| Nowadays,with the continued technological advances,traditional sheet metal forming processes face more and more challenges in both forming quality control and forming cost control.These challenges are especially serious for extreme manufacturing areas,such as aviation and aerospace industries.In aviation and aerospace industries,there are lots of sheet metal parts with characteristics of large-size,thin-walled,deep-cavity,complicated curved-surface.The increasing forming cost and the difficult forming quality control in manufacturing of these sheet metal parts nearly pushes the traditional sheet metal forming processes to their limits.Lots of researches indicated that,electromagnetic forming(EMF)—as one of high energy rate and high speed forming process shaping sheet metal with high pulsed Lorentz force—has lots of advantages over the traditional quasi-static forming process,such as enhanced forming flexibility,improved forming limit,inhibited wrinkling,reduced spring-back,and effectively lowered forming cost.Therefore,EMF is one of the most promising forming technologies that can break through the abovementioned challenges.However,the high complexity of the EMF process leads to the lack of effective design criterion for EMF;meanwhile,the high electromagnetic and mechanical loadings in EMF process put forward high requirements on the forming facility.These two reasons significantly limit the widespread applications of EMF in sheet metal forming.In order to overcome these obstacles,this dissertation proposed a multi-space-time high pulsed magnetic field based electromagnetically sheet metal forming method.From methodology aspect,the new method introduces a multi-stage(in time dimension)and multi-direction(in space dimension)pulsed Lorentz force to flexibly alter the deformation behavior of the workpiece,thus accurately controlling the forming quality.And from technological realization aspect,high magnetic field pulsed magnet and high energy pulsed power source technologies were introduced to develop high performance EMF system with multiple driving coils and multiple power sources,thus producing the multi-stage and multi-direction pulsed Lorentz force.Based on this method,the dissertation systematically investigates the design and implementation of the multi-stage and multi-direction pulsed Lorentz force,and the mechanism of the multi-stage and multi-direction force on driving and altering the workpiece deformation behavior.These investigations establish key foundations to realize the manufacturing of sheet metal parts with extremely large size and complicated shape.Firstly,the dissertation developed a circuit-magnetic-structure coupled numerical model of EMF.This model is the foundation to analyze the complicated dynamic deformation behavior driven by multi-space-time magnetic field,and is also the key to optimize the EMF system.In this coupled model,the electromagnetic model was realized based on equivalent circuit method.The structure model was realized with finite element software ANSYS.And the coupling between the electromagnetic and structure models was realized with a data interface program.Compared with most of the existing EMF numerical models which simulate electromagnetic field with finite element method,the proposed model need not to update the air mesh.As consequences,the simulation is more effective;and more importantly,this enables the structure model to handle the high speed impact between the workpiece and the die,conveniently and effectively.Therefore,the proposed model can simulate the complicated dynamic deformation behavior of the workpiece-die system more effectively and more accurately.Secondly,aim at the state of art that EMF is hard to form sheet metal part with complex shape,due to the onefold spatial distribution of the generated Lorentz force,the dissertation proposed the process of axial-radial Lorentz force driven EMF method with a dual-coil EMF system.Different to the traditional EMF,which can be regarded as axial Lorentz force driven EMF,the proposed method introduced an additional radial inward Lorentz force on the sheet flange to enhance the plastic flow of the sheet flange,therefore introducing the deformation mode of deep drawing.In this dissertation,theoretical analysis,experimental investigations,and numerical simulations were conducted to prove the feasibility of the method,to demonstrate the flexibility of the method on altering the deformation behaviors of the workpiece,and to reveal the underlying mechanism of the radial and axial Lorentz forces on driving the changes of the deformation behaviors.Thirdly,aim at the state of art that EMF is hard to shape large size sheet parts,due to the limitations of the forming equipment and other reasons,the dissertation proposed a light-weight and flexible EMF method capable of shaping large size sheet metal.In this study,this method was applied to form an aluminum alloy sheet with a diameter of 1378 mm into partial-ellipsoid shape.The numerical simulations were carried out to optimize the forming coil system.The design and implementation of the forming facility was discussed in detail.The fabricated forming facility is very light-weight,the characteristic length of which is 1840 mm,only 1.34 times of the workpiece diameter,which is much lighter than those of traditional quasi-static forming equipment.An experiment was performed to evaluate the forming quality obtained by the proposed facility.The workpiece to be formed was with thickness of 3.945 mm,the manufacturing of which is very difficult with traditional forming process,due to the thickness-diameter ratio is lower than 0.3%.The experimental results show sound forming quality.The maximum thinning over the ellipsoid surface of the formed sheet was only about 9.5%.The maximum deviation between the deformed sheet and the die is about 4.2 mm.The results show the great advantages of the proposed method on improving the forming quality in large-scale sheet metal forming,providing a new approach for integrated manufacturing of large-scale light-weight alloy sheet parts in aviation and aerospace industries. |
PDF Full Text Request