Effect of swirling to enhance chemical homogeneity in in-situ alloying and promote strength-ductility-synergy in twin wire arc additive manufacturing

This research investigates the in-situ alloying of austenitic stainless steel 316 L (SS) and low-carbon steel ER70S-6 (LCS) utilizing a twin-wire arc additive manufacturing (T-WAAM) process. Two different scanning techniques, linear scan deposition (LSD) and interweaving scan deposition (ISD), were employed to address the enduring problems of elemental segregation, martensitic transformation, and brittle interfacial phases commonly observed in dissimilar steel deposition. The process optimization enabled a stable bead geometry for balanced SS-LCS in-situ alloying. Further, microstructures, chemical distributions, phase evolution, hardness, and tensile performance of the fabricated walls were examined. Also, EBSD analysis was performed to characterize the structural properties of the fabricated intermixed walls. The LSD intermixed walls exhibited rapid cooling-induced martensitic transformation, chemical segregation, high hardness (∼420 HV), and high strength (∼808 MPa), but showed limited ductility (∼11 %). On the other hand, the ISD intermixed walls promoted rotational reheating and swirling-induced melt homogenization, resulting in bainite–pearlite transformation, enhanced elemental diffusion, chemical uniformity, reduced dislocation density, higher high-angle grain boundary fraction, enhanced grain growth recovery, and a desirable strength ductility synergy (∼711 MPa UTS and ∼ 22 % elongation). However, no intermetallic phases were detected in the intermixed wall, and XRD confirmed a dual-phase structure consisting of FCC and BCC phases. This study investigates the in-situ alloying of austenitic stainless steel 316 L (SS316L) and low-carbon steel ER70S-6 (LCS) using a twin-wire arc additive manufacturing (T-WAAM) process. Two deposition strategies, linear scan deposition (LSD) and interweaving scan deposition (ISD) were employed to mitigate elemental segregation, martensitic transformation, and brittle interfacial phase formation commonly associated with dissimilar steel deposition. Process optimization enabled stable bead geometry for balanced SS–LCS alloying. The fabricated intermixed walls were characterized through microstructural, chemical, phase, hardness, tensile, and EBSD analyses. The LSD walls exhibited rapid cooling-induced martensitic transformation, pronounced chemical segregation, high hardness (∼420 HV), and superior tensile strength (∼808 MPa), but limited ductility (∼11 %). In contrast, the ISD walls promoted rotational reheating and swirling-induced melt homogenization, leading to bainite–pearlite transformation, improved elemental diffusion, enhanced chemical uniformity, reduced dislocation density, higher high-angle grain boundary fraction, and improved grain recovery. Consequently, the ISD walls achieved a desirable strength–ductility synergy with ∼ 711 MPa ultimate tensile strength and ∼ 22 % elongation. No intermetallic phases were detected, while XRD confirmed a dual-phase FCC–BCC structure.