1. Overview
With the rapid development of my country's national economy, science and technology, aerospace and aviation industries have ushered in new opportunities for development 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 development of the national economy and has broad development prospects. In order to continuously improve the advancement, reliability, and applicability of aircraft, and increase the competitiveness of domestic aircraft in the international market, civil aviation manufacturing enterprises have higher and higher requirements for the selection of aviation manufacturing materials; the main characteristics of titanium alloys are small specific gravity, high strength, and good heat resistance and corrosion resistance.
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 thermoplasticity of α and α+β-type alloys has 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 transformation temperature.
2.1 Conventional forging of titanium alloy
Commonly used deformed titanium alloys are usually forged below the β transformation temperature, which is called conventional forging. According to the heating temperature of the billet in the (α+β) phase zone, it can be subdivided into upper two-phase zone forging and lower two-phase zone forging.
2.1.1 Forging in the lower two-phase region
Forging in the lower two-phase region is generally heated and forged at 40-50°C below the β transformation temperature. At this time, the primary α phase and β phase participate in the deformation at the same time. The lower the deformation temperature, the more α-phase involved in the deformation. Compared with the deformation in the β region, the recrystallization process of the β phase in the lower two-phase region is sharply accelerated, and the new β grains formed by recrystallization not only precipitate along the deformed original β grain boundary, but also appear in the β interlayer between the β grain boundaries and the α sheets. The forging produced by this process has high strength and good plasticity, but its fracture toughness and creep properties still have great potential.
2.1.2 Forging in the upper two-phase region
It is forged at a temperature of 10-15°C below the β/(α+β) transformation point. The final structure after deformation contains more β-transition structure, which can improve the creep performance and fracture toughness of the structure; make the titanium alloy have both plasticity, strength and toughness.
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 in which the billet is heated in the β zone, and the forging is started and completed in the β zone; the second is the process method in which the blank is heated in the β zone, forged in the β zone, and the forging is completed in the two-phase zone with a large amount of deformation controlled, referred to as "sub-beta forging". Compared with forging in two-phase region, β forging can obtain higher creep strength and fracture toughness, and is also beneficial to improve the fatigue performance of titanium alloy.
2.3 Isothermal die forging of titanium alloy
This process uses the superplasticity and creep mechanism of the material to produce more complex forgings, and requires the mold to be preheated and kept within the range of 760-980°C; 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 instead, it is not necessary to use as fast moving beams to avoid quenching. Many forgings used in aircraft have the characteristics of thin wall and rib height, so this process has been applied in aviation manufacturing, such as the isothermal precision die forging process of TB6 titanium alloy for a certain type of domestic aircraft.
3. Defect analysis and process improvement of GR5 forgings
3.1 Occurrence and analysis of defects in GR5 forgings
When a factory carried out trial production of GR5 forgings according to navigation marks, it was detected that several forging performance indicators of the test pieces were unqualified, among which the "notch stress fracture" index was less than 5 hours. To solve this problem, we should first analyze the metallographic structure of GR5, and then find the reason from the forging process.
3.1.1 Metallographic structure and morphological characteristics of GR5
GR5 titanium alloy metal α + β titanium alloy, the composition is Ti-6AL-4V, the annealed structure is α + β phase, contains 6 α stable element aluminum, the strength of α phase is improved through solid solution strengthening, and the ability of vanadium to stabilize β phase is small, so the number of β phase in the annealed structure is small, accounting for about 7-10.
Under different heat treatment and hot working conditions of GR5 alloy, the ratio, properties and morphology of the basic phase α and β are very different. The β-transition temperature of GR5 alloy is about 1000°C. If GR5 is heated to 950°C, the microstructure obtained after air cooling is the primary α+β transformation structure; if it is heated to 1100°C and air-cooled, a coarse and completely transformed β-phase structure is obtained, which is called Widmanstatten structure. If the heating and deformation act at the same time, the effect is more obvious. The GR5 alloy is heated above the β transformation temperature, but the deformation is small, that is, the Widmanstatten structure is formed. Its organizational characteristics are: low plasticity and impact toughness, but good creep resistance. If the starting deformation temperature is above the β transformation, but the degree of deformation is large enough, the obtained microstructure features are: the β grain boundary delineated by the α phase is crushed, and the strip α phase is distorted, which is called a basket structure. It is characterized by better plasticity and impact toughness than Widmanstatten structure, similar to equiaxed fine-grained structure, and better high temperature durability and creep performance. If the heating temperature is lower than the β transformation temperature and the degree of deformation is sufficient, an equiaxed structure is obtained. It is characterized by good comprehensive properties, especially high plasticity and impact toughness. If the α+β phase region is partially deformed at high temperature and then annealed at high temperature to form a mixed structure, its comprehensive performance is good.
From the above analysis of the metallographic structure, it can be judged that if the performance of GR5 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 forging is not large enough.
3.1.2 GR5 forging process analysis
The effect 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 transformation), the β grains become larger, while the elongation and reduction of area become smaller, and the plasticity decreases; in order to ensure good comprehensive properties of GR5 forgings, it should be forged below the β transformation temperature. Titanium alloys have high deformation resistance, but poor thermal conductivity; under the violent flow of the alloy and heavy hammering during forging, the deformation may cause the temperature of individual parts of the forging to exceed the β transformation temperature, and factors such as excessive or small deformation will cause coarse grains and reduce performance. Based on the above, the reasons that may cause the unqualified performance of GR5 forgings can be preliminarily determined:
①The temperature of this batch of forging blanks is too high when heated, exceeding the β transformation point;
②The single hammer blow is too heavy during forging, so that the degree of single deformation is too large, causing local overheating and aggregation recrystallization, which reduces the performance.
③The heat treatment temperature after forging is too high, so that the temperature of the GR5 forging exceeds the β transformation point, forming a Widmanstatten structure and reducing the performance of the forging.
3.2 Changes of GR5 forging process parameters and test results
3.2.1 Selection of test parameters and results
In view of the above analysis, change the GR5 forging process parameters and pay attention to light strike and quick strike when forging. (Note: blanking size ￠50×113, forging size 50×65×65)
Test results: All performance indicators are qualified, and the "notch stress fracture" indicators are all greater than 5 hours.
3.2.2 Analysis of test results
(1) Judging from the furnace temperature and initial forging temperature, the heating temperature is not too high, and qualified parts can still be forged even if it exceeds 20 °C.
(2) In the test, a single hammer blow is used to strike lightly and quickly, and the performance of the test forging reaches the standard, which proves that lightly striking and quickly striking is an important factor to improve the performance of forgings.
(3) The heat treatment temperature after forging is lowered by 20°C than the original parameter, which may also be a factor to improve the performance, because from the temperature point of view, if the furnace temperature reaches 795°C due to the temperature control deviation, which exceeds the 780°C specified in the production specification, it will lead to a decline in the performance of the forging.
3.2.3 Test result verification and conclusion
In order to further verify the test results, another test was carried out in combination with production, and the method of light hitting and quick-hitting was still maintained during hammering; the results of the forgings were all qualified, and the "notch stress fracture" indicators were all greater than 5 hours.
The mechanical properties of GR5 titanium alloy forgings before and after the test are shown above. Through the test, it is concluded that the forging process parameters should be strictly controlled during the production of GR5 titanium alloy forgings; firstly, attention should be paid to beating lightly and quickly during forging to reduce the deformation of a single hammering, and secondly, the theoretical value of the heat treatment temperature after forging should be set within the range of 760-770°C, so as to ensure the forging quality of GR5 forgings.
4. The development prospect of titanium alloy forging process
The forging process of titanium alloy is widely used in aviation and aerospace manufacturing, and the isothermal forging process has been used to produce engine parts and aircraft structural parts; it is also more and more popular in industrial sectors such as automobiles, electric power and ships. In foreign countries, the application of titanium alloys has been developed to a very high level, and TiAL alloys and intermetallic compounds used at higher temperatures have been paid attention to, and a lot of research has been carried out; in order to better apply these materials, many studies have also been done on their deformation processes. People are also paying more and more attention to the research of higher strength sub-β titanium alloys. The application of titanium alloy and the research of forging process will still be a hot topic.
