Microstructure evolution and mechanical properties of laser additive manufactured Ti6Al4V alloy under nitrogen-argon reactive atmosphere

It is known that the reactive nitrogen atmosphere can effectively modulate the microstructure of laser powder bed fusion (L-PBF) of Ti6Al4V alloys. In the present work, high nitrogen contents of 15 and 25 vol% were applied in argon atmospheres to investigate the influence on microstructure and mechanical performances. The enhanced nitrogen solid solution was achieved with nitrogen content ranging 374 ppm and 456 ppm (parts per million) in the Ti6Al4V alloys, respectively. Besides, the EBSD (electron back-scattered diffraction) characterization shows the significant refinement of acicular martensite alpha' and the EBSD phase reconstruction shows the interrupting epitaxial growth of the beta phase with the increasing nitrogen content in the atmosphere. The KAM (kernel average misorientation) diagrams with an increasingly higher value indicate an increased micro-strain and dislocation density at higher nitrogen content in the atmosphere. Meanwhile, the high magnification TEM further confirms the martensite alpha' refinement and also the formation of beta lamella at 25 vol% nitrogen content in L-PBF atmosphere. Such microstructure evolution can be mainly attributed to the more severe thermal cycling with a higher thermal gradient and cooling rate under the atmosphere with higher nitrogen content. By applying the 25 vol% nitrogen in the atmosphere, the L-PBF Ti6Al4V alloys exhibit a significant increase in maximum compressive strength of ~1885 MPa and maximum compressive strain of ~ 10%, respectively. The underlying strengthening mechanism can be attributed to the refinement of martensite structure, solid solution, and higher dislocation density. The present work shows that the atmosphere can effectively modulate the microstructure and improve the mechanical property of L-PBF alloys.