Origin of localized hardness softening associated with layer band microstructures in multilayer tungsten inert gas (TIG)-welded TC4ELI alloy

Multilayer tungsten inert gas (TIG) welding is widely used for the fabrication of thick-section Ti-6Al-4V extra low interstitial (TC4ELI) alloy structures used in marine equipment. However, repeated thermal cycling during multilayer deposition can generate pronounced microstructural heterogeneity and localized mechanical degradation within the welded joint. In this work, the spatial distribution of microstructure, crystallographic characteristics, geometrically necessary dislocation (GND) density, and Vickers microhardness in a multilayer TIG TC4ELI-welded joint was systematically investigated using continuous scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and microhardness testing. Along the transverse direction, the welded joint exhibited significant microstructural heterogeneity from the base metal to the heat-affected zone and fusion zone, accompanied by a progressive increase in hardness associated with the transformation from lamellar α+β to acicular martensite. More importantly, characteristic layer band microstructures were identified along the thickness direction of the fusion zone. Localized hardness softening was consistently observed within these layer band regions. Detailed microstructural analyses revealed that the layer band region was characterized by coarse α colonies exhibiting a Burgers orientation relationship and reduced GND density. The localized hardness softening was attributed to the combined effects of grain coarsening, formation of crystallographically aligned α colonies, and reduced GND density. These findings establish a direct correlation between the layer band and localized hardness softening, providing guidance for the welding process design of large-scale structural components in marine and deep-sea engineering applications.