【作者】 王少卿；

【导师】 于化顺；

【作者基本信息】 山东大学 ， 材料加工工程， 2010， 博士

【摘要】 Al-Zn-Mg-Cu高强铝合金以其高强、轻质、高韧、耐腐蚀等优良特性,被广泛应用于航空航天工业和现代高速列车制造工业。本文以充分挖掘现有铝合金型材生产设备潜力为前提,采用快速凝固雾化制粉、粉末热挤压成形、后续热处理的技术路线,制备了高强铝合金棒材。本课题的研究目的是希望获得晶粒和析出相尺寸细小且分布均匀的合金组织,从而显著提高铝合金的强度及塑性,同时改善其抗应力腐蚀性能,最终开发一种新的适合于工业化生产的低成本高强铝合金制备技术。本文研究了采用氮气雾化技术制备的Al-6.0Zn-2.7Mg-1.3Cu(1#粉末)和Al-10.OZn-3.2Mg-2.3Cu-0.2Zr(2#粉末)两种高强铝合金粉末的物相、组织及其热稳定性等特性；采用粉末热挤压法制备了合金棒材,并对挤压工艺参数(挤压温度、挤压比、粉末粒度)进行了优化；优化了挤压棒材的热处理工艺；对合金的断裂机制、强化机制和回复再结晶等机制进行了研究。研究结果表明,氮气雾化高强铝合金粉末的组织是细小的枝晶,其物相主要是α-Al和MgZn2相。快速凝固技术大幅提高合金元素在基体中的过饱和度,1#合金粉末和颗粒尺寸小于75μm的2#-C组粉末基体元素过饱和度最高。在400℃保温1h后粉末组织仍为细小的枝晶结构,在保持粉末快速凝固特征的同时,能够满足除气和顺利挤压的要求。挤压合金组织由剪切变形条带组织和沿挤压方向流线分布的析出相构成,物相主要是玖-Al、η(MgZn2)和少量的η’(MgZn2)和Al7Cu2Fe相。挤压过程中变形条带组织主要发生多边形化,即动态回复。提高挤压温度和挤压比,组织中的析出相颗粒粗化。粉末粒度影响挤压变形晶粒和析出相颗粒的尺寸,粒度最小的C组粉末挤压合金的变形晶粒和析出相颗粒尺寸最为细小。粉末热挤压合金的力学性能研究结果表明,1“合金的最优挤压工艺为采用C组粉末挤压,挤压温度300℃,挤压比为25,在此工艺条件下合金的σb、60.2和6分别为575MPa、547MPa和7.4%。1#-C组粉末在挤压温度300℃、挤压比λ=36的条件下挤压,合金σb和60.2最高达到650MPa和630MPa,但是6较低,仅为5.6%。2#合金的最优挤压工艺为采用C组粉末,挤压温度400℃,挤压比为25,在此工艺条件下合金的σb、σ0.2和δ分别为384MPa、275MPa和8.2%。粉末挤压合金的断裂机制是粉末粒度、挤压比等工艺参数和析出相尺寸、数量及分布综合作用的结果。挤压比较小时合金断裂机制为沿晶脆性断裂；采用大粒度粉末或高挤压比时,其合金断裂机制是内颈缩扩展和剪切扩展两种微孔聚集方式混合的韧窝型韧性断裂；当断裂机制为内颈缩扩展微孔聚集型韧性断裂时,合金具有良好的综合力学性能。合金的力学性能是细晶强化、沉淀强化和位错强化等多种强化机制共同作用的结果。粉末在较低温度挤压成形(1#-C组粉末在280～320℃挤压,挤压比λ=36),合金的动态回复机制以稳定多边形化为主；粉末在较高温度以较小的挤压比挤压成形(2#-C组粉末在400℃挤压,挤压比λ=25),组织的动态回复机制以再结晶前多边形化为主；粉末在较高挤压温度和挤压比条件下挤压成形(2#-C组粉末在400℃挤压,挤压比λ=36),组织发生完全动态再结晶,降低合金性能。挤压合金组织的再结晶形核机制包括晶界弓出形核、亚晶形核和位错塞积形核。出现完全动态再结晶的原因是大变形导致的合金畸变能增加、粗大析出相造成的位错塞积形核和变形热导致的挤压温度升高。挤压合金的热处理工艺研究表明,1#合金适宜的固溶热处理工艺为460℃／20min,2“合金适宜的固溶热处理工艺为460℃／2.5h。固溶后进行单级时效处理,研究合金表现出显著的时效硬化特性,其时效硬化曲线具有双峰时效特征,分别对应GP区和η’强化。1#合金适宜的峰值时效工艺为120℃／20h,在此工艺条件下,合金的6b、σ0.2和6分别为695MPa、675MPa和11.3%。2#合金适宜的峰值时效工艺为12℃／24h,在此工艺条件下,合金的σb、σ0.2和6分别为731MPa,670MPa和6.2%。合金在具有超高强度的同时,保持了良好的韧性。1“合金固溶后进行双级时效处理,其适宜的双级时效工艺为120℃／3h+160℃／18h,在此工艺条件下,合金的σb、60.2和6分别为582MPa、542MPa和15.1%,电导率达到38.7%IACS。在损失部分强度性能的前提下,伸长率和电导率相对于T6合金分别提高33.6%和18.3%,合金具有优良的综合性能。合金经过固溶及时效热处理后获得微晶组织,晶粒尺寸小于5μm。合金在热处理过程中的组织变化规律与传统热处理有所不同。1#合金120℃／3h+160℃／18h时效处理、2#合金120℃／24h时效处理后组织表现出回归再时效(RRA)合金的特征。分析认为主要由于晶粒细化导致的合金元素结构敏感性扩散,使研究合金可以在简化的热处理工艺条件下获得RRA组织。Zr元素在合金时效初期抑制GP区的形核,削弱了合金的时效硬化特性。随时效时间延长Zr以亚稳态Al3Zr粒子形式析出,促进η’强化相析出,合金得到强化。对位错的钉扎作用是亚稳态Al3Zr粒子实现细晶强化、弥散强化和形变强化机制的本质原因。更多还原

【Abstract】 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.更多还原