Effect of Microstructure and Strain Rate on Thermomechanical Behavior of Additively Manufactured Titanium Alloy

Abstract

This research examines the effects of post-heat treatments on the thermomechanical characteristics and microstructure of a Ti-6Al-4V Extra Low Interstitials (ELI) alloy produced via Selective Laser Melting (SLM). The study utilized two post-SLM heat treatments to produce different microstructures: Solution Treatment and Aging (STA) for a bimodal microstructure and Beta Annealing (BA) for a Widmanstätten microstructure. Thermomechanical compression tests were conducted at 550 °C with strain rates of 0.01 s⁻¹ and 1 s⁻¹utilizing a Gleeble-3800 thermomechanical simulator. The microstructures were analyzed utilizing Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-Ray Diffraction (XRD). The findings indicated that the compressive yield strength (CYS) of the bimodal microstructure reached 724 MPa and 740 MPa at strain rates of 0.01 s⁻¹ and 1 s⁻¹, respectively, representing a 20 % and 12 % increase over the as-printed martensitic microstructure (604 MPa and 662 MPa). In comparison to the Widmanstätten microstructure (422 MPa and 438 MPa), the CYS of the bimodal microstructure was 71 % and 69 % greater at the corresponding strain rates. Adiabatic Shear Bands (ASBs) were present in all microstructures at both strain rates, significantly influencing the failure mechanisms. The martensitic microstructure displayed minimal cracking under compression at both strain rates. The bimodal microstructure showed predominantly intergranular fractures along the grain boundaries of thick primary α (αₚ) phases. Conversely, the Widmanstätten microstructure exhibited both intergranular and translamellar fractures. Intergranular cracks originated and advanced along the continuous grain boundary α (αɢʙ), while translamellar fracture, occurring within the α colonies, exhibited cracks traversing across lamellar α/β interfaces.