Ultrasonic vibration-induced microstructural and textural changes governing fatigue anisotropy in cold metal transfer-wire arc additively manufactured 316L stainless steel

Abstract

Fatigue anisotropy remains a key limitation in wire-arc additive manufacturing (WAAM) due to the formation of columnar grain structures and strong crystallographic texture, which result in direction-dependent mechanical behaviour. Although ultrasonic vibration (UV) has shown promise as an in-process grain refinement technique, its influence on fatigue anisotropy in WAAM-fabricated 316L stainless steel has not been systematically investigated. This study evaluates the effect of UV on microstructure, crystallographic texture, residual stress, and high-cycle fatigue behaviour of 316L stainless steel fabricated by WAAM. UV transformed the microstructure from columnar to near equiaxed, reducing the grain aspect ratio from 3.57 to 1.98, increasing the recrystallized grain fraction, and decreasing the texture intensity from 51.41 to 13.97 MRD. UV weakened the initial Cube {001}<100> texture and suppressed the development of a dominant Goss {110}<001> deformation texture during fatigue, instead promoting a distributed deformation texture that enabled more homogeneous plastic deformation. It also enhanced compressive residual stress from −83.19 MPa to −275.88 MPa. These microstructural modifications significantly improved fatigue resistance by delaying crack initiation and propagation. Kernel Average Misorientation (KAM) analysis near the fracture regions revealed higher dislocation density in UV-assisted specimens, supporting the observed enhancement in fatigue performance. While vertically built specimens exhibited superior fatigue life in the absence of UV due to columnar grain alignment, UV provided a substantially greater improvement in horizontally built specimens, leading to a remarkable reduction in fatigue anisotropy from 62% to 8%. These findings provide new mechanistic understanding of how UV governs the microstructure–texture–fatigue anisotropy relationship in wire-arc additively manufactured 316L stainless steel. © 2026 Elsevier B.V.