Investigation into the occurrence of recrystallization during the additive manufacturing of a thick Ti6Al4V deposit

A thick deposit made of superimposed layers with a constant track length strategy has been manufactured under a laser cladding process of Ti6Al4V powders. The cladded material exhibits
columnar grains with epitaxial growth. The matrix inside the grains is composed of both martensite and Widmanstätten structures within the regions corresponding to the solidified melt pool.
After laser cladding, conventional quenching tests have been carried out on the cladded material that aim at better enhancing conditions for the occurrence of recrystallization phenomenon.
The heat treatments correspond to a furnace heating stage up to a temperature above the Beta transus (tr) followed by a defined soaking time prior to water quenching (WQ) to yield a fully
martensitic structure. Two different temperatures are chosen for the annealing stage, which are 1273K and 1323K respectively.
The microstructures of the cladded deposit and after subsequent quenching are characterized under both optical and scanning electron microscopes. Vickers hardness measurements are also
performed to complete the metallographic characterization and to help comparing the as-cladded state with the cladded and quenched one.
The thermal history on selected points of interest (POIs) within the cladded deposit is simulated thanks to a validated thermal model, thus leading to the determination of both heating rates prior
to remelting, and cooling rates during solidification stage when laser cladding goes on.
The conditions for the occurrence of recrystallization, in terms of temperature and critical rates during the reheating stages are discussed while taking into account the time necessary for diffusion
and initial dislocation density within the parent martensitic phase. In addition, correlations are established between the cooling rate above the critical value allowing the displacive transformation
of prior  into ’ lath-martensite, and the hardness of the latter phase.