Interlayer cooling time control of microstructure, hardness and corrosion behaviour in WAAM of Ti6Al4V

Wire Arc Additive Manufacturing (WAAM) of Ti6Al4V is strongly governed by thermal history, which controls phase transformation, microstructural evolution and functional performance. This study evaluates the effect of interlayer cooling time (tdwell) on thermal behaviour, microstructure and corrosion resistance. A finite-element thermal model was implemented and validated using thermocouples and infrared pyrometry, achieving a correlation coefficient of ≥ 85% between predictions and experiments. Cooling strategies of 30 s and 100 s, as well as a variable approach based on the number of deposited layers, were analysed. Increasing tdwell enhanced heat dissipation, promoted α-phase refinement from 1.3 to 0.6 µm, and increased α/β interfacial density, improving hardness to 505 HV. High-temperature X-ray diffraction confirmed a more homogeneous β→α transformation at longer cooling times, reducing thermal accumulation. Electrochemical testing in 3.5% NaCl showed a direct correlation between microstructure and corrosion behaviour. The substrate exhibited the lowest polarization resistance (1.34 × 105 Ω·cm2) and highest corrosion rate (0.002258 mmpy), while WAAM samples showed significant improvement. The 100 s condition provided the most stable electrochemical response, whereas the variable strategy achieved the lowest corrosion rate (0.000516 mmpy) but with higher instability. These results demonstrate that microstructural refinement and homogeneity, controlled by tdwell, govern passive film stability and corrosion resistance.