| The selective laser melting technology can directly form functional structural materials with high precision and high performance,therefore,it is widely used in the production of complex shaped structural parts.However,due to the high price of fine powder raw materials on the market,complicated thermal history and process depended defects in the printing process,the printing cost of the deposited 316 L structural parts is high,the tissue defects of the sample are different,and the mechanical properties are unstable.In order to explore the effects of different initial particle sizes and processing parameters on the density,microstructure and tensile mechanical properties of the samples,and study the formation of unique micron-scale subgrain structure formed by SLM and the effect of subsequent heat treatment on subgrain structure.In this paper,316 L stainless steel powder was employed as raw material to study the microstructures and properties of the SLM-made samples under different parameters.Two kinds of powder with different particle sizes were mixed in different proportions as raw material.The phase composition,surface morphology,microstructure and mechanical properties were studied before and after inducing different amounts of coarse powder.It revealed that the phase composition of the powder remained unchanged after mixing,and both of them were single austenite.With the increase of the proportion of large particle size,the top scanning track morphology changes from flat to curved,the size of the side micro-molten pool decreases,the subgrain structure characteristics gradually weaken,and the unmelted particles wereobserved,and the composition segregation phenomenon exists in the unmelted particles and the introduction of O element.After mixing 20% coarse powder,SLM-made samples with mechanical properties up to 662 MPa and 47% can be fabricated.The influence of different processing parameters on the density of SLM-made samples was studied.It was found that the scanning speed was the main factor,followed by the scanning distance and the scanning layer thickness.In the stable forming range,the density of SLM-made samples increases firstly and then decreases with the increase of volume energy density.Considering the properties and deposition efficiency of SLM-made samples,the optimum forming parameters are determined as follow:scanning speed 600 mm/s,scanning spacing 80 m,scanning layer thickness 25 m.By comparing the density,microstructure and mechanical properties of SLM-made samples under different processing parameters,it is found that the effect of density on mechanical properties of SLM-made samples is greater than that of microstructure.Microstructure observation showed that the morphologies of strip,elongated and hexagonal honeycombs micron-scale subgrain structures caused by Marangoni convection,and all these structures existed in the top of the scanning track and in the side of "ripple" micro-pool.Some grains grew along <001> orientation,while most grains grew between <101> and <111> orientation.The fluctuation of local temperature gradient and surface tension gradient could cause the transformation of subgrain structure.There are spherical silica-rich oxide particles in the cellular structure.The ultimate tensile strength decreases from 673 MPa to 615 MPa and the elongation did not change after annealing at 800?+5 h+with furnace cooling.The impact energy of annealed samples is 77.68 J,which is lower than that of SLM-made samples.The micron-scale subgrain structure is an important factor to improve the mechanical properties of the SLM-made samples.However,after the heat treatment,the subgrain microstructure disappears,and the "grid-like" grain structure and columnar crystals appear resulting in a decrease in mechanical properties. |
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