Alpha-case Formation in Ti–6Al–4V in a Different Oxidizing Environment and Its Effect on Tensile and Fatigue Crack Growth Behavior

Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process

“…In the present work, the different cooling rates can be correlated to the different sample thicknesses between the tensile sample (6 mm) and the bar (10 mm) because both are simultaneously cooled with Argon gas for 1 h. Heat treatment at 1050 °C/60′ also induces the formation of an α-case surface layer with a thickness of 100 ÷ 150 μm and hardness of (400 ± 10) HV0.1 in both the tensile sample (Figure 7a) and bar (Figure 7b). Similar results are shown in [49,52] for the cross-sections of the 45°-sample and 45°Z-bar.…”

“…In this case, it is the microstructural observation in Figure 6 which confirms that the diffraction peaks of the samples heat-treated at 1050 °C belong to the α- Heat treatment at 1050 • C/60 also induces the formation of an α-case surface layer with a thickness of 100 ÷ 150 µm and hardness of (400 ± 10) HV 0.1 in both the tensile sample (Figure 7a) and bar (Figure 7b). Similar results are shown in [49,52] for the cross-sections of the 45 • -sample and 45 • Z -bar.…”

“…In order to characterize the coarsening of the α-phase, its thickness was measured as a function of the heat-treatment conditions, and it increases from (540 ± 60) nm at 704 • C/120 to (700 ± 10) nm at 740 • C/130 due to the different temperatures and the furnace cooling from 740 to 520 • C in 90 . The same results were obtained in [29,48,49]. For example, Haar et al [29] discussed the same phenomenon during the heat treatments in the low-SSTR (Solid Solution Temperature Region), i.e., at temperatures below 800 • C.…”

“…Wu et al [66], however, showed the recrystallization process already after 1000 • C/40 , followed by water quenching. Due to this microstructural change, together with the α-laths in a basketweave structure (Figure 6), the UTS and YS values decrease (Figure 9) up to 918 ± 7 MPa and 817 ± 6 MPa, respectively; meanwhile, the elongation increases only up to 12.75 ± 1.01% due to both the α-laths rather than α-colonies in a basketweave structure (Figure 6a,c) and the possible detrimental effect of the α-case [12,20,49,67]. The same tensile properties are confirmed by [19,32,68,69].…”

“…However, in relation to the samples heat-treated at 1050 • C/60 , the ASTM F3001-13a is not satisfied due to the presence of the α-case layer. In fact, post-processing operations are required to remove the α-case due to its worsening effects on fatigue life and ductility in aerospace components [2,49]. At the same time, the samples heat-treated at 704 and 740 • C, which are characterized by higher tensile properties than the solution heat-treated samples, also satisfy the F2924-14 standard specification (UTS of 895 MPa, YS of 825 MPa and ε of 10%, [70]); this, however, is related to the Ti6Al4V alloy.…”