Dynamic mechanical properties and failure characteristics of electron beam melted Ti-6Al-4V under high strain rate impact loadings

Abstract

This study presents an investigation on the effects of building direction on microstructure, dynamic mechanical properties, and deformation mechanisms of electron beam melted Ti-6Al-4V (EBM-Ti64) cylindrical rods. Microstructural features were characterized using optical microscopy (OM) and scanning electron microscopy (SEM). The initial microstructure in both directions consists of transformed α+β phases and grain boundary-α (αGB)along prior β-grain boundaries. The vertically built cylindrical specimens have a finer grain structure, including lower interlamellar spacing and finer α-laths compared to the horizontally built ones. Dynamic impact tests using Split-Hopkinson Pressure Bar (SHPB) were conducted on both horizontally and vertically built samples at strain rates ranging between 1150 and 2700 s−1. Dynamic mechanical properties are strain-rate sensitive; the maximum flow stress of 1960 MPa (at 2100 s−1) and 2160 MPa (1650 s−1) were obtained for horizontal and vertical specimens, respectively. Horizontal specimens fragmented when deformed at 2700 s−1, whereas the vertical specimens failed at a much lower strain rate (1900 s−1). At a given strain rate, vertical specimens exhibited better dynamic strength and lower strain (total strain) due to their finer microstructure. The temperature rise during deformation primarily governs flow softening at all conditions, which led to the formation of adiabatic shear bands (ASBs). Microstructures of deformed specimens revealed thermal softening features such as voids formation. These voids coalesced and grew, leading to crack initiation and propagation along ASBs. Fractographic examination of the fragmented specimens under impact loading revealed ductile dimples and smoother surfaces, which indicate a combination of both ductile and brittle fracture. The contribution of twinning and pyramidal <c+a> slip systems is the primary deformation mechanism during high strain rate impact loading of EBM-Ti64. The experimentally obtained flow curves are in good agreement with the Chang-Asaro equation-based constitutive modeling results.