Forging test block and cooling technology of titanium alloy

orging and cooling process of titanium alloy after forging

Titanium has an allocrystal transition in the solid state. Below 882.5℃, it is a close-packed hexagonal lattice, called α-titanium; Above 882.5 ° C, it is a body-centered cubic lattice, called β-titanium. α-titanium has high strength and good heat resistance, but the plasticity is poor, the deformation resistance is large, and the plastic forming is more difficult. The heat resistance of β-titanium is poor, but the process plasticity is good, and the deformation workability is good.

1

Process plasticity of titanium alloys

The carbon, alloying elements, gas impurities, especially oxygen in titanium alloys (especially as cast titanium alloys) have a great influence on the plasticity of titanium alloys. The plasticity of as-cast titanium alloy will be greatly improved after pretreatment and deformation.

Titanium and titanium alloys have two allotropes: below 885℃ titanium has a close-packed hexagonal lattice structure α phase; When the temperature exceeds 885℃, the α phase is transformed into the β phase with body-centered cubic lattice structure. At low temperature, the number of slip planes in the hexagonal lattice is limited, so the plastic deformation is difficult. When the temperature increases, the slip planes in the hexagonal lattice increase, so the plasticity of titanium and titanium alloy increases with the increase of temperature. When the temperature exceeds the phase transition point and enters the β phase region, the structure of the metal is transformed from a hexagonal lattice to a body-centered cubic lattice, and the plasticity of titanium and titanium alloys is high, therefore, titanium alloys are generally processed under hot pressure.

In the forging process, recrystallization and work hardening are carried out simultaneously. The increase of deformation speed sometimes makes recrystallization cannot be fully carried out, and the result is that the plasticity of titanium alloy decreases and the deformation resistance increases. Therefore, the deformation rate of titanium alloy can not be too large. According to the upsetting test of the as-cast TA3 titanium alloy, the allowable deformation degree of the upsetting on the hammer is not more than 45%, while the allowable deformation degree of the upsetting on the hydraulic press is up to 60%. Therefore, titanium alloy forging on the press is more suitable.

2

Specification for heating titanium alloys before forging

The pre-forging heating specification of titanium alloy has great influence on the microstructure and some properties of titanium alloy forgings. α and (α + β) titanium alloy ingot billet, because there are subsequent plastic deformation and heat treatment processes, so the forging heating temperature is preferable in the β phase zone, and the final forging is taken in the α + β phase zone.

The heating temperature of (α + β) titanium alloy conventional forging is generally lower than the β transition temperature of 10 ~ 30℃, which can ensure that the microstructure of titanium alloy after forging contains a primary equiaxed α phase with a volume fraction of 20% ~ 30%, so that titanium alloy has good comprehensive mechanical properties. However, in order to improve the fracture toughness, high temperature lasting strength and high temperature creep resistance of titanium alloy, while not reducing the room temperature plasticity index (section shrinkage) too much, the heating temperature of titanium alloy before forging can be increased to about 10 ° C below the β transition temperature, which can ensure the microstructure and properties of titanium alloy after forging, but also improve its malleability.

The forging heating temperature of α titanium alloy is generally slightly higher than the β transition temperature, so as to expand the forging temperature range of titanium alloy and improve the process plasticity.

The forging heating temperature of β titanium alloy is generally higher than the β transition temperature, and is equal to or slightly lower than the recrystallization temperature.

The holding time of titanium alloy blank at heating temperature is generally calculated according to 0.7 ~ 0.8min/mm.

3

Influence of forging process parameters on forging properties of titanium alloy

The core of titanium alloy hot deformation process is a good combination of deformation temperature and deformation degree, which plays a decisive role in the formation of titanium alloy structure.

1) The formation process of three kinds of titanium alloy structures

The formation process of the Weil structure (coarse needle structure) : When the titanium alloy performs plastic deformation in the β phase region and terminates the deformation above the transition temperature, the formation process of the structure goes through two stages of grain formation and intrachrystalline formation. With the increase of deformation degree, β grains are elongated and flattened along the direction of metal flow, β grains recrystallize and aggregate recrystallize, β grains grow even beyond the original grain size, completing the process of grain formation. After the deformation is finished at a temperature higher than β, the β →α transformation occurs when the temperature drops to β phase transformation temperature during the cooling process. Firstly, strip α phase is precipitated along the boundary of the original β grain, and then cross-parallel sheet α phase is precipitated along the grain according to different orientation, that is, the in-grain formation process is completed.

The formation process of equiaxed α grains: When titanium alloy is plastic deformed in the α +β two-phase region, for titanium alloy with the original structure of sheet structure, the original β grains and sheet α simultaneously produce plastic deformation, they are simultaneously flattened and elongated and broken along the direction of metal flow, and the difference between the strip α phase of the grain boundary and the sheet α phase in the crystal gradually disappears. When the deformation degree is greater than 60% ~ 70%, the trace of lamellar α phase disappears completely. Therefore, under the appropriate deformation temperature and deformation conditions, the strip α phase and sheet α phase in titanium alloy recrystallization, because the recrystallization of the α phase is faster than the recrystallization of the β phase region, and the spherical α recrystallization grain is obtained, which is called the primary equiaxed α grain.

The formation process of netted basket structure: Although the titanium alloy deforms above the β transition temperature, the deformation degree is large enough, and the deformation terminates in the α+β two-phase region, that is, the strip α phase and the sheet α phase are precipitated in the dynamic deformation process, so the strip α phase precipitated along the β grain boundary is deformed and distorted, and is cut by the deformed sheet α phase and becomes less complete. At the same time, the lamellar α phase in the crystal is stretched and distorted. The original regular orientation and parallel arrangement are changed, and the morphology is close to strip shape, which retains the mixture of α+β.

2) Influence of deformation temperature on the properties of titanium alloy

After plastic deformation in the (α+β) phase region, the ratio of primary equiaxed α phase and β phase content in the microstructure of titanium alloy determines the mechanical properties of titanium alloy.

Effect of primary α phase content on mechanical properties of TC4 titanium alloy at room temperature and high temperature. Primary α phase content has little effect on tensile strength index Rm at room temperature, but has great effect on plastic index A and Z, especially the section shrinkage. The volume fraction of the primary α phase is in the range of 20% to 80%, and the section shrinkage is always above 40%. The volume fraction of the primary alpha phase begins to decline when it is below 20%. If the volume fraction of the primary alpha phase is less than 10%, the section shrinkage will be lower than the general technical requirements (30%). Therefore, in order to ensure that the room temperature plasticity index is not too low, the volume fraction of the primary α phase should be controlled above 20%. There was no significant relationship between notch tensile strength sensitivity (Rm/ReL) and primary α content. strike

There was also no significant relationship between toughness aK and primary phase content. However, the influence of primary α content on high temperature mechanical properties shows that the high temperature durability and creep strength decrease significantly with the increase of primary α content

This is due to the fact that strip α has better durability and creep strength than the isometric α phase. There is no obvious regularity between the tensile strength and the primary α phase content at 400℃. The fatigue performance increases with the increase of primary α phase content and the decrease of primary α phase size.

In order to obtain good strength and plasticity, β titanium alloy must be forged at or slightly below the recrystallization temperature. After forging at higher than recrystallization temperature, the α phase is mainly precipitated along the grain boundary, and the strength, especially the plasticity and impact toughness are decreased.

3) Influence of deformation degree on microstructure and properties of titanium alloy

When forging process is established, the reasonable deformation degree is one of the important conditions to ensure that the titanium alloy forging can obtain certain performance requirements.

The deformation degree has a significant effect on the microstructure of titanium alloy: when the deformation degree is greater than 30% ~ 40%; The apparent refinement of the organization began;

In the α + β two-phase region, the degree of deformation should not be less than 60% ~ 70% in order to make the acicular coarse-crystal structure fully refined and transformed into spherical structure. When the degree of deformation is small, it forms an intermediate structure between acicular and equiaxial. The higher the deformation temperature, the greater the degree of deformation required to obtain fine crystal structure, however, if the alloy in the β phase before deformation in the α+β phase region after plastic deformation, in the subsequent β phase region deformation only given a small deformation (30% to 40% deformation degree) can make the structure is greatly refined. The reason is that when the titanium alloy forged in the α+β phase zone is deformed in the β phase zone, there is a recrystallization of new grains (β grain recrystallization), so deformation in the β phase zone is more effective for grain refinement than deformation in the α+β phase zone. The degree of deformation not only changes the grain size of titanium alloy, but also has a great effect on the change of intracrystalline needle (sheet) structure. With the increase of deformation degree, the intracrystalline structure is refined. The most obvious effect of deformation degree on the intracrystalline structure is the forging at the temperature of α+β phase region, when the existing α phase is also plastically deformed, thus changing the morphology of needle (sheet) α, but this effect is weakened with the increase of deformation temperature.

4) Influence of deformation speed on microstructure properties

Titanium alloys can be freely forged and die forged on the hammer and press. The quality comparison between the two kinds of forging equipment and other kinds of forgings shows that the structure and mechanical properties of the two are similar. This shows that the movement speed of the mold during forging is in the range of (0.5 ~ 0.8m/s) ~ (6 ~ 8m/s), and the degree of deformation has no significant impact on the quality of the forging. However, titanium alloy forging is still hoped to use a smaller deformation speed of the press. The main reason is that the deformation heat effect is large when forging on the hammer, and the risk of metal overheating is large, which may cause coarse structure and plastic decline.

The size of the heat effect during forging is related to the temperature of the metal, forging speed and forging ratio. When TC6 titanium alloy is forged at 940 ~ 950℃ with 50% ~ 60% deformation degree, the temperature rises to 40 ~ 60℃ due to thermal effect. The degree of deformation is reduced to 20% ~ 30%, the temperature rise caused by the thermal effect is reduced to 10 ~ 20℃, and when the degree of deformation is increased to 80% ~ 90%, the temperature rise caused by the thermal effect is to reach 100 ~ 140℃. When forging titanium alloy forgings with I-section on the hammer, the temperature rise of the spoke part of the forging is 100℃ higher than that of the flange part due to the thermal effect.

When forging on the hammer, the thermal effect causes the local coarse crystal structure of the forging, reduces the plasticity and fatigue strength at room temperature, and the mechanical properties are also very unstable.

In order to avoid local overheating of the forging blank on the hammer, the forging temperature can be lowered or the forging can be carried out by tapping. However, the reduction of forging temperature will cause the deformation resistance of the metal to increase and require the use of large tonnage equipment. The use of light hitting is bound to prolong the contact time between the blank and the mold, the blank quickly becomes cold, and the need for repeated heating, thereby extending the residence time of the blank in the furnace, causing the α layer of the forging surface to thicken, not only reducing the plasticity and lasting strength of the alloy, but also reducing labor productivity.

4

Forging process characteristics of titanium alloy

1) Large deformation resistance

The deformation resistance of titanium alloy is higher than that of steel at forging temperature. At the same time, the deformation resistance of titanium alloy increases with the decrease of temperature much faster than that of steel. When forging titanium alloy, even if the forging temperature is slightly reduced, the deformation resistance will be greatly increased.

For some (α + β) titanium alloys, this sensitivity of deformation resistance to temperature is mainly more obvious at temperatures below the (α + β) / β phase transition.

The deformation speed has a great influence on the deformation resistance of titanium alloy, and the unit pressure when deformed on the hammer is several times higher than that when deformed on the press. Therefore, from the point of view of reducing die forging is energy consumption

Look, it's better on a press than on a hammer.

2) Poor thermal conductivity

The thermal conductivity of titanium alloy is worse than that of steel, aluminum and other metals, and the thermal conductivity of titanium alloy is 1/5 of iron and 1/12.5 of aluminum. As a result, the surface of the forged billet cools quickly after it is released. If the operation is slow, it will cause a large internal and external temperature difference. This often leads to the phenomenon of cracking in the forging process and aggravates the uneven distribution of deformation degree inside and outside the blank. In order to reduce the cooling rate of the billet surface, it is very important to fully preheat the forging die, clamp and other tools in direct contact with the billet.

3) Large viscosity, poor liquidity

Compared with steel, titanium alloy has large viscosity and poor fluidity. When forging (including extrusion) must be strengthened lubrication, otherwise it is easy to produce mucous membrane phenomenon, and the extrusion pressure of forging will also be significantly increased due to the increase in friction. In addition, the forging may be torn when the module or hammer head is returned. The test shows that the friction coefficient of coarse titanium alloy is as high as 0.5 when no lubricant is used, and the friction coefficient is reduced to 0.04 ~ 0.06 when glass lubricant is used.

5

Cooling and heat treatment of titanium alloy after forging

Titanium alloy forgings are usually air-cooled after forging. However, depending on the type of titanium alloy and the strength and plasticity required for the part, heat treatment is sometimes performed after forging. There are mainly the following kinds of post-forging heat treatment of titanium alloy:

(1) Incomplete annealing This annealing is mainly used to eliminate the residual stress after forging, and air cooling after annealing.

(2) Complete annealing This annealing can effectively eliminate the residual stress after forging.

(3) isothermal annealing This annealing is suitable for two-phase titanium alloys with a high content of β-stable elements. This annealing can not only eliminate the stress generated in forging, but also change the phase composition of titanium alloy and its role

It is to stabilize the structure and properties of titanium alloys. The annealing consists of two stages: first, the titanium alloy is heated to a temperature below the allotropic transition temperature of 20 ~ 160℃, and the heat is kept at this temperature; ② Then it is transferred to the furnace where the temperature is lower than the allotropic transition temperature of 300 ~ 450 ° C, and the heat is kept at this temperature. Finally take out air cooling.

(4) Double annealing The effect of double annealing is similar to that of isothermal annealing, which is also mainly used for (α + β) two-phase titanium alloys. The process is the same as isothermal annealing, except that the forged billet treated with high temperature is cooled to room temperature in the air, and then put into the low-temperature furnace for treatment. With double annealing, a higher strength limit can be obtained, but the plasticity index should be reduced.

(5) Quenching, aging quenching, aging is a kind of enhanced heat treatment, which can ensure that titanium alloy has the maximum strength. Quenching is the heating of titanium alloy forgings to the temperature of the β phase zone, and cooling in water after holding heat to obtain the metastable β phase and martensitic phase α 'and α ". The effect of aging is to decompose the metastable β phase. The forgings are cooled in the air after aging.

In order to make (α+β) titanium alloy forgings have better strength and plastic comprehensive properties after quenching and aging, it is best to have a uniform or network-basket structure before quenching and aging. However, this rapid cooling is not beneficial to the mechanical properties of all titanium alloys. For example, Ti-2.5Al-7.5Mo-1.0Cr-1.0Fe titanium alloys, although also belonging to (α+β) titanium alloys, are slowly cooled after deformation at the temperature of the β phase region. Because the degree of alloying of this titanium alloy is high, the α needle in the tissue is thin, and it must be slowly cooled to achieve a state of equilibrium and make the needle thicker to the optimal size.

For titanium alloys with low alloying degree, such as TC4, TC6, TC9 and other titanium alloys, after deformation at the temperature of the β phase region, to make the fast cooling effect, the cooling speed must be controlled within a certain range. if

Cooling is too fast, and the α needles in the tissue are too thin, which may greatly reduce the plasticity index. At the same time, the rapid cooling will prevent the recrystallization of β phase, so that the coarse crystal structure is retained on the forging.

In summary, when choosing the cooling system of the forgings after β forging, the microstructure and intrachrystalline structure of titanium alloys should be taken into account.

6

Titanium alloy forging defects and their prevention

When forging titanium alloy, there may be various defects in forging parts due to improper process specifications and lax quality control of raw materials. Common lack of flame has the following types:

(1) Beta brittleness Beta brittleness is caused by overheating of forgings. α and (α+β) titanium alloys, especially (α+β) titanium alloys, if the forging heating temperature is too high, exceeding the β transition temperature, resulting in low microstructure and large grains of forging, is equiaaxial; In the microstructure, the α phase precipitates along the grain boundary of the coarse original β grain and in the grain. The result is a reduction in the plasticity of forgings at room temperature, a phenomenon called beta brittleness.

Overheating defects in titanium alloy forgings cannot be repaired by heat treatment methods, but must be repaired by reheating to a plastic deformation below the β transition temperature (if the forgings allow). In order to prevent overheating, when titanium alloy is heated, the furnace temperature should be strictly controlled, the temperature of the qualified area of the furnace should be regularly determined, and the loading level and loading amount should not be much. When resistance heating is used, baffles should be set on both sides of the furnace to avoid overheating caused by excessive proximity of the billet to the silicon carbide rod. Measuring the actual β transition temperature of each furnace number alloy is also an effective measure to prevent overheating.

(2) When the local coarse crystal is forged on the hammer or press, due to the poor thermal conductivity of titanium alloy, the temperature of the billet surface is reduced a lot during the contact process with the mold, and the friction between the billet surface and the upper and lower die of the mold is added

The middle part of the billet is strongly deformed, the surface deformation degree is small, so that the organization of the raw material is retained, and a new local coarse crystal is formed.

In order to avoid the local coarse crystal defect of titanium alloy, the following measures can be taken: adopt the pre-forging sequence to make the deformation uniform during the final forging; Strengthen lubrication, improve the friction between billet and mold; Fully preheat the mold to reduce billets

The temperature of the material drops during forging.

(3) Cracked titanium alloy forging surface cracks are mainly produced when the final forging temperature is lower than the full recrystallization temperature of titanium alloy. In the process of die forging, the contact time between the billet and the die is too long, because of the poor thermal conductivity of titanium alloy, it is easy to cause the surface of the billet to cool below the allowed final forging temperature, and also cause surface cracks of the forging. In order to control the occurrence of cracks, glass lubricant can be used when forging on the press, or forging on the hammer

To minimize the contact time between the billet and the lower die.

(4) Residual casting structure When forging titanium alloy ingot, if the forging ratio is not large enough or the forging method is improper, the forging will remain the casting structure. The method to solve this defect is to increase the forging ratio and adopt repetition

Upset.

(5) Bright strips The so-called bright strips in titanium alloy forgings are visible strips with abnormal brightness that exist in low-power tissues. Due to the difference in light Angle, the bright strip can be brighter than the base metal, but also darker than the base metal. In cross section, it is point-like or flaky; In the longitudinal section, it is a smooth strip, whose length varies from more than ten millimeters to several meters. There are two main reasons for the production of bright bars: one is the chemical composition of titanium alloy segregation, and the other is the deformation thermal effect of the forging process. The glitter has a certain effect on the properties of titanium alloys, especially on the plasticity and high temperature properties. The measures to prevent the appearance of bright strips are to strictly control the segregation of chemical components in smelting; Correct selection of forging thermal specifications (heating temperature, deformation degree, deformation speed, etc.) to avoid forging temperature due to deformation thermal effect and the difference is too large.

(6) α embrittlement layer α embrittlement layer is mainly titanium alloy at high temperature through the loose oxide skin, the diffusion of oxygen and nitrogen to the inside of the metal, so that the content of oxygen and nitrogen in the surface metal increases, so that the number of α phase in the surface tissue increases. When the oxygen and nitrogen content of the surface metal reaches a certain value, the surface tissue may be completely composed of α phase. In this way, the surface of the titanium alloy forms a surface layer with more α or completely α phase. The surface layer composed of this α phase is often called the α embrittlement layer. The α embrittlement layer on the surface of titanium alloy billet is too thick, which may cause billet cracking during forging.

The thickness of the α embrittlement layer is closely related to the type of heating furnace used in forging or heat treatment, the nature of the gas in the furnace, the heating temperature of the blank or part and the holding time. The thickness increases with the increase of heating temperature and holding time. It thickens with the increase of oxygen and nitrogen content in furnace gas. Therefore, in order to avoid this embrittlement layer is too thick, the heating temperature, holding time and furnace gas properties of forging or heat treatment must be properly controlled.

α, β and (α + β) titanium alloys may form α embrittlement layers. However, α titanium alloy is particularly sensitive to the formation of α embrittlement layer, while β titanium alloy will form α embrittlement layer when heated to 980℃ or above.

(7) Hydrogen embrittlement Hydrogen embrittlement has two types: strain time type and hydride type. Under the action of stress, the hydrogen atoms in the lattice gap diffuse and accumulate to the stress concentration gap after a certain time. Due to hydrogen atoms

The interaction with the dislocation makes the dislocation pinned and unable to move freely, which makes the matrix brittle, which is called strain aging hydrogen embrittlement. The hydrogen dissolved into the solid solution at high temperature precipitates out in the form of hydride with the temperature dropping, and the phenomenon that makes the titanium alloy brittle is called hydride hydrogen embrittlement. Both types of hydrogen embrittlement can occur in titanium and titanium alloys.

Hydrogen embrittlement problem is caused by excessive hydrogen content in titanium alloy. Therefore, the hydrogen content in industrial titanium alloys must be controlled within 0.015%. In order to prevent or reduce hydrogen embrittlement, the furnace should be slightly oxidizing when forging or heat treatment, and vacuum annealing can be carried out to eliminate hydrogen embrittlement for titanium alloy parts with hydrogen content exceeding the regulations and important parts.