Research Of Plastic Flow Behavior And Microstructure Mechanism Of An Al-Zn-Mg-Cu Alloy

Al-Zn-Mg-Cu alloy is a kind of aluminum alloy with high strength,good ductility and corrosion resistance,which is mainly used in the manufacture of aircraft and its main structural parts.Cutting is one of the most common processing methods for this material and its relevant products.During the cutting process conducted at room temperature,tensile stress exerted by the cutting tool and heat generated by friction form a thermal-mechanical coupling field around the chip,resulting in high temperature plastic deformation of material and the formation of chips with different morphologies.The service time and the surface quality of the workpiece highly depend on the morphology of chips.Therefore,the systematic research of high temperature plastic flow behavior of Al-Zn-Mg-Cu alloy is able to provide reference for the improvement of cutting technologies performed at room temperature.In addition,cryogenic extrusion cutting and crystal anisotropy-dependent cutting technologies are gradually emerging in recent years.However,the development of these promising cutting technologies is relatively slow at current stage,mainly due to the lack of in-depth study of cold temperature plastic flow behavior and its effects on the anisotropy of Al-Zn-Mg-Cu alloy.Based on current development of cutting technology of Al-Zn-Mg-Cu alloy and the urgent need of breaking through constraints of newly key technologies,this dissertation has systematically studied mechanical properties,micro deformation mechanisms and its evolution behavior of Al-Zn-Mg-Cu alloy during its high and cold temperature plastic flow deformation by uniaxial tensile test,respectively.Thus,according to this technical route,current dissertation has mainly conducted innovative research in the following four parts:（1）High temperature plastic flow behavior of Al-Zn-Mg-Cu alloy has been systematically studied in a wide range of deformation temperatur.To be specific,evolution process of its mechanical properties,microstructure deformation mechanisms and fracture behavior with the change of deformation temperature and strain rate have been investigated,respectively.Results demonstrate that work hardening mechanism dominates the plastic deformation of Al-Zn-Mg-Cu alloy from25^oC to 100^oC.At 200^oC,dynamic recrystallization and dynamic recovery mechanisms mainly control the deformation of this material at lower and higher strain rates,respectively.From 300^oC to 400^oC,the plastic deformation of Al-Zn-Mg-Cu alloy is primarily controlled by dynamic recrystallization mechanism.At temperature above 200^oC,the coalescence of microvoids and dimples leads to the reduction of elongation at lower strain rates.Besides,mechanical properties and microstructure of such material begin to present strain rate sensitivity,which becomes more remarkable with the increasing of deformation temperature.A constitutive model of high temperature plastic flow stress behavior of Al-Zn-Mg-Cu alloy has been established based on Arrhenius model,showing good fitting results.（2）Plastic flow behavior of Al-Zn-Mg-Cu at cold temperature environment has been thoroughly studied.Mechanical responses of this material at different deformation temperature and strain rate have been obtained.The microstructure deformation mechanism and fracture morphology of this material evolving with various deformation temperature and strain rate have been revealed.It has been found that Al-Zn-Mg-Cu can maintain excellent strength and plasticity at the same time in cold temperature environment.Fine grain strengthening mechanism results in the phenomenon that material’s strength increases with decreasing deformation temperature.For the outstanding plasticity of Al-Zn-Mg-Cu alloy exhibited in cold environment,lower temperature is able to inhibit the coalescence of microvoids and dimples,which restrains the generation and propagation of microcracks.While lower strain rate weaken the texture intensity of this material.In addition,at the same strain rate,the reduction of deformation temperature facilitates abundant generation of texture within material,indicating that the anisotropy of this material has been enhanced.Meanwhile,the strain rate sensitivity of this materials increases with decreasing of deformation temperature.（3）According to the above researches,specimens have been extracted along 45^oand 90^o direction with rolling direction of Al-Zn-Mg-Cu sheets to perform flow stress tests at cold temperature.Evolution behaviors of mechanical properties and fracture morphologies of these two kinds of specimens changing with different temperature and strain rate in cold environment have been revealed.Results show that the mechanical strength of 45^o and 90^o specimen increases with decreasing temperature,while the plasticity of two kinds of specimens reduces with the reduction of temperature,respectively.To be specific,45^o specimen possesses the highest fracture strain and the lowest peak stress,however,which is opposite for 90^o specimen.Within the scope of-75～25^oC and 10^-4～10^-1 s^-1,for both 45^o and 90^ospecimen,work hardening mechanism and dynamic strain aging mechanism dominates the plastic deformation at lower and higher strain rate,respectively.However,the plastic deformation of 0^o specimen at this temperature and strain rate range is mainly controlled by dynamic strain aging mechanism.（4）Effects of plastic flow behavior at cold temperature on the anisotropy of mechanical properties and fracture behaviors of Al-Zn-Mg-Cu alloy have been investigated.Results demonstrate that 45^o specimen possesses the highest value of strain rate sensitivity,90^o specimen ranks as the second,and the lowest value belongs to 0^o specimen.The fracture modes of three kinds of specimens are brittle-ductile mixed mode,which the proportion of intergranular fracture characteristic rises with decreasing temperature.The fracture morphology of 45^o specimen is covered with lots of dimples and microvoids,besides,slip behaviors can also be observed on the fracture surface and cleavage step surface of this kind of specimen.The number of microvoids on the fracture surface of 90^o specimen remarkably reduces with decreasing temperature.Comparing the thickness of cleavage steps shown on the fracture morphologies,it has been found that 90^o specimen possesses the highest value,followed by of 0^o specimen,while the lowest value belongs to 45^o specimen.Such phenomena are consistent with the aforementioned results of mechanical properties for these three kinds of specimens.In conclusion,current dissertation performed uniaxial tensile tests and thoroughly studied the plastic flow behavior of Al-Zn-Mg-Cu alloy at high and cold temperature,respectively.The tendency of mechanical properties changing with deformation temperature and strain rate has been obtained,and the relevant microstructure evolution behaviors of this material have been revealed.In addition,effects of plastic flow behavior on anisotropy of mechanical properties and fracture morphologies of this material have been thoroughly investigated.Thus,the current research is able to provide a solid foundation for optimizing room temperature cutting technology of Al-Zn-Mg-Cu alloy and the further development of a series of newly promising cutting technologies,such as cryogenic extrusion cutting technology and crystal anisotropy-dependent cutting technology.