Mechanisms For Optimizing The Microstructures And Properties By Thermomechanical Treatments Based On Strain-induced Precipitation In High-strength Aluminum Alloy

High strength aluminum alloys with excellent comprehensive properties like 7055 are developed as the development of aerospace industry and the requirement of their applications.But the existing micro alloying and heat treatment cannot satisfy the need of modern industry.High strength,toughness,corrosion resistance and damage tolerance indubitably become the main development direction of 7000 series alloys.Microstructure refinement has been proved to improve strength/plasticity and formability of alloys simultaneously.The intermediate thermomechanical treatment(combination of plastic deformation and heat treatment)is the most effective method to refine grains of 7000 series alloys by enhanced deformation storage energy caused by precipitates.Moreover,the size and distribution of grain boundary precipitates can significantly affect the corrosion resistance of 7000 series alloys.The final thermomechanical treatment(combination of deformation and precipitation)can not only enhance strength but also improve corrosion resistance by ameliorative size/distribution of grain boundary precipitates.In present work,based on deformation induced precipitation,DIP,new thermomechanical treatments to refine grains or improve matrix precipitates/grain boundary precipitates of Al-Zn-Mg-Cu alloys were designed to improve the plasticity and corrosion resistance of Al-Zn-Mg-Cu alloys while maintaining high strength.The precipitates could be affected by deformation and over aging.In present work,thermal strain induce the precipitation of MgZn2 in 7055 A1 alloy and promote their spheroidization and refinement.Massive crystal defects(such as dislocations,vacancy,etc.)introduced by hot deformation can generate more precipitation nucleation to raise the nucleation rate and refine precipitates.These precipitates could be spheroidized under the drive of interfacial energy.The effect of strain parameters on microstructures of 7055 Al alloy was studied systemically via control various methods.The sizes of precipitates and dislocation cells increase and their amount decreases/dislocation density decreases/grain width increases gradually with the increase of deformation temperature and decrease of strain rate while with the increasing deformation amount,precipitates were more and refined/dislocation density increased firsly and then decreased/grain width decreased firstly and then increased.Based on this,two short cycled thermomechanical treatments(improved intermediate thermomechanical treatment and final thermomechanical treatment)were proposed to raise the plasticity(by grain refinement)and corrosion resistance(by improved MPts and GBPs).Fine spherical dispersed precipitates were introduced into A1 matrix by DIP.During the intermediate thermomechanical treatment deformation process,low angle grain boundaries would transferr to high angle grain boundaries(fine grains were formed)due to the pinning effect of these precipitates.A grain refinement model was set up to clarify that dislocation slipping and boundary migration were hindered by the pinning effect of the DIPs so as to form high density dislocation cells.During the subsequent short-term intermediate annealling,numerous ordered polygonal subgrains were generated via static recovery from these dislocation cells.In the final hot rolling process,subgrain boundaries were still pinned by DIPs.So with the increasing accumulated strain,subgrain boundaries absorbed dislocations and finally low angle grain boundaries transferred into high angle grain boundaries leading grain refinement.Plasticity can be improved remarkablely while maintaining high strength.In the improved final thermomechanical treatment,the re-dissolution/discontinuous distribution of GBPs and precipitation of fine dispersed MPts would be accelerated by "deformation induced precipitation" at overaging temperature.So strength and corrosion resistance of 7055 Al alloy are improved simultaneously.