Computational Study of Fatigue in Sub-grain Microstructure of Additively Manufactured Alloys

Abstract

Additively manufactured (AM) materials experience shorter fatigue lives compared to their wrought form. Shorter fatigue life can be related to different effects like defects, residual stresses, surface finish, geometry, size, layer orientation, and heat treatment. One of the main contributors to the shorter fatigue life of AM alloys is their unique and complex microstructure. In this paper, we study this challenge from a novel perspective in which the interaction between the microstructure and fatigue life is explored. Among different microstructural features in the AM alloys, here we focus on the cells which form inside the grains during fabrication. While this microstructural feature is not always the prominent site for the fatigue initiation, it always has a significant role in the fatigue failure, particularly in high cycle fatigue because it occupies a high percentage of the volume in the material. A fatigue damage model is developed and verified to predict the life of cellular microstructures present in the AM metal microstructure. It is shown that the life of a cellular microstructure, which is composed of an arrangement of cells and cell boundaries is lower than a single-phase material without such an arrangement. We investigate how the arrangement if cells can govern the fatigue life, and analyze different cellular geometries to find the best performing cellular microstructure. By changing the geometrical parameters, the considerable variation in life can be as high as 95% in some strain amplitudes. Since the microstructure of cells in AM alloys can be tailored by changing the processing parameters, our results can be used as a guide to additively manufacture alloys with improved fatigue-resistance.