Study Of The Effects Of Zn Content And Ageing Process On Microstructures And Properties Of 6082 Aluminum Alloy

Ageing hardenable 6xxx aluminum alloy is widely used in structural components for transportation vehicles and one of the preferred materials for vehicle lightweight due to its high specific strength,good corrosion resistance,formability and weldability.Because 6xxx aluminum alloy has medium strength,the study on the improvement of the strength could bring great application value.Due to the contradiction between high Zn content in recycled aluminum and high performance of aluminum alloys,the application of this series of alloy is restricted.In order to improve the strength of 6xxx series aluminum alloy and broaden the application of recycled aluminum,this paper focused on the optimizing design of chemical compositions of the Al-Mg-Si-Zn alloy.In this study,the effect of Zn contents on microstructures and properties of 6082 aluminum alloy was systematically studied,and the content of Zn addition was optimized.Based on the specific Zn content,the effect mechanism of different ageing processes on the microstructures and properties of 6082 aluminum alloy with Zn addition was revealed.The ageing process to obtain excellent mechanical properties and corrosion resistance of the alloy was determined.The main conclusions are as follows:(1)The effect of different Zn contents on the microstructures of 6082 aluminum alloy was investigated.The addition of Zn reduces the liquidus temperature and increases the content of primary phases at the grain boundary of as-cast alloys.The higher diffusion rate of Zn in Al makes it easier to segregate at the grain boundary and form Zn-rich phases.(2)The effect of different Zn contents on the microstructures and properties of 6082 aluminum alloy after traditional ageing was revealed.Compared with the alloys with 0wt.%,0.2wt.% and 2.0wt.% Zn,when Zn content is 0.5wt.%,the higher number density of β " precipitates is formed in the grains,smaller β precipitates and wider precipitation-free zone(PFZ)are formed at the grain boundary.Therefore,the strength of the alloy is significantly improved,while the intergranular corrosion(IGC)resistance is slightly decreased.The IGC resistance of the alloy with 1.0wt.% Zn is slightly improved due to smaller grain boundary β precipitates and narrower PFZ,but its strength is only comparable to that of the alloy with0.5wt.% Zn.When Zn content increases to 4.0wt.%,high number density of β" precipitates and a small amount of η’ phases are formed in the grains,coarse η phases and wider PFZ are formed at the grain boundary.Although the strength of the alloy is increased,the IGC resistance is decreased obviously.Finally,the Zn addition is determined to be 0.5wt.%.At peak ageing of traditional ageing,the ultimate tensile strength(UTS),yield strength(YS)and elongation(EI)of the 0.5Zn alloy are 371 MPa,344MPa and 15.0%,respectively,and the maximum IGC depth is 124.87μm.(3)The effect of interrupted ageing on the microstructures and properties of 6082 aluminum alloy with 0.5wt.% Zn was investigated.Compared with the traditional ageing,the interrupted ageing promotes the precipitation of fine β" precipitates with higher number density,decreases the size and increases the interspacing of grain boundary β precipitates and reduce the width of the PFZ of the alloy at peak ageing.This microstructure results in the improvement of the mechanical properties and IGC resistance of the alloy.At peak ageing of interrupted ageing,the UTS,YS and EI of the alloy are 384 MPa,354MPa and 18.1%,respectively,and the maximum IGC depth is 88.36μm.(4)The mechanism of the effect of rolling deformation introduced in interrupted ageing on the comprehensive properties of 6082 aluminum alloy with 0.5wt.% Zn was revealed.After the introducing of rolling deformation,the interrupted ageing promotes the precipitation of fine β"precipitates of the alloy at peak ageing,average Taylor factor of texture and dislocation density in the grains increase.Thus,the precipitation strengthening of the alloy improves significantly,but the elongation decreases.And the strengthening effect of asymmetrical rolling deformation is more significant than that of symmetrical rolling deformation.After introducing symmetrical rolling deformation,the IGC resistance of the final alloy is slightly reduces as a result of coarser grain boundary precipitates(GBPs).While after introducing asymmetrical rolling deformation,almost no GBPs are formed and thus the IGC resistance of the final alloy is more excellent.In conclusion,in this study,the alloy has the best comprehensive properties after introducing asymmetrical rolling in interrupted ageing.The UTS,YS and EI of the alloy can reach 425 MPa,406MPa and 9.7%,respectively,and the maximum IGC depth is only 65.49μm.