| Age-treated7075aluminum alloy with high-strength and good corrosion resistant hasbeen widely used in aerospace,aviation,military and automotive industries.However,itsfatal flaw that high-strength aluminum alloy has been generally considered difficult tobe the fusion welding process due to the poor solidification microstructure and porosityin the fusion zone has had great impact on its wide application. Friction Stir Weldingwhich is a new type of solid state welding method makes the7075Al welding possibleand solve the fusion welding problems. Current research on7075aluminum FSW andFSP has made many achievements. There is much less research on microstructureevolution in the process of7075aluminum alloy friction stir processing.In this paper,through different rotation speed and stop-action technology to process the ageing of7075aluminum alloy sheet during friction stir processing, the microstructures evolutionand mechanical properties were studied after the processing.The main conclusionsincluded:â‘ Through microhardness test,metallographic microscopy and back scatteredelectron imaging technique, microstructures and mechanical properties around thekeyhole RD-ND section and at the joint TD-ND section were studied.The results showthat:The hardness profile likes an arched door shape around the keyhole RD-ND section.Microstructures around the keyhole which were squashed and stretched graduallyoccurred recrystallization and fully recrystallized behind the stir pin. The hardnessprofile likes taper shape and pour eight in the NZ and TMAZ respectively at the jointTD-ND section.The nugget zone happens fully recrystallization,from the top to thebottom in the NZ grain size gradually decreases and the size and distribution of theintracrystalline dispersed second phase are same,but the size and density of the secondphase at grain boundary gradually increased.The interface between the NZ and theTMAZ is clear and sharp at the advancing side.There are tunnel holes and kiss defectsat the interface,however the interface transit easily without defects at the retreating side.â‘¡Microstructures around the keyhole RD-ND section and at the joint TD-NDsection were characterized by electron back scattering diffraction technology. Initialmicrostructure under the stir needle shear and compression flows along the stir needlecylinder tangent direction around the keyhole RD-ND section. Material of TMAZ flowsalong the stir needle axial at the keyhole center ND-TD section. Grain size at the top of the nugget zone is slightly flattened due to the shoulder pressure.There is a few lowangle grain boundaries in the grain. In the same grain there is a different degree oforientation.â‘¢Mechanical properties of the material which was processed at same rotatingspeed and different rotational speed in the nugget zone were compared and analyzedthrough the room temperature tensile test.1#plate from the top to the bottom at thewelding zone, the tensile strength decreases.2#,3#and4#plate the5mm and3mmlayer in welding zone happened sudden rupture due to defective materials.only1#platehas no obvious defects after friction stir processing which show that for7075frictionstir processing, rotation speed600r/min and the welding speed100mm/min is a set ofoptimization parameters.Ageing of7075aluminum alloy mainly increases the strengthof the material by the intracrystalline dispersed phase.However, the coarsening secondphase at grain boundary can reduce the strength of the material. Fine grain structure cansignificantly increase the plasticity but the coarsening second phase at grain boundarycan reduce the plasticity. Eventually the coarsening second phase at grain boundary hasbad effects on the comprehensive mechanical properties of materials.The problems willbe solved by different processing parameters and subsequent heat treatment. |
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