Microstructural causes and mechanisms of crack growth rate transition and fluctuation of additively manufactured titanium alloy

Wire and arc additive manufacturing (WAAM) enables rapid near-net-shape fabrications of large-size parts and in-situ remanufacturing in many industry sectors. A comprehensive understanding of the fatigue failure mechanism of WAAM titanium alloys is a prerequisite for their widespread use in critical structural components subject to fatigue load. Here, the fatigue crack growth behavior of WAAM TA15 material is investigated. Fatigue crack growth tests are performed using compact tension specimens sampled from different locations and with different crack orientations of the WAAM TA15 block. The fatigue crack growth rate (FCGR) data exhibit two governing rates separated by a transition stress intensity factor value, $\Delta K_n$, and the degrees of fluctuation of the FCGR data in the two regimes are notably different. A piecewise log-linear model is first proposed by incorporating the Heaviside step function and $\Delta K_n$ into the classical Paris’ model, allowing for the transition $\Delta K_n$ to be determined by the data. The potential causes of the transition $\Delta K_n$ are phenomenologically inferred via fractography and surface roughness profiling results. The critical microstructure affecting the value of $\Delta K_n$ is identified by relating the crack tip cyclic plastic zone size at $\Delta K_n$ to the sizes of main microstructures. The cause of different degrees of fluctuations in the two regimes separated by $\Delta K_n$ is inferred by examining the microstructures within the plastic zone. The microstructural mechanisms of the local FCGR reduction and fluctuation are further identified and explained.