Dislocation Behavior Under The Coupling Of Electron Wind Force And Joule Heating In Plastic Forming Of Al-Zn-Mg Alloy

Current assisted plastic forming refers to the application of current to the forming sample in the traditional forming process.When the current flows through the forming sample,the drifting electrons can interact with the defects in the sample,which is called electron wind force.At the same time,due to the resistance of the formed sample,a part of heat will be generated,which is called Joule heating.Compared with the traditional hot forming,the current assisted plastic forming has more advantages and potential than the traditional hot forming because of the additional electronic wind force used to promote the high quality and stability of the forming at the same forming temperature.The mechanism of currentassisted plastic forming is the interaction between dislocations and the coupling field of electronic wind and Joule heating.Current physical detection methods can observe the phenomenon of dislocation density reduction,texture and precipitates change under the action of current,but most research stopped here and did not make a more systematic and in-depth analysis of the interaction between dislocations and the coupling field of electron wind and Joule heating,leading to "rich phenomena in the field of currentassisted plastic forming,but lack of basic theories and related mathematical models".This has seriously hindered the maturity of currentassisted plastic forming technology.The purpose of this study is to investigate the dislocation behavior under the coupling field of electron wind force and Joule heating:1)relationship between the promotion and inhibition of electron wind force on dislocation motion,2)the relationship between current and dislocation density reduction,and the establishment of related mathematical models,3)the difference of dislocation behavior in the two pinning models of "solid solution atom-dislocation" and"precipitate-dislocation" under the action of current,4)the competition behavior between the pinning of solid solution atoms and dislocation debinding under the action of current and the establishment of related mathematical models.The research methods and main results are summarized as follows:(1)According to the characteristics of axial strain at the center and radial strain at the edge of the cylinder during compression,an experimental system was designed,in which the relative motion direction of drift electron and dislocation satisfiesvertical and parallel conditions at the same time.The relationship between the promotion and inhibition of drift electron on dislocation slip was explored.It is found that in a fixed system,the effect of drift electrons on the promotion and suppression of dislocation motion is approximately in accordance with the law of energy conservation,that is,the sum of the energy consumed by drifting electrons in promoting dislocation motion and the energy consumed in suppressing dislocation motion is approximately close to a certain value.It is also found that the coupling field of electron wind and Joules introduced by current can significantly reduce the dislocation density.(2)Based on the distinguishing of the intergranular strengthening and the grain boundary strengthening in the Hall-Petch model,the reduction of grain boundary strengthening under the action of electric current is obtained.The mathematical model of the relationship among the current density,the initial dislocation density and the reduction of dislocation density caused by current is established.It is found that the interaction coefficient Cew(d)between drift electrons and dislocations increases with the decrease of average grain size.The decrease of dislocation slip resistance in the intergranular caused by drift electrons is independent of grain size,and the softening stress of grain boundary caused by Joule heatingincreases with the decrease of the average grain size.According to the established mathematical model,it could be found that with the decrease of grain size and the increase of initial dislocation density,the reduction of dislocation densitycaused by current increases.It is also found that it is feasible to distinguish the softening behavior of grain boundary and intergranular under the action of electric current by usingHall-Petch relationship,which may providea new methodin the field of current assisted plastic forming.(3)Based on the above method of distinguishing the behavior of intragranular softening and grain boundary softening under the action of electric current,the difference of dislocation behavior in two pinning models of"precipitate-dislocation" and "solid solution atom-dislocation" was studied.It is found that in the "solid solution atom-dislocation" pinning model,the drifting electrons will be scattered by the solid solution atoms,which will greatly change the direction of their motion,and then consume part of the electronic wind force,resulting in the reduction of the effect between drifting electrons and the dislocation.In the "precipitate-dislocation" pinning mode,the current flows around the precipitated phase,and the drifting electrons will continue to act on the dislocations,therefore,the softening degree of drift electrons in the "precipitate-dislocation" pinning model is greater than that in the "solid solution atom-dislocation" pinning model.(4)Based on the characteristics of the interaction between solid solution atoms and dislocations in the process of yield platform and the Portevin Le-Chatelier effect,the competition law of the behavior of solution atoms pinning and dislocation debinding under the action of electric current was studied,and the relevant mathematical model was established.It is found that the advantages and disadvantages of the two behaviors are not only affected by the current density,but also affected by the deformation stage.For example,when the current density is 55A/mm2,the dislocation debinding behavior is dominant in the yield platform stage,while in the work hardening stage,the pinning behavior of solution atoms is dominant in the competition.The research also finds that the decrease of dislocation waiting time is linear with the current density,and the absolute value of the linear coefficient increases with the increase of dislocation density.