Titanium alloy has a small specific gravity (about 4.5), a high melting point (about 1600 ° C), good plasticity, high specific strength, strong corrosion resistance, and long-term work at high temperatures (currently hot-strength titanium alloy has been used at 500 ° C). Therefore, it has been increasingly used as an important bearing component 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: for example, large deformation resistance, poor thermal conductivity, high notch sensitivity (about 1.5), and changes in microstructure have a significant impact on mechanical properties, resulting in smelting, forging, and heat treatment Complexity. Therefore, the use of non-destructive testing technology to ensure the metallurgical and processing quality of titanium alloy products is an important issue. The following mainly introduces the defects that are easy to occur in the flaw detection of titanium alloy forgings:
- Segregation-type defects
In addition to β-segregation, β-spots, titanium-rich segregation, and strip-shaped α-segregation, the most dangerous is the gap-type α-segregation (I-type α-segregation), which is often accompanied by small holes and cracks, containing oxygen, nitrogen and other gases , Greater brittleness. There are also aluminum-rich α-stable segregation (II-type α-segregation), which is also a dangerous defect due to cracks and brittleness.
Is mostly high melting point, high density metal inclusions. It is formed by the high melting point and high density elements in the titanium alloy composition that are not fully melted and left in the matrix (such as molybdenum inclusions), and there are also cemented carbide tool chips or inappropriate electrode welding processes mixed with smelting raw materials (especially recycled materials) ( Titanium alloy smelting generally uses vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusions, in addition to titanium inclusions.
The presence of inclusions can easily lead to the occurrence and propagation 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 0.5 mm must be given during titanium alloy X-ray inspection recording).
- Residual shrinkage
Holes do not necessarily exist alone, but may also exist in multiple concentrations, which will accelerate the growth rate of low-cycle fatigue cracks and cause early fatigue damage.
Mainly refers to forging cracks. Titanium alloy has high viscosity, poor fluidity, and poor thermal conductivity. Therefore, during forging deformation, due to large surface friction, significant internal deformation unevenness, and large internal and external temperature differences, it is easy to produce shear bands inside the forging ( Strain line), when severe, it will cause cracking, and its orientation is generally along the direction of maximum deformation stress.
The thermal conductivity of titanium alloy is poor. In addition to improper heating in the hot working process, which causes overheating of the forgings or raw materials, it is also easy to cause overheating during the forging process due to the thermal effect of deformation, causing microstructure changes and overheating Weiss structure.