Analysis of Distribution of Nonmetallic Inclusions in Aluminum DC-Cast Billets and Slabs

Inclusion distribution was studied in commercial aluminum DC-cast billets and slabs using a newly developed deep-etching method. Analyses revealed a nonuniform distribution of nonmetallic inclusions across billet diameters and lengths, and also across slab thicknesses and widths. In as-cast billets, more inclusions were found at the beginning and end of the billet length; more were present near the cross-section center than near the surface. In slabs, inclusions were located mostly within 13 mm of the surface and in a band between the centerline and the surface. Few inclusions were found 60 to 100 mm from the slab surface or at the centerline. In addition, comparing slab quality after casting using three types of ceramic foam filters (CFFs; i.e., 30 ppi, 50 ppi, and 50 ppi + HF) revealed significant differences in inclusion size, number, and distribution. Casting slabs using a finer pore-size filter (50 ppi) reduced the number of non-metallic inclusions greatly. The inclusion distribution patterns observed in the solidified slabs are discussed in terms of melt flow during casting.     

A Model of Foam Filters

Filtration with foam filters is reported as a successful method to remove inclusions from top-cut silicon scrap. Inclusions in top-cut silicon scrap are needle-like Si₃N₄ particles and round SiC inclusions. A high filtration efficiency of more than 99 pct for 30-ppi SiC filters is achieved. The inclusions that remain are mainly SiC particles smaller than 10 μm. Possibly these particles are primarily secondary inclusions. The filtration efficiency increases with decreasing filter pore size. The main factor that plays a role in deep-bed filtration seems to be interception. Various models are considered to estimate the removal efficiency of foam filters by this mechanism. Here, we propose a new model called “the branch” model. This model gives a high filtration efficiency and agrees the best with the experimental results.     

Deep filtration of molten aluminum using ceramic foam filters and ceramic particles with active coatings

The inclusions in molten aluminum were removed using the deep filtration of ceramic foam filters and ceramic particles with active coatings. The results of tensile tests showed that the elongation of the filtered tensile specimen S₆ increases by 17.93 pct, but the tensile strength does not. The scanning electron microscope (SEM) examination showed that the secondary cracks and dimples in the filtered tensile specimen S₆ were finer and more homogeneous than those in the unfiltered tensile specimen S₀. In addition, metallographic examination showed that there were only a few inclusions of approximately 6 μm in diameter in the filtered specimen S₆, but more inclusions of approximately 40 μm in diameter were found in the unfiltered specimen S₀. The filtration efficiency of the coated ceramic particles was higher than that of the coated ceramic foam filters. The active coating could effectively capture the inclusions and dissolve Al₂O₃ in them during filtration.     

Preparation of Aluminum Metal Matrix Composite with Novel In situ Ceramic Composite Particulates, Developed from Waste Colliery Shale Material

A novel method is adapted to prepare an in situ ceramic composite from waste colliery shale (CS) material. Heat treatment of the shale material, in a plasma reactor and/or in a high temperature furnace at 1673 K (1400 °C) under high vacuum (10^?6 Torr), has enabled in situ conversion of SiO₂ to SiC in the vicinity of carbon and Al₂O₃ present in the shale material. The composite has the chemical constituents, SiC-Al₂O₃-C, as established by XRD/EDX analysis. Particle sizes of the composite range between 50 nm and 200 μm. The shape of the particles vary, presumably rod to spherical shape, distributed preferably in the region of grain boundaries. The CS composite so produced is added to aluminum melt to produce Al-CS composite (12 vol. pct). For comparison of properties, the aluminum metal matrix composite (AMCs) is made with Al₂O₃ particulates (15 vol. pct) with size <200 μm. The heat-treated Al-CS composite has shown better mechanical properties compared to the Al-Al₂O₃ composite. The ductility and toughness of the Al-CS composite are greater than that of the Al-Al₂O₃ composite. Fractographs revealed fine sheared dimples in the Al-CS composite, whereas the same of the Al-Al₂O₃ composite showed an appearance of cleavage-type facets. Abrasion and frictional behavior of both the composites have been compared. The findings lead to the conclusion that the in situ composite developed from the colliery shale waste material has a good future for its use in AMCs.     

Separation of Non-metallic Inclusions from a Fe-Al-O Melt Using a Super-Gravity Field

An innovative method for separating non-metallic inclusions from a high temperature melt using super gravity was systematically investigated. To explore the separation behavior of inclusion particles with densities less than that of metal liquid under a super-gravity field, a Fe-Al-O melt containing Al₂O₃ particles was treated with different gravity coefficients. Al₂O₃ particles migrated rapidly towards the reverse direction of the super gravity and gathered in the upper region of the sample. It was hard to find any inclusion particles with sizes greater than 2 μm in the middle and bottom areas. Additionally, the oxygen content in the middle region of the sample could be reduced to 0.0022 mass pct and the maximum removal rate of the oxygen content reached 61.4 pct. The convection in the melt along the direction of the super gravity was not generated by the super-gravity field, and the fluid velocity in the molten melt consisted only of the rotating tangential velocity. Moreover, the motion behavior of the Al₂O₃ particles was approximatively determined by Stokes’ law along the direction of super gravity.     

Removal of Inclusions from Aluminum Through Filtration

Filtration experiments were carried out using both an AlF₃ slurry-coated and an uncoated Al₂O₃ ceramic foam filter to study the removal of nonmetallic inclusions and impurity elements. The results showed that the 30-ppi ceramic foam filter removed up to 85 pct inclusions from aluminum. Several pictures of two- and three-dimensional morphologies of both nonmetallic and intermetallics inclusions also have been presented. The following contributing mechanisms for the removal of nonmetallic inclusions in the deep-bed filtration mode are proposed: (1) collision with walls and interception effect and (2) the formation of both intermetallic and nonmetallic inclusion bridges during filtration. Fluid dynamics modeling of inclusion attachment to the filter walls showed that most inclusions, especially those with larger sizes, are entrapped at the upper part of the filter, whereas smaller inclusions are dispersed well throughout the filter. The calculated inclusions removal fractions for the 30-ppi filter showed that almost all inclusions >125 μm are removed, and inclusions ~5 μm in size are removed up to 85 pct. The interfacial energy between two collided same-size inclusions was calculated, indicating that a strong clustering of inclusions may result within the filter window. Magnesium impurities were removed up to 86 pct by the AlF₃ slurry-coated filter. The filter acted in active filtration mode in addition to the contribution of the air oxidation of dissolved [Mg], which was calculated to be 13 pct. The total mass transfer coefficient of dissolved [Mg] to the reaction interface was calculated to be 1.15 × 10⁻⁶ m/s.     

Modification of cast aluminum-matrix composite materials by refractory nanoparticles

The effect of SiO₂ and Al₂O₃ oxide ceramic nanoparticles on the solidification of model samples based on a commercial D16 alloy is studied. The composite samples are fabricated by reaction casting when titanium, nickel, and ceramic powders are mixed with an aluminum melt. The grain size in a matrix, the size and shape of Al₃Ti intermetallic inclusions, and the interphase distances in eutectics are determined with optical and scanning electron microscopes. A certain modifying effect of oxide ceramic nanoparticles on the structure of model CMs during their in situ formation is detected, and the inoculation effect of SiO₂ added to a melt on the reaction products is most pronounced.     

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.     

Correlation of abrasive wear with microstructure and mechanical properties of pressure die-cast aluminum hard-particle composite

Aluminum hard particle composites were synthesized by the solidification processing technique and the composite melt was solidified using gravity and pressure die castings. An aluminum-silicon alloy (A 332.1) has been used as the matrix and silicon carbide particles (quantity: 10 wt pct, and size: 50 to 80 μm) have been used as reinforcement for synthesis of the composite. The microstructure of the pressure die cast composite is found to be finer than those of the gravity cast ones. Additionally, the distribution of SiC particles in the Al alloy matrix is found to be more uniform in the pressure die-cast composites compared to the gravity die-cast ones. The mechanical properties such as ultimate tensile strength, hardness, and ductility are observed to be superior in the case of pressure die-cast composites compared to the gravity-cast one. The two-body abrasive wear resistance of the Al-composite is also noted to be greater in the pressure die-cast composite than in the gravity-cast one. The effects of injection pressure on the mechanical properties and wear resistance of the pressure die-cast composites are examined. It is observed that the wear resistance (inverse of wear rate), hardness, and strength of the Al-SiC composites increase with the increase in injection pressure during pressure die casting. This may be due to the finer microstructure, the absence of casting defects, and the stronger interfacial bonding between the matrix and hard dispersoid in pressure die-cast composites. The wear rate of the alloys and composites is studied as a function of their hardness, strength, and Young’s modulus. It is noted that the wear rate is primarily controlled by hardness even though other mechanical properties influence the wear behavior of the materials to some extent. An attempt is made to establish an empirical relation to correlate the wear rate of material with the mechanical properties such as hardness, ultimate tensile strength, and elongation.     

Electron and laser beam welding of high strain rate superplastic Al-6061/SiC composites

The welding characteristics of a fine-grained 6061 Al and three 6061/1, 5, and 20 pct SiC composites under high energy electron beam welding (EBW) and laser beam welding (LBW) were examined. The three composites exhibited high strain rate superplasticity (HSRS). The 6061 Al was more readily welded by EBW than by LBW, and the situation was reversed in the reinforced composites. In the reinforced composites, the fusion zone contained the once fully melted matrix and fully reacted SiC, and the heat affected zone (HAZ) contained the partially melted matrix and nearly unreacted SiC. This effect was particularly apparent in the 20 pct SiC composite. With increasing SiC content from 0 to 20 pct, the reflection of the laser beam decreased, and the melt viscosity increased due to the increasing amount of Al₄C₃ compounds. For the HSRS fine-grained 6061/20 pct SiC composite, there formed a sharp V-notch under EBW. The high viscosity or low fluidity of the melt inside the fusion zone of 6061/20 pct SiC resulted in incomplete backfill and notch formation. The postweld mechanical performance and joint efficiency both became seriously degraded. The original fine structures in the HSRS composites could not be restored after welding.     

Development of Porous Ceramic Preforms Based on Magnesia Partially Stabilised Zirconia with Additions of MgAl2O4, MgO,Al2O3 and Ti

The present paper deals with the development of metal matrix composites with improved mechanical properties based on metastable austenitic TRIP‐steel and magnesia partially stabilised zirconia. The ceramic material is characterised regarding the influence of different additives (MgAl₂O₄, MgO, Al₂O₃) on the monoclinic/tetragonal/cubic phase composition. First results for the infiltration of ceramic preforms are shown, on the one hand performed by conventional steel casting technology and on the other hand by submerging of preforms in a metal melt using a steel casting simulator.     

Optimization of Mold Flux for the Continuous Casting of Cr-Contained Steels

To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr₂O₃ but decrease with the addition of B₂O₃. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr₂O₃, and then further decreases to 25.3 pct with the addition of B₂O₃; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B₂O₃-1.10 pct MnO-2.10 pct Cr₂O₃ shows the best properties for the continuous casting of Cr-contained steels.     

Enhancing physical and mechanical properties of Mg using nanosized Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particulates as reinforcement

A magnesium-based composite with 1.1 volume percentage of nanosized Al₂O₃ particulates reinforcement was fabricated using an innovative disintegrated melt deposition technique followed by hot extrusion. Al₂O₃ particulates with an equivalent size of 50 nm were used as reinforcement. Microstructural characterization of the materials revealed grain refinement of magnesium matrix due to incorporation, retention, and uniform distribution of reinforcement. Physical properties characterization revealed that the addition of nano-Al₂O₃ particulates as reinforcement improves the dimensional stability of pure magnesium. Mechanical properties characterization revealed that the presence of nano-Al₂O₃ particulates as reinforcement leads to a significant increase in microhardness, dynamic elastic modulus, 0.2 pct yield strength (YS), ultimate tensile strength (UTS), and ductility of pure magnesium. The results revealed that the combined tensile properties of these materials are superior when compared to Mg reinforced with much higher volume percentage of SiC. An attempt is made in the present study to correlate the effect of nano-Al₂O₃ particulates as reinforcement with the microstructural, physical, and mechanical properties of magnesium.     

The behavior and effect of rare earth CeO<Subscript>2</Subscript> on <Emphasis Type="Italic">in-situ</Emphasis> TiC/Al composite

Rare earth CeO₂ was investigated as an additive for in-situ preparation of TiC/Al composites using XD (exothermal dispersion) + casting technology. Experiment results showed that an optimum CeO₂ addition of 0.5 wt pct promotes the generation and refinement of TiC particles, prevents the formation of Al₃Ti, increases the wettability between the TiC ceramic particles and the Al matrix, and improves the mechanical properties of composite. A corresponding thermodynamic model was proposed for the mechanism.     

Structure and properties of hybrid composite materials

The structure and interfacial interaction are studied in the hybrid aluminum-matrix composite materials fabricated by reactive casting combined with mechanical mixing of fillers with a metallic melt. The following types of hardening are considered: hardening by ceramic particles and by the phases formed as isolated inclusions or coatings on ceramic particles during in situ reactions. The hardness and tribological properties of the composite materials as functions of their compositions are discussed.     

Mechanical behavior of Al−Li/SiC composites: Part III. Micromechanical modeling

A micromechanical model is developed to compute the stress-strain curve of particle-reinforced metal-matrix composites under monotonic and cyclic deformation. The composite was modeled as a three-dimensional array of hexagonal prisms, each containing an intact or fractured reinforcement. The average stresses acting on the intact and damaged cells—as well as on the ceramic particles —were computed from the finite-element analysis of axisymmetric cylindrical cells, and the overall composite response was then calculated through an isostrain approach. The model was validated against the experimental results, reported in Parts I and II of this article, for an 8090 Al alloy reinforced with 15 vol pct SiC particles,^[1,2] where the matrix and reinforcement properties were obtained from mechanical tests on the unreinforced alloy and from quantitative microscopy analyses of the fraction of broken reinforcements in the composite. The critical mechanisms which controlled the deformation and damage processes in the composite during monotonic and cyclic deformation are discussed in light of the model results.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

Mechanical behavior of Al-Li/SiC composites: Part III. Micromechanical modeling

A micromechanical model is developed to compute the stress-strain curve of particle-reinforced metal-matrix composites under monotonic and cyclic deformation. The composite was modeled as a three-dimensional array of hexagonal prisms, each containing an intact or fractured reinforcement. The average stresses acting on the intact and damaged cells — as well as on the ceramic particles — were computed from the finite-element analysis of axisymmetric cylindrical cells, and the overall composite response was then calculated through an isostrain approach. The model was validated against the experimental results, reported in Parts I and II of this article, for an 8090 Al alloy reinforced with 15 vol pct SiC particles,^[1,2] where the matrix and reinforcement properties were obtained from mechanical tests on the unreinforced alloy and from quantitative microscopy analyses of the fraction of broken reinforcements in the composite. The critical mechanisms which controlled the deformation and damage processes in the composite during monotonic and cyclic deformation are discussed in light of the model results.     

Refining of Aluminum and Steel Melts by the Use of Multi-Cellular Extruded Ceramic Filters

Abstract

Laboratory prepared melts of steel containing Al₂O₃ inclusions and aluminum containing TiB₂ inclusions have been successfully filtered using multicellular extruded ceramic filters. Relatively high inclusion removal efficiencies have been achieved in both low temperature and high temperature melt systems —68 % inclusion removal efficiency in the Al-TiB₂ system (1020 K) using cordierite multicellular filters and 96 % inclusion removal efficiency in the steel–Al₂O₃ system (1873 K). The results have been analysed using Apelian and Mutharasan's kinetic model for filtration of the metallic melts [1]. The inclusion capture kinetics and filtration characteristics of the porous media used in this investigation are discussed.