Research On Magnetorheological Flexible-die Forming Technology And Forming Limit For Aluminium 1060 Sheet

Sheet flexible-die forming is a fast developing branch in sheet forming,which use gas,liquid,viscous medium or urethane rubber as a pressure-carrying medium,applying pressure on the sheet to obtain the desired shape.It has certain advantages in forming parts with complex shapes.For different forming methods,flexible-die determines the suitable material property and structural characteristics of formed parts.For a forming method,the property of flexible-die has a large impact on parts quality.At different forming stages,stress states of sheet metal should match different flexible-die material properties.In traditional sheet flexible-die forming,property of flexible-die can not be changed.If it can be varied to accommodate the sheet stress state during the forming process,it will benefit the deformation and improve sheet formability.To solve the above problem,a kind of smart material-magnetorheological(MR)fluid is used as s flexible-die in this paper.Its property and force transmission characteristics can be changed by applying magnetic fields,which can control the loading paths in forming process and improve sheet formability and parts quality.The sheet magnetorheological flexible-die forming broadens the range of flexible-die material.The effective adjustment of the flexible-die properties during the forming process can be achieved,which provides a new way for intelligent control of sheet flexible-die forming.Firstly,sheet magnetorheological flexible-die forming is proposed,which use MR fluid as a new flexible-die.By adjusting the applied magnetic fields adjustment of flexible-die property can be achieved.An experimental device including magnetic field generating device,force loading system and data acquisition and analysis system is selfdesigned,which can meet the experimental requirement.By using squeezing test of MR fluid combined with FEM analysis,the constitutive relationship of MR fluid is obtained,which is suitable for flexible-die forming.In squeeze test of MR fluid,the experimental force increases with increasing magnetic flux density and the magnetic particle content.The constitutive relationship of MR fluid transform from linear relationship to power law relationship.For the same strain rate and applied magnetic field,the stress of MR fluid with high magnetic particle content can increase more than 10 times.Secondly,sheet magnetorheological flexible-die forming of Al1060 sheet is conducted under constant magnetic fields.The effect of magnetic particle content in MR fluid and the external magnetic field conditions on load curve,the specimen configuration and strain distribution are studied.The results showed that when the magnetic particle content and the applied magnetic field strength are larger,the bulging force under the same experimental condition is greater.When the content of the magnetic particles is small,strain and the maximum height of the bulging specimen increase with increasing magnetic field strength.For higher magnetic particle content,strain and the maximum bulging height of the specimens first increase and then decrease.Through finite element analysis,the velocity and stress distribution of MR fluid under different magnetic fields are obtained,which further explained the effect of different MR fluid properties on the configuration and strain distribution of specimen.Changing loading speed is a traditional method to adjust the loading path.This paper presents a new way to adjust the pressure by changing the property of MR fluid(constitutive relation curve).In this way,the dual control of the loading path can be achieved.After adjusting the external magnetic fields,the mechanical properties of MRF changes rapidly,which affects friction lubrication conditions and force transmission,resulting in different stress states of the sheet.Magnetorheological flexible-die forming of Al1060-O sheet is conducted under changing magnetic field.By varying the applied magnetic fields,it can change the loading paths effectively.Using a MR fluid with higher magnetic particle content,the effect of magnetic fields on load curves is more pronounced.Compared with changing loading path in earlier stage of bulging process,changing magnetic field in the latter stage has a greater effect on loading paths.Load path change has an important impact on the specimen configuration and strain distribution.When using a 43%vol.MR fluid and applying magnetic flux density of 0.13 T at the latter stage of bulging process,the maximum bulging height of specimen with a thickness of 0.42 mm increases 5.18% than the case of applying a magnetic flux density of 0.13 T in earlier stage,and the maximum thickness strain improved 8.48%.Finally,magnetorheological flexible-die forming of Al1060-O sheet with two kinds of thickness were conducted using different elliptic dies under constant and changing magnetic fields,and the forming limit diagrams of Al1060-O sheet under different experimental conditions are obtained.Considering the effect of magnetic fields,FLDs which can intuitively reflecting the influence of magnetic fields on sheet formability are established.There exists an optimal property of MR fluid,by which the sheet forming limit can be improved.When the magnetic particle content of MR fluid is low(eg.30%),the property of MR fluid under higher magnetic flux density helps to improve the sheet forming limit.When the magnetic particle content of MR fluid is high(eg.43%),the sheet forming limit can be improved when the applied magnetic field is small.But as the magnetic flux density increases to a certain extent,MR fluid changes into quasi-solid state,which is not good for the sheet formability.