Study of Fracture Toughness and Notch Toughness under Impact loading of Electron Beam Melted Ti-6Al-4V alloy

Abstract

This thesis presents a comprehensive investigation into the fracture toughness and the notch toughness behavior of Electron Beam Powder Bed Fused (EB-PBF) Ti-6Al-4V alloy, with particular focus on the effects of build orientation, key mechanical properties and microstructural characteristics. Vertical compact tension specimens as well as Horizontal and vertical Charpy Impact specimens were fabricated using optimized processing parameters. Notch toughness was evaluated using instrumented Charpy impact tests, while fracture toughness was measured using linear-elastic fracture mechanics on fatigue-precracked specimens. The fracture surfaces were examined using profilometry, scanning electron microscopy, microhardness measurements, and microstructural analysis. The findings were correlated with the mechanical properties of similarly fabricated specimens under identical conditions.Vertical specimens demonstrated superior toughness performance across both test regimes. In fracture toughness testing, Y-Z specimens exhibited the highest toughness (65.81 MPa√m), while X-Z showed the lowest (63.10 MPa√m). Post-machining generally improved toughness values by mitigating surface defects and enhancing crack resistance. In impact testing, vertical specimens absorbed ~50% more energy than horizontal specimens due to greater ductility, lower microhardness, and crack propagation paths that intersect prior-β grains more frequently. Surface profilometry confirmed ~35% higher shear lip heights, ~50% larger shear areas, and ~15% rougher cracking regions in vertical specimens, indicating increased plastic deformation. The study highlights the critical role of build orientation and post-processing in tailoring the toughness of EB-PBF Ti6Al4V, offering valuable guidance for design and qualification of AM components in structural applications.