Microstructural control across multiple length scales in additively manufactured Ti-6Al-4V via cyclic heat treatments

Abstract

Ti-6Al-4V is a premier titanium alloy widely used across various industrial sectors, thanks to its versatile properties arising from diverse microstructures tailorable via thermomechanical processing (TMP). In contrast, Ti-6Al-4V made by laser powder-bed fusion (LPBF) additive manufacturing (AM) lacks the same microstructural diversity and precise in-process microstructural control, primarily due to rapid thermal cycling inherent to LPBF. In the as-built state, the microstructure predominantly comprises acicular α′ martensites within columnar prior-β grains, which often fails to achieve mechanical properties comparable to those obtained through TMP. This necessitates the use of post-heat treatments as a critical step to ensure superior and reliable mechanical performance. The present study explores cyclic heat treatment (CHT) as an effective strategy for AM-specific microstructural control across multiple length scales, including prior-β grains, primary α, and secondary α. By varying peak temperature, number of cycles, and cooling rate, the initial microstructure dominated by α′ martensite in columnar prior-β grains rapidly evolves into diverse microstructures comparable to those achieved via TMP. These include lamellar α+β in equiaxed prior-β grains, globular α in near-equiaxed prior-β grains, and bimodal microstructure comprising a mixture of globular α and lamellar α+β/acicular α′. The accelerated microstructural evolution driven by the repetitive α↔β phase transformations induced by CHT facilitates processes like epitaxial recrystallisation and α globularisation. The developed CHT protocol provides a framework for microstructural engineering, enabling mechanical property optimisation and supporting broader industrial adoption of LPBF Ti-6Al-4V.