Thermal Dynamic Evolutions Of The Molten Pool During SLM Scanned Single Tracks:Simulation And Experiment

During selective laser melting process,the molten pool metallurgy plays a vital role on the part quality.However,owing to rapid moving and variation of the molten pool,it is hard to capture its dynamic evolution by experimental methods,which is based on trial and error,and can be costly and full of uncertainty.So,numerical simulation becomes a significant tool to reveal the molten pool behaviors and characteristics.The objective of the present research is to investigate the different factors that may affect the characteristics of the molten pool and its thermal dynamic evolutions.The main work and results are listed below.To establish the numerical model,the metal powder model and the physical model of the molten pool were built by the discrete element method(DEM)and the computational fluid dynamic(CFD)method,respectively.The volume of fluid(VOF)model was used to calculate the evolution of the gas/liquid interface.Additionally,the laser heat source and the recoil pressure were incorporated in the model based on the ray-tracing algorithm.Firstly,the effects of laser scanning speed on the molten pool evolution were studied.The state of the molten pool can be classified into three types: unstable state,transition state and stable state,based on the characteristics of temperature and velocity fields,surface morphology of single tracks(continuity and uniformity)and defects(balling and porosity).The influence of different molten pool states on the bulk density was also studied.Besides,it was discovered that the recoil pressure has dual effects on the bulk density: On the one hand,the recoil pressure balances the surface tension,stabilizing the molten pool;one the other hand,it may produce a keyhole,resulting in residual pores.Aiming at the dual effects of the recoil pressure,the variation of the recoil pressure and its effects on the molten pool profile and evolution were investigated specifically.It was found out that when the linear energy density was constant,the molten pool depression depth increased with the increased scanning speed,while the width of the solidified track decreased.When the scanning speed was constant,the molten pool depression depth increased with the elevated laser power,which the width of the solidified track was not influenced.The depression depth fluctuated within a certain range.The smallest value of the range combined with an as small value of depression depth as possible is the desirable,on which the process parameters should be chosen based.The role of Gaussian laser and flat-top laser into the molten pool evolution were also investigated.It was observed that the molten pool evolution mechanism under different laser types were totally different.Under the flat-top laser,the melting process started from the top region of the powder.The melt would be locally condensed into a spherical shape under the surface tension.If the baseplate could be timely fused,it would connect the spherical melt to produce a uniform and flat molten pool.Additionally,the effect of two laser types on the grain directional growth were studied.It was found out that the melt velocity was larger under the Gaussian laser than that under the flat-top laser.The latter was approximately 0.2 m/s in the half bottom of the molten pool cross section.There was several circumfluence in the molten pool under the Gaussian laser;whereas,no circumfluence was found in the molten pool under the flat-top laser.It was revealed that the flat-top laser is more advantageous for the continuous and directional epitaxial growth.