Microstructure and mechanical properties investigation of CP titanium processed by selective laser melting (SLM)

The aim of this study was the characterization of the microstructure and the mechanical properties of commercially pure titanium (CP Ti) processed by selective laser melting (SLM) in a regulated reactive atmosphere with a slight addition of oxygen (0.2-0.4 vol.%) to enhance the mechanical properties of the material. This work is one of the first extensive studies of the influence of the SLM process on the anisotropic material properties of printed Ti elements. Microstructure and mechanical properties were investigated both in the building platform plane (XY), as well as in the direction of the element's growth (XZ). The tested sample, fabricated using a power density of only 75 J/mm3, had a density close to the theoretical density of titanium (98.7%) and 0.27-0.50 wt.% oxygen. Observations carried out by light and scanning electron microscopes revealed some micropores typical for laser melting processes. The total porosity was evaluated using X-ray computed microtomography (μ-CT), and was different in the XY and XZ directions. Additional STEM study allowed us to determine the lattice parameters of the dominant martensitic phase (α′). It was shown that the obtained material had a random crystallographic orientation with a texture factor close to 1, due to phase transformation during the manufacturing process. The average roughness Ra parameter was 10.36 μm and 9.11 μm for the top and side surfaces, respectively. The range of the tensile strength of the tested specimens was between 690 and 830 MPa in the XY plane, and 640-740 MPa in the XZ plane. The maximum elongation at break showed high anisotropy, and was in a range of 16-22% and 8-12% for the XY and XZ planes, respectively. The determined mechanical properties exceed those found in many conventionally obtained titanium alloys due to oxygen solution strengthening.