Research On Hot Compression Behavior And Hot Extrusion Process Of Al-7.5Zn-1.5Mg-0.2Cu-0.2Zr Alloy

Automotive lightweight is one of the main development directions for energy saving and emission reduction.One important way is to use lightweight high-strength materials,and aluminum alloys are typical light alloy with excellent performance.7000 series aluminum alloys have high specific strength,specific stiffness,excellent fracture toughness and fatigue resistance.7000 series aluminum alloys are widely used in aerospace,vehicle and other fields,and hot extrusion is one of the main processings for its profile production.In this paper,an Al-7.5Zn-1.5Mg-0.2Cu-0.2Zr alloy was taken as the experimental materials.Flow behavior under different deformation conditions of the alloy was studied by hot compression simulation.A constitutive equation describing the relationship between flow stress and deformation conditions was extablished and hot processing maps were constructed to find out the optimum processing parameters for the studied alloy.Then,the effect of extrusion ratio on microstructure and mechanical properties of the alloy was studied by hot extrusion process.Finally,the effect of one-step and two-step aging on microstructure and mechanical properties of the hot extruded rod was studied.The results of hot compression simulation show that flow stress increase with increasing strain rate or decreasing deformation temperature.At the initial stage of the deformation(ε=0.02-0.1)flow stress increases rapidly with increasing strain;and then the second deformation stage can be divided into two situations:when the strain rate≤0.1 s-1,flow stress will decrease after reaching the peak value;when the strain rate ≥ 1 s-1,flow stress will be stable after reaching the peak value.The main softening mechanism of the studied alloy during hot compression is dynamic recovery accompanied with continuous dynamic recrystallization,particularly at low Z deformation conditions.Hot processing maps were established.The instable domains are located at the high strain rate(≥1 s-1)region and become larger by expanding to the low deformation temperature with increasing true strain.Adiabatic shear bands and flow localization occur when the alloy are deformed at instable domain.The peak value of power dissipation efficiency is 38-39%under the deformation conditions of 500％-0.01 s-1,at which the studied alloy will undergo dynamic recrystallization and the microstructure will be more homogeneous.According to the hot processing maps,the optimum deformation temperature and strain rate of the studied alloy ranges from 450 to 500 ℃ and 0.003 to 0.1 s-1,respectively,with high power dissipation efficiency of 34-39%.The activation energy for deformation of the studied alloy is Q= 164.075 kJ·mol-1,and its Arrhenius constitutive equation is as follows:ε= 3.58×1010[sinh(0.021σ)]4.44 exp(-1.641×105/RT)After hot extrusion,the original equiaxed grains of the alloy are elongated along the extrusion direction,and the average grain size decreases with the extrusion ratio increasing.The dislocation density of the extruded rod was calculated by Williamson-Hall(WH)method,and the dislocation density increases with the increase of the extrusion ratio.TEM microstructure shows that there are a large number of dispersed precipitation phases in the alloy rod.The strength of the extruded rod is enhanced by grain refinement strengthening,precipitation strengthening、dislocation strengthening and deformation texture.The strength of the extruded rod is significantly higher than that of the as cast alloy,and the strength increases with increasing extrusion ratio.When the extrusion ratio is 69.4,the tensile strength and elongation of the alloy are 758.2 MPa and 8.6%,respectively.Aging peak occurs when the extruded rod is heat treated at 120℃ for 16 h in one-step aging,the strength is mainly affected by η’ phase.The tensile strength,micro-hardness and elongation were 735.8 MPa,182.6 HV and 9%,respectively.The fracture mechanism combines brittle and ductile fracture.The extruded rod shows good comprehensive properties under 120 ℃/6 h-160 ℃/8 h two-step aging.The tensile strength,micro-hardness and elongation were 651 MPa,164.5 HV and 12.7%,respectively.The fracture mechanism is ductile fracture.