Study On Ultra-high Strength Al Alloy Containing Sc During Friction Stir Processing

The 7055 and different Sc and Zr contents of the ingot were prepared by the melting and casting process,respectively.The alloy ingots were homogenized,rolled,and solution treated,and the T6 treated alloys were subjected to friction stir processing?FSP?under different process parameters.OM,SEM,TEM,XRD,DSC,Vickers hardness tester Analytical techniques were used to study the influence of Sc and Zr content on the microstructure and mechanical properties of 7055 alloy during solidification,homogenization,rolling,T6 treatment,FSP,and F6 after TSP treatment.The results show that the as-synthesized Al₃?Sc,Zr?phase can be formed during the solidification process of the 7055 alloy with Sc and Zr added.This phase can act as a heterogeneous nucleus to promote the nucleation of the alloy,which in turn significantly refines the as-cast alloy grains.And promote equiaxed crystal formation.Secondary Al₃?Sc,Zr?precipitated phase is formed during the homogenization annealing.This nano-phase size can pin dislocations and grain boundaries,thereby inhibiting dislocation migration and grain during the subsequent deformation and heat treatment of the alloy,and thenincreases the thermal stability of the alloy.The contents of Sc and Zr have a great influence on the microstructure and mechanical properties of 7055 alloy.The present study shows that the addition of0.25%of Sc can make the best combined mechanical properties of 7055 alloy,and its maximum tensile strength,yield strength and elongation reach to 679MPa,600MPa and 14.3%,respectively.T6 process of 7055 aluminum alloy is solid solution 470°C,2h,artificial aging 120°C,24h.The high-density metastable??phase is the main strengthening phase of the T6-treated alloy.The grain refinement and the nano Al₃?Sc,Zr?phase also further optimize the mechanical properties of the alloy.The 7055 alloy and 7055Sc alloy are processed by friction stir to form fine equiaxed crystals.Adding Sc and reducing the processing rotation speed can further reduce the grain size of the FSP sample.The temperature rise caused by the FSP process causes the?'phase to transform into the steady state?phase.Therefore,the strength of the FSP sample is significantly lower than that of the T6 state alloy.To obtain a high-density?'phase,the FSP sample was solution-aged.Microscopic structural observations revealed that the grain size of FSP 7055 alloy was seriously coarsened after solid solution aging.The average grain size is as high as 12.4?m,but the nano-Al₃?Sc,Zr?precipitates can significantly inhibit the coarsening of the grain size during the solid solution aging treatment of the FSPed 7055Sc sample,thereby retaining a large number of fine equiaxed grains.The average grain size of TSP-treated FSP 7055-0.25Sc alloy is only 3.4?m.Therefore,the alloy has both high-density?'phase and fine equiaxed grain structure.This structure makes the FSP7055-0.25Sc alloy with excellent mechanical properties and excellent damping performance after T6 treatment,which further expands the engineering application of high-strength aluminum alloys.