Effect of gas nitriding process on the microstructure and properties of titanium Grade 2 specimens produced using direct metal laser sintering

The paper presents the effect of a gas nitriding process on the microstructure and properties of Titanium Grade 2 specimens produced by means of the Direct Metal Laser Sintering method (DMLS). Microstructure, texture, and residual stress investigations showed that produced using the DMLS technique possess highly textured, fine-crystalline structures containing α’ martensite laths, twin boundaries and compressive stresses. Phase and stress analyses demonstrated that the nitrided layer produced in a mixture of 25 vol% nitrogen and 75 vol% hydrogen at 760 °C and containing nitrides exhibits higher compressive stress than the Titanium Grade 2 specimens produced by DMLS technique without nitriding. Tensile tests conducted at room temperature show that nitrided specimens reach a much lower yield strength and ultimate tensile stress than Titanium Grade 2 specimens without nitriding. Corrosion resistance tests conducted in Ringer's solution show that the electrochemical behaviour of nitrided specimens differs due to the transition of nitrides to the protective layer of titanium oxide, unlike the passive response and the high corrosion resistance of the as-built Ti specimens without nitriding.