| With the development of the automobile and aerospace industries, increase in the complexity of the parts and emphasis on the low formability and expensive Lightweight materials further promotes the application of the sheet hydroforming. Based on the following equation of the cup body during the hydrodynamic deep drawing assisted by radial pressure, the prime reasons of improving the sheet formability were analyzied. Afterwards, a new method named the hydrodynamic deep drawing process with inward flowing liquid is proposed, considering the effect of friction between the punch and sheet, the effect of radial pressure loaded on the blank rim. After a preliminary validation of this new method, FEM was used to analysis the deep drawing process. Finally, a lid of gasoline tank of a helicopter was successfully made using this new method. The main conclusions can be outlined as follows:A new die was desined and manufactured to validate the proposed method and the charateristics of the pressures in the die cavity and around the blank rim were analyzed. The preliminary experiment validated the feasibility, stability, and easy controlling of the new proposed process. A cup of a drawing ratio of 2.85 was successfully formed using this new method, which is made by 2A12-O allumium alloy.FEM is a powerful tool to analyse the sheet hydroforming. Successful applicaton of FE simulation in sheet hydroforming depends on the right inputs of the material properties, constitutive equation, and boundanary conditions. At first, the material properties was measured by use of the uni-axial test; and then the constitutive equation was obtained based on the data come from the the uni-axial test; and last, the boudanary conditions for liquid pressure was studied in detail. How to load the boundary conditions of the liquid pressure on the bottom of the blank in the FEM was provided by the subroutine of Marc. The effects of pramaters such as cavity pressure, radial pressure, gap between blank holder and die on the process were studied, as well as the moment that the cacity pressure is up to the maximum. Under prerequisite conditions that the liquid could float the blank and the inward following of liquid is initiated, lower cavity pressure could gain thinner thickness distribution; and the radial pressure significantly influenced the forming process, higher radial pressure could prevent the balank from cracking. Just as gap between the blank holder and the die was concented, small gap would benefit to suppress the wrinkling. There are two-valley values on the thickness distribution, which are related with the two puch peak force and gained with smaller gap. Furthermore, the second valley point has some relation to the increasing fricion causing by the thickening of the flange between the die and the flange. It could affect the forming process when the maximum cravity pressure reached at certain maximum cavity pressure. The results shows that it is a better proper process that cavity pressure is up to the maxmum at the moment of the punch stroke h = r_P + r_D + t.The proper loading path of the cavity pressure is the curve of the pressure that can just float the blank from die corner during the hydrodynamic deep drawing with inward flowing liquid. The math analytical method of cavity pressure is provided according to the condition which could float th blank, and which could provide reference for processs design and production. A new method is proposed to determine the the optium loading path of cavity pressure by adaptive FE method combined with Marc subroutine.A new blank holder method with certain blank holder gap and hold force is proposed, and which could solve the thinning of the second valley point, and actually the blank holder method is a combination of BHG and BHF. The blank holder gap would increase with the flange thicking, which could improve the drawing ratio during hydrodynamic deep drawing process with inward flowing. Furthormore, the FEM was used to analyze the effect of the blank gap and blank hold force on the second valley point.Sheet hydrofoming with punch is used firstly to form the lid of gasoline tank of helicopter with conical wall and inclined flanges by experiment and FEM. The research shows that the formed part meets the design requirements, but one wave appears at the bottom of the cup wall because of the contact asynchronous of blank and the bottom surface of punch. Furthermore, this wave could not be eliminated by adjusting the loading curve.Another new method named soft-punch hydroforming with radially inward flowing liquid was propsed to solve problem mentioned above. The soft-punch hydroforming proecess of the part was carefully studied by FEM. The part could be succeffuly formed withoud the wave defect when pressure difference of the radial pressure and the cavity pressure is more than 15MPa. |
