Vacuum annealing process of TC4 alloy sheet

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TC4 titanium alloy belongs to the typical a+β type titanium alloy of Ti-AI-V series. This alloy accounts for about 40% of the market amount of titanium alloy series, and it is very difficult to cold work. The main reason is that the alloy has poor plasticity and high yield-strength ratio, and its mechanical properties and service performance depend to a large extent on the microstructure, and its microstructure mainly depends on the heat treatment process. Therefore, different vacuum annealing methods are studied. The influence of the process on the properties plays a decisive role in the subsequent processing. In addition, it is necessary to study the dehydrogenation effect index of different vacuum annealing to reduce the hydrogen content of the titanium alloy surface layer to a safe concentration, to eliminate the possibility of hydrogen embrittlement, and to avoid the occurrence of cracks during processing.

1 method

1.1 Sample preparation

The sponge titanium, high-purity aluminum (99.99%) and aluminum-vanadium alloy are smelted in a vacuum consumable furnace according to a certain proportion. It is forged and rolled into a semi-finished plate with a thickness of 3.7mm. The chemical composition is shown in Table 1, and the chamber tensile properties are shown in Table 2.

1.2 Original tissue analysis

In hot working, the heating temperature is 980℃~1020℃, and the microstructure obtained by hot deformation with a deformation rate of 95% is shown in Figure 1.

It can be seen from Figure 1 that the original β grain boundary is very obvious, the grain boundary a phase is very obvious, and the intragranular a phase is regularly arranged in a coarse needle shape. The main reason for this kind of structure is that the heating or deformation of the original billet is carried out in the β phase region, or the original billet is carried out in the β phase region, and when the a+β phase region is deformed, the amount of deformation taken is small. . The grain boundary a is left over by the a phase on the grain boundary that is not recrystallized due to insufficient deformation. This structure has poor plasticity and high strength. It must be recrystallized and annealed in the middle to improve its plasticity and reduce the strength, so as to create good deformation conditions for cold working.

The grains in the fully deformed region in Figure 1 are fine and significantly elongated.

2 process

The original 3.7mm sample was annealed in a vacuum annealing furnace, and 4 samples were taken near the 2# sample plate with the worst plastic properties. , Annealing system 820℃+2℃, annealing system 830℃±2℃, vacuum degree ≤0.02Pa, heat preservation for 2 hours, 200C released. After the furnace was released, the chamber tensile properties and hydrogen content were measured.

3 Results and Discussion

From Table 3 and Figure 2, it can be seen that Rp0.2 and A% of the four annealing regimes are the lowest value of 846MPa and the highest value of 15.5% after holding at 800C soil for 2 hours, which are 288MPa and 7% lower than the original sample respectively. The average H content is about 0.009% lower than the original sample. It is confirmed by cold rolling that the vacuum heat treatment system of 800℃±2℃ for 2 hours can effectively reduce the strength and improve the plasticity, and the rolling deformation rate can reach 10% ~ 15%; the hydrogen embrittlement phenomenon of edge cracks and surface cracks caused by hydrogen embrittlement did not occur during rolling, indicating that the hydrogen concentration of the titanium alloy surface layer was reduced to a safe level that would not cause hydrogen embrittlement (chronic fracture).

Figure 3 shows the microstructure after annealing at 800°C ± 2°C. It can be seen that since 800°C ± 2°C is a solid solution treatment below the phase transition point, the a phase is not transformed into a β phase during heat treatment, but is Remaining, the microstructure is a stable network structure of a + β phase composed of spherical or strip phase + intergranular β phase. The cold-rolled 3.0mm TC4 alloy sheet is shown in Figure 4, and the performance, shape and size indicators can meet the standard requirements.

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