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Fatigue Properties of a Titanium Alloy (Ti-6Al-4V) Fabricated Via Electron Beam Melting (EBM): Effects of Internal Porosity and Residual Stress

Published

Author(s)

Nikolas W. Hrabe, Thomas H. Gnaeupel-Herold, Timothy P. Quinn

Abstract

A deeper understanding of microstructural influences on the fatigue properties of Ti-6Al-4V made using electron beam melting (EBM) is necessary before this material can be safely implemented in medical devices and aerospace components. Investigation of two potential influences, residual stress and internal porosity, was achieved in this work by characterizing fatigue properties (20 Hz, R=0.1) in three different conditions: as-built, stress relieved, and hot isostatic pressed (HIPed). No difference was observed in fatigue endurance limit between as-built and stress relieved conditions (200 MPa to 250 MPa). This was supported by residual stress measurements (neutron scattering) showing no residual stresses in any of the three conditions. It is thought that the relatively high build plate temperature (minimum 600 ˚C throughout the entire build), which is achieved by build plate and powder layer preheating, relieves any residual stress formed during building. HIPed material showed over 100 % improvement in endurance limit (550 MPa to 600 MPa) compared to as-built material, suggesting closure of internal pores during HIPing has a significant effect on fatigue properties. Localized incomplete melting voids were formed due to unacceptably long electron beam scan lengths and were observed in all as-built and stress relieved specimens and none of the HIPed specimens. Fractography showed fatigue cracks initiated at voids, suggesting these voids lowered the endurance limits for as-built and stress relieved material and enhanced the measured improvement in fatigue properties due to internal pore closure during HIPing. Microstructure coarsening during HIPing most likely affected fatigue properties, but the estimated effect was small (approximately 2 %) for the observed change in α lath thickness (approximately 0.8 µm as-built to 1.6 µm HIPed).
Citation
International Journal of Fatigue
Volume
94

Keywords

Additive manufacturing, electron beam melting, EBM, fatigue, microstructure, mechanical characterization, microstructure characterization, titanium alloy, residual stress

Citation

Hrabe, N. , , T. and Quinn, T. (2017), Fatigue Properties of a Titanium Alloy (Ti-6Al-4V) Fabricated Via Electron Beam Melting (EBM): Effects of Internal Porosity and Residual Stress, International Journal of Fatigue, [online], https://doi.org/10.1016/j.ijfatigue.2016.04.022 (Accessed March 1, 2024)
Created January 1, 2017, Updated November 10, 2018