Additive manufacturing of compositionally graded laser deposited titanium-chromium alloys

Ti-xCr (9 <= x <= 28) at% graded alloys fabricated via laser engineered net shaping (LENS (TM)) are potential candidate materials for graded implants in spinal fixation surgeries due to their tunable Young's modulus. This is achievable by various phase transformations on the laser deposited graded alloys by subjugating them to different beta-solutionizing heat treatments. The microstructure of the as-deposited (AD) Ti-xCr alloy was comprised of a metastable beta-Ti(Cr) matrix phase along with alpha-Ti and TiCr2 (an intermetallic Laves phase) at different regions of the graded alloy. In comparison, the microstructure of the rapidly air cooled (RAC) Ti-xCr alloy contained omega and TiCr2 precipitate phases, while the slowly furnace cooled (SFC) Ti-xCr alloy has alpha-Ti and TiCr2 precipitate phases along different regions of the graded alloys. An extreme change in micro-hardness values at different regions of the RAC and SFC alloys due to these various phase transformations indicates that the alloy fabricated may be qualified as a potential graded implant material. Additionally, the phase evolution along the compositional gradient of the AD Ti-xCr alloy was correlated to theories of reheating in the lower build layers, thermal gradient effect, rapid solidification rates, and enthalpy of mixing between Ti-Cr powders. A detailed explanation to variation in microstructures and mechanical properties observed at compositions of Ti similar to 29at%Cr (a lower build layer) and Ti similar to 28at%Cr (an upper build layer) in the AD Ti-xCr graded alloy is also provided.