Forging technology of titanium alloy

With the continuous progress of modern society, in order to meet the healthy development of China’s national economy and the major needs of national defense modernization construction, large tonnage, high precision and high efficiency forging deformation equipment with international advanced level has been manufactured. Energy saving and environmental protection heating equipment has also been widely used in forging production. At the same time, advanced forging technologies such as hot die forging, isothermal forging, superplastic forging, multi-directional die forging and powder forging have been widely used in production. Special forging technologies such as roll forging, cross rolling, radial precision forging, extrusion forging, rolling and upsetting have also been widely used. New forging technologies such as forging CAD / CAM / CAE have been applied in engineering, and intelligent forging technology is being comprehensively developed Engineering Research on life prediction technology of forging die and forging performance. With the development of advanced forging technology, the development trend of titanium alloy forging technology includes:

• ① Precision forging technology for complex ultra-thin forgings;
• ② Near net deformation technology of large integral forging;
• ③ High reliability and low cost forging technology;
• ④ Forging technology of new material forging;
• ⑤ Composite manufacturing technology based on forging deformation;
• ⑥ Forging Intelligent Technology (including numerical simulation, forging knowledge system, production automation line and equipment control), etc.

Forging process characteristics of titanium alloy

The purpose of forging deformation of titanium and titanium alloy is to obtain the shape and size of forgings meeting the design requirements, and the other is to make the microstructure and performance of forgings meet the requirements of design specifications. However, the quality of titanium alloy forgings is mainly determined by the forging process, that is to say, the bad microstructure formed during forging deformation of titanium alloy is difficult to be improved by heat treatment process. Therefore, it is necessary to understand the forging process characteristics of titanium alloy before making forging process. The forging process characteristics of titanium alloy mainly include the following three aspects.

Large deformation resistance

High deformation resistance is one of the remarkable characteristics of titanium alloy forging deformation. Compared with Cr Ni Mo alloy structural steel, the deformation resistance of Ti alloy is higher when forging deformation temperature is the same, which increases rapidly with the decrease of forging deformation temperature. Therefore, due to the high deformation resistance of titanium alloy, even if the forging deformation temperature is slightly reduced, the deformation resistance of titanium alloy will increase significantly, as shown in Fig. 1. Therefore, the first task of titanium alloy forging is to select reasonable forging deformation temperature.

Fig.1 Effect of forging deformation temperature on deformation resistance of titanium alloy and Cr Ni Mo alloy structural steel

Poor thermal conductivity

Poor thermal conductivity is another significant feature of titanium alloys. The thermal conductivity of some titanium alloys is shown in Table 1. Poor thermal conductivity makes the surface cooling of titanium alloy billet after heating out of the furnace faster than that of the internal part. If the operation is improper, it will cause relatively large temperature difference inside and outside the billet, aggravate the inhomogeneity of deformation inside and outside the billet in the process of forging deformation of titanium alloy, and even crack, which will seriously affect the service life and reliability of titanium alloy forgings. Therefore, it is very important to fully preheat the tools directly in contact with titanium alloy blanks, such as forging dies and clamps.
Table 1 thermal conductivity of some titanium alloys

Mark	Temperature / ℃
	20	100	200	300	400	500	600
TA2	19.3	18.9	18.4	18	18	18	18
TA7	8.8	9.6	10.9	12.2	13.4	14.7	15.9
TA15	–	8.8	10.2	10.9	12.2	13.8	15.1
TC4	6.8	7.4	8.7	9.8	10.3	11.8	–
TB10	–	8.4	10.9	12.3	13.5	15.2	16.5
TB2	–	8.2	10.8	11.9	13.1	14.7	16.3
TB7	7.1	7.5	10	10.7	13.6	15.3	16.8

High viscosity and poor liquidity

Due to the high viscosity and poor fluidity of titanium alloy, it is required to strengthen lubrication during forging deformation of titanium alloy. Otherwise, there will be sticky die and material backflow phenomenon. At the same time, the deformation resistance will increase significantly due to the increase of friction force, and sometimes the forging will be torn due to die sticking. The results show that the friction coefficient of titanium alloy is 0.5 without lubricant and 0.04 ~ 0.06 with glass lubricant. Therefore, the reasonable lubricant is an important measure to ensure the quality of titanium alloy forging.

Classification of titanium alloy forging process

Like other metal materials, titanium alloy parts with high reliability should be manufactured by forging process. The common forging methods of titanium alloy include free forging, hot die forging and special forging.

Free forging

Open die forging (ODF) of titanium alloy is a forging method that uses external force to deform titanium alloy between upper anvil and lower anvil to obtain forgings with certain microstructure, properties, shape and size. It is especially suitable for the production of large forgings or extra large forgings in heavy machinery. The degree of forging deformation is an important process parameter in the free forging process, and it is a necessary condition to refine the microstructure of titanium alloy. When the deformation degree is less than 30%, the casting structure can not be broken or only slightly broken; when the deformation degree is greater than 30%, the microstructure can be obviously refined. Generally, in order to refine the acicular microstructure of titanium alloy and transform it into spherical structure, the forging deformation temperature should be in the a + β phase region, and the deformation degree should be greater than 60%.

Hot die forging

Hot die forging (HDF) of titanium alloy is a forging method which uses external force to deform the titanium alloy blank in the die cavity to obtain forgings with certain microstructure, properties, shape and high dimensional accuracy. Hot die forging is suitable for producing titanium alloy forgings with complex structure, high dimensional accuracy and small machining allowance. In order to make the microstructure and properties of titanium alloy forgings meet the design requirements, hot die forging is widely used in titanium alloy forging. Hot die forging of titanium alloy can be divided into three forging processes: Die Forging (DF), isothermal forging (IDF) and superplastic forging (SPF). The forging process has a significant effect on the dimensional accuracy of titanium alloy forgings. When the common forging is used, the maximum height width ratio of common die forging is 6:1, and that of precision forging is 15:1; when isothermal forging or superplastic forging is adopted, the maximum height width ratio of precision forging rib is 23:1, the minimum width of reinforcement is 2.5mm, and the minimum thickness of web is 2.0mm. Ordinary forging is generally used to produce titanium alloy forgings with simple shape, isothermal forging is generally used to produce titanium alloy forgings with complex shape and high dimensional accuracy requirements, and superplastic forging is generally used to produce titanium alloy forgings with extremely complex shape, large cross-section variation and high performance requirements.

Special forging

Special forging (SF) of titanium alloy is a forging method that uses external force to deform titanium alloy blank on special equipment to obtain forgings with certain microstructure, properties, shape and size. The production efficiency of special forging is high, which is suitable for the production of large quantities of titanium alloy forgings. For example, the production of screws on upsetting machines and thread rolling machines has doubled the production efficiency. However, a special forging equipment can only produce one kind of forgings, which has limitations.
Forging deformation temperature, deformation degree and deformation speed are the key control parameters in titanium alloy die forging process design. In order to reduce the energy consumption of forging deformation and make full use of the plasticity of titanium alloy, the higher the initial forging temperature is, the better. For example, the flow stress of Ti6A14V alloy during hot die forging is 1200Mpa, that of isothermal forging is 150MPa, and that of superplastic forging is 40MPa. When the deformation temperature is 980 ℃ and the deformation rate is 1 mm / s, the minimum wall thickness of Ti6A14V alloy nose ring is 6.3 mm; when the deformation rate is 0.04 mm / s, the wall thickness of Ti6A14V alloy nose ring forging at the same section reaches 1.52 ~ 1.87 mm. However, if the initial forging temperature exceeds the β phase transformation temperature of titanium alloy, the widmanstatten structure is easy to be formed due to the rapid growth of β grains, which will result in the low room temperature plasticity of titanium alloy forgings. The phenomenon that the initial forging temperature of titanium alloy is higher than that of β phase transformation temperature leads to grain growth and decrease of plasticity, which is called β brittleness of titanium alloy. Therefore, in order to avoid the β brittleness of α + β alloy and make α + β alloy forging have excellent comprehensive properties, forging should be carried out below β transformation temperature. For β – alloy, the forging deformation temperature is higher than that of titanium alloy, and β – brittleness may also occur. However, on the one hand, due to the high alloying degree of β alloy, its β phase transformation temperature is low (700 ~ 800 ℃), if forging below the β transformation temperature, the deformation resistance is too large; on the other hand, due to the high alloying degree of β alloy, if forging under the β transformation temperature, the growth rate of β grain will be lower than that of α + β alloy and α alloy. Therefore, the initial forging temperature of β – alloy is always higher than that of β phase transformation temperature, but in order to avoid β – brittleness, the initial forging temperature of β – alloy should not be too high.
Forging deformation degree is an important factor to determine the service performance of titanium alloy forgings. The results show that when the forging deformation degree is 2% ~ 10%, the grain size of titanium alloy after deformation is very coarse. When the deformation degree exceeds the forging deformation degree, the larger the deformation degree, the finer the grain size. When the forging deformation is more than 85%, the grains of the titanium alloy are very coarse due to the aggregation and recrystallization. In addition, increasing the deformation degree can reduce the anisotropy of titanium alloy during forging. For example, when the deformation temperature is 800 ~ 1000 ℃ and the deformation degree is 75% ~ 80%, the anisotropy in the microstructure of TA2 alloy reaches the minimum; when the deformation degree is about 90%, the anisotropy in the microstructure of TA6 alloy and TC6 alloy reaches the minimum.
Recrystallization and work hardening occur simultaneously in the forging process of titanium alloy. When the forging deformation rate is increased, the recrystallization of titanium alloy can not be fully carried out, resulting in the decrease of plasticity and the increase of deformation resistance. Therefore, the deformation degree of each stroke should be larger and the deformation speed should not be too large. For the common forging equipment, the deformation speed of the press is relatively slow. Choosing the titanium alloy forging on the press can reduce the deformation resistance of the titanium alloy and reduce the energy consumption. Moreover, the relatively low deformation speed will make the plasticity of the titanium alloy higher and the filling easier.
According to the forging deformation temperature of titanium alloy, it can be divided into four forging processes: α + β forging, β forging, near β forging and quasi β forging.

• (1) α + β forging is a typical equiaxed structure obtained by heating and forging at 30 ~ 50 ℃ below the β phase transformation temperature. After α + β forging deformation, the plasticity and room temperature strength of titanium alloy forgings are higher, and the high temperature properties and fracture toughness are lower.
• (2) β forging is to obtain basket structure or widmanstatten structure by heating and forging at 50 ℃ or higher than the β transformation temperature. After β forging deformation, the creep resistance, fracture toughness and impact resistance of titanium alloy forgings are high. Due to “β brittleness” and “microstructure heredity”, the plasticity and thermal stability of titanium alloy forgings are low, so they are rarely used.
• (3) Near β forging (or sub β forging) is to obtain 10% ~ 20% equiaxed α + 50% ~ 60% lamellar α + β transformation matrix structure by heating and forging at 10 ~ 15 ℃ below the β phase transformation temperature. After nearly β forging deformation, the comprehensive properties of titanium alloy forgings such as plasticity, high temperature performance, fatigue property and fracture toughness are good.
• (4) Quasi – β forging is a typical basket structure obtained by heating and forging deformation at 5 ~ 10 ℃ higher than the β phase transformation temperature. After quasi – β forging deformation, the creep resistance, fracture toughness and impact resistance of titanium alloy forgings are higher, and the plasticity and thermal stability are lower.

Typical applications

Titanium alloy forgings are widely used in aviation, aerospace, warship, marine engineering, weapons, energy, automobile, metallurgy and petrochemical fields, and are key parts related to operation safety. For example, turbine disk and blade of aeroengine, landing gear and wing beam of aircraft, crankshaft and front beam of automobile, rotor, impeller and shaft of generator set. In this paper, the typical application of titanium alloy equiaxed technology is introduced combining with precision forging blade and large integral forging.

Precision forging blade

There are thousands of blades in aeroengine, of which 1 / 2 of them are forged by titanium alloy. In particular, the compressor blades are required to have high strength, good crack resistance and long service life. Therefore, improving the performance of titanium alloy forged blades and reducing their manufacturing costs have been highly valued in the field of forging technology at home and abroad. High performance titanium alloy precision forging blades (precision forged blades refer to the blades after zirconium forging can be used as mechanical parts without machining) ）Instead of the traditional titanium alloy die forging blade, it has been widely used. The precision Forged Blade of Ti6Al4V alloy produced by Rolls Royce for Trent 900 aeroengine is shown in Fig.2. Compared with the titanium alloy forged blade manufactured by hot die forging process, the strength, plasticity and impact toughness of ti6.2al2.5mo1.5cr0.2si0.5fe alloy precision forged blade manufactured by superplastic deformation process in Russia have been significantly improved, as shown in Table.2. Compared with hot die forging process, the superplastic deformation process is used to manufacture Ti6Al4V alloy blade with a projection area of 9000m2 in the United States, which saves 40% of raw materials and reduces the manufacturing cost by about 20%. In China, northwest Polytechnic University and Xi’an Aero Power Co., Ltd. of China Aero Engine Group Co., Ltd. have overcome the key technologies of precision forging process of titanium alloy blade, computer-aided design, precision billet making, forging billet lubrication, blade correction, chemical milling, three-dimensional inspection, multi-scale coupling numerical simulation and deformation process parameter optimization. TC4, TC6, tc8 and TC11 titanium alloy precision forged blades have been widely used in aviation, aerospace and marine power plants. Compared with the original forging process, the material utilization rate is increased from 15% to more than 30%, and the machining cost is saved by about 50%.

Fig.2 Ti6Al4V

Table.2 mechanical properties of ti6.2al2.5mo1.5cr0.2si0.5fe alloy blade

Technology		Tensile strength σb/MPa	Elongation δ/%	Reduction in area ψ/%	Impact toughness/(J/cm2)
					U-notch	Fatigue crack
Superplastic deformation	Portrait	1190	18	54	55	16
	Axial	1150	16	55	51	16
Hot die forging	Portrait	1080	16	35	55	10
	Axial	945	14	38	43	19

Large integral forging

Due to the limitation of technical level and production conditions, the original process of manufacturing large-scale structural parts first adopts block die forging, and then adopts the way of butt welding, riveting or bolt fastening. The mechanical processing of the components takes a long time, which will reduce the strength and reliability of the components, increase the weight of the aircraft structure, and put forward high requirements for the matching accuracy. The forging technology of titanium alloy large-scale integral parts can change the traditional multi-component components into integral structural parts, which greatly reduces the structural weight of the aircraft and improves the structural efficiency of the aircraft and the safety and reliability of the parts. For example, BT22 alloy heavy forgings are used in Russia for Su27 (Su27), il76 (il76), il86 (il86), il96 (il96), an124 (A124) and figure 204 (tu204) and other aircraft body and landing gear; France uses 7 m long ti1023 alloy heavy forgings for the main landing gear of Airbus 380; the landing gear of Boeing 777 aircraft adopts ti1023 alloy heavy forgings, which reduces the structure weight by about 270Kg; the United States F-22 fighter plane adopts Ti6Al4V alloy integral spacer frame forging, which is 3.8m long, 1.7m wide, 1590kg in weight and 5.16m2 in projection area.
Large integral forgings have been widely used in China’s aerospace field. By adopting isothermal forging technology, Baosteel Group Co., Ltd., Baosteel Special Steel Co., Ltd. and northwest Polytechnic University, etc., key technologies such as near net forging deformation of large and extra large titanium alloy forgings, numerical simulation of the whole forging process, die design and manufacturing, and design and manufacturing of special heating equipment, etc., have been overcome. TC17 alloy blisk and TC4 alloy journal have been successfully applied in national major projects, as shown in Fig.3. Therefore, the development of advanced titanium alloy forging technology will expand the application field of titanium alloy.

Fig.3 TC17 blisk and TC4 alloy Journal

Other aspects

Using titanium alloy forgings as steam turbine blades for thermal power generation can increase the length of steam turbine blades, thus improving power generation efficiency and reducing rotor load. As early as 1991, Ti-6Al-4V alloy blade with 1m length has been used in the last stage of high-speed rotating steam turbine. In sports equipment, titanium alloy forgings can be used on golf clubs. Due to the high strength of β – type titanium alloy forgings, forgings with plate thickness less than 3 mm can be used as the hitting sphere, so that the elastic hitting surface can store or release energy to slow down the impact through a long impact time, so that players can hit the ball far without swinging the club. The spherical forging of titanium alloy golf club is shown in Fig. 6. In addition, titanium alloy forgings are widely used in marine and offshore areas, automobile industry, construction industry and medical equipment industry.

Fig.4 Spherical forging of titanium alloy golf club

Common forging defects of titanium alloy

The tissue is not uniform

In the process of metal forging, due to the influence of external friction and other factors, uneven deformation will occur, which has an important impact on the realization of forming and the microstructure and properties of the material after forming. When the deformation temperature is 800 ℃ ~ 950 ℃, the grain size of titanium alloy is refined, but the crystallization volume fraction is small; when the deformation temperature is 950 ℃ ~ 1150 ℃, the dynamic recrystallization is more sufficient, and the microstructure uniformity is improved correspondingly. However, when the temperature exceeds 1050 ℃, the grains grow excessively and the microstructure coarsens seriously. See Figure 7 for details. Compared with the fine normal α – bar in basket structure, the crystal interface of this kind of coarse α – block is rough and uneven. It grows from grain boundary to crystal interior in morphology, and the crystal interface of normal α – bar is smooth, which affects the quality of forging.

Fig.5 Microstructure of titanium alloy forgings

Impact on Performance

If the preheating temperature of the tool is too low, the impact speed of the equipment is low and the deformation degree is large, X-shaped shear band is often formed on the longitudinal section or cross section. This is especially true for non isothermal upsetting on hydraulic press. This is because the tool temperature is low and the contact between the blank and the tool causes the surface of the metal billet to be chilled. During the deformation process, the deformation heat generated by the metal has no time to conduct heat around, and a large temperature gradient is formed from the surface layer to the center, resulting in the formation of a strong flow strain band. Secondly, there is residual casting structure. Titanium alloy. When the forging has residual casting structure, the center of transverse macrostructure is dark gray, without metallic luster, with network structure and no obvious streamline in longitudinal direction; the dendrite in high-power structure is complete, and the main branches and trunks form 90 ° each other. The residual forging microstructure of superalloy is columnar crystal in macrostructure, and the branches are not broken; the grains in high-power structure are very coarse, and there are some broken fine grains.

Crack defect

It mainly refers to forging cracks. Titanium alloy has high viscosity, poor fluidity and poor thermal conductivity. Therefore, in the process of forging deformation, due to large surface friction, obvious internal deformation heterogeneity and large temperature difference between inside and outside, it is easy to produce shear band (strain line) in the forging, which will lead to cracking in severe cases, and its orientation is generally along the direction of maximum deformation stress. The cracks produced by forging may be forging folding or quenching cracks after forging. Prepare the transverse metallographic specimen of the crack to see whether there is overheated and overheated structure near the crack, and analyze the composition of oxide on the surface of crack fracture.
Source:  China Titanium Alloy Flange Manufacurer: www.titaniuminfogroup.com