Comparison of Strain Rate Sensitivity between As-Built and Stress-Relief Annealed Additively Manufactured 316L Stainless Steel

ABSTRACT

The strain rate sensitivity of additively manufactured 316L stainless steel is compared between material in the as-built condition and after being subjected to a stress-relief anneal. The mechanical properties are assessed using tensile testing at strain rates spanning the orders of 10−3–103 s−1, utilizing a hydraulic load frame for low-rate tests and a tensile split-Hopkinson bar apparatus for the dynamic tests. The rate sensitivity is quantified using three different parameters: the Johnson-Cook c parameter, the rate sensitivity parameter m which relates the change in flow stress to the strain rate using an exponential function, and the rate sensitivity parameter β that is related to the material activation volume. Optical microscopy and X-ray diffraction measurements indicate that the dislocation motion because of the thermal activation energy from the annealing process results in an increase in the sub-grain crystallite size as the dislocations are rearranged. This causes an increased rate sensitivity in the material after the stress-relief annealing process. In the as-built condition, the material exhibits strain rate sensitivity parameters of c=0.022, m=0.018, and β=11.6 MPa, whereas after stress-relief annealing, these parameters increase to c=0.028, m=0.025, and β=12.1 MPa.