Impact of build process on microstructural evolution in laser directed energy deposition of Ti-8.5Cu Alloy: An in situ synchrotron X-ray imaging and post-build electron microscopy study

Understanding the microstructural formation and evolution in a dynamic temperature field during the metal additive manufacturing process has been a significant challenge, particularly in alloy systems containing solute(s) with high diffusivity at temperatures where solid-solid phase transformations take place, such as the eutectoid decomposition in Ti-Cu alloys. This study explores the influence of the build process on the microstructural evolution in Ti-8.5Cu alloy processed using Directed Energy Deposition – Laser Beam/Metals (DED-LB/M). Utilising in situ synchrotron X-ray radiography, changes in melt pool geometry for each layer during the DED-LB/M process are analysed. The results reveal a substantial increase in the length, depth and volume of the melt pool from the bottom to the top layers and with the increase of laser power. Such changes reflect the rise in residual temperature, and these data are used to calibrate the classical Rosenthal model, enabling it to accurately describe the temperature profile across the entire build. Post-build electron microscopy studies reveal that the volume fraction of pearlite exhibits an unusual ‘V-shaped’ dependence on laser power. This dependence is well explained by the competition between the nucleation rate and the growth rate of pearlite. This finding offers valuable insight into microstructure manipulation of Ti-8.5Cu alloy during DED-LB/M processing. It also serves as an excellent example of how in situ synchrotron X-ray imaging can be fully utilized to understand microstructure evolution in a complex and challenging manufacturing process.