| Selective laser melting(SLM)is a high-end digital manufacturing technology based on the principle of preset powder laying,layered manufacturing and superimposed forming.This technology has received extensive attention in the field of complex precision component forming processing for it has obvious advantages in user-defined shape design,high machining accuracy and short processing cycle.During SLM process,the flow and heat transfer behavior of molten pool and other thermal fluid behavior directly affect the quality of the forming parts.However,the thermal fluid behavior of molten pool and its related physical mechanisms are currently not very clear,thus it is difficult to stably control the forming process and there are typical metallurgical defects such as pores and cracks in the forming parts,which limit the wide application of SLM technology and its forming parts.Therefore,studying the thermal fluid behavior of molten pool,investigating the influence of environmental conditions,operating conditions,laser processing parameters and other factors on it,and understanding deeply of the physical mechanism of SLM process can lay a theoretical foundation for reducing defects and improving product quality,and it can also provide the necessary theoretical guidance for the design and optimization of process parameters.Due to the molten pool is small(100?200?m)and the cooling rate of melt is very high(10~7K/s),it is difficult to analyze the complex physical mechanism in dynamic SLM forming process through experiments at present,and numerical simulation is an effective way to solve this problem.Therefore,in this paper,a three-dimensional mesoscopic model that can distinguish single-particle powder is established for SLM of 316L stainless steel.Through this model,the simulation research is carried out for single track and double tracks forming under the continuous exposure scan pattern,and the single pass forming under the point exposure scan patten.The study revealed the formation mechanism of droplet spatter and humping,and clarified the influence of ambient pressure,substrate preheating temperature,laser processing parameters,etc.on the thermal fluid behavior of the molten pool and the solidification morphology of melt track during SLM of 316L stainless steel.The reliability of the research results is verified through experiments.The main results are as follows:(1)The effects of ambient pressure and substrate preheating temperature on the flow and heat transfer behavior of molten pool in the single-track forming process are studied.It is found that the recessed keyhole caused by evaporation in the molten pool becomes more obvious and the maximum flow velocity of the melt under the keyhole increases gradually as ambient pressure decreases from 10~5 Pa to 10~1 Pa,which is beneficial to the transfer of heat from molten pool to the molten pool depth.Moreover,the average keyhole depth increases from 48?m to 83?m,but the fluctuation of the keyhole depth does not increase as the decrease of the ambient pressure.Compare with atmospheric pressure(Pabm=10~5Pa),the change of laser processing parameters(laser power and scanning speed)has a greater influence on the keyhole depth under the subatmospheric pressure(Pabm=10~3Pa).In addition,the substrate preheating temperature can promote the downward flow of melt and reduce the temperature gradient in the forming process,as well as the change of substrate preheating temperature has a certain influence on the morphology of the melt track.(2)The effects of laser processing parameters on the thermal behavior and solidification morphology of melt track in the single-track forming process are studied.Results show that the maximum cooling rate of the melt in solidifying mushy zone decreases from 0.4×10~7 K/s to 0.31×10~7 K/s as the laser power increases from 85 W to125 W,and the maximum cooling rate of the melt in solidifying mushy zone decreases from 0.84×10~7 K/s to 0.31×10~7 K/s as the scanning speed decreases from 1200 mm/s to400 mm/s.The decrease of cooling rate is disadvantageous to grain refinement in molten pool.Moreover,the width of the welt track increases gradually and the rippling on the melt track surface becomes more obvious with the increase of laser power and the decrease of scanning speed.The humping formation is periodic in the process,increasing the input laser energy and reducing the particle size of the powder can reduce the humping formation.The change of morphology of the melt track with scanning seepd predicted by simulation is in good agreement with the experimental results,and the error of the melt track width between simulation and experiment is 9%.The suitable processing parameters confirmed by this study for the single-track SLM of 316L stainless steel as follows:the laser power is 125 W,scanning speed is 400 mm/s,laser diameter is 60?m and powder layer thickness is 50?m.(3)The molten pool behavior with evaporation(evaporated molten pool)and the formation mechanism of droplet spatter in SLM process are revealed,as well as the influence of typical laser processing parameters on the droplet spatter formation is clarified.Results show that the the molten pool surface has periodic oscillation under the action of recoil pressure as evaporation takes place in the molten pool.The rippling formation on the melt track surface is related to the periodic oscillation of the molten pool surface.The droplet spatter formation at rear of molten pool experienced four stages:a recessed keyhole is formed in molten pool with evaporation,the melt at the back wall of the keyhole gushes upward and forms a bump on the molten pool surface,a liquid column is formed between the bump and the surrounding melt,and the liquid column formed a neck and gradually pinched off,resulting in the droplet spatter.Moreover,the appearance time of each stage of droplet spatter formation become shorter with the increase of laser power.The melt track morphology predicted by simulation is in accord with the experimental result.These results enhance the understanding of reducing droplet spatter and improving forming quality.(4)The effects of hatch spacing,scanning speed and scanning mode on the thermal behavior,density and surface morphology of during the double track forming process are studied.With the increase of hatch space,the preheating effect and remelting phenomenon between two adjacent melt tracks are weakened,the trend of flow and heat transfer of molten pool to sintered area,the surface quality of the overlapping track as well as the temperature gradient along the overlapping direction decrease gradually.With the increase of scanning speed,the height and density of overlapping track decrease gradually,and the overlapping track is also prone to necking.In addition,the S-shaped scanning mode with the opposite scanning direction and the Z-shaped scanning mode with the same scanning direction have less influence on the morphology of overlapped track in the middle area as other parameters remain unchanged.The overlapped track predicted by simulation is in good agreement with the experimental result.Through this research,the change mechanism of the thermal behavior and the morphology of the overlapped track with the process parameters in the lapping process is clarified,which has a reference value for preparation of dense sintered layer products.(5)The thermal behavior and the influence of different process parameters on it during the single-track SLM of 316L stainless steel in a point exposure scan pattern are investigated.It is found that the surface size of molten pool gradually reaches a relatively stable state with the increase of exposure points in the process.In addition,as the exposure time increases from 30?s to 90?s,the trailing phenomenon on the molten pool surface gradually weakens,the maximum temperature and the duration of liquidus of the melt at different exposure positions increase,and the flowability and spreadability of the melt enhance accordingly.Simultaneously,the maximum temperature gradient of the molten pool along the scanning direction also increases from 2.77×10~7 K/m to 8.01×10~7 K/m with the increase of exposure time.The influence of the change of laser power and exposure time on the thermal behavior of melt is consistent,while the influence of the change of point distance on the thermal behavior of melt is more complex.The change of molten pool depth with exposure time obtained by simulation are in good agreement with the experimental results.The linear energy density confirmed by melt track morphology and molten pool depth for the single-track SLM of 316L stainless steel in point exposure scan pattern should be no less than 371 J/m.The research results can provide a theoretical basis for the establishment and optimization of SLM process parameters under the point exposure scan pattern. |
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