Heat treatment characteristics of titanium alloy
(1) The martensitic phase transformation does not cause significant changes in the properties of titanium alloys.
This feature is different from the martensitic phase transformation of steel, where the heat treatment strengthening of titanium alloys can only rely on the aging decomposition of the sub-stable phase (including the martensitic phase) formed by quenching.
Moreover, the method of heat treatment for pure a-type titanium alloys is basically ineffective, i.e., the heat treatment of titanium alloys is mainly used for α+β-type titanium alloys.
(2) Heat treatment should avoid the formation of ω phase. Formation of the ω phase will make titanium alloy brittle, the correct choice of the aging process (such as the use of a higher aging temperature) can make the ω phase decomposition.
(3) It is difficult to refine titanium alloy grains using repeated phase transformations.
This is also different from steel materials.
Most steels can use the repeated phase transformation of austenite and pearlite (or ferrite, cementite) to control the nucleation and growth of new phases to achieve the purpose of grain refinement.
There is no such phenomenon in titanium alloys.
(4) Poor thermal conductivity.
Poor thermal conductivity can lead to the poor hardenability of titanium alloys, especially α+β titanium alloys.
The quenching thermal stress is large, and the parts are easy to warp during quenching.
Due to the poor thermal conductivity, the deformation of the titanium alloy easily causes local temperature rise, which may cause the local temperature to exceed the β transformation point and form the Widmanstatten structure.
(5) Lively chemical properties
During heat treatment, the titanium alloy easily reacts with oxygen and water vapor, forming a certain depth of oxygen-rich layer or scale on the surface of the workpiece, which reduces the performance of the alloy.
At the same time, titanium alloys tend to absorb hydrogen during heat treatment, causing hydrogen embrittlement.
(6) The β transition point is very different.
Even if it is the same composition, due to different smelting furnaces, the β transformation temperature sometimes varies greatly.
(7) When heating in the β phase region, β grains tend to grow larger.
The coarsening of β grains can make the plasticity of the alloy drop sharply, so the heating temperature and time should be strictly controlled, and the heat treatment for heating in the β phase region should be used with caution.
Type of heat treatment of titanium alloy
The phase transformation of titanium alloy is the basis of titanium alloy heat treatment,
In order to improve the performance of titanium alloys, in addition to reasonable alloying, it is necessary to cooperate with appropriate heat treatment.
There are many types of heat treatments for titanium alloys. Commonly used are annealing treatment, aging treatment, deformation heat treatment and chemical heat treatment.
Annealing is suitable for various titanium alloys, and its main purpose is to eliminate stress, improve alloy plasticity and stabilize the structure.
The forms of annealing include stress relief annealing, recrystallization annealing, double annealing, isothermal annealing and vacuum annealing, etc.
The annealing temperature range of titanium alloy in various ways is shown in Figure 1.
Figure 1 Schematic diagram of the annealing temperature range of various methods in titanium alloy
(1) Stress relieving annealing.
Stress relieving annealing can be used to eliminate the internal stress generated in the process of casting, cold deformation and welding.
Stress relieving annealing temperature should be lower than the recrystallization temperature, generally 450 ~ 650 ℃.
The time required depends on the cross-sectional size of the workpiece, processing history and the degree of stress relief required.
(2) Ordinary annealing.
Its purpose is to make the titanium alloy semi-finished product to eliminate the basic stresses, and has high strength and plasticity in accordance with the technical conditions required.
The annealing temperature and recrystallization temperature is generally equivalent to the start or slightly lower, such annealing process is generally metallurgical products used at the factory, so it can also be called factory annealing.
(3) Complete annealing.
The purpose is to completely eliminate the process hardening, stabilizing the organization and improve plasticity.
This process occurs mainly recrystallization, it is also known as recrystallization annealing.
The annealing temperature is preferably between the recrystallization temperature and the phase transition temperature.
If the temperature exceeds the phase transition temperature, Widmanstatten structure will be formed and the properties of the alloy will deteriorate.
For various kinds of titanium alloys, the type of annealing, temperature and cooling methods are different.
(4) Double annealing.
In order to improve the plasticity of the alloy, fracture toughness and stability of the organization, double annealing can be used.
After annealing, the alloy organization is more uniform and close to the equilibrium state.
In order to ensure the stability of the structure and performance of heat-resistant titanium alloys under high temperature and long-term stress, this type of annealing is often used.
Double annealing is to heat and air cools the alloy twice.
The heating temperature of the first high-temperature annealing is higher than or close to the end temperature of recrystallization, so that the recrystallization can fully proceed without the crystal grains growing significantly, and the volume fraction of the ap phase is controlled.
The structure is not stable enough after air cooling, and second low-temperature annealing is required.
The annealing temperature is lower than the recrystallization temperature, and the temperature is kept for a long time to fully decompose the metastable β phase obtained by high-temperature annealing.
(5) Isothermal annealing.
Isothermal annealing can obtain the best plasticity and thermal stability.
This type of annealing is suitable for dual-phase titanium alloys with a high content of β-stabilizing elements.
Isothermal annealing adopts hierarchical cooling, that is, after heating to a temperature above the recrystallization temperature, it is immediately transferred to another lower temperature furnace (generally 600~650℃) for insulation, and then air-cooled to room temperature.
Quenching aging is the main way to strengthen titanium alloy heat treatment, using phase change to produce strengthening effect, which is also known as heat treatment strengthening.
Titanium alloy heat treatment to strengthen the effect is determined by the nature of the alloy element, concentration and heat treatment specifications
Because these factors affect the type, composition, quantity and distribution of the metastable phase obtained by alloy quenching, as well as nature, structure, and dispersivity etc. of the precipitated phase during the decomposition of the metastable phase, which is related to alloy composition, heat treatment process specifications and original structure.
For alloys with a certain composition, the effect of aging strengthening depends on the selected heat-treatment process.
The higher the quenching temperature, the more obvious the effect of aging strengthening, but when the quenching is higher than the β transformation temperature, it will cause brittleness due to excessively coarse grains.
For two-phase titanium alloys with a lower concentration, higher temperature quenching can be used to obtain more martensite.
The two-phase titanium alloy with a higher concentration should be quenched at a lower temperature to obtain more metastable β-phase, so that the maximum aging strengthening effect can be obtained.
The cooling method is generally water-cooled or oil-cooled, and the quenching process should be rapid to prevent the decomposition of the β phase during the transfer process and reduce the aging strengthening effect.
The aging temperature and time should be chosen to obtain the best overall performance criteria with a general aging temperature of 500 ~ 600 ℃ in α + β-type titanium alloy and time of 4 ~ 12h.
The aging temperature of β-type titanium alloy is 450-550℃, time is 8-24h, and the cooling method is air cooling.
Deformation Heat Treatment
Deformation heat treatment is an effective combination of pressure processing (forging, rolling, etc.) and heat treatment technology, which allows both deformation strengthening and heat treatment strengthening to obtain organization and comprehensive performance that cannot be obtained with a single strengthening method.
A common deformation heat treatment process is shown in Figure 2.
Different types of thermomechanical heat treatment are classified according to the relationship between deformation temperature and recrystallization temperature and phase transition temperature.
According to the deformation temperature, it can be divided into:
Figure 2 Schematic representation of the deformation heat treatment process in titanium alloy.
(a) High-temperature deformation heat treatment;
(b) Low-temperature deformation heat treatment
- 1 – heating;
- 2 – water cooling;
- 3 – aging;
- 4 – high or low temperature deformation; tβ-β: phase transition point; t: re-recrystallization temperature
(1) High temperature thermomechanical treatment
It is heated to above the recrystallization temperature, deformed by 40% to 85%, then quenched quickly, and then subjected to conventional aging heat treatment.
(2) Low temperature thermal mechanical treatment
Deformation is about 50% below the recrystallization temperature, followed by conventional aging treatment.
(3) Compound thermomechanical treatment
It is a process that combines high temperature thermomechanical treatment and low temperature thermal mechanical treatment.
Chemical Heat Treatment
Titanium alloys have a large friction coefficient, poor wear resistance (generally about 40% lower than steel), which are prone to adhesion on contact surfaces and cause frictional corrosion.
Titanium alloys are more resistant to corrosion in oxidizing media, but less resistant to corrosion in reducing media (hydrochloric acid, sulfuric acid, etc.).
To improve these properties, electroplating, spraying and chemical heat treatment (nitriding, oxygenation, etc.) can be used.
The hardness of the nitrided layer after nitriding is 2 to 4 times higher than that of the surface layer without nitriding, thus significantly improving the wear resistance of the alloy and at the same time improving the corrosion resistance of the alloy in reducing media;
Oxygen infiltration can increase the corrosion resistance of the alloy by 7-9 times, but the plasticity and fatigue strength of the alloy will be lost to different contents.
Microstructure characteristics of titanium alloy
In titanium alloys, especially α+β duplex titanium alloys, a wide variety of tissues can be observed.
These structures are different in morphology, grain size and intragranular structure, which mainly depend on alloy composition, deformation process and heat treatment process.
In general, the organization of titanium alloys has two basic phases: α-phase and β-phase.
The mechanical properties of titanium alloys largely depend on the proportion, morphology, size and distribution of these two phases.
The organization types of titanium alloys can be basically divided into four categories: Widmanstatten structure (lamellar structure), basketweave structure, bimodal structure and isometric structure.
Figure 3 shows the typical morphological characteristics of the various types of titanium alloys.
Table 1 gives the alloy performance indexes of TC4 titanium alloy in four typical tissue states, which shows that the performance of different tissues varies greatly.
Table 1: Influence of four typical tissues on the performance in TC4 alloy
|Mechanical property||Compressive strength σ
|Impact toughness αk
|Fracture toughness KIC
Figure 3 Typical organization in titanium alloys
(a) lamellar tissue; (b) basketweave tissue; (c) bimodal tissue; (d) isometric tissue
It is characterized by coarse original β crystal grains and complete grain boundary α phase, forming large-sized “bundles” in the original β crystal grains, and there are more in the same “bundles”.
The slices are parallel to each other and in the same orientation, as shown in Figure 3(a).
This kind of microstructure is the structure formed when the alloy is not deformed or deformed after heating in the beta phase region, and it is slowly cooled down from the beta 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 very poor.
They vary with the size of the α “bundle” and the thickness of the grain boundary α.
The α “bundle” becomes smaller, the grain boundary α becomes thinner, and the overall performance is improved.
Its characteristic is that the original β grain boundary is destroyed during the deformation process, no or only a small amount of dispersed granular grain boundary α appears, and the α slices in the original β grain become shorter.
The size of α “bundle” is small, and the clusters are arranged staggered, like a woven basket, as shown in Figure 3(b).
This kind of microstructure is generally formed when the alloy is heated or begins to deform in the β-phase region, or the amount of deformation in the (α+β) dual-phase region is not large enough.
The fine mesh basket structure not only has better plasticity, impact toughness, fracture toughness and high cycle fatigue strength, but also has better thermal strength.
Its characteristic is that unconnected primary α is distributed on the matrix of p-transformation tissue, but the total content does not exceed 50%, as shown in Figure 3(c).
When the heating temperature of thermal deformation or heat treatment of the titanium alloy is less than the β transformation temperature, a dual-state structure can generally be obtained.
Bimodal issue refers to the α-phase in the organization has two forms, one is the primary α-phase equiaxed; the other is the lamellar α-phase in the β-transformed organization, which corresponds to the primary α.
The phase is also called secondary α phase or secondary α phase.
This structure will be formed when the alloy is at a higher temperature and greater deformation in the (α+β) dual phase zone.
Its characteristic is that a certain amount of transformed β structure is distributed on the primary α-phase matrix with a uniformly distributed content of more than 50%, as shown in Figure 3(d).
The deformation processing and heat treatment of the titanium alloy are all carried out in the (α+β) dual phase zone or the α phase zone,
And when the heating temperature is much lower than the β transformation temperature, an equiaxed structure can generally be obtained.
Compared with other issues, this issue has better plasticity, fatigue strength, notch sensitivity, and thermal stability, but its fracture toughness, durability, and creep strength are worse.
Because this issue has a better overall performance, it is currently the most widely used.
Effect of Heat Treatment Process on Microstructure Evolution of Titanium Alloy
The heat treatment process of the titanium alloy is shown in Figure 4.
The main controlled parameters are solid solution temperature, solid solution time, cooling method (including water quench, oil quench, air cooling) and furnace cooling, aging temperature and aging time.
Figure 4 Process diagram of a typical heat treatment
Effect of solid solution temperature on the microstructure of TC21 alloy
Figure 5 is the microstructure of TC21 alloy at different solid solution temperatures.
It can be seen from Figure 5 that as the solid solution temperature increases, the volume fraction of the αp phase decreases.
When the solid solution temperature is higher than Tβ, the αp phase disappears.
In the solution treatment at 940°C, due to the obstruction of the equiaxed αp phase, the grain boundaries of the β grains bend and bow, as shown by the arrow in Figure 5(c).
When applying solution treatment (>Tβ) at 1000℃, the αp phase disappears.
As the obstacles to the movement of the β grain boundaries disappear, the β grains grow up sharply, with an average diameter of about 300 μm, as shown in Figure 5(d).
It can be seen that the solution temperature has a significant effect on the microstructure of TC21 alloy.
When the (α+β) dual-phase region is solid solution, the size, morphology and distribution of αp phase will directly affect the size of β crystal grains.
The αp phase and β grain size of the titanium alloy play a vital role in the mechanical properties of the alloy.
In order to avoid the rapid growth of β grains, the solid solution temperature of TC21 alloy should be selected below Tβ, so that a relatively suitable grain size can be obtained, and a dual-state structure composed of primary and secondary phases can be obtained.
Figure 5 The effect of solution temperature on the microstructure of TC21 alloy
(a)850℃/AC; (b)910℃/AC; (c)940℃/AC; (d)1000℃/AC
Effect of Solution Time on Microstructure of TC21 Alloy
Figure 6 shows the microstructure of TCIZ alloy after solution treatment and air cooling for 4 hours.
From Figure 6 and Figure 5(a) and (b), it can be seen that with the increase of the solution time, the volume fraction and distribution of the ap phase in the TC21 alloy did not change significantly.
It can be seen that when the solution treatment reaches a certain time, the microstructure of TC21 alloy is not sensitive to the solution treatment time, but the solution treatment temperature plays a decisive role in the solid solution structure of the alloy.
Figure 6 The effect of solution time on the microstructure of TC21 alloy
(a)850℃/4h, AC; (b)910℃/4h, AC
Effect of Cooling Method on Microstructure of TC21 Alloy
Figure 7 shows the effect of cooling methods on the microstructure of TC21 alloy.
It can be seen from Figure 7 that the cooling method has a significant effect on the microstructure of TC21 alloy after solution treatment.
Under WQ and OQ conditions, due to the faster cooling rate, only metastable β is formed but no βT is formed.
Under AC conditions, a certain amount of βT is formed;
The size of the αp phase obtained under WQ and OQ conditions is slightly smaller than that obtained under AC conditions.
This difference is due to the slow cooling rate of AC, and the αp phase in the alloy can fully grow during the cooling process (causing the αp phase content in the alloy to increase and aggregate growth under AC conditions).
The β phase at high temperature can also be fully transformed to form βT in the slower cooling process.
Figure 7 The effect of the cooling method on the microstructure of TC21 alloy
(a)910℃/1h, WQ; (b)910℃/1h, OQ; (c)910℃/1h, AC
Effect of Aging Temperature on the Structure of TC21 Alloy
Fig. 8 is a microstructure photograph of TC21 alloy aged at 500°C and 600°C.
It can be seen from Figure 8 that the structure of the alloy after aging is αp phase and βT phase.
As the aging proceeds, the secondary α-phase grows up and merges;
With the increase of the aging temperature, the secondary α-phase gradually increases.
As shown in Figure 8(a), (b) and (c), at 500 °C aging, due to the low aging temperature, the sub-stable β obtained from the solid solution treatment lacks the driving force for decomposition during the aging process, and fewer secondary phases are formed.
Figure 8 Effect of aging temperature on the structure of TC21 alloy
- (a) 910℃/1h，WQ+500℃/6h,AC；
- (b) 910℃/1h，OQ+500℃/6h,AC
- (c) 910℃/1h，AC+500℃/6h,AC；
- (d) 910℃/1h，WQ+600℃/6h,AC
- (e) 910℃/1h，OQ+600℃/6h,AC；
- (f) 910℃/1h，AC+600℃/6h,AC
Effect of Aging Time on the Structure of TC21 Alloy
Figure 9 shows the microstructure photos of TC12 alloy aged at 550°C for different times.
It can be seen from Figure 9 that with the prolongation of the duration of time, βT keeps increasing, while the size of the αp phase has not changed significantly, but the phenomenon of merger and growth has appeared.
The larger secondary strip-like α phases also appear to merge and grow.
Figure 9 The effect of aging time on the structure of TC21 alloy
- (a) 910℃/1h，WQ+500℃/2h,AC；
- (b) 910℃/1h，WQ+550℃/12h,AC
- (c) 910℃/1h，AC+500℃/2h,AC；
- (d) 910℃/1h，OQ+550℃/12h,AC
- (e) 910℃/1h，OQ+600℃/2h,AC；
- (f) 910℃/1h，AC+550℃/12h,AC
Effect of Heat Treatment on the Microstructure of Typical Titanium Alloy
By controlling the heat treatment process conditions of TC12 alloy and Ti60 alloy, two major types of lamellar microstructure and bimodal microstructure are obtained, as shown in Figure 10.
Figure 10 The effect of heat treatment on the microstructure of a typical titanium alloy
- (a) TC21 970℃/1h，FC；
- (b)TC21 910℃/1h，AC+550℃/6h,AC
- (c) TC21 910℃/1h，FC+550℃/6h,AC；
- (d)Ti600 1020℃/2h，AC+650℃/8h,AC
- (e)Ti600 1005℃/2h，AC+650℃/8h,AC；
- (f)Ti600 AC+600℃/100h,AC
It can be seen from Figure 10(d) and (e) that the solid solution temperature of Ti600 alloy is selected above and below Tb (1010°C) to obtain LM and BM structures, respectively.
The lamella thickness of LM tissue is 2 to 3 μm; the volume fraction of αp phase in BM tissue is about 20%, and its average diameter is about 15 μm.
Figure 10(f) shows the microstructure of Ti600 alloy of BM structure after thermal exposure (TE) at 600℃ for 100h.
The differences between BM and BM + TE tissues cannot be distinguished from the microscopic tissues shown in Figure 10(e) and (f) alone.
High-temperature titanium alloys readily precipitate the α2 (Ti3Al) phase in their Al-rich αp phase during long-term aging or thermal exposure.
By transmission electron microscopy, the α2 phase was found in the αp phase of BM tissue Ti600 alloy after thermal exposure, as shown in Figure 11.
Figure 11 TEM morphology and selected area electron diffraction pattern of α2 phase in Ti600 alloy after thermal exposure
(a) TEM topography; (b) selected area electron diffraction pattern