Analysis and application examples of titanium alloy forging process in the aviation industry

Analysis and application examples of titanium alloy forging process in the aviation industry

1.summarize

With China's national economy, science and technology development, aerospace, aviation industry in recent years ushered in a new development opportunity, especially in the national "big plane" project, civil aviation manufacturing industry will become a new economic growth point leading the development of the national economy, has a broad development prospects. Civil aviation manufacturing enterprises in order to continuously improve the advanced nature of the aircraft, reliability, applicability, increase the competitiveness of domestic aircraft in the international market, the choice of aviation manufacturing materials more and more demanding; titanium alloys are mainly characterized by a small specific gravity, high strength, and at the same time has a good heat resistance, corrosion resistance, to become the main choice of materials for the modern aircraft components, greatly reducing the weight of the aircraft, of which the TC4 (Ti-6AL-4V) is the main material of the aircraft. 6AL-4V) and TB6 titanium alloy forgings in aviation manufacturing applications.

2.Classification of titanium alloy and forging process

According to the room temperature microstructure, titanium alloys can be divided into three types: α-type alloys, α + β-type alloys and β-type alloys, of which α and α + β-type alloys thermoplasticity and deformation speed is not very close to the relationship between α and β-type alloys have a good forging, but the temperature may be caused by the low temperature of the α-phase precipitation. The forging process of titanium alloy is categorized into conventional forging and high temperature forging according to the relationship between forging temperature and β-transition temperature.

2.1 Conventional Forging of Titanium Alloys

Commonly used deformed titanium alloys are usually forged below the β-transition temperature, called conventional forging. According to the billet heating temperature in the (α + β) phase zone, can be subdivided into the upper two-phase zone forging and lower two-phase zone forging.

2.1.1 Lower two-phase zone forging

Lower two-phase zone forging is generally in the β transformation temperature below 40 ~ 50 ℃ heating and forging, when the primary α-phase and β the same time to participate in the deformation. The lower the deformation temperature, the more the number of α phase involved in deformation. Compared with the β zone deformation, in the lower two-phase region of the β phase recrystallization process is dramatically accelerated, recrystallization of the formation of new β grains not only along the deformation of the original β grain boundaries precipitation, but also in β grain boundaries and α lamellae between the β interlayer appears. Produced by this process of forging high strength, good plasticity, but its fracture toughness and creep properties have great potential.

2.1.2 Upper two-phase zone forging

It is in the β / (α + β) phase transition point below 10-15 ℃ temperature of the beginning of forging. The final organization after deformation contains more β transformation organization, can improve the organization of the creep properties and fracture toughness; so that the titanium alloy plasticity, strength, toughness and both.

2.2 High temperature forging of titanium alloys

Also known as "β forging", divided into two kinds: the first is the billet in the β zone heating, in the β zone to start and complete the forging process; the second is the billet in the β zone heating, in the β zone to start forging, and control the deformation of a large amount of deformation in the two-phase zone to complete the forging process, referred to as "sub-beta forging Sub-β forging". Compared with the two-phase zone forging, β forging can get higher creep strength and fracture toughness, but also conducive to the improvement of titanium alloy fatigue performance.

2.3 Isothermal Die Forging of Titanium Alloys

The process makes use of the superplasticity of the material and the creep mechanism to produce more complex forgings, the requirements of the mold preheating and maintained in the range of 760 ~ 980 ℃; hydraulic press to a predetermined value of the pressure, the working speed of the press by the deformation of the blanks of the resistance to automatic adjustment. Because the mold is changed to heating, do not need to use so fast moving beam to avoid rapid cooling. Aircraft with many forgings have thin-walled and rib high characteristics, so the process has been applied in the aviation manufacturing, such as a domestic aircraft TB6 titanium alloy isothermal precision die forging process.

3, TC4 forgings defect analysis and process improvement

3.1 TC4 forging defects and analysis

A factory according to the beacon TC4 forging test production, test pieces several forging performance indicators failed, including "notched stress fracture" indicator is less than 5 hours, for this problem, first of all should be analyzed from the TC4 metallurgical organization and morphology, and then from the forging process to find the reason.

3.1.1 TC4 metallographic organization and morphology characteristics

TC4 titanium alloy is α + β type titanium alloy, composed of Ti-6AL-4V, annealed organization for α + β phase, containing 6? of α-stabilizing element aluminum, solidification strengthening of the α-phase to improve the strength of vanadium stabilization of the β-phase ability is small, so the number of β-phase in the annealed organization is small, accounting for about 7-10?

TC4 alloy in different heat treatment and thermal processing conditions, the proportion of the basic phase α, β, the nature and morphology is very different. β transformation temperature of TC4 alloy in 1000 ℃ or so, if the TC4 heated to 950 ℃, air cooling after the organization of the primary α + β transformation of the organization; such as heated to 1100 ℃, air-cooled, it is a coarse fully transformed β-phase organization, known as the Weiss organization. If the heating and deformation at the same time, the effect is more obvious, the TC4 alloy heated to β transition temperature above, but the deformation is small, that is, the formation of Wei's organization. Its organizational characteristics are: plasticity, impact toughness is lower, but better creep resistance. If the beginning of the deformation temperature in the β transition above, but the degree of deformation is large enough, then the organization is characterized by: α-phase delineation of the β grain boundaries are partially crushed, striped α-phase partially distorted, known as the net basket-like organization. Characterized by plasticity, impact toughness is better than the Wei's organization, similar to the equiaxed fine crystal organization, high temperature persistence and creep performance is better. If the heating temperature is lower than the β-transition temperature, and the degree of deformation is sufficient, that is to get the equiaxial organization. It is characterized by better overall performance, especially high plasticity and impact toughness. If the α + β phase area in the high-temperature part of the deformation and high-temperature annealing on the hybrid organization, its comprehensive performance is good.

From the above analysis of the metallographic organization can be judged if the TC4 performance decline, may be caused by the forging process of two links:
① heating temperature is too high, reaching or exceeding the β transition temperature;
② The degree of deformation of the forging is not large enough.

3.1.2 Analysis of TC4 forging process

Forging temperature on the α + β titanium alloy β grain size and room temperature performance is with the increase in temperature (β phase transition above) β grain size, while the elongation and section shrinkage becomes smaller, plasticity decreases; in order to ensure that the TC4 forgings have a good overall performance, should be forged below the β transition temperature. Titanium alloy deformation resistance is higher, but poor thermal conductivity; forging in the alloy flow and heavy hammering, the resulting deformation may make the forging individual parts of the temperature exceeds the β-transition temperature, as well as deformation of the degree of oversize, too small, and other factors will cause grain size, so that the performance of the decline. Comprehensive the above can be initially determined may cause TC4 forgings unqualified performance reasons: ① the batch of forging billet heating.

① the batch of forging billet heating temperature is too high, more than β transition point; ② forging a single time during the forging, the temperature is too high, more than β transition point.

② forging a single hammering too heavy, so that a single deformation degree is too large, causing local overheating and gathering recrystallization, so that the performance decline.

③ after forging heat treatment temperature is too high, so that the TC4 forging temperature exceeds the β transition point, the formation of Wei's organization, reducing the performance of forgings.

3.2 TC4 forging process parameters change and test results

3.2.1 Selection and results of test parameters

For the above analysis, change the TC4 forging process parameters (Table 1) at the same time when forging, pay attention to light hit fast hit. (Note: material size ¢ 50 × 113, forging size 50 × 65 × 65)

Test results: all performance indicators are qualified, of which "notched stress fracture" indicators are greater than 5 hours.

3.2.2 Test results analysis

(1) From the furnace temperature and the beginning of forging temperature, the heating temperature is not too high, even if more than 20 ℃ can still be forged qualified parts.

(2) test using a single hammer blow light hit fast, test forging performance up to standard, proving that light hit fast is to improve the performance of forgings is an important factor.

(3) forging heat treatment temperature than the original parameters to reduce 20 ℃, may also be a factor to improve performance, because from the temperature point of view, if the furnace temperature due to temperature control deviation reaches 795 ℃, which exceeds the production specification of 780 ℃, will lead to a decline in the performance of forgings.

3.2.3 Test results verification and conclusion

In order to further verify the results of the test, and with the production of a test (Table 2), in the hammering still maintain the method of light beating fast; results of the forging test all qualified, "notch stress fracture" indicators are greater than 5 hours.

Test before and after the mechanical properties of TC4 titanium alloy forgings see above (Table 3). Through the test concluded that: in the production of TC4 titanium alloy forgings, should strictly control the forging process parameters; first of all, pay attention to the forging in the light hit fast, reduce the amount of deformation of a single hammer blow, and secondly, the theoretical value of the post forging heat treatment temperature should be set in the range of 760 ~ 770 ℃, so as to ensure that the forging quality of TC4 forgings.

4.The development prospect of titanium alloy forging process

Titanium alloy forging process is widely used in aviation, aerospace manufacturing industry, isothermal forging process has been used in the production of engine parts and aircraft structural components; also more and more by the automotive, electric power and naval and other industrial sectors welcome. In foreign countries, the application of titanium alloy has been developed to a very high level, applied to higher temperatures of TiAL alloys and intermetallic compounds have been emphasized, and a lot of research; in order to better apply these materials, and at the same time its deformation process has also done a lot of research. People also pay more and more attention to the higher strength of sub-beta type titanium alloy research. The application of titanium alloys and forging process research will remain a hot topic.