Abstract

Wire Arc Additive Manufacturing (WAAM) has emerged as a promising technology for the production of large-scale metallic components owing to its high deposition rates and relatively low manufacturing costs. However, the complex thermal history inherent to layer-by-layer deposition poses significant challenges to achieving homogeneous microstructures and reproducible mechanical properties. Although numerous studies have investigated the effects of processing parameters on ER70S-6 steels, the synergistic influence of electrical parameters, deposition rate, and cumulative thermal history under nearly constant heat input conditions remains insufficiently understood. Accordingly, this study investigates the influence of deposition parameters on the microstructural evolution and mechanical behavior of ER70S-6 steel fabricated by WAAM-GMAW using a multiscale characterization approach. Three processing conditions were evaluated by systematically varying welding current, arc voltage, wire feed speed, and travel speed while maintaining the effective linear heat input within a narrow range of 0.461-0.480 kJ/mm. Microstructural and mechanical characterization was performed using optical and scanning electron microscopy, X-ray diffraction with Rietveld refinement, nanoindentation, digital image correlation, and mechanical testing, enabling correlations to be established among processing conditions, microstructural evolution, and material properties. The results revealed a progressive increase in grain size from 10.57 +/- 1.59 mu m to 17.79 +/- 2.43 mu m with increasing processing parameters, accompanied by a decrease in the pearlite fraction from 7.37 +/- 0.92% to 5.99 +/- 0.68%, indicating a shift toward increasingly ferrite-dominated microstructures. Microhardness values ranged from 180 to 200 HV, whereas nanoindentation measurements identified ferritic regions with hardness values between 2.7 and 3.8 GPa and localized pearlitic regions reaching up to 5.5 GPa, revealing pronounced mechanical heterogeneity at the microscale. Mechanical testing showed that the 20 V condition provided the best overall performance, achieving an ultimate tensile strength of 541.17 +/- 9.99 MPa, an elongation of 39.96 +/- 0.57%, a Charpy impact energy of 57 +/- 7 J, and the highest product of strength and elongation (21.63 GPa%). These findings demonstrate that, even under nearly constant linear heat input conditions, deposition rate, instantaneous arc power, and cumulative thermal effects play critical roles in governing ferrite-pearlite transformation kinetics and, consequently, the mechanical response of WAAM-fabricated components. The results further demonstrate that linear heat input alone is insufficient to fully describe microstructural evolution in multilayer WAAM-GMAW components, providing a basis for the development of more comprehensive process-control and optimization strategies for additively manufactured low-carbon steels.