Heat Treatment of TC4 (Ti-6Al-4V)

GR5 is one of the most widely used titanium alloys. It is usually used in the annealed state. Ti-6Al-V can undergo stress relief annealing, recrystallization annealing, and solution aging treatment. After annealing, the structure consists of a coexistence of α and β phases, but the β phase content is relatively small, accounting for about 10%. The recrystallization temperature of grade 5 is 750℃. The recrystallization annealing temperature is generally selected to be 80–100℃ above the recrystallization temperature (but in actual applications, it can be determined according to specific conditions, as shown in Table 5-26). After recrystallization annealing, the structure of Gr5 becomes equiaxed α phase + β phase, providing good overall performance. However, the annealing treatment of TC4 is mainly a phase stabilization treatment. To fully exploit its excellent performance, strengthening treatment should be carried out.

The α+β/β phase transformation temperature of TC4 alloy is 980–990℃, and the solution treatment temperature is generally selected to be 40–100℃ below the α+β/β transformation temperature (depending on specific requirements). Solution treatment within the β phase region produces a coarse Widmanstätten structure, which has the advantages of high creep strength and high fracture toughness but results in low tensile ductility and fatigue strength. Solution treatment within the α+β phase region avoids these disadvantages.

 

Heat Treatment	Temperature (℃)	Time (min)	Cooling Method
Stress relief annealing	550–650	30–240	Air cooling
Recrystallization annealing	750–800	60–120	Air cooling or furnace cooling to 590℃, then air cooling
Vacuum annealing	790–815	-	-
Solution treatment	850–950	30–60	Water quenching
Aging treatment	480–560	4–8 hours	Air cooling

 

Aging treatment involves heating TC4 after solution treatment to a medium temperature, maintaining it for a certain period, and then air cooling. The purpose of aging treatment is to eliminate the α' phase formed during solution treatment, which is detrimental to overall performance. The quenched martensitic α' produced by solution treatment rapidly decomposes during aging (the phase transformation is complex), leading to an increase in strength. There are two main views on this phenomenon:

 

It is believed that the decomposition of α' into α + β phases results in a dispersion strengthening effect, thereby increasing the strength of GR5

It is believed that during aging, the β phase decomposes to form the ω phase, causing an increase in strength. As aging progresses, strength decreases. Regarding this decrease, there are two further views:

 

The aggregation of the β phase reduces strength (corresponding to viewpoint 1 above).

The decomposition of the ω phase is a softening process (corresponding to viewpoint 2 above).

 

The selection of aging temperature and time should aim to achieve the best comprehensive performance. Within the recommended solution and aging treatment ranges, it is best to determine the optimal process through an aging hardening curve (as shown in Figure 5-28, which presents the aging hardening curve of Gr5 at different temperatures after solution treatment at 850℃).

 

Low-temperature aging (480–560℃) is preferable to high-temperature aging (above 700℃). In terms of tensile strength, rupture and creep strength, fracture toughness, and notch tensile properties at high temperatures, low-temperature aging yields better comprehensive properties than high-temperature aging.

 

The comprehensive performance of Grade 5 after solution treatment is superior to that after annealing at 750–800℃. It should be noted that the original structure of TC4 in the processed state has a significant influence on the microstructure and mechanical properties after heat treatment. For the mesh-like structures formed by deformation above the phase transition temperature, heat treatment cannot alter them. After annealing at 750–800℃, the original structure is largely retained.

 

 

For the α and β phase structures formed by processing below the phase transition temperature, annealing at 750–800℃ results in equiaxed primary α phase and transformed β phase. The tensile ductility and reduction in cross-sectional area of the former are lower than those of the latter. However, the high-temperature resistance, fracture toughness, and resistance to hot salt stress corrosion of the former are superior.

 

Shaanxi Yuanzekai Metal Technology Co., Ltd. produces and sells gr5 titanium bars, titanium plates, titanium tubes, titanium parts, titanium bolts and other products for aerospace, military, chemical, medical, marine, sporting goods, automotive and other application fields.