Revealing the evolution of microstructure and mechanical properties with energy density to achieve high-strength Ti-6wt%Cu alloy by laser metal deposition

Abstract

In this work, Ti-6wt%Cu alloys were fabricated by laser metal deposition (LMD) technology to understand how the energy densities impact the microstructure and consequent mechanical properties. The results show that a bimodal microstructure of ultrafine eutectoid lamellae and Widmanstätten-α can be obtained at low energy density (118 J/mm3), while higher energy density produces coarse colony-α structure. The multi-scale microstructural refinement is achieved due to the fast cooling rate and low heat accumulation in low energy density deposits, resulting in a sound combination of property with tensile strength of 1063 MPa and elongation of 10%. Based on the solidification theories and accurate solidification parameters in the LMD process, the columnar to equiaxed transition (CET) was proposed to elucidate the refinement mechanism of near-equiaxed prior-β grains. Furthermore, detailed microstructural characterization identified the effect of Cu segregation on the formation of eutectoid lamellae. The Cu aggregation at low energy density plays a crucial role in phase transition and microstructural refinement, as well as account for orientation heterogeneity of lamellae. In addition, the strengthening mechanisms were discussed by considering microstructural evolutions due to different energy densities.