Titanium alloy has small specific gravity (about 4.51 kg /CM³), high melting point (about 1650°C), good plasticity, high specific strength, good corrosion resistance, can work at high temperature for a long time (currently hot strong titanium alloy has been used for 450°C) and with other advantages, so it has been used as an important bearing part of aircraft and aircraft engines.
At present, the application proportion of titanium alloy materials in aircraft has reached about 30%, titanium alloy materials in aviation, navigation, oil, electricity, electronics and other fields of the application with a broad future.However, titanium alloy also has the following defects: high deformation stress, poor thermal conductivity, greater notch sensitivity (about 1.5), the change of microstructure has a significant impact on mechanical properties, which leads to the complexity of smelting, forging and heat treatment.
Therefore, it is an important link to adopt nondestructive testing technology to ensure the processing quality of titanium alloy materials.
The followings defects are easy occuring in titanium alloy material Ultrasonic detection :
1. Segregation defect
The common segregation defects are β segregation, β spots, titanium-rich segregation and barαsegregation, and the most dangerous gap type α stable segregation (type I α segregation), which is often accompanied by small holes, cracks, containing oxygen, nitrogen and other gases, brittleness.There are also aluminium-rich alpha stable segregation (type II αsegregation), which also present dangerous defects with cracks and brittleness.
2. Inclusions are mostly metal inclusions with high melting point and density.By titanium alloy composition of the high melting point, high density 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 (titanium alloy smelting generally use vacuum self-consuming furnace electrode dissolving method), such as tungsten arc welding, the high density inclusions, such as tungsten inclusions, and titanium compound inclusions, etc.Inclusion can easily lead to the occurrence and expansion of cracks, so it is not allowed to exist defects.
3. Residual shrinkage cavity
4. The existence of holes may not be single, but may be multiple and dense, which may accelerate the growth rate of cycle fatigue crack and lead to fatigue damage in advance.
5. Crack, mainly refers to forging crack.Titanium alloy of the viscosity, liquidity is poor, and thermal conductivity is bad, so 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 line (strain), serious when leading to cracking and its orientation along the direction of the maximum deformation stress.
6. Overheating, poor thermal conductivity of titanium alloy, in the process of thermal processing in addition to improper heating caused by the forging or raw material overheating, in the process of forging is also easy to cause overheating due to the thermal effect of deformation, causing changes in microstructure, resulting in overheating weiderweiders.
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