The effect of hot isostatic pressure on the corrosion performance of Ti-6Al-4 V produced by an electron-beam melting additive manufacturing process

Abstract

Additive manufacturing (AM) is a rapidly growing technology that enables the fast production of complex and near-net-shaped (NNS) components. Among the many applicable AM methods (particularly powder bed technologies), electron-beam melting (EBM) is gaining increased interest mainly in aerospace and medical industries, due to its inherent advantages for the printing of Ti-6Al-4V alloy. Although major strides have been made towards understanding the effect of hot isostatic pressure (HIP) on the mechanical properties of Ti-6Al-4V produced by AM, its effect on corrosion performance remains relatively unexplored. To date, the reported corrosion studies remain essentially limited to the selective laser melting (SLM) process, while the corrosion behavior of EBM Ti-6Al-4V and particularly HIPed EBM Ti-6Al-4V have not been fully realized. This paper provides a detailed analysis of this corrosion performance, including the stress-corrosion susceptibility of EBM Ti-6Al-4V in as-build condition and after HIP heat treatment. Microstructure and phase identifications were examined by scanning electron microscopy (SEM) and X-ray diffraction analysis. Corrosion performance was evaluated by electrochemical measurements, including open-circuit potential (OCP), potentiodynamic polarization analysis and impedance spectroscopy (EIS), as well as stress-corrosion examination in terms of slow strain-rate testing (SSRT). All of the corrosion tests were carried out in a 3.5 wt.% NaCl solution at ambient temperature. Owing to the natural excellent corrosion resistance of Ti-6Al-4V, the obtained results revealed that the HIP process has only a slight positive effect on the corrosion resistance of Ti-6Al-4V produced by EBM. This minor improvement may be related to the improved efficiency of the passivation layer that was attributed to the increased β-phase content and the reduction of α/β interfaces. In terms of stress corrosion sensitivity, the HIPed specimens exhibited extended time-to-failure (TTF) at the low strain rate at 2.5•10-7 1/sec, where the effect of the corrosive environment was more dominant.