等径角挤压制备AZ91D镁合金的组织与性能

研究了铸态AZ91D镁合金在等径角挤压(Equal Channel Angular Extrusion,ECAE)后的室温力学性能和微观组织特征.在力学性能方面,铸态AZ91D镁合金经过1道次ECAE变形后,室温力学性能(屈服强度、抗拉强度、延伸率、弹性模量)由86.3 MPa,146.3 MPa,1.84%,42.5...     

等通道角挤压AZ91D镁合金热压缩变形行为

在应变速率为0.005~1 s~(-1)、温度200~275℃条件下,利用Instron-5500热模拟机,对经过等通道角挤压(Equal Channel Angular Extrusion,ECAE)后的AZ91D镁合金的高温压缩特性进行了研究,得到了ECAE-ed态AZ91D镁合金真实应力-应变曲线,分析了挤压温度、应变速率等对其的影响,得出本构方程的一系列常量,建立了ECAE-ed态AZ91D镁合金在高温压缩中的本构方程关系式,并与铸态AZ91D镁合金进行了对比。结果表明:热压缩过程中,ECAE-ed态AZ91D镁合金与铸态一样,流动应力随温度的升高而降低,随应变速率的升高而升高;流动应力也可以用双曲正弦函数来描述,且双曲正弦值随Zener-Hollomon参数的自然对数的升高呈线性升高;两者同为正应变速率敏感材料,但ECAE-ed态AZ91D镁合金要比铸态应变速率敏感性小,其指数从铸态的m=0.14下降为0.096,变形激活能从182.65 kJ/mol上升为227.14 kJ/mol。研究结果对AZ91D镁合金进一步塑性成形和应用具有指导意义。     

AZ91铸态镁合金中Mn含量对其微观组织及耐蚀性能的影响

目前,关于Mn对AZ91铸态镁合金耐蚀性能影响的研究并不充分,尤其是腐蚀机制方面仍不清楚。采用静态失重、电化学及盐雾腐蚀法研究了Mn含量对AZ91铸态镁合金微观组织及耐腐蚀性能的影响。利用金相显微镜(OM)、扫描电镜(SEM)观察AZ91铸态镁合金的微观组织和腐蚀形貌,采用能谱(EDS)仪、X射线衍射仪(XRD)分析了其物相组成。结果表明:AZ91铸态镁合金中的Mn主要以颗粒状的低电化学活性物质Al2Mn形式存在,阻碍了其腐蚀过程的进行;Mn能使AZ91铸态镁合金中的有效腐蚀相β-Mg17Al12减少,分布更弥散,从而使合金的腐蚀电位升高,腐蚀电流降低,腐蚀速率降低,耐腐蚀性能增强;含0.8%Mn的铸态镁合金的耐腐蚀性能最好。     

等径道角 挤压工艺对铸态 AZ91D 镁合金组织的影响

目的介绍等径道角挤压的原理及其对铸态AZ91D镁合金的组织产生的作用。方法通过确定的试验工艺参数,对AZ91D镁合金进行了等径道角挤压变形试验。使用金相显微镜和扫描电镜(SEM),对变形前后的材料进行了显微组织的观察。结果通过进行ECAE挤压后,AZ91D镁合金中的黑色共晶相(Mg17Al12)产生了回溶,在机械剪切和动态再结晶的综合作用下,晶粒得到了细化。结论通过等径道角挤压,能明显改善铸态AZ91D镁合金的组织。     

铸态AZ31镁合金板材等温轧制工艺及组织性能研究

为研究铸态AZ31镁合金轧制工艺及轧制后组织性能,通过试验得到不同道次和变形量对铸态AZ31镁合金板材显微组织和力学性能的影响规律,并采用扫描电子显微镜研究了轧制后板材组织.结果表明,铸态AZ31镁合金板材经等温4道次、等变形量轧制后,板材厚度由20mm变化到4.8 mm,抗拉强度和屈服强度分别达到275 MPa和18...     

Nd对AZ91镁合金组织和高温力学性能的影响

研究了Nd对AZ91镁合金组织和高温力学性能的影响.结果表明,稀土Nd(1%～4%(wt))的加入明显细化了AZ91镁合金的铸态组织,减少了β(Mg17Al12)相的析出.随着稀土Nd含量的增加,室温、150℃和250℃等3个温度下的强度都是先升后降,Nd含量为1%时合金的强度均达到最大值,特别是150℃下其强度高达203MPa;Nd含量的增加对AZ91镁合金的延伸率影响规律也是先升后降,高温下当Nd含量为2%时合金的延伸率达到最大值,表现出良好的韧性,同时也具有较高强度.     

Mg_2Si/AZ91D复合材料阻尼性能

以AZ91D镁合金为基体,采用搅熔铸造法将球磨后的粉煤灰漂珠颗粒加入到熔融态基体中,设置球磨漂珠质量分数(2%、6%和10%)和搅拌时间(3min和6min),成功制备了Mg2Si/AZ91D复合材料。采用金相分析、XRD分析和动态机械热分析等方法研究了铸态和固溶态Mg2Si/AZ91D复合材料的显微组织、成分及阻尼性能。研究表明:与AZ91D镁合金相比,加入球磨漂珠颗粒后制备的Mg2Si/AZ91D复合材料中生成了Mg2Si相,而且随着漂珠质量分数的增加,Mg2Si相呈现不规则形状,固溶后Mg2Si相呈现均匀块状。随着漂珠质量分数的增加,Mg2Si/AZ91D复合材料的阻尼性能越好,搅拌时间6min制备的复合材料阻尼性能高于搅拌时间3min制备的复合材料的阻尼性能,并且固溶态的阻尼性能优于铸态。在室温下,Mg2Si/AZ91D复合材料阻尼性能可用位错理论来解释。     

AZ91D镁合金凝固过程分析

采用差热法以及冷却曲线测定AZ91D镁合金的相变温度,通过金相试验,研究了AZ91D镁合金凝固过程中显微组织的变化,为制定合理的AZ91D镁合金热处理工艺,提高合金力学性能提供了依据.     

Mg-9Al-1Zn-(0-0.6)Sr镁合金铸态组织及晶粒细化研究

对含Sr(0.05%~0.6%)AZ91铸态镁合金组织进行了观察,并且采用"边-边匹配"模型研究了其细化机制。结果表明,微量Sr对AZ91铸态组织有明显的晶粒细化效果,Sr含量为0.05%时细化效果最为显著。通过"边-边匹配"模型计算,得到[021]Al4Sr∥[1-1 0]Mg17Al12,(200)Al4Sr∥(332)Mg17Al12,表明微量Sr对AZ91镁合金铸态晶粒细化主要是Al4Sr相为Mg17Al12相的形成提供有效的异质形核核心,从而加速Mg17Al12相对α-Mg枝晶长大的阻碍作用。     

Mg-Gd-Y-Zr 活塞微观组织及力学性能研究

目的研究Mg-Gd-Y-Zr镁合金活塞本体的微观组织和力学性能。方法采用金属型重力铸造工艺制备镁合金活塞,利用光学显微镜和扫描电镜分析了铸态、固溶态(T4)和固溶时效态(T6)活塞本体的显微组织,利用岛津材料试验机和硬度计测试活塞本体的力学性能。结果铸态Mg-Gd-Y-Zr镁合金活塞本体组织中大量的第二相分布于晶界处,T4处理后大部分固溶到基体中,T6处理后晶粒内部出现麻点状和细条状的析出相。活塞裙部和顶部经T6处理后的抗拉强度随着拉伸温度的升高而逐渐降低,在300℃拉伸时活塞裙部抗拉强度达到226.38 MPa;活塞裙部和顶部的伸长率随着拉伸温度的升高而增加,在350℃拉伸时活塞裙部伸长率达到23.65%。结论镁合金活塞裙部的室温和高温抗拉强度好于活塞顶部,裙部尺寸较均匀。     

Characterization of fold defects in AZ91D and AE42 magnesium alloy permanent mold castings

Casting premium-quality magnesium alloy components for aerospace and automotive applications poses unique challenges. Magnesium alloys are known to freeze rapidly prior to filling a casting cavity, resulting in misruns and cold shuts. In addition, melt oxidation, solute segregation and turbulent metal flow during casting contribute to the formation of fold defects. In this research, formation of fold defects in AZ91D and AE42 magnesium alloys cast via the permanent mold casting process was investigated. Computer simulations of the casting process predicted the development of a turbulent metal flow in a critical casting region with abrupt geometrical transitions. SEM and light optical microscopy examinations revealed the presence of folds in this region for both alloys. However, each alloy exhibited a unique mechanism responsible for fold formation. In the AZ91D alloy, melt oxidation and velocity gradients in the critical casting region prevented fusion of merging metal front streams. In the AE42 alloy, limited solubility of rare-earth intermetallic compounds in the α-Mg phase resulted in segregation of Al₂RE particles at the leading edge of a metal front and created microstructural inhomogeneity across the fold.     

Damping capacity of sand cast magnesium alloy AZ91

Abstract

Measurements of damping properties have been carried out on sand cast specimens of the most widely used commercial magnesium alloy AZ91. Damping properties of specimens in both as cast and heat treated (T4 and T6) conditions were determined by measurements of the logarithmic decrement of free bending beam vibration at various frequencies. The best damping properties were found in the as cast specimens. This is directly related to the microstructure, which consists of α magnesium, large inclusions of β phase (Mg₁₇Al₁₂), as well as lamellar eutectoid Mg₁₇Al₁₂ precipitating as a result of low cooling rate. For homogenised and annealed specimens, the maxima of the hardness and of the damping capacity as a function of annealing time was found. The measurements of the logarithmic decrement can be explained using a model of Granato and Lücke.     

Influence of neodymium on microstructure and mechanical properties of AZ31B wrought magnesium alloy

Abstract

The influences of rare earth neodymium on microstructure and mechanical properties of as cast and hot rolled AZ31B wrought magnesium alloy were investigated. The results show that the mechanical properties of both as cast and hot rolled AZ31B alloys decrease due to Nd addition. Nd reacts with Al to form Al₂Nd phase when Nd is added. Bulky and brittle Al₂Nd intermetallic degrades the mechanical properties. Moreover, the addition of Nd weakens the grain refining effect of Al on as cast AZ31B alloy, resulting in grain coarsening. Coarse grains also cause the decline of the mechanical properties of as cast AZ31B–Nd alloy. The negative influence of the bulky and brittle intermetallics on mechanical properties of AZ31B alloy can be relieved by large deformation because the intermetallics can be sufficiently broken up during the deformation process.     

Rotating cylinder manufacturing method and investment casting of SiCp/AZ91HP magnesium composites

Abstract

This paper describes the rotating cylinder method for manufacturing, and investment casting for forming, composite slurry. Microstructural features, such as SiC particle distribution and grain refinement of the as cast composites, were investigated. Also the effect of SiC particle fraction and size, and process parameters on the microstructure and the mechanical properties are discussed. Attempts were made to evaluate the thermal stability of oxides against molten AZ91HP magnesium alloy. The oxides examined included CaO, CaZrO₃, and silica bonded Al₂O₃ and zircon flour. Finally, the tensile properties, hardness, and wear resistance of the as cast composites were evaluated and the results are compared with those of the as cast alloy.     

Effects of Ca addition on the microstructure and mechanical properties of AZ91magnesium alloy

The effects of Ca addition on the microstructure and mechanical properties of AZ91 magnesium alloy have been studied. The results show that the Ca addition can refine the microstructure, reduce the quantity of Mg₁₇Al₁₂ phase, and form new Al₂Ca phase in AZ91 magnesium alloy. With the Ca addition, the tensile strength and elongation of AZ91magnesium alloy at ambient temperature are reduced, whereas Ca addition confers elevated temperature strengthening on AZ91 magnesium alloy. The tensile strength at 150°C increases with increasing Ca content. The impact toughness of AZ91magnesium alloy increases, and then declines as the Ca content increases. The tensile and impact fractographs exhibit intergranular fracture features, Ca addition changes the pattern and quantity of tearing ridge, with radial or parallel tearing ridge increasing, tensile strength, elongation and impact toughness reduce.     

Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Microstructure evolution and mechanical properties of Al<Subscript>2</Subscript>O<Subscript>3sf</Subscript>/AZ91D magnesium matrix composites fabricated by squeeze casting

The squeeze casting process was used to fabricate Al₂O_3sf/AZ91D magnesium matrix composites before thixoforging. The microstructural evolution process in Al₂O_3sf/AZ91D was investigated during partial remelting. Tensile mechanical properties of thixoforged automotive component were determined and compared with those of squeeze casting formed composites. The results show that the microstructural evolution during partial remelting exhibited four stages: the formation of liquid, structural fragmentation, the spheroidization of solid particles, and final coarsening. As the holding time increases, the size of solid particles decreases initially and then increases. However, the size of solid particles decreases monotonously as the temperature increases. Increasing holding time or temperature promotes the degree of spheroidization. It is also shown that the cylindrical feedstock of the Al₂O_3sf/AZ91D composites can be thixoforged in one step into intricate shapes in the semi-solid state. The tensile tests indicate that the yield strength and ultimate tensile strength for Al₂O_3sf/AZ91D thixoforged from starting material fabricated by squeeze casting and partial remelting are better than those of Al₂O_3sf/AZ91D fabricated by squeeze casting. This research confirms that thixoforging is a practical method for the near net shape forming of magnesium matrix composites.     

In situ observation of ageing process and new morphologies of continuous precipitates in AZ91 magnesium alloy

An in situ observation of the precipitating process of γ-Mg₁₇Al₁₂ phase in die-cast AZ91 magnesium alloy, was carried out with a transmission electron microscope equipped with a heating stage maintained at 473 K for 8 h. In addition to the thin plate-shaped continuous precipitates, continuous precipitates with rod-shaped and the Potter orientation relationship were observed and analyzed with transmission electron microscopy including high-resolution transmission electron microscopy techniques. It was also observed firstly that there exist plate-shaped continuous precipitates with the Pitsch-Schrader orientation relationship in the die-cast AZ91 magnesium alloy.     

AZ91D/TiB2 Nanocomposites Fabricated by Solidification Nanoprocessing

AZ91D, as one of the most widely used casting magnesium alloys, still suffers from inadequate mechanical performances for various applications. Nanoparticles could be used to form high‐performance magnesium matrix nanocomposites. Among all nanoparticles, TiB₂ has great potentials to enhance the mechanical property of AZ91D. This paper studies the microstructures and mechanical property of AZ91D‐TiB₂ nanocomposites fabricated through solidification nanoprocessing. TiB₂ nanoparticles with a diameter of 25 nm are effectively fed into the AZ91D melt through a newly developed automatic nanoparticle‐feeding system. Ultrasonic cavitation is used to disperse these nanoparticles in AZ91D melt for casting. With 2.7 wt% (about 1.0 vol%) of TiB₂ nanoparticles addition, the mechanical property of AZ91D is much enhanced (by 21, 16, and 48% for yield strength, tensile strength, and ductility, respectively). Microstructural analysis with optical microscope, SEM, and S/TEM show that α‐Mg grain and a network of massive brittle intermetallic phase (β‐Mg₁₇Al₁₂) are simultaneously refined and modified. Further study suggests that the enhancement of mechanical properties of AZ91D is attributed not only to primary phase grain refinement, but also to the modification of intermetallic β‐Mg₁₇Al₁₂ by TiB₂ nanoparticles.     

Laser–tungsten inert gas hybrid welding of dissimilar AZ based magnesium alloys

Abstract

The microstructure and mechanical properties of dissimilar AZ based magnesium alloys subjected to laser–tungsten inert gas (TIG) hybrid welding have been investigated. The results show that magnesium alloys can be readily welded as dissimilar joints using this process. The microstructure of the dissimilar magnesium alloy joints is composed of primary α phase (Mg) and β phase (Mg₁₇Al₁₂), based on electron probe microanalysis (EPMA) and X-ray diffraction (XRD) data. In addition, the tensile strength of AZ31B–AZ61 and –AZ91 joints is equal to that of AZ31B base metal. It has also been found that the presence of β phase has a severe influence on the tensile strength and mirohardness of dissimilar magnesium alloy joints.