Investigation on evolution mechanisms of site-specific grain structures during metal additive manufacturing

A multiscale model is developed to investigate the evolution mechanisms of site-specific grain structures during additive manufacturing (AM) of metallic alloys, using the selective electron beam melting (SEBM) fabrication of Ti-6Al-4V as an example. Specifically, finite-element method is utilized to predict the thermal response at macroscale during SEBM, and the extracted thermal information is then input into a temperature-dependent phase-field model to simulate the grain growth at mesoscale. The grain epitaxial growth, grain selection, grain nucleation and layer-by-layer manufacturing fashion are incorporated, in order to accurately predict grain structure development and relevant physical processes during AM. It is found that, the development of the predominant grain structures in the thick and thin walls, i.e., the large vertical columnar <001>_β//N_z grains and slanted columnar grains with various grain orientations, respectively, can be attributed to the competition and collaboration between the thermal gradient and the crystallographically preferred grain growth, as shown from the different growth stages in the simulations. Good agreements in the final grain structures and textures are achieved between the experimental observations and numerical simulations. The present study potentially offers valuable insights and guidance toward designing AM conditions to tailor the grain structures and textures.