Formability And Microstructure Evolution Of 7075 Aluminum Alloy Under Instantaneous Current And Electromagnetic Force

7075 aluminum alloy possesses the advantages of low density,high specific strength and good corrosion resistance.However,there are some associated limitations such as poor formability and high spring back at room temperature,which restrict its wide application in aerospace and other fields.Pulse current can reduce the deformation resistance of materials,improve the plastic deformation ability and reduce the spring back of parts,and is widely used in the field of metal forming.However,the loading time of pulse current in traditional electrically assisted forming process is tedious,and there are some relative challenges,such as high material temperature,low performance,and difficulty in design of current distribution on demand.Electromagnetic forming is a unique energy field manufacturing method which uses pulsed strong magnetic field to generate transient strong electromagnetic force and large current on the workpiece to drive high-speed deformation of the workpiece.It is believed that the inertia caused by high-speed deformation was the main factor to improve the forming limit of materials,but the influence of instantaneous current in the forming process on the properties and microstructure of materials was rarely studied.Considering 7075high-strength aluminum alloy sheet as the research object,this thesis proposes a new technology of applying induction or passing instantaneous current to deformed sheet and combining instantaneous current and electromagnetic force to drive high-speed deformation of workpiece.Theoretical analysis,multi-field coupling simulation,macro and micro experiments and parts manufacturing were adopted in this study.The single/combined effects of instantaneous current and electromagnetic force on mechanical properties,spring back control,rapid temperature rise and microstructure of 7075 high-strength aluminum alloy were studied,and the formability of 7075 high-strength aluminum alloy was significantly improved and spring back was eliminated.The originality of the work and conclusions of this thesis are as follows:(1)Considering the issue of poor plasticity of 7075 high-strength aluminum alloy at room temperature,an induced current processing device suitable for uni-directional tensile samples was designed,and a composite process of "solid solution-uniaxial pre-tensile-induced electropulsing treatment-artificial aging" was proposed.A coupling simulation model of electromagnetic field and temperature field was established in ANSYS software and the current density and temperature distribution on the samples were analyzed.The results showed that the yield and tensile strength of 7075 aluminum alloy samples decreased by6.4% after 5 k V induced electropulsing treatment,and the total elongation increased by 14.7% compared with the original solution state.This is due to the fact that the induced current promoted the pre-precipitation of Gunier-Preston zone in the supersaturated solid solution and reduced the dislocation density.With respect to conventional deformation heat treatment,after induced electropulsing and aging treatment,there was a finer and denser second phase present in the grain,the precipitated phase at the grain boundary was more discrete,and the intergranular precipitation-free zone appeared narrower.As a result,the strength of the material after aging remained unchanged while the elongation was increased by 12.15%,improving the corrosion resistance of the material.(2)Targeting the challenges of complex heat treatment process and low material elongation improvement,a composite process of single-pass and multi-pass "uniaxial tensile-induced electropulsing treatment" for7075-T6 high-strength aluminum alloy was proposed.The results revealed that the total elongation of the sample increased by 22.81%compared with the original state by using the 5% tensile and 5 k V induced current treatment.After three cycles of 4% tension and multiple cycles of 5 k V discharge induced electropulsing treatment,the total elongation of the sample increased by 49.30% compared with the original state.The microstructure observation showed that the dislocation density decreased,the sub-boundaries increased,the grains and the second phase grew slightly,the Cube texture decreased,and the Goss texture increased after induction current treatment.The current density and temperature distribution on the samples were analyzed,and the thermal annealing experiments were conducted.It was observed that the change in mechanical properties and microstructure were the results of the joint action of the thermal effect and athermal effect.(3)In view of the unknown plasticizing mechanism of high-strength aluminum alloy under electromagnetic forming(induced current and electromagnetic force coupling)condition,ANSYS and ABAQUS software were used to analyze the dynamic deformation,fracture behavior and bending spring back process of 7075-T6 aluminum alloy under electromagnetic forming condition,and the accuracy of the simulation results was verified by experiments.The results showed that the current density on the sheet was small during the forming process,and the sample had a spring back angle of about 3.1° after electromagnetic forming.Under the condition of electromagnetic forming,the energy required for sheet metal fracture was bigger than that of quasi-static stamping,which increased the maximum fracture strain from0.116 to 0.147.With one part of the material being fractured,the other parts continue to deform under the action of inertia caused by electromagnetic forming,and finally form multiple cracks.With the increase in discharge voltage,the strain rate of sheet metal increased with corresponding increase in fracture strain.(4)Considering the issue of 7075-T6 aluminum alloy large spring back after forming,a method of reducing spring back of bending parts by passing instantaneous current was proposed.The results showed that the spring back of the bending part decreased with increasing pulse current,and the spring back of 7075 aluminum alloy is completely eliminated when the discharge voltage is 10 k V.In order to decouple thermal and athermal effects,hot stamping experiments were conducted in a controlled environment.It was found that the spring back of the sample under pulse current was smaller than that under hot stamping,so the thermal effect and athermal effect together reduced the spring back.The strength of the material decreased and the elongation increased under the action of current.The dislocation reduction and the second phase dissolution caused by the pulse current were the main reasons for the change in mechanical properties.The decrease in spring back was related to the decrease in flow stress and stress release.(5)Aiming at the challenge of limited improvement of forming limit of 7075-T6 aluminum alloy under traditional electromagnetic forming,the electrically assisted electromagnetic forming process(coupling of induction and incoming current with electromagnetic force)was proposed.The coupling model of electromagnetic field,temperature field and deformation field was established,and the current and temperature distribution of samples were predicted.The results showed that the ultimate principal strain of 7075-T6 aluminum alloy was increased by76.4% under the condition of electrically assisted electromagnetic forming compared with quasi-static forming.Compared with traditional electromagnetic forming,the ultimate principal strain of electrically assisted electromagnetic forming was increased by 42.3%.After passing the instantaneous current,the rebound angle of the parts decreased from6.4° to 0.4°.The current promoted dislocation recovery and directional alignment,enhanced dislocation slip ability and enhanced atomic diffusion flux,which were the main reasons for the improvement of plastic deformation ability of materials.In summary,the formability and microstructure evolution of 7075 aluminum alloy under instantaneous current and electromagnetic force were studied in this thesis.The issues of poor formability and large rebound of 7075 aluminum alloy at room temperature were resolved.The plasticity of 7075 aluminum alloy was significantly improved and the spring back was reduced by the action of high density instantaneous induced current,incoming current and electromagnetic force.The influence mechanism of instantaneous current on dislocation,second phase,grain and texture was revealed,which provided a new method for plastic forming and property control of high strength aluminum alloy.