Investigation of the relationship between nano-macro mechanical properties, microstructure, and process parameters in the additive manufacturing of 316L alloy

Owing to its admirable printability and the applications in high corrosive environment, the 316L alloy has been widely employed via LB-PBF. Though, the processing of such alloys through LB-PBF can produce fully dense parts with mechanical properties better than those found in conventionally produced components, the parts typically possess high residual stresses as well as mechanical anisotropy. The extraction of mechanical properties of such components via instrumented indentation test (IIT) at various scales, may also suffer distinctively due to the process-induced stresses and anisotropy. In this dissertation the macroand nanoscale IIT tests have been carried out on the LB-PBF 316L and the deviation of the obtained indentation properties from the tensile properties of similar or conventional components have been investigated. Two samples were sliced from the mid-length of the as-build block. Prior to the indentation tests, the two samples were mirror polished and etched as per guidelines for microstructure observations via optical microscope, scanning electron microscope and electron back scatter diffraction. The microstructure of the etched samples was observed before indentation tests, showing large grains and the microstructure morphologies consistent with the applied process parameters. Relatively large grains of the sample (of one of the sample) were macro indented (with a Vickers indenter) using a peak load of 150 N, 1 N/s loading rate, 30 s holding and 4 N/s unloading time). A The 10-repetitions cyclic indentation were with the constant peak load were used for macro-IIT. Each of the XY and YZ planes of the same sample were probed with three macro-IIT. The indentation properties from the XY plane show that it was softer as well as more compliant as compared to YZ plane. The other sample was indented using a Berkovich nanoindenter at a peak load of 100 mN, loading and unloading rate of 0.5 mN/s, and a dwell time of 10 s. A total of 264 number of nanoIIT tests were performed along five distinct directions. These five directions were part of a performance lines nanoindentation (PL-nIIT) methodology. The results show a strongest and stiffest region of the deposit closer to the deposit-substrate interface (EIT > 195) as compared to the regions located at the periphery of the deposit. The core of the deposit represents the average effects of the dilution from the deposit-substrate interface, convecting edges, and microstructure morphologies of the distinct layers. The softest and most compliant region was spotted at E1 zone (EIT = 128 GPa). The nano-IIT results were consistent with the extensive EBSD measurements of the same sample, showing correlation with the concentration of LAGBs in the relevant zones. The nano-IIT is highly sensitive to local changes such as anisotropy and residual stresses and to get a component-scale response a certain methodology such as PL-nIIT becomes helpful. Whereas, macro-IIT produces a large indent, covering several grains and is itself can provide the component scale or Tensile-like properties.