Effect Of Travelling Magnetic Fields Coupling With Sequential Solidification On The Microstructure And Defects Of Al-Cu Based Alloys

ZL205A alloy is a representative Al-Cu-based alloy having the highest strength and the best comprehensive performance currently,and has been widely used in aviation,aerospace,weapons,automobile and other fields.However,the large solidification intervals of Al-Cu-based alloys leads to a large mushy zones during the solidification process,which can easily cause the excessive dendrite network to form.As a result,the feeding channels and exhaust channels are blocked,resulting in serious shrinkage porosity,gas porosity and other defects,so that,the technological performance are badly decreased and the casting difficulty of these alloys is largely increased.In this study,the travelling magnetic fields coupling with sequential solidification(Travelling magnetic fields&Sequential solidification,TMF&SS)was used to conduct contactless,non-pollution,and easily controlled continuous treatment on Al-Cu-based alloys with large solidification intervals,aiming to achieve real-time continuous regulation and improvement on the solidification behavior in mushy zone during the solidification process,so as to reduce the solidification defects and improve the mechanical properties of the alloys.In this study,the technology scheme of TMF&SS was designed and perfected,and the related laws of the key technologies were studied.It can be obtained that the magnetic intensity distributed in the TMF generator presents a trend of"large in the middle and small at both ends"along the axial.The direction of axial magnetic force in alloy melt can be controlled by changing the phase sequence of excitation current.By making the pull-down velocity of alloy melt equal to the upward movement velocity of the solid-liquid interface,the mushy zone in the solidification process can be stabilized in the effective area of TMF.Firstly,the microstructure of Al-5wt.%Cu binary model alloy was regulated by TMF&SS,and the relevant mechanism was studied.It can be determined that the strong and long range directional circulation in alloy melt induced by TMF is t he most important factor affecting the solidified microstructure.Specifically,Down-TMF can result in the strong and long range circulation in a specific direction during the solidification,which can equivalently increase the solute distribution coeffici ent and reduce the solidification intervals.Meanwhile,Down-TMF can improve the growth of primary dendrite along the axial direction,reduce the primary dendrite arm spacing and the formation of second dendrite,as well as the segregation of second phases.As a result,the refined and uniformed microstructure,and the improved mechanical performance can be obtained by the Down-TMF process.Conversely,compared with Down-TMF process,Up-TMF can generate opposite directional circulations,which will lead to a series of negative effects on the microstructure and properties of alloys.Then,the porosity defects in Al-5wt.%Cu binary model alloy were reduced by using TMF&SS,and the mechanism was studied from the perspective of thermodynamics and dynamics respectively.It can be determined that Down-TMF can optimize the microstructure,feeding channels and exhaust channels,and stabilize the solid-liquid interface,so as to decrease the difficulty of feeding and exhaust during the solidification process.In addition,Down-TMF can improve the feeding pressure and exhaust driving force,leading to a decrease in porosity and an increase in mechanical performance.On the contrary,Up-TMF can produce a series of negative effects compared with the Down-TMF process.Based on the relevant results and mechanisms already obtained,the analysis of TMF&SS optimizing defects,microstructure and performance of ZL205A alloys with multi-element and multi-phase were performed.It can be found that TMF will influence the solidification behavior according to the solidification sequence of each phase in ZL205A alloys.Specifically,both the Down-TMF and Up-TMF can refine and uniform the Al₃Ti formed in the first place,which are the nucleated particle of?-Al.Additionally,Down-TMF can promote the growth consistency of primary dendrite and the other phases,and reduce the formation of second dendrite,dendrite net,Al₆Mn and Al₂Cu.Meanwhile,the addition of Down-TMF can effectively reduce the shrinkage porosity in the alloys.However,Up-TMF will cause the growth of primary dendrite to deflect,aggravate the formation and aggregation of Al ₆Mn and Al₂Cu,and promote the formation of second dendrite and dendrite net.Besides,Up-TMF will increase the shrinkage porosity.Finally,by using the relevant results and mechanisms already obtained,the solidification behavior and Ni-rich phases of Al-5wt.%Cu-1wt.%Ni model alloys were regulated by TMF&SS.It is concluded that TMF can break the?-Al which grows preferentially according to the solidification sequence of the alloy and refine the matrix.And then the Al₃Ni₂ phase will be refined and homogenized,so that the formation of refined Al₇Cu₄Ni can be promoted,and the properties of the alloy at both room temperature and high temperature can be improved.Besides,the Down-TMF process can reduce the formation and aggregation of Al₂Cu.Oppositely,the Up-TMF process will increase the formation and aggregation of Al₂Cu,resulting in a certain limitation in the improvement of room temperature performance.