Research On Hydroforming Of 5A02 Aluminum Alloy Curved Thin-walled Tube With Polygonal-sections

Aluminum alloy curved thin-walled tube with polygonal-sections is widely used in aircraft, aerospace and other areas as typical lightweight component, while having broad application prospects in automobile. Because of the structure characteristics such as three-dimensional curved axis, high diameter-thickness ratio and the material characteristics like low elongation and Young's modulus, large friction coefficient compared with steel, the hydroforming of aluminum alloy tube is more difficult than that of steel tube. So, the forming regularity and deformation mechanism need to be studied further.In this dissertation, for 5A02 aluminum alloy curved thin-walled tube with polygonal-sections, the filling behavior of corner and regularity of plastic deformation in corner zone in hydroforming of rectangular section tubes were analyzed, thin-walled tubes were formed by hydro-bending, and hydroforming process of curved thin-walled tube with polygonal-sections was studied.With the combining of mechanics analysis and incremental constitutive relation, theoretical model about the relationship between limiting corner radius and friction coefficient was established, and the mechanism of limiting corner radius was revealed. With low friction coefficient and short straight zone, small limiting corner radius is achieved.With experimental device, quantitative relationship between corner radius and internal pressure was gotten during corner filling in hydroforming of rectangular section tube, the effects of three lubricants on corner forming internal pressure and limiting corner radius were analyzed, and the theoretical value of corner forming internal pressure was verified. It is shown that lubrication conditions have significant influence on corner filling, and good lubrication condition makes the internal pressure lower and the limiting corner radius smaller. For 5A02 aluminum alloy tube lubricated by polyethylene film, the relative corner radius is up to 2.2.In order to illustrate the complex three-direction stress state at the late stage of corner filling in hydroforming of rectangular section tube better, the filling behavior of corner was simulated by DEFORM-2D. The effects of friction coefficient, expansion ratio, and shape of preform on corner filling were discussed. And three-dimensional representation of stress state of corner zone and stress locus on yield cylinder revealed variation regularity of stress state. Three-dimensional representation of stress state demonstrates the strain state of corner zone is elongation in hoop direction and shortage in thickness direction. Then the transition point enters plastic zone firstly and its equivalent strain is always the greatest. Stress locus on yield cylinder of typical points inside shows, with increasing of internal pressure, the axial stress of the transition point is always tensile stress.Hydro-bending is proposed to resolve wrinkling inside, which is a main defect of aluminum alloy thin-walled tube bending. According to the theoretical value of internal pressure, the effects of internal pressure on wrinkling inside, cross-section distortion and thinning outside were studied by experiment, and 5A02 aluminum alloy bending parts whose diameter-thickness ratio is up to 63 were gotten. And then sound preform was provided for further hydroforming. The strain state of bending inside and mechanism of wrinkling were obtained by numerical simulation. The axial tensile stress provided by the internal pressure acting on the closed ends reduces the axial compressive stress value. When the internal pressure is greater than the critical value, wrinkling inside can be avoided.The hydroforming process of curved thin-walled tube with polygonal-sections was studied by experiment, and sound specimens were gotten by hydro-bending, preforming and hydroforming. The whole process of hydro-bending, preforming and hydroforming was studied, the regularity of wall thickness distribution and the process of contacting die of middle cross-section that is the most difficult to form were provided, and the mechanism of splitting outside was analyzed by multi-stage numerical simulation. The place that is seriously thinned after bending contacts die last in the following hydroforming. Then local deformation inhomogeneity and excessive strain lead to excessive thinning until splitting.