium alloy are studied.A powder model with different particle sizes and random distribution was established.VOF method was applied to trace the gas-liquid interface and the laser heat source is a combination of a parabolic rotating body heat source(lower half of the powder layer)and a cylindrical heat source(upper half of the powder layer)in physics model.Moreover,by quoting the C_VOF multiplication factor into the heat source,the laser heat source is loaded only in units containing both metal and gas phases.A number of single channel scanning experiments were carried out and the accuracy of the model is verified by the simulated results with different parameters agree well with that of experiments.The size of molten pool,molten metal flow and gas-liquid interface evolution of a set of laser scanned with different laser powers,scanning speeds,powder layer thicknesses and scanning distance during SLM were studied.The temperature distribution of molten pool and the surface morphology of molten pool were obtained under different technological parameters.The variation of molten pool size under different process parameters is analyzed,and the relationship between the depth and width of the molten pool with the line energy density(the ratio of laser power and scanning speed)is fitted.For the laser power of 300 W,the scanning speed of 1.5m/s,the thickness of the powder layer of 40 um,when 300 us,the molten pool size is basically stable,and the depth to width ratio of the molten pool is about 0.5.In addition,with the increase of laser power and the decrease of scanning speed,the depth and width of the molten pool are increasing.And with the increase of scanning speed,the depth and width of the molten pool decrease exponentially.When the actual powder thickness is 40 um,the threshold of line energy density is about 170J/m in order to ensure the molding quality.With the increase of the thickness of the powder layer,the flatness of the surface of the melt decreases.When the powder thickness is greater than 80 um,there are many dents in the melt.Through the study of the flow process of molten metal near the gas-liquid interface during single channel and double channel scanning,it can be seen that for single channel scanning,the main factors affecting the formation of the pores are the scanning speed and the thickness of the powder layer.The rapid scanning speed or the excessive thickness of the powder layer is not conducive to the escape of the gas body in the powder void.For double channel scanning,the main factor affecting the formation of the pores is scanning distance.With the increase of scanning distance,the number of pores increases.For the process parameters of the powder layer thickness of 60 um,the laser power of 300 W and the scanning speed of 1.5m/s,the scanning distance should be controlled below 130 um in order to ensure the quality of the molding.The numerical model is used to quantitatively study the effect of different parameters on the build quality.It has a certain guiding function for the setting of actual process parameters.