Effect of printing parameters on the structure and high strain rate deformation behavior of additively manufactured 316L stainless steel

Abstract

High strain rate deformation is characterized by strain localization and formation of adiabatic shear bands (ASBs), as opposed to the movement of slip systems seen in quasistatic deformation. Several studies have been done on the effect of printing parameters on the structure and properties of 316L stainless steel, little attention has been paid to the high-strain rate deformation behaviour of this material. In this study, the effect of printing parameters on the microstructure and high strain rate deformation behavior of 3D-printed 316L stainless steel alloy was investigated. Printing parameters such as hatch spacing, laser power, scan speed and build direction, and their effect on the microstructure, density, hardness, and high strain rate compressive behaviour of 316L stainless steel alloy are established. The results show that even though increasing laser power leads to a reduction in part porosity and better part consolidation (density), scan speeds had a significant effect on porosity production regardless of laser power and hatch spacing. A scan speed of 750 mm/s generally resulted in the lowest porosities regardless of laser power and hatch spacing (150 W ≤ P ≥ 250 W and 0.08 mm ≤ h ≥ 0.14 mm respectively). Scan speeds of 1250 mm/s would have to be coupled with higher laser powers (P ≥ 200 W) to get relatively similar porosities obtained at 750 mm/s. Regardless of the laser power and hatch spacing, scan speeds ≥1750 mm/s resulted in higher porosities. In addition, increasing hatch spacing, generally, resulted in increasing levels of porosity in the printed samples, with a hatch spacing of 0.08 mm resulting in the lowest levels of porosities while hatch spacing of 0.14 mm resulted in the highest levels of porosities. In terms of Volumetric Energy Density (VED), VEDs between 44 and 139 J/mm3 resulted in the lowest porosities, highest densities, and hardness, regardless of the build directions. In addition, increasing laser power resulted in thicker and tightly packed sub-grain structures whereas, an increase in hatch spacing and scan speed resulted in thinner/loosely packed sub-grain structures, and thinner/tightly packed sub-grain structures, respectively. Regardless of the printing parameters, samples perpendicular to the build direction had higher maximum flow stresses and lower strains when compared with the samples parallel to the build direction during high strain rate impact. Also, any combination of individual printing parameters that result in finer sub-grain structures resulted in a relatively brittle deformation behaviour during high strain rate impact. The formation of ASBs is characterized by refined grains within the region of the ASBs and coarser grains away from the ASBs region.