Research On Cryogenic Incremental Forming Process And Microscopic Mechanism Of Aluminum Alloy Based On Strength/Plasticity Cooperative Increasing Effect

Aluminum alloy has the advantages of high specific strength/stiffness and good corrosion resistance.It is an important structural material for aerospace and automotive lightweight parts.However,aluminum alloy has poor plasticity at room temperature,which is difficult to meet the forming requirements of complex parts.Although the hot forming process can improve the forming plasticity,there are still problems such as uncontrollable forming quality,long process time and narrow process window due to the strength loss caused by the softening behavior at high temperature.At present,the excellent mechanical properties of aluminum alloys at cryogenic temperatures have attracted widespread attention.The deformation of materials at cryogenic temperatures is conducive to the homogenization of multi-system slip and dislocation distribution,and promotes the synergistic growth of strength and plasticity.In order to overcome the shortcomings of traditional forming processes,cryogenic sheet metal forming technology has emerged to meet current needs.This technology uses the double increase effect of aluminum alloy at cryogenic temperature to affect the sheet metal deformation mechanism,while taking into account the formability and high strength requirements of the parts,providing a new way for the forming of complex curved structural parts.This study combines cryogenic technology with incremental forming technology.Based on the independently developed cryogenic incremental forming experimental platform,the feasibility of the new process has been verified.By analyzing the macro deformation behavior and microstructure evolution of aluminum alloys,the deformation mechanism of aluminum alloys at cryogenic temperatures was revealed.The main contents and conclusions are as follows:(1)Taking 6061-T6 aging hardening aluminum alloy sheet as the research object,the experimental research of cryogenic incremental forming based on variable angle truncated cone model was carried out.The influence of forming temperature on the forming limit,forming force,microhardness and surface quality of the workpiece was investigated.The results show that the cryogenic incremental forming can significantly improve the formability of 6061-T6.When the temperature is reduced to 113K,the ultimate forming height of the specimen increases by 32.4%compared to 295K,and the specimen has greater deformation resistance and higher microhardness value.The deformation mechanism of 6061 aluminum alloy at different temperatures was revealed through microstructure characterization.The severe planar slip mode at 295K causes rapid concentration of dislocations at grain boundaries,resulting in deformation localization.At 113K,due to the suppression of local planar slip,the slip distance is shortened.A large number of dislocations are generated and uniformly distributed within the grain center,resulting in a significant improvement in the plasticity of the sheet.Texture analysis shows that the sheet texture changes along βorientation line at cryogenic temperature,and the {011}<211>Brass texture is evolved in the final forming stage,which is conducive to deformation and improves the formability of sheet.(2)Taking 2219 high strength aluminum alloy sheet as the research object,the effect of tempering conditions under different heat treatment processes on the formability of the sheet at cryogenic temperature was investigated.The effects of cryogenic incremental forming on the aging hardening effect,formability and microstructure of the specimens were investigated.The results show that the deformation behavior of high strength aluminum alloy 2219 at cryogenic temperature is sensitive to the microstructure of tempered under different conditions.The ductility of 2219-W sheet in solid solution state at cryogenic temperature is significantly improved,and the elongation of 2219-W sheet increases from 25.1%to 46.1%at 103K,with an increase of 83.7%.The elongation of 2219-T4 sheet in natural aged state at cryogenic temperature increased slightly,but it had higher strength,with the tensile strength of 737.8MP at 103K.By constructing the aging dynamics image,the influence of cryogenic incremental forming process on the aging hardening effect of the specimen is analyzed.The aging dynamics of 2219-WQ alloy at 113K is significantly improved.Through the microstructure characterization,the cryogenic deformation mechanism of high-strength aluminum alloy was further revealed from the perspective of slip behavior and dislocation evolution.