Titanium alloy has the advantages of small specific gra […]
Titanium alloy has the advantages of small specific gravity (about 4.5), high melting point (about 1600°C), good plasticity, high specific strength, strong corrosion resistance, and long-term work at high temperatures (currently heat-strength titanium alloys have been used at 500°C), etc. Therefore, it has been increasingly used as an important load-bearing component of aircraft and aircraft engines. In addition to forgings of titanium alloy materials, 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, large 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 a very important topic. The following mainly introduces the defects that are easy to appear in the flaw detection of titanium alloy forgings:
1. segregation defect
In addition to β segregation, β spot, titanium-rich segregation and stripe α segregation, the most dangerous is interstitial α stable segregation (I type α segregation), which is often accompanied by small holes and cracks, containing oxygen, nitrogen and other gases. , The brittleness is greater. There are also aluminum-rich α stable segregation (type II α segregation), which also constitutes dangerous defects due to cracks and brittleness.
are mostly metal inclusions with high melting point and high density. The high melting point and high density elements in the titanium alloy composition are not fully melted and left in the matrix (such as molybdenum inclusions). There are also cemented carbide tool chips mixed in the smelting raw materials (especially recycled materials) or improper electrode welding processes ( Titanium alloy smelting generally uses vacuum consumable electrode remelting method), such as tungsten arc welding, leaving high-density inclusions, such as tungsten inclusions, and titanium inclusions.
The existence of inclusions can easily lead to the occurrence and propagation of cracks, so it is a defect that is not allowed (for example, the Soviet Union's 1977 data stipulates that high-density inclusions with a diameter of 0.3 ~ 0.5 mm must be found during the X-ray inspection of titanium alloys. recording).
3. residual shrinkage cavity
Holes do not necessarily exist individually, but may also exist in multiple dense, which will accelerate the growth of low-cycle fatigue cracks and cause premature fatigue failure.
mainly refers to forging cracks. Titanium alloy has high viscosity, poor fluidity, and poor thermal conductivity. Therefore, during the forging deformation process, due to the large surface friction, the obvious internal deformation unevenness and the large internal and external temperature difference, it is easy to produce shear bands inside the forging ( Strain line), which leads to cracking in severe cases, and its orientation is generally along the direction of maximum deformation stress.
Titanium alloy has poor thermal conductivity. In addition to overheating of forgings or raw materials caused by improper heating during hot working, the forging process is also prone to overheating due to thermal effects during deformation, causing microstructure changes, resulting in overheated Widmanstatten structure.