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Analysis of Hot Pressing Deformation Behavior of TC4 Titanium Alloy

Abstract: A simulation experiment was conducted on the compression deformation of TC4 alloy in three temperature ranges below 700 ℃ using a thermal simulator. The macroscopic mechanical properties and microstructure evolution during the deformation process were studied through metallographic experiments, in order to provide theoretical reference for the medium temperature deformation of TC4 alloy. The research results indicate that TC4 alloy is difficult to deform and undergo dynamic recrystallization below 500 ℃. After low-temperature deformation, the equiaxed grains increase and the microstructure tends to be uniform.

[Keywords] Medium temperature deformation microstructure dynamic recrystallization

Preface

Titanium alloy has the advantages of low density, high specific strength to stiffness, good corrosion resistance, excellent high-temperature mechanical properties, outstanding fatigue and creep resistance, and non-magnetic weldability, which are widely used in aviation, aerospace, chemical, weapons, ships, energy and other fields.

The deformation below the phase transition point of titanium alloy at lower temperatures can to some extent suppress the dynamic recovery and recrystallization behavior of titanium alloy, improve microstructure, and enhance the comprehensive mechanical properties and strength of the material. However, due to the poor low-temperature plastic deformation ability of titanium alloy, it is prone to cracking and other processing defects. Therefore, research on the behavior of relatively low-temperature materials below the phase transition point of titanium alloy has been relatively scarce [2-3].

The microstructure and deformation behavior of titanium alloy sensitive materials are closely related, and have a significant impact on the macroscopic material properties, which requires special research [4-6].

In response to the above issues, this article explores the material behavior below the phase transition point of TC4 titanium alloy at relatively low temperatures through physical simulation methods. Through experiments, the stress-strain behavior and microstructure evolution characteristics of TC4 titanium alloy during deformation are studied, in order to provide theoretical reference for the control of low-temperature plastic forming of titanium alloy.

2 Experimental research

2.1 Experimental Materials and Processes

The material used in the experiment is titanium alloy rods obtained through rolling, which are made by wire cutting Φ 8X12 sample. The temperature control system of the Gleeble-1500 experimental machine was used to conduct experiments on the thermal coarsening of TC4 at three different temperature ranges. The compression process was a free cooling process. When the termination temperature was reached, deformation stopped and water quenching was immediately carried out to freeze its structure, ensuring the purity of the dynamically crystallized grains. The specific experimental parameters are shown in Table 1.

2.2 Analysis of experimental results

It is the stress-strain curve of TC4 compression at different temperatures under the same strain rate conditions. From the graph, it can be seen that the higher the deformation temperature, the smaller the flow stress. When the starting temperature of deformation drops from 700 ℃ to 600 ℃, the peak flow stress increases by about 100Mpa. When it drops to 500 ℃, the peak flow stress increases by about 200Mpa. This is mainly because as the deformation temperature increases, the thermal activation effect of the alloy is enhanced, and dynamic recrystallization can proceed. As the volume fraction of recrystallization increases, the dislocation density decreases, and the flow stress of the alloy decreases, indicating that TC4 is a temperature sensitive material. On the contrary, the lower the deformation temperature, the more difficult it is for dynamic recrystallization to occur, and the flow stress of the alloy increases.

At the same time, the experimental results also showed that as the temperature decreased, the plastic deformation ability of TC4 also decreased. The deformation rate of the 600-500 ℃ compression test did not reach 50%, but 41.6%, and the deformation rate of the 500-400 ℃ compression test was only 33.3%.

Here are photos of the microstructure before and after deformation, with Figure 2a showing the microstructure before deformation, and Figures 2b, 3c, and 3d showing the deformed microstructure at 700 ℃, 600 ℃, and 500 ℃, respectively. From the graph, it can be seen that the original grain structure has a narrow and elongated shape, with an average grain length along the rolling direction of about 80 microns. After deformation, the grains gradually transform from elongated to equiaxed, and the grain boundaries gradually decrease, which is mainly caused by dynamic recrystallization. At the same time, it can be seen from the deformed microstructure that there is not a significant difference in grain size between 700 ℃ and 600 ℃ after deformation. In addition to equiaxed grains, there are also some original flat and narrow grains, while there are more equiaxed grains in the 500 ℃ deformed microstructure. Due to the difficulty of dynamic recrystallization at lower temperatures, it indicates that TC4 alloy may have some other grain refinement mechanism during deformation at medium and low temperatures.

3 Conclusion

Although TC4 alloy is not easily deformed at low temperatures, it is possible to obtain finer and more evenly distributed equiaxed grains after deformation compared to medium and high temperature deformation.

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