Influence of post deposition annealing on microstructure and properties of laser additively manufactured titanium copper alloys

Abstract

While copper is a potent strengthener in titanium alloys, its use in commercial alloys has been severely restricted due to the strong tendency for segregation during solidification, leading to heterogeneous microstructures and what has often been referred to as the “beta-fleck” problem. This problem can be largely obviated by using additive manufacturing (AM) for processing Ti-Cu alloys. This study focuses on AM of a binary Ti-4Cu and a ternary Ti-4Cu-4Al alloy using the laser engineered net shaping (LENS) process. The influence of post-deposition annealing treatments and the subsequent cooling rate on the microstructure and tensile properties of these alloys has been investigated in detail. The phase fraction of the eutectoid alpha + Ti2Cu product is dependent on the cooling rate from above the beta transus temperature. Additionally, the Ti2Cu phase exhibited a far-from equilibrium composition in case of the water-quenched Ti-4Cu-4Al alloy. Both the yield stress (∼550−650 MPa) as well as the ductility (∼15–18 %) were also higher in case of the ternary alloy. The high strengths exhibited by the water-quenched samples of both alloys, while maintaining appreciable tensile ductility, could be attributed to clustering of Cu within the α laths, revealed by atom probe tomography.