Research On Serrated Plastic Instability Behavior And Deformation Mechanism Of 5086 Aluminium Alloy Under Electric-assisted Tensile Test

Aluminium(Al)alloy is widely used in mechanical manufacture,vehicle,and aerospace for its high strength and toughness,damage resistance and easy to recycling.However,plastic instability(namely PLC effect)is easy to occur during the plastic deformation of Al alloy.PLC effect is manifested as the serrated stress curve and strain localization,which greatly affects the surface quality and forming performance.So it should be suppressed or eliminated.The electric-assisted forming can effectively reduce material deformation resistance and improve material formability.In this work,uniaxial tensile tests of 5086 Al alloy were conducted at different conditions.The threshold value of restricted PLC effect was obtained.By comparing with the heating tensile test,the influence of athermal effect on plastic deformation behavior of Al alloy were studied with the help of microhardness,EDS,TEM and EBSD.The mechanisms of the serrated yielding suppression due to the applied pulse current were discussed from the point of view of microstructure evolution.One electroplastic constitutive model based on dislocation density was constructed.The main research contents and conclusions are as follows:The uniaxial tensile tests were conducted at different strain rates,electric-assisted and constant temperature.The change of serration type and PLC macroscopic characteristic at different conditions were analysed.It was found that the stress decreased and elongation increased slightly with increasing current densities.The PLC effect is first strengthened and finally eliminated.The threshold value of pulse current to restrict PLC effect was 7.46 A/mm2?8.09 A/mm2.The threshold value of direct current to restrict PLC effect was 9.07 A/mm2.Combined with the analysis of the diffusion coefficient,it can be found that pulse current was superior to direct current on suppression of PLC effect.The diffusion coefficient increases exponentially with increasing current densities,which had a smaller difference at the beginning of plastic deformation and PLC effect was restricted under threshold parameter,indicating that pulse current can accelerate diffusion of solute atoms.The athermal effect of pulse current was the important reason on suppression of PLC effectThe uniaxial tensile tests were conducted in heating furnace with a warming trend similar to the threshold parameter.It was found that the stress-strain curve still exhibited serrated plastic instability during heating stretching.It was more intuitively showed that the major contributor to restricted PLC effect was the athermal effect.TEM and EBSD analysis results showed that the dislocation density increased with increasing current density.The slip mode changed from plane slip to cross slip.The dislocations were piled up near the precipitations and the precipitations were difficult to cut by dislocations without current.However,the precipitations can be cut by dislocations when the pulse current applied.It is indicated that pulse current promoted the motion of the dislocations.KAM results show that the current can make the dislocations move more uniform.The electroplastic behavior of 5086 Al alloy was studied.Based on Johnson-Cook constitutive equation,the Arrhenius exponential expression was used and modified parameter s was added to construct the temperature-affected constitutive equation.One constitutive model based on current density was proposed on this basis,which can use to describe the rheological behavior of 5086 Al alloy under pulse current,and the curve fitting result was good.