Refined grain formation behavior and strengthening mechanism of α-titanium with nitrogen fabricated by selective laser melting

Customized titanium 3D parts with good mechanical performance have attracted considerable attention for application in structural industries. In this study, alpha-Ti materials with superior tensile strength and good ductility were fabricated via additive manufacturing (AM). To improve the mechanical properties, core-shell structured Ti-(N) powder was used to introduce nitrogen atoms into selective laser melting (SLM) fabricated Ti materials. The microstructural analysis of as-fabricated Ti materials revealed an a microstructure with various grain morphologies such as irregular/massive grains, equiaxed grains, and refined acicular martensite grains (alpha') when different energy densities (E) from 83 J/mm(3) to 278 J/mm(3) were applied. Furthermore, we observed that grain refinement occurred with increasing solute nitrogen atoms. The mechanical properties were evaluated based on interrelated aspects, including nitrogen content and energy density, by changing the scanning parameters. Our results showed that as the nitrogen content in the as-fabricated Ti materials increased, a significant improvement in tensile strength and hardness was observed. The yield strength (sigma(ys)) and ultimate tensile strength (sigma(UTS)) increased up to 1072 MPa and 1126 MPa, respectively, when the nitrogen content was approximately 0.5 wt.%, which was approximately three times that of SLM-processed pure Ti (similar to 0.01 mass% N). The increment in yield strength was further discussed mainly in terms of solid solution strengthening using the Labusch model and grain refinement strengthening using the Hall-Petch equation. This calculation raises the ability to quickly predict the yield strength of Ti materials with nitrogen solid solution produced by selective laser melting.