The common defects in production of titanium alloy forgings
The common defects in production of titanium alloy forgings
1. Segregation type defect
In addition to β segregation, β spot, titanium-rich segregation and stripe α segregation, the most dangerous is interstitial α stable segregation (type I 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 alpha stable segregation (type II alpha segregation), which also constitutes dangerous defects due to cracks and brittleness, and also reduces the thermal stability of the alloy.
2. Inclusions
There are inclusions on the surface of the blank, and cracks are often formed along the inclusions during forging, or obvious foreign bodies appear after the forging is corroded, most of which are 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 [ii]), such as high-density inclusions left by tungsten arc welding [iii], such as tungsten inclusions, in addition to titanium inclusions, etc. Titanium alloy forgings with inclusions are not allowed to be put into use.
3. Hole
The holes do not necessarily exist individually, but may also exist in multiple dense ones, which will accelerate the growth of low-cycle fatigue cracks and cause premature fatigue failure.
4. Cracks
Mainly refers to forging cracks. Titanium alloys have high viscosity, poor fluidity, and poor thermal conductivity. Therefore, during the forging deformation process, due to the large surface friction, the uneven internal deformation and the large temperature difference between the inside and the outside, it is easy to be inside the forging. Shear bands (strain lines) are produced, and cracks occur along the direction of maximum deformation stress in severe cases.
5. Overheating
Titanium alloys have poor thermal conductivity. In addition to overheating of the forgings or raw materials caused by improper heating during the hot working process, the forging process is also prone to overheating due to the thermal effect during deformation, causing microstructure changes, resulting in overheated Widmanstatten structure[iv ].
In order to ensure the quality of titanium alloy forgings, in addition to strictly controlling the quality of raw materials, attention should also be paid to the ultrasonic flaw detection of the forging blanks and semi-finished products to prevent some deformation and physical characteristics that will change during the subsequent heating process.
