Development and properties of aluminium silicon alloy fly ash composites

Abstract

Aluminium (Al) silicon alloy fly ash composites were developed using powder metallurgy technique. Aluminium silicon alloy powder was homogenously mixed with various weight percentages of fly ash (5–15%) and uniaxially cold pressed at pressures ranging between 200 and 515 MPa, and the green specimens were sintered at temperatures between 575 and 625°C. The various properties of the Al alloy fly ash composites were studied and compared with that of base alloy. The density of Al alloy fly ash composites was lower than that of the base alloy. The sintered density of the Al alloy fly ash composites and Al alloy slightly decreased when compared to green density. The hardness of the Al alloy fly ash composites was higher than that of base alloy and it increased with the increase in weight percentage of fly ash content upto 12 wt-%. Compressive strength of the composites was lower than that of base alloy and it decreased with increasing weight percentage of fly ash. The electrical resistivity and corrosion rate of the composites were higher than that of base alloy and they increased with increasing weight percentage of fly ash. Scanning electron microscope was used to examine the microstructural characteristics of the composites. X-ray diffraction pattern was used to identify various phases present in Al alloy fly ash composites.     

Age-hardening characteristics of aluminum alloy-hollow fly ash composites

The aging characteristics of aluminum alloy A356 and an aluminum alloy A356 containing hollow spherical fly ash particles were studied using optical microscopy, transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, hardness tests, and compressive tests. The variation of hardness and compressive strength as a function of aging time for the composite have been reported. Since the density of the composite is lower than that of the base alloy due to the presence of hollow particles, the composites have a higher specific strength and specific hardness compared to the matrix. Even though the hardness of the as-cast composite was higher than that of the base alloy, no significant change in the aging kinetics was observed, due to the presence of spherical fly ash particles in the matrix. Aging times of the order of 10⁴ to 10⁵ seconds were required to reach the peak hardness (92 HRF) and compressive strength (376 MPa) in both the A356-5 wt pct fly ash composite and the matrix alloy. The possible effects of shape and hollowness of particles, the interface between the matrix and the particles, the low modulus of the particles, and the microcracks formed on the surface of hollow fly ash particles on the kinetics of the age hardening of aluminum alloy A356 are discussed.     

Effects of SiC whiskers and particles on precipitation in aluminum matrix composites

The age-hardening precipitation reactions in aluminum matrix composites reinforced with discontinuous SiC were studied using a calorimetric technique. Composites fabricated with 2124, 2219, 6061, and 7475 alloy matrices were obtained from commercial sources along with unreinforced control materials fabricated in a similar manner. The 7475 materials were made by a casting process while the others were made by powder metallurgy: the SiC reinforcement was in the form of whiskers or particulate. It was found that the overall age-hardening sequence of the alloy was not changed by the addition of SiC, but that the volume fractions of various phases and the precipitation kinetics were substantially modified. Precipitation and dissolution kinetics were generally accelerated. A substantial portion of this acceleration was found to be due to the powder metallurgy process employed to make the composites, but the formation kinetics of some particular precipitate phases were also strongly affected by the presence of SiC. It was observed that the volume fraction of GP zones able to form in the SiC containing materials was significantly reduced. The presence of SiC particles also caused normally quench insensitive materials such as 6061 to become quench sensitive. The microstructural origins of these effects are discussed.     

Compaction and sintering behaviour of A356–fly ash composites: a preliminary investigation

Abstract

In the present study, A356–fly ash metal matrix composites were developed through powder metallurgy route. The composites were mixed by using the ball milling technique, shaped through uniaxial and cold isostatic compaction, and then sintered at 520°C. Scanning electron microscopy and X-ray diffraction were used for microstructure and phase characterisation. The density and microhardness of the composites were evaluated as a function of fly ash content, compaction pressure, sintering time and age hardening time. Uniaxial cold compaction of the composites increased their green density and cold isostatic compaction of the compacts led to a further increase in the density. At a constant compaction pressure, the density decreased with increasing fly ash content, resulting in light weight composites. The microhardness of the composites increased with the addition of 10 wt-% fly ash while it decreased with the addition of 20 and 30 wt-% fly ash. Sintering at 520°C increased the density of the composites and the grain size of the α-Al phase of the matrix. The matrix alloy and the composite containing 10 wt-% fly ash showed some response to age hardening at 160°C. However, no response to age hardening was observed at 200°C.     

Chemical reactions between aluminum and fly ash during synthesis and reheating of Al-fly ash composite

Thermodynamic analysis indicates that there is the possibility of chemical reactions between aluminum melt and cenosphere fly ash particles. These particles contain alumina, silica, and iron oxide, which, during solidification processing of aluminum-fly ash composites or during holding of such composites at temperatures above the melting temperature of aluminum, are likely to undergo chemical reduction. These chemical reactions between the fly ash and molten aluminum have been studied by metallographic examination, differential thermal analysis (DTA), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and X-ray analysis after holding the aluminum-fly ash composites for different periods above the liquidus temperature. The experiments indicate that there is progressive reduction of silica and mullite in the fly ash, and formation of alumina with holding time of composites at a temperature of 850 °C. The walls of the cenosphere fly ash particles progressively disintegrate into discrete particles as the reaction progresses. The rate of chemical reaction was high at the start of holding the composite at a temperature of 850 °C, and then the rate significantly decreased with time. The reaction was almost complete after 10 hours.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.     

Interfacial Reactions in Al-Mg (5083)/SiCp Composites during Fabrication and Remelting

The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06 wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800 °C for 30 minutes. The reaction mechanisms between SiC _p and liquid Al and between the SiO₂ layer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAl₂O₄) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO₂ layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing the SiC _p from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness of the initial SiO₂ layer.     

Determination of Optimum Ingredients for Phosphogypsum Composite Stability under Marine Conditions-Response Surface Analysis with Process Variables

The construction of mixture designs and the methods of response surface analysis of mixture data are discussed and applied for determining the optimum ingredients for stabilized phosphogypsum (PG) composites conducive to marine application. Of particular importance is the ability of the composites to maintain physical integrity when submerged. Therefore, potential indicators for the survivability of the stabilized PG composites were also screened. The triangular coordinate system was used to present the three ingredient components of the PG composites as well as their dependent variables. The augmented simplex centroid design with pseudocomponents was used in determining mixture ingredient composition. A quadratic model with two process variables was used to analyze the experimental results and predict the optimum ingredient composition. The model predicts that a series of PG: class C fly ash:portland type II cement ingredients, such as 62%:35%:3% PG:class C fly ash:portland type II cement and 65%:31%:4% PG:class C fly ash:portland type II cement composites with class C fly ash content (30.7–36.6%) and portland type II cement content (2.7–4.2%), can survive in marine environments for more than two years. The indicator screening for the survivability of the stabilized PG composites in the marine environment showed that the minimum wet leached surface hardness, minimum wet control surface hardness, and maximum effective diffusion coefficient may serve as indicators.     

Preparation and casting of metal-particulate non-metal composites

A new process for the preparation and casting of metal-particulate non-metal composites is described. Particulate composites of ceramic oxides and carbides and an Al-5 pet Si-2 pct Fe matrix were successfully prepared. From 10 to 30 wt pct of A1₂O₃, SiC, and up to 21 wt pct glass particles, ranging in size from 14 to 340 ώ were uniformly distributed in the liquid matrix of a 0.4 to 0.45 fraction solid slurry of the alloy. Initially, the non-wetted ceramic particles are mechanically entrapped, dispersed and prevented from settling, floating, or agglomerating by the fact that the alloy is already partially solid. With increasing mixing times, after addition, interaction between the ceramic particles and the liquid matrix promotes bonding. Efforts to mix the non-wetted particles into the liquid alloy above its liquidus temperature were unsuccessful. The composite can then be cast either when the metal alloy is partially solid or after reheating to above the liquidus temperature of the alloy. End-chilled plates and cylindrical slugs of the composites were sand cast from above the liquidus temperature of the alloy. The cylindrical slugs were again reheated and used as starting material for die casting. Some of the reheated composites possessed “thixotropy.” Distribution of the ceramic particles in the alloy matrix was uniform in all the castings except for some settling of the coarse, 340ώ in size, particles in the end-chilled cast plates.     

XRD and XANES study of reactions in particulate Al–Mg alloy composites

Abstract

Reactivity in stir cast Al Uju Mg alloy composites reinforced with fly ash was investigated by means of X‐ray diffraction (XRD) and X‐ray absorption near edge structure (XANES) spectroscopy at Al and Si K‐edges. High purity Al, α‐Al₂O₃, MgAl₂O₄ (spinel), MgO and SiO₂ powders were used as reference materials in the XANES investigation. The XRD and XANES data acquired from the raw fly ash used in making the composites were compared with fly ash particles extracted from the as cast composites. The major reaction product formed during fabrication of the composites was spinel. The XANES technique was useful in verifying the presence of various oxides in raw and extracted fly ash particles.

On a investigué la réactivité de composites de l’alliage Al–Mg renforcé avec des cendres volantes et moulé par agitation, au moyen de la diffraction des rayons X (XRD) et de la spectroscopie de structure près du front d’absorption des rayons X (XANES) aux seuils K de l’Al et du Si. On a utilisé des poudres d’Al, d’Al₂O₃‐α, de MgAl₂O₄ (spinelle), de MgO et de SiO₂ de haute pureté comme matériaux de référence dans l’investigation par XANES. On a comparé les données de XRD et de XANES acquises à partir des cendres volantes brutes utilisées dans la fabrication des composites, avec les particules de cendres volantes extraites des composites de brut de coulée. Le produit de réaction majeur formé lors de la fabrication des composites était le spinelle. La technique de XANES était utile dans la vérification de la présence d’oxydes variés dans les particules de cendres volantes brutes et extraites.     

Friction and abrasion resistance of cast aluminum alloy-fly ash composites

The abrasive wear properties of stir-cast A356 aluminum alloy-5 vol pct fly ash composite were tested against hard SiC _p abrasive paper and compared to those of the A356 base alloy. The results indicate that the abrasive wear resistance of aluminum-fly ash composite is similar to that of aluminum-alumina fiber composite and is superior to that of the matrix alloy for low loads up to 8 N (transition load) on a pin. At loads greater than 8 N, the wear resistance of aluminum-fly ash composite is reduced by debonding and fracture of fly ash particles. Microscopic examination of the worn surfaces, wear debris, and subsurface shows that the base alloy wears primarily by microcutting, but the composite wears by microcutting and delamination caused by crack propagation below the rubbing surface through interfaces between fly ash and silicon particles and the matrix. The decreasing specific wear rates and friction during abrasion wear with increasing load have been attributed to the accumulation of wear debris in the spaces between the abrading particles, resulting in reduced effective depth of penetration and eventually changing the mechanism from two-body to three-body wear, which is further indicated by the magnitude of wear coefficient.     

Effect of δ alumina fibers on the aging characteristics of 2024-based metal-matrix composites

The age-hardening precipitation reaction in aluminum matrix composites reinforced with discontinuous alumina fibers was studied using the differential scanning calorimetry (DSC) technique, microhardness tests, and transmission electron microscopy (TEM) observation. Composites fabricated with the 2024 alloy matrix were infiltrated through a ceramic preform using a squeeze-casting process. The alumina fibers had a considerable effect on the aging response of the matrix alloy in composites. Alumina fibers caused suppression of Guinier—Preston (GP) zone formation in composite that reduced the peak hardening during artificial aging. The suppression of GP zone formation in composites is believed to be due to the fiber-matrix interface, which acts as a sink for vacancies during quenching. Moreover, the presence of reinforcement does not alter the kinetics of the subsequent artificial aging of these Al₂O₃/2024Al composites.     

Mechanical Properties and Dry Sliding Wear Behaviour of Molybdenum Disulphide Reinforced Zinc–Aluminium Alloy Composites

ZA-27 alloy is a lightest alloy which offers excellent bearing and mechanical properties in automobile and industrial applications. In this study, the MoS₂ particles with 0.5, 1 and 1.5 (wt%) weight percentages were reinforced in ZA-27 alloy to form composites, which were fabricated by using ultrasonic assisted stir casting method. The ZA-27/MoS₂ composite specimens were examined for chemical composition with the aid of XRD technique and EDS. Microstructure analysis of the ZA-27/MoS₂ composites was studied using SEM. Tests were conducted for mechanical properties such as tensile strength and hardness on ZA-27/MoS₂ composites samples as per ASTM standards. Dry sliding wear behavior of the composites was tested at various operating conditions by using pin-on-disc apparatus. Microstructural images of the ZA-27 composites reveal that there is a uniform dispersion of the MoS₂ particles in the base material. From the results it is observed that the mechanical properties increases with ZA-27 reinforced with 0.5 wt% MoS₂ composite and further decreases with increase in the filler content. The enhanced wear resistance is observed in ZA-27 reinforced MoS₂ composites as compared to the unreinforced alloy. The wear rate of the ZA-27 composites decreases with the increase in filler content, further the worn surfaces as examined using SEM reveals the wear mechanism explaining the improved wear resistance of the particulate composites.     

常压H2气氛烧结W-TiC合金的致密化行为和性能

采用粉末冶金方法在常压H2气氛下制备W-TiC合金,研究W-TiC合金的烧结致密化行为,并对合金的性能和组织结构进行分析.结果表明:添加微量强化烧结元素可改善W-TiC合金的烧结活性,在1700℃烧结120min后其相对密度达到99.2％;随着烧结温度的升高,W-TiC合金的拉伸强度提高,在2000℃烧结120 min...     

高铝粉煤灰钙矿相转化行为研究

从高铝粉煤灰的矿相特性入手,研究转化剂的种类及用量、温度、时间、液固比等对高铝粉煤灰钙矿相转化行为的影响及其反应机理,并提出了高铝粉煤灰制备铝硅合金新工艺。结果表明,高铝粉煤灰中Al2O3和SiO2含量之和达到70%以上,Ca元素主要以CaSO4形式存在,还有一部分以2CaO·Al2O3·SiO2形式存在。粉煤灰中钙矿相转化为碳酸钙后对后续酸浸除杂过程中氧化铝损失影响不大,但能够大幅度提高除钙率。高铝粉煤灰钙矿相转化最佳条件为:转化剂Na2CO3添加量为2倍理论值、转化温度60℃、反应时间1.5h、液固比4,在此条件下除钙率能够达到90%以上。     

The effects of hot pressing parameters on the strength of aluminum/stainless steel composites

Stainless steel fiber reinforced aluminum matrix composites have been fabricated by hot pressing under different conditions of temperature, pressure, and time. The variation in tensile strength of these composites has been studied in detail, and the hot pressing parameters have been optimized in order to fabricate composites having maximum strength at any fiber volume fraction. Microprobe analysis and Scanning Electron Microscopy of these composites have revealed interesting features of fiber/ matrix interface. These features have been found helpful in explaining the dependence of strength of the composites on hot pressing parameters.     

Effect of Graphite and SiC Addition into Cu and SiC Particle Size Effect on Fabrication of Cu–Graphite–SiC MMC by Powder Metallurgy

Here we have reported individual and combined effect of graphite and SiC into Cu matrix during fabrication of Cu–graphite–SiC hybrid metal matrix composite by powder metallurgy. Mechanical properties of the composites are enhanced by simultaneous addition of 1, 3, 5, 10 and 15 vol. % of graphite along with 2, 5 and 10 wt. % of SiC into pure Cu, whereas electrical conductivity deteriorates. Composites are fabricated by cold compaction of composite powder mixture followed by conventional sintering in a tubular furnace at 900 °C for 1 h in argon atmosphere. For comparison, SiC powder size of 5 and 50 µm are used to study the effect of SiC particle size on microstructure, mechanical and electrical properties of the composites. Optical microscopy and scanning electron microscopy reveal the homogeneous distribution of graphite and SiC in matrix and good compatibility between Cu–graphite and Cu–SiC particles. Hardness of the composites decreases with increase in graphite and increases with increase in SiC content. Composites containing fine SiC particles show higher hardness value as compared to coarse particles. Maximum Vickers hardness value of 75 is obtained for Cu-1 vol. % graphite-10 wt. % SiC composite. Electrical conductivity decreases with increase in both graphite and SiC content. Composites containing coarse SiC particles exhibit higher electrical conductivity than fine SiC.     

Oxidative Decolorization of Reactive Dye Solution Using Fly Ash as Catalyst

This paper deals with the potential of coal fly ash as a heterogeneous catalyst in peroxidative decolorization of aqueous solution of several reactive drimarene dyes using hydrogen peroxide (H2O2). The effects of various parameters (source of fly ash, fly ash loading, temperature, pH, initial concentration of dye and hydrogen peroxide, and deactivation of catalytic effect of fly ash) were studied. The rate of decolorization is zero order with respect to concentration of dye in the range of 50–200 mg∕L and first order with respect to concentration of H2O2 in the range of 375–2,500 mg∕L. It was found that at 333 K and pH 2.0, only 2% (weight/volume) fly ash loading can completely decolorize 100 mg∕L dye solution within 180–1,770 s for an initial H2O2 concentration of 1,500 mg∕L, depending on the source of fly ash and dye used. But maximum 25% decolorization was noticed after 3 h duration of reaction without fly ash. The negligible homogeneous catalytic action of trace amount of metals like Fe, Mn, and Cu leached from the fly matrix confirmed the sole heterogeneous catalytic nature of fly ash particles in decolorization reactions. The deactivation of fly ash was found to be <30% in nine repeated uses. Adsorption dynamics and mechanism of dyes onto fly ash also were studied in the absence of H2O2.     

Strength Characteristics of Class F Fly Ash Modified with Lime and Gypsum

This paper presents the shear strength characteristics of a low lime class F fly ash modified with lime alone or in combination with gypsum. Unconfined compression tests were conducted for both unsoaked and soaked specimens cured up to 90 days. Addition of a small percentage of gypsum (0.5 and 1.0%) along with lime (4–10%) enhanced the shear strength of modified fly ash within short curing periods (7 and 28 days). The gain in unsoaked unconfined compressive strength (qu) of the fly ash was 2,853 and 3,567% at 28 and 90 days curing, respectively, for addition of 10% lime along with 1% gypsum to the fly ash. The effect of 24?h soaking showed reduction of qu varying from 30 to 2% depending on mix proportions and curing period. Unconsolidated undrained triaxial tests with pore-pressure measurements were conducted for 7 and 28 days cured specimens. The cohesion of the Class F fly ash increased up to 3,150% with addition of 10% lime along with 1% gypsum to the fly ash and cured for 28 days. The modified fly ash shows the values of Skempton’s pore-pressure parameter, Af similar to that of over consolidated soils. The effects of lime content, gypsum content, and curing period on the shear strength parameters of the fly ash are highlighted herein. Empirical relationships are proposed to estimate the design parameters like deviatoric stress at failure, and cohesion of the modified fly ash. Thus, this modified fly ash with considerable shear strength may find potential use in civil engineering construction fields.