The wire laser additive manufacturing (WLAM) process enables the production of large-scale components. Therefore, it is essential to understand the fatigue behavior at high and very high number of cycles, as well as the scale effects and the mechanisms of fatigue crack propagation in parts produced by this process. Part of this study involves in-situ multimodal instrumentation of the process and a parametric analysis to establish the link between manufacturing parameters and the presence of defects (such as porosity and lack of fusion). The second part of the study consists of a fatigue testing campaign, both conventional (at 20 Hz) and ultrasonic (at 20 kHz), on samples with varying elemental volumes, ranging from a single layer to multiple layers, to better understand the scale and frequency effects on low-alloy steels produced via additive manufacturing (WLAM). The overall objective of these two parts is to establish a link between the fatigue behavior at a very high number of cycles and the parameters of the WLAM process.