Influence of Size Design on Microstructure and Properties of 304L Stainless Steel by Selective Laser Melting
As one of the most promising metal additive manufacturing methods,selective laser melt-ing(SLM)is very attractive for fabricating complex-shaped structure components of austenitic stainless steels in the nuclear field.SLMed austenitic 304L stainless steel has been demonstrated to have excel-lent mechanical properties and superior corrosion resistance due to the unique hierarchical microstruc-ture produced by the ultrafast cooling rate and high-thermal gradient.Scanning tracks(T)and depositing layers(L)are key factors for geometry design by affecting the processing efficiency and the solidification structure of the components.Hence,it is essential to clarify the effecting mechanism of sample size and geometry on material performance.In this work,samples of different sizes are designed to study the influ-ence of geometry on the microstructure and mechanical properties of 304L stainless steel components made via SLM.Various solidification conditions are achieved by varying the temperature gradients and cooling rates by adjusting T and L.Metallographic microscopic observations in samples with various T×L combinations demonstrate that a columnar structure is formed along the build direction,which is signifi-cantly impacted by the geometry effect.The columnar grains grow preferentially along the heat dissipa-tion direction with an increase in the sample size.The columnar grains gradually change from having a low length-diameter ratio(LDR)with a rice grain shape to a higher LDR with a short rod-like shape and then a long strip shape.Grain coarsening could also be identified along with the formation of"long strip"columnar grains.Moreover,consistent microstructure evolution behavior is observed in large-sized sam-ples.The influence of geometry on the mechanical properties is examined via tensile testing to demon-strate the decrease in yield strength and increased plastic elongation with the rise in sample size.As the sample size increases,the mechanical properties become consistent.The comprehensive analysis con-cludes that grain size and columnar grains play critical roles in determining the mechanical properties ac-cording to the Hall-Petch relationship.In larger-sized samples,"long strip"columnar grains with a high proportion could lead to a decrease in material strength and an increase in plasticity.The geometry mech-anism affecting the solidification process,microstructure formation,and mechanical properties of 304L stainless steel processed by SLM is explored by combining the solidification rate and thermal gradient simulation results using ANSYS ADDITIVE.
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