Study On The Deformation Mechanism Of Incremental Metal Sheet Forming

As a ‘3D printing’ method in sheet forming field,incremental forming of sheet metal is an advanced flexible technique for rapid thin-walled prototype production.Incremental sheet forming(ISF)uses a standard tool to make sheet metal blank deform into various shapes.Due to the characteristics of local and incremental deformation,ISF can enhance the formability of sheet metal to get complex shapes which usually cannot be produced by conventional stamping process.Therefore,ISF can substitute traditional process in the low-volume or customized production,especially in the new model development of automotive body-in-white,medical customization and aerospace field.At present,the study of incremental sheet forming focuses on its process optimization and industrial application.Few researches on the deformation mechanism of incremental sheet forming have been conducted.In order to reveal the forming mechanism of ISF in micro scale level,advanced microstructural characterization techniques are used to comprehensively study the microstructural evolution of incremental sheet forming parts and test their regular mechanical properties in this paper.This paper gratefully acknowledges the support from the National Natural Science Foundation of China(51675332)and National Natural Science Foundation of China-China Aerospace Science and Technology Corporation Aerospace Advanced Manufacturing Technology Research Joint Funding Project.Based on the microstructure and mechanical results,this paper presents the grain refinement mechanism and the dislocation structure model.The main research contents are as follows:1.Explored the grain size change of iron sheets during multi-pass incremental sheet forming by electron backscattered diffraction technique in scanning electron microscope.By analyzing the changes of grain size during different passes,it was found that the grain size was more refined with increasing ISF passes: the original grains of 20μm were refined to 1.2μm after seven passes.At the same time,there was a gradient distribution of grain size in the thickness direction due to the frictional contact between the forming tool and sheet.2.According to the characteristics of ISF,the key process parameters such as single point incremental forming/two points incremental forming(SPIF/TPIF)and forming angle were selected to be studied by analyzing the microstructure evolution of ISF for AA1100 and DC05 sheets.The experimental results showed that compared with SPIF,the microstructure of TPIF was more uniform;the forming angle of ISF was larger,the grain size was smaller.The original grains of aluminum in 11.5μm were refined to 1.5μm after one ISF pass.3.The samples were taken from AA1100 and DC05 pyramidal ISF parts for tensile test at room temperature and microhardness test.The mechanical test results showed that the tensile strength of ISF parts increased and the elongation decreased,which proved to be work hardening.Compared with SPIF,the working hardening effect of TPIF was stronger.4.Transmission electron microscopy(TEM)was used to investigate microstructural features of multi-pass ISF parts.TEM images showed that dislocations clustering and high density dislocation walls contributed to the grain refinement.Considering ‘repeated forming’ and ‘local forming’ of ISF finite element simulation,a dislocation structure model was proposed to explain the process from dislocation bundle to sub-structure.