Study On The Second Phase And Grain Evolution Behavior Of 7055 Aluminum Alloy During Hot Working By Spray Forming

The AlZnMgCu alloy has irreplaceable application in aerospace,high-speed rail transportation and new energy vehicles,duo to its excellent strength,toughness and corrosion resistance.The 7055 aluminum alloy formed by sprayed forming technology has many advantages such as higher Zn/Mg ratio,fine equiaxed grains and uniform composition without macro-segregation.Therefore,the component of spray-formed 7055 aluminum alloy made by reasonable deformation and heat treatment has broad application prospect.It is of great theoretical value to study the evolution of the second phase and grain structure of7055 aluminum alloy during hot working.In this thesis,OM,SEM,EBSD,TEM,DSC,XRD,and hardness,electrical conductivity,tensile,exfoliation corrosion tests were used to study the behavior of the evolution of Al₃Zr,thermal deformation and solution in as-sprayed7055 aluminum alloy.The processing parameters of optimized pre-deformation heat treatment,thermal working and solution were proposed.The main conclusions are as follows:（1）The microstructure of as-sprayed 7055 aluminum alloy consists of fine AlZnMgCu quaternary phase and irregularly shaped Fe phase.The AlZnMgCu phase will transform into Al₂Cu Mg phase（S phase）after pre-heating treatment at lower than 450℃,which is not conducive to the improvement of solid solubility.After 470℃preheating,the primary AlZnMgCu phase is basically dissolved.After preheating at 450℃and 470℃,The Al₃Zr particles were precipitated from the alloy.The two-stage preheating before deformation contributes to the fine dispersion precipitation of Al₃Zr particles,and the dissolution of primary AlZnMgCu phase depends on the second stage temperature of the two-stage preheating.The optimized pre-heating technology prototype is 300℃/12 h+470℃/12 h.（2）The thermal deformation activation energy of as-spray formed 7055 aluminum alloy is 129.45 k J/mol.The constitutive equation of thermal deformation and the thermal working diagram containing the"Zener-Hollomon parameter"were established.The optimized thermal working parameters were as follows:the deformation temperature was400～450℃and the strain rate was less than 0.1s^-1.The dynamic softening mode of 7055aluminum alloy is dynamic recovery and dynamic recrystallization,and dynamic recovery is dominant at low temperature and dynamic recrystallization is dominant at high temperature.The dynamic recrystallization mechanism of the alloy is"primary grain boundary bowing out"and"the migration of subgrain boundaries"at low temperature.The recrystallization mechanism at high temperature is"the migration of subgrain boundaries"and"the rotation of sub-grains"mechanism at original high-angle grain boundary（3）After solution treatment,the second phase of 7055 aluminum alloy basically dissolved back,but the Al₇Cu₂Fe phase couldn’t be eliminated.With the increase of solution temperature,the solid solubility of alloy increases gradually,and the recrystallization fraction also increases.The static recrystallization mechanism of the alloy during solution treatment is"original grain boundary bending out"and"sub-grain rotation merging",and the optimal solution treatment is 470℃/2 h.