Laser-based direct energy deposition of compositionally graded titanium alloys: Microstructural evolution, hardness, and tensile properties

Laser-based direct energy deposition (DED) technology allows researchers to alter feeding materials while fabricating products and offering opportunities for producing compositionally graded materials such as graded alloys for structural applications. Many graded alloys suffer from their poor tensile properties due to asperities formed at the interface of two materials, arising from microstructural mismatches and varying thermal properties. Towards the load-bearing applications of lightweight dissimilar titanium alloys, we employ the DED method to fabricate a compositionally graded material consisting of the Ti-6Al-4V (TC4) alloy and Ti-6.5Al3.5Mo-1.5Zr-0.3Si (TC11) alloy. We investigate the effects of heterogeneous microstructure and compositions on the hardness and tensile properties of the graded TC11-TC4 alloy, as well as heat treatment effects, to unveil their microstructure evolution and deformation mechanisms. The smooth change of chemical compositions across the graded TC11-TC4 alloy interface leads to a smooth change of interfacial hardness from -320 HV0.3 on the TC4 side to -380 HV0.3 on the TC11 side. The as-deposited and heat-treated specimens show comparable yield strength (806-860 MPa), ultimate tensile strength (921-937 MPa), and ductility (7.8-8.6 %) despite different microstructures. We demonstrate a correlation between the microstructural features and microdeformation mechanisms in each region of the graded Ti-alloy, showing that the graded interfacial region resists plastic strain localization. The micro- and macro-mechanical properties of the graded alloy are associated with the distribution, size, morphology, and orientation of the alpha and beta phases.