Investigation Of Microstructure And Mechanical Properties Of 15-5PH Stainless Steel Produced By Additive Manufacturing

With excellent mechanical properties and corrosion resistance,precipitation hardening(PH)stainless steels are widely used in aircraft components,marine facilities,and chemical and petroleum industries.The high strength of PH steel is originated from the precipitation of Cu phase in martensite matrix upon heat treatment.However,there are challenges in machining PH steel,due to its high hardness.Therefore,there is significant interest in using selective laser melting(SLM)and direct energy deposition(DED)methods to produce near net-shape parts with minimal subsequent steps of machining and processing.SLM technology was employed to produce 15-5PH.The effect of heat treatment(H900)on microstructure evolution and mechanical properties was studied.The results showed that numerous nearly spherical oxide inclusions,ranging from a few to several tens of nanometers,existed in the SLM sample.Additionally,a large volume fraction of retained austenite was observed in the as-built SLM 15-5PH,which resulted in a lower yield strength(YS)and ultimate tensile strength(UTS),but an improvement in elongation to failure(Ef)when compared to the as-received wrought 15-5PH with fully martensitic.SLM and wrought 15-5PH exhibited similar precipitation behaviors after heat treatment.However,after heat treatment,most of the retained austenite was transformed to martensite.Additionally,a finer grain structure,concentration of dislocations around the inclusions and at the submicron grain boundaries,and strain hardness promoted heat treatment SLM 15-5PH to demonstrate a higher strength(YS:1370 MPa;UTS:1495 MPa)than that of the wrought materials(YS:1296 MPa;UTS:1410 MPa)while still retaining a high Ef level of 14.4%.The effects of the scanning strategy on the density,microstructure,texture,and flowing mechanical behavior of the material were evaluated.Owing to the different thermal history,samples processed by SLM with different scanning strategies contained different austenite/martensite proportions.A weak ? fiber crystal texture along the build direction was observed,and the texture level was closely related to the scanning strategies.By using the island scanning strategy,high-densification samples with a fine grain structure can be obtained,improving strength(YS:892 MPa;UTS:1191 MPa)and elongation(Ef:18.7%).High-performance 15-5PH can be achieved even without the application of a precipitation hardening treatment.The results contribute to the tailoring of the densification,microstructure,texture,and mechanical properties of 15-5PH steel produced by additive manufacturing.The effects of building direction,heat treatment(H1025)and hot isostatic pressing(HIP)on microstructure and fatigue properties of SLM 15-5PH were studied.The results show that the building direction affects not only the microstructure of the material,but also the size and shape of the hole defects in the material,which in turn affects the monotonic tensile and fatigue properties.The fatigue limit was improved by heat treatment.After the HIP process,the number of hole defects was reduced and the fatigue life was improved.The fatigue crack of all samples originated from the hole defects near the surface.The distance from the hole defects to the surface,the shape and size of the hole rather than the number of the holes determine the fatigue life.Laser-based directed energy deposition(DED)was used to fabricate 15-5PH,and the geometry-dependent,location-dependent and direction-dependent microstructure and the mechanical properties of the resulting materials were investigated.Two different geometric components,deposition cuboid(DC)and deposition wall(DW)were fabricated,and the microstructure and mechanical properties of the samples were measured at different locations and in different directions.Finite element simulation was employed to interpret the thermal history of DED processing.The experimental and simulation results indicated that DED components presented different microstructure and mechanical properties in different geometries,directions,and locations,due to complex thermal history during DED processing.The use of post-heat treatment(H900)significantly decreased the amount of retained austenite and resulted in homogeneous grain structure,leading to uniform mechanical properties.However,after heat treatment,the part distal from the baseplate contained coarser grain than the part closer to the baseplate,leading to reduced mechanical properties.