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

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

Summary: It mainly introduces titanium alloy and forging technology. Taking the forging defects of TC4 forgings found in aviation production and the process of improving the process as an example, it analyzes the process characteristics of titanium alloy forging and its application and development prospects in the aviation industry.

1. Overview

With the great development of our country's national economy and science and technology, the aerospace and aviation industries have ushered in new development opportunities in recent years, especially after the establishment of the national “large aircraft” project, the civil aviation manufacturing industry will become a new economic growth point leading the national economic development, with broad prospects for development.In order to continuously improve the advanced, reliability and applicability of aircraft and increase the international market competitiveness of domestic aircraft, civil aviation manufacturing enterprises have higher and higher requirements for the selection of aviation manufacturing materials; The main characteristics of titanium alloy are its small specific gravity, high strength, and good heat resistance and corrosion resistance. It has become the main material of choice for modern aircraft force components, greatly reducing the weight of the aircraft, among which TC4 (Ti-6AL-4V) and TB6 titanium alloy forgings are more used in aviation manufacturing.

2. Classification of titanium alloys and forging processes

According to the microstructure at room temperature, titanium alloys can be divided into three types: α-type alloys, α+β-type alloys and β-type alloys. Among them, the thermoplastics of α and α+β-type alloys have little relationship with the deformation speed, while β-type alloys have good malleability, but too low temperature may cause α-phase precipitation.The forging process of titanium alloy is divided into conventional forging and high temperature forging according to the relationship between forging temperature and β transition temperature.

2.1 Conventional forging of titanium alloy

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

2.1.1 Forging in the lower two-phase zone

Forging in the lower two-phase zone is generally heated and forged at 40~50℃ below the β transition temperature. At this time, the nascent α phase and β participate in deformation at the same time.The lower the deformation temperature, the greater the number of α phases involved in the deformation.Compared with the deformation of the β region, the recrystalization process of the β phase in the lower two-phase region is accelerated sharply. The new β grains formed by recrystalization not only precipitate along the deformed original β grain boundary, but also appear in the β intermediate layer between the β grain boundary and the α sheet layer.The forgings produced by this process have high strength and good plasticity, but their fracture toughness and creep properties still have great potential.

2.1.2 Forging in the upper two-phase zone

It is forged at a temperature of 10-15℃ below the β/(α+β) phase transition point.The final tissue after deformation contains more β-transformed tissue, which can improve the creep performance and fracture toughness of the tissue; the plasticity, strength and toughness of titanium alloy can be combined.

2.2 High temperature forging of titanium alloy

Also known as ”β forging“, it is divided into two types: the first is the process method of heating the blank in the β zone, starting and completing the forging in the β zone; the second is the process method of heating the blank in the β zone, starting the forging in the β zone, and controlling a large amount of deformation to complete the forging in the two-phase zone, referred to as ”sub-β forging".Compared with two-phase zone forging, β forging can obtain higher creep strength and fracture toughness, which is also conducive to the improvement of the fatigue properties of titanium alloys.

2.3 Isothermal die forging of titanium alloy

This kind of process uses the superplasticity and creep mechanism of the material to produce more complex forgings, which requires the mold to be preheated and maintained in the range of 760~980℃; the hydraulic press applies pressure at a predetermined value, and the working speed of the press is automatically adjusted by the deformation resistance of the blank.Since the mold is heated, there is no need to use such a fast movable beam to avoid rapid cooling.Many forgings used in aircraft have the characteristics of thin walls and high ribs, so this kind of process has been applied in aviation manufacturing, such as the TB6 titanium alloy isothermal precision die forging process of a certain type of domestic aircraft.

3. Defect analysis and process improvement of TC4 forgings

3.1 Occurrence and analysis of defects in TC4 forgings

When a factory conducted the trial production of TC4 forgings according to the beacon, several performance indicators of the forgings were detected to be unqualified. Among them, the index of “notch stress fracture” was less than 5 hours. In view of this problem, the metallographic structure of TC4 should first be analyzed, and then the reason should be found from the forging process.

3.1.1 The morphological characteristics of the metallographic organization of TC4

TC4 titanium alloy is an α+β titanium alloy, the composition is Ti―6AL-4V, the annealed structure is α+β phase, containing 6?The α-stabilized elemental aluminum improves the strength of the α-phase through solid melt strengthening, and vanadium has a smaller ability to stabilize the β-phase. Therefore, the number of β-phases in the annealed tissue is small, accounting for about 7-10?。

Under different heat treatment and thermal processing conditions, the proportions, properties and morphology of the basic phases α and β of TC4 alloys are very different.The β transition temperature of TC4 alloy is about 1000℃. If TC4 is heated to 950℃, the resulting tissue after air cooling is nascent α+β transition tissue; if heated to 1100℃ and air-cooled, a thick and completely transformed β-phase tissue is obtained, which is called Wei's tissue.If heating and deformation act at the same time, the effect is more obvious. The TC4 alloy is heated above the β transition temperature, but the deformation is small, and the Wei's tissue is formed.Its organizational characteristics are: low plasticity and impact toughness, but good creep resistance.If the initial deformation temperature is above the β transition, but the degree of deformation is large enough, the resulting tissue characteristics are: the β grain boundary part drawn by the α phase is crushed, and the striped α phase part is distorted, which is called a mesh-like tissue.It is characterized by better plasticity and impact toughness than Wei's organization, similar to isometric fine crystal organization, long-lasting high temperature and good creep performance.If the heating temperature is lower than the β transition temperature and the degree of deformation is sufficient, an isometric structure is obtained.It is characterized by good overall performance, especially high plasticity and impact toughness.If the high-temperature part of the α+β phase region is deformed and then annealed at high temperature to form a mixed structure, its overall performance is good.

From the above analysis of the metallographic organization, it can be judged that if the performance of TC4 decreases, it may be caused by two links in the forging process.:

①The heating temperature is too high, reaching or exceeding the β transition temperature;

②The degree of deformation of the forgings is not large enough.

3.1.2 TC4 forging process analysis

The influence of forging temperature on the β grain size and room temperature properties of α+β titanium alloy is that as the temperature increases (above the β phase transition), the β grain becomes larger, while the elongation and cross-section shrinkage become smaller, and the plasticity decreases; in order to ensure that TC4 forgings have good overall performance, they should be forged below the β transition temperature.Titanium alloy has high deformation resistance, but poor thermal conductivity. During forging, under the violent flow of the alloy and heavy hammering, the deformation may cause the temperature of individual parts of the forging to exceed the β transition temperature, and the degree of deformation is too large, too small and other factors will cause the grain to be thick, which will reduce the performance.Based on the above, it can be preliminarily determined that the reasons for the substandard performance of TC4 forgings may be caused.:

①The temperature of the batch of forged blanks is too high when heated, exceeding the β transition point;

②when a single hammer is too heavy during forging, the degree of deformation of a single hammer is too large, causing local overheating and aggregation and recrystalization, and the performance is degraded.

③The heat treatment temperature after forging is too high, so that the temperature of the TC4 forging exceeds the β transition point, forming a Wei's tissue and reducing the performance of the forging.

3.2 TC4 forging process parameter changes and test results

3.2.1 Selection and results of test parameters

In view of the above analysis, change the TC4 forging process parameters (Table 1) and pay attention to light and fast forging when forging at the same time.(Note: Cutting size￠50×113, forging size 50×65×65)

Test results: All performance indicators are qualified, of which the ”notch stress fracture" index is greater than 5 hours.

3.2.2 Analysis of test results

(1) Judging from the furnace temperature and the initial forging temperature, the heating temperature is not too high, even if it exceeds 20℃, qualified parts can still be forged.

(2) In the test, a single hammer strike was used to hit the quick punch lightly, and the performance of the test forgings was up to standard, proving that the light punch and quick punch were an important factor in improving the performance of the forgings.

(3) The heat treatment temperature after forging is 20℃ lower than the original parameter, which may also be a factor in improving performance, because from the temperature point of view, if the furnace temperature reaches 795℃ due to the temperature control deviation, this exceeds the 780℃ specified in the production manual, which will lead to a decrease in the performance of the forgings.

3.2.3 Verification and conclusion of test results

In order to further verify the test results, a test was conducted in combination with production (Table 2), and the method of light and quick beating was still maintained during hammering; the result was that all the forgings passed the test, and the index of “notch stress fracture” was greater than 5 hours.

The mechanical properties of TC4 titanium alloy forgings before and after the test are shown above (Table 3).Through the test, it is concluded that when producing TC4 titanium alloy forgings, the process parameters of forging should be strictly controlled; first of all, pay attention to the light and fast forging in the forging to reduce the deformation of a single hammer, and secondly, the theoretical value of the heat treatment temperature after forging should be set in the range of 760~770℃, so as to ensure the forging quality of TC4 forgings.

3. Development prospects of titanium alloy forging technology

The forging process of titanium alloy is widely used in the aviation and aerospace manufacturing industries. The isothermal forging process has been used in the production of engine parts and aircraft structural parts; it is also becoming more and more popular in the automotive, electric power and naval industries. Welcome.In foreign countries, the application of titanium alloys has developed to a very high level, and the application of higher temperature TiAL alloys and intermetallic compounds has been valued by people, and a lot of research has been carried out; in order to better apply these materials, at the same time, many studies have been done on their deformation technology.People are also paying more and more attention to the research on higher-strength sub-β titanium alloys.The application of titanium alloy and the research of forging technology will still be a hot topic.