Study Of Thermal Stability And Properties Of 7075 Aluminum Alloy During Friction Stir Processing

As a kind of serve plastic deformation,equiaxed fine grains can be obtained by friction stir processing,which is the initial material for super-plastic processing.Thermal Stability of microstruture is vital in super-plastic processing.However,abnormal grain growth was observed after heat treatment.Therefore,it is necessary to study microstruture thermal stability in the friction stir welding/processing.And study of thermal Stability of microstruture during FSP is a guidance for the parameters of heat treatment in FSW..In this paper,AA7075 –T6 were friction stir processed(FSPed)under load control at a rotational speed of 400 rpm and a translational speed 80 mm/min),and FSPed sample was followed by heat treatments.Abnormal grain growth were investigated.The main results are as follows:(1)The EBSD data shows that after friction stir processing,the five typical regions of the processing area are the products of different stages in the dynamic re-crystallization process.The three regions in the RD direction have high uniformity in local strain,recrystallization and grain orientation distribution,and these in SFZ and SZ are similar.(2)The coarse grains are observed in five typical zone during heat treatment.And the direction and degree of grain growth are different.The direction of grain growth is consistent with the direction of distribution of the second phase in these region.As the heat treatment time or temperature increases,the grain size of the five typical regions of the weld nugget increases,and the direction of grain coarsening does not change.(3)There are three states in the second phase of the processing region,the fine dispersed phase,the coarse undissolved phase in the grain,and the discontinuous second phase in the grain boundary,and the second phase is streamlined.As the heat treatment time and heat treatment temperature increase,the precipitation phase distribution is more uniform,while some of the undissolved second phase is more coarse.(4)The tensile test and the hardness test showed that,after heat treatment,the strength and elongation of the sample were improved,and the "W" characteristic of the hardness distribution disappeared.The hardness values of different zones in the same layer are basically equal.