Factors affecting the flaw detection of titanium alloy forgings and titanium forgings

Titanium alloy has the advantages of small specific gravity (about 4.5), high melting point (about 1600 ℃), good plasticity, high specific strength, strong corrosion resistance, and long-term work at high temperatures (currently thermal strength titanium alloy has been used for 500 ℃), so it has been increasingly used as an important bearing part of aircraft and aircraft engines, in addition to titanium alloy forgings, There are also castings, plates (such as aircraft skins), fasteners and so on. The weight ratio of titanium alloy used in modern foreign aircraft has reached about 30%, which shows that the application of titanium alloy in the aviation industry has a broad future. Of course, titanium alloys also have the following shortcomings: such as large deformation resistance, poor thermal conductivity, large notch sensitivity (about 1.5), and changes in microstructure have a significant impact on mechanical properties, resulting in complexity in smelting, forging processing and heat treatment. Therefore, the use of non-destructive testing technology to ensure the metallurgical and processing quality of titanium alloy products is a very important topic. The following mainly introduces the defects prone to occur in the inspection of titanium alloy forgings:

1. Segregation defects
In addition to β segregation, β spot, titanium rich segregation and strip α segregation, the most dangerous is the gap type α stable segregation (type I α segregation), which is often accompanied by small holes, cracks, containing oxygen, nitrogen and other gases, brittle. There is also an aluminum-rich α-stabilized segregation (type II α-segregation), which is also a dangerous defect due to its accompanying cracks and brittleness.

2. Inclusions
Most of them are metal inclusions with high melting point and high density. By the titanium alloy composition of high melting point, high-density elements are not fully melted to form in the matrix (such as molybdenum inclusion), there are also mixed in smelting raw materials (especially recycled materials) carbide tool chips or inappropriate electrode welding process (titanium alloy smelting generally uses vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusions, tungsten inclusions. In addition, there are titanium inclusions.

The existence of inclusions can easily lead to the occurrence and expansion of cracks, so it is not allowed to exist defects (for example, the Soviet Union's 1977 data stipulates that high-density inclusions with a diameter of 0.3 to 0.5mm must be recorded during X-ray inspection of titanium alloys).

3. Residual shrinkage hole

4. Holes
The existence of holes is not necessarily single, but may be multiple dense, which will accelerate the propagation rate of low-cycle fatigue cracks and cause premature fatigue failure.

5. Cracks
Mainly refers to forging cracks. The viscosity of titanium alloy is large, the fluidity is poor, and the thermal conductivity is not good, so in the forging deformation process, due to the large surface friction, the internal deformation is not uniform and the internal and external temperature difference is large, it is easy to produce a shear band (strain line) in the forging, which leads to cracking when it is serious, and its orientation is generally along the direction of the maximum deformation stress.

6. Overheating
The thermal conductivity of titanium alloy is poor, in addition to improper heating in the hot processing process caused by forging or raw materials overheating, in the forging process is also easy to cause overheating because of the thermal effect of deformation, resulting in changes in microstructure, resulting in overheating Weischel structure.