Defect analysis and fatigue strength prediction of as-built Ti6Al4V parts, produced using electron beam melting (EBM) AM technology

Abstract

Ti6Al4V alloy has been highly employed in numerous applications of aerospace, medical, and chemical industries because of having excellent corrosion resistance, fracture toughness and high specific strength. Various studies on fatigue have shown that under cyclic loading fatigue cracks preferentially initiate and propagate from both the external surface defect or internal defect depending on severity and size of the defect. Although, electron beam melting (EBM) provides excellent prospect to the additive manufacturing (AM) technology in terms of energy, cost, and time efficiency, it produces rougher surfaces as compared to the other AM processes. For this reason, fatigue performances of as-built EBM parts could be poor. In the current study, the surface roughness profile on different sides of the as-built EBM Ti6Al4V parts with different orientations has been analyzed to determine anomaly of surface roughness parameters resulting from sample orientations. The average surface roughness parameter (Ra) was observed to fluctuate from 6 µm to 27 µm among different sides and build orientations of the samples. Furthermore, internal porosity of these parts was analyzed using X-ray computed tomography (CT) and found to vary between 0.05% to 0.17% in different build orientations. Based on this defect analysis, the fatigue strength of these parts has been calculated nondestructively using a quantitative method, Murakami's square root of area parameter model and Basquin's model. Besides, the severity of internal and external defects on fatigue strength have been evaluated and the external surface defects were rendered to be more critical as potential crack initiation sites.