Melting Pool And Microstructure Evolution Of Martensitic Stainless Steel In Selective Laser Melting

Selective Laser Melting(SLM),as a kind of additive manufacturing,has the advantages of high production efficiency,short cycle,complex parts forming,good surface quality,etc.It has a wide application prospect in aerospace,automotive electronics,biological medical and other fields.However,due to the intense temperature variation and unstable flow of the molten pool,it is easy to lead to the problems of balling effect and uneven microstructure,which affect the surface quality and service performance of the parts.Therefore,the molten pool flow and microstructure evolution during SLM processing should be studied in depth.The economical and fast numerical simulation method provides an effective research approach.However,most of the numerical models only focus on one or two aspects of temperature field,molten pool dynamics and microstructure evolution in SLM process,lacking of continuity and unity between the models.It is difficult to reveal the typical characteristic changes during the SLM process from all aspects and multi-scale.Therefore,the study took 6511 martensitic stainless steel as the research object,and established a simulation framework including a macroscopic temperature field model,a mesoscopic molten pool dynamics model,and a microstructure evolution model.They were independent of each other while transmitting data to keep them connected.Based on the simulation framework,the flow of molten pool,the morphology of molten track and the evolution of microstructure during SLM processing were studied comprehensively.First,a mesoscale powder spreading model was established by the discrete element method.The linear contact model was used to simulate the interaction between powders.Then,the influence of the bottom roughness on the quality of powder bed was investigated.Using average coordination number and packing density quantified the quality of powder bed.The model was verified by powder spreading experiment.A macroscopic single-layer multi-track temperature field model was established by the finite volume method.Gaussian body heat source attenuation model was used as the energy input in the model.The packing density obtained in the mesoscale powder spreading model was used as the boundary condition to distinguish the differences in the thermal conductivity between solid and powder.Thus,the scanning surface temperature field distribution was explored and the heat source parameters were calibrated.The temperature field model was verified by block forming experiment.Then,combined with the powder bed data in the mesoscopic powder spreading model and the heat source calibrated in the temperature field model,the mesoscopic dynamics of molten pool was established based on computational fluid dynamics.The study explored the effects of bottom roughness and laser power on the flow in the molten pool and track morphology by the model.Using laser confocal microscope observed the track morphology to verify the reliability of the model.Finally,in order to explore the influence of laser power on grain size and morphology,combined with the cellular automata method and the accurate temperature data obtained from the mesoscopic molten pool dynamics model established the microstructure evolution model.The reliability of the model was verified by observing the microstructure of the parts with ultra-depth of field microscope.The research results show that the bottom roughness has an important influence on the morphology of the powder bed and the melt track.With the increase of the bottom roughness,the powder content and the density of powder bed increase;the size of the molten pool increases accordingly,the internal flow of the molten pool is turbulent,and the balling effect of the track is gradually serious.With the increasing of laser power,the temperature and size of the molten pool increase,the internal flow speed up,the wettability and stability of the molten pool increase,and the balling effect of the melt track decreases gradually.The crystal size increases with the laser power and the crystal morphology is columnar,which grow to the top perpendicular to the boundary of the molten pool.