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

A clear understanding of the fatigue properties of Ti-6Al-4V manufactured with electron beam melting (EBM) is needed to ensure performance in critical applications in the medical device and aerospace industries. In this work, the effects of residual stress and internal defects (pores and voids) on fatigue properties of EBM Ti-6Al-4V material in as-built, stress-relieved, and hot isostatic pressed (HIPed) conditions were evaluated. Conventional techniques were used to measure the chemical composition and quantify microstructures, and neutron diffraction was utilized to measure residual stresses arising from the EBM process. Post-processing did not alter chemical composition. Compared to the as-built condition, microstructure was unchanged for stress-relieved material and coarser for HIPed material. No significant residual stresses were measured for any of the three conditions. This indicates build platform and layer preheating lead to sufficient process temperatures to achieve full stress relief in-situ. The fatigue strengths at 10 cycles measured for the as-built and stress-relieved conditions were statistically similar and were measured to be 200 MPa to 250 MPa. A significantly higher fatigue strength at 10 cycles of 550 MPa to 600 MPa was measured for the HIPed condition. The increase in fatigue endurance limit was attributed to a reduction in internal porosity and void content. INTRODUCTION Electron beam melting (EBM), an additive manufacturing (AM) process, shows great promise for making medical devices and aerospace components through excellent shape control via computer aided design input. In these industries, it is of utmost importance to characterize and fully understand all influences on the fatigue properties as components are exposed to fatigue loading, which could lead to fatigue failures [1]. In previous EBM Ti-6Al-4V work, hot isostatic pressing (HIPing) has been shown to close internal porosity and improve fatigue endurance limit by approximately 20 % [2]. It was assumed that this improvement in fatigue properties was due solely to closure of stressconcentrating internal pores, but it is possible that other microstructural changes, such as residual stress relief, occur during HIPing and contribute to the improvement in fatigue properties. Residual stresses are known to affect Ti-6Al-4V fatigue properties both beneficially (compressive residual stresses) and deleteriously (tensile residual stresses) [3]. It is likely that the rapid heating and cooling of additive manufacturing processes [4] create residual stresses in as-built material. This has been predicted by theoretical modeling of additive manufacturing processes [5, 6], as well as experimentally verified in laser-based additively manufactured material [5]. Previous experimental and numerical modeling results showed that build platform and powder layer preheating reduce residual stresses and negative effects such as part warping [5]. In this work, the relative effects of internal pore closure and residual stress relief on fatigue properties were determined by analyzing and comparing three conditions: as-built (contains residual stresses and internal pores), stress-relieved (internal pores only), and HIPed (neither residual stresses nor internal pores). MATERIALS AND METHODS Sixty-four cylinders (24 mm diameter x 155 mm height, oriented vertically with long axis in the build-direction, a.k.a. z-direction) were built in a single build using Arcam S12 EBM equipment (software version 3.2.114.13836, accelerating voltage 60 kV, layer thickness 70 μm, speed factor 35, standard Arcam build theme for Ti-6Al-4V) and gas-atomized Ti-6Al-4V powder (average particle size 70 μm). After additive manufacturing, one third of the parts were stress-relieved (5 h at 650 ̊C, air, furnace cooling), one third were HIPed (2 h at 900 ̊C, argon, 12 ̊C/min heating and cooling rates, 100 MPa), and one third were left in the as-built condition. Chemistry, microstructure, and residual stress (neutron diffraction, gage volume 4 mm x 4 mm x 4 mm) were measured for each condition in addition to fatigue testing (tensile-tensile, load ratio R=0.1). The vertical orientation (tensile/fatigue stress axis in z-direction) was chosen as it represents the worst case for fatigue strength [2]. RESULTS No significant differences in chemistry were observed between the three conditions (Table I). This eliminates the possibility that differences in chemistry between the conditions would be the cause of differences in fatigue results. Table I. Chemistry results (in wt%) shown for all three conditions compared to ASTM F2924. Uncertainties are given when available.