Papers by Author: David J. Lloyd

Abstract: Two AA5754 sheets have been processed by cold rolling with 83% thickness reduction, one at room temperature and another with liquid nitrogen as coolant. The sheets were subsequently annealed at 220-275°C for 1 hour. The development of grain structure and texture was studied by optical microscope, scanning electron microscopy (SEM), X-ray diffraction and electron backscatter diffraction (EBSD) in SEM, and the mechanical property by micro-hardness testing. It has been demonstrated that the as-rolled sheets have the same micro-hardness, but the grain structures and textures are very different. Compared to the sheet processed with liquid nitrogen, the one rolled at room temperature has stronger shear texture and finer grain structure.

Abstract: The application of 6000 series alloys is widespread and of particular importance to the automotive sector. Their functionality depends on the detailed behaviour of the strengthening phases. In this study, transmission electron microscopy (TEM) supplemented with a variety of mechanical tests were used to examine the precipitates and their role in aspects such as the Bauschinger effect, damage and fracture events, and in recovery and recrystallization processes.

Abstract: Novelis Inc. recently released its first new innovative technology which opens new opportunities in the clad aluminum product marketplace, where a combination of mechanical and physical properties can be obtained which are superior to the monolithic material alone. Clad aerospace and brazing products are well known commercial products which are provided by commercial roll bonding processes, but which can now be produced with the new Novelis technology. This paper discusses the new technology, e.g., the casting, fabrication, the properties of clad sheet are reported and it is established that the clad-core interface is comprised of a high strength, oxide free zone. This technology enables a new family of clad products with clad/core combinations which cannot be produced by the conventional roll bonding process.

Abstract: A three-dimensional damage percolation model, which captures the effect of microstructural heterogeneity on damage evolution, has been developed to model damage initiation and propagation in materials containing second phase particles. It considers the three phenomena preceding ductile rupture of the material: void nucleation, growth, and coalescence. Threedimensional X-ray tomography is used to obtain measured three-dimensional second phase particle distributions in aluminum alloy sheet. Material damage evolution is studied within a tensile test simulation and compared to measured damage from an in situ tensile test utilizing X-ray tomography. Experimental and simulation results for material damage initiation and evolution are in good agreement.

Abstract: In this paper, stretch flange forming experiments were performed on the AA5182 and AA5754 Al-Mg sheet materials. A triple-action servo-hydraulic press, developed at the University of Waterloo, was used in the experiments. A z-flange tooling, which incorporates mating drawbeads on the main and backup punches, was employed. Drawbeads are used in commercial stretch flange operations to control or limit the rate of cutout expansion. Of interest in the current research are the flange formability and the damage development induced by the bending-unbending of the sheet as it passes through the drawbeads. Both AA5182 and AA5754 were tested with thickness of 1.6 mm. Further tests were performed using 1.0 mm AA5182 to examine the effect of thickness. To examine the effect of cutout size on the formability, cutout radii in the range 88 to 98 mm in increments of 2 mm were tested to failure.

Abstract: The numerical simulation of the stretch flange forming operation of Al-Mg sheet AA5182 was conducted with an explicit finite element code, LS-DYNA. A Gurson-Tvergaard- Needleman (GTN)-based material model was used in the finite element calculation. A strain controlled void nucleation rule was adopted with void nucleating particle fraction measured directly from the as-received Al-Mg sheet. Parametric study was performed to examine the effect of void nucleation strain on the predicted onset of ductile fracture. Critical porosity levels determined through quantitative metallurgical analysis were adopted to predict the commencement of void coalescence in the GTN model. The numerical results were compared to the experimental ones and an applicable void nucleation strain was suggested.

Abstract: AA6111 sheet alloy has been used in automotive panel applications in North America and Europe for several years. This alloy exhibits an excellent combination of strength, formability, ageing response and surface appearance following forming and painting operations. Such a combination of properties is obtained by carefully tailoring the processing route to obtain the desired microstructure of the alloy. In recent years, the ability to predict the phase stability in different alloys has improved significantly, and it is now relatively easy to predict the particles that could form in complex multi component alloys during different processing steps. The accuracy of the predictions is dependent on whether or not the free energy expressions used in the calculations are correct. In this study, the AA6111 alloy was subjected to various annealing treatments that are reflective of different phase fields computed by the Thermo-Calc software. The particles were extracted using the phenol extraction technique and were identified using energy dispersive analysis. The interrelation of the particle analyses with the computed phase stability in AA6111 is presented.

Abstract: The role of Cu in precipitation behavior of 6000 series aluminum alloys has been investigated by isothermal calorimetry and transmission electron microscopy. The newly developed analytical techniques have been used to evaluate the precipitation kinetics in alloys with or without an initial pre-aging history. It has been found that Cu addition results in increasing the rate of precipitation when artificial aging is applied immediately after solutionizing and quenching. However, Cu has no significant effect on the kinetics of precipitation in alloys with the pre-aging history, while it produces finer microstructures in the pre-aged and then artificially aged alloys. These observations have been explained by the effect of Cu on increasing the rate of precipitate nucleation in 6000 series alloys. It has also been suggested that the effect of Cu on nucleation arises from the role of Cu on the cluster formation during and/or immediately after quenching.

Abstract: The development of grain structures after asymmetric rolling (ASR) and annealing was investigated in Al-Mg alloys AA5754, AA5182 and AA5083. It has been demonstrated that a fine grain structure could be produced through continuous recrystallization, but it is strongly affected by the presence of large second phase particles. In AA5754 the volume fraction of large particles is relatively low and continuous recrystallization is able to occur throughout the sheet thickness, resulting in a fine grain structure of 2μm mean grain size. In AA5182 the fraction of large particles increases to a level that the continuous recrystallization occurs only in the sheet surface, whereas the sheet centre undergoes discontinuous recrystallization. The discontinuous recrystallization due to particle stimulated nucleation (PSN) is dominant in AA5083 so that no continuous recrystallization has been observed. The fully recrystallized grain structure is slightly finer in the ASR processed AA5083 than the conventionally rolled one.

Abstract: This paper presents results from quasi-static and high rate tensile testing of three aluminum sheet alloys, AA5754, AA5182 and AA6111, all of which are candidates for replacing mild steel in automotive bodies. Tests were performed at quasi-static rates using an Instron apparatus and at strain rates of 600 to 1500 s-1 using a tensile split Hopkinson bar. Additionally, an in-depth investigation was performed to determine the levels of damage within the materials and its sensitivity to strain rate. The constitutive response of all of the aluminum alloys tested showed only mild strain rate sensitivity. Dramatic increases in the elongation to failure were observed with increases in strain rate as well as greater reduction in area. Additionally, the level of damage was seen to increase with strain rate.