The Study Of Thermal-Mechanical Processing Technique Of High Strength, High Toughness And Damage Tolerant Titanium Alloy

This paper carried out systematic study on the forging technique and post-forging heat treatment of TC21 titanium alloy. The results shows that TC21 alloy could get optimum overall mechanical properties and damage tolerant properties adopting the quisi-β forging temperature of Tβ+15°C for the first time forging and Tβ for the second time forging; The best post-forging heat treatment under study is 900℃×1h+(570℃-590℃)×4h which could keep basketweave at maximum extent and get fine and compact second precipitating α phase. The microstructures of TC21 alloy undergone near-β forging and quisi-β forging were bimodel and typical basketweave respectively. Quisi-β forging technique could get better fracture toughness while keeping strength, plasticity and fatigue crack propagaton rate at the same level with near-β forging technique. Further investigation on quisi-β forging technique focuses on five different compound quisi-β forging parameters. It is found that the strength of TC21 alloy grows higher when the forging temperature of the second time forging rises, and the trend of plasticity is contrary to that of strength, but there is little difference for fracture toughness and fatigue crack propagation rate of the two techniques. Moreover, great deformation amount to 70% didn't change the basketweave. In the experimental manufacture of forged pieces, two scheme were adopted: the temperature of first time forging was both Tβ+15°C, but the temperature of second time forging were T β +15°C(scheme 1) and T β (scheme 2). After comparing mechanical properties, microstructures and fatigue crack propagation rate of the two schemes, it was found that scheme 2 was superior to scheme 1 because scheme 1 had the combination of high strength, excellent plasticity and toughness. Solutioning temperature greatly affects the amount and shape of pre-α. As the solutioning temperature rising, the pre-α gradually spheroidizing from the original basketweave. Solutioning at Tβ-50°C could keep basketweave at maximum extent, thus acquiring excellent combination of overall mechanical properties and damage tolerant properties. The shape of second precipitation α grows longer and thicker as the aging temperature rises. Hence, the strength of TC21 alloy drops while tensile plasticity grows as the aging temperature rises. Aging at 570℃ and 590℃ could get fine and compact second precipitating α phase. Aging time also influences the amount, dimension and shape of the second precipitating α phase, but the effect was not so evident as that of aging temperature. Aging at 590℃ for 4 hours could get the proper dimension and shape of second precipitating α phase. After that, second precipitating α phase begins to become thicker. When aging time lasts beyond 16 hours, no observable change happens. Thus, the aging time should be around 4 hours.