Thermal history-tailored microstructure, microhardness, and tensile properties in heterogeneous double-wire arc directed energy deposited martensitic stainless steel

Martensitic stainless steel (MSS) is renowned for its exceptional performance. However, current researches on the additive manufacturing of MSS rely on pre-alloyed materials with fixed compositions, limiting the flexibility. This study utilized a heterogeneous double-wire arc directed energy deposition (DED) technique to in situ fabricate MSS wall structures, employing a hybrid feedstock of 55 % ER2209 (duplex stainless steel) and 45 % ER70-G (low-alloy steel). Two distinct inter-layer cooling strategies were implemented. Under shortened inter-layer cooling intervals, pronounced thermal accumulation elevated inter-layer temperatures above the martensite start transformation (M_s) temperature, delaying phase transformation until post-deposition cooling. This regime produced coarser lath martensite with retained δ-ferrite, resulting in higher strength and hardness but poorer plasticity. Conversely, prolonged cooling durations allowed inter-layer temperatures to approach room temperature, facilitating thermal cycling-induced microstructure-property transformation. This transitioned initial lath martensite/δ-ferrite to a tempered martensite-austenite dual-phase architecture and promoted microstructural refinement. Consequently, improvement in elongation was achieved despite moderate reductions in strength and hardness. Furthermore, this investigation elucidated the intrinsic correlation between thermal history and phase transformation mechanisms through a synergistic combination of experiment and simulation, reproducing the evolution of microstructure during each deposition. This work demonstrates the feasibility of fabricating MSS using common heterogeneous welding wires and clarifies the regulation mechanism of thermal history on the in situ fabricated MSS. This study establishes a transferable theoretical framework applicable to diverse material systems in DED research.