Study On Microstructure And Properties Of Al-Mg-Sc Alloy In Friction Stir Processing And Thermo-mechanical Treatment

As the most effective alloying element in aluminum alloy,the addition of element Sc will make Al-Mg alloy to produce a qualitative leap,which can effectively improve the microstructure and mechanical properties of the alloy.The application of advanced processing technology,friction stir processing（FSP）,to Al-Mg-Sc alloy can greatly refine the grain size of the alloy.However,few researchers have tried to combine the FSP alloy with the deformation and subsequent heat treatment process to further improve the mechanical properties of the alloy.This study took Al-3Mg-0.2Sc alloy as the research object,combined friction stir processing with traditional rolling and heat treatment technology,studied the changes of microstructure and mechanical properties of the alloy in the whole process in detail,aiming at improving the comprehensive mechanical properties of alloy,and providing theoretical and experimental basis for the research and development of aluminum alloy containing scandium.After the initial sample passed through FSP,observation of the section of the sample showed that the grain size and grain boundary characteristics of the alloy was not significantly different from each other in the transversal direction of the stirring zone（SZ）.However,along the normal direction,the microstructure of the SZ1 region at the top of the stirring zone was obviously different from that of SZ3,SZ5 and SZ6,which was caused by the influence of the shoulder driven zone.In addition,in the thermo-mechanically affected zone（TZ）,the grains of TZ1 were completely recrystallized.Then the misorientation distribution of TZ2 was mainly low angle grain boundaries,indicating that TZ2 was related to the plastic flow of the surrounding material forced by the stirring pin.TZ3 and TZ4 showed a coarse grain distribution,indicating that TZ3 and TZ4 were only affected by the thermal cycle.Finally,using the mathematical model proposed by previous researchers,it was determined that continuous dynamic recrystallization（CDRX）was the main mechanism of grain refinement in the stirred zone.The FSPed samples was rolled by 80%at room temperature,then measuring its hardness after high temperature annealing.For the FSPed+R80%sample,after 330℃ annealing,the annealing hardness peak was observed within the range of 50 min to 130min.Subsequently,the annealing temperature was raised to 400℃,and the annealing hardening peak was observed in the annealing interval from 40 min to 130 min,with the hardness peak appearing earlier.From the equivalent diameter of Al₃Sc particles,it was observed that the diameter of Al₃Sc particles decreased slightly after rolling,while the diameter presented an overall rising trend after annealing.Especially after annealing 70min,the equivalent diameter increased significantly.Finally,this study analyzed the hardness evolution mechanism in the whole process.The stage I is given priority to with recrystallization nucleation and growth,including the formation of subgrain and subsequent rotating and aggregation.But the Al₃Sc particles near the grain boundary would hinder the movement of subgrain or grain boundary,delaying the recrystallization softening effect.Hence,on the macro,hardness of stage I drops sharply,then the speed slows down.Stage II is dominated by grain growth,while the pinning effect of Al₃Sc particles at grain boundaries has been enough to inhibit grain boundary migration,as shown by the increased hardness of materials.Finally,for stage III,when the annealing time exceeds 70 min,the equivalent diameter of Al₃Sc particles increases significantly,indicating that the coarsening rate of Al₃Sc particles increases,and further showing that the coherent between Al₃Sc particles and matrix was decreased,which results in a decrease in the pinning effect.Hence,the pinning effect is insufficient to restrict the migration of grain boundaries,resulting in a decrease in the hardness of materials.