Study On Microstructure And Mechanical Properties Of High-strength Al-Zn-Mg-Cu Alloy Prepared By MA-SPS

Al-Zn-Mg-Cu alloys are the essential structural materials in aerospace,national defense and transportation fields.With the rapid development of giant aircraft and rail transit industry in China,higher requirements are put forward for the comprehensive properties of Al-Zn-Mg-Cu alloys.Due to the limitation of low solubility of alloying elements,coarse grain size and intergranular secondary phase,the strength of the alloys prepared by traditional casting technology is difficult to be improved.The Al-Zn-Mg-Cu alloys prepared by powder metallurgy process have a series of advantages,such as high solute content,fine grain size,uniform microstructure,good plastic formability and aging strengthening effect,and excellent mechanical properties.However,the pre-alloyed gas atomized powders are currently used for powder metallurgy Al-Zn-Mg-Cu alloys,the coarse powder particles,high hardness and low sintering activity limit its widespread applications.In this paper,the high-strength Al-6.0Zn-2.5Mg-2.5Cu（wt.%,Abbr.Al-Zn-Mg-Cu）alloy was prepared by mechanical alloying（MA）and spark plasma sintering（SPS）using elemental powders of Al,Zn,Mg and Cu as raw materials.The morphology and microstructure evolution of the powders during mechanical alloying and the microstructure and properties of block alloy prepared by SPS were studied,meanwhile the strengthening mechanism of Al-Zn-Mg-Cu alloy prepared by MA-SPS was discussed.The effects of thermal compression deformation and aging treatment on the microstructure and properties of MA-SPS Al-Zn-Mg-Cu alloy were investigated.The following research results were obtained.Nanocrystalline Al-Zn-Mg-Cu alloy powders were prepared by high energy ball milling,and the morphology and microstructure evolution of the powders were investigated.In the range of 0～40 h,with the increase of milling time,the evolution of powders morphology went through five stages including flattening,welding,crushing,particles equiastration and dynamic equilibrium.The mechanism of grain nanocrystallization was that annihilation,merger or recombination of the dislocations in the powders resulted in cellular structure or substructure,which was transformed into nanocrystalline.While the formation of supersaturated solid solution was facilitated by a large number of grain boundaries and defects in powders.30 h milled powders reached dynamic equilibrium,and had the smallest particle size of 3.02μm and grain size of 22.3 nm,and the largest micro-strain of 0.183%.Nanocrystalline Al-Zn-Mg-Cu powders were consolidated by SPS to prepare bulk Al-Zn-Mg-Cu alloy.During SPS process,a certain degree of grain growth and solid solution decomposition occurred for the supersaturated solid solution powders.The grain size of bulk alloy sintered at temperature of 500℃and pressure of 50 MPa by using 30 h milled powders was 53.5 nm,which increased by 140%compared with the powders.The secondary phase Al₂Cu and Mg Zn₂ precipitated from the supersaturated solid solution during SPS process.The density,compressive strength,yield strength and Vickers hardness of Al-Zn-Mg-Cu alloys gradually increased with the extension of milling time,while the fracture strain gradually decreased.The sintered samples of 30 h milled powders had the relative density of 96.1%,while the compressive strength,yield strength,fracture strain and the Vickers hardness were 908.1 MPa,853.2 MPa,8.1%and 235 HV,respectively.The compression fracture mode of the sample was the quasi-cleavage fracture,and the micropores and second phase particles promoted the initiation and propagation of cracks.The grain size of Al-Zn-Mg-Cu alloy prepared by SPS increased with the increase of sintering temperature,while the sintering pressure had no obvious effect on the grain size.The grain size of the sample sintered at 550℃and 50 MPa was 84.7 nm,and the samples sintered had the relative density of 98.6%,while the compressive strength,yield strength,fracture strain and Vickers hardness were 683.8 MPa,634.2 MPa,11.2%and 211 HV,respectively.Compared with the samples sintered at 500℃,although the relative density increased,the grain size increased,leading to a decrease in strength and hardness,while the fracture strain increased significantly.The strengthening mechanism of MA-SPS Al-Zn-Mg-Cu alloy was mainly attributed to the fine-grain strengthening of nanocrystals,followed by dislocation strengthening and Orowan strengthening of the second phase particles,while the effect of solid solution strengthening was very limited.The hot compression deformation significantly increased the relative density of the Al-Zn-Mg-Cu alloy.When the compression ratio was 30%,the relative density increased from98.6%of the sintered sample to 99.6%;but when the compression ratio exceeded 50%,the relative density decreased due to the cracks in the samples.During the process of hot compression,the second phase aggregated and grew,and was arranged along the direction of deformation.The main deformation mechanism ofα-Al grain was slipping,accompanied by dynamic recovery and dynamic recrystallization,resulting in partial grain growth,while the hot deformation mechanism of ultra-fine nanocrystallines was intergranular sliding.With the increase of solution temperature and solution time,the number of the second phase in the Al-Zn-Mg-Cu alloy decreased and the Vickers hardness increased gradually.The Vickers hardness of the sample solution at 470℃/2 h reached the maximum of 202 HV.The aging temperature of the peak hardness corresponding to this solution process was120℃.The high grain boundary fraction,high solid solubility and crystal defect density in the microstructure accelerated the aging process of the MA-SPS alloys.The peak aging time was 5 h,which is significantly earlier than that of the alloys formed by casting and spray,and the peak hardness was 238 HV.The aging precipitate series were as follows,supersaturated solids（SSSS）→G.P.region→metastable precipitates semi-coherent with matrix（η’phase）→equilibrium precipitates incoherent with the matrix（η-Mg Zn₂）.In the precipitation process,the GP formation corresponded to the peak I of the harding curve,while the phaseη’was the main age strengthening phase,corresponding to the peak aging.The bending strength and bending strain of 470℃/2 h+120℃/5 h solution aged Al-Zn-Mg-Cu alloy were 1105.5 MPa and 0.50%,respectively,which were increased by 54.1%and233.3%,compared with those of hot compression deformed specimens.The bending fracture mode of the aged sample was quasi-cleavage fracture.