Simulation-assisted investigation on the formation of layer bands and the microstructural evolution in directed energy deposition of Ti6Al4V blocks

Additive manufacturing (AM) of titanium alloy entails severe microstructural heterogeneity and layer bands due to diverse thermal histories. While the thermal-microstructure relationship in AM has been reported, the details on how complex thermal histories influence the microstructural evolution have not been so addressed, and the formation of layer bands in multi-layer multi-pass builds is still unclear. To undertake such investigation, a thermal model is firstly calibrated using two part-scale blocks fabricated on differently sized substrates, and then used to study the relationship between key microstructural characteristics and the thermal cycling involved. Results show that the different evolutions of the temperature ranges just underneath the β-transus temperature (T _β) controlled by the printing path are responsible for the different band distributions at the centre and corner of the blocks. Also, the α sizes in the normal region are closely linked to the integral area obtained from the thermal curve as temperature fluctuates between T _β and α dissolution temperature, which helps linking AM variables to metallurgy. This further demonstrates that the α coarsening during thermal cycles is primarily driven by multi dissolution and precipation transformations instead of Ostwald ripening. Finally, the quantitative thermal-microstructure-microhardness relationship is established, helpful for the microstructural design.