| Al-Zn-Mg-Cu high strength aluminum alloy have been widely applied to aerospace industry and modern high speed strain industry because of the excellent characteristics, such as high strength, low density, good toughness, corrosion resistance, and so on. Based on the best use of the existing aluminum alloy production equipment, ultra-high strength aluminum alloy bars were fabricated using rapid solidification atomization, powder hot extrusion process, subsequent heat treatment technology. The objective of this research is to get fine grain and homogenously distributed precipitates to significantly improve the strength, plastic, resistance to stress corrosion, and ultimately to develop a new low-cost high-strength aluminum alloy fabrication technology that suitable for industrial production.This paper studies the phase structure, microstructure, thermostability and other characteristics of two kinds of ultra-high strength aluminum alloy powder fabricated by nitrogen atomization technology, which composition are Al-6.0Zn-2.7Mg-1.3Cu (1#) and Al-10.0Zn-3.2Mg-2.3Cu-0.2Zr (2#), respectively. The powder hot extruded alloy bars were prepared by above mentioned raw materials, and extrusion process parameters, i.e. extrusion temperature, extrusion ratio, powder size, and heat treatment process were optimized. The fracture mechanism, strengthen mechanism, dynamic recovery and recrystallization mechanisms were studied as well.The results show that the microstructure of nitrogen atomizing ultra-high strength aluminum powder is fine dendrite, and the phases are mainlyα-Al and MgZn2. The rapid solidification process greatly increased the solubility of alloy elements in solid solution,1# and 2#-C group alloy powder have the highest degree of saturation. The fine dendrite is retained after powder are exposed to high temperature 400℃for 1h, which can meet the request for degassing and extrusion successfully.The microstructures of extruded alloys are mainly made up by shear bands and densely distributed precipitates distributed along the extrusion direction. The phase of extruded alloys is mainlyα-Al,η(MgZn2) and a spot ofη'(MgZn2), AlCu2Fe phase. Shear bands polygonized during extrusion, thus, dynamic recovery occurs. The precipitates coarsen with improving the extrusion temperature and extrusion ratio. The size of deformed grains and precipitate particles are influenced by granularities. The smallest size of deformed grains and precipitates in the extruded alloy are observed in group C powder.Mechanical properties of powder hot extrusion alloys show that the optimum preparation parameters of 1# alloy are using group C powder, extrusion temperature at 300℃and extrusion ratio of 25, theσb,σ0.2 andδof extruded alloy are 575MPa, 547MPa and 7.4%, respectively. After the 1#-C group of powder are extruded at 300℃by extrusion ratioλof 36, theσb andσ0.2 up to 650MPa and 630MPa, but the elongation 8 is only 5.6%. The optimum preparation parameters of 2# alloy are using group C powder, extrusion temperature at 400℃and extrusion ratio of 25, theσb,σ0.2 andδof extruded alloy are 384MPa,275MPa and 8.2%.Fracture mechanisms of the extruded alloys are synthetically determined by granularity, extrusion ratio and the size, quantity and distribution of precipitates. When extrusion ratio is relatively small, the fracture mechanism is intergranular brittle fracture. When using large size powder or high extrusion ratio, the fracture mechanism is mix model of dimple fracture consist of internal necking and shearing spread microvoid coalescence. When the fracture mechanism of alloy is internal necking microvoid coalescence dimple fracture, the alloys have good overall performance. Dynamic recovery mechanisms of alloys extruded at lower temperature by 1#-C group powder and extruded at higher temperature with higher extrusion ratio by 2#-C group powder are stabilized polygonization and polygonization previous to recrystallization, respectively. The dynamic recrystallization occurs in the alloy made by 2#-C group powder because the severe deformation can increase the distortional strain energy, coarse precipitates promote piled-up dislocation to form nucleus, and thermal deformation cause the rise of extrusion temperature. Nucleation mechanisms of extruded alloy include projecting grain boundary nucleation, sub-grain nucleation and dislocation pileup nucleation. Heat treatment process of extruded alloys shows that the suitable solution treatment of 1# alloy is 460℃/20min, and that of 2# alloy is 460℃/2.5h. After solution and single-stage aging treatment, the alloys show significant age-hardening characteristics. The age-hardening curves of two kind of alloys have two aging peaks, corresponding to GP zones andη'strengthening, respectively. The suitable peak aging process of 1# alloy is 120℃/20h, and theσb,σ0.2 and 8 are 695MPa,675MPa and 11.3%, respectively. The counterpart of 2# alloy is 120℃/24h, and theσb,σ0.2 andδare 731MPa,670MPa and 6.2%. Alloys after solution and aging have ultra-high strength and maintain good toughness. The appropriate two-stage aging process of 1# alloy is 120℃/3h+160℃/18h, and theσb,σ0.2 andδare 582MPa,542MPa and 15.1%, conductivity reaches 38.7% IACS. On condition of loss part of the strength properties the alloy has excellent overall performance, whose elongation and electrical conductivity relative to T6 alloy are increased by 33.6% and 18.3%.Solution and aging treated alloys have fine grains, whose size is less than 5μm. The research shows the evolution of the microstructure of powder hot extruded alloys is different from that of traditional alloys during heat treatment. The microstructure s of 1# alloy treated by 120℃/3h+160℃/18h and 2# alloy treated by 120℃/24h show the characteristic of retrogression and reaging (RRA) microstructure. It is indicated that grain refinement lead to structural sensitivity diffusion of alloying elements, which make the RRA microstructure can be required by simplified heat treatment process. Zr elements suppress the nucleation of GP zone in the early stage of aging, which weakens the age-hardening characteristics of alloys. Zr elements precipitates in the shape of metastable Al3Zr particles with prolonging of aging time, promoting the form ofη'phase precipitates and strengthening the alloys. Pinning of metastable Al3Zr particles on dislocations is the essential reason to achieve fine-grain strengthening, dispersion strengthening and deformation strengthening mechanisms.
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