Quantitative metallographic assessment of the electromagnetic casting influence on the microstructure of 7075 Al alloy

This study presents an attempt to obtain the better quality of an aluminum super-high strength alloy by application of electromagnetic field during the casting process. The conventional continuous casting process of aluminum alloys causes many defects, such as surface imperfections, grain boundary segregation, non-uniform grain size, and porosity. The better ingot surface along with the homogeneous fine-grained microstructure, and hence the better mechanical properties of the ingot, can be achieved by applying the electromagnetic casting process. The microstructure characterization, accompanied by quantitative metallographic assessment, reveals that it is possible to avoid or decrease many defects of as cast ingots during electromagnetic casting process. In this article, the microstructure of the samples of as cast 7075 aluminum alloy, obtained with and without electromagnetic field influence, was analyzed by optical microscope and the variation of key alloying elements content, i.e., Zn and Mg, through the ingot cross section was examined by chemical analysis. Besides, the microstructural parameters such as dendrite arm spacing, interdendritic space width, as well as eutecticum and intermetallic phases volume fraction, were measured using linear method. The electromagnetic field influence on the microstructure of the as cast 7075 Al alloy was evaluated based on measured quantitative metallographic data.     

真空微重力熔炼铝锂合金

在微重力电磁模拟装置中进行铝锂合金的熔炼与铸锭。考察了坩埚材料、加热温度、真空度、保温时间、气氛保护等工艺因素对铝锂合金铸锭质量的影响。实验证明,通过采取适宜的工艺手段,合金中的杂质可以得到有效的控制,提高了合金的铸锭质量。     

Intermetallic and dispersoid structures in aa3104 Aluminum alloy during two-step homogenisation

During homogenisation of the AA3104 cast ingot, a phase transformation of intermetallic particles from β-Al₆(Fe,Mn) orthorhombic phase to harder α-Al_x(Fe,Mn)₃Si₂ cubic phase occurs. The large constituent intermetallic particles, regardless of phase, assist in the recrystallisation nucleation process through particle stimulated nucleation (PSN). Ultimately, this helps to refine grain size. The sub-micron dispersoids act to impede grain boundary migration through a Zener drag mechanism. For this reason, the dispersoids that form during homogenisation are critical in the recrystallisation kinetics during subsequent rolling, with smaller dispersoids being better suited to reverse rolling mills. This work simulates an industrial two-step homogenisation practice with variations in the peak temperature of the first step between 560 °C and 580 °C. The effect of this temperature variation on the intermetallic particle-phase evolution is investigated. The aim is to identify the ideal intermetallic phase balance and the dispersoid structure that are best suited for hot rolling on a single stand reversing mill, in order to minimise recrystallisation during rolling through maximising Zener drag and maintaining galling resistance. The results indicate a trend where an increase in homogenisation temperature from 560 °C to 580 °C yields, firstly, an increase in the volume fraction of the α-phase particles to greater than 50% of the total volume fraction at both the edge and the center of the ingot and, secondly, it yields an increased dispersoid size. Thus, in the context of a reverse rolling operation, a lower temperature homogenisation practice produces a near-ideal combination of intermetallic particle-phase distribution, as well as dispersoid size, which is critical for Zener drag and the minimization of recrystallisation during the hot rolling processes.

Graphical abstract

SEM BEI images and corresponding EDS maps, highlighting the variation in intermetallic particle phase balance, size and morphology after homogenisation at different temperatures. With a focus on the exaggerated differences seen between material the center of and at the edge of a DC cast ingot of AA3104 Aluminum alloy.

     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

Production and mechanical properties of SiC<Subscript>p</Subscript> particle-reinforced 2618 aluminum alloy composites

In this study, 2618 aluminum alloy metal matrix composites (MMCs) reinforced with two different sizes and weight fractions of SiC_p particles upto 10% weight were fabricated by stir cast method and subsequent forging operation. The effects of SiC_p particle content and size of the particles on the mechanical properties of the composites such as hardness, tensile strength, hot tensile strength (at 120 °C), and impact strength were investigated. The density measurements showed that the samples contained little porosity with increasing weight fraction. Optical microscopic observations of the microstructures revealed uniform distribution of particles and at some locations agglomeration of particles and porosity. The results show that hardness and tensile strength of the composites increased, with decreasing size and increasing weight fraction of the particles. The hardness and tensile strength of the forged composites were higher than those of the cast samples.     

Abrasive wear of Al2O3-reinforced aluminium-based MMCs

The effects of volume fraction, Al₂O₃ particle size and effects of porosity in the composites on the abrasive wear resistance of compo-casting Al alloy MMCs have been studied for different abrasive conditions. It was seen that porosity in the composites is proportional to particle content. In addition, process variables like the stirring speed, and the position and diameter of the stirrer affect of the porosity content in a way similar to that observed for particle content. In addition, the abrasive wear rates of composites decreased more rapidly with increase in Al₂O₃ volume fraction in tests performed over 80 grade SiC abrasive paper than in tests conducted over 220 grade SiC abrasive paper. Furthermore, the wear rates decreased with increase in Al₂O₃ size for the composites containing the same amount of Al₂O₃. Hence, it is deduced that aluminium alloy composites reinforced with larger Al₂O₃ particles are more effective against abrasive wear than those reinforced with smaller Al₂O₃ particles. At the same time the results show that the beneficial effects of hard Al₂O₃ particles on wear resistance far surpassed that of the sintered porosity in the compocasting metal-matrix composites (MMCs). Nevertheless, the fabrication of composites containing soft particles such as graphite favors a reduction in the friction coefficient. For this reason graphite and copper were used in the matrix in different amounts to detect their effect on wear resistance. Finally, it was seen that wear rate of the composites decreased considerably with graphite additions.     

饱和松砂与结构接触面反复剪切下邻近土体性状数值分析

该文采用颗粒离散元软件PFC2D,研究了不排水条件高频循环剪切作用下饱和松砂与板桩接触面附近的砂土特性,分析了模型参数对饱和松砂孔隙率和体应变的影响,以及剪切过程中超静孔压的变化规律和砂土颗粒的运动轨迹。结果表明:当颗粒摩擦系数、颗粒刚度、剪切振幅和剪切频率越小或边界围压和接触面摩擦系数越大,饱和松砂的剪缩现象越明显。砂土的孔隙率在作用瞬间减少、然后略有增加后呈下降趋势至稳定;随循环次数增加,砂土中的超静孔压总体上呈增长趋势;砂土颗粒的变位随离开接触面距离的增加而减小。     

Processing-microstructure relationships in compocast magnesium/SiC

Compocasting experiments were conducted to investigate the feasibility of the process as applied to the AZ91 D magnesium alloy-SiC particles system. Processing-macro/ microstructure relationships were examined. Three temperature-time processing sequences were investigated: stirring temperature maintained above liquidus; stirring temperature in the semi-solid temperature range; and lastly, an imposed temperature rise above the liquidus after stirring in the mushy zone. Stirring temperature and particle size significantly affect spatial particle distribution and porosity level. The easy incorporation and even dispersion of particles in the matrix suggest good wetting of SiC particles by the magnesium matrix. Impact fracture surfaces show strong bonding at the particle/matrix interface. A reaction takes place at the matrix/particle interface whilst stirring at temperatures above the liquidus. Reaction products have been identified. Finally, the mechanical properties of a compocast ingot which was extruded have been studied and are reported. This work clearly points out that there is a preferred procedure to follow during compocasting to obtain an optimum microstructure. The procedure is to add the reinforcing materials to the semi-solid alloy followed by stirring above the liquidus temperature.     

多孔不锈钢材料的制备及孔隙控制

利用凝胶注模工艺结合微波烧结的方法制备孔隙可控的多孔不锈钢,实验研究了凝胶注模参数、微波烧结参数、粉末形状和粉末粒径等因素对孔隙结构的影响.研究表明:体积固相含量达56%,明胶和海藻酸钠含量分别为1%和0.8%,混合液p H值为7时,浆料流动性好,干燥后的坯体强度较高;烧结温度达1 200℃,保温时间为30 min,孔隙形貌较好;原料的粉末形状因子越大,粒径越小,则多孔不锈钢的孔径和孔隙率越小,分布越均匀;采用粒径35~60μm,形状因子0.85~1.0的粉末,制备出的多孔不锈钢孔隙率为20%~35%、孔径为10~30μm,接近现有透气模具钢水平.通过选取不同粉末形状因子和粒径的粉末,以及合理的凝胶注模和微波烧结工艺参数,可以准确控制多孔不锈钢材料的孔隙率与平均孔径.     

High-strength and highly-uniform composites produced by compocasting and cold rolling processes

Silicon carbide reinforced aluminum alloy composite materials produced by casting methods are increasingly used in many engineering fields. However, these materials suffer from poor distribution of the reinforcement particles in the matrix and high content of porosity. The effect of subsequent cold rolling process with different reductions on the porosity, microstructure and mechanical properties of cast Al6061/10 vol.% SiC_p composite was investigated in this study. Composites fabricated by compocasting method were rolled at five different reductions of 30, 60, 75, 85 and 95%. The rolled specimens exhibited reduced porosity as well as a more uniform particle distribution when compared with the as-cast samples. Microscopic investigations of the composites after 95% reduction showed an excellent uniform distribution of silicon carbide particles in the matrix. During cold rolling process it was observed that the tensile strength and ductility of the samples increased by increasing the reduction content. After 95% reduction, the tensile strength and elongation values reached 306.7 MPa and 7.9%, which were 4.6 and 3.3 times greater than those of the as-cast composite, respectively.     

影响聚苯乙烯离聚体水基微乳液颗粒大小和稳定性的因素

以乙酰磺酸为磺化剂备磺化度为３ｍｏｌ％－１５ｍｏｌ％的磺化聚苯乙烯,并加水将ＳＰＳ化成水包油的稳定水基微乳液。研究了ＳＰＳ溶液的乳化初始浓度,中和方式、离子含量和乳液放置时间ＳＰＳ水基微乳液中分散相颗粒尺寸和乳液稳定性的影响,也研究了影响分散相颗粒尺寸分布的因素。     

颗粒随机分布复合材料的细观构造对有效热传导系数的影响

结合建立的颗粒随机分布复合材料的微结构模型和基于均匀化理论统计的双尺度计算方法, 针对氮化硼颗粒增强高密度聚乙烯(BN/HDPE)复合材料, 研究了颗粒形状、体积分数和空间分布参数等微结构特征对复合材料有效热传导系数的影响。结果表明: 颗粒体积分数的增加将导致有效热传导系数升高; 球形颗粒的位置参数、长椭球颗粒的取向程度都对有效热传导系数有重要影响。数值试验表明, 材料的微结构特征对复合材料的有效热传导系数具有极大影响。     

Improvement of mechanical properties of AA1070 aluminium by the addition of borax under different holding conditions

In this study, mechanical properties of Al‐B alloys, prepared by the addition of different amounts of borax to 99.70% pure commercial aluminum, were improved. The main objective is to study the influence of borax addition and holding conditions on the mechanical properties of cast Al‐B alloys. In this process, amount of borax, holding temperature and holding time were realized as affecting parameters. By considering these parameters three specimens were prepared for each condition and they were subjected to mechanical tests such as tensile, impact and hardness tests. As a result of tests, tensile strength of the alloy with the incorporation of borax was found 33% increase in comparison to pure aluminum while 21% increase in impact energy and 54% in hardness value. A substantial quantity of aluminum‐boride flakes were observed in the microstructural investigation of the samples. Microscopic observations of alloyed samples reveal homogeneous distribution of aluminum boride particles and fewer porosity levels.     

AN EXPERIMENTAL STUDY OF THE TREATMENT OF ALUMINUM-CELL GASES FOR POLLUTION CONTROL USING FLUIDIZED-BED TECHNOLOGY

This paper describes the two-stage treatment of tars and hydrogen fluoride contained in emitted aluminum-cell gases for pollution control and recovery of useful components. The treatment involves combustion of tar and then chemisorption of hydrogen fluoride on alumina particles in a fluidized-bed reactor of 25 cm inner diameter. Calcined coke was used to support the combustion of tar. Smelter-grade alumina was used as the bed material with average particle size of 66.71 μm. Stability and temperature uniformity of the bed were confirmed for different operating conditions. The tar was removed with combustion efficiency of up to 98% at a bed temperature of 700-900°C. Heat removal from the bed by water cooling tubes was achieved with a heat transfer coefficient as high as 450 W/m² · K. The hydrogen fluoride was captured by chemisorption with a scrubbing index of up to 0.75 kg^-1 depending on the bed temperature, distributor design, fluidizing velocity, and bed height. Good reactor performance could be obtained with a distributor fitted with 113 nozzles each having six inclined holes.     

Microstructure and abrasive wear behaviour of B<Subscript>4</Subscript>C particle reinforced 2014 Al matrix composites

In this study, the microstructure and abrasive wear properties of varying volume fraction of particles up to 12% B₄C particle reinforced 2014 aluminium alloy metal matrix composites produced by stircasting method was investigated. The density, porosity and hardness of composites were also examined. Wear behaviour of B₄C particle reinforced aluminium alloy composites was investigated by a block-on-disc abrasion test apparatus where the samples slid against the abrasive suspension mixture (contained 10 vol.% SiC particles and 90 vol.% oil) at room conditions. Wear tests performed under 92 N against the abrasive suspension mixture with a novel three body abrasive. For wear behaviour, the volume loss and specific rate of the samples have been measured and the effects of sliding time and the content of B₄C particles on the abrasive wear properties of the composites have been evaluated. The dominant wear mechanisms were identified using SEM. Microscopic observation of the microstructures revealed that dispersion of B₄C particles was generally uniform while increasing volume fraction led to agglomeration of the particles and porosity. The density of the composite decreased with increasing reinforcement volume fraction but the porosity and hardness increased with increasing particle content. Moreover, the specific wear rate of composite decreased with increasing particle volume fraction. The wear resistance of the composite was found to be considerably higher than that of the matrix alloy and increased with increasing particle content.     

Fluidity and microstructure of an Al–10% B<Subscript>4</Subscript>C composite

The fluidity evolution of an Al–10 vol.% B₄C experimental composite during long holding periods has been investigated by using a vacuum fluidity test. It was found that the fluidity of the composite melt decreased with the increase of the holding time. The microstructure of the fluidity samples was examined by optical metallography, quantitative image analysis, and electron microscopy. Two secondary reaction-induced phases were identified and the volume fraction changes of the solid phases during the holding periods were quantified. The relationship between the fluidity, volume fraction, and surface area of solid phase particles was established. In addition, the particle distribution along the entire length was examined in the fluidity samples. The mechanism of the particle redistribution during flow and solidification is presently discussed.     

真空感应熔炼Nb-16Si-22Ti-2Hf-2Cr-2Al合金经高温热处理后的组织和性能

用真空非自耗电弧熔炼法制备了Nb-16Si-22Ti-2Hf-2Cr-2Al母合金锭, 用Y₂O₃坩埚真空感应熔炼对母合金锭重熔, 浇注在温度梯度约为4 ℃/mm的模壳里, 制备出Φ 60 mm×170 mm铸锭, 研究了1500 ℃/100 h真空热处理后铸锭的组织特征对室温抗拉强度和高温压缩性能的影响。结果表明, 真空感应熔炼Nb-16Si-22Ti-2Hf-2Cr-2Al合金经过1500 ℃/100 h热处理后, 合金锭的组成相为Nb_SS枝晶, Nb₅Si₃层片或不规则颗粒和残留Nb₃Si块。块状Nb₃Si的尺寸越大, 发生完全共析转变需要的时间越长、温度越高。热处理合金锭的室温抗拉强度在208~355 MPa之间, 室温延伸率变化不大, 均小于0.3%。残留的粗大块状Nb₃Si和热处理过程中析出的HfO₂是导致合金锭拉伸性能较低的重要原因。合金锭的高温压缩强度受到组织中硅化物相含量的影响, 压缩强度与硅化物含量成正比。      

Microstructural features induced by spray forming of a ternary Pb-Sn-Sb alloy

An alloy containing Pb-12% Sn-12% Sb with small addition of copper and arsenic was spray deposited employing two different atomization gas pressure and nozzle to substrate distances. The temperature of the spray-deposit was measured during deposition at a distance of 2 and 10 mm above the substrate-deposit interface. Thermal profile data indicated small variation in temperature with time during deposition stage whereas during post deposition stage an exponential decrease in temperature was recorded. Second phase particle size along the thickness of the deposit varied from 4 to 8 μm compared to 70 to 80 μm size of these particles in the as cast alloy. Maximum porosity occurred in the section of the deposit near the contact surface of the substrate and also in its peripheral regions. X-ray diffraction analysis exhibited the formation of additional Cu₂Sb phase in the spray-deposit and CuSn and Cu_3.3Sb phases in atomized powders compared to that of the as cast alloy. The microstructural evolution during spray deposition of this alloy is discussed.     

碳化钨粉末对聚晶金刚石复合片钻头胎体组织与力学性能的影响

以铸造碳化钨粉末混合镍粉作为骨架粉末,采用无压浸渍工艺制备了聚晶金刚石复合片（PDC）钻头胎体材料。研究了碳化钨的粉末粒形、粒度及质量分数对PDC钻头胎体的微观组织和力学性能的影响。结果表明,碳化钨的粉末粒形、粒度及质量分数是影响胎体微观组织和力学性能的重要因素。与破碎碳化钨相比,粉末粒度适中的球形碳化钨作为骨架制备的胎体组织更均匀、更致密,胎体的力学性能明显提高。采用150~180 μm的球形碳化钨混合13wt%的镍粉作为骨架粉末制备的胎体力学性能优于石油天然气行业标准SY/T 5217—2000,其硬度、冲击韧性和抗弯强度分别为HRC 34、6.7 J/cm2和820 MPa。      

Revealing Precipitate Development During Hot Rolling and Cooling of a Ti–Nb Micro-Alloyed High Strength Low-Alloy Steel through X-Ray Scattering

High-energy synchrotron X-ray small-angle scattering (SAXS) is used to study the precipitate development during hot rolling and cooling of a commercial Ti–Nb micro-alloyed, high-strength, low-alloy (HSLA) steel. To study precipitation during hot rolling conditions, Gleeble and dilatometer trials are made. Samples are then studied at room temperature using SAXS in conjunction with transmission electron microscopy (TEM). TEM is used to determine the morphology and composition of the precipitates, whilst both TEM and SAXS are used to study the particle sizes. One major advantage with high-energy SAXS is the ability to make measurements after a minimum of sample preparation and in transmission geometry, as opposed to just at prepared surfaces, plus the possibility to determine volume fractions of the precipitates. The measurements show that after deformation at high temperature, particle coarsening occurs and the volume fraction of precipitates increases after holding for 20 s at 900 °C which confirms strain-induced precipitation at finishing rolling conditions. The measurements show that holding at 600 or 650 °C for one hour gives a larger volume fraction of nanosized particles. Coiling simulations with slow cooling from 600 to 470 °C show coarsening of particles and an increase in the volume fraction of the smaller particles compared to holding at a constant temperature.