Abstract: A novel experimental method has been developed by amalgamating a pencil beam X-Ray diffraction (XRD) technique with the recently developed grain boundary tracking (GBT) technique. XRD and GBT are both non-destructive in-situ analysis techniques for characterizing bulk materials, which can be carried out close to the point of fracture. DAGT provides information about individual grain orientations and 1-micron-level grain morphologies in 3-dimensions (3D) together with high-density local strain mapping. An Al-3 mass % Cu model alloy was used to investigate its deformation behavior under tension. The morphology of the grains was determined by the X-ray microtomography (XMT) imaging and the liquid metal wetting technique, after which GBT provided an accurate description of the position and morphology of all the grains in a region of interests. Diffraction spots in the XRD experiments were related to grains, making it possible to describe crystallographic orientation of all the grains. It has been revealed that deformation is localized at both microscopic and meso-scopic levels. Inhomogeneous deformation was observed in each individual grain. In addition, a group of a few grains coordinately interacts and specific grain boundaries thereby exhibit intense strain localization. Hydrostatic tension was also observed at quadruple junction points and its mechanism was analyzed.
Abstract: The high-strength weldable 7xxx series of aluminium alloys are of great importance to the aeronautics industry. Only recently, the complex structures of the AlZnMg hardening precipitates have been solved by HAADFSTEM imaging and first-principles calculations. However, perfect models of precipitate structures are often insufficient as several elements may be mixed into precipitate compositions. We have investigated this effect by STEMEELS spectrum imaging with an aberration-corrected microscope. In a 7449 alloy, Cu and Al were found to replace atoms at certain sites in both metastable and equilibrium ZnMg precipitates.
Abstract: Aluminium-Silicon (Al-Si) alloys are the most extensively used Al foundry alloys and are widely used in gravity die-casting (GDC) of automotive components. Reference dies are used to characterize the tensile properties of the castings. Among the various die configurations, the ASTM B-108 (also called the Stahl Mold), and the AA Step Mold are most popular in the foundries. Several modifications to the standard dies have been suggested in the scientific literature to obtain better mechanical properties in minimizing casting defects. This work reviews the scientific literature on the effect of different reference dies on the porosity and tensile properties of the Al alloy castings.
Abstract: The automotive industry is willing of employing a greater amount of light metals like aluminium, magnesium and titanium, in order to reduce the total weight of the cars and, consequently, fuel consumption. Even though some casting aluminium alloys are commonly employed for automotive structural applications, their mechanical performances can be improved by means of appropriate grain refining by heterogeneous nucleation. This practise is well established for wrought aluminium alloys by using Al-Ti-B master alloys but they are not effective in casting alloys due to the much higher silicon percentage (poisoning). A novel chemical composition which, actually, refines appropriately Al-Si alloys was developed at Brunel University. In this study the combined effect of casting temperature and addition of this novel grain refiner (NGR) on the microstructure of binary Al-Si alloys was considered. The addition of this NGR leads to the formation of finer primary α-Al grains, finer eutectic Al-Si intermetallics and less and smaller primary Si particles.
Abstract: In order to obtain accurate chemical compositions in as-cast billets and ingots the sampling methods for the analysis have to give reproducible results with high precision. OES analysis made on samples at certain milling depths does not always show the desired nominal composition, and especially the segregation profiles within the samples can show significant variations. The present work examine the influence of main sampling parameters, such as the volume of the melt, the melt temperature, the mould design and the method of filling the mould, on the segregation. The results point out the importance of the convection in the mould during solidification, and thus the technique of pouring the melt into the sampling mould.
Abstract: The effect of ultrasonic melt treatment (UST) on the microstructure of hypereutectic Al-17 wt.% Si alloys was investigated. UST applied to a melt at a temperature range of 750 - 800 °C refines the primary Si dramatically but has little influence on the grain size of primary Al. The solidification behaviour was characterised by thermal analysis and a mechanism responsible for the primary Si refinement was suggested. Whilst UST has no effect on the grain refinement, a significant increase in the matrix hardness as well as the tensile strength in the as cast condition is possibly associated with solid solution hardening. Detailed microstructure analysis was carried out and characteristics of the intermetallic formation in the Al-17 wt.% Si alloys were further discussed in the view point of Cu solubility in the Al matrix, which is considered to increase with UST.
Abstract: A set-up for tensile testing in the mushy zone allowing for studies of semi-solid mechanical behavior is available at SINTEF. A hot-tearing experimental set-up has recently been developed allowing for investigation of the hot-tearing susceptibility of industrial aluminium alloys and effects of e.g. alloying composition and grain-refiner. Load and temperature are registered during constrained solidification giving information on the mechanical behavior of the alloy during solidification. Two crack-prone alloys in the 3xxx-series (A and B) have been investigated using both techniques and the results analyzed using information about solidification path from a thermo-physical model. Alloy B is found to be mechanically weaker in the interval most susceptible to hot-tearing in agreement with cast-house experience. This study shows that the experimental techniques combined with thermo-physical modeling and characterization allow for a better understanding of the hot-tearing sensitivity of the alloys.
Abstract: Ultrasonic melt processing enjoys the revival of interest in the last 1015 years. Although the main fundamental works as well as lab-scale and pilot-scale demonstrations date back to the 1950-1980s, the ultrasonic melt processing of light metals has not become a major technology. Recently the deficiencies of current technological approaches brought back the interest to ultrasonic treatment for degassing, grain refinement and composite materials. The current attempts to repeat the earlier results, to gain more fundamental insight using advanced means available and to up-scale the positive effects to the industrial scale show frequent lack of understanding of the basic controlling mechanisms. This paper describes the main mechanisms of ultrasonic melt processing, shows frequent mistakes, and gives some guidelines for technology up-scaling. The paper is illustrated with the latest experimental results.
Abstract: Effect of potential on hydrogen evolution during simulated pitting of aluminum has been investigated. Aluminum wire in 1.0 mm diameter was mounted in resin, immersed in 0.6 kmol.m-3 NaCl solution and anodically dissolved in axial direction by applying a constant potential from-0.6 to-0.1 VAg/AgCl for 86.4 ks. In addition, a resistance of the solution in the crevice of the resin where the aluminum wire corroded was simultaneously measured by superimposing high-frequency alternating potential (Ep-p=10 mV) to the main potential. As higher main potential was applied, the corrosion depth increased. Hydrogen evolved at the corroded site although the anodic potentials were applied. The amount of hydrogen evolution increased as higher main potential was applied to the wire. An interfacial potential (Eint) at just the wire surface was calculated from applied main potential (Eapp), solution resistance (Rsol) and current (I) as Eint = Eapp - I.Rsol. The interfacial potential was about-0.7 to-1.0 VAg/AgCl, and became lower as the higher main potential was applied. The lower interfacial potential may cause hydrogen evolution in this case.
Abstract: In this paper, the effects of process parameters on the absorption rate of P for hypereutectic Al-Si alloy (A390) which modified with Cu-14%P master alloy were systematically studied. And the mechanism that the process parameters play a role in effecting on the absorption rate of P was deeply investigated. The results indicated that the absorption rate of P increased with the increase of modified time and temperature, and decreased with the increase of the amount of added P. Under the same process condition, P added in form of Cu-14%P particles can cause to obtain the higher absorption rate of P than that of Cu-14%P powders. According to the experimental results, an equation on changes of the absorption rate of P was proposed, and the results show that good agreement is obtained between calculated and experimental values. The process parameters played a role in effecting on the absorption rate of P mainly through affecting the reaction rate of forming AlP particles in the melt, as well as the degree of reaction reached.