Towards Understanding the Influence of Surface Integrity on Mechanical Behavior of Additively Manufactured Ti-6Al-4V

Abstract

Additively manufactured components often exhibit shorter fatigue lives in their unmachined state compared to machined or wrought counterparts mainly due to the presence of surface micro-notches. While post-processing treatments can improve performance, they increase lead times, may not always be feasible or desired, and can compromise the advantages of AM, such as near-net shape fabrication. This dissertation aims to investigate the influence of surface integrity on the mechanical behavior of laser powder bed fused (L-PBF) Ti-6Al-4V specimens. First, a novel framework was developed to quantify surface micro-notches using X-ray computed tomography, where key geometrical features such as width, depth, opening angle, and radius of curvature were captured. These features were then combined in the fatigue stress concentration formula, Kf, to assess the fatigue criticality of each notch. It was found that the crack-initiating features ranked within the top 1% of all notches based on the calculated Kf. Fatigue life predictions, employing the fatigue notch factor approach that incorporated localized geometrical features of micro-notch, demonstrated reasonable accuracy for unmachined L-PBF Ti-6Al-4V specimens. This framework was then further validated for conditions where process parameters were systematically varied (i.e., specifically, combination of different infill and contour parameters), and a wide range of surface and near-surface conditions were produced. Moreover, the modelling approach was further applied to partially treated specimens to assess its broader applicability. The results demonstrated that the developed fatigue modelling framework could reasonably predict lives across a wide range of surface textures including partially treated specimens. In addition toiii modelling, the influence of a wide range of surface conditions on the mechanical performance was investigated. Results showed that avoiding contour passes increased surface roughness and reduced tensile ductility due to early void nucleation. While contour passes enhanced fatigue life for default and underheated infill settings, this beneficial effect was diminished for overheated conditions due to increased roughness and near-surface defects. Moreover, the impact of surface material removal methods, including direct polishing, shallow machining and polishing, deep machining (DM), and deep machining with polishing (DM+P) on the mechanical performance was evaluated. The results showed that polishing alone did not improve tensile behavior and fatigue resistance due to persistence of surface valleys, while DM and DM+P improved tensile ductility and fatigue life by removing surface and near-surface defects.