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Study On Vacuum Annealing Process Of Gr5 Alloy Sheet
Mar 19, 2018

Gr5 Titanium is also called TC4 in China, TC4 titanium alloy contains 6% alpha stable element Al and 4% beta stable element V, which belongs to the typical alpha + - beta titanium alloy of Ti-Al-V system. The alloy of titanium alloy series about the amount of market 40%, cold processing is very difficult, the main reason is the plasticity of the alloy, yield ratio, and its mechanical properties and performance depends largely on the microstructure, the microstructure mainly depends on the heat treatment process, the vacuum annealing process of different the influence on the performance of subsequent processing will play a decisive role. 

Besides, it is necessary to study the dehydrogenation effect of different vacuum annealing, reduce the hydrogen content in the surface layer of titanium alloy to the safe concentration, eliminate the possibility of producing hydrogen embrittlement, and avoid machining cracks.

Ti Gr5 block.jpg

The sponge titanium, high purity aluminum (99.99%) and Al vanadium alloy were melted in a vacuum self consumable furnace in a certain proportion. The semi-finished sheet of 3.7mm is formed by forging and rolling.

In hot processing, the heating temperature is 980℃ to 1020℃. It is observed that the original beta grain boundary is very obvious in the microstructure obtained by hot deformation with a deformation rate of 95%, the alpha phase in the grain boundary is very obvious, and the alpha phase in the crystal is thick, needle like and regular. The main reason for such an organization is the heating or deformation of the raw billet in the beta phase area, or the original blank is carried out in the beta phase zone, while the deformation taken place in the phase of the alpha + beta phase is smaller. 

The grain boundary is due to the lack of recrystallization in the grain boundary due to the lack of deformation. The microstructure is of poor plasticity and high strength. It is necessary to undergo intermediate recrystallization annealing to improve its plasticity and strength, and create favorable deformation conditions for cold working. It is also found that the grain in the completely deformed area is fine and obviously elongated.

The original 3.7mm samples were annealed in a vacuum annealing furnace, and 4 pieces of sample were selected with the worst plastic properties. The annealing treatment was 780℃ ± 2 ℃, 800 ℃ ± 2 ℃ and 820 ℃± 2 ℃ and 830 ℃ ±2 ℃, the vacuum degree is ≤ 0.02Pa, 2 hours, 200 ℃ out. The tensile properties and hydrogen content at room temperature were measured. The test results show that:

(1) the annealing system is 800℃ ± 2 ℃ and 2 hours of heat preservation, which can achieve lower yield strength and the highest plastic properties.

(2) the annealing system is 800℃ ± 2 ℃ and 2 hours of heat preservation, and a stable alpha + beta phase network can be obtained.

(3) according to the existing vacuum furnace vacuum degree ≤0.02Pa, with 800℃ ± 2 ℃, heat for 2 hours, the average hydrogen content decreased by 0.009% compared with the original sample, to hydrogen effect, reached the following cold processing will not produce hydrogen embrittlement (chronic fracture) level of security.

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