Effect of hot extrusion process on the microstructure and mechanical properties of TA15 titanium alloy tube
The nominal chemical composition of TA15 titanium alloy is Ti-6.5AI-2Zr-1Mo-1V. It is an alpha titanium alloy successfully developed by the former Soviet Union in the mid-1960s. The brand name is BT20. This alloy is currently used as a typical near-alpha titanium alloy with two characteristics. The medium room temperature, strength and high temperature strength of phase titanium alloy are higher than those of Ti-6AL-4V alloy, with good thermal stability and welding performance, higher specific strength, creep resistance and corrosion resistance. It can be used for up to 3000 hours in an environment below 5500°C. Due to its good processability, it is widely made into plates, bars, forgings, die forgings, pipes, profiles and other products. It is widely used in high-performance structural parts in the aerospace field, so TA15 alloy has good room temperature properties and must also have good high-temperature properties. In this paper, the mechanical properties of φ170mm×30nn×LTA15 titanium alloy extruded tube blanks formed by different hot extrusion and processing techniques at different temperatures were studied. Analysis and microstructure research are conducted to analyze the impact of different hot extrusion processes and the formed tube blank structure on its mechanical properties.
Provide a theoretical basis for the production process of TA15 titanium alloy tube blanks.
1.Experimental materials and methods
(1)Experimental materials The TA15 alloy used in the experiment is first-grade sponge Ti, industrial pure AI wire and zirconium chips, and aluminum-vanadium master alloy as raw materials. It is carried out in a three-time vacuum consumable electric arc furnace. The ingot specification is φ700mm×L. Table 1 is the chemical composition of the alloy.
(2)experiment method
The billet was forged 6 times on a 3150t hydraulic press, and the finished product specifications were φ220×650mm. It was extruded on a 315t hydraulic press. The extrusion temperatures were 950°C and 1050°C, and the holding time was 60 minutes. The specifications of the finished tube blank were: φ170mm×30mm×L. Use a resistance furnace for 750℃/2h, AC return processing, use a wire EDM machine to cut metallographic and tensile specimens from the alloy tube blank, and use HFHN03:H20 for 1:3:10 corrosion for microstructure analysis, and use OLYMPUSPMG3 Observe with a metallographic microscope, and temperature tensile specimens are conducted in accordance with GB/T228.1-2010. High temperature tensile specimens are conducted in accordance with GB/T4338-2006. The tensile test temperatures were 25°C, 100°C, 200°C, 300°C, 400°C and 500°C respectively. The tensile test was conducted on a TC-12-031 tensile machine.
2.results and analysis
(1)Microstructure analysis at different hot extrusion temperatures
Figure 1 shows the apparent structure of TA15 alloy φ17mm×30mm×L pipe under different hot extrusion temperatures. The heating temperature of 950°C is in the upper part of the two-phase zone, and the holding time is 60 minutes. The tissue obtained after extrusion is isometric. The α+β two-phase structure can be considered a two-state structure, with an average of 14.5 μm so far. In the β transformation structure, the α laths are distributed in an interlaced or clustered shape. The thickness of the laths is about 35 μm. The heating temperature is 1050°C, which belongs to the β phase. In the processing area, the holding time is 60 minutes. The structure obtained after extrusion is a β-processed structure, which is a lamellar structure. The size of the initial β grains is about 445μm, the width of the α phase at the grain boundary is about 1.5um, and the thickness of the α lamellae distributed on the β grains is 2.5 μm, and the local α phase appears to be about 60 μm wide. The shapes are clustered and arranged side by side in the same direction.
(2)High temperature performance analysis at different hot extrusion temperatures
As shown in the figure, it is the high-temperature mechanical properties of TA15 alloy φ17mm×30mm×L pipe under different hot extrusion temperature conditions. As can be seen in Figure 2, the room temperature mechanical properties of TA15 alloy pipe are obtained by extrusion at 950°C, with a tensile strength of 980MPa and an elongation of 12.5%. Comparing the figure above, the two-state structure is obtained by extrusion at 950°C. Its room temperature and high temperature mechanical properties are lower than those of the lamellar structure obtained by extrusion at 1050°C. As can be seen from Figure 2, as the experimental temperature increases, the tensile strength and yield strength of the two tissue states show an obvious downward trend. Comparison shows that the high-temperature tensile curve of the TA15 titanium alloy extruded pipe with dual-state structure is steeper, while the lamellar structure is relatively gentle. It shows that extrusion in the β phase region has higher high-temperature mechanical properties.
Conclusion display
The structure of TA15 titanium alloy pipe obtained at different extrusion temperatures has a significant impact on its high-temperature mechanical properties. The high-temperature mechanical properties of the lamellar structure of TA15 alloy pipe extruded in the beta phase zone are higher than those of the two-phase two-state structure.
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