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Aluminum Transactions __________________________________________________

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 Abstract

The effect of damage by particle cracking on the strain hardening rate and the onset of tensile instabilities in an Al-7Si-0.4Mg casting alloy has been modeled combining current theories of dispersion hardening with Weibull statistics. The effect of spatial distribution and aspect ratio of the eutectic Si particles on the rate of particle cracking with the applied strain has been considered. The model predicts that at low strains, the aspect ratio of the Si particles determines the rate of generation of damage, while the scale of particle clustering is important at large strains. This is in broad agreement with the experimental results. However, the model predicts global tensile instabilities that are not observed experimentally, suggesting that the actual effect of damage on the strain-hardening rate is very small. Fracture seems to be caused by the attainment of a critical amount of damage along the dendritic boundaries. The role of particle clustering on the overall ductility is discussed.

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In the present study, Al/SiC composites containing three different weight percents of SiC particulates were synthesized using an innovative disintegrated melt deposition route. Microstructural characterization studies conducted on the composite samples following extrusion revealed minimal porosity, fairly uniform distribution of SiC particulates and good SiC-Al interfacial integrity. The cumulative volume fraction of SiC particulates and porosity when plotted against tensile ductility revealed a linear correlationship. The results were rationalized placing particular emphasis on the role of SiC particulates and porosity to serve as crack initiation sites under tensile loading. In addition, their cumulative influence to adversely affect ductility in the aluminum based discontinuously reinforced composite formulations is addressed.

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The tensile response and fracture characteristics of aluminum alloy 6061 discontinuously-reinforced with particulates of Al₂0₃, is presented and discussed. Microstructural analysis of the composite revealed a non-uniform spatial distribution of the particulate reinforcements. The elastic modulus and strength of the composite was better than that of the unreinforced metal matrix. However, their strength decreased with an increase in test temperature. Tensile fractures, on a microscopic scale, were comprised of particulate cracking and decohesion at the interfaces. Final fracture of the composite resulted from crack propagation through the matrix between particles. The intrinsic mechanisms and kinetics governing the tensile fracture process are discussed in light of competing and mutually interactive influences of composite microstructural effects, matrix deformation characteristics and test temperature.

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Vacuum compo-casting of A356 aluminum alloy composites, reinforced with SiC particulates, were investigated in this study. The processing parameters that improve the microstructures and mechanical properties of these composites were established. At the stirring temperature of 610^oC and the stirring speed of 400 rpm under 10^-2 torr vacuum, good quality Al-SiC_p composites having relatively homogeneous microstructure and sound Al/SiC interfacial bonding were obtained. However, a small amount of micropores were present in the castings. The microstructures and mechanical properties of these composites were compared with those of Duralcan's commercial composites. Enhanced distribution of SiC particulates and reduction in the sizes of micropores were found in composites processed by vacuum compo-casting. Minimization of micropores, fast cooling rate and homogeneous distribution of SiC particulates and eutectic Si particles were found to improve the mechanical properties of the A356 Al-SiC_p composites.

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The fracture strain of a composite, as measured in a tension test, is an indication of limit to which the composite can be deformed. The fracture strain is in turn related to the work hardening behavior of the composite. In the case of particulate reinforced composites with an age hardenable aluminum alloy matrix, the microstructure and mechanical properties can be altered by a suitable aging treatment. In particular, aging can be used to suitably alter the strength and the ductility of the composite. The strength was maximized in the peak-aged condition while the as solutionized condition had relatively higher ductility. The under aged and over aged conditions exhibited intermediate levels of both strength and ductility. In this paper, a modified continuum model was used to relate the work hardening behavior of the composite to microstructural parameters. The model was then used to predict the fracture strain of the composite for the four aging conditions studied. The model was shown to predict the fracture strain of the composite quite accurately for all aging conditions.

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The in-situ reinforcement method for producing aluminum composites, in which reinforcing particulates are formed in-situ during the process, opens a promising avenue for the production of metal matrix composite (MMC) components. However, to incorporate the particles into molten aluminum alloy, this process is confronted with the difficulty of a long processing time at high temperatures. To eliminate such a difficulty arising from poor wettability, ultrasonic vibration along with electromagnetic stirring were applied to the in-situ processing of the aluminum-magnesium matrix composite with SiO₂ particles as a source of oxygen for generating the reinforcing oxide particulates. As a result, within a short processing time, the SiO₂ particles were incorporated and reacted with the molten Al-Mg alloy to produce MgAl₂O₄ and Al₂O₃. In addition, since the application of ultrasonic vibration forcibly improves the wettability between the particles and the molten Al-Mg alloy, it helps prevent the formation of gas porosity. The selective incorporation of reinforcing particulates in a specific portion of composites, for the purpose of such functions as wear resistance, is strongly desired in certain applications. To achieve this goal, the oxide particles are forced to segregate to the outer periphery of composite samples by applying a magnetic field with alternating current during solidification of the composite slurry. The selective incorporation of particulates was mainly affected by the following factors: the electromagnetic buoyancy of the magnetic field that was applied to the particle motion toward the outer periphery, the circulating flow of particulates that was originated from the difference in the magnetic flux density, preventing the particles from sedimentation and the relative diameter of the solidifying sample with respect to the current frequency.

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The aim of the present study was to determine the elastic modulus of aluminum based composites containing different volume fractions of SiC particulates using an innovative free-free beam type impact-based technique. This technique is based on the classical vibration theory, by which the geometry and material properties of the metal matrix composites are related to the resonant frequency of the test specimen. With the fundamental resonant frequency obtained from the experiment and the density determined by the Archimedes' principle, the elastic modulus values were determined. In addition, a finite element model was proposed for different SiC weight percentage samples for the determination of the dynamic elastic modulus using the first natural frequency corresponding to the flexural mode. The elastic modulus values obtained from the impact-based experiments were found to be in close agreement with predictions made using the finite element method and in better agreement when compared to theoretical methods such as the Halpin-Tsai method.

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Embedding optical fibers into structural components is a promising technology for real-time monitoring of some defects and properties of materials. There is much interest for producing such components at a low cost in the mechanical, civil and aerospace industry. In this work, silica based optical fibers with aluminum and polyimide coating were successfully embedded in various aluminum alloys by utilizing conventional casting techniques. Mechanical and optical transmission tests as well as metallographic examination were carried out to highlight the fiber integrity and observe the fiber/matrix interface after the embedding process. Preliminary interferometric tests showed that the embedded optical fiber could eventually be used to sense and measure the variations of properties and macrodefects in the matrix materials.

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Functionally-graded materials (FGMs) are a new class of materials in which the composition and/or the microstructure varies in one specific direction. One of the promising methods for the fabrication of FGM is the centrifugal method, which is an application of the centrifugal casting technique. In this method, a centrifugal force applied to the mixture of molten metal and dispersed material, such as ceramics powders or intermetallic compounds, leads to the formation of the desired gradation. The composition gradient is then achieved mainly from the difference in the centrifugal force produced by the difference in density between the molten metal and particles. The centrifugal method has the advantage of a possible application to the mass production of small and large FGM specimens. The physical properties of aluminum-based FGMs are also investigated using relatively large samples. In the present review, we first describe the fabrication method and microstructures of aluminum based FGMs and then discuss some recent results about the physical properties of FGMs fabricated by the centrifugal method.

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Recycling has grown to become an important global issue with the purpose of saving land space and optimizing natural resources. In this study, an aluminum-copper composite with reinforcing silicon carbide particulates was recycled using the technique of disintegrated melt deposition. Microstructural characterization studies on the recycled composite samples revealed minimal porosity, a near uniform distribution of the reinforcing SiC particulates, an equiaxed grain structure and good interfacial integrity between the reinforcing particulates and metal matrix. The recycled composite also showed good combinations of strength and ductility. The resulted tensile responses were rationalized in light of the intrinsic microstructural characteristics of the recycled composite sample.

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 Abstract

Some recent results from investigating the creep behavior of discontinuous aluminum alloy matrix composites processed by powder metallurgy are evaluated. The true threshold creep behavior indicates that the temperature dependence of the true threshold stress is stronger than that of the shear modulus of the matrix alloy. This accounts for the strong temperature and applied stress dependence of both the apparent activation energy of creep and the apparent stress exponent of the minimum creep strain rate. The creep strain rate is controlled by the diffusion in the composite matrix. The athermal detachment of dislocations model for fine alumina particles in the composite matrix is adopted in this study as the creep strain rate controlling mechanism. The temperature dependence of the true threshold stress is discussed showing that the Arzt-Wilkinson's relaxation factor () increases as a function of increasing temperature. Disappearance of the true threshold stress at high temperatures is indicative of the transition from athermal to thermally activated detachment of dislocations from fine alumina particles in the composite matrix.

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The Erosive-corrosive wear of LM13 aluminum alloy and its composite with Al₂O₃ and SiC particulates was studied in the as-cast and T6 heat-treated conditions. The corrosion rate of the as-cast aluminum alloy was two to three times higher than that of the cast aluminum composites. The corrosion rate of the aluminum composites was reduced by 75-80% due to the T6 heat treatment process. The erosive-corrosive wear rate of the alloy under investigation and its composites in a slurry containing 40 wt.% sand (size 100 - 200 m m) and synthetic mine water (0.1 cc H₂S0₄ + 0.4 gm NaCl per liter of water) was very similar. The erosive-corrosive wear rate of the aluminum alloy was 50% higher than that of the cast aluminum composites. On the other hand, the erosive-corrosive wear rates of the cast composites were 5 to 10% higher than the wear rate of the heat treated composites. The corrosion rate and erosive-corrosive wear rate were relatively low in Al-SiC_p composites as compared to that in Al-Al₂O₃ composite either in the as-cast or heat-treated conditions. The corrosion rate of Al-Al₂O₃ composite is 50% and 100% more than those of Al-SiC_p composite in the as-cast and heat-treated conditions, respectively. Whereas, the difference in the erosive-corrosive wear rate in these materials was only about 10-15%. This is due to the dominating mechanism of erosion during the erosive-corrosive wear of materials.

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 Influence of Load and Abrasive Size on the Abrasive

The two-body abrasive wear behavior of LM13 aluminum alloy and aluminum composites with 10 and 15 wt.% SiC particle were studied. The effect of sliding distance and abrasive grit size on the wear rate of these components was investigated. In most of the cases, the alloy or composite wear rate was found to be invariant with sliding distance and increases with applied load. Abrasive particle sizes between 25 and 80 m m do not have any effect on the wear rate of the composites. However, in the case of the Al-Si alloy, a finer particle size showed a lower wear rate. It was also found that the wear rate of the aluminum alloy is mainly governed by a plowing action. On the other hand, surface grinding and fracturing are dominant during the wear of composites.

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Stress corrosion cracking of Al-5Zn-1Mg alloy with two different tempers, T4 and T6, was investigated using constant load and slow strain rate in corrosive solutions of NaCl, NaCl+HCl, NaCl+As2O3 and NaCl+HCl+As2O3. Recorded results of fracture energy and plastic properties have shown that susceptibility of this alloy to stress corrosion cracking of this alloy depends on its aging conditions and the composition of corrosive solution. The positive effect of prolonged aging on the susceptibility of Al-Zn-Mg alloys to the stress corrosion cracking is likely caused by three phenomena associated with the appearance of grain-boundary precipitates, as follows: a) the change in deformation mode, b) high deep trapping of hydrogen at the grain boundaries and decrease in its lattice concentration and c) the changes in electrochemical behavior of the alloy resulting in the cathodic protection of the grain-boundary area that limits the intercrystalline corrosion. The effect of solution composition is fully consistent with the hydrogen-related models proposed for stress corrosion cracking. The effect of As2O3 addition on the susceptibility of the alloy to stress corrosion cracking may be attributed to the promotion of hydrogen entry in the alloy and the effect of HCl addition to the increase in rate of localized corrosion and higher quantity of evolved hydrogen.

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 Abstract

A kinetic model for the removal of magnesium from molten aluminum by the use of submerged Na₂SiF₆ powder injection was developed and experimentally examined. Different chemical reaction rates for the permanent contact reaction and the transitory reaction were calculated by this model. The reaction rate between the free powder particles and the magnesium dissolved in the melt, as well as the reaction rate between the particles entrapped by the bubbles formed by the carrier gas and the dissolved magnesium were determined. The reactions that occur after the decomposition of Na₂SiF₆ into NaF (solid) and SiF₄ (gas), over the temperature range of magnesium removal, were also discussed. The magnesium removal rate, under various experimental conditions such as the ratio of the reactive powder flow rate to carrier gas flow rate, temperature of the bath, residence time of the particles discharged inside the melt and powder size, was predicted with this model. The model allowed the calculation of the efficiencies of the different reactions and confirmed that the highest efficiency corresponds to the reaction between the magnesium dissolved in the melt and the SiF₄ gas released by the decomposition of the Na₂SiF₆ powder. The results predicted by the model were fitted to those obtained during an experimental stage and showed a good agreement for all the experimental conditions.

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The kinetics of the precipitation in Al-16 at.%Mg and Al-18 at.%Mg alloys was investigated utilizing differential scanning calorimetry (DSC). The developed precipitates were characterized by using scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometry (EDS). Analysis of non-isothermal DSC scans at different heating rates were carried out to evaluate the overall activation energies associated with the transformation processes during continuous heating of the quenched alloys. An average activation energy associated with the dissolution of GP-zones was determined to be 70.88 kJ/mol, which implies that the dissolution process is controlled by the migration of magnesium atoms through the aluminum matrix. The average activation energies of b' and b-phase precipitation were 89.88 and 87.762 kJ/mol for Al-l6 at.%Mg and Al-18 at.%Mg, respectively. The activation energies obtained for the dissolution of b' and b-phase precipitates were 268.72 and 222.535 kJ/mol for Al-l6 at.%Mg and Al-l8 at.%Mg, respectively. The reaction orders of all the processes indicate that they occur three-dimensionally throughout the aluminum matrix.

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A number of studies have suggested that increased mixing improves the results obtained during the degassing of aluminum melts. However, the role of stirring has not yet been systematically studied. This paper reviews the available technical information on the role of mixing and then examines the theoretical basis of the degassing process. The effects of mixing during the degassing of aluminum melts are classified. It appears that mixing is best characterized by a specific mixing intensity or power density. Equations are given for specific mixing intensity by rotary stirrers and by the flow of gas through a melt and calculations are made for a commercial degassing system.

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Several mechanisms have been proposed in the literature to explain the origin of equiaxed grains in castings. However, there is an underlying assumption in the grain refinement literature, that the addition of grain refiners will dominate these mechanisms by providing potent nucleant particles for nucleation throughout the melt. The work presented in this study indicates that in grain refined castings of aluminum there are two sources of aluminum grains: the mold wall and the bulk of the melt. It was observed that the mechanism of nucleation at the mold wall is as significant as the mechanism of nucleation in the bulk of the melt. The contribution of the wall nucleation mechanism to the formation of grains relative to the formation of grains in the bulk of the melt was found to increase when grain refiners containing TiB₂ nucleant particles were added. The results also suggest that the wall grains are more likely to re-melt when they have to travel further in the castings. These observations are explained by the consideration of the role of potent nucleants and solute elements in the context of existing theories about nucleation at or near the mold wall.

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The influence of typical alloying elements on the as-cast grain size of aluminum alloys was investigated. Nominally pure aluminum with various amounts of Fe, Si, Mg, Mn, Cu, Cr and Ti as well as various commercially available wrought aluminum alloys were studied. The addition of Ti, Cr, Mg, Si and Cu resulted in a reduction of the grain size (listed by decreasing effect), while the addition of Mn produced a coarser grain structure. These results correspond well with the constitutional growth restriction caused by these elements. The grain size of the specimens was modeled by a semi-empirical approach, taking the alloy composition and grain refiner addition in to account. The predictions of grain sizes obtained by this model were in good agreement with the experimental results. However, the experimental data for multi-component alloys deviated somewhat from the predictions. This is attributed to the synergistic effects and formation of phases that resulted from the added elements. A transition from a decreasing to an increasing grain size occurred, when added elements exceeded a certain limit. Thus, a U-shaped curve could be fitted on the grain size data as a function of the added elements. This transition was strongly element dependent and it was suggested that the transition was due to the formation of pre-coherency eutectics. The formation of pre-coherency eutectics led to less efficient nucleation of a-aluminum, assumably due to the release of excess heat during the formation of the eutectics and thus the grain sizes increased. By such a mechanism, it is possible to explain why elements such as Fe and Mn reach the transition at a very low addition level, while very large additions of elements like Mg and Zn would be necessary in order to observe the U-shaped curve.

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A new method was developed for studying features such as, morphology, thickness and chemical composition of the oxide film on molten aluminum alloys. This method is based on the introduction of bubbles into the molten metal and study the triple layer consisting of metal and oxide films (the Oxide-Metal Sandwich), which forms in the contact region between impinged bubbles. Air bubbles in liquid Al-7Si-Mg and Al-5Mg alloys were impinged to study the structure of the contact area. This fragile area of double oxide film was sufficiently thin to allow its structure and the entrapped residual liquid alloy to be investigated in the scanning electron microscope.

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 Abstract

A numerical algorithm was developed to simulate the movement, breakup, and entrapment of the oxide film inclusions encountered during the casting of aluminum alloys. The flow field was solved using the Marker and Cell method by a time marching process. The Volume of Fluid method was also used to track the free surface boundaries. A kinematic approach was employed to track the movement and breakup of the oxide films on the free surface or in the bulk liquid metal. A series of computer simulations of two-dimensional mold filling have been carried out. The computer program based on the proposed algorithm was able to model the oxide film movement, breakup, entrapment and flow behavior in the mold cavity during casting of aluminum.

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The measurements of positron annihilation lifetime technique (PALT) have been performed on a set of nine Al-Si alloys. It has been shown that positrons can become trapped at imperfect locations in solids and their mean lifetime can be influenced by changes in the concentration of such defects. No change has been observed in the mean lifetime values at a saturation of defect concentration. The mean lifetime and trapping rates for the samples deformed at 40.9 percent have been studied as a function of isochronal annealing temperature. Three stages have been observed, two of them for recovery and the third for formation thermal vacancy.

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Recent advances in understanding the effects of folded oxide film filling defects entrained in aluminum alloys have shown that the existence of such defects significantly affects both static and fatigue reliabilities of castings. Such defects are usually the consequence of an inappropriate filling system for castings, where metal flow exceeds a critical velocity, approximately 0.5 m/s. As a result, the surface turbulence can be clearly observed during the filling. To reduce or eliminate the surface turbulence, which takes place during the time that liquid metal enters the mold cavity, a vortex-flow runner filling system has been proposed. The effects of the adoption of this runner in comparison with the conventional runner bar have been investigated using both computer modelling and real-time x-ray radiography techniques. The results showed that the conventional rectangular bar runner system gave an ingot velocity in the range generally above the critical value of 0.5 m/s. In contrast, it was shown that the ingate velocity for the vortex-flow runner system was in the range of 0.3 to 0.4 m/s, greatly reducing the occurrence of surface turbulence occurrence in the casting cavity. In particular, it was found that the use of the vortex-flow runner could eliminate the rolling back-wave (a constrained hydraulic jump) normally encountered in the priming of conventional runners. Experimental studies including four-point bend tests on the final castings demonstrate greater strength and consistency, illustrating the overall reduction in the oxide film entrapment damage.

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The aluminum industry is under continual pressure to improve metal quality and at the same time reduce costs. It is also desirable to reduce emissions to the environment. The usual way to remove alkali metals that are dissolved in aluminum is to flux the melt with chlorine gas, but this process is often associated with a significant amount of chloride emissions. As environmental regulations become stricter, the traditional chlorine fluxing processes may no longer be viable. In this review, an analysis is given for the chemistry of the removal of dissolved alkali metals. The analysis shows that chlorine is especially favorable from a chemical point of view. However, the oxidation of alkali metals at the melt surface is also possible, even though simple thermodynamic calculations suggest that this reaction would not be favorable. The reasons for this apparent anomaly are considered.

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The increasing demands for identical components necessary for mass production of parts for the transportation, construction and electronics industries, has made the die casting process the predominant method for producing aluminum alloy castings. This dominance is reflected in the tonnage of die castings produced in the United States and Japan. For example, in the U.S. in 1994 the shipments of aluminum die castings reached 984,000 tons, which represented about 58% of the total shipments of aluminum alloy castings [1]. In Japan, in 1996, this figure was at about 64.4% [2]. Despite this ever growing demand for aluminum die castings, development and research efforts in aluminum die casting alloys has lagged behind. Recent research work on aluminum die casting alloys has been, for the most part, concentrated on the 380 aluminum alloy; understandably so, since this is the most commonly used aluminum die casting alloy. The data on this alloy, however, is scattered and sometimes contradictory. It is the purpose of this review to summarize and present the available knowledge on this prevalent alloy.

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The effect of polyvinyl alcohol (PVA) content on the characteristics of a granulated aluminum alloy produced by wet-fluidized-bed granulator was investigated. The flowability of the granulated particles was remarkably improved as compared to the primary aluminum powder and it was equivalent to that of conventional iron powder. The granulated particles had spherical shapes with a mean particle size of 75-l00 m m. The apparent density of the granulated aluminum decreased compared with that of the primary powder because of both the uniform particle size distribution and spherical shape of the granulated aluminum. Dimensional tolerance of the granulated aluminum in the green compact test was much improved as compared to the green compact of the primary aluminum powder. This was due to the good flowability of the granulated particles during the process of filling the die.

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In-situ SEM observations of fracture toughness tests have been used to characterize the fracture behavior of a wrought aluminum alloy (AA2091). Particular attention was given to the fracture strength of the coarse inclusions associated with this alloy. Interfacial cavitation or debonding did not occur during inclusion fracture of particles larger than ten micrometers. The fracture domain of CuAl₂ and Al₂CuMg particles with respect to the main crack was also investigated. Crack propagation was conducted by ductile failure of matrix ligaments between the fractured particles and the main crack, thereby producing slight crack deflection. The driving forces for variations of local crack with crack extension were estimated utilizing the approximate approach on microcrack shielding. Coarse Al₃Zr and Al₃Ti particles, even those adjacent to the fracture surface, remained intact. A combination of HRR singularity and Eshelby type internal stress analysis was used to estimate the in-situ strength values of various inclusion particles. It was concluded that the fracture strength of the inclusion particles has strong dependence on their diameter. The lower bound estimates of the fracture strength of the intact CuAl₂ and Al₂CuMg inclusions were determined.

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The addition of Mn and Cr is often used in the 6xxx-series aluminum alloys. During high temperature annealing, dispersoids containing Mn and/or Cr are formed, which stabilize the microstructure and improve the mechanical properties of the alloys. The kinetics of the formation of the dispersoids in the Al - 0.6 wt.% Mg - 0.9 wt.% Si alloy with Mn and/or Cr additions were metallographically studied. Special emphasis was placed on the spatial distribution of the dispersoids after different heat treatment processes. An inherent tendency towards a non-uniform spatial distribution of dispersoids was related to the nucleation mechanisms of dispersoids and the microsegregation of the alloying elements after non-equilibrium solidification. It was demonstrated that a prerequisite for a homogenous spatial distribution of dispersoids is a heat treatment procedure that produces an abundant density of b '-Mg₂Si - needles in a temperature range between 250ºC to 350ºC.

Keywords: Al-Mg-Si Alloys, a -Al(MnCrFe)Si Dispersoids, Spatial Distribution, Kinetics.

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The influence of replacing toxic lead with tin was studied in wrought Al-Cu-Bi based alloys. It was shown that after solution treatment, Pb and Bi had a negligible influence on the decomposition process of the binary Al-Cu system. Whereas, only a small amount of Sn in the matrix caused significant deviations, especially in the early stages of decomposition. The formation of Cu-rich GP zones was suppressed by the presence of Sn atoms and the precipitation of Q ‘(Al₂Cu) particles was preceded by the formation of modulated structure. Direct TEM observations revealed the presence of progressively developing fringes contrast in the matrix and satellites near the fundamental matrix reflections, indicating the presence of the spinodal decomposition process.

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Pure aluminum is one of the lowest strength metals. The yield stress of pure aluminum indicates that the creep deformation can take place even at a lower stress level. Creep experiments of 99.999% pure aluminum were performed under a constant stress at 293 K, 373 K, 473 K and 573 K. The creep strain curves were examined using mechanical models to analyze the creep properties of pure aluminum. The crept aluminum contained a dual structure, which was composed of a hard region and a soft region. The creep strain rate was quasi-constant against the creep time even at a low temperature. The stress dependence of the creep strain rate and creep life was found to be compensatory. The product of the minimum strain rate and the creep life was constant, regardless of the temperature and the applied stress. The cell structure developed in the pure aluminum that was crept at a constant stress is observed using transmission electron microscopy.

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The fracture behavior of Al-Si alloys with varying silicon contents under resonant vibration was evaluated. The tested materials were prepared by varying the silicon contents of the alloy within the range of 4.5 wt.% to 10.9 wt.%. Experimental results indicated that the critical number of cycles to failure increased with increasing the silicon contents. This was attributed to the suppression of the deflection amplitude by silicon particles. Quantitative analysis of the crack propagation paths showed that all of the specimens had a similar ratio of the projected crack intercepted density to the line intercepted density of the silicon particles (PCID/LID). Thus, these specimens had similar probabilities of crack propagation towards the silicon particles.

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The influence of notch geometry and test temperature on the impact behavior of three aluminum alloys, 6061, 2024 and 7055, is presented and discussed. Notch angles of 45^o, 60^o, 75^o and 90^o were chosen for a Charpy V-notch type impact test. For each angle, specimens were prepared with two notches positioned perpendicular to the applied load. The dynamic fracture toughness of aluminum alloy 6061, for a given angle of the notch increased as a function of increasing temperature. At a given test temperature, the impact toughness of this alloy decreased with an increase in notch severity. For the least severe notch, the dynamic fracture surfaces revealed an occurrence of localized mixed-mode deformation at elevated temperatures. In the case of aluminum alloys 7055 and 2024, for a given angle of the notch, the increase in dynamic fracture toughness with test temperature is most significant for the least severe of the notches. An increase in notch severity resulted predominantly in the mode I-dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for these two alloys. The impact fracture behavior of the alloys is rationalized in light of intrinsic microstructural features, fracture-governing mechanisms and the deformation field ahead of the propagating crack.

Keywords: Aluminum Alloys, Impact Fracture, Notch Geometry

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Casting an Al-7Si-0.35Mg alloy under suitable conditions with a low superheat produces material suitable for semisolid processing. Important features of the microstructure include a fine grain-size with a globular dendritic morphology. When partially remelted and held isothermally for a few minutes, the as-cast microstructure develops into a structure with globular a-phase particles. The effect of casting temperature on the as-cast microstructure was investigated, and the materials so produced were compared with electromagnetically stirred, DC cast material. A direct shear test, which provides a measure of semisolid forming behavior, was used to assess the relative performance of the developed microstructures. The materials prepared with a low casting temperature had a fine grain size and exhibited very low resistance to shear deformation after isothermal holding in the semisolid state, while the larger grain size material exhibited a higher shear resistance.

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Characterization of the mechanical properties, mainly yield stress and creep behavior, of semisolid metals has not been fully addressed. This work reviews different aspects of the yielding phenomena in semisolid (viscoplastic) materials. The explicit concepts of time dependence and shear-rate dependence of yield stress have been presented to clarify conflicting claims and to elucidate discrepancies in previous investigations. Recent studies of the rheological behavior of semisolid metal slurries at Metal Processing Institute (MPI) have shown the existence of a distinct yield stress and also its role in accurately describing rheological behavior. Though the theoretical and modeling work is applicable to metal alloy systems, the experimental work has been carried out with the Al-Si based casting alloys. In regions where the applied stress is smaller than the yield stress, the material does not deform and hence its microstructure remains constant. In regions where the applied stress is higher than the yield stress, the material does deform and non-uniform properties result due to microstructural changes. Additionally, the models developed at MPI are capable of predicting the degree of agglomeration and/or dis-agglomeration of solid metal particles in the slurry as a function of time. Finally, current methods used to measure yield stress of viscoplastic materials and their limitations are reviewed. It is pointed out that the selection of the measurement method and the associated experimental conditions requires consideration of the time and shear-rate dependence of the semisolid metal slurry.

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The deformation behavior of can body aluminum alloys is mainly determined by crystallographic texture. The original hot band texture and the cold rolling process have significant influence on the ultimate texture and thus on the formability of wrought aluminum alloys. AA3104 aluminum alloy was used in this investigation. In order to obtain a fully recrystallized microstructure, the materials were annealed prior to cold rolling process. Aluminum hot band without annealing was also examined in cold rolling process for comparison. Samples at different cold rolling reductions were prepared for earing cup testing and crystallographic texture determination. By comparing the texture components and the corresponding earing results, a better understanding of the relation between crystallographic texture components and earing was obtained. Several degrees of reduction were employed; especially reductions near the critical point that lead to a change of earing from 0/90^o to 45^o.

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 Investigation of Low Resolution Texture Analyses for on-Line R-Value Determination of Aluminum Can Body Materials

Aluminum alloys for can body making are produced by hot and cold rolling. The hot band may be cold rolled to final gauge or annealed before being cold rolled to final gauge. Texture transition occurs during each processing step. In order to get excellent deep drawability, it is necessary to control the texture evolution in each stage. On-line texture determination is the best method to determine or control this evolution. In the present work, AA 3104 aluminum alloy is used to demonstrate an on line texture determination process, which obtains the volume fraction of texture components from one or two x-ray diffraction peaks produced by the C-{112}<111> component and the cube {100}<001> component. The result shows that this method is effective and provides an economical way for on line texture determination. The R-value can also be derived from this on line process.

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The mechanical anisotropy of direct chill (DC) cast AA3003 hot band was determined by measuring the earing behavior for different thermomechanical processes. The corresponding crystallographic texture was determined by pole figure measurements and analyzed by the orientation distribution function (ODF) method. The texture characterization was correlated with the variation in earing percentage. The as-received hot band had a 45^o earing and high deformation texture components, while annealing the hot band produced 90^o earing. The annealed hot band had a recrystallized texture with a high volume fraction of the cube component {001}<100>. When the annealed hot band was cold rolled to different strains, its 90^o earing percentage decreased with an increase in the degree of cold rolling reduction and the earing changed from 90^o to 45^o at a cold rolling reduction of 45%. Accordingly, the softening texture components decreased and the deformation texture components increased with an increase in strain. However, the as-received material, annealed after cold rolling, had a critical change in mechanical anisotropy at a plastic strain of 70%, which was higher than the annealed hot band materials subjected to cold rolling. The former material showed a lower volume fraction of the cube component than the annealed hot band. On the other hand, the annealed hot band after cold rolling and subsequent annealing exhibited 45^o earing instead of 90^o earing as would normally be expected. This material showed a high randomly recrystallized texture with little accumulation of any particular orientation and a low cube component and a high R-cube component.

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The addition of scandium to aluminum alloys results in a sharp increase in recrystallization temperature and ensures the production of heat-treated semiproducts, including cold-rolled sheets, with non-recrystallized structures and improved strength properties. Sheets with a non-recrystallized structure are anisotropic, as a rule. In the case of Sc-bearing aluminum alloys, anisotropy noticeably increases because of the appearance of shear bands that occur during rolling in a sheet structure. The causes of shear band formation are discussed in this paper. A technique for preventing the shear band formation and producing sheets with minimum mechanical anisotropy was developed and substantiated.

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In this study, the effect of eutectic silicon morphology on the workability of hypoeutectic Al-Si strips under the tensile deformation was investigated. The strips had two different silicon contents including 6.35 wt.% (poured at 630° C) and 10.67 wt.% (poured at 600° C and 630° C). The experimental results indicated that the Al-Si strips possessed a multi-layer structure with two types of eutectic silicon particles (namely acicular and granular). Both acicular and granular particles were observed in the center of the strip. In addition, the shape of eutectic silicon particles was mainly granular near the surface of strips. According to the tensile test result, cracks in the strip were initiated from broken acicular particles in the center region of the strip. This reduced the strain-hardening exponent of the strips as the tensile strain increased. The presence of acicular particle was a major factor for the inferior ductility and workability of the strips. Furthermore, this study confirmed that the acicular particles in the center of the strip could be modified into granular particles, to improve the ductility and workability of the strips.

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The conventional forming limit diagram (FLD) separates those states of strain that will cause failure from those that are safe in sheet metal forming operations. This tool is often used in sheet stamping to determine the manufacturing feasibility of a particular part. However, the FLD is typically constructed under proportional loading conditions and therefore ignores the effects of strain path on the prediction of limit strains. That is, the FLD considers only the final state of strain and not the strain path in which the material experiences to arrive at this final strain state. In this study, the effect of strain path on the formability of aluminum sheet was investigated. FLDs were determined on sheets that were prestrained in either tensile or equibiaxial tension and then compared with FLDs determined from as-received material. This technique allows for the determination of the effect of both a pre-stretch and pre-draw on the subsequent formability of material over a wide array of proportional strain paths. Two Al-Mg alloys, AA5754 and AA5182, were chosen for this work because of their application in automotive body structures. It was determined that a tensile prestrain to approximately 5% strain does little to affect the overall formability, however, an equibiaxial prestrain of 4% can greatly reduce the subsequent formability in these alloys.

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Bolted and riveted joints find extensive application in aluminum structures that are used in building structures, roofing systems, railcars, intermodal shipping containers, cladding systems, and trailer bodies. A sound and efficient design of the bolted and riveted joints requires appropriate limit states. In this study, the bearing capacity and fastener hole deformation of thin sheet AA5052 alloy were evaluated and recommendations for safe design established. Determination of bearing yield of this alloy showed limited relevance to behavior of bolted joints. Limiting bearing strength of this alloy was equal to twice the ultimate tensile strength of the connected material. Ultimate failure by tensile rupture of the AA5052-H32 alloy occurred at a bearing stress around four times of its ultimate tensile strength. Design recommendations include elimination of the check on bearing yield strength and development of an allowable bearing stress equal to twice the ultimate tensile strength of the alloy.

Keywords: AA5052 Aluminum, Bearing Strength, Bolted Joint

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The packaging material, aluminum alloy AA3004, was industrially produced by the strip cast process. The hot band was subjected to various preheat treatments. The evolution of the microstructures and their influence on the recrystallization behavior and texture behavior of the cold rolled sheets was investigated. The results show that preheat treatment, especially performed at high temperature, gave rise to breaking-up and coalescence of primary intermetallics, while slow cooling or soaking at low or medium temperature resulted in the precipitation of numerous dispersoids. These metallurgical factors significantly promoted recrystallization behavior. In turn, the formability of the material was improved greatly by reducing 45-degree earing. No 90-degree earing was observed in any of the cases. This study on texture shows that a random recrystallization textures was formed in the recrystallized material that was preheat treated at a high temperature. The development of the Cube texture was restrained. In addition, an uncommon texture component {110}<554> was found in the recrystallized material that was not subjected to preheat treatment.

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The consequence of thermal treatment on the microstructure and subsequent mechanical properties of an Al-2.6wt.%Li-0.09wt.%Zr alloy was studied. The alloy was solution heat treated and artificially aged for a series of aging times and temperatures at various precipitation hardening conditions. The underaged, peak-aged and overaged TEM microstructures of the alloy were analyzed. Quantitative particle size measurements were performed to determine the size, distribution, morphology and coarsening rate for both d'(Al₃Li) and d'(Al₃Li)/Al₃Zr precipitates. The average particle size, distribution, spacing and volume fraction of the intermetallic precipitates were related to the heat treating process, overall deformation behavior and composition of the alloy. For all of the aging times studied, the d'(Al₃Li)/Al₃Zr particles were much larger in size than the d'(Al₃Li) and Al₃Zr-free particles. The particle coarsening rate, determined from the Lifshitz, Slyozov and Wagner coarsening rate theory, was more accelerated for the d'(Al₃Li)/Al₃Zr particles than for the d'(Al₃Li) precipitates. The presence of the Al₃Zr phase was found to accelerate the aging kinetics of the alloy. Therefore, a small amount of zirconium in the alloy resulted in a faster particle coarsening rate of the overall combined particle size distribution and thus led to more rapid precipitation aging more rapidly. Increasing the aging time resulted in a smaller percentage of d'(Al₃Li) precipitates that deviated from an approximately spherical morphology.

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The friction-stir welding of 1100, 2024, 2195, and 6061 aluminum alloys as well as dissimilar alloy systems of 2024/1100, 2024/2195, and 2024/6061 were studied for comparison. The Vickers microhardness of these alloys ranged from 27 VHN to 147 VHN. The microstructures, including grain and sub-grain structures, of the base metals were compared with the weld zone and heat affected zone microstructures using optical metallography and transmission electron microscopy. The correlation of residual microhardness profiles extending from the base metals through the weld zones to the residual microstructures was determined. It was demonstrated that the weld zones were characterized by dynamic recrystallization producing fine equiaxed grain structures. However, the heat- affected zone exhibited annihilation of the dislocations and precipitates in its microstructures. The friction-stir welding zones in the dissimilar metal systems exhibited complex intercalation lamellae, which produced hardness fluctuations throughout. The age-hardenable 2xxx and 6xxx series aluminum alloys experienced significant hardness loss near the friction-stir welding zone boundaries and general softening within the recrystallized friction-stir-welding zone.

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A process of silver plating on aluminum alloys using the sequence of immersion zincating, copper plating, silver striking, silver plating and antitarnish treatment with 1-phenyl-5-mercaptotetrazole (PMTA) was optimized and evaluated for space telecommunication components. The plated deposits were characterized by the thickness measurement, microhardness evaluation, morphological studies, evaluation of electrical performance, measurement of optical properties, adhesion, humidity and tarnishing resistance tests. The space worthiness of the coatings has been evaluated by thermal cycling, thermo-vacuum and thermal stability tests. The silver plating procedure described herein has been found to reduce the insertion loss by 20-50 percent.

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Literature on non-chromate based conversion coatings for application on aluminum surfaces was reviewed with special attention given to mechanisms of formation and corrosion protection performance. Present commercial applications and potential future applications are discussed and comparisons are made with the traditional chromate-based coatings. Some recent trends are described.

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The effect of up to 3% aluminum addition to the galvanizing bath on the weight and thickness of the galvanized layer, at 450°C galvanizing temperature, was investigated. The effect of aluminum addition to the molten zinc bath is characterized by the two stages of fast and nil growth. An empirical formula characterizing the first stage was developed. It was found that aluminum addition is only effective in the range of 0.1 to 0.3 percent. The effect of copper addition to the molten zinc bath was also studied for comparison. It was noticed that aluminum addition influences the weight growth of the galvanized layer more than copper additions.

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An improved current interruption technique and a special cell configuration were used to measure the anodic overpotential of carbon anodes in cryolite-alumina melts. The parameters affecting the anodic overpotential and the precision of its measurements were studied and analyzed. A substantial increase in the anodic overpotential was observed by bubbling CO through the melt. The anodic overpotential was also influenced by the composition of electrolyte and the initial presence of metallic aluminum in the electrolyte. The impact of carbon materials on the anodic overpotential is discussed.

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The Butler-Volmer expression for the electrode kinetics of the carbon anode in the Hall-Heroult cell is derived using the concept of the transition state theory in the case of either a two-electron process producing CO or a four-electron process producing CO₂. It is demonstrated that the forward and backward transmission coefficients have to be equal in order to satisfy the Nernst equation at equilibrium. The evaluation of the electrode kinetic parameters from experimental voltage-current curves is also discussed.

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Using a novel interpretation of cell potential vs. current density data for the Hall-Heroult process in terms of the Butler-Volmer (BV) equation, a model with two-electron and four-electron reactions running in parallel is discussed. The first reaction is related to the production of CO and the second reaction to the production of CO₂. At a given cell potential, each process has its own overpotential. A high cell potential favors the anodic production of CO₂. The chemical potential of the surface carbon is lower than the chemical potential of equilibrium graphite, especially after pre-electrolysis, because of insertion of electrochemical reaction products in the graphite lattice. This leads to higher zero-current cell potentials for the two anode reactions. Baked Søderberg paste anodes and graphite anodes do not differ in this respect. The influence of the iron and pitch content upon the Butler-Volmer parameters of the Søderberg anodes is also investigated.

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  Parallel Butler-Volmer Reactions at the Carbon Anode in an Aluminum Reduction Cell, Producing CO and CO₂. III. Method of Second Harmonic

Dewing and van der Kouwe's method of second harmonic amplitudes was compared with the double Butler-Volmer (BV) model of the carbon anode kinetics in the Hall-Héroult cell. The results obtained by Dewing and van der Kouwe were found to be compatible with the two parallel BV reactions on the surface of the carbon anode, one producing CO and another producing CO₂.

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The two step Butler-Volmer electrode kinetic model, described in Part II of this series for the reactions at the carbon anode in a laboratory Hall-Héroult cell was fitted to voltage-current curves at three different bath temperatures. The influence of the cryolite melt temperature on the CO/CO₂ ratio of the anode gas was determined by the fitted Butler-Volmer parameters. It was found that low bath temperature and high cell potentials favor the anodic production of CO₂. The experimentally observed CO/CO₂ ratios in the gas leaving the cell are higher than the values calculated from the Butler-Volmer expressions. This is due to the back reaction between aluminum and CO₂ and the Boudouard reaction between CO₂ and carbon. General relations between gas composition, electrode kinetics, the kinetics of the back reaction and the Boudouard reaction, current efficiency and carbon consumption were studied. The mechanism of electrocatalysis on the carbon surface is also discussed.

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 Abstract

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In aluminum treatment furnaces, refractories are subjected to corrosion either by the reductive action of aluminum or by direct oxidation from the atmosphere. The effect of temperature, partial pressure of oxygen, alloy composition and presence of cryolite on corrosion of the refractories in aluminum melting and holding furnaces is investigated in this paper. The results presented show that the corrosion of the refractory lining in such furnaces, at and below the metal line, mainly takes place by the reduction of the oxides under the action of the molten metal. Such reduction is favored when the partial pressure of oxygen decreases, the metal temperature increases, the refractory pre-firing temperature increases, and cryolite is present.

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