Materials Science Forum Vols. 618-619

Abstract: Precipitation hardening, or aging hardening, is one of the most widely adopted techniques for strengthening of aluminium alloys. During the precipitation process, three major mechanisms are involved: i.e. nucleation, growth and coarsening. Kampmann and Wagner have developed a powerful and flexible numerical approach (KWN model) for dealing with concomitant nucleation, growth and coarsening and thus capable of predicting the full evolution of the particle size distribution. KWN model has been successfully applied to a number of aluminium alloy systems, such as 2xxx, 6xxx and 7xxx. However, most of these modelling works were focused on the wrought aluminium alloys, few had applied to the casting aluminium alloys. In the present modelling work, the microstructure evolution is modeled based on the KWN model and then a strength model based on the well established dislocation theory is used to evaluate the resulting change in hardness or yield strength at room temperature. Then the modelling is applied to casting aluminium alloys A356 and A357. And the modelling results are validated by comparing with own experimental results and the results obtained from the open literature.

Abstract: Grain boundaries significantly influence the properties of polycrystalline metallic materials, particularly with grain sizes in the nano-metre range. The effect of grain boundaries on plasticity, fracture and corrosion resistance is well documented experimentally. The aim of this paper is to show that recent progress in modelling of the role of grain boundaries in metallic materials offers new possibilities for optimizing their properties.

Abstract: The proposed work makes use of an inverse analysis approach to identify the mechanical properties of a material sample which can be described by means of the elastic modulus, yield stress and strain-hardening parameter. The particularity of the proposed work is the exploitation of the indentation curve as well as the imprint left by the indenter at the end of the test as input information to the inverse analysis. The numerical simulations required by the analysis are carried out by means of the finite element software Abaqus. This paper describes initial work carried out to validate the robustness of the inverse procedure for stainless steel samples using computer-generated data.

Abstract: New bimetal magnesium/aluminium macrocomposites containing millimeter-scale Al based core reinforcement were fabricated using solidification processing followed by hot coextrusion. The initial macrocomposite consisted of a combination of pure Mg shell and pure Al core. Some problems encountered with the macrocomposite were Mg and Al grain coarsening, an inadequate Mg-Al interface (macrointerface) and consequent reduction in strength, compared to monolithic Mg. To rectify these problems, three approaches were taken in the following order primarily to widen (strengthen) the Mg-Al interface: (a) pouring of pure Al at 900°C (higher temperature approach), (b) pure Mg shell substitution with AZ31 shell (single substitution approach) and (c) pure Mg shell and pure Al core substitution with AZ31 shell and AA5052 core, respectively (double substitution approach). The evolution (strengthening) of the Mg-Al interface and its effect on microstructure and mechanical properties in each macrocomposite is investigated in this paper.

Abstract: It is shown that wrought magnesium alloys display a number of significant types of deformation inhomogeneities. These are influenced by the variation in the ease of basal slip amongst grains, micro-textures, shear banding, twinning and grain boundary sliding. Key features of each of these effects are examined and their engineering consequences and challenges are identified.

Abstract: The suitability of a twin-roll cast (TRC) age-hardenable alloy for wrought applications is explored. A Mg-4Zn (wt.%) alloy, 3mm thick, was cast using the TRC route. Deviating from the traditional practice of homogenization followed by age-hardening in die/sand cast parts, the TRC sheet, cut into small strips, was hot rolled and annealed after homogenization. They were then deep drawn and subsequently age-hardened. The rollability, mechanical properties and microstructure of the alloy at different stages of processing and after forming, are presented and discussed.

Abstract: The magnesium alloy AZ31 was processed by severe hot rolling and annealing. This processing was optimised to produce recrystallised grain sizes as small as 2.2μm. Specimens in the as-rolled condition had a grain size of 0.5μm, and exhibited a yield strength in excess of 350MPa. In the fully recrystallised condition, with a grain size of 2.2μm, the material had a yield strength of 260MPa which is almost twice that of the as-received plate. The ductility of the annealed specimens was also increased compared to the as-received condition. The combination of specific strength and ductility brings this newly processed material into a new property space compared to the other light metals.

Abstract: The use of two-piece billets in aluminium extrusion is often viewed as a necessary evil. A heating system that provides two-piece billets to the press can provide better ingot utilization than a heater which provides single-piece billets. The issues associated with the handling and extruding of two-piece billets are accepted as the price of improved ingot utilization. With the rotary welder, the end of each log segment is joined to the front of the following log by friction welding. The combination of the rotation of the short piece and the pressure applied creates a bond that will maintain one-piece integrity as the billet is conveyed to the press and loaded into the container. The handling issues associated with two-piece billets are eliminated. A unique re-cut cycle removes contaminants from the two weld faces immediately prior to welding. The two ends are precisely aligned and the saw makes a cut that removes the face of the remaining piece and the face of the following log. This re-cut removes debris, oxide and potential trapped air from a less than perfect cut at the cast house. The welding is then performed with the two freshly cut faces. The metallurgical performance is improved by the removal of debris and oxide on the faces. This allows the use of welded billets in some applications that previously required single-piece billets to achieve the necessary structural integrity.

Abstract: The mechanical properties of extruded ZA (Mg-Zn-Al) alloys with different Al contents were examined. The effects of Al on deformation behaviour were examined by tensile and compressive tests. The changes in texture due to plastic deformation were examined using the X-ray diffraction method. It was found that the basal poles of the extruded ZA alloy were parallel to the normal direction with a slight tendency to incline toward the extrusion direction. The degree of inclination increased with increasing Al content. The inclination degree of the basal poles was found to be closely related to the change in the deformation behaviour of the ZA alloy. Computer simulations were also carried out using the VPSC (visco-plastic self-consistent) theory in order to predict the contributions of various deformation modes to the plastic deformation behaviour of the ZA alloy.

Abstract: An XPS investigation was carried out on the surface film formed by exposure to high-purity water, on mechanically polished Mg and the two Mg-Al intermetallic compounds: Al3Mg2 and Mg17Al12. The result for mechanically polished pure Mg indicates that a film of MgO covered by a Mg(OH)2 layer, formed by the reaction of MgO with water vapour in the air. On immersion in distilled water, this film was hydrated to a duplex film with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. For both intermetallics, there was preferential dissolution of magnesium from the mechanically ground surface and also during aqueous immersion. After immersion, there was a 10 nm thick, stable film on the surface; the film composition on Al3Mg2 was whilst that on Mg17Al12 was .