On the microstructural evolution and failure mechanism in laser powder bed fusioned Ti-6Al-4V during low cycle fatigue at room and elevated temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti-6Al-4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 degrees C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti-6Al-4V. The evolution of a' grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti-6Al-4V has greater tensile strength than conventionally manufactured wrought Ti-6Al-4V due to its microstructure, with fine a' needles which provide small slip lengths. For cyclic loading at ET, the interaction be-tween the dislocations increases which in-turn increases the ability of material to over-come the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior a' grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough sur-face, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.(c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

The microstructural features and their evolution during cyclic deformation have a direct impact on the low cycle fatigue life of additively manufactured LPBF Ti-6Al-4V. Tensile and strain controlled LCF tests were conducted to study the cyclic softening behavior and failure mechanism. The presence of fine a' needles in LPBF Ti-6Al-4V provides higher tensile strength and increases the ability to overcome obstacles to dislocation motion. The evolution of dislocation substructures and the formation of low angle boundaries during cyclic deformation contribute to fatigue fracture.