Analysis of Titanium Alloy Forging Process in Aviation Industry

With the rapid development of China's national economy and science and technology, the aerospace industry has seen new 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 driver leading the development of the national economy, with broad development prospects. To continuously improve the advancement, reliability, and applicability of aircraft and increase the international market competitiveness of domestically produced aircraft, the requirements for aerospace materials selection are becoming increasingly stringent. Titanium alloys have become the primary material for modern aircraft structural components due to their low density, high strength, and excellent heat and corrosion resistance. Among them, TC4 (Ti-6AL-4V) and TB6 titanium alloy forgings are widely used in aerospace manufacturing.

Classification of Titanium Alloys and Forging Techniques
Based on room temperature microstructure, titanium alloys can be classified into three types: α alloys, α+β alloys, and β alloys. The hot plasticity and deformation speed of α and α+β alloys are not significantly affected, while β alloys have good forgeability but may cause α phase precipitation at low temperatures. The forging techniques of titanium alloys can be categorized into conventional forging and high-temperature forging, based on the relationship between forging temperature and β transformation temperature.

2.1 Conventional Forging of Titanium Alloys

Commonly used deformable titanium alloys are usually forged below the β transformation temperature, known as conventional forging. According to the heating temperature of the billet in the (α+β) phase region, it can be further divided into upper two-phase region forging and lower two-phase region forging.

2.1.1 Lower Two-Phase Region Forging

Lower two-phase region forging is generally conducted at 40-50°C below the β transformation temperature, where the primary α and β phases are involved in deformation simultaneously. Lower deformation temperatures result in a higher amount of α phase participating in the deformation. Compared to deformation in the β region, the recrystallization process of the β phase is significantly accelerated in the lower two-phase region, leading to the formation of new β grains not only at the original β grain boundaries but also within the β interlayer between α lamellae. Forgings produced using this process exhibit high strength and good ductility, but there is still potential for improving fracture toughness and creep performance.

2.1.2 Upper Two-Phase Region Forging

This technique involves initial forging at temperatures 10-15°C below the β/(α+β) phase transformation point. The resulting microstructure contains a higher proportion of β transformation structure, which improves creep resistance and fracture toughness of the titanium alloy, striking a balance between plasticity, strength, and toughness.

2.2 High-Temperature Forging of Titanium Alloys

Also known as "β forging," this can be divided into two types: the first type involves heating the billet to the β region, starting and completing the forging process in the β region, while the second type, known as "sub β forging," involves heating the billet to the β region, initiating forging in the β region, and controlling a significant deformation in the two-phase region. Compared to two-phase region forging, β forging can achieve higher creep strength, fracture toughness, and improved fatigue performance of titanium alloys.

2.3 Isothermal Die Forging of Titanium Alloys

This technique utilizes the material's superplasticity and creep mechanisms to produce complex forgings. It requires preheating the die and maintaining it within a range of 760-980°C, with the hydraulic press applying a predetermined pressure, and the press's working speed being automatically adjusted based on the deformation resistance of the billet. Many forgings used in aircraft have thin walls and high ribs, making this technique suitable for aerospace manufacturing, such as the isothermal precision die forging process for domestically produced aircraft TB6 titanium alloy.

Analysis of TC4 Forging Defects and Process Improvement
3.1 Occurrence and Analysis of TC4 Forging Defects

When a certain factory conducted TC4 forging trial production following the aviation standard, several performance indicators of the forgings were found to be unqualified, particularly the "notch stress fracture" indicator being less than 5 hours. To address this issue, the analysis started with the metallographic structure of TC4 and then explored the reasons in the forging process.

3.1.1 Metallographic Characteristics of TC4

TC4 titanium alloy is an α+β titanium alloy with the composition of Ti-6AL-4V. Its annealed microstructure consists of α+β phases, containing 6% aluminum as an α stabilizing element, and the β phase is strengthened by solid solution strengthening, resulting in a small amount of β phase in the annealed structure, approximately 7-10%.

The proportion, properties, and forms of basic phases α and β in TC4 alloy vary significantly under different heat treatment and hot working conditions. The β transformation temperature of TC4 alloy is around 1000°C. Heating TC4 to 950°C and then air-cooling results in a primary α+β transformation structure. Heating it to 1100°C and then air-cooling leads to coarse fully transformed β phase structure, known as Widmanstätten structure. Simultaneous heating and deformation have a more pronounced effect; if TC4 is heated above the β transformation temperature but undergoes a small deformation, it forms Widmanstätten structure. In this process, the plasticity and impact toughness decrease, but the creep resistance improves. If the initial deformation temperature is above the β transformation temperature but with sufficient deformation, it forms a mesh structure. In this case, the α phase delineated by the β grain boundaries is shattered, and the lamellar α phase is distorted, resembling an equiaxed fine-grained structure with better plasticity, impact toughness, and high-temperature creep performance. If the heating temperature is below the β transformation temperature, and the deformation is sufficient, it results in an equiaxed structure, exhibiting overall good properties, especially high plasticity and impact toughness. If the deformation is followed by high-temperature annealing in the α+β phase region, a mixed structure with good comprehensive properties is obtained.

Based on the above analysis of metallographic structures, it can be inferred that the performance decrease in TC4 may be caused by two factors in the forging process:

The heating temperature is too high, reaching or exceeding the β transformation temperature.
The deformation degree of the forging is insufficient.
3.1.2 Analysis of TC4 Forging Process

Forging temperature affects the β grain size and room temperature properties of α+β titanium alloys. As the temperature increases above the β transformation temperature, the β grain size increases, while the elongation and cross-sectional shrinkage decrease, leading to reduced plasticity. To ensure that TC4 forgings have good comprehensive properties, forging should be conducted below the β transformation temperature. Titanium alloys have high deformation resistance but poor thermal conductivity. During forging, severe flow and heavy hammering may cause localized overheating and recrystallization, resulting in grain coarsening and decreased performance. From the above analysis, the possible reasons for the unqualified TC4 forging performance can be preliminarily determined as follows:

The heating temperature of the batch of billets is too high, exceeding the β transformation point.
The single forging impact is too heavy, causing excessive deformation and resulting in local overheating and recryst