Composition Optimization And Hot Working Microstructural Control Of The Spray Deposited Al-Zn-Mg-Cu Alloy

Based on the characteristics of deep undercooling and rapid solidification,the spray-deposited Al-Zn-Mg-Cu series alloys have wide applications in aeronautics and astronautics fields.At present,many achievements have been made in the research of spray-deposited highly alloyed Al-Zn-Mg-Cu alloys,however,there are still some shortcomings in composition optimization,microstructural control and so on.In this research,the alloy composition was designed based on the thermodynamic calculation method of phase diagram,and the highly alloyed Al-Zn-Mg-Cu series alloys were prepared by spray deposition.The effect of alloy composition on the microstructure,phase composition and properties of the alloys in the preparation processes(spray deposition,densification and heat treatment)were systematically studied.The corresponding relationships between the alloy composition and the microstructure and properties were then established.Subsequently,the forging experiments were carried out on the alloy with optimized composition,and the stress,strain behavior and the formation mechanism of defects in the deformation process were emphatically analyzed.The above research is of great significance for the development of new alloy system with independent intellectual property rights and the application of the spray-deposited highly alloyed Al-Zn-Mg-Cu alloys.The thermodynamic calculation results show that Zn/Mg ratio is the most important factor in determining the type of precipitates in Al-Zn-Mg-Cu series alloys.When Zn/Mg>4.5,with increasing Cu content,only ?(Mg Zn2)and ?(Al2Cu)phases can form in the alloy.When Zn/Mg<4.5,T(Al2Mg2Zn3)and S(Al2Mg Cu)phases appear in the alloy with increasing Cu content.Microstructural observation of the alloys with different composition shows that the special solidification condition of spray deposition leads to the lack of residual liquid phase in the eutectic reaction,thus the grain boundary structure presents divorced eutectic morphology.Influenced by the velocity difference of the solute diffusion,the solidification path of the alloy shifts to low Cu direction,but the eutectic reaction of liquid??-Al+?(Mg Zn2)still occurs.The eutectic phases generated by liquid phase reactions are all ?(Mg Zn2).The amount of the secondary phases in the grain interior is mainly determined by Mg content,and the size of the secondary phases at the grain boundary is determined by the total content of(Mg+Cu).The addition of trace Sc element in the alloys reacts with Zr to form the primary Al3(Sc Zr)particles which can not only refine the grains,but also refine the grain boundary divorced eutectic structures by heterogeneous nucleation.The solid state of the atomization droplets and the sedimentary layer is the key factor to determine the type of pores.In the HIP(hot isostatic pressing)process,the densification of the billet is divided into the stage of plastic deformation and creep diffusion.The optimal HIP parameters are temperature 465/4h to 485/h,pressure 130 MPa,and the density of the billet is more than 99.9% after treatment.The pores of the deposited alloy disappear completely after hot extrusion,and the secondary phases in the grain interior and at the grain boundary are broken and dispersed along the extrusion direction.Weak surfaces are generated parallel to the extrusion direction in the extruded alloy.After extrusion,the high Mg content alloy has larger amount of secondary phases,with increasing the Sc content,the size of the secondary phases decreases gradually and the microstructure is further refined.After the two-stage solution treatment,most of the secondary phases are dissolved into matrix,and the low Mg content alloy is easier to enter the single ?(Al)phase region.Aging at 120?,the age hardening curves of the alloys present double peak characteristic.High Mg content increases the dissolving driving force of the GP zone and the nucleation rate increases during aging.High Cu content can effectively prevent the precipitate from coarsening and transformation,thus is beneficial to maintain the peak aging state of the alloy.The main precipitation phases in the peak aging alloys are ?? phase,with thickness of 2~3nm.With increasing Mg content,the volume fraction of the precipitates increases significantly,the precipitation area fraction of the grain boundary increases,also,the strength increases and the plasticity decreases.Cu content plays little role on the size and volume fraction of the precipitates in the grain interior.For the alloy with high Mg content,increasing Cu content will increase the amount of undissolved coarse intermetallic compounds,which promotes the occurrence of intergranular fracture and further reduces the ductility of the alloy.The alloy with medium Zn/Mg ratio and Cu content shows the best comprehensive mechanical properties: tensile strength 811 MPa,yield strength 778 MPa,elongation 6.8%.The addition of trace Sc in the alloy generates a large number of fine Al3(Sc Zr)precipitates,which can stabilize the substructures and improve the mechanical and creep resistance properties of the alloy.The influence of deformation speed and temperature on the deformation process of the forgings was studied by numerical simulation.The optimum deformation speed was determined to be 1.0mm/s~3.0mm/s,and the optimum temperature was 430?.The deformation schemes were designed according to the size and shape characteristics of the target component and the difference state of the preforms,and the disc-shape forgings were prepared in the optimum range of deformation parameters.The HIPped billet is subjected to secondary tensile stresses in the free upsetting process,and serious cracks will be generated at the outer edge of the preform.When the height to diameter ratio of the extruded preform is large,the defects of the flow line folding and the eddy current are easy to occur,and the uneven distribution of flow lines leads to anisotropy of mechanical properties of the forgings.After heat treatment,the canned forging shows the best mechanical properties: with tensile strength of 727 MPa,yield strength of 690 MPa and elongation of 5% along the radial direction;tensile strength of 735 MPa,yield strength of 693 Mpa and the elongation of 6.3% along the tangential direction of the plate,and the fracture toughness along the double directions are higher than 30MPam1/2.