Research on Segregation Phenomenon in TC17 Titanium Alloy

Titanium alloy, as an important structural material, is widely used in the aerospace field due to its excellent comprehensive properties (specific strength, corrosion resistance, and heat resistance). TC17 alloy is a near-β type alloy with excellent comprehensive properties. This alloy not only has high strength, fracture toughness, thermal stability, and fatigue performance, but also has high hardenability and good hot working performance. Since the 1970s, this alloy has gradually entered the aviation manufacturing field due to its excellent performance and has been used as the material for compressor discs and other forgings in high-thrust engines. Due to the addition of a large amount of alloying elements in this alloy, segregation is prone to occur if the melting process is not properly controlled. Therefore, it is necessary to study the morphology, enrichment status, and distribution law of segregation in this alloy to optimize the current melting process and lay a theoretical foundation for the production of high-quality TC17 titanium alloy.

The ingot diameter was Φ620mm and was formed by three vacuum consumable arc melting processes. Samples were taken from the upper, middle, and lower points of the ingot circumference for analysis, and the chemical composition met the requirements of G/BT3620.1-2007 standard.

The phase transformation point of the ingot was measured by metallographic method to be 875℃ to 880℃. After the ingot was descaled, the riser and bottom were removed and then transferred to the forging process. The ingot was forged in the β single-phase region by a 2500t press, elongated in the near-β region, and rounded. The rounded billet was precisely forged to Φ60mm in the α+β two-phase region by an SX-16 precision forging machine. Finally, the billet was rolled at 40℃ to 50℃ below the phase transformation point by a 250 rolling mill to obtain two specifications of rods, Φ32mm and Φ17mm, and then machined into Φ30mm and Φ15mm finished rods. Low-magnification structure inspection was conducted on the finished rods, and high-magnification samples were prepared for the parts with problems in the low-magnification inspection. The microstructure of the samples was observed under a metallographic microscope, and the abnormal areas were analyzed by energy spectrum.


The experiment found that the bright spots observed in the low magnification of the two specifications of TC17 titanium alloy rods appeared as β spots in the high magnification. The formation of β spots was caused by the segregation of Cr elements. The content of Cr elements in the segregation area was significantly higher than that in the matrix, resulting in a lower phase transformation point in the segregation area than in the matrix. Segregation was mainly concentrated in the center and head of the ingot.

According to the titanium alloy phase diagram and the solidification theory of alloys, under normal solidification conditions, alloy elements with a segregation coefficient K≥1 are not prone to segregation, unless the alloy elements and master alloys are not fully homogenized during the melting process. For alloy elements with K＜1, even if the alloy is uniform in the molten state, there are still differences in the composition of the solid phase and liquid phase at the same temperature during solidification, and the content of elements in the liquid phase is always higher than that in the solid phase, leading to the easy formation of segregation in the center and head of the ingot. In addition, the elements that segregate and the degree of segregation determine the actual β phase transformation temperature in the segregation area. Within the range of Cr mass fraction of 0% to 7%, for every 1% increase in Cr mass fraction, the phase transformation point decreases by 26℃. Therefore, when the forging is heated during hot working or subsequent heat treatment, if the heating temperature is close to the phase transformation point, it may cause the segregation area to exceed the phase transformation point, resulting in β spots.

Therefore, the appearance of β spots (bright spots) in the center area of the two specifications of rods in the experiment is related to the melting process and subsequent hot working temperature. To improve the uniformity of the ingot, an effective method is to reduce the melting current, slow down the melting speed, and make the molten pool depth smaller and flatter to reduce the equiaxed crystal area. At the same time, the ingot should not be too large, as the larger the diameter, the larger the equiaxed crystal area. To avoid the appearance of β spots, the maximum diameter of the alloy should not exceed 720mm. During hot working and heat treatment, the upper limit of heating should be strictly controlled to reduce the deformation heat effect, so that the segregation area lacks the necessary conditions for the formation of β spots, thereby reducing the harm.