Influence of vacuum stress relieving annealing on residual stress and microstructure properties of TC18 titanium alloy

The different temperature and time parameters were adopted in the vacuum stress relieving annealing processes of TC18 titanium alloy. The evolution of mechanical properties and residual stress was researched, the microstructures and SEM images of fracture surfaces were also observed. The results show that, after the vacuum stress relieving annealing, the tensile and yield strengths of TC18 titanium alloy decrease, while the charpy toughness, fracture toughness, elongation and area reduction increase. The removing effect of residual stress is remarkable. Along with the increases of the temperature of stress relieving annealing, the microstructure of the alloy changes regularly. When the temperature exceeds 650°C, α-phase decreases obviously, and the metastable β-phase increases dramatically. Thus, the strength of the alloy decreases, which coincides with the result of mechanical properties. Meanwhile, the plasticity increases, which coincides with the fracture appearance. Finally, the vacuum stress relieving annealing schedule is obtained to be 600-650°C and 1-4 h.