The THF compression test for friction estimation: study on the influence of the tube material

In the Tube Hydroforming (THF) process, a tube, placed in closed dies, is expanded by a high pressure liquid and two punches push its edges in order to feed the material into the expansion zones. Because of the high pressure and the contact area involved in the process, high friction stresses act on the tube walls restricting the material flow so reducing the amount of fed material, the part formability and affecting the soundness of the final part and its geometry. In fact, previous studies showed that the lower the friction coefficient at the tube-die interface, the more uniform the friction distribution and, therefore, the more uniform the stresses acting on the tube walls. The tube deformation being dependent on the stresses, its final thickness is influenced by friction. Starting from this premise, the authors proposed a reference test which is able to highlight the effect of friction on the final tube thickness. In this test, namely the THF Compression Test, a tube is placed in a cylindrical die having the same diameter as the outside of the tube, it is pressurized and then compressed by the punches. In this way, the tube has no expansion and its final thickness depends on the process parameters (tube material, pressure, punch stroke, tube material and geometry). Using FE simulations, it is possible to express the friction coefficient as a function of the process conditions and to use it in combination with experimental results. In the present paper, the previously validated FE model is used to investigate the influence of the tube material on the compression test results. Therefore, a simulation study was performed using different values of strength and hardening coefficients showing how the method is affected by the tested material thereby giving further indication of the test sensitivity.