The paper addresses the fatigue crack growth behaviour of untreated and heat-treated WAAM ER70S-6 carbon steel. Specimens were extracted from the printed wall along different directions (vertical and horizontal) and tested under mode-I loading at two stress ratios (R=0.05 and R=0.25). Crack closure was measured using Digital Image Correlation (DIC). The microstructure of the untreated material mainly consisted of polygonal ferrite and intergranular lamellar pearlite. After heat treatment, pearlite decomposed, allowing ferrite to grow and reducing hardness. The load ratio influenced fatigue crack growth rates due to variations in crack closure levels. However, the loading direction relative to the print layer orientation did not significantly affect the crack growth rate. Fracture surfaces were examined by scanning electron microscopy to identify the main fatigue crack growth mechanisms associated with the different loading orientations and material conditions. Fractographic analysis revealed a mixed fracture mechanism, characterised by cleavage in the harder pearlite-rich regions and fatigue crack propagation striations in the softer ferrite-dominant areas. Minor manufacturing defects, such as inclusions and porosity, were also observed. The tested WAAM carbon steel exhibited slightly lower performance than conventional steels of a similar grade, aligning closely with the existing literature for WAAM ER70S-6 carbon steel.

Wire Arc Additive Manufacturing (WAAM) has gained popularity due to its speed and cost-effectiveness. However, the current knowledge on the cyclic deformation behaviour of WAAM materials is limited. To address this issue, this study investigates the cyclic deformation behaviour of WAAM ER70S-6 carbon steel. Coupons were extracted from printed walls with horizontal and vertical orientations and from surface and interior locations. Low-cycle fatigue tests were performed under fully reversed strain-controlled conditions, spanning strain amplitudes from 0.20 % to 1.50 %. Regardless of the group, the material exhibited cyclic hardening for strain amplitudes above 0.60 % and cyclic softening for smaller strain amplitudes. Significant non-Masing hysteretic behaviour was also observed. Cyclic stress–strain curves and fatigue-life relationships written in terms of stress, strain and energy-based parameters were derived for all groups. Fatigue life was not significantly influenced by printing orientation or location across thickness. However, horizontal orientation resulted in a slightly superior fatigue response. A statistical analysis revealed no significant difference in the accuracy of fatigue-life relationships between the groups. Finally, a comparative study between WAAM materials and conventional steels showed promising results. Yet, conventional steels outperform WAAM carbon steel, at least doubling the fatigue life for the same value of the damage parameter.