Influence of Powder Microstructure on the Microstructural Evolution of As-Sprayed and Heat Treated Cold-Sprayed Ti-6Al-4V Coatings

Abstract

Cold spray repair of high-value titanium alloy components has gained considerable interest; however, the influence of deposition conditions on the resulting microstructure and properties is not well established. This work examines the influence of feedstock powder type on the microstructural evolution of cold spray-deposited Ti-6Al-4V powders following deposition and after low-temperature heat treatment. Plasma-atomized, gas-atomized, and hydride de-hydride Ti-6Al-4V powders were deposited on Ti-6Al-4V substrates using cold spray technology and subsequently annealed at 550 °C for 5 h. Powders and cold spray depositions were characterized using x-ray diffraction, optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Atomized and hydride de-hydride powders were characterized by a martensitic alpha and equiaxed alpha microstructure, respectively. Phase analysis revealed hydride de-hydride powders to contain beta phase regions near alpha grain boundaries; however, beta phase was not observed in atomized powders. Atomized coatings retained their powder microstructure in particle interiors but demonstrated ultra-fine grain formation near particle boundaries, likely due to dynamic recrystallization. In contrast, hydride de-hydride powders showed a larger increase in microstrain after deposition, without ultra-fine grain formation. Heat treatment resulted in recovery and recrystallization for all coatings and, in the case of atomized coatings, resulted in beta phase precipitation in regions that experienced large plastic strains.