Titanium rotating Components
• Type I hard alpha inclusion remained in the Sioux City fan disk from manufacture. How could the manufacturing process have been modified to achieve defect free production?
The type I hard alpha inclusion is due to the nitrogen absorbed by the titanium only in the molten state. To improve the manufacture we could do different things. On the one hand to avoidthe concentration of nitrogen in one point, we could increase the vacuum melts from two to three, and increasing the temperature and the time in the molten pool where the material is in a liquid state, because hard alpha inclusions have a melting point significantly greater than the normal structure. On the other hand to avoid the inclusions of nitrogen that take place in the furnace, the solutioncould be to do an exhaust control of the atmosphere into the furnace to avoid the contamination of the titanium.
Then to avoid that one billet with type I hard alpha inclusion was sent to the titanium rotating components industry, we should reduce the diameter of the billet to allow a more sensitive immersion-ultrasonic inspection of the billet.
• Could changes to alloy composition or to TMPhave improved the fatigue resistance?
The type I hard alpha inclusion is the most frequent cause of fatigue failure in titanium rotating parts, but also there is two defects which are common in titanium, the high density inclusions and the segregation, so it will be good to produce without these defects.
These defects produce an embrittlement in a zone, and normally this zone is where the crackinitiates due to the different composition in the surface.
To improve the fatigue resistance we could change the alloy adding some chemical element to avoid the concentration of nitrogen with some chemical reaction. Also to improve the fatigue resistance it should be good to do a nitriding to make the surface uniform and in this way increasing the fatigue resistance .
• Are titanium rotating partsmade differently today?
The manufacture of titanium rotating parts follow these steps: sponge processing, melting, billet conversion, component forging, and inspection of the finished component. During the manufacturing process appropriate controls and inspections are in place to minimize the occurrence of and to maximize detection of, such anomalies based on the best available technologies.
Theconventional melting process for titanium alloys has been the Vacuum Arc Remelting process (VAR). Triple VAR has been recommended as the current standard for critical rotating component use. Improvements in VAR technology have resulted in a significant reduction in the occurrence of melt related defects since the mid 1980's. The newer Cold Hearth Melting (CHM: process where the metal is meltedand then maintained molten as it traverses a specified hearth distance, allowing inclusion elimination either by dissolution in the molten pool, or by density separation into the skull) technology virtually eliminates the risk of having high density inclusions survive through the melting cycle.
• Include in you discussion other relevant fatigue initiating metallurgical defects in Ti 6A1 4V alloys.Another type of defects that affect the life of Ti 6A1 4V components is the porosity; there are three major types of porosity defects. The first one is gas-induced porosity. The pores are spherical in shape and range in diameter from 0.02 to 1 mm. The second type is small shrinkage porosity. These have indefinite shapes and range in size from 0.005 to 0.02 mm. The third type is large shrinkagepores. These pores could reach 6mm in size.
Pore-related failures used to initiate from a defect located close to the specimen surface. Defects nearest to the surface have a higher probability of initiating fatigue cracks. Therefore, sometimes a relatively small defect near the surface will initiate a crack even though larger defects may exist within the material. Near-surface defect crack...
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