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Vacuum heat treatment process of TC11 titanium alloy pipe

Aviation and weapons have high requirements on the strength and plasticity of materials, requiring tensile strength Rm ≥ 1030MPa, yield strength Rp0.2 ≥ 910MPa, elongation A ≥ 8%, and section shrinkage rate Z ≥ 23%. Using the solution + aging heat treatment process, vacuum heat treatment was performed on Φ180mm×25mm×LTC11 titanium alloy tubes formed by hot extrusion. The influence of vacuum heat treatment system on the microstructure and mechanical properties of the material was studied, and the law of influence between them was discussed . The results show that the content of primary α phase in the alloy gradually decreases with the increase of solution temperature under the same aging regime under the same aging regime, and the grain size gradually increases, while the content of β phase increases. When the temperature reaches 1040 °C, coarse original β grains appear, and there are continuous α-phase slender flaky α-phases on the original β-grain boundaries; the mechanical properties of solid solution alloys at 950-970 °C can meet the requirements of the material.

The extrusion forming Φ180mm×25mm studied in this article requires its tensile strength Rm≥1030MPa, yield strength Rp0.2≥910MPa, elongation A≥8%, section shrinkage rate Z≥23%, and its strength and plasticity are required to be high matching. . Due to the high degree of alloying of TC11 and the large amount of hot extrusion deformation, the microstructure and properties are more sensitive to the vacuum heat treatment process. Relationship between properties and vacuum heat treatment process.

1 method

The TC11 titanium alloy ingot is made of first-grade sponge titanium and industrial pure aluminum wire as raw materials, and is smelted into an ingot with a diameter of Φ700mm by three vacuum consumable electric arc furnaces.

The β-transition point of the alloy was measured by metallographic method to be 1010°C. The ingot was cast on a 3150t hydraulic press and repeatedly forged to a size of Φ292×650mm; it was extruded on a 3150t hydraulic press at a temperature of 950°C and kept for 20 minutes. , the specification of extruded tube blank is Φ180mm×25mm×L. The metallographic and tensile specimens were cut from the alloy tube blank by a wire electric discharge machine. The tests were selected respectively: solution-aging heat treatment: 950℃, 960℃, 970℃, 1040℃/30min, AC+ 530℃/6h, AC; metallographic analysis was carried out on OLYM PU SPM G3 metallographic microscope, HF: HNO3: H2O is 1:3:10 (volume ratio) etchant for etching. The tensile test specimen is a cylindrical sample with a working area diameter of 5 mm and a length of 90 mm. The tensile test is carried out on an IN-ST RON 1185 universal tensile machine.

2 Results and discussion

2.1 Effect of vacuum heat treatment on microstructure

After adopting four different solution temperatures for TC11 hot extrusion tube blank, adopt 530℃/6h, after aging treatment, under the same aging time, with the increase of solution temperature, the primary α phase in the alloy The content of β phase decreases gradually, the grain size gradually increases, and the content of β phase increases. The morphological change of the primary α phase is due to the gradual decrease of the thermodynamically stable content of the α phase in the alloy as the solution temperature gradually increases and approaches the transformation point. Comparing the two solid solution temperatures of 950 °C and 970 °C, it can be found that most of the α phase distributed on the β matrix phase dissolves, and the grain boundaries become clearer. Grain boundaries grow at 60°.

When the alloy is solution-aged at 1040℃, the coarse equiaxed spherical primary αp phase disappears, and the coarse primary β grains appear on the primary β grain boundaries. There are continuous α phase slender flaky α phases on the primary β grain boundaries. The grain boundaries and grains are precipitated, and some of the flaky α are parallel to each other to form a bundle, forming a certain angle with the grain boundary. When the equal primary phase αp in the alloy becomes coarse, the phase interface between the β matrix and the primary αp phase decreases, and the phase interface acts as a defect to hinder the movement of dislocations. The strength decreases and the elongation increases; however, as the secondary phase αs increases, it becomes finer and more dispersed, and the interface between the two phases increases, resulting in the enhancement of the second phase strengthening effect, thereby increasing the strength of the alloy and the elongation. decline.

2.2 Relationship between tensile properties and vacuum heat treatment temperature

The alloy can obtain tensile strength Rm≥1030MPa, yield strength Rp0.2≥910MPa, elongation A≥8%, section shrinkage rate Z≥ 23%, which meets the mechanical performance requirements of TC11 titanium alloy for aviation and weapon applications. It shows that the TC18 alloy extruded tube is aged at the solid solution temperature in the range of 950℃～970℃ and kept at 530℃ for 6 hours. With the increase of the solution temperature, the strength of the alloy gradually increases, and the plasticity decreases slightly. The difference in strength and plasticity is mainly caused by the difference in the content of primary α phase in the microstructure of each vacuum heat treatment system. However, the mechanical properties of the obtained alloys can meet the mechanical properties of the developed and applied pipes. Comparing the strength and plasticity values of the alloys at the three solution temperatures, it can be seen that the strength and plasticity of the alloys are well matched at 970°C/30min solution and 530°C/6h aging.

The mechanical properties obtained after aging heat treatment at 1040°C/30min, AC, +530°C/6h can also meet the requirements of tensile strength Rm≥1030MPa, yield strength Rp0.2≥910MPa, and elongation A≥8% for aviation and weapon applications. requirements, the performance margin is not large. However, the area shrinkage rate cannot meet the requirement of Z≥23%. The main reason is that the microstructure formed by vacuum heat treatment at this temperature is already a Widmandelstein microstructure, especially the area reduction rate is much lower than that of the first three vacuum heat treatment systems. This is due to its original The β grains are larger than other vacuum heat treatment systems, and there are network grain boundaries α.

3 Conclusion

(1) With the increase of solution temperature, the content of primary α phase in the alloy gradually decreases, the grain size gradually increases, and the content of β phase increases. When the temperature reaches 1040 °C, coarse primary β grains appear, and there are continuous α-phase slender flaky α-phases on the primary β-grain boundaries.

(2) The mechanical properties of 950-970℃/30min solid solution/AC+alloy 530℃/6h/AC can meet the material requirements.