Study On Spatter Behavior And Molten Pool Characteristics In Selective Laser Melting Based On High-speed Photography Technique

Selective laser melting(SLM)is a precise metal additive manufacturing technology,which has great advantages in high quality,high performance and integrally forming of sophisticated metal parts.Meanwhile,diversified process parameters and complex manufacturing process of SLM often lead to defects such as balling,pores and lack of fusion of forming parts.However,the off-line method is mostly adapted for detecting the forming defects in SLM currently,which has the disadvantages of hysteresis and inability to intervene in advance.The detection accuracy of traditional vision and thermal imaging technology is difficult to meet the application requirements of SLM.Therefore,based on high-speed photography technology,the mechanism of spatter generation and its transformation law,the evolution law of molten pool and its influence on the morphology of the molten track during SLM were studied in this thesis.The main contents and results are as follows:(1)An in-situ high-speed and high-resolution microscopic imaging system of SLM based on high-speed photography technology was designed and built independently.The system achieved high-resolution monitoring of fast moving micro-molten pool and high-speed flying micron-scale spatter during SLM with a time resolution of up to 2.5?s and a spatial resolution of up to 3.34?m.(2)The physical phenomena such as melting,boiling,depression,protrusion and droplet column ejection during the laser-matter interaction of SLM have been studied in a wide range of laser power.With the increase of laser energy density,the occurrence time of the above physical phenomena is advanced,and the time required decreases exponentially.The laser power(with constant laser mode and laser spot size)determines the required accumulate time of the physical phenomenon during SLM,while the scanning speed decides whether the dwell time of laser beam is sufficient to form those phenomena.(3)Based on the methods of"statistical characteristics of growth rate for spatters"and"kinematic analysis of typical spatter trajectory",the spatter behavior and its generation mechanism during single and double laser SLM are studied.During single-laser SLM,the dominant mechanism of spatter generation changes from the“unstable molten pool”into“vapor entrainment effect”.With the mechanism changes,the growth rate of spatters decreases.With the increase of the laser volume energy density,the growth rate of spatters increases,and the corresponding time of transition starting point is advanced.The spatters generated by the vapor entrainment is more directional,with accelerations of up to~10~4 m/s~2.During double-laser SLM,the mechanism of spatter generation transforms many times from the above-mentioned methods in single-laser as the double-laser scanning mode changes.The spatters gradually converge toward the double-laser metal vapor overlap region,leading to an increase in the number of spatters in the middle of double-laser interaction zones,while the spatter ejection trajectory is deflected and eventually tends to be vertically upward.(4)The evolution law of molten pool and its influence on the morphology of the molten track during SLM processing GH4169 and CuZn10 alloys are studied,respectively.The results show that during SLM processing GH4169 alloy,the threshold of escape velocity for droplet columns determines whether it can break away from the molten pools.The back surge of molten pools and the ejection behavior of droplet columns affect the morphology of the initial molten tracks.During SLM processing CuZn10 alloy,based on theoretical calculation and on-line observation,the formation and healing mechanism of molten track pits caused by the burst of molten pools were studied.The internal relationship between"laser energy density-temperature in molten pool-saturated vapor pressure of Zn"is explored.When the saturated vapor pressure formed by volatilization is larger than the ambient pressure of SLM,the vapor expansion leads to the burst of molten pools and the formation of pits in molten tracks.However,when the dwell time of liquid in molten pools determined by the scanning velocity is long enough,the melt in molten pools can backfill into the pits and the flatness of molten tracks can be improved greatly.Finally,the theoretical calculation is in good agreement with the experimental results.