1. Heat treatment characteristics of Titanium Alloy (1) martensitic transformation does not change the properties of titanium alloy significantly. This characteristic is different from the martensitic transformation of steel, the heat treatment strengthening of titanium alloy can only depend on the aging decomposition of the metastable phase (including martensitic phase) formed by quenching, and the heat treatment method of pure a type titanium alloy is basically not effective, that is, the heat treatment of titanium alloy is mainly used for α + β titanium alloy. (2) the formation of ω phase should be avoided during heat treatment. The formation of ω phase will make the titanium alloy brittle, and the proper aging process (for example, higher aging temperature) can make ω phase decomposition. (3) it is difficult to refine titanium alloy grains by means of repeated phase transformation. This is also different from steel materials, most of the steel can use austenite and pearlite (or ferrite, cementite) of the repeated phase transformation control of the new phase nucleation and growth, to achieve grain refinement goal, titanium alloys do not have this effect. (4) poor thermal conductivity. Poor thermal conductivity can lead to poor hardenability of titanium alloys, especially α + β titanium alloys, high quenching thermal stress and easy warpage of parts during quenching. Because of the poor thermal conductivity, the local temperature of titanium alloy will rise too high, and the local temperature may exceed β transition point and form widmanstatten structure. (5) chemically active. During heat treatment, the Titanium Alloy Reacts With Oxygen and Water Vapor, forming an oxygen-rich layer or oxide scale on the surface of the workpiece, which reduces the properties of the alloy. At the same time, it is easy to absorb hydrogen and cause hydrogen embrittlement during heat treatment of titanium alloy. (6) big differences in Beta transition points. Even the same composition, but due to different smelting furnace, its β conversion temperature sometimes very different. (7) when the β phase is heated, the β grain tends to grow up. Because β grain coarsening can decrease the plasticity of the alloy rapidly, the heating temperature and time should be strictly controlled, and the heating treatment in β phase zone should be used cautiously.

Phase transformation of titanium alloy is the basis of titanium alloy heat treatment. In order to improve the properties of titanium alloy, it is necessary to adopt proper alloying and heat treatment. There are many kinds of heat treatment of titanium alloy, such as annealing treatment, aging treatment, deformation heat treatment and chemical heat treatment. 1. Annealing treatment annealing is suitable for all kinds of titanium alloys, its main purpose is to eliminate stress, improve alloy plasticity and stable structure. The forms of annealing include de-stress annealing, recrystallization annealing, double annealing, isothermal annealing and vacuum annealing, and so on.

(1) stress relief annealing. In order to eliminate the internal stress in the process of casting, cold deformation and welding, stress relief annealing can be used. The temperature of stress relief annealing should be lower than that of recrystallization, generally 450 ~ 650 °C. The time required depends on the cross-section size of the workpiece, processing history and the degree of stress relief required. (2) ordinary annealing. The aim is to make the semi-finished product of titanium alloy free from the basic stress and have higher strength and plasticity in accordance with the technical requirements. Annealing temperature and recrystallization start temperature is generally equal to or slightly lower, this annealing process is generally used when metallurgical products factory, so it can be called factory annealing. (3) complete annealing. The aim is to completely eliminate work hardening, stabilize the structure and improve plasticity. This process mainly occurs recrystallization, it is also known as recrystallization annealing. The annealing temperature should be between the recrystallization temperature and the phase transformation temperature. If the annealing temperature is above the phase transformation temperature, the widmanstatten structure will be formed and the properties of the alloy will deteriorate. For All kinds of titanium alloys, the annealing type, temperature and cooling mode are different. (4) double annealing. In order to improve the plasticity, fracture toughness and stable structure of the alloy, double annealing can be used. After annealing, the microstructure of the alloy is more uniform and close to equilibrium state. In order to ensure the stability of microstructure and properties of heat-resistant titanium alloys under high temperature and long-term stress, this kind of annealing is often used. Double Annealing is performed by twice heating and air cooling the alloy. The first high temperature annealing heating temperature is higher than or close to the recrystallization end temperature, so that recrystallization can be fully carried out without obvious grain growth, and the volume fraction of AP phase can be controlled. After Air Cooling, the microstructure is not stable enough, so the second low temperature annealing is needed, the annealing temperature is lower than the recrystallization temperature, and the holding time is longer.ISOTHERMAL annealing. The best plasticity and thermal stability can be obtained by ISOTHERMAL annealing. This kind of annealing is suitable for the dual-phase titanium alloy with high content of β stable element. The ISOTHERMAL annealing is carried out by step cooling, that is, heating to a temperature above the recrystallization temperature, then immediately transferring to a lower temperature furnace (usually 600 ~ 650 °c) for heat preservation, and then air cooling to room temperature.

2. Quenching and aging is the main way of strengthening titanium alloy by heat treatment. It is also called strengthening heat treatment because of phase transformation. The strengthening effect of heat treatment of titanium alloy depends on the properties, concentration and heat treatment specification of alloy elements, because these factors affect the type, composition, quantity and distribution of metastable phase, and the nature, structure and dispersion of precipitates in the process of metastable phase decomposition, which are related to the composition, heat treatment process and original structure of the alloy. For the alloy with certain composition, the effect of aging strengthening depends on the heat treatment process chosen. The higher the quenching temperature is, the more obvious the effect of aging strengthening is, but the bigger the β transformation temperature is, the more brittle the grains are. For two-phase titanium alloys with lower concentration, more martensite can be obtained by quenching at higher temperature, while for two-phase titanium alloys with higher concentration, more metastable β phases can be obtained by quenching at lower temperature, this allows for maximum age hardening. The cooling method is usually water or oil cooling, and the quenching process should be rapid to prevent the decomposition of β phase in the transfer process and reduce the effect of aging strengthening. Generally, the aging temperature of α + β type titanium alloy is 500 ~ 600 °C for 4 ~ 12h, while the aging temperature of β type titanium alloy is 450 ~ 550 °C for 8 ~ 24h, and the cooling method is air cooling. 3. Deformation Heat treatment deformation heat treatment is an effective combination of pressure processing (forging, rolling, etc.) and heat treatment, and can play the role of deformation strengthening and heat treatment strengthening simultaneously, the microstructure and comprehensive properties can not be obtained by single strengthening method. Common Deformation Heat treatment processes are shown in figure 2. Different types of deformation heat treatment are classified according to the relationship between deformation temperature and recrystallization temperature and phase transition temperature:

(1) high temperature deformation heat treatment. Heating to recrystallization temperature above, deformation 40% ~ 85% after rapid quenching, and then conventional aging heat treatment. (2) low temperature deformation heat treatment. The deformation is about 50% below the recrystallization temperature, and then the conventional aging treatment is carried out. (3) composite deformation heat treatment. A process that combines high temperature thermomechanical treatment with low temperature thermomechanical treatment. 4. Chemical Heat Treatment Titanium Alloy Friction Coefficient is large, poor wear resistance (generally about 40% lower than steel) , in contact surface easy to produce bonding, cause friction corrosion. The corrosion resistance of titanium alloy is strong in oxidizing medium, but poor in reducing medium (hydrochloric acid, sulfuric acid, etc.) . In order to improve these properties, plating, spraying and chemical heat treatment (nitriding, oxygen) and other methods can be used. The hardness of nitrided layer after nitriding is 2 ~ 4 times higher than that of the surface layer before nitriding, so the wear resistance of the alloy is improved obviously, and the corrosion resistance of the alloy in reducing medium is also improved, however, the plastic and fatigue strength of the alloy will have different degrees of loss.

Microstructure characteristics of titanium alloys various microstructures can be observed in titanium alloys, especially in α + β dual-phase titanium alloys. These microstructures are different in morphology, grain size and intragranular structure, which mainly depends on alloy composition, deformation process and heat treatment process. There are two basic phases in the microstructure of titanium alloys, α phase and β phase. The mechanical properties of titanium alloy depend on the proportion, shape, size and distribution of the two phases to a great extent. The microstructure of titanium alloy can be divided into four types, namely, widmanstatten structure (lamellar structure) , basket-net structure, bi-state structure and equiaxed structure. Fig. 3 shows the morphological characteristics of various typical microstructures of the titanium alloy. Table 1 gives the corresponding properties of TC4 titanium alloy in four typical microstructure states. It can be seen that the properties of TC4 titanium alloy in different microstructure are quite different.

1. The lamellar structure is characterized by coarse original β grain and complete grain boundary α phase, and large “Clusters”are formed in the original β grain, and there are more in the same “Clusters”. The sheets are parallel to each other and oriented in the same direction, as shown in Fig. 3(a) . This microstructure is formed when the alloy is cooled from β phase region slowly without deformation or with little deformation after heating in β phase region. When the alloy has this structure, its fracture toughness, durability and creep strength are good, but its plasticity, fatigue strength, notch sensitivity, thermal stability and thermal stress corrosion resistance are poor, they vary with the size of the α “Beam”and the thickness of the grain boundary α. The α “Beam”decreases, the Grain Boundary α thins, and the comprehensive properties improve. 2. The net-basket microstructure is characterized by the destruction of the boundary of the original β grain during deformation, the absence or only a small amount of dispersed granular grain boundary α, the shortening of the α sheet in the original β grain, the smaller size of the α “Cluster”and the staggered arrangement of the bundles of α sheets, it looks like a woven basket, as shown in figure 3(B) . This microstructure is generally formed when the alloy is heated or deformed in the β phase region, or when the amount of deformation in the (α + β) duplex region is not large enough. The fine net basket microstructure not only has good plasticity, impact toughness, fracture toughness and high cycle fatigue strength, but also has good thermal strength. 3. The bicomponent structure is characterized by the distribution of disconnected primary α on the Matrix of p-transformed tissue, but the total content is not more than 50% , as shown in Fig. 3(c) . When the heating temperature of hot deformation or heat treatment of titanium alloy is lower than β transformation temperature, two-state structure can be obtained. Two-state tissue means that the α phase in the tissue has two forms, one is equiaxed primary α phase and the other is a lamellar α phase in beta-transformed tissue, which corresponds to the primary α, and this lamellar form. Phase is also called Secondary α phase or secondary α phase. This structure is formed when the alloy is deformed at a higher temperature in the (α + β) biphase region.

4. Equiaxed microstructure is characterized by the distribution of a certain amount of transformed β microstructure on the primary α-phase Matrix with a uniform distribution of more than 50% , as shown in fig. 3(d) . The equiaxed microstructure can be obtained when the deformation processing and heat treatment of titanium alloy are all carried out in (α + β) biphase zone or α phase zone, and the heating temperature is lower than β transformation temperature. Compared with other microstructure, this kind of microstructure has better plasticity, fatigue strength, notch resistance sensitivity and thermal stability, but worse fracture toughness, durability and creep strength. Because of the good comprehensive performance of this kind of organization, the most widely used at present. Effect of heat treatment process on microstructure evolution of titanium alloy heat treatment process of titanium alloy is shown in Fig. 4. The main control parameters are solution temperature, solution time, cooling mode [ including water quench (WQ) , oil quench (Oq) , air cooling (AC) and furnace cooling (FC)] , aging temperature and aging time.

1. The effect of solution temperature on the microstructure of TC21 alloy Fig. 5 shows the microstructure of TC21 alloy at different solution temperature. As can be seen from Fig. 5, the volume fraction of αp phase decreases with the increase of solid solution temperature, and disappears when solid solution temperature is higher than T β. At 940 °C, the grain boundaries of β grains bend out due to the equiaxed αp phase, as shown by the Arrow in Fig. 5(c) . At 1000 °C Solid Solution Treatment (& GT; T β) , the αp phase disappeared. As the barrier of grain boundary migration disappeared, the β grain grew rapidly, and the average diameter of β grain could reach about 300 ΜM, as shown in Fig. 5(d) . The results show that the solution temperature has a significant effect on the microstructure of TC21 alloy. The size, morphology and distribution of αp phase will directly affect the size of β grain when (α + β) phase is dissolved in solid solution. Titanium Alloy. αp phase and β grain size play an important role in the mechanical properties of the alloy. In order to avoid the rapid growth of β grains, the solution temperature of TC21 alloy should be lower than t β, so that the suitable grain size can be obtained and the two-state structure can be mixed by primary and secondary phases.

2. The effect of solution time on the microstructure of TC21 alloy Fig. 6 shows the microstructure of TCIZ alloy after solution treatment for 4 hours. As can be seen from Fig. 6 and Fig. 5(a) and (b) , the volume fraction and distribution of AP phase in TC21 alloy did not change significantly with the increase of solution time. It can be seen that the microstructure of TC21 alloy is not sensitive to the solution treatment time, but the solution treatment temperature plays a decisive role in the solution treatment.

3. Effect of cooling mode on microstructure of TC21 alloy Fig. 7 shows the effect of cooling mode on microstructure of TC21 alloy. It can be seen from Fig. 7 that cooling mode has obvious effect on microstructure of TC21 alloy after solution treatment. Under the conditions of WQ and Oq, because of the rapid cooling rate, only metastable β is formed but no β t is formed, while under the condition of AC, there is a certain amount of β t formation The size of αp phase obtained under WQ and Oq conditions is slightly smaller than that obtained under AC conditions. The reason for this difference is that the cooling rate of AC is slow, and the αp phase in the alloy can grow fully during cooling (resulting in the increase of αp phase content and the aggregation of αp phase in the alloy under AC condition) . The β phase at high temperature can also be transformed sufficiently to form βt during the slow cooling process.

4. The effect of aging temperature on the microstructure of TC21 alloy Fig. 8 shows the microstructure of TC21 alloy aged at 500 °C and 600 °C. It can be seen from Fig. 8 that the microstructure of the alloy after aging is αp phase + βt phase. With the increase of aging temperature, the secondary α phase grew and merged, and the secondary α phase increased gradually. As shown in figures 8(A) , (b) and (c) , metastable β obtained by solid solution treatment during aging at 500 °C lacks the driving force of decomposition and is less secondary in the aging process due to low aging temperature.

5. The effect of aging time on the microstructure of TC21 alloy Fig. 9 shows the microstructure of TC12 alloy aged at 550 °C for different time. As can be seen from Fig. 9, with the increase of aging time, βt increases continuously, while the size of αp phase does not change obviously, but appears to merge and grow, and the larger size of secondary strip α phase also appears to merge and grow.

6. The influence of heat treatment on the microstructure of typical titanium alloy through controlling the heat treatment process conditions of TC12 alloy and TI60 alloy, two kinds of LM and BM were obtained, as shown in Fig. 10.

Source: Caitong

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