Ultrasonic vibration frequency as a governing mechanism for microstructure-property relationships in cold metal transfer-based wire arc directed energy deposited 316 L stainless steel

In wire-arc directed energy deposited (wire-arc DED) 316 L stainless steel, heterogeneous grain structures, porosity, and strength anisotropy have long been critical factors limiting performance. Recent studies have shown that ultrasonic vibration can refine microstructures and improve material properties, yet the influence of vibration frequency on melt dynamics, microstructure, fretting wear, and corrosion behaviour remains insufficiently explored. In this study, ultrasonic vibration (USV) at frequencies of 15, 25, and 40 kHz was applied during cold metal transfer (CMT)-based wire-arc arc directed energy deposition to systematically evaluate its effects. The results reveal that 25 kHz produced the finest average grain size (∼34 µm), significantly reduced Cr, Ni, and Mo segregation at grain boundaries, and enhanced structural homogeneity. This condition achieved the highest hardness (221 HV, +22 % over the without-ultrasonic vibration condition), improved ultimate tensile strength, and reduced tensile anisotropy by ∼29 %. Wear resistance and corrosion performance were also maximized at 25 kHz, whereas 40 kHz led to grain coarsening and degraded properties due to excessive heat input. Mechanistic analysis indicates that property improvements at the optimal frequency arise from three coupled effects: (i) grain refinement strengthening through vibration-enhanced nucleation, (ii) defect reduction via acoustic-driven melt stirring and porosity suppression, and (iii) enhanced corrosion resistance through uniform solute distribution and refined passive film formation. Beyond this case study, the findings establish ultrasonic frequency as a transferable process parameter for microstructural engineering, offering generic insights applicable to steels and other alloys with comparable melting behaviour.