Pulsed robotic wire arc additive manufacturing of 316L stainless steel for forging preforms–processing evaluation

This study investigates the microstructural characteristics and mechanical properties of 316 L stainless steel fabricated using Robotic Pulsed Wire Arc Additive Manufacturing (WAAM), with a focus on its properties for designing forging preforms. A wall structure measuring 280 × 152 × 19 mm was successfully deposited using a robotic pulsed GMAW-based WAAM system. Key mechanical and physical properties, including density, microhardness, and phase composition, were evaluated. Further, the immersion UT scan and Resonance Ultrasonic Spectroscopy (RUS) were employed to characterize the 316 L specimens. The measured density values ranged from 7.907 to 7.952 g/cm², closely matching the nominal density of wrought 316 L stainless steel. Microhardness varied between 166.6 HV and 184.6 HV, with an average of 173.73 HV, aligning with expected values. Metallographic analysis revealed columnar dendritic morphology with directional solidification and epitaxial growth, attributed to the thermal gradients inherent to the WAAM process. EBSD and SEM-EDS analyses confirmed the predominance of austenitic phase (γ), with delta ferrite (δ) along interdendritic regions, and minor sigma phase formation. Due to the high heat input and relatively slow cooling rates inherent to WA-DED, the formation of both δ-ferrite and σ-phase is anticipated in the as-built microstructure. The delta ferrite content was further analyzed with Schaeffler’s and WRC diagrams along with the elemental mapping, which resulted primarily as austenitic stainless steel, with less (~ 10%) ferrite formation. The lower flow stress of the WAAM sample (590 MPa) compared to the extruded rod (700 MPa) at room temperature demonstrated the forgeability of the WAAM 316 L sample.