Quantifying the Microstructure and Mechanical Property Differences between Bulk and Thin-wall Additively Manufactured Inconel 718

Abstract

Multiple thermal excursions and rapid, uncontrolled cooling rates are inherent to additive manufacturing (AM). Differences in thermal conductivity between solidified metal and uncoalesced powder leads to disparate microstructures and properties near areas of high geometrical complexity relative to bulk regions. To quantify this effect for Inconel 718 (IN718) a series of micro-tensile dogbones were printed in 0.6 mm and 0.8 mm thicknesses using a contour + hatching strategy with either an aligned or rotated beam path and compared to a larger ‘bulk’ sample from the same print. The driving hypothesis for this study is that the lower cooling rates observed for areas in the thin-wall regime results in a coarser microstructure but with reduced texture and a resulting increase in strength due to a more uniform microstructure especially in the rotated case. Samples were characterized via micro-tensile testing and scanning electron microscopy. It was observed that thin-wall samples possessed significantly smaller grains than the Bulk samples (31.45 μm vs > 255.96 μm) by preventing the epitaxial growth of columnar grains that span many build layers as was observed in the Bulk samples. This difference led to the Bulk samples possessing an average ductility 30% higher than the thin-walls but a decrease in ultimate strength of over 25%. Quantifying and accounting for the geometry-dependent variability of microstructure and properties such as is shown in this study will be necessary to optimally design and build geometrically complex AM parts.