Study on the Bright Spot in the Microstructure of TC17 Titanium Alloy Rods

TC17 alloy is a near-β type alloy with excellent comprehensive properties. Its nominal composition is Ti-5Al-2Sn-2Zr-4Mo-4Cr (mass fraction, %). This alloy can be forged in both the α+β phase region and the β phase region. It not only has high strength, good hardenability and fracture toughness, but also has good thermal stability, high fatigue performance, and good hot working properties. Since the 1970s, it has gradually entered the aerospace manufacturing field and has been used as forging parts such as compressor discs in engines with higher thrust. Researchers have studied the influence of forging temperature on the bright spots in the low-magnification structure of TC17 titanium alloy through three forging temperatures.

The test used a Φ140mm TC17 titanium alloy bar. The microstructure of the bar was a β matrix with uniformly distributed equiaxed α phases, with the α phase content being approximately 50%. The phase transformation point (α+β/β) of the alloy was measured by metallographic method to be 880-900℃.

At three temperatures of 950℃, 850℃ and 830℃, the billet was deformed by about 60% on a 2500T rapid forging machine to form a Φ90mm bar. The low-magnification structure of the bars produced at the three forging temperatures was inspected to observe the distribution of bright spots. Microstructure observation was conducted using an Olympus GX71 metallographic microscope, and the bright spots were analyzed using energy dispersive spectroscopy (EDS) and microhardness testing. The mechanical properties and microstructure of the bars after treatment at 840℃/2h.AC + 800℃/4h.WC + 630℃/8h.AC were also tested.

Forging temperature determines the microstructure and properties of the titanium alloy. The β phase transformation during the forging of the β phase of the titanium alloy is an inevitable phase transformation process, but if it occurs in the two-phase forging, the β spots are an unacceptable organizational defect. By choosing a reasonable forging temperature, β spots can be effectively avoided. Existing studies have shown that the larger the area of β spots, the greater the impact on the room temperature tensile properties and low-cycle fatigue properties. The maximum area of β spots in the Ti-10V-2Fe-3A1 alloy in the United States is 0.762mm×0.762mm, and in the Ti-5553 alloy in Russia, it is 0.75mm×0.75mm. Below this value, the influence of β spots on mechanical properties is not significant.

For TC17 titanium alloy products forged near the β phase, the uniformity of deformation and the strain rate should be controlled as much as possible to avoid the formation of β spots due to excessive local temperature rise. The following conclusions were drawn:

(1) The non-uniformity of micro-component is the essential factor causing β spots in TC17 titanium alloy, and the forging temperature is the main inducing factor for the formation of β spots. Producing below the phase transformation point by 30℃ will result in different degrees of β spots in the low-magnification structure of the bar. This is because the uneven distribution of micro-components leads to a lower transformation temperature in this area than the matrix area, causing local temperatures to exceed the phase transformation point prematurely during forging, thereby forming β spots. Producing below the phase transformation point by 50℃ can effectively avoid the formation of β spots.

(2) The area of the β spots observed in the study was approximately 0.35mm×0.30mm. Local small areas of β spots did not have a significant impact on the overall mechanical properties of the TC17 titanium alloy bar and could meet the usage requirements.