Abstract
Additively manufactured (AM) Ti-6Al-4V titanium alloy is increasingly utilized in high-performance aerospace and thermal power applications where structural components experience complex spectrum loading and thermal gradients. This study presents a systematic experimental investigation into the fatigue crack propagation (FCP) behavior of laser powder bed fusion (LPBF) Ti-6Al-4V subjected to variable amplitude loading (VAL)—specifically single tensile overloads and block spectrum sequences—at ambient (25°C) and elevated temperatures (350°C and 500°C). Compact tension specimens manufactured in vertical and horizontal orientations were subjected to hot isostatic pressing (HIP) to homogenize the microstructure into an α+β lamellar matrix with negligible residual porosity. In-situ direct current potential drop (DCPD) measurements demonstrated that single tensile overloads induce pronounced transient crack growth retardation, the magnitude and duration of which depend strongly on the overload ratio (OLR) and operating temperature. Elevating the temperature from 25°C to 500°C accelerated baseline crack growth rates by up to an order of magnitude and reduced the delay cycles following an overload by 42–65%. This reduction is attributed to thermally assisted residual stress relaxation within the overload plastic zone and crack-tip oxidation embrittlement competing against oxide-induced crack closure. Fractographic analysis confirmed a transition from transgranular striation-dominated fracture at room temperature to combined interlamellar shearing and oxide-assisted micro-void coalescence at 500°C.
