Effect of interlayer temperature on the microstructure and mechanical properties of ultra-high-strength steel fabricated by ultrasonic impact-assisted wire arc additive manufacturing.

This study investigates the microstructure and mechanical properties of 14Ni3Cr3Mo2Mn ultra-high-strength steel components fabricated by ultrasonic impact-assisted wire arc additive manufacturing (UI-WAAM) at four interlayer temperatures: 80 °C, 150 °C, 200 °C, and 300 °C. Results reveal that interlayer temperature non-linearly regulates microstructural evolution through thermal cycling. The average grain size in the top region increased from 32.9 μm at 80 °C to 49.0 μm at 200 °C and then decreased to 40.2 μm at 300 °C owing to the expanded effective action range of UI. The middle region experienced significant grain coarsening, reaching 84.6 μm at 300 °C due to heat accumulation. The phase composition varied with interlayer temperature, while the middle region generally retained a higher fraction of austenite due to repeated thermal cycling and elemental redistribution. Mechanical properties deteriorated as temperature increased. Optimal performance was achieved at 80 °C, with ultimate tensile strengths of approximately 1230 MPa, elongations between 18.0 % and 20.0 %, and a microhardness of 315.3 HV0.5. Simulations showed that elevated interlayer temperatures widened and prolonged the effective UI action zone between 800 °C and 1450 °C. While UI dominates strengthening at low temperatures, thermal accumulation diminishes its benefits at high temperatures. This work clarifies the temperature-UI coupling mechanism, providing critical insights for optimizing the manufacturing process.