| In the modern industry, one of the key concerns is to promote the formability of lightweight sheet metal. The stress state during the deformation process is the most crucial factor affecting formability. The scheme of applying hydrostatic stress has been successfully adopted in many manufacturing techniques. It is well known that the high hydrostatic pressure tends to reduce the tensile loads acting on microstructure of sheet metal, suppresses void growth, thereby delaying fracture. So the transverse normal pressure is expected to play a significant role in the formability of sheet metal. Viscous Pressure Forming (VPF) technology is a recently developed sheet metal flexible-die forming technology, which uses a highly viscous but flowable semi-solid medium as the soft die. Due to the flowability of the viscous medium, the sheet metal can be deformed under pressure entirely during the whole forming process. Besides, the viscous medium can be introduced in both sides of the sheet. So VPF provides a possible way to make full use of the promotion of hydrostatic stress on the formability of sheet metal. In this paper, the double-sided viscous pressure bulging of sheet metal is proposed. Several key problems met in practical application of double-sided viscous pressure forming are deeply and systematically studied. So as to lay a foundation for the further development and application of double-sided pressure forming technique.Firstly, based on the stress space forming limits ( FLDσ), the concept of formability increase factor,Φ(the ratio of forming limit strain under triaxial stress condition to that of plane stress condition) is given out, which possesses the guiding significance for test scheme design of sheet metal double-sided pressure forming. According to the practical limitations to the influence of through-thickness normal stress on sheet metal formability, the theoretical analysis of sheet metal deformation during the double-sided pressure bulging process is carried out, and an analytical approximation for normal pressure loading path that secures out-of-plane stress ratio,γ, continuously increasing is presented. The results show that the required lateral pressures are closely related to the strain hardening curve of material. Hereby, two kinds of regulating pressure bulging test scheme are proposed——viscous pressure bulging of overlapping sheets and double-sided viscous pressure bulging.Secondly, Al1060 is chosen as the material investigated, the overlapping sheet is austenitic stainless steel 1Cr18Ni9Ti. The VPB test of single Al1060, overlapped Al1060 with lubrication and overlapped Al1060 without lubrication were carried out. Based on the pressure range determined in VPB of overlapping sheet metal, the double-sided viscous pressure bulging test of Al1060 under different constant counter pressure (0MPa, 5MPa, 10MPa and 25Mpa) were also carried out. The effects of different transverse normal pressure loading mode and interface contacting condition on sheet metal limit dome height, geometry and strain distribution are analyzed experimentally.Subsequently, the FEM analysis of double-sided pressure bulging of sheet metal was carried out. It is shown that the magnitude and distribution of transverse normal pressure are dependent on the overlapping sheet metal strain hardening exponent n-, strength coefficient K- and thickness t- values. So the VPB test of overlapping sheet metal provides a flexible way to study the effect of transverse normal pressure loading mode on the deformation behavior of sheet metal. Beside the effect of interface friction of the overlapping sheet metals is also studied. In the double-sided viscous pressure bulging, the effect of transverse normal pressure magnitude, loading velocity, the viscous adhesive stress and viscous media properties on sheet metal bulging are all discussed, and emphatically analyzes the cause and influence factors of non-uniform of viscous media pressure.To clarify the effect of double-sided pressure on failure mechanism and microstructure evolution of aluminum sheets, the fractured surfaces of the single and overlapped aluminum sheets were observed using scanning electron microscope (SEM), and microstructures were studied by optical microscope (OM) and transmission electron microscopy (TEM). The results show that during the double-sided pressure bulging process, the void growth and conjunction in thickness direction is restrained by compressive normal stress, the fracture is delayed, and so large strain is permissible. When reaching a critical deformation degree, the stored energy of Al1060 releases through recovery, which reduces the dislocation density along grain boundary and the internal and restrains the microcrack formation conversely. The deformed microstructure under double-sided pressure is improved and planar anisotropy of mechanical property is also weakened.Finally, the average void radius and void volume fraction at the fracture surfaces under different deformation condition are calculated using the existing modes. Through the comparison of calculation results and SEM observation, the requirement for improvement of void growth model of sheet metal under triaxial stress state is discussed. The stress triaxiality region of Al1060 is divided according to the different ductile fracture mode. The limiting failure curve in ( )εf ?σmσspace is constructed by the combination of the experiment and finite element simulation, and the proper damage factor is also given out. The forming limit of sheet metal during the double-sided pressure bulging test is predicted by this model, and compared with the experimental results. The predicted limit dome height and fracture position are in good agreement with the measured ones. |
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