Study On Hot Deformation Behavior And Microstructure And Properties Of A Dispersion Strengthened 6XXX Al Alloy

6XXX?Al-Mg-Si-Cu?aluminum alloy belongs to heat-strengthable alloy.It is of high specific strength,corrosion resistance and excellent formability.Therefore,it is one of the main materials to reduce vehicle weight.Addition of transition elements such as Mn/Cr into the Al-Mg-Si-Cu alloy combined with homogenization heat treatment process can form nano-scale?-Al?Mn Cr?Si dispersoids in the matrix and further enhance the comprehensive mechanical properties of the alloy through dispersoid strengthening,which facilitate expanding application range of Al-Mg-Si-Cu alloy.In this paper,the traditional 6061 and the novel Mn/Cr element-added WQ1 alloy are used as research objects.The homogenization heat treatment process of WQ1 alloy is optimized.The formation mechanisms of the dispersoid within WQ1 alloy during single-stage and double-stage homogenization heat treatment are proposed.The rheological behavior and microstructural evolution of 6061 and WQ1 alloys during hot deformation are quantitively analyzed and compared with each other.The influence of dispersoid on hot deformation behavior and strengthening mechanism are discussed.The microstructure controlling mechanisms of hot deformed alloy after solution treatment for 6061 and WQ1 are proposed.The overall research provides experimental basis and theoretical analysis for development of new types of high strength,toughness and high performance aluminum alloy,which is beneficial to alloy development and its practical engineering application.The detailed research contents and conclusions are as follows:For the novel WQ1 aluminum alloy,single-stage and double-stage homogenization heat treatment systems are designed and optimized,in order to characterize and adjust the distribution characteristics of?-Al?Mn Cr?Si dispersoid in the alloy and ensure the effect of homogenization heat treatment.During the single-stage homogenization heat treatment,the distribution of?-Al?Mn,Cr?Si dispersoid is of non-uniform phenomena.There are coarse dispersoids zone?CDZ?within the grain center and the dendrite areas,which are haracterized by OM,SEM,and TEM.Dispersoid in the CDZ region are withi lower number density and larger size compared to adjacent regions.Compared to single-stage homogenization heat treatment,double-stage homogenization heat treatment can reduce the CDZ area fraction from 7%to 2.5%.When the first-stage homogenization temperature increases from 175°C to 300°C,the CDZ area fraction is further reduced.The distribution of metastable phases?Mg₂Si and Q?is improved during the first-stage homogenization heat treatment,which provides a uniform nucleation sites for diserpsoid and promotes the uniform distribution of the dispersoids.The double-stage homogenization heat treatment system is determined to be 300°C*8h+550°C*10h.Based on the EPMA results,the formation mechanisms of the dispersoids of WQ1 alloy during single-stage and double-stage homogenization heat treatment are proposed.The homogeneity kinetics theory is used to analyze and prove the rationality of the homogenization heat treatment system of WQ1 alloy.Isothermal hot compression experiments are conducted on 6061 and WQ1 alloys under different deformation conditions via Gleeble-3500 hot deformation equipment.During the hot compression of the alloy,the flow stress starts to descent with the increase of deformation temperature and it increases with the uprising strain rates.The evolution of the peak stress of the alloy with the deformation conditions is established.Through constitutive analysis,the material parameters and constitutive equations of the WQ1 alloy are obtained.The evolution diagram of the deformation activation energy of the alloy with the deformation conditions is established.The deformation activation energy begins to decrease with the growing strain rate and the increasing deformation temperature.Due to the dispersion strengthening effect,the deformation activation energy of WQ1 alloy is about 50k J/mol higher than that of 6061.Based on dynamic material model,hot processing map of WQ1 alloy is established.The influence of strain on hot processing map is discussed.Deformation parameters within instability zones and safety zones under different strains are also established.And the optimal processing window for WQ1 alloy is determined;analysis of instability zones And the organizational evolution of safe areas.The main softening mechanism of the WQ1 alloy in the safe area is the dynamic recovery.TEM results confirmed that the?-Al?Mn Cr?Si dispersoid can effectively pin sub-grain rotation,sub-grain boundaries migration and dislocation motion,therefore dynamic recrystallization is also effectively suppressed.There are micro defects,including stress concentration,secondary phase cracking and de-bonding,micropores and matrix cracking,etc.,in the microstructure of alloy deformed within the instability zone.Cracks prone to grow along the micropores and the second phase.OM,EBSD and TEM observation and analysis methods are used to systematically compare the microstructure and dispersoids distribution characteristics of 6061 and WQ1alloys and analyze the rheological behavior of the two alloys.The microstructure evolution of 6061 and WQ1 alloys under different deformation conditions is quantitatively characterized.The main softening mechanism of WQ1 alloy during thermal deformation is dynamic recovery,while the softening mechanism of 6061 alloy changes from dynamic recovery?DRV?to dynamic recrystallization?DRX?when temperature is higherh than500°C.The sub-grains formed during the dynamic recovery of 6061 alloy are transformed into dynamic recrystallized grains by rotation between adjacent sub-grains.The dispersoids in WQ1 alloy can effectively pin the dislocation movement and restrict the subgrain rotation.Therefore,the dynamic recovery process in the WQ1 alloy is suppressed.The misorientation of the sub-grains in the deformed WQ1 alloy is mainly concentrated at 1-5°.The strengthening effect and strengthening mechanism of the dispersoid are analyzed.The theoretical correspondence between the dispersoid and the flow stress increment is established,which is able to predict the flow stress behavior of the WQ1 alloy according to the dispersoid distribution.The theoretical value of flow stress of WQ1 alloy is in good agreement with the experimental results.The thermal deformation parameters?deformation temperature,strain,and strain rate?of 6061 and WQ1 alloys have a significant effect on their static recrystallization behavior during solution treatment.The static recrystallization behavior of the two alloys is strengthened with the decrease of deformation temperature,the increase of strain and the increase of strain rate.Under the same deformation conditions,although the static softening driving force of 6061 alloy is lower than that of WQ1 alloy,the static recrystallization behavior of 6061 alloy during solution treatment is significantly stronger than that of WQ1alloy.It shows that the dispersoid effectively inhibits the static recrystallization behavior of WQ1 alloy during solution and aging.The microstructure controlling models of deformed6061 and WQ1 alloy alloys after T6 treatment are established.It shows that when the ln Z of6061 alloy is higher than 23.84,the structure is completely static recrystallized after solution and artificial aging.When ln Z is lower than 17.05,static recrystallization does not occur after dissolution and artificial aging in 6061 deformed alloy.For WQ1 alloy,when ln Z is lower than 29.42,the deformed alloy does not undergo static recrystallization after solid solution and artificial aging,and when ln Z is greater than 37.98,the deformed alloy is transformed into completely static recrystallization after solid solution and artificial aging.Applying the control models of deformed alloy,the microstructure of the deformed 6061and WQ1 alloy after T6 heat treatment can be controlled by adjusting the thermal deformation parameters.