Anisotropy Of 316L Stainless Steel Fabricated By SLM And Its Influence On Properties

Additive manufacturing technology has developed rapidly in the past 30 years due to its free formability,short production cycle,and material saving.It has achieved a very important position in the material forming methods.316 L austenitic stainless steel has good corrosion resistance and good mechanical properties,and is widely used in machinery manufacturing,marine ships,petrochemicals and biomedicine.In this paper,the selective laser melting molding process is used to prepare high-density additive manufacturing parts with 316 L stainless steel powder as the material,and the internal mechanism of the anisotropy is studied by analyzing the anisotropy of its microstructure.At the same time,study the transfomation rule of different annealing temperature on the microstructure and the improvement mechanism of anisotropy.In addition,the influence of the anisotropy of the additively manufactured 316 L stainless steel structure on its corrosion resistance under sulfuric acid conditions and wear resistance under dry friction conditions was explored.The research in this paper will have important practical significance for the development of additive manufacturing stainless steel with excellent comprehensive performance.In this paper,through the porosity,microstructure morphology,grain size,grain shape and grain orientation,The anisotropy of the 316 L stainless steel prepared by SLM in the XY plane perpendicular to the printing direction and the YZ plane parallel to the printing direction is characterized in detail.;the results show that : There is no significant difference in surface porosity between the XY plane sample and the YZ plane sample.In terms of the micro morphology,the micro morphology of the XY plane sample is mainly composed of strip-shaped molten pools at an angle of 67 °;the micro morphology of the YZ plane samples is mainly composed of fan-shaped molten pools overlapping each other.In both samples,columnar structure growing perpendicular to the boundary of the molten pool and a honeycomb cell structure with a size of about 500 nm inside the molten pool were found.The width of the columnar structure of the XY plane sample is about 500 nm and the length is 10-30 ?m,which grows inside the strip molten pool;the width of the columnar structure of the YZ plane sample is about 300-400 nm,which can grow through several layers of molten pool,and the length can reach 100 microns level.In terms of the average grain size,the average grain size of the XY plane sample is 7.35 ?m,which is smaller than the 9.51 ?m of the YZ plane sample.In terms of the shape of the crystal grains,the crystal grains of the YZ plane sample are more like columnar crystals than the XY plane sample.In terms of grain orientation,the(001)and(101)planes are more in the crystal grains in the XY plane sample,compared with the(111)plane in the YZ plane samples.The recrystallization temperature of the 316 L stainless steel sample prepared by SLM is 876.6 ?.The strip or fan-shaped molten pool boundary of the sample begins to disappear at 800 °C,and the molten pool boundary almost disappears at 1000 °C.The growth of crystal grains is the cause of the disappearance of the molten pool boundary.The growth of grains does not significantly improve the orientation of the grains,but it can make the equiaxes of the XY plane sample and the YZ plane sample grains closer,which effectively improves the anisotropy of the material.On the basis of studying the anisotropy of 316 L stainless steel prepared by SLM,further exploration of the anisotropy of its corrosion resistance has been carried out.The results show that SLM specimens have higher corrosion resistance than bars,This is mainly due to the fact that there are a large number of fine grains inside the SLM sample to improve its corrosion resistance.The corrosion resistance of 316 L stainless steel prepared by SLM decreases with the increase of annealing temperature.The reason is that the annealing treatment will release the residual compressive stress on the surface of the sample,the growth of crystal grains,and the precipitation of the second phase.These factors will reduce the corrosion resistance of the material.The anisotropy of the structure of 316 L stainless steel prepared by SLM also has an impact on its wear resistance.SLM specimens have better wear resistance than bars,mainly due to the large number of fine grains inside the SLM specimens that improve their wear resistance.The XY plane sample have higher hardness and wear resistance than the YZ plane specimens,which is attributed to the flat specimens with finer grains and smaller grain aspect ratios.This is because the columnar crystals have worse wear resistance under this friction condition,and the wear resistance of YZ plane samples with more columnar crystals will be worse.The wear resistance of 316 L stainless steel prepared by SLM will decrease as the annealing temperature increases.The main reason is that the annealing treatment will release the residual compressive stress on the surface,the honeycomb cell structure will disappear and the crystal grains will grow.These factors will reduce the hardness of the material and make it easier for cracks to initiate and propagate.