Titanium alloy has a small specific gravity (about 4.5), a high melting point (about 1600 ℃), good plasticity, high specific strength, strong corrosion resistance, and can work at high temperatures for a long time (currently, thermal strength titanium alloy has been used at 500 ℃). Therefore, it has been increasingly used as an important bearing part of aircraft and aircraft engines. In addition to titanium alloy forgings, there are castings, plates (such as aircraft skin), fasteners, and so on. The weight ratio of titanium alloy used in modern foreign aircraft has reached about 30%, which shows that titanium alloy has a broad prospect in the aviation industry. Of course, titanium alloys also have the following disadvantages: such as large deformation resistance, poor thermal conductivity, large notch sensitivity (about 1.5), and significant influence of microstructure changes on mechanical properties, which leads to complexity in smelting, forging, and heat treatment.
Therefore, it is a very important subject to adopt nondestructive testing technology to ensure the metallurgical and processing quality of titanium alloy products. The following mainly introduces the defects that are easy to occur in the flaw detection of titanium forgings such as titanium blocks and titanium rings:
1. Segregation defect
Except β Segregation β Spot, titanium-rich segregation, and strip α In addition to segregation, the most dangerous is interstitial type α Stable segregation (type I α Segregation), which is often accompanied by small holes and cracks, containing oxygen, nitrogen, and other gases, and is relatively brittle. And aluminum-rich type α Stable segregation (type II α Segregation), which is also accompanied by cracks and brittleness and constitutes dangerous defects.
2. Inclusion
Most of them are metal inclusions with high melting points and high densities. The high melting point and high-density elements in the titanium alloy composition are not fully melted and left in the matrix to form (such as molybdenum inclusion), and there are also carbide tool chips mixed in the smelting raw materials (especially recycled materials) or improper electrode welding process (the smelting of titanium alloy generally adopts the vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusion, in addition to titanium inclusion, etc.
The existence of inclusions can easily lead to the occurrence and propagation of cracks, so they are not allowed defects (for example, according to the Soviet Union's data in 1977, high-density inclusions with a diameter of 0.3~0.5mm found in the X-ray inspection of titanium alloy must be recorded).
3. Residual shrinkage cavity
See example.
4. Hole
Holes do not necessarily exist individually, but may also exist in a number of densely, which will accelerate the low cycle fatigue crack growth rate and cause early fatigue failure.
5. Crack
Mainly refers to forging cracks. Due to the high viscosity, poor fluidity, and poor thermal conductivity of titanium alloy, it is easy to produce shear bands (strain lines) in the forging due to the large surface friction, obvious internal deformation non-uniformity and large internal and external temperature difference during the forging deformation process, which will lead to cracking in severe cases, and its orientation is generally along the direction of maximum deformation stress.
6. Overheat
The thermal conductivity of titanium alloy is poor. In addition to the overheating of forgings or raw materials caused by improper heating during hot processing, it is also easy to cause overheating due to the thermal effect during deformation during the forging process, resulting in the change of microstructure and the formation of overheated Widmanstein structure.
Titanium alloy has large deformation resistance, and poor thermal conductivity and the change of microstructure has a significant impact on mechanical properties, which leads to the complexity in smelting, forging, and heat treatment.
