A unified approach to model the effect of porosity and high surface roughness on the fatigue properties of additively manufactured Ti6-Al4-V alloys

Abstract

In this article, we propose a model that can account for the effect of porosity and high surface roughness on the fatigue crack initiation of AM Ti6Al4V alloys in moderate and high cycle fatigue regimes. Within these fatigue regimes, the applied force to the component is below the yield stress, however, defective features, viz., porosity and high surface roughness, can act as stress raisers. As a consequence, local plasticity can occur. To capture this phenomenon, a nonlinear isotropic kinematic hardening elasto-plasticity model is employed in our finite element (FE) model. For creating the geometry of the FE models, inputs from fractography analyses and surface roughness measurements are needed. From fractography analyses, the shape of pores formed by gas bubbles during manufacture appears quite regular. Thus, these pores are modeled as circles in FE models. The size of these pores and their distance to a free surface of the tested specimens are extracted from Scanning Electron Microscope images. Moreover, it has been mentioned in the literature that statistical parameters of surface roughness cannot fully describe the detrimental effect of this type of defect to the fatigue life of the associated component. Thus, in our FE model, the surface topography, which was measured using stylus-based profilometer, is explicitly modeled. The finite element results are then post-processed by our in-house software to extract the Smith–Watson–Topper (SWT) fatigue indicator parameter (FIP). The SWT parameter is calculated at each element centroid of the FE mesh, i.e., the local indicator. Afterward, an average value of the SWT parameter over a so-called critical area whose center is located at the considered centroid is also calculated, i.e., the average indicator. The results show that the local SWT indicator is too conservative in predicting the fatigue life of the AM Ti64 alloys while the average SWT one can provide good results.