Microstructure evolution of sub-critical annealed laser deposited Ti–6Al–4V alloy

Abstract

Laser deposited titanium alloy Ti–6Al–4V was sub-critically annealed in the α+β phase region and the microstructure evolution process and microstructure characteristics were systematically investigated as functions of annealing temperature and time during the sub-critical annealing process. A unique bi-modal microstructure consisting of coarse crab-like primary α and fine lamellar transformed β is obtained by sub-critical annealing in the upper part of α+β phase region. The crab-like morphology of the primary α becomes more manifest with the increasing anneal temperature up to its β-transus temperature. The annealing treatment has significant effect on the volume fraction, size and aspect ratio of primary α, while the aging treatment mainly affects the volume fraction and aspect ratio of primary α and the width and volume fraction of secondary α. Formation of the crab-like primary α is due to the preferential epitaxial growth of secondary α on the edge of primary α along with the 〈1210〉 preferred crystallographic direction during the diffusive β-decomposition process. The sub-critically annealed alloy exhibited the maximum microhardness by further aging at approximately 600°C, which is 9% higher than that of the as-annealed one and 4% higher than the as-deposited one.