Study On 6082 Aluminum Alloy Subjected To Surface Mechanical Attrition Treatment And Thermal Stability

Surface Mechanical Attrition Treatment(SMAT)is one of the most efficient methods,which was developed for synthesizing a nanostructure surface layer on metallic materials in order to upgrade the overall properties and performance,in all severely plastic deformation(SPD)processes.The basic idea of SMAT,as a type of surface self-nanocrystallization,is to transform the surface layers of the materials into nanocrystalline while keeping the overall composition and/or phases unchanged.In this paper,the effects of SMAT time on the microstructure and mechanical properties of the 6082 aluminum alloy were first studied.And then the aging process and thermal stability of the SMAT 60 min samples were studied.The microstructure of the samples was characterized by optical microscope(OM),X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electron microscope(TEM).The mechanical properties of the samples were investigated by tensile test and hardness measurement.Strengthening mechanisms,aging behaviors and thermostability were analyzed.The main results are as follows:(1)Nanocrystalline layer and plastic deformation microstructure layer were formed on the surface of 6082 aluminum alloy via SMAT process.And on the topmost surface,equiaxed nano grains,of which the grain size is about 25 ?m,was formed.Meanwhile sub-micron grains were observed within the region of 25-70 ?m below the topmost surface.With longer time SMAT,it leads to more plastic deformation on the surface and thickening the nanocrystalline layer.The tensile strength of the samples increased with the SMAT process going on.On the contrary,the ductility of the samples decreased gradually.(2)After SMAT process,a gradient variation in the microstructure and properties along the depth from top surface were observed.The grain size varied from nano scale to micron scale,the hardness varied as well.The hardness of the topmost surface was about 2-4 time than that of the deep matrix,and with the depth from top surface increasing,the hardness decreased gradually.(3)The aging behavior of the SMAT 60 min samples was different from the coarse-grained 6082 aluminum alloy greatly.For the coarse-grained samples,it reached the peak-aged state after aging at 175? for 6 hours,when the strength and hardness was highest along all the aging process.However,for the SMAT 60 min samples,its strength after aging for 6 hours was lower than that before aging process.And the hardness on the top surface and sub surface showed unusual characteristics during the aging process.As recrystallization is a typical atom diffusion-controlled process,longer time isothermal holding provides sufficient time for atoms to diffuse,resulting in nucleation of new grains and grain growth.(4)After annealing at 100? and 200? for 1 hour,the microstructure and the mechanical properties of those samples was similar to that of SMAT 60 min samples,and the deformation features can still be clearly observed.And even the tensile strength was higher than the SMAT samples.To sum up,the SMAT samples was stable at low temperature and had high thermal stability.(5)After annealing at 300? and 400? for 1 hour,noticeable equiaxed grains form within the SMAT affected zone,grains coarsening occurred as well.Besides,the tensile strength and hardness was worse than the SMAT samples,which means the SMAT 6082 aluminum alloy cannot service well at relatively high temperature.