Effect Of Post-annealing On The Structural Stability And Mechanical Properties Of Additively Manufactured 316L Stainless Steel

The trade-off of "strength-ductility" has always been a dilemma in the material field.Recent studies have shown that most of the mechanisms used to enhance the strength of crystalline materials,such as reducing the size of grains and introducing the interface of grain boundaries,result in reduced ductility.Many steel-strengthening methods usually sacrifice the useful ductility of the material.New metallurgical processes provide great chances to overcome this problem.In this study,the process the traditional forging process to prepare 316 L stainless steel was abandoned.While,the additive manufacturing process-selective laser melting(SLM)technology to prepare 316 L stainless steel parts was adopted to overcome the problem of the “strength-ductility” trade-off.It is demonstrated that austenitic 316 L stainless steel prepared by selective laser melting technology exhibits a combination of yield strength and ductility over conventional forged 316 L stainless steel.In this paper,316 L stainless steel samples were processed by the SLM technique,and then isothermally annealed at different temperatures.The mechanical properties of the samples were tested by microhardness testing and tensile testing;and the microstructures were characterized by optical microscopy,electron backscatter diffraction,transmission electron microscopy,and three-dimensional atom probe techniques,respectively.An optimized heat treatment process to increase the yield strength of the 316 L stainless steel without sacrificing ductility was achieved and the effects of annealing on the microstructural stability and the mechanical properties were studied.The results of the present study can provide a way for the development of high-performance metals through additive manufacturing processes.The main conclusions are as following.(1)316L stainless steel was successfully prepared by SLM technology,and the microhardness of the sample was 252 Hv at room temperature.The SLM samples were annealed at different temperatures.After annealing at 400°C for 1h,the microhardness increased to 291 Hv;after annealing at 800°C for 1h,the microhardness decreased to 228 Hv.(2)The yield strength of the original printed sample is 586 MPa.After annealing at 400°C for 1h,the yield strength increases to 642 MPa;after annealing at 800°C for 1h,the yield strength of the sample drops to 482 MPa.Despite the changes in strength of the samples in three states,the elongations do not change too much,around 35%.(3)The SLM 316 L stainless steel exhibited a multi-scale structure composed of grains,unit cells,dislocations and nano-sized particles.Annealing at 400°C for 1h increases the yield strength by 10%,due to the thermal stability of the cellular subgrains and dislocation structures,as well as the nano-silicate particles precipitation.The retention of the cellular subgrain and the dislocation structure maintains the yield strength and elongation of the prepared material,while the silicate particles contribute to the strength of the 56 MPa increased by the Orowan strengthening mechanism for the sample annealed at 400°C for 1h.However,annealing at temperatures above 400°C results in a constant decrease in material strength because the cellular subgrains and dislocation structures are no longer stable and dissolution of the silicate particles occurs.