Microstructure And Fsw Weldability Of 7085Al Composities Reinforced By In-situ ZrB₂ Nanoparticles And Rare Earth Sc

7085Al alloy is a new generation of super high strength aluminum alloy for aviation,which has high strength and good forming performance.However,the low toughness and poor welding performance of 7085Al alloy are the bottlenecks restricting its development.In recent years,foreign researchers had introduced nanoparticles to improve the weldability of 7075 alloy welded joints.On the one hand,nanoparticles have the effect of grain refinement,which can reduce the grain size of the joint and inhibit the formation of thermal cracks.On the other hand,the mechanical properties of the joints are strengthened by nanoparticles with high temperature stability.However,there are few reports on the strength,toughness and weldability of the new generation of 7085Al alloy for aviation in China.With the rapid development of the new generation of manned spaceflight,large aircraft and advanced fighter aircraft,how to improve the mechanical properties and weldability of 7085Al alloy is an important topic that needs to be studied urgently.In this paper,a new method for the synergistic strengthening of in-situ Zr B₂nanoparticles and rare earth Sc was proposed.The in-situ Zr B₂nanoparticles and rare earth Sc were prepared by using Al-Zr-B system and Al-2Sc master alloy to strengthen 7085Al composite.The effects of in-situ Zr B₂nanoparticles,rare earth Sc and synergistic strengthening of Zr B₂and Sc on the microstructure of 7085Al alloy and friction stir welding（FSW）joints were investigated.The mechanical properties and FSW welding properties of the composites were studied,and the strengthening and toughening mechanism and welding mechanism of in-situ Zr B₂nanoparticles and rare earth Sc on the composites were revealed.The specific research conclusions were as follows:Study on microstructure of composites shows that:The in-situ synthesized Zr B₂particles were small,with an average size of 73.9 nm.The Zr B₂particles were quadrilateral or hexagonal,and the interface between the particles and the matrix was non-pollution and well bonded.Nano Zr B₂particles had a refinement effect on the matrix grain,and the particles were distributed in the form of clusters in the matrix.With the increase of particle content,the particle cluster size increases.When the particle content was 3 wt.%,the distribution of Zr B₂particles was the most uniform,and the matrix grain size was the smallest,with an average of about 46.9μm.When rare earth Sc was added to 7085Al alloy,Al₃Sc and Al₃（Sc,Zr）particles were formed,which were spherical and had obvious refining effect on matrix grains.When the Sc content was 0.5 wt.%,the grain size of the alloy decreases from 129.3μm to 26.1μm.When the content of rare earth reached 0.7 wt.%,the primary Sc-containing metal compounds were formed,which splited the matrix and reduced the performance of the alloy.In situ nano Zr B₂particles and rare earth Sc synergistically strengthened 7085Al composites,the matrix grain was further refined and the hole defects were significantly reduced.Study on microstructure evolution of FSW joint of composites shows that:Compared with7085Al alloy,the grains in all regions of FSW joints of in-situ nano Zr B₂particles reinforced7085Al composite decreased,and the grain size near the particles was finer.The addition of Sc also makes the grains of FSW joints of the composites smaller than those without rare earth addition,and reduces the growth trend of grains after welding thermal cycle.Particle distribution of 1 wt.%in-situ nano Zr B₂particles and 0.5 wt.%rare earth Sc synergistic strengthening7085Al composite joint in the nugget zone（NZ）was more uniform.After friction stir welding,the precipitates in the heat affected zone（HAZ）and NZ of the joint decreased,and the dislocation density decreased.Study on mechanical properties of composite materials shows that:The addition of in-situ nano Zr B₂particles was beneficial to improve the mechanical properties of composites.The simultaneous improvement of elongation and strength could be realized when adding 1 wt.%Zr B₂.Rare earth Sc also showed obvious strengthening effect on 7085Al alloy.When the rare earth content was 0.5 wt.%,the strength of the composite increased by 45.7 MPa.The tensile strength of 7085Al composite reinforced by 1 wt.%Zr B₂particles and 0.5 wt.%rare earth Sc was further improved to 562.6 MPa.The strengthening of 7085Aluminum alloy by in-situ Zr B₂particles and rare earth Sc were mainly realized by fine grain strengthening,Orowan strengthening,CTE strengthening and load-bearing strengthening.The synergistic effect of in-situ Zr B₂particles and rare earth Sc was attributed to"the adsorption of rare earth Sc by the stress field around Zr B₂particles",and the Sc-containing wetting layer was formed,which improved the wettability between ceramic particles and matrix.The dispersion of particles was improved and the strengthening effect was improved.The study on the mechanical properties of FSW joints of composites shows that:Compared with 7085Al alloy joint,the tensile strength of 1 wt.%Zr B₂reinforced 7085Al composite joint increased by 71.2 MPa,and the joint coefficient increased from 59.95%to 72.39%.When the Zr B₂reinforcement content continues to increased to 3 wt.%or 5 wt.%,the particles in NZ of the joint accumulated.The weak bonding between particles made the crack easy to initiate and propagate,resulting in the decrease of joint performance.After the addition of 0.5 wt.%rare earth Sc,the tensile strength of the FSW joint of the composite increased by 55.5 MPa compared with that without addition.After the composite addition of 1 wt.%Zr B₂reinforcement and 0.5wt.%rare earth Sc,the tensile strength of the FSW joint of the composite increases by 114.8MPa,and the joint efficiency reached 72.72%.The tensile fracture of the joints containing 0.5wt.%or 0.9 wt.%rare earth Sc was in NZ,which was mainly caused by the uneven grain size and obvious stress concentration in the thermo-mechanically affected zone（TMAZ）and NZ of the advancing side,which made NZ become a new weak performance zone and finally causes tensile fracture.