Small proportion of titanium alloy (4.5), high melting point (1600), plasticity is good, is better than high strength, corrosion resistance is strong, can work for a long time under high temperature (500) heat strong titanium alloy at present has been used to wait for an advantage, and thus has more and more important bearing used in aircraft and aircraft engine components, in addition to the forging of titanium alloy material, and castings, plate (such as aircraft skin), fasteners and so on.The weight ratio of titanium alloy used in modern foreign aircraft has reached about 30%, it can be seen that the application of titanium alloy in aviation industry has a broad future.Of course, titanium alloys also have the following disadvantages: such as deformation resistance, poor thermal conductivity, large notch sensitivity (about 1.5), microstructure changes on the mechanical properties of more significant impact, resulting in the smelting, forging processing and heat treatment of complexity.Therefore, it is a very important task to use nondestructive testing technology to ensure the metallurgical and processing quality of titanium alloy products.The following is a brief introduction to the defects easily found in flaw detection of titanium alloy forgings:
1. Segregation defects
In addition to severe segregation, bright spot, titanium-rich segregation and stripwise segregation, the most dangerous is the clearance type stable segregation (type I segregation), which is often accompanied by small holes and cracks around, containing oxygen, nitrogen and other gases, brittle.There are also aluminum-rich stable segregation (type II segregation), which is also a dangerous defect due to cracks and brittleness.
Most of them are metallic inclusions with high melting point and high density.High melting point, high density by titanium alloy composition elements not fully melted in matrix form (such as molybdenum inclusions), there are mixed in the smelting of raw materials (especially the recycled material) chip carbide cutting tools collapse or improper electrode welding process commonly used (titanium alloy smelting vacuum consumable electrode remelting process), such as tungsten arc welding, the high density inclusions, such as tungsten inclusions, and titanium compound inclusions, etc.
The presence of inclusions is likely to lead to the occurrence and propagation of cracks, and is therefore an inadmissible defect (for example, the Soviet data of 1977 stipulated that high density inclusions with diameters of 0.3~0.5mm must be recorded when detected on X-ray examination of titanium alloys).
1. Residual shrinkage
See the examples.
The holes may not exist singly, but may also exist in multiple clusters, which will accelerate the propagation rate of low-cycle fatigue crack and cause premature fatigue failure.
Mainly refers to forging crack.Titanium big viscosity, liquidity is poor, and thermal conductivity is not good, therefore in the process of forging deformation, high friction on the surface, the internal deformation inhomogeneity and inside and outside temperature difference obviously, easier to produce in forging internal shear zone (strain), serious when that lead to crack, its orientation along the direction of the maximum deformation stress.
The thermal conductivity of titanium alloy is poor, in the process of hot processing, in addition to improper heating caused by the forging or raw materials overheating, in the forging process is also easy to cause overheating due to the thermal effect of deformation, resulting in the microstructure changes, the generation of overheating wei's organization.