Research On Service Properties Of 316L Stainless Steel By Selective Laser Melting Additive Manufacturing

Selective laser melting（SLM）,a mature technology of additive manufacturing,has attracted extensive attention in aerospace,medical implants,art manufacturing and other fields due to its advantages of high material utilization,low cost and easy customization of complex parts.As a typical austenitic stainless steel,316L stainless steel has been successfully used in manufacturing,construction and medical parts manufacturing.However,the research on the high temperature properties,impact properties,fatigue properties and anisotropy of 316L stainless steel prepared by SLM is still insufficient,which limits its further application.In this paper,the research status of the related fields of 316L stainless steel prepared by SLM is reviewed.The influence of process parameters on the forming quality and mechanical properties of the samples is summarized,and the corrosion and high-cycle fatigue properties are studied to evaluate its practical application prospects.The main research contents are as follows:（1）The effects of scanning speed and laser power of SLM on relative density,surface roughness and surface morphology were investigated experimentally.It was found that with the increase of laser energy density,the relative density of the SLM samples increased firstly and then stabilized.The top surface roughness was most sensitive to process parameters and had a large fluctuation range,while the left side and rear side were less affected by process parameters.When the scanning speed was fixed at 300 mm/s,the forming quality of the top surface could be optimized by increasing the laser power appropriately.However,when the sample size was large,the excessive laser power would damage the surface quality.The phase composition and microstructure of SLM samples were characterized by XRD and SEM,and the effect of SLM preparation process on the microstructure and phase composition of 316L stainless steel was studied.The samples prepared by SLM were mainly austenitic phase,and the content of（200）crystal plane was obviously increased compared with the forgings.Compared with the forgings,the grains were finer and there were a lot of columnar crystals and ultrafine equiaxed crystals in the microstructure.（2）The influence of process parameters on Vickers hardness was systematically studied by adjusting the laser power and scanning speed during SLM preparation.With the increase of laser energy density,the Vickers hardness of SLM sample increased firstly and then stabilized at about 220 HV,and the highest was 246.1 HV,which was much higher than that of the forgings（170.3 HV）.Based on the relationship between process parameters and relative density,316L stainless steel samples with laser power of 200W and four different scanning speeds were prepared,and tensile tests at room temperature,tensile tests at high temperature（400℃）and impact tests at room temperature were carried out respectively.It was found that the room temperature tensile strength of the SLM samples was up to 704.0 MPa,which was slightly higher than that of the forgings of 646.4 MPa.The highest elongation at break was 55.19%,far less than 69.56%of the forgings.There were a lot of dimples in the fracture,which was obviously plastic fracture.At 400℃,the highest tensile strength of the SLM samples was 448.6 MPa,which was slightly lower than 468.9 MPa of the forgings,and the highest elongation at break was 43.85%,which was far lower than the elongation at break of the forgings measured at 100.19%.Through fracture morphology analysis,although the fracture was rougher than room temperature,a large number of dimples were still observed,but a small number of areas were transformed into brittle fractures.The impact toughness of the SLM samples under room temperature impact could reach up to 119.8 J/cm~2.The SEM analysis of the fracture morphology showed that there were a lot of dimples and some peeling morphology at the fracture.Different scanning strategies were designed to study the influence of scanning strategies on mechanical properties of samples.It was found that scanning strategies had little influence on tensile strength at room temperature,but had great influence on elongation at break.（3）The corrosion samples and high cycle fatigue samples with laser power of 200W and scanning speed of 400 mm/s were prepared by SLM.The corrosion properties of the samples in 3.5 wt.%Na Cl solution and high cycle fatigue properties at r=-1 were tested.It was found that after immersed in 3.5 wt.%Na Cl solution,the corrosion sites and areas of the SLM samples were less than those of the forgings,but the degree was more serious.The R_ct values of different characteristic surfaces of the SLM sample were analyzed,and the results showed that the left side>the rear side>the top,with the value of the forgings located between the left and the rear.After immersed for 8 weeks,the R_ct of the rear surface increased slightly,while the R_ct of the forgings decreased significantly.E_pit values were shown as the top>the rear>the left>the forgings,which was same to i_pit.After immersed for 8 weeks,the E_pit and i_pit of the rear surface decreased significantly.The fatigue limit of the SLM samples was 257.5 MPa at r=-1,which was higher than that of coarse static 316L stainless steel in reference.There were cleavage and brittle fractures without any dimples at the fracture.