Characterizing the tensile behavior of additively manufactured Ti-6Al-4V using multiscale digital image correlation

Abstract

The tensile behavior of an additively manufactured (AM) Ti-6Al-4V alloy is examined at multiple length scales using two-dimensional digital image correlation (DIC) techniques. The work focuses on characterizing the relationship between microstructure and bulk mechanical properties of the AM Ti-6Al-4V specimens and understanding how unique microstructural features pertaining to AM metals, such as columnar grain morphology and nonequilibrium phase structures, influence deformation behavior at the microscale (i.e., the length scales of those features). Lower magnification in situ DIC experiments were performed at the macroscale during tension experiments to assess the bulk properties, and a higher magnification ex situ DIC study was performed at the microscale using a microscope to observe heterogeneous strain accumulation at that scale. At the macroscale, the AM Ti-6Al-4V exhibited greater strength than conventionally processed Ti-6Al-4V, due to its acicular, or needle-like, grain structure. At the microscale, alternating regions of high and low strains were observed to accumulate along these needles, or laths. The effects of build orientations (related to microstructure anisotropy), powder bed layering thicknesses (30 and 60 µm), and heat treatment, on the properties were also examined. A decrease in strength was observed in samples subjected to annealing heat treatment compared to as-built specimens with no heat treatment. Also, the AM specimens exhibited higher tensile strength when the direction of applied load was perpendicular to the AM build orientation. This observed anisotropy is thought to be related to interfaces of consecutively deposited layers because regions of high strain at the microscale were observed to accumulate at changes in structure associated with the deposition of layers.