Research On Fracture Limits In Electrically-assisted Double Side Multi-ponit Incremental Sheet Forming Process For Aluminum Alloy Sheet

Many parts in the aerospace,automotive and other fields have special-shaped components and local structures.The traditional sheet metal forming process has a long manufacturing cycle and high trial production costs,so it is difficult to adapt to the requirements of irregular structures,small batches,and trial production.Double side multi-point increnmental sheet forming process combines local and progressive forming features with the advantages such as high forming precision,high production flexibility,and high fracture limit,so it is suitable to form complex parts.In this paper electrically-assisted forming was introduced,and the electrically-assisted double side multi-ponit incremental sheet forming(E-DMISF)process was proposed to further improve the formability based on the electroplastic effect.However,the E-DMISF process is a complicated electro-thermal-structure multi-field coupling process,the rheology of the material in the forming process is very complicated.So it is urgent to discuss the material formability and fracture limit under the E-DMISF process conditions.The aim is to achieve a reasonable process path planning of the E-DMISF process in order to guide the forming process design for thin-walled components.In this research,the finite element simulation model and experimental platform for E-DMISF process of 2024-T3 aluminum alloy were established.The hyperbolic truncated cone and hyperbolic truncated pyramid were chosen as study objects and the stress triaxiality was selected as the characteristic function of ductile fracture criterion.Based on all above,the fracture limit experiments and theoretical studies in E-DMISF process were performed.The influence of the electric field parameters on the fracture limit and the internal mechanism of the increase of the fracture limit of E-DMISF process were revealed.The main work includes:The stress analysis of the material deformation zone in the E-DMISF process was performed,and the specific expression of the stress triaxiality is derived from the function of the Hertz contact theory and the pulse current influence,which is the function of the current density,forming force and equivalent strain and other process parameters.The "electric-thermal-structure" multi-field coupling simulation modeling method for E-DMISF process was studied based on ABAQUS software.Hyperbolic truncated cone and hyperbolic truncated pyramid were chosen as the study objects.According to the simulation result,the equivalent strain,at which the material enters and leaves the compression zone from the tensile zone during the forming process was determined.The E-DMISF process experimental platform was designed the fracture limit experiments of the E-DMISF process were carried out based on the hyperbolic truncated cone and hyperbolic truncated pyramid.The forming force,the maximum forming angle and maximum forming height of the material under different process conditions were obtained.The fracture limit experimental curves under different current parameters were analyzed,and the fitting equation about the fracture limit of E-DMISF process was proposed.The stress triaxiality curves under different process conditions were analyzed.The fracture criterion using stress triaxiality as the characteristic function in the E-DMISF process was established.Through experimental and theoretical analysis,it was found that the introduction of current can indeed increase the fracture limit,and the influence law of the current on the fracture limit in E-DMISF process was obtained and verified.