| Al-Zn-Mg-Cu alloy is a kind of heat treatment strengthening aluminum alloy,which has the advantages of high specific strength and hardness,good thermal processing and excellent stress corrosion resistance.It is widely used in aerospace,military,transportation and other fields.Due to the uneven distribution of temperature field and flow field of Al-Zn-Mg-Cu alloy during non-equilibrium solidification,the solidification structure is coarse,which limits its application.The conventional heat treatment has the disadvantages of high energy consumption and long time.To further refine the solidification structure and optimize the heat treatment process of Al-ZnMg-Cu alloy is an alternative way to further improve the mechanical properties of the alloy and reduce the production cost.The influence of electric pulse,electromagnetic field and other physical fields on the solidification behavior of alloys and the effect of electric pulse on the microstructure evolution of alloys have attracted extensive attention of metallurgical and casting workers.It has shown a broad application prospect in the field of materials science and engineering.In this paper,the effects of electric pulse,rotating magnetic field and coupled field on the solidification microstructure and mechanical properties of Al-Zn-Mg-Cu alloy are studied by means of finite element simulation and experiment.The effects of electric pulse heat treatment on microstructure and mechanical properties of Al-ZnMg-Cu alloy are studied by comparing with conventional heat treatment.Then the alloy samples solidified under the external fields are subjected to electric pulse heat treatment.The synergistic effect of grain refinement by external fields and electric pulse strengthening is studied.The aim is to explore a feasible way to improve the properties and process optimization of Al-Zn-Mg-Cu alloy.The effect of grain refinement and mechanical properties of Al-Zn-Mg-Cu alloy are improved to varying degrees by applying electric pulse,rotating magnetic field and coupled field during solidification.The results show that the refinement effect of solidification microstructure is all obvious under electric pulse,rotating magnetic field and coupled field.The mechanical properties of the alloy are improved most obviously by solidification under rotating magnetic field.The average grain size decreases from140.3 ?m to 102.9 ?m after electrical pulse solidification treatment with pulse current of 700 A and pulse frequency of 50 Hz.The tensile strength of the alloy increases to210.5 MPa,which is 20.6% higher than that of conventional solidification.Due to the effect of electric pulse,on the one hand,the convection generated by the melt accelerates heat loss and increases the cooling rate.On the other hand,it promotes the diffusion of solute atoms and significantly increases the degree of grain boundary segregation.The nucleation barrier of crystal decreases,which increases the number of crystal nuclei in the melt and refines the grains.The average grain size decreases to96.4 ?m under rotating magnetic field solidification with magnetic field current of 100 A and magnetic field frequency of 14.0 Hz.The tensile strength and elongation increase to 219.3 MPa and 9%,respectively.Due to the centrifugal movement of the melt under Lorentz force generated in the rotating magnetic field,the cooling rate of the alloy is further increased.The number of crystal nuclei formed per unit time increases and more fine equiaxed crystals are obtained.Because magnetic field current and frequency are the main factors to obtain fine grains.Applying a larger magnetic field current increases the cooling rate of the melt.At the same time,the columnar crystals at the interface are more prone to fracture due to larger Lorentz force.With a small magnetic field frequency,the magnetic field penetration ability is enhanced and the grain refinement effect is more significant.The solidification microstructure parameters of Al-Zn-Mg-Cu alloy treated by electric pulse and rotating magnetic field are optimized by orthogonal experiment.The average grain size decreases to 95.8 ?m after coupled field solidification with pulse current of 700 A,pulse frequency of 30 Hz,magnetic field current of 150 A and magnetic field frequency of 3.5 Hz.The tensile strength significantly increases to 211.3 MPa.It is found that the strengthening effect of electric pulse heat treatment is better than that of electric pulse assisted heat treatment.The tensile strength and elongation of Al-Zn-Mg-Cu alloy increase significantly to 505.7 MPa and 7.35 % after solution treatment(134 AŚ60 min).Compared with conventional heat treatment,the tensile strength of the alloy increases significantly by 40.7 MPa and the solution time is shortened by 1 h.The directional movement of electrons promotes the diffusion of solute atom.The vacancy concentration of the alloy increases,which creates favorable conditions for more precipitated phases during aging treatment.The thermodynamic energy barrier required for nucleation of the precipitated phase decreases and the nucleation rate of the precipitated phase increases during the electric pulse aging treatment.The tensile strength and elongation of the alloy is increased by 14.1 MPa and 4.45% after aging treatment(64 AŚ4 h),and the aging time is shortened by 12 h.The Al-Zn-Mg-Cu alloy samples solidified under external fields are subjected to electric pulse heat treatment and electric pulse assisted heat treatment.The results show that that the tensile strength of Al-Zn-Mg-Cu alloy solidified under rotating magnetic field increases most significantly after electric pulse heat treatment.The tensile strength increases from 423.4 MPa to 488.7 MPa.It verifies the synergistic effect of grain refinement by external fields and electric pulse strengthening.The microstructure and mechanical properties of the alloy are further improved. |
PDF Full Text Request