Continuous cooling transformation of L-PBF Ti64

A systematic investigation to construct the continuous cooling transformation (CCT) behaviour of Ti6Al4V alloy, manufactured by Laser Powder Bed Fusion (L-PBF), was conducted across cooling rates from 0.1 °C/s to 1000 °C/s. This study examines the effects of varying cooling rates on phase transformations, identifying phase boundaries and transformation mechanisms from near-equilibrium to non-equilibrium conditions. Hot-stage and optical microscopy, SEM, and EBSD analyses were used to evaluate the transformation mechanisms and resulting microstructures. The results show that cooling rates below 5 °C/s favour long-range diffusion-controlled β→α+β transformations, forming stable lamellar microstructures. At intermediate cooling rates (10 °C/s to 190 °C/s), displacive and short-range diffusional transformations occur, resulting in formation of Widmanstätten α and massive α phases. At cooling rates above 500 °C/s, shear-dominated diffusionless transformation results in fully martensitic α'. The lamellar characteristics of the as-printed L-PBF samples were preserved during reheating and cooling, despite grain morphology changes from columnar to equiaxed. An empirical model was developed to correlate cooling rates with lamellae and needle widths, offering a predictive tool for controlling microstructure and estimating cooling rates. These findings contribute to optimizing heat treatments for L-PBF-manufactured Ti6Al4V components, supporting tailored properties for aerospace and biomedical applications. • A comprehensive Continuous Cooling Transformation (CCT) study was conducted on L-PBF Ti6Al4V alloy, delineating phase transformations across cooling rates from near-equilibrium to extreme non-equilibrium. • Microstructural evolution showed distinct transformation sequences: β → α+β at slow cooling rates, β → Widmanstätten α and massive α at intermediate rates, and β → α' martensite under rapid cooling. • Detailed analysis with SEM, EBSD of dilatometry samples and as-print L-PBF samples enabled the understanding of microstructural variation due to L-PBF in addition to morphological preservation in L-PBF Ti64 after heat treatment. • Developed a predictive empirical model linking cooling rates with lamellae or needle widths, enabling estimation of cooling conditions based on microstructural examination.