Six Defects Easily Appeared in Titanium Alloy Forgings
1. Segregation defects
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.
2. Inclusions are mostly metal inclusions with high melting point and high density. It is formed by insufficient melting of high melting point and high density elements in the titanium alloy composition to remain in the matrix (such as molybdenum inclusions), and there are also carbide tool chippings mixed in 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 expansion of cracks, so they are not allowed defects (for example, the Soviet Union’s 1977 documents stipulate that high-density inclusions with a diameter of 0.3~0.5mm must be found during the X-ray inspection of titanium alloys. record).
3. Residual shrinkage cavity
See examples.
4. Hole
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.
5. Crack
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.
6. Overheating
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.
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