Research On Formability And Microstructure Of Aa6061 Tubes At Elevated Temperatures

Aluminum alloy parts with irregular cross-sections are applied more and more in the automotive and aircraft. However, poor formability at room temperature of aluminum alloy restricts seriously the application of aluminum alloy light weight parts. Recently, hydroforming at elevated temperature which has been developed for the forming of light-weight materials is an effective way to solve the problem.In this paper, uniaxial tensile tests are conducted to investigate the mechanical properties of AA6061-O at elevated temperatures. The relationship between flow stress and deformation temperature as well as strain rate is analyzed. The deformation constitutive equation at elevated temperature is established by Fields & Backofen model. The results show that the flow stress gets larger with decreasing temperature and increasing strain rate; the deformation behavior of aluminum alloy at elevated temperatures can be described by the formula. The fracture surface after tensile tests at different temperatures are analyzed. Ductile porous aggregate fracture can be seen at different temperatures, and a hybrid tough and brittle fracture character appears at 550℃.To evaluate the formability of AA6061 extruded tubes at elevated temperatures, hydro-bulge test is carried out at different temperature from 350℃up to 500℃. Bursting pressure and maximum expansion ratio (MER) of the tube are obtained. The fracture surface after bursting are analyzed and compared with tensile fracture. Results show that: with increasing temperature, bursting pressure is monotone decreasing; the MER is first increasing and then decreasing, and the maximum can be obtained at 425℃. The number of dimples in fracture surface became larger, and dimple got bigger and deeper with increasing temperature. At 500℃, burning occurs with some oxides appear on fracture surface.The microstructure along axial and hoop direction are analyzed by EBSD before and after hydro-bulge test. The hardness of tubes is also tested. The results show that: in the hot bulging process, the initial fine equiaxed grains are obviously brought up; as the temperature increases, grain orientation becomes more and more obvious, which is elongated along the deformation direction, and fibrous tissue appears at 450℃; recrystallized grains at crystal boundaries grow and is elongated as the temperature increases; a lot of unevenly distributed sub-structure is produced in grains because of deformation. After bulging, the hardness of deformed parts are higher than that of nearly the tube ends along axial direction, and the hardness of nearly fracture part is higher than that far from the fracture. At different test temperature, the hardness is almost unchanged.By axial tensile test and hydro-bulge test, formability of AA061-F extruded tube is studied preliminarily. The results show that: strength of AA061-F extruded tube improves after deformation, but it has almost no plastic formability at room temperature. The thickness of tube is also very uneven. But as the test temperature increases, formability of AA061-F extruded tube is significantly improves, the MER is more than 60% at 500℃.