Evaluating the flow stress of aerospace alloys for tube hydroforming process by free expansion testing

In order to obtain accurate tube hydroforming (THF) simulation results, one of the important inputs in the finite element model (FEM) of the process is the mechanical response of the material during THF. Generally, the mechanical response is defined by the stress–strain behavior that can be determined from tensile testing of the specimens extracted either from the sheet used for roll forming of the tubes or directly from the tubes. More recently, free expansion testing has been used to characterize the mechanical response of the material for hydroforming applications. The free expansion test can emulate process conditions similar to those found during THF, and as such, can be used to obtain reliable and accurate information on the mechanical response/properties of the tubular material. The aim of this research is to present an approach for evaluating the stress–strain behavior of different materials using a 3D deformation measurement system in conjunction with an analytical model. Here, to characterize the mechanical response of the materials, free expansion and tensile testing were used for austenitic stainless steel types 321 (SS 321) and 304L (SS 304L), INCONEL® alloy 718 (IN 718), and aluminum alloy 6061 in the annealed “0” temper condition (AA 6061-0). The mechanical response of each material, measured through free expansion testing of tubular forms, was compared to the respective stress–strain behavior determined from the uniaxial tensile test using ASTM E8 geometry specimens extracted from the tubes. For each material studied in this work, the two flow stress behaviors were distinct, indicating that the test method can have a noticeable effect on the mechanical response. Finite element analysis (FEA) of the free expansion of each material was also utilized to simulate the THF process with the flow stress curves obtained experimentally; the predicted expansion and burst pressure results were close to the experimental data indicating that the approach developed and described in this work has merit for characterizing the mechanical response of aerospace alloys for hydroforming applications.