Study On The Microstructure,Properties And Friction Stir Welding Of 7N01Al Alloy Synergistically Reinforced By In-Situ ZrB₂ Nanoparticles And Rare Earth Sc

7N01Al alloy is a typical medium-high strength and weldable aluminum alloy,widely used in the welding structural parts of rail transit vehicle such as high-speed train,subway and maglev.However,as the requirements of new generation rail vehicles to be high speed,lightweight,low energy consumption keep rising,traditional 7N01Al alloy can’t meet the demands of being high strength and toughness,high safety and good welding property.Therefore,it is of great significance to further improve the strength of 7N01Al alloy and maintain or improve its toughness and welding property.At present,in-situ nanoparticle strengthening and rare earth microalloying are important ways to improve the strength,toughness and welding property of aluminum alloy.But the agglomeration of in-situ nanoparticle and excessive rare earth could generate coarse precipitates or being expensive make single reinforcement method limit the increase of property and promotion of application.Therefore,this thesis proposed a reinforcement method which make in-situ nanoparticles and rare earth reinforce7N01Al alloy synergistically.In situ ZrB₂ nanoparticles and rare earth Sc are introduced into 7N01Al alloy simultaneously to fabricate ZrB₂/7N01Al-Sc composites reinforced by in situ ZrB₂ nanoparticles and rare earth Sc synergistically.The effects of in-situ ZrB₂ particle,rare earth Sc and their synergistic effects on microstructure,mechanical properties and welding property were studied by OM CPLM,SEM,TEM,XRD,electronic universal material testing machine and microhardness tester.The mechanisms of their synergistic effects are also discussed.The study of microstructure indicates that there are amounts of particle clusters exist in ZrB₂/7N01Al composites.The amount and size of clusters rise by the increase of ZrB₂ contents.ZrB₂ particle exhibits hexagonal structure with round edge and its average size is 61.7 nm.The interface between particles and Al matrix is clean and the interfacial bonding is great.The introduction of ZrB₂ particles can also refine grain and the minimum size of grain is 44.3μm when the content of ZrB₂ is 3 wt.%;The microstructural study of 7N01Al-Sc alloy indicates that the introduction of Sc can refine grain more greatly than ZrB₂ particles and the minimum grain size is 33.7μm when the content of Sc is 0.2 wt.%.The grain refinement is related to the Al₃Sc and Al₃（Sc,Zr）phases precipitate after the introduction of Sc.The coarse precipitates Al₃（Sc,Zr）form in the matrix when the content of Sc exceeded 0.2 wt.%and the Al₃Sc phases also become bigger at the same time.The addition of Sc also changes the coarse and continuously distributed phase into fine and discontinuously distributed Sc-containing phase;The simultaneous introduction of in-situ ZrB₂ nanoparticle and rare earth Sc further refine the grain of 7N01Al alloy and the average grain size of 3 wt.%ZrB₂/7N01Al-0.2 wt.%Sc composite is 24.2μm.Besides,the agglomeration phenomenon is alleviated by improving the wettability between ZrB₂ particles and Al melt after adding Sc into ZrB₂/7N01Al composites.The ZrB₂ particles promote the precipitation of rare earth phases around it by helping solute atoms and vacancy to diffuse towards the interface and reducing the activation energy for nucleation.The study of mechanical property indicates that the tensile properties of ZrB₂/7N01Al composite reach the highest when the content of particle is 3 wt.%and the YS,UTS and elongation are 451.5 MPa and 482.2 MPa,which increase by 11.6%and 6.9%compared with 7N01Al alloy for YS and UTS,but decrease 12.6%for elongation.For 7N01Al-Sc alloy,the tensile properties reach the highest when the content of Sc is 0.2 wt.%,and the YS,UTS and elongation are 462.1 MPa,493.2 MPa and 10.2%,respectively,which increase by 14.4%,9.3%and 43.7%compared with7N01Al alloy.Combining the optimum contents of in-situ ZrB₂ nano particles and rare earth Sc to fabricate 3 wt.%ZrB₂/7N01Al-0.2 wt.%Sc composites.The YS,UTS and elongation of composite are 477.8 MPa,506.4 MPa and 9.8%,increase by 18.1%,12.2%,38.0%compared with 7N01Al alloy.The fracture morphology indicates that the fracture mechanism of composite is ductile fracture.Strengthening and toughening mechanisms analysis shows that the strengthening mechanisms of composite are mainly grain refinement strengthening,dislocation strengthening,Orowan strengthening,load bearing strengthening and the toughening mechanisms relate to the work-hardening generated by the introduced nanoparticles and rare earth precipitates.The study of friction stir welding（FSW）property indicates that the welding joint of 3 wt.%ZrB₂/7N01Al-0.2 wt.%Sc composite is divided into heat affected zone（HAZ）,thermal mechanical affected zone（TMAZ）and nugget zone（NZ）.HAZ was only affected by input heat and its microstructure still stay as BM condition;TMAZ is the boundary of HAZ and NZ.The grain in this area is elongated and ZrB₂ clusters distribute in band-shape along the boundary;NZ undergone high temperature thermal cycling and severe plastic deformation.The particle clusters in NZ are broken and disperse uniformly and some particles with big size are cracked.EBSD analysis indicates that the dynamic recrystallization in NZ of 7N10Al alloy happen and make grain in this area equal-axis and fine.The grain size in NZ of composite is only 1/6 of that in 7N01Al alloy because the recrystallization is restrained due to the pinning effect of reinforcements and sub-structure is preserved.Hardness tests show that NZ of composite is 29.9%higher than that of 7N01Al alloy and HAZ is still the zone which has the lowest hardness though it is also be reinforced.Tensile tests of welding joints show that the welding efficiency of 3 wt.%ZrB₂/7N01Al-0.2 wt.%Sc composites is92.3%and 12.8%higher than 7N01Al alloy.The strengthening mechanisms of NZ are further grain refinement strengthening,Orowan strengthening and dislocation strengthening.The softening mechanism of HAZ is related to the coarseningη’（Mg Zn₂）generated by high temperature process during FSW.The lost strength during FSW process is compensated by the introduced in-situ ZrB₂nanoparticles and Al₃Sc,Al3（Sc,Zr）phases.