Basic Research Of Sheet Metal Precision Forming Technology Based On Subregional Flexible-die Control

The flexible-die forming technology of sheet metal has certain advantages in the forming of complex thin-wall components in many varieties and small batches,because of its advantages of low die processing cost,good filling ability,high surface quality of parts and small rebound.However,the properties of traditional force transfer media(such as water,compressed gas,hydraulic oil,viscous media,etc.)remain the same throughout the forming area,so it is difficult to meet the differentiated demands of flexible-die performance of complex thin-walled parts in different deformation areas.In order to solve these problems,this paper proposed the sheet metal precision forming technology based on subregional flexible mode control.Magnetorheological fluid was used as the force transfer medium,and the force transfer performance of magnetorheological fluid flexible mode in different regions was adjusted by designing and regulating magnetic field distribution,so as to promote the uniform flow and coordinated deformation of sheet metal.Firstly,a method was proposed to establish and regulate the subregional magnetic field in the forming process by the combination of iron core and coil.The feasibility of establishing subregional magnetic field was verified by the combination of experimental measurement and ANSYS finite element simulation.On this basis,the distribution and law of the magnetic field in the bulging process and step forming process were studied.The results showed that the gradient controllable distribution of the magnetic field in the forming area could be realized by adjusting the current and designing the geometry of the core.Two kinds of subregional magnetic fields,namely fringing-dominated magnetic field and central-dominated magnetic field,were realized in the forming process of bulging and step forming parts.For example,in the bulging process,when the current is 15A,the gradient magnetic field difference under the dominant magnetic field of the center reached 360mT,and the maximum magnetic flux density reached 706mT.The magnetic sensing lines were mainly concentrated in the upper and lower core.Secondly,the experiment and simulation of sheet bulging process under subregional flexible mode control were carried out,and the influence law of external magnetic field environment on loading curve,bulging part configuration,strain distribution and rupture form were studied.The results showed that with the change of current intensity under the fringing-dominated magnetic field,the loading curve changed more than that under the central-dominated magnetic field.There were great differences in the configurations of the bulge parts obtained under the two kinds of regional magnetic fields.The comparison of the parts configurations with ovality showed that the larger the ovality,the fuller the configuration would be,and the flatter the configuration would be.The ovality of the central dominant magnetic field remained at about 0.85,while the fringingdominated magnetic field of the edge decreased by 44.56%from 0.846 at 0 A to 0.469 at 15 A.Part configuration tended to be flat.With the increasing of the current intensity,the maximum strain value of the component was transferred from the central position to the transition area of the magnetic field,and the fracture position was corresponding to it,which changed from the central area to the transition area of the magnetic field.The maximum position of the fringingdominated magnetic field strain at the edge was directly transferred from the central area to the rounded corner of the die,and the fracture position was also directly transferred from the central position to the rounded corner of the die.The subregional magnetic field changed the stress-strain curves of the magnetorheological fluid in the central region and the edge region,and the properties of magnetorheological fluid and the friction relationship between magnetorheological fluid and plate in the two regions were changed,resulting in changes in the deformation and stress state of the plate,and two kinds of bulging parts were obtained.Finally,according to the results of bulging test,the forming test of step part was studied by using subregional magnetic field.Two kinds of loading methods were used in the test:the integral loading path and the variable loading path.The use of different subregional magnetic fields in the integral loading path would lead to the change of the main deformation area and the change of wall thickness distribution.The minimum wall thickness of the first fillet of the edge dominant magnetic field was 0.533mm,and the wall thickness of the point of the center dominant magnetic field was 0.78mm.Under the condition of edge dominant magnetic field,the third fillet wall thickness is maintained at 0.56mm,while under the condition of center dominant magnetic field,the thinnest wall thickness was 0.35mm.The local wall thickness decreased sharply under both magnetic fields.Using appropriate variable loading path could effectively improve the wall thickness thinning phenomenon at the above two positions and effectively improved the part formability.