Assessment of Kitagawa-Takahashi diagram for Ti-6Al-4V manufactured by DED-LB/CW: Evaluation of the interaction between defects and microstructure

Abstract

This work presents comprehensive fatigue test results utilizing machined specimens extracted horizontally from Ti-6Al-4V blocks manufactured by Direct Energy Deposition using Laser Beam and Coaxial Wire (DED-LB/CW). The experimental results act as basis to establish defect- and microstructure-based models applying the Kitagawa-Takahashi (K-T) approach and the modifications by El Haddad, Murakami and Chapetti. Three batches manufactured in two different DED-LB/CW systems were considered to achieve a variation in microstructure and micropore sizes. Fractographical analyses were executed to examine the size, location and shape of the crack-initiating features. Due to the parameter optimization carried out, Lack of Fusion (LoF) defects were avoided and only shrinkage and keyhole pores were detected. Fatigue Crack Growth (FCG) tests were performed using Compact Tension (CT) specimens to determine the long and effective crack growth thresholds. Kitagawa-Takahashi, El-Haddad, Murakami and Chapetti models were calculated combining fatigue strength, fractographical analysis, as well as fracture mechanics characteristics, achieving good fit with the experimental results. It can be concluded that the interaction between the defects and the microstructure is not negligible. Hence, an application of defect- and microstructure-based models using simplified pore size parameters such as Murakami’s √area, enable a proper assessment of K-T diagrams for life predictions of Ti-6Al-4V manufactured by DED-LB/CW based on FCG behavior.