Study On Microstructure,Properties And Heat-treatment Process Of Al-Zn-Mg-Cu Alloy By Ultrasonic Treatment And Squeezed Casting

At present,the realization of automotive lightweight has become the focus of the entire automotive industry,which requires the structural parts in light weight and its strength to meet the requirements.Al-Zn-Mg-Cu alloy is the highest strength aluminum alloy at room temperature,so it naturally becomes the first choice material to achieve lightweight.However,Al-Zn-Mg-Cu alloy has the characteristics of high alloying degree and wide solidification temperature range,which lead to the defects of ingot,such as shrinkage hole,porosity and hot cracking.Therefore,ingots need generally plastic processing to reduce the defects.The Al-Zn-Mg-Cu alloy is mainly rolled into plates for use at present,which is difficult to be processed into complex parts,greatly restricting the application range of Al-Zn-Mg-Cu alloy.Squeeze casting is a process to make the melt solidify under high pressure and produce a small amount of plastic deformation,which can effectively reduce the shrinkage cavity and porosity in the casting process,increase the density of the casting significantly,and thus improve the mechanical properties of the casting.Squeeze casting can be directly used to make products with complex shapes and is considered to be the preferred method to achieve the substitution of casting for forging.In addition,Zr element is one of the important grain refiners of Al-Zn-Mg-Cu alloy,so it is of great significance to research the influence of its content.Al-Zn-Mg-Cu alloy samples with different Zr content were prepared by squeeze casting process(SC),and the effect of Zr content on squeeze casting Al-Zn-Mg-Cu alloy was studied.The results show that Zr can refine the grain,but when the addition amount of Zr is too high(w(Zr)>0.15wt.%),forming coarse primary Al₃Zr phase,the refining effect is weakened,and the grain refinement is not uniform,leading to a large difference in grain size.In addition,with the increase of Zr content,the composition segregation becomes more serious,and a large number of second phase structures appear in the grain.Then,Al-Zn-Mg-Cu alloy samples with different Zr contents were prepared by ultrasonic treatment and squeeze casting process(UT-SC).The results show that the ultrasonic treatment can effectively improve the composition segregation of Al-Zn-Mg-Cu alloy,and can inhibit the segregation of Zr element during the solidification process to prevent the formation of primary Al₃Zr phase.Under the action of ultrasound,the addition amount of Zr element can be increased,and the grain refinement effect can be further enhanced,which is beneficial to generate more metastable Al₃Zr particles and improve strength of alloy.This paper studied the effect of ultrasonic treatment on the solution process of Al-Zn-Mg-Cu alloy by comparing the microstructure and properties of the samples of SC and UT-SC after solution treatment,determined the optimal peak aging process of UT-SC alloy to ensure the maximum strength,and studied its aging behavior.The results show that the transformation of eutectic phase into insoluble phase with higher melting point occurs in both SC and UT-SC alloy samples during solution treatment.Two-stage solution treatment is needed to eliminate the residual phase,improve the supersaturation degree of the alloy and promote aging strengthening.Under the same solution conditions,the solution efficiency of UT-SC alloy samples is higher.After the two-stage solution treatment at 470?×24h+480?×8h,the residual second phase in UT-SC alloy samples has been basically eliminated(only a small amount of Al Cu Fe phase residual).The optimum peak aging process for UT-Sc alloy samples is 120?×20h.After aging treatment,it can be observed that fine GP region,?'phase and Al₃Zr particles are uniformly distributed in the grain.The yield strength of the alloy can reach 521MPa,the tensile strength can reach 579MPa,and the elongation can reach8%.However,the grain boundary segregation phenomenon is obvious in the alloy.It can be observed that coarsening?phase(Mg Zn₂)is easily formed at the grain boundary.The extruded Al-Zn-Mg-Cu alloy samples were prepared by squeeze casting+hot extrusion(SC-EX)and ultrasonic squeeze casting+hot extrusion(UT-SC-EX),respectively,and the effects of the initial microstructure of Al-Zn-Mg-Cu alloy before deformation on the microstructure and mechanical properties after deformation were studied.The results show that after hot extrusion,the coarse non-equilibrium eutectic phases in the cast microstructure are crushed and polymerized into fibrous microstructure distributed along the extrusion direction,and these coarse microstructure will accelerate the accumulation of dislocations during hot deformation.Therefore,these large-sized second-phase structures with stored energy and strain around them can stimulate the nucleation of recrystallization and promote the occurrence of recrystallization,making the region preferentially grow new grains.However,after hot extrusion,the second phase microstructure of UT-SC alloy samples is thin,the alloying elements are evenly distributed,and dislocations cannot accumulate.Therefore,dynamic recovery is the dominant mode in the process of hot deformation.In addition,the coarse second phase microstructure in the SC-EX alloy samples mainly gathered at the grain boundary,impeding the dislocation movement and grain boundary migration,which would lead to the increase of recrystallization temperature.Therefore,under the same solution conditions,the solution efficiency and static recrystallization degree of SC-EX alloy samples are lower than those of UT-SC-EX alloy samples,which leads to lower strength and plasticity of SC-EX alloy samples than those of UT-SC-EX alloy samples.Compared with the extruded casting alloy,the strength and elongation of the extruded alloy are improved after the peak aging treatment,and the yield strength and tensile strength of UT-SC-EX alloy are 583MPa and 634MPa,respectively.The ductility of UT-SC-EX alloy is improved most obviously,and the elongation of UT-SC-EX alloy reaches 18.2%.It can be seen from the fracture morphology that the fracture form changes from brittle fracture to ductile fracture.