Study On The Effect Of Microstructure Orientation On Properties Of 316L Stainless Steel By Selective Laser Melting

Selective laser melting(selective laser melting,SLM)is an important branch of metal additive manufacturing technology,which has outstanding advantages in the processing of complex shaped parts and rapid prototyping of workpieces.For the engineering application of this kind of complex parts,it is of great scientific significance to study the relationship between the characteristics of the microstructure and the mechanical properties.In this paper,gas atomized 316 L powder is used as the raw material for sample preparation,and the forming process,microstructure,mechanical properties and post heat treatment are studied.(1)The influence of scanning speed on microstructure and property is explored.Too fast scanning speed will result in shallower molten pool,and it is easy to cause insufficient melting of powder,resulting in large defects such as Lack of fusion.When the scanning speed is too slow,the molten pool is deeper and the powder is fully melted,but the amount of liquid is too large and the surface tension cannot be fully spread,causing defects such as spheroidization and forming pores.When the scanning speed is 400 mm/s,the microstructure is more uniform,the density is better,the strength and plasticity reach a better combination,and the optimal process is obtained.(2)The effect of the directionality of the microstructure on the property is studied.The microstructure has an obvious hierarchical structure,including micron-level columnar grains and molten pools,and nano-level cellular substructures.The columnar grains and molten pool are parallel to the building direction and have obvious directionality.The tensile specimens processed in different directions perpendicular to the building direction(transverse)and parallel to the building direction(longitudinal)have obvious anisotropy observed in the tensile test.The in-situ tensile test shows that the geometric relationship between the molten pool boundary and the tensile force is the cause of the anisotropy of tensile properties.When the loading direction is parallel to the boundary of the molten pool,or at an acute or obtuse angle,the sliding deformation of the boundary of the molten pool will be greatly restricted,and the cellular substructure will coordinate greater deformation until it ruptures.On the contrary,when the loading direction is perpendicular to the boundary of the molten pool,the boundary of the molten pool will tear quickly,resulting in lower yield strength and elongation.(3)The effect of post heat treatment on the directionality of the microstructure and the property was studied.After the original state(371°C annealed)of the 316 L stainless steel sample was annealed at 600°C/2 h,the yield strength and elongation of the transverse and longitudinal samples did not change significantly,and the hardness of the top and lateral surfaces did not change significantly.After annealing at 850°C/2 h,the change in strength and plasticity is still not obvious,and the hardness decreases slightly.After annealing at 1100°C /2 h,the yield strength of the transverse specimens decreased by about 200 MPa and the plasticity increased by about 13%,while the yield strength of the longitudinal specimens decreased by about 150 MPa,and the plasticity increased by about 17%.Secondly,the hardness of the top and lateral surfaces also decreased by approximately 60 HV.The reason may be that after annealing at 1100°C /2 h,the dislocation density decreases,the molten pool boundary and cellular substructure disappear,and the mixed grain of fine grains and columnar grains is formed,resulting in the decrease of hardness,yield strength and plasticity.