Effect Of Retrogression And Reaging Treatment On Grain Boundary And Matrix Precipitation Of 7150 Aluminum Alloy

7xxx series aluminum alloy is widely used in aerospace,weapons and large pressure vessel production since it has many advantages such as light weight,high specific strength,high specific stiffness,excellent corrosion resistance and good machinability.The Retrogression and Reaging treatment was used to 7150 aluminum alloy to obtain the microstructure characteristics that the grain boundary precipitates are coarsening,however the matrix precipitates are homogeneous and fine which is similar to the state of peak aging,which can ensure the 7150 aluminum alloy has high strength,good plasticity,good toughness and also excellent intergranular corrosion resistance and exfoliation corrosion resistance simultaneously.Recently,most of the study on the RRA treatment of 7150-T6 aluminum alloy were focused on optimizing the aging progress.There is little research attention into the evolution of both the grain boundary precipitates and the matrix precipitates of 7150 aluminum alloy during retrogression treatment.The retrogression treated of 7150 aluminum alloy sheets were heated at a temperature of 140??280? for 4 s?12 h in this paper.The evolution of the grain boundary precipitates and matrix precipitates of the 7150 aluminum alloy sheets during retrogression treatment was studied by conductivity,hardness,microhardness test as well as the microstructure analysis before and after they were reaging treated at 120? for 12 h.The main conclusions are as follows:1.When the aging temperature is 120?,the grain boundary precipitates and matrix precipitates have no obvious change with increase of the aging time.The size of the grain boundary precipitates are about 15 nm?30 nm,the distribution of the grain boundary precipitates are incomplete discrete.The matrix precipitates are fine dot-like precipitates.The dissolution temperature of the GP zone and ?' phase of 7150 aluminum alloy sheet aging at 120? for 12 h is about 138? and 225? during continous heating.2.When the retrogression temperature is less than 160?,the retrogression time has less effect on the hardness of the T6 state aluminum alloy before and after reaging.When the retrogression temperatures are 180?,200? and 240?,the hardness of the sheet before reaging decrease first,then increase and finally decrease with the retrogression time increasing.Higher the retrogression temperature is,shorter the time of the hardness is at valley and peak value,the time of the hardness at valley value is about 24 min,3 min and 10 s.When the retrogression temperatures is 180?,the hardness of the sheet after reaging decrease first,then increase and finally decrease with the retrogression time increasing.When the retrogression temperatures are 200? and 240?,the hardness of the sheet after reaging increase first and then decrease with the retrogression time increasing.With the retrogression time increasing,the hardness of the sheet retrogressed at 280? before reaging decrease while the hardness of the sheet retrogressed at 280? before reaging increase first and then decrease.3.The grain boundary precipitates of the T6 state 7150 aluminum alloy sheet before and after reaging are continuous growth,coarsening and become more discrete with the retrogression time increasing when the retrogression temperatures are 180? and 200?.When the retrogression time lower than 3 min,some matrix precipitates of the alloy retrogressed at 180? dissolve into the aluminum matrix after retrogression treatment,for the alloy retrogressed at 200?,this phenomenon happens at the time lower than 30 s.When the size of the matrix precipitates in the alloy retrogressed at 180? is the same as the size of the matrix precipitates in T6 state alloy,the retrogress treatment time is less than 48 min.When the size of the matrix precipitates in the alloy retrogressed at 200? is the same as the size of the matrix precipitates in T6 state alloy,the retrogress treatment time is less than 3 min.When the retrogress time over 48 min,the grain boundary precipitates of the alloy retrogressed at 180? before and after reaging start become coarsening.When the retrogress time over 3 min,the grain boundary precipitates of the alloy retrogressed at 200? before and after reaging start become coarsening.4.When the retrogression time is 4 s and the retrogression temperature between 140?and 200?,the matrix precipitates of the alloy before and after reaging are similar.When the retrogression time is 4 s and the retrogression temperature over 240?,the matrix precipitates growth rapidly.When the retrogression time is 4 s,the matrix precipitates growth slowly when the retrogression temperature increase from 140? to 160?,the matrix precipitates growth rapidly when the retrogression time over 180?.5.When the retrogression times are 4 s,48 min and 12 h,the grain boundary precipitates of the T6 state 7150 aluminum alloy sheet before and after reaging are continuous growth,coarsening and become more discrete with the retrogression temperature increasing.