Deformation Behavior of Sheet Metal in Multi-Point Forming by Adjusting Force-Displacement

Author:Jia Bin Bin

Supervisor:yuan shi jian wang wei wei


Degree Year:2018





The sheet metal parts in aviation,aerospace,shipbuilding and other industries have the characteristics of small batch and multiple varieties,and are suitable for multi-point die forming technology for production.In order to solve the problem of large springback and wrinkling inclination in traditional multi-point die forming,a new technology of multi-point forming by adjusting force-displacement is proposed.The core idea of the technology is to control the force and displacement separately,thus the plates are always deformed under normal constraints,so as to realize the precision forming of curved parts.In this paper,theoretical analysis,numerical simulation and experiment are used to study cylindrical parts and saddle parts.The laws of deformation and springback under different loading and displacement loading conditions are clarified,and the mechanism of wrinkle and springb ack suppression under the normal constraint condition is revealed.The aim is to provide theoretical guidance and technical support for the realization of multi-point forming by adjusting force-displacement for curved parts.Through the mechanical analysis of multi-point forming by adjusting force-displacement,the stress state of the plate in different forming stages is clarified,and the upper and lower limits of the normal binding force are given respectively.The upper limit is inversely proportional to the friction coefficient between the indenter and the plate,and the crack appears when the binding force is higher than the upper limit.The lower limit value is directly proportional to the thickness of the plate,and the defect occurs when the binding force is lower than the lower limit.The formula of the plate at bending moment under normal constraint condition is deduced,and the mechanism of normal constraint on reducing the springback is revealed.Under the action of tangential tensile stress,the tensile stress zone of the plate increases,the compressive stress zone decreases,and the neutral layer moves inward,which leads to the reduction of the elastic bending moment,thus significantly reducing the resilience of the plate.The deformation law of plates under two-way bending is obtained.Two-way bending makes the plate repeatedly subjected to tensile stress and compressive stress,which can significantly increase the plastic deformation of the inner and outer layers,and its distribution characteristics are smaller in the central area and larger in the marginal area.The critical wrinkling stress model of sheet metal is established by energy method,and the restraining mechanism of normal constraint on wrinkling is revealed.Due to the effect of normal constraint,the anti-buckling ability of sheet metal is obviously increased,and the critical wrinkling stress is proportional to the normal constraint force.The parameterized modeling of the numerical model was realized by the secondary development of the Python language.The numerical model of multi-point forming by adjusting force-displacement was established.The stress-strain state and deformation behavior of the cylindrical parts and saddle parts under different loading conditions were analyzed.The plates are subjected to piecewise and bidirectional repeated bending deformation rather than the whole bending deformation of traditional multi-point forming process,and the whole deformation is realized through the accumulation of local bi-directional bending deformation at different stages.Compared with the traditional multi point die forming,the elastic bending moment decreased by 17%,and the rebound amount was reduced significantly.Compared with the loading mode of the center forming after the first edge forming,the plastic deformation and the thickness distribution of the plate are more uniform in the loading mode of the edge forming after the center forming.Compared with the symmetrical arrangement of the indenter,the plastic deformation and thickness distribution of the asymmetrical arrangement of the indenter are more uniform.In the multi-point forming by adjusting force-displacement,the longitude direction and the latitudinal direction of the center part of the saddle piece is a tensile stress state,so there is no wrinkling defect.A multi-point forming by adjusting force-displacement experiment device is developed,and the defect mechanism under different binding conditions is studied by experiment.A binding loading forming window is established.The defects in the cracking area is caused by the step deformation of the plate between two columns.In the insufficient deformation area,there plates in the transition zone and the central area cannot be completely in contact with the upper punch,resulting in insufficient deformation of the sheet metal.Experimentally research of cylindrical parts in multi-point forming by adjusting force-displacement were studied.The law of wall thickness and springback distribution under different displacement loading ways and punch arrangement are obtained.The thickness of the plate shows the same distribution trend,that is,the numerical value in the center area is la rge,and the value in the edge area is small.With the increase of binding force,the thinning rate is also increased.The maximum thinning rate is only 2.2%,indicating that the parts produced by multi-point forming by adjusting force-displacement have good wall thickness distribution uniformity.For aluminum alloy and low carbon steel,the resilience decreased by 83.7% and 43.6% respectively compared with traditional multi-point forming.A springback compensation method based on sectional shape adjustment is established,and the springback compensation law of plates with different materials is obtained.Springback compensation can significantly improve the matching effect between workpiece and die,and improve the curvature distribution uniformity of workpiece.The wrinkling behavior of saddle parts in multi-point forming by adjusting force-displacement is experimentally studied,and the mechanism of wrinkle inhibition is revealed.When the saddle parts are deformed by the multi-point forming by adjusting force-displacement,the sheet is restrained by the normal force during the whole forming process,and there is no suspension zone,therefore the wrinkling phenomenon of the plate is greatly suppressed.When the normal force is lower than the lower limit,there is a defect of insufficient deformation.The experimental study on different arrangement methods of punch was carried out,and the deformation rules of saddle parts under different contact conditions were obtained.Compared with the symmetrical arrangement of the punch,the fold of the saddle part is significantly weakened when the punch is asymmetrically arranged.