Mechanical Behavior Assessment of Ti-6Al-4V ELI Alloy Produced by Laser Powder Bed Fusion

Abstract

The present work correlates the quasi-static, tensile mechanical properties of additively manufactured Ti-6Al-4V extra low interstitial (ELI, Grade 23) alloy to the phase constituents, microstructure, and fracture surface characteristics that changed with post-heat treatment of stress relief (670 °C for 5 h) and hot isostatic pressing (HIP with 100 MPa at 920 °C for 2 h under an Ar atmosphere). Ti-6Al-4V ELI alloy tensile specimens in both the horizontal (i.e., X and Y) and vertical (Z) directions were produced by the laser powder bed fusion (LPBF) technique. Higher yield strength (1141 MPa), higher tensile strength (1190 MPa), but lower elongation at fracture (6.9%), along with mechanical anisotropy were observed for as-stress-relieved (ASR) samples. However, after HIP, consistent and isotropic mechanical behaviors were observed with a slight reduction in yield strength (928 MPa) and tensile strength (1003 MPa), but with a significant improvement in elongation at fracture (16.1%). Phase constituents of acicular α′ phase in ASR and lamellar α + β phases in HIP samples were observed and quantified to corroborate the reduction in strength and increase in ductility. The anisotropic variation in elongation at fracture observed for the ASR samples, particularly built in the build (Z) direction, was related to the presence of “keyhole” porosity.

Additive Manufacturing of AlSi10Mg and Ti6Al4V Lightweight Alloys via Laser Powder Bed Fusion: A Review of Heat Treatments Effects

“…In this scenario, not only the influence the α laths coarsening but also the , as reported by Wycisk et al [ 301 ], who showed a α laths coarsening from ~1 μm to 4 μm. The same results are shown by Mahmud et al [ 111 ], who reported a linear increase with time. To avoid an excessive grain coarsening, Herzog et al [ 302 ] proposed an increase in pressure to 200 MPa and a consequent decrease in temperature up to 820 °C.…”

“…In this scenario, the as-built sample formed by a fully α′ martensite structure does not show any portion of the β-phase in the XRD spectrum, as shown in Figure 34 . The same results are illustrated by [ 111 , 223 , 224 , 225 ]. On the other hand, increasing the presence of the α + β phase, the XRD spectrum starts showing small peaks related to the β-phase, as highlighted in the red spectrum related to the Ti6Al4V stress-relieving sample.…”

“…The same authors also showed an increase of c/a ratio from 1.595 to 1.598 considering the α′ and α phases, respectively ( Figure 41 b). All of these reflect on the results obtained by the X-Ray diffraction analysis where the α and α′ phases are considered together due to the peaks overlap [ 111 , 227 , 230 , 258 , 259 ]. In this scenario, Sallica-Leva [ 229 ] reported the analysis of the FWHM to discriminate the presence of the α′ and α phases.…”

“…In this scenario, the hot isostatic pressing (HIP) HT can be used not only to densify the material but also to reduce the residual stress [ 105 , 106 ]. At the same time, if the fatigue resistance is increased due to the pore’s reduction, the tensile properties of Ti6Al4V samples decrease, as can be observed in Figure 7 [ 107 , 108 , 109 , 110 , 111 ]. In contrast, the ductility increases.…”

“…On the other hand, the greatest coarsening of the α laths starting from α′-martensite was obtained after 1050 °C × 2 h + 100 MPa rather than after 920 °C × 2 h + 100 MPa, as shown in the study of Leuders et al [ 110 ]. The vast majority of the HIP heat treatments reported on different research were performed in the temperature and pressure ranges of 900–950 °C and 100–150 MPa, respectively; the α + β lamellar structure is the microstructure obtained [ 110 , 111 , 223 , 268 , 288 , 300 ]. In this scenario, not only the influence the α laths coarsening but also the , as reported by Wycisk et al [ 301 ], who showed a α laths coarsening from ~1 μm to 4 μm.…”