Hot working processes of titanium and titanium alloys

A systematic summary and explanation of China’s hot working processes of titanium and titanium alloy materials were made.

Titanium and titanium alloys have excellent comprehensive properties such as high specific strength, specific modulus, toughness, high corrosion resistance, high temperature resistance, weldability, and no magnetism. They are widely used in various fields such as aviation, aerospace, shipbuilding, petroleum, chemical, weapons, electronics, and medical treatment. In the 1960s, industrialized countries such as the United States, the United Kingdom, and the former Soviet Union extensively used titanium and titanium alloy materials in aircraft and aviation engine manufacturing. The application of titanium and titanium alloy materials in China’s aviation industry started relatively late. It was in the 1980s that a small amount of titanium and titanium alloy materials were gradually used in aircraft and aviation engine manufacturing. However, after entering the 21st century, the application level of titanium and titanium alloy materials in China’s aviation industry has significantly improved. After nearly 40 years of efforts in China, significant progress has been made in alloy microstructure, processing technology, alloying, and other technologies. Especially in the past 20 years, through engineering practice research and exploratory testing by researchers from various research institutes, professional manufacturers, and universities, China’s titanium and titanium alloy hot processing has formed a relatively complete process technology system.

1. Classification of titanium and titanium alloys

The classification methods of titanium and titanium alloys include McGellen classification, β stability coefficient classification, and definition classification in GB/T 6611-2008. There are some differences in the literature for the classification of titanium and titanium alloys, but the general classification is consistent. Here is a brief introduction to the McGovern classification. This classification method is McGovern’s classification method in 1956 according to the phase composition of titanium and titanium alloy annealing state. Initially, titanium and titanium alloys were roughly divided into three categories: α type alloys, α + β type alloys, and β type alloys. However, with the rapid development of the research and application of titanium and titanium alloys, there are more and more types of titanium alloys, and the limitations of this McGovern classical classification method are getting larger and larger. Therefore, the descendants have improved on this basis and divided titanium and titanium alloys into five categories: the basic structure of titanium and titanium alloys after annealing is α phase, which is called α type alloy. The basic structure after annealing is α phase + β phase, but the α phase is mainly called near α type alloy. The basic structure after annealing is α phase + β phase, which is called (α + β) type alloy. The basic structure after annealing is the β phase, but there is a certain α phase called near β type alloy. The basic structure after annealing is the β phase, which is the β type of alloy.

2. Structure of titanium and titanium alloys

2.1 Microstructure of near α type and α + β type titanium alloys

2.1.1 Weinstein organization

Generally, it refers to the as-cast structure of titanium and titanium alloy, or titanium alloy deformation start temperature and end temperature are in the β phase region, the deformation is not very large (generally less than 50%), or when the alloy is heated to the β phase and then slowly cooled, the widmanstatten structure will be obtained. The Widmanstatten structure is characterized by coarse original β grains, and clear grain boundary α is distributed on the original β grain boundary. The original β crystal is a lamellar α beam domain, and the lamellar α is a β phase, as shown in Fig.1 (a).

2.1.2 Net Basket Organization

Titanium alloy deforms near the β transition temperature or begins to deform in the β phase region but terminates the deformation in the two-phase region, and the deformation amount is 50%-80%. The basketweave structure is characterized by the destruction of the original β grain boundary during the deformation process; no or only a small amount of granular grain boundary α appears, and the α sheet in the original β grain becomes shorter (i.e., the aspect ratio is small), the size of the α beam domain is small. The clusters are interlaced, as shown in Fig.1 (b).

2.1.3 Mixed organization

The titanium alloy is deformed at the upper part of the two-phase region, or after deformation in the two-phase region; it is heated to the upper temperature of the two-phase region and then air-cooled to obtain a mixed structure. The mixed structure is characterized by the distribution of unconnected primary α particles on the β transformation matrix, and the number is less than 40% (defined as 50% in the literature). There are two forms of α in the mixed structure: one is the primary equiaxed α particles, and the other is the secondary strip α on the transformed β matrix. Most of the literature is called two-state organization. Still, due to the development of a three-state organization based on the two-state organization in the 1990s, it is more appropriate to call it a mixed organization, which includes a two-state organization and a three-state organization. Three-state organization expression: (α + α + β) organization, characteristics: α ≈ 10%-20%, α ≈ 60%-70%, and chaotic interweaving, as shown in Fig.1 (c).

2.1.4 Isometric structure

The equiaxed structure can be generally obtained when titanium alloy is deformed in the two-phase region below the formation temperature of the bimodal structure (about 30-60 °C below the β phase transition point). The equiaxed structure is characterized by a uniform distribution of equiaxed primary α matrix with a content of more than 40%, and there is a certain amount of β structure. The lower the deformation temperature, the greater the primary α and dislocation density. The content of equiaxed primary α mainly defines the mixed structure and equiaxed structure. It is defined in the literature that the content of equiaxed primary α is more than 50%. However, it is currently recognized that the content of primary equiaxed α is more than 40%, even higher to 70%-80%. The morphology of primary equiaxed α includes spherical, elliptical, olive-shaped, rod-hammer-shaped, and long strip. Equiaxed structure = (α + β) = (α + α + β). The matrix of β includes fine widmanstatten α, and the black bottom between the fine strips is residual β, as shown in Fig.1 (d).

2.2 Microstructures of α type and β type titanium and titanium alloys

The typical microstructure of α type titanium and titanium alloy is a single α grain, as shown in Figure 1 (e); the typical microstructure of β type titanium alloy is a single β grain, as shown in Fig.1 (f). The properties of titanium and titanium alloys are determined by their microstructure, and the microstructure of titanium and titanium alloys is mainly determined by thermal processing (forging, heat treatment).

3. Forging of titanium and titanium alloy

3.1 Purpose of forging

3.1.1 Improvement of microstructure and properties

The most original billet for titanium and titanium alloy forging is ingot. During the forging process, the as-cast structure undergoes deformation and recrystallization. The original coarse dendrites and columnar grains become equiaxed recrystallized structures with fine grains and uniform size. At the same time, the original segregation, porosity, pores, and cracks in the ingot make the structure of the metal denser and improve the plasticity and mechanical properties of the metal.

3.1.2 Change the shape and size

In order to obtain the required blank specifications and dimensions, it needs to be achieved through appropriate forging deformation.

3.2 Free forging of titanium and titanium alloys (billet production)

The so-called free forging is a forging method in which the flow of metal is not restricted or not completely restricted in the direction perpendicular to the deformation force. The main advantage of free forging is that no forging die is needed, the parts are put into production quickly, and the size and weight of the parts are not limited. The disadvantage is that forgings with complex shapes and high dimensional accuracy requirements cannot be forged, the material utilization rate is low, and the production efficiency could be higher. Free forging is suitable for the production of single piece and small batch forgings.

Fig.1 Typical microstructures of titanium and titanium alloys
The free forging of titanium and titanium alloy mainly includes ingot forging, bar forging, slab forging, ring forging, and blank forging. In producing titanium and titanium alloy products, rings, forgings, etc. Are further hot-worked (forged, etc.) based on qualified rods. Therefore, the production of bars is the key to the production of titanium and titanium alloy products.

3.2.1 Heating of titanium and titanium alloy forging

Titanium and titanium alloys have the characteristics of allotrope transformation. When titanium and titanium alloys are in a solid state, the crystal structure of their atomic arrangement will change with the temperature change (microstructure transformation). At room temperature, the α-Ti (α phase) of the close-packed hexagonal crystal is dominant, and the β-Ti (β phase) of the body-centered cubic crystal is dominant at high temperature. When the heating temperature exceeds a certain temperature, the α-Ti (α phase) of the close-packed hexagonal crystal in titanium and titanium alloy is completely converted into the β-Ti (β phase) of the body-centered cubic crystal, which is called the β transition temperature (phase transition point). There is only one crystal plane with the densest atomic arrangement, the {0001} plane, for the hexagonal close-packed crystal. A slip plane can have three slip directions, so the number of slip systems is 1 × 3 = 3. Because titanium and titanium alloys are mainly α-Ti (α phase) of hexagonal close-packed crystal at room temperature, most titanium and titanium alloys have poor cold processing performance. The β-Ti (phase) is a body-centered cubic crystal. There are six {110} crystal planes with the densest arrangement of atoms in the body-centered cubic crystal; each has two slip directions, so the number of slip systems is 6 × 2 = 12. In the body-centered cubic crystal, there are 48 main slip planes and secondary slip planes. Therefore, the plasticity of titanium and titanium alloys becomes better, and the deformation resistance is greatly reduced after heating to high temperatures, which is beneficial to forging deformation.
Therefore, almost all titanium and titanium alloys are hot forging. During the forging of titanium and titanium alloys, due to the course as-cast structure, poor plasticity, too large ingot shape, and large deformation stress surface, it is generally heated at 100-200 °C above the β transition temperature to effectively improve the hot working plasticity and greatly reduce the deformation resistance per unit area. In the subsequent forging process, the heating temperature is gradually lower than the blank forging until the β transition temperature is below. The process method is characterized by high yield, uniform structure, and high requirement for equipment tonnage. However, some titanium and titanium alloy manufacturers adopt the process of heating and forging deformation directly into the two-phase zone after the completion of the billet opening. The disadvantage of this process is that the yield rate is low and the structure is uneven, but it is suitable for small equipment production. The specific process of titanium and titanium alloy product manufacturers will be different. Most of the specific processes are based on the practical experience and test data accumulated by the manufacturers themselves. Due to the poor plasticity of the widmanstatten structure or basket structure obtained by deformation above the β transition temperature, the microstructure of the near α type and α + β type titanium alloy bars are generally required to be an equiaxed or mixed structure with good plasticity in most product technical standards. Therefore, 2-3 fires or even more in the later stage of titanium and titanium alloy bar forging are heated and forged within 20-60 °C below the phase transition point.
The thermal conductivity of titanium is small, which is 1/15 of that of aluminum and 1/5 of that of iron. Therefore, it is necessary to preheat the titanium and titanium alloy billets with a diameter (thickness) of more than 300 μs at low temperatures to prevent internal cracking of the billet. At the same time, titanium and titanium alloys are not easy to hold for too long at high temperatures to avoid structure deterioration. Therefore, when heating titanium and titanium alloys, the temperature of the empty furnace is generally raised to a predetermined temperature and then heated in the loaded billet. The heating holding time of titanium and titanium alloy is generally calculated according to the thickness of 0.5-0.8min/mm; the longest holding time is generally not more than 1.2min/mm, and the hot state is halved. Since the heating temperature is sensitive to the transformation of titanium and titanium alloys, titanium and titanium alloys forging is generally heated by a resistance furnace with a temperature control accuracy of (±10-15) °C. However, due to the coarse as-cast structure formed by direct solidification of liquid metal, the temperature control accuracy of the heating furnace is not high. Therefore, the billet forging can be heated by a solid fuel reverberatory furnace, oil furnace, natural gas furnace, etc., but it must be the oxidizing atmosphere. 

3.2.2 Forging deformation per fire of titanium and titanium alloy 

The forging deformation of titanium and titanium alloy is generally more than 60%-75%, the forging deformation of intermediate fire is generally controlled at 40%-75%, and the forging deformation of the finished product is controlled at more than 20%. 
When titanium and titanium alloys are forged by a free forging hammer with fast forging speed, the thermal effect in the forging process must be considered. Due to the poor thermal conductivity of titanium and titanium alloys, and the temperature is very sensitive to its microstructure, the deformation heat of the core of the bar during the forging process on the hammer makes it easy to make the local temperature of the alloy close to or exceed the β transition temperature, resulting in the overheating structure of the core. Therefore, the billet cannot be continuously hit hard when the hammer forging process is used for titanium and titanium alloy products with organizational and performance requirements.

3.2.3 The cooling method after forging of titanium and titanium alloy forging 

In the traditional process, the cooling method of titanium and titanium alloy after forging is air cooling. After heating and forging deformation at the phase transition point, the microstructure of near α type titanium alloy and α + β type titanium alloy is a small amount of primary equiaxed α and a large amount of β phase at high temperatures. When the forging is air-cooled after forging, with the decrease of forging temperature, most of the stable β phase in the microstructure at high temperature gradually transforms to the stable α phase at room temperature. That is to say, during the air cooling process after forging, a small amount of primary equiaxed α in the microstructure gradually grows or aggregates with the decrease in temperature. A large amount of high-temperature β phase gradually transforms into strip α phase and residual β phase with the decrease of temperature. The strip α phase gradually grows, thickens, and even aggregates. This phenomenon is not conducive to the improvement of microstructure and properties. In addition, the cooling rate between the surface and the core of the billet is very different, and the consistency of microstructure and properties on the billet’s cross-section is poor. Similarly, for single-phase titanium and titanium alloy, air cooling will make its single-phase grain grow, and the consistency of internal and external grain size becomes worse.
Given the above shortcomings, in recent years, some titanium and titanium alloy manufacturers have adopted the water cooling process after forging titanium and titanium alloy billet and intermediate billet production. After forging, the near α titanium alloy and α + β titanium alloy are cooled by water, which has fast cooling speed and large undercooling. On the one hand, a small amount of primary equiaxed α in the high-temperature structure does not have enough time to grow; on the other hand, rapid cooling makes a large number of high-temperature β phases too late to transform into stable strip α phases gradually, and there is no time to make the new strip α phases grow and coarsen. Therefore, the primary equiaxed α phase and strip α phase in the microstructure produced by the water cooling process after forging are smaller than those produced by the air cooling process after forging. It can be seen from the experiment that water cooling after forging can not only refine the high-fold structure of titanium and titanium alloy but also refine its low-fold structure. The crystal defects (dislocations, subgrains) and the deformed microstructure with increased dislocation density were completely or partially fixed to room temperature by water cooling after forging, which increased the large number of crystal cores for recrystallization during subsequent heat treatment. During the subsequent heat treatment, the precipitation mechanism of the β phase changed from the induced nucleation mechanism under air cooling to the independent nucleation mode, and the fine, chaotic, and intertwined strip primary α and secondary α were obtained, which could significantly improve the strength. Similarly, for single-phase titanium and titanium alloy, water cooling will inhibit its single-phase grain growth, and the internal and external grain size consistency is good. 

3.2.4 Control of phase in near α type and α + β type titanium alloys

The free forging products of near α type titanium alloy and α + β type titanium alloy generally require that their microstructures are equiaxed or mixed, and the content of the primary α phase in them is required. For example, GJB493-88 stipulates that the primary α content of the bar for the rotor blade should not be less than 30%, the primary α content of the bar for the stator blade should not be less than 25%, and the size of the long strip α is required. The long strip α of the bar for the rotor blade is less than 0.06 mm, and the long strip α of the bar for the stator blade is less than 0.08 mm. For example,” HB5432-89 aircraft TC4 titanium alloy forgings” stipulates that the primary α content in the high-magnification structure of free forgings should not be less than 15%, the primary α content in the high-magnification structure of die forgings should be 15%-55%, and the long strip α should not exceed 0.2 mm. For example, GJB1538 stipulates that the long strip α in the high magnification tissue with a diameter greater than 150 does not exceed 0.25 mm. Taking the production of α + β two-phase TC4 titanium alloy bar as an example, the key to obtaining the bar that meets the requirements of GJB493-88 is the control of forging heating temperature and deformation: the closer the forging heating temperature of the finished product is to the β transition temperature, the less the number of primary α in the microstructure of the alloy, the worse the plasticity and the higher the strength; the more forging times and the larger deformation of the alloy below the β transition temperature, the finer, more uniform and closer to the spherical shape of the primary α in the microstructure of the alloy.
Most titanium alloy bars, rings, cakes, and other products require ultrasonic testing. The α of Widmanstatten and basketweave structures are coarse lath-shaped, and its grain boundary is more obvious. In ultrasonic testing, the alloy’s attenuation and the bottom wave’s reflection level easily exceed the requirements of the defect wave in the standard, and it is more difficult to detect the metallurgical defects in the alloy. Therefore, most technical standards do not allow titanium alloy products to be delivered in these two structural states. For the equiaxed and mixed structures, the large deformation during the forging process makes the grain boundary α completely broken and spheroidized, the primary α in the crystal is also completely spheroidized, and the secondary lath α becomes very short. The grain boundary of the alloy almost no longer exists (the fuzzy crystal is presented in the low-magnification inspection). Hence, the attenuation is very small during the ultrasonic flaw detection process. It is easy to detect the small metallurgical defects required by the standard. The equiaxed or mixed structure is the most desirable in producing titanium materials. 

3.3 Die forging (near α type and α + β type titanium alloy) 

The so-called die forging is the method of forging the metal blank in the forging model groove. The advantages of die forging are that it can forge forgings with complex shapes and high dimensional accuracy, high material utilization, high production efficiency, excellent mechanical properties, and good quality stability, which is suitable for mass production. The disadvantage is that the need to use professional mold manufacturing cycles is long, and equipment and mold investment is high.
Die forgings mostly process the parts used in the aviation industry, so the die forging of titanium alloy is widely used in the aviation industry. The conventional α + β forging, near β forging, sub β forging, and full β forging of titanium alloy often mentioned in various kinds of literature generally refers to the finished die forging process of (near α type, α + β type) titanium alloy. The billets used in the die forging of titanium alloy finished products are generally qualified billets with equiaxed or mixed microstructure produced by free forging. 

3.3.1 Conventional α + β forging

The conventional α + β forging process of titanium alloy in HB/Z 199-91 is defined as forging after heating in the α + β phase region below the β phase transition temperature of 25-50 °C or lower. Conventional forging generally obtains an equiaxed or mixed structure with good room-temperature plasticity and thermal stability but poor high temperature performance, fracture toughness, and crack propagation resistance. At present, α type titanium alloy and α + β type titanium alloy forgings are mostly produced by this process. More than 80% of titanium alloy forgings in China”s aero-engines are produced by this conventional α + β forging process.

3.3.2 Near β forging

The definition of titanium alloy’s near β forging process in the navigation mark HB/Z 199-91 is forging after heating at 10-15 °C below the β phase transition point. Titanium alloy forgings produced by conventional α + β forging have high room-temperature plasticity and thermal stability but poor high temperature performance, fatigue performance, and fracture toughness. Therefore, in the 1980s, the conventional α + β forging, near β forging, and β forging process of TC11 titanium alloy compressor disc of aero-engine were studied in Aerospace 148 factory. The microstructure obtained by the near β forging test is about 20% equiaxed primary α phase distributed on the matrix of short, fine, and messy basket structure, which is a mixed structure. This structure’s plasticity and thermal stability are not lower than those of conventional forging, but its high temperature performance, fatigue performance, and fracture toughness are greatly improved compared with conventional forging, and it has excellent comprehensive mechanical properties. Moreover, the heating temperature of this forging process is higher than that of conventional forging, and the alloy’s deformation resistance is small, which improves the thermal processing performance. At the same time, the heating temperature does not exceed the β transition point, thus avoiding the shortcomings of β forging. Based on a successful test, WP13 aero-engine TC11 compressor disk die forgings were produced by 148 factory using a near β forging process and installed in the machine for installation assessment, and the use is good. Later, Zhou et al. conducted in-depth research on the near-β forging process and introduced the strengthening and toughening treatment technology. The structure obtained by the near-β forging process was named a three-state structure (about 10%-20% equiaxed α, 50%-60% basket-shaped α, and a margin of transformation β matrix). When studying the near β forging process of TC11 titanium alloy, it is found that this structure can improve the high temperature performance, fracture toughness, and low cycle fatigue performance of the material without reducing the plasticity and thermal stability and can improve the service temperature of the material by 20-50 °C.

3.3.3 Sub-β forging

The definition of the sub-β forging process for titanium and titanium alloy in HB/Z 199-91 is as follows: forging is carried out after heating at 10-40 °C above the β phase transition point, and the deformation is all or mainly completed in the α + β two-phase region below the β phase transition point. Through the preferential crushing of the boundary α, the strip α in the boundary region is equiaxed, and the basketweave structure is obtained. The basic feature of the basketweave structure is that the grain boundary α is broken, and the intragranular α is basketweave, but this structure is difficult to obtain. This forging process generally produces forgings with high requirements for fracture toughness. 
TC17 titanium alloy discs for aero-engines have been produced by hot die/isothermal sub-β forging process in China Aviation Industry Forging Factory and Baosteel Special Steel Branch (formerly Shanghai No.5 Steel Plant) and have entered engineering applications. As the aircraft design concept gradually changes from the past pure static strength to safety-life, damage-safety, and even to the modern damage tolerance design concept, advanced titanium alloy materials are gradually developing towards damage-tolerant titanium alloys with high fracture toughness and low crack growth rate. The development and application of damage-tolerant titanium alloys in the United States are at the forefront. For example, the titanium used in the fourth generation fighter F-22 in the United States mainly includes medium-strength damage-tolerant Ti-6Al-4VELI titanium alloy and high-strength damage-tolerant Ti-6Al-2Sn-2Zr-2Cr-2Mo-S titanium alloy. It is reported that the finished forgings of these two titanium alloys are produced by β hot processing technology. Since the” Eleventh Five-Year Plan”, based on the domestic situation, China has independently developed TC4-DT medium-strength damage-tolerant titanium alloy and TC21 high-strength damage-tolerant titanium alloy and their β thermal processing technology and has been applied in the key bearing structural parts of advanced fighters.

3.3.4 Full β forging

The definition of full β forging process for titanium alloy in HB/Z 199-91 is that forging is carried out after heating at 50 °C or higher temperature above the β phase transition point, and the deformation is all or mainly completed in the β phase zone. The grain refinement is achieved by the dynamic recrystallization of the β phase, and the lamellar structure is obtained. The microstructure of the products produced by this process is mostly a widmanstatten structure, and the plasticity and thermal stability are too poor, so the process method is almost not used in producing finished forgings. 

3.3.5 Key elements of titanium alloy die forging production

Similar to the free forging process, the main control factors in the die forging process of titanium alloy are heating temperature, fire deformation, final forging temperature, and cooling method after forging. Among them, the most critical is the heating temperature. “HB 5355-94 forging process quality control” stipulates that II-IV should be selected for titanium and titanium alloy forging heating furnaces. For near-β forging and sub-β forging, II resistance heating furnaces with good temperature control accuracy and temperature uniformity should be selected. The influence of fire deformation, final forging temperature, and cooling mode after forging on the microstructure and properties of die forgings is consistent with that of free forging mentioned above, which can be flexibly adjusted according to the specific process.

4. Heat treatment of titanium and titanium alloy

4.1 Heat treatment of α type titanium and titanium alloy

This kind of titanium and titanium alloys are generally treated by ordinary annealing. When it is necessary to maintain the effect of cold deformation strengthening, stress relief annealing is used. Recrystallization annealing can be used when plasticity needs to be restored. 

4.2 Heat treatment of near α and α + β titanium alloys 

The heat treatment of this kind of titanium alloy mainly includes ordinary annealing, double annealing, quenching + aging, and so on. 
Double annealing aims to improve the alloy’s fracture toughness, stabilize the structure, and obtain good strength and plasticity matching. It is generally suitable for titanium alloys working at high temperatures. The purpose of secondary annealing is to fully decompose the metastable β phase obtained by primary annealing, produce a certain degree of aging strengthening effect, and obtain a structure with similar strength to ordinary annealing, high fracture toughness, and stable structure at high temperature.
The main characteristics of quenching + aging are as follows: Martensitic transformation occurs during quenching, that is, isomerism transformation. The plasticity and toughness of the alloy increase slightly, and the strength and hardness decrease slightly. In the subsequent aging process, due to the decomposition of the metastable phase and the production of the intermediate phase, the hardness and strength of the alloy increase, and the plasticity and toughness decrease. Suppose the content of β stable element in titanium alloy is not high. In that case, the quenching is that the β phase will be transformed from body-centered cubic lattice to hexagonal lattice by shear, and this supersaturated solid solution with hexagonal lattice is called hexagonal martensite α’. If the content of β stable element in titanium alloy is high, the lattice shear resistance is large. During quenching, the metastable phase formed by the β phase from body-centered cubic lattice to orthorhombic martensite α’ will decompose during aging, and α’ → α + β, α’ → α + β will obtain dispersed α and β phases, resulting in dispersion strengthening effect. This is the basic principle of heat treatment strengthening (quenching and aging strengthening) for most dual-phase titanium alloys. 

4.3 Heat treatment of β titanium alloy 

β type titanium alloy is generally used in the state of solid solution + aging heat treatment. This alloy is first dissolved near the phase transition point and then subjected to long-term artificial aging at a lower temperature. After aging treatment, a large amount of α phase will be dispersed in the β grain boundary and grain interior of the alloy microstructure, which plays a role in dispersion strengthening and ultimately improves the alloy’s strength and reduces the alloy’s plasticity. 
Author: Zhang Lijun