TC4 titanium alloy (Ti-6Al-4V) is a typical α+β dual-phase titanium alloy, composed of 6% Al and 4% V, consisting of α phase of hexagonal close-packed structure (HCP) and β phase of body-centered cubic structure (BCC). The microstructure can be regulated by heat treatment. It has excellent comprehensive properties, such as high strength, good corrosion resistance, low density, and high specific strength. It is widely used in important fields such as aerospace (engine blades, fuselage structure), medical devices (artificial joints), and marine engineering.
The microstructure and mechanical properties of TC4 titanium alloy are highly dependent on hot working technology. The TC4 titanium alloy hot working represented by forging is an important means to improve the performance of TC4 alloy. Common forging methods of titanium alloy include free forging, hot die forging, and special forging.
(1) Free forging
Free forging refers to placing the blank between the upper and lower anvils and using the upper anvil to apply impact load on the forging to cause a certain deformation. Usually, the forging temperature should be in the α+β phase region, and the deformation should be greater than 60% to refine the titanium alloy microstructure and obtain titanium alloy forgings with good performance.
(2) Hot die forging
Hot die forging is placing the titanium alloy billet in a die with a specific shape and using an external force to deform it to obtain a forging with a certain shape. It is often used to produce forgings with complex structures and high precision. To make the microstructure and performance of titanium alloy forgings meet the design technical index requirements, hot die forging is widely used in titanium alloy forging. Hot die forging is divided into ordinary die forging, superplastic forging, and isothermal forging. Ordinary forging is often used to produce forgings with low dimensional accuracy and simple shapes. Superplastic forging is suitable for forgings with complex three-dimensional dimensions and high-performance requirements. Isothermal forging is used for forgings with high precision requirements and complex shapes.
(3) Special forging
Special forging is a processing method in which titanium alloy billets are placed on special equipment for deformation. TC4 titanium alloy hot working production efficiency is high, suitable for mass production of similar forgings, but because a device can only produce a specific forging, it has certain limitations.
Forging temperature and deformation are two key forging process parameters of titanium alloy. From the perspective of reducing the forging energy of titanium alloy, the higher the initial forging temperature, the better, but too high will lead to coarsening of β grains and reduce the plasticity of forgings. When the initial forging temperature is higher than the β phase transformation temperature, grain growth and plasticity reduction occur. To avoid this phenomenon, forging should be performed below the β phase transformation temperature of titanium alloy.
According to the forging temperature, titanium alloy forging is divided into α+β forging, β forging, near β forging and quasi-β forging.
1) α+β forging: the heating temperature is α+β two-phase region 30-100℃ below the β transformation temperature. Its forging process deforms the primary α phase and β phase simultaneously during the TC4 titanium alloy hot working, and a typical equiaxed structure or dual-state structure can be obtained in complex structure forgings and large deformation.
(2) β forging: The heating temperature is 30-100℃ above the β transformation temperature. Unlike the α+β forging process, the temperature difference of the β forging process is relatively large. Within its temperature range, it can greatly improve the deformation resistance of titanium alloy during TC4 titanium alloy hot working, reduce the forming flow stress, and thus improve the die life, and improve the creep resistance, impact resistance, and fracture toughness of forgings. However, improper process control is prone to occur during the forging process, resulting in a significant decrease in the plasticity of the titanium alloy metal, resulting in the unqualified fatigue strength of the forging.
(3) Near-β forging: The heating temperature is controlled at 10-20℃ below the β transformation temperature for forging. After forging, water quenching and solid solution aging treatment are used to obtain a three-state structure composed of a small amount of equiaxed α and strip α mesh basket and β transformation matrix. Without reducing plasticity and thermal stability, the high-temperature performance, low-cycle fatigue performance, and fracture toughness of the material are improved.
(4) Quasi-β forging: preheat the billet at 20-40℃ below the β transformation temperature, and then quickly heat the billet to 10-20℃ below the phase transformation temperature for forging to obtain a basketweave structure, thereby obtaining a forging with high plasticity, good toughness, and excellent fatigue performance.
In addition to the forging temperature, the deformation is an important factor that determines the performance of titanium alloy forgings. Studies have shown that when the forging deformation is 2%~10%, the grain size of the forging is coarse. After exceeding the above forging deformation, the grain size gradually decreases with the increase of the deformation. When the deformation is greater than 85%, the grain size of the forging grows due to recrystallization. Some researchers have studied the effects of forging temperature, deformation, and strain rate on TC4 titanium alloy and found that the deformation and strain rate have a significant effect on the mechanical properties of the forging. Under medium deformation and large strain rate, the microstructure of the forging is fine and uniform, with high yield strength and tensile strength; when the forging temperature is slightly lower than the phase transformation point, the forging parameters have a significant effect on the strength and plasticity of the forging. The study of the isothermal forging of TC18 titanium alloy in the β phase region shows that the microstructure of the alloy is sensitive to the change in isothermal forging temperature. When the deformation reaches 60%, the microstructure is uniform and fine, and the fracture toughness is high.
The TC4 titanium alloy hot working significantly improves its comprehensive performance by regulating the microstructure, but the parameters need to be optimized for different application scenarios. Developing low-energy consumption and high-precision composite processing technology, combining machine learning to establish a process-structure-performance prediction model, and exploring the influence of new cooling media on phase transformation will be of great significance for further improving the performance of TC4 forgings in the future.
