Heat Treatment and Properties of Titanium Grade 5 Ti6Al4V Titanium Alloy
Feb 29, 2024
Titanium Grade 5 Ti6Al4V is one of the most widely used titanium alloys, typically used in its annealed state. It can undergo stress relief annealing, recrystallization annealing, and solution aging treatment. The annealed structure of TC4 consists of a mixture of α and β phases, with the β phase content being around 10%. The recrystallization temperature of Grade 5 Ti6Al4V is 75°C. Recrystallization annealing temperature is usually selected to be 80-100°C above the recrystallization temperature (though specific conditions may vary). After recrystallization annealing, Grade 5 Ti6Al4V exhibits an equiaxed α phase + β phase structure with excellent comprehensive properties. However, annealing of Grade 5 Ti6Al4V is merely a phase stabilization treatment. To fully exploit its excellent properties, it requires strengthening treatment.
The α+β/β phase transformation temperature of Grade 5 Ti6Al4V alloy is 980-990°C, and the solution treatment temperature is generally selected to be 40-100°C below the α+β/β transformation temperature. While solution treatment in the β phase region results in coarse Widmanstätten structures with high permanent strength and fracture toughness, it leads to low tensile plasticity and fatigue strength. Solution treatment in the α+β phase region does not have this drawback. The specifications, types, temperatures (°C), times (min), and cooling methods for various heat treatments of Grade 5 Ti6Al4V are as follows:
Stress relief annealing: 550-650°C for 30-240 min, air cooling.
Recrystallization annealing: 750-800°C for 60-120 min, air cooling or cooling in the furnace to 590°C followed by air cooling.
Vacuum annealing: 790-815°C.
Solution treatment: 850-950°C for 30-60 min, water quenching.
Aging treatment: 480-560°C for 4-8 hours, air cooling.
Aging treatment involves heating the Grade 5 Ti6Al4V solution-treated material to a moderate temperature, holding it for a certain period, and then air cooling. The purpose of aging treatment is to eliminate the unfavorable α' phase produced during solution treatment. The coarse martensitic α' formed during solution treatment rapidly decomposes during aging (the phase transformation is quite complex), resulting in increased strength. There are two perspectives regarding this phenomenon:
The dispersion strengthening effect of the decomposition products of α' into α+β leads to increased Grade 5 Ti6Al4V strength.
During aging, β phase decomposition forms ω phase, leading to TC4 strengthening.
As aging progresses, the strength decreases, and there are two different views on this phenomenon:
Aggregation of β phase leads to strength reduction (corresponding to perspective 1 above).
Decomposition of ω phase during aging is a softening process (corresponding to perspective 2 above).
The selection of aging temperature and time should aim to achieve the best comprehensive properties. Within the recommended solution and aging ranges, the optimal process should ideally be determined through aging hardening curves. Low-temperature aging (480-560°C) is preferable to high-temperature aging (>700°C) because low-temperature aging generally outperforms high-temperature aging in terms of tensile strength, creep strength, fracture toughness, notch tensile properties, etc. The comprehensive properties of Grade 5 Ti6Al4V after solution treatment are better than those after annealing at 750-800°C. It should be noted that the initial structure of Grade 5 Ti6Al4V alloy in the as-worked state has a significant impact on the microstructure and mechanical properties after heat treatment. For the lamellar structure formed by different deformations above the phase transformation temperature, it cannot be changed by heat treatment and basically maintains its original structure after annealing at 750-800°C. For the α and β phase structures obtained by working below the phase transformation temperature, equiaxed primary α phase and transformed β phase can be obtained after annealing at 750-800°C. The former has lower tensile elongation and reduction of area than the latter; however, it exhibits higher high-temperature performance, fracture toughness, and resistance to thermal salt stress corrosion.
