Experimental and numerical investigation of the behavior of additively manufactured Ti-6Al-4V alloy under high strain rates

In this study, Ti-6Al-4V alloy produced by Laser Powder Bed Fusion was examined in four main stages: powder characterization, microstructure and mechanical properties after heat treatment, deformation behavior under high strain rates, and Johnson-Cook material modeling and its correlation with microstructure parameters. In the first stage, three types of Ti-6Al-4V powders, virgin, recycled, and spatter, were morphologically, microstructurally, and chemically analyzed. Morphologically, it was determined that the virgin and reused powders were suitable for manufacturing, except for the spatter powder. In the microstructure, all powder types exhibited α′ martensite phase, with phase thickness ranging between 0.2 and 0.27 µm, and dislocation density varying between 1.72 and 9.38 × 1015 m-2. In the second stage, Ti-6Al-4V samples produced by the Laser Powder Bed Fusion method were annealed at 725–950 °C for 2 hours. While ultra-fine α phase was observed in the as-built microstructure, α+β phases were present after annealing. Through heat treatment, the α phase thickness increased from 0.32 µm to 1.09 µm, and the β phase fraction reached to 17%. The dislocation density was measured as 3.94×1014 m-2 in the as-built condition, which decreased with heat treatment (2.08×1014 m-2 at 800 °C) and then increased again at 950 °C (3.20×1014 m-2). Although ductility generally increased after annealing, the limited increase observed at 875 and 950 °C was attributed to the coarsening of the α phase lath and the increase in the β phase fraction. In the third stage, dynamic compression tests were conducted using the Split-Hopkinson Pressure Bar system at strain rates of 400–1000 s⁻¹. The as-built specimen, containing ultra-fine α phase, fractured at 1000 s⁻¹, whereas the annealed samples did not. This was due to the ultra-fine α phase restricting dislocation motion and deformation. Due to high strain rate, α phase thickness of samples manufactured with laser powder bed fusion reduced by 11%. On the other hand, due to strain hardening and thermal softening effects α phase thickness of heat treated samples show limited deformation relative to the as-built sample. In the fourth stage, the Johnson-Cook material model parameters A, B, n, C, and m were determined using experimental results, and SHPB tests were simulated in LS-Dyna using these parameters. High consistency between simulation and experimental results was achieved (R² = 0.812–0.929). Furthermore, a correlation matrix was established between microstructural features such as α phase thickness, c/a ratio, phase content, and dislocation density, and the Johnson-Cook material parameters. Based on this correlation, empirical regression equations were developed, demonstrating that Johnson-Cook material parameters can be predicted directly from microstructural features.