Titanium alloys are characterized by their low density, high melting points (around 1600°C), good plasticity, high specific strength, excellent corrosion resistance, and the ability to work at high temperatures for extended periods. As a result, they are increasingly used as critical load-bearing components in aircraft and their engines, including forgings, castings, fasteners, and more. The weight of titanium alloys on modern foreign aircraft has reached approximately 30%, demonstrating the vast prospects for their application in the aviation industry. However, titanium alloys do have certain drawbacks: such as high resistance to deformation, poor thermal conductivity, high notch sensitivity (about 1.5), and significant changes in microstructure affecting mechanical properties, which lead to complexity during smelting, forging, and heat treatment. Therefore, using non-destructive testing techniques to ensure the metallurgical and processing quality of titanium products is a crucial issue. The following primarily introduces the defects commonly found in the detection of titanium forgings:

1. Segregation defects

In addition to β segregation, β spots, titanium-rich segregation, and strip-like α segregation, the most dangerous is the interstitial α stabilization segregation (Type I α segregation), which is often accompanied by tiny holes and cracks, containing gases such as oxygen and nitrogen, and has high brittleness. There is also the aluminum-rich α stabilization segregation (Type II α segregation), which also constitutes a dangerous defect due to the presence of cracks and brittleness.

Item 2: Inclusions

High-melting point and high-density metallic inclusions, such as those formed by insufficient melting of high-melting point and high-density elements in titanium alloys (e.g., molybdenum inclusions), or from the breakdown of hard alloy cutting tools mixed with metallurgical raw materials (especially recycled materials), such as improper electrode welding processes (commonly vacuum arc remelting for titanium alloys, e.g., tungsten electrode arc welding), which leave high-density inclusions like tungsten inclusions. Additionally, titanium compounds and other inclusions are also present.

The presence of inclusions can easily lead to the occurrence and expansion of cracks, hence they are considered unacceptable defects (for instance, according to Soviet documentation from 1977, any high-density inclusions with a diameter of 0.3 to 0.5 mm detected during X-ray inspection of titanium alloys must be documented).

3. Residual shrinkage holes

4. Holes

Holes may not exist individually but can also occur in clusters, which can accelerate the propagation speed of low-cycle fatigue cracks, leading to premature fatigue failure.

5. Cracks

The primary concern is forging cracks. Titanium alloys have high viscosity and poor fluidity, coupled with poor thermal conductivity. During the forging deformation process, due to high surface friction, significant internal deformation inhomogeneity, and large temperature differences between the inside and outside, shear bands (strain lines) are prone to form within the forging. In severe cases, this leads to cracking, with the orientation generally along the direction of the maximum deformation stress, Z.

6. Overheating

Titanium alloys have poor thermal conductivity. During the heat treatment process, not only can improper heating cause the forging or raw material to overheat, but it is also prone to overheating due to the thermal effect during deformation, leading to microstructure changes and the formation of overheated Widmanstätten structures.