Titanium Alloy specific gravity is small (about 4.5), high melting point (about 1600 ℃), good plasticity, has the advantages of high specific strength, corrosion resistance, can work at high temperatures for a long time (at present the hot strength of the titanium alloy has been used for 500 ℃), and so on, and therefore has been more and more used as an important bearing parts of aircraft and aircraft engines, in addition to titanium alloy material forgings, there are also casting, plate (such as aircraft skin), fasteners and so on. In addition to Titanium Alloy Forgings, there are castings, plates (such as aircraft skins, fasteners and so on. Modern foreign aircraft using titanium alloy weight ratio has reached about 30%, visible titanium alloy application in the aviation industry has a broad future. Of course, titanium alloy also exists the following shortcomings: such as deformation resistance, poor thermal conductivity, notch sensitivity (1.5 or so), microstructure changes on the mechanical properties of the more significant impact, which leads to 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 that are easy to appear in the flaw detection of titanium square, titanium ring and other commonly used titanium forgings:
1, segregation-type defects
In addition to β segregation, β spot, titanium-rich segregation and striated α segregation, Z dangerous gap type α stable segregation (I type α segregation), which is often accompanied by small holes and cracks around it, containing oxygen, nitrogen and other gases, brittleness is larger. There are also aluminum-rich α-stable segregation (type II α segregation), also due to cracks and brittleness and constitute a dangerous defect.
2、Inclusions
Mostly high melting point, high density metal inclusions. By the titanium alloy composition of high melting point, high density elements are not sufficiently melted to stay in the matrix formation (such as molybdenum inclusions), but also mixed in the smelting of raw materials (especially recycled materials) in the tungsten carbide cutting tool chip or improper electrode welding process (titanium alloy smelting is generally used in the vacuum self-consumption electrode remelting method), such as tungsten electrode arc welding, leaving high-density inclusions, such as tungsten inclusions, in addition to the titanium inclusions. The presence of inclusions can easily lead to cracking.
The presence 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 in 1977, the information provided by the titanium alloy X-ray radiography inspection found that the diameter of 0.3 ~ 0.5mm of high-density inclusions must be recorded).
3、Residual shrinkage
See examples.
4、Hole
Holes do not necessarily exist individually, there may be more than one dense presence, will make the low-week fatigue crack expansion speed up, resulting in early fatigue damage.
5、Crack
Mainly refers to forging cracks. Titanium alloy viscosity, poor fluidity, coupled with poor thermal conductivity, and thus in the forging deformation process, due to the surface friction, internal deformation unevenness is obvious, as well as the temperature difference between the inside and outside, etc., easy to produce in the forging internal shear band (strain line), which leads to cracking in severe cases, its orientation is generally along the direction of the Z large deformation stress.
6、Overheating
Titanium alloy thermal conductivity is poor, in the thermal processing process in addition to improper heating caused by forgings or raw materials overheating, in the forging process is also prone to deformation due to the thermal effect caused by overheating, causing microstructural changes, resulting in overheating Weiss organization.
