A study on the grain refinement mechanism of Ti-6Al-4V alloy produced by wire arc additive manufacturing using hydrogenation treatment processes

• The microstructure of as-fabricated Ti-6Al-4V samples is columnar β grain with lamellar α cluster inside. • Coarse α″ martensite and twins appear in Ti-6Al-4V-0.48H alloy after quenching. • Equiaxed structure is formed in the Ti-6Al-4V-1.38H alloy after HVC hydrogenation treatment. • The average lamellar α size is refined from 5.78 µm to 0.84 µm. The effect of the hydrogen content of 0.48 wt% and 1.38 wt% on the refinement of the microstructure of Ti-6Al-4V alloy produced by wire arc additive manufacturing was studied, and the microstructure evolution of Ti-6Al-4V alloy after hydrogenation, heat treatment and dehydrogenation was revealed in this work. The result shows that the Ti-6Al-4V alloy prepared by wire arc additive manufacturing technology consist of coarse columnar β grains with the lamellar α cluster inside. After hydrogenation treatment, the α clusters inside the β grains become smaller, and hydrides were produced. Then αʹ, αʹʹ and metastable phase β M were formed in the microstructure during the quenching from the temperature of Tp+ 10 °C. At last, the metastable phase and hydride were decomposed during the subsequent aging and dehydrogenation process, which significantly refines the α grains in the columnar β grains, forming a large area of a fine-grained microstructure. The shape and size of the original and hydrogen-removed grains was observed by electron backscatter diffraction (EBSD). The result shows that the average size of the original lamellar α grains has been refined from 5.78 µm to 0.84 µm, especially, there were no original α grains with size above 4 µm left. With the hydrogen content of 0.48 wt%, the morphology of α grains after hydrogen removal is fine needle-like, while, when the hydrogen content increases to 1.38 wt%, the morphology of α grains is equiaxed.