【作者】 张振亚；

【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】 镁合金具有高比强度、比刚度,良好的阻尼减震性,优良的机加工性能和易于回收利用等优点,被誉为“21世纪绿色金属结构工程材料”,是近年来国内外材料界研究的热点之一。进一步提高镁合金的综合性能,满足某些结构件“以镁代铝”甚至“代钢”的性能要求；改善镁合金的塑性、减少各向异性,显著改善其塑性成形能力；简化镁合金制品的制备工艺,降低其生产成本,扩大镁合金的应用范围,是国内外研究的主要方向。本课题从细化晶粒提高镁合金力学性能角度出发,采用离心雾化法制备ZK60及AZ91合金粉末,显著细化粉末组织,减少成分偏析,扩展合金元素在基体中的固溶度；针对快速凝固／粉末冶金(RS／PM)制备工艺繁琐、成本高昂等不足,结合粉末特性及粉末热挤压数值模拟结果,提出一种短流程、适于工业化生产的快速凝固／粉木冶金(RS／PM)简化工艺,并成功制备出超细晶ZK60及AZ91高性能镁合金。在此基础上,系统地研究了挤压温度对超细晶材料组织及性能(如强度、塑性、耐腐蚀性等)的影响,并分别对合金细化机制、热稳定性、氧化粒子引入及强化机制等进行细致研究。离心雾化法所得ZK60及AZ91合金粉末球形度高、表面光洁,冷却速度高达104～105K／s。研究结果表明,合金粉末的组织主要为等轴晶,晶界上分布着细小的第二相,快速凝固的急冷作用致使合金粉末的晶粒度及第二相尺寸比传统铸锭法的组织更为细小,分布均匀性显著改善,同时基体处于过饱和固溶态。RS合金粉末表面自然氧化膜主要由2nm MgO膜及0.5nm污染层构成,在100～200℃下短时加热,氧含量变化不大。在空气气氛下双向温压5～10min可制备出相对密度0.8～0.9的ZK60、AZ91粉末生坯,有效地回避了包套、真空除气等工序,降低能耗及制备成本。采用粉末挤压有限元数值模拟对模具结构和挤压工艺参数进行优化,结果表明,凹模塑性变形区和缩颈区对粉末起同样致密化作用,前者使粉末发生大塑性变形,后者则使粉末完全焊合；模具入口角θ=60°,凹模过渡角半径r=2mm时试棒在保持较高的变形量时应变分布均匀；适量的摩擦对获得均匀应变并保持较大变形量是积极有利的；大挤压速度(≥10mm／s)、高挤压温度会导致挤压件表面出现缺陷；生坯的初始相对密度对挤压致密效果无明显的影响。RS／PM简化技术制备所得ZK60及AZ91合金,晶粒度0.3～2.11μm,其中ZK60合金拉伸强度高达348～424MPa,延伸率9-22%；而AZ91合金中存在弥散第二相“钉扎”晶界,拉伸强度更佳(363-516MPa)。随挤压应变增加,粉木发生宏观变形并最终焊合,粉末表面氧化膜被破碎成纳米氧化粒子弥散分布于焊合部位,对合金起到一定复合强化作用；微区细化机制以连续动态再结晶为主,旋转动态再结晶为辅,镁合金中第二相的细化机制主要为机械破碎。同时发现超细晶ZK60合金的热稳定性与挤压温度密切相关,低于挤压温度保温1h时,组织发生回复及再结晶,但晶粒长大甚微；随着退火温度提高,则晶粒长大明显；RS／PM合金的强化来源于晶粒细化、织构强化和第二相强化三种机制的叠加效应。通过浸泡试验和电化学试验首次研究了超细晶ZK6O及AZ91合金的腐蚀性能。在NaCl溶液浸泡及电化学腐蚀试验中,RS／PM合金平均腐蚀速率略高于铸锭AZ91D合金。超细晶RS／PM合金由于晶界比例提高,缺陷增多,致使表层各处的化学活性高,腐蚀性能降低,此外,RS／PM AZ91合金中第二相沿晶界弥散分布,微电池腐蚀严重,因此其抗腐蚀能力最差。相比于腐蚀速率变快的劣势,均匀腐蚀成为RS／PM合金腐蚀性能的最主要的优势。更多还原

【Abstract】 Magnesium alloys, as "21st century green metallic structural materials", have the most promising development in material filed, for their high specific strength and specific stiffness, good damping capacity, excellent machinability and high recycling rate. The principal focus on enhancing integrated properties of magnesium alloys is to satisfy the requirements of some structural materials, such as aluminum, or even steel. Researches which aim to increase the elongation, decrease the anisotropy and improve the plastic deformation capability, simplify fabrication techniques, reduce manufacturing cost and expand the application of magnesium alloy also receive global attention.It is well known that the mechanical properties increase rapidly with refining on grain size for magnesium alloys. Centrifugal atomization, which can effectively refine grain and second-phase size, reduce segregation, and expand solid solubility of matrix, was initially employed to prepare ZK60 and AZ91 alloy powders.Combination of powder characteristics and powder extrusion simulation results, this paper proposed a high cost-effective process to promote wide application of rapid solidified/powder metallurgy (RS/PM) magnesium alloys. Finally, ultrafine grained ZK60 and AZ91 bars were successfully prepared.Based on laboratory scale, the effect of extrusion temperature on properties (such as strength, toughness and corrosion resistance, etc) were investigated. The refinement mechanism, thermal stability, oxidation particles and strengthening mechanism were also discussed in detail.Spherical-shaped ZK60 and AZ91 powders with smooth surface were prepared by centrifugal atomization technology. The average cooling rate was calculated to be 104～105K/s. These powders essentially exhibit an equiaxed grain morphology with fine intermetallic compounds along grain boundaries. Due to higher cooling rate, the grain size of RS powders is much smaller than that of conventional cast alloys; second phase particles distribute more uniformly; and quenching effect keep supersaturated solid solution state in matrix. In other ways, the native oxide layers of RS powders are composed of 2nm MgO film and 0.5nm contamination film.When exposure to air at 100～200℃for short time, the increase in oxygen content can be negligible. Therefore, powder compacts with relative density of 0.8～0.9 can readily prepared in double compression for 5-10min at low temperature, without aid of canning, vacuum degassing and other steps.The die structure and extrusion process parameters were optimized by FEM simulation. The result shows that there are similar densification behaviors to powders from deformation area and necking region of concave die. However, the former induces severe plastic deformation of powders, while the latter generates complete welding between powders. In order to keep a relatively uniform distribution and maintain high level in effective plastic strain, the entrance angle of die and the radius of transition angle are suggested to choose 60°and 2mm, respectively. Moreover, the normal friction is benefit to obtain the above function. The surface defects could be found under high extrusion speed(≥10mm/s) or higher extrusion temperature. Besides, the initial relative density of powder compacts shows less significant influence on extruded bars.The grain refinement is significant by simplified RS/PM process, and the mean grain size of ZK60 and AZ91 alloys can be refined to 0.3-2.1μm with different extrusion temperature. To ZK60 alloys, the tensile strength and elongation are 348-424MPa and 9-22%,while AZ91 alloys show higher strength (363-516MPa) due to more dispersion of second phase particles inhibiting grain movement. With strain increasing, interparticle mechanical interlock combined with mass transfer across the boundaries, develops a good adhesive bonding. The nano-oxide particles, broken from oxide layer of powder, locate at welding position, which enhances the strength of alloys further. During extrusion, the refining mechanism is dominated by continuous dynamic recrystallization (CDRX) and assisted by rotation dynamic recrystallization (RDRX). For the second phase, it is mechanical cracking refining mechanism. The thermal stability of ultrafine ZK60 alloy is closely related with the extrusion temperature. When annealing temperature lower than extrusion temperature, recovery and recrystallization happen, but no grain coarsens. In present work, the strengthening mechanisms of RS/PM ZK60 and AZ91 alloys are originated from overlapping effect of grain refinement, texture and second phase strengthening.Through immersion corrosion tests and electrochemical tests, the corrosion properties of ultra-fine grained ZK60 and AZ91 alloys were researched for the first time. During both tests in NaCl solution, the mean corrosion rate of RS/PM alloys is higher than that of cast AZ91D alloy. The reasons is attribute to the increase of boundary fraction, internal energy and defects in material after RS/PM process, all of which would result in high chemical activity throughout the surface. Moreover, because of abundant second phase particles discontinuously pinning grain boundaries in RS/PM AZ91 alloys, which introduces serious micro-cell corrosion, the worst corrosion resistance was found in RS/PM AZ91 alloys extruded at 210℃. However, as compared with the disadvantage of higher corrosion rate, the uniform corrosion is the most prominent advantage for corrosion properties of RS/PM magnesium alloys.更多还原