The improvement of the microstructure and properties of Ti-6Al-4V titanium alloy by annealings

Titanium and its alloys are widely used in aerospace, automotive, chemical, and shipbuilding industries due to their advantages such as low density, high specific strength, and good corrosion resistance. Ti-6Al-4V titanium alloy contains 6% α-phase stabilizing element Al and 4% β-phase stabilizing element V, belonging to the typical α+β type duplex hot strong titanium alloy. It has good mechanical and process properties and can be processed into semi-finished products such as rods, profiles, plates, and forgings, and is increasingly favored by people.

At present, domestic research mainly focuses on the high-temperature performance, creep performance, and thermal stability performance of Ti-6Al-4V. There is still relatively little research on how to formulate reasonable heat treatment processes to meet its actual usage performance. This paper studies the influence laws of heat treatment processes on the microstructure and mechanical properties of Ti-6Al-4V plates through different heat treatment processes. It has important theoretical and practical significance. Sponge titanium, high-purity aluminum (99.99%), and aluminum-vanadium alloy are melted in a vacuum water-cooled copper crucible non-self-consuming arc furnace at a certain ratio, with electromagnetic field stirring and argon gas protection. The alloy after melting has the composition (mass fraction, %): 6.29 Al, 4.14 V, 0.029 Fe, 0.023 C, 0.19 O, and the remainder is Ti. To ensure the uniformity of the sample chemical composition, Ti-6Al-4V rods are prepared by three times of flipping and remelting, then rolled into a 3mm thick titanium plate, and undergo 650℃×4h stress relief annealing treatment. The processed plates are processed into microstructure observation samples and tensile samples, and undergo different heat treatments: annealing (790℃×3h), solution quenching (980℃×1h, water cooling), solution aging (980℃×1h, water cooling + 580℃×8h, furnace cooling). The heat-treated samples undergo tensile performance testing.

After annealing of Ti-6Al-4V and furnace cooling, both phases undergo recrystallization. The α phase undergoes recrystallization, and small polygonal grains are precipitated in the deformed matrix. Secondary α precipitates are formed in the recrystallized β phase, resulting in a matrix with α phase distributed on the β phase transformation structure, and the structure is relatively uniform. Due to the elimination of internal stress, the plasticity and structural stability are improved, but the strength and hardness are slightly reduced. After solution quenching, the aspect ratio of α sheets decreases, the straight α sheets are distorted, and the continuous β phase boundaries are destroyed, forming thin sheets or basket-shaped α. The β phase rapidly cools from the high-temperature zone and fails to transform into α phase in time, forming metastable β phase. The room-temperature structure is martensite α' and metastable β phase, with increased strength and hardness, but reduced plasticity. After solution aging, some of the martensite α' and metastable β phase decompose, transforming into stable dispersed α phase and β phase, with higher strength and hardness than furnace cooling, but lower plasticity than furnace cooling. The comprehensive performance of the titanium alloy is improved.