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Study on plastic forming technology of thick wall titanium tee

Thick wall titanium alloy tube parts have been widely used in aerospace and other fields due to their strong weight ratio, excellent corrosion resistance and fatigue resistance. The thick-walled titanium alloy pipe fittings obtained by plastic forming process have the characteristics of good plasticity and high strength (such as extrusion, spinning and drawing), and have become the main method for processing titanium tee of titanium alloy pipe parts.

The analysis of plastic deformation behavior of pipe is the premise and foundation to ensure the accurate plastic forming of pipe, and the reliability of deformation analysis often depends on the mechanical properties of the material during deformation, especially the plastic stress-strain relationship. Because the plastic stress-strain relationship of materials is related to the stress state, it is necessary to select suitable test method according to the stress state of materials in the concrete forming process to confirm the plastic parameters of materials.

For the plastic forming process of thick-walled titanium tube, such as spinning and extrusion, it is necessary to confirm the stress-strain relationship under compression. However, due to the hollow structure of pipe, the traditional axial compression test method of cylindrical specimen is difficult to confirm the compressive mechanical properties of pipe. Therefore, how to accurately confirm the stress-strain relationship of thick-walled titanium pipe under compression has become a key problem to accurately analyze the plastic deformation behavior of thick-walled titanium tee.

Force strain relationship. The local cut compression sample is directly cut from the pipe wall, which is greatly affected by the wall thickness of the pipe, and is easy to lose stability in the compression process. The arc stacked sample is suitable for thin-wall pipe, and its principle is the same as the cutting sample. The axial compression test of integral annular specimen is more stable than the compression test of block cutting and stack, and is more close to the real stress state in the plastic forming process of pipe, so it has been widely used.

However, under the influence of friction, the integral annular specimen will have uneven deformation along the radial direction during the compression process, resulting in bulging phenomenon. The hollow structure of pipe makes it difficult to remove the shape of the sample. Therefore, when using this test method, only the compressive stress-strain relationship of materials with small strain range before bulging occurs can be obtained. After bulging occurs, the calculated stress and strain data differ greatly from the actual value. However, the plastic forming of pipe generally belongs to the process of large deformation, which requires the stress-strain relationship curve of large strain range.

In view of the above problems, some scholars proposed to confirm the stress-strain relationship of materials by combining test with analytical formula (or finite element method) and optimization algorithm. The essence of the inverse method is to calculate the failure parameters of 5052 aluminum alloy material by using unidirectional tensile test and numerical simulation.

The strength coefficient and strain hardening index in the strengthening equation of titanium tee pipe are confirmed by inverse calculation. The method adopts too many assumptions in the process of establishing the analytical relationship between material parameters and force-displacement curves, so the accuracy of its analytical representation has a great impact on the identification accuracy of material parameters.