Achieving strength-ductility synergy in wire-arc additively remanufactured part by in-situ hot rolling and thermal history engineering: A case study on 42CrMo low-alloy steel motor shaft

Wire-arc additive manufacturing (WAAM) is extensively employed in remanufacturing owing to high efficiency and cost-effectiveness. However, it often results in coarse grain structures and considerable residual stress, ultimately deteriorating mechanical properties and fatigue life of remanufactured components. This study addresses this issue by utilizing a customized in-situ hot rolling-assisted WAAM (HR-WAAM) system, specially designed for rotational components, to remanufacture a motor shaft using a high-strength low-alloy steel wire. The rolling temperature was maintained within the ferrite phase region. Comparative analyses of single-layer, double-layer, and multi-layer samples were conducted to uncover the combined effects of in-situ hot rolling and thermal cycling on microstructure and mechanical properties. In-situ hot rolling introduces substantial low-angle grain boundaries and dislocations, serving as nucleation sites for recrystallization. Thermal cycling during subsequent deposition not only provides necessary activation energy to enhance grain boundary mobility and thus promotes recrystallization, but also induces various solid-state phase transformations to facilitate grain refinement and microstructural homogenization. An optimal processing window was identified with a rolling temperature of 600–700 °C and 23 % rolling strain. The yield strength of HR-WAAM low-alloy steel increased from 604 MPa to 786 MPa while maintaining an elongation of 20 %, comparable to that of WAAM samples. Moreover, the high-cycle fatigue strength substantially increases from 428 MPa to 501 MPa. These enhancements primarily result from grain refinement and the introduction of compressive residual stress. This work demonstrates that HR-WAAM can effectively tailor microstructures to achieve strength-ductility synergy and provides a technical reference for its application in shaft remanufacturing.