Heat accumulation dictates microstructural evolution, tensile anisotropy and wear behavior in cold metal transfer-wire arc additive manufacturing of high-strength low-alloy steel

Wire arc additive manufacturing (WAAM) enables cost-effective fabrication of large metallic components, but the repeated thermal cycles during deposition strongly affect microstructure and mechanical properties. This study combined numerical simulation and experimental characterization to examine a 150 mm long, 60-layer ER100S-G steel wall produced by cold metal transfer (CMT)-WAAM. The simulation revealed pronounced heat accumulation: the interlayer temperature rose from about 75 °C to 680 °C and the cooling rate fell from ~266 °C/s to ~1.5 °C/s between the 2nd and 50th layers. The microstructure was predominantly ferritic-bainitic and coarsened along the build direction, evolving from fine acicular ferrite and bainite near the substrate to coarser polygonal and lath ferrite with bainite in the upper layers, with the average grain size increasing from ~3.40 to ~5.47 µm. Microhardness decreased from 237 to 200 HV0.1 (≈15.8%) from bottom to top, consistent with the coarsening. Tensile behavior was anisotropic: horizontal specimens (parallel to the deposition direction) reached 372 MPa yield strength, 542 MPa ultimate tensile strength, and 22% elongation, versus 343 MPa, 522 MPa, and 17% for vertical specimens. All the tensile specimens showed predominantly ductile fracture. Dry-sliding tests performed on a coupon extracted from the middle region gave a stable friction coefficient of about 0.51 and a wear rate of 1.08×10⁻4 mm3/(N·m). These results clarify how heat accumulation governs the microstructural gradients, the tensile anisotropy and the wear response of WAAM HSLA steel and inform thermal control and process optimization.