Formability and microstructure of AA6061 Al alloy tube for hot metal gas forming at elevated temperature

Free bulging test was carried out at different temperatures ranging from 350 °C to 500 °C to evaluate the formability of AA6061 extruded tube, which can provide technology foundation for complex structures forming in hot metal gas forming (HMGF) process. Maximum expansion ratio (MER) and bursting pressure were obtained to evaluate directly the formability at heated conditions. Vickers hardness at different positions was measured. The fracture surface after bursting was observed with scanning electron microscope (SEM), and the microstructure change along axial and hoop directions was analyzed by electron backscattering diffraction (EBSD). The results show that the largest MER value is 86% at 425 °C. Bursting pressure decreases from 4.4 MPa to 1.5 MPa with temperature increasing. The Vickers hardness of fracture position is a little higher than other positions after gas bulging. The fracture mechanism is still the micro-pore aggregation fracture at elevated temperature, while overheated structure appears seriously at 500 °C. The initial fine equiaxial grain grows as temperature increases, which is elongated simultaneously in both axial and hoop directions.