Research on the Plastic Forming Process of Thick-Walled Titanium Three-way Connector

Thick-walled titanium alloy pipe parts have been widely used in aerospace and other fields due to their high specific gravity, excellent corrosion resistance and fatigue resistance. The thick-walled titanium alloy pipe components obtained through plastic forming processes have good plasticity and high strength (such as extrusion, spinning, and drawing), and have become the main method for processing titanium alloy pipe parts and titanium elbows.

The analysis of the plastic deformation behavior of pipe materials is the prerequisite and foundation for ensuring the accurate plastic forming of pipe materials. The reliability of the deformation analysis often depends on the mechanical properties of the material during deformation, especially the plastic stress-strain relationship. Since the plastic stress-strain relationship of the material is related to the stress state it is subjected to, therefore, the appropriate test method should be selected according to the stress state of the material during the specific forming process to confirm the plastic parameters of the material.

For the plastic forming processes of thick-walled titanium tubes that mainly involve compressive deformation, such as spinning and extrusion, it is necessary to determine the stress-strain relationship of the material in the compressed state. However, due to the hollow structure of the tube, the traditional axial compression test method for cylindrical specimens is difficult to be used to confirm the compressive mechanical properties of the tube. Therefore, how to accurately determine the stress-strain relationship of thick-walled titanium tubes in the compressed state has become a key issue for accurately analyzing the plastic deformation behavior of thick-walled titanium elbows. Stress-strain relationship. Among them, the local cut block compression specimen is directly cut from the pipe wall, which is affected greatly by the pipe wall thickness and is prone to instability during the compression process. The arc-shaped stacked specimen is suitable for thin-walled pipe materials, and its principle is the same as that of the cut block specimen. Different from the cut block and stacking compression tests, the axial compression test of the overall ring specimen has better stability and is closer to the actual stress state during the plastic forming process of the pipe material, and has been widely applied.

However, due to the influence of friction, the overall annular specimen undergoes non-uniform deformation radially during the compression process, resulting in a bulging phenomenon. The hollow structure of the pipe material makes it difficult to perform the shape correction of the specimen to eliminate the bulge. Therefore, when using this test method, only the compressive stress-strain relationship of the material within a small strain range before the bulge occurs can be obtained. After the bulge occurs, the calculated stress and strain data differ significantly from the actual values. And the plastic forming of the pipe material generally belongs to a large deformation process, requiring a stress-strain relationship curve within a large strain range.

In response to the above issues, some scholars proposed to confirm the stress-strain relationship of the material through an inverse method that combines experiments with analytical formulas (or finite elements) and optimization algorithms. The essence of the inverse method is to obtain the failure parameters of 5052 aluminum alloy through experiments, combined with uniaxial tensile tests and numerical simulations.

By reverse calculation, the strength coefficient and the hardening index of strain in the titanium three-way pipe strengthening equation were confirmed. This method overly relies on assumed conditions when establishing the analytical relationship between material parameters and force-displacement curves, thus the accuracy of its analytical expression has a significant impact on the identification accuracy of material parameters.