Papers by Author: David J. Lloyd

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Authors: O. Orlov, Éric Maire, Jérôme Adrien, Michael J. Worswick, David J. Lloyd
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.
Authors: J. Sarkar, T.R.G. Kutty, David S. Wilkinson, J. David Embury, David J. Lloyd
Authors: S.F. Corbin, E. Ansah-Sam, David J. Lloyd
Abstract: The objective of this study was to investigate and compare the influence of Mn and Fe additions on the fracture behaviour of AA6000 series alloys in under, peak and overaged conditions. Testing was completed under uniaxial tension and the microstructures of the alloys were observed using optical and Transmission Electron Microscopy. Alloys with very low levels of both Mn and Fe underwent a transition from transgranular to intergranular fracture and a reduction in strain-tofracture when heat treated from the under aged (UA) to peak aged (PA) condition. Increasing Mn and Fe content prevents this transition in fracture mode such that the stain-to-fracture is similar in the UA and PA states. Despite the change in fracture mode, when comparing the strain-to-fracture for a given ageing condition, increasing Fe systematically reduces the strain-to-fracture. Conversely, increases in Mn systematically increase the strain-to-fracture for a given ageing condition.
Authors: Hai Ou Jin, Pei Dong Wu, 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.
Authors: A.K. Gupta, P.H. Marois, David J. Lloyd
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.
Authors: Zeng Tao Chen, Michael J. Worswick, David J. Lloyd
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.
Authors: Xi Wang, Warren J. Poole, J. David Embury, David J. Lloyd
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.
Authors: Hai Ou Jin, David J. Lloyd
Abstract: AA5754 sheet has been processed by asymmetric rolling and the development of grain structure and texture in subsequent annealing studied at 240-500°C. It has been found that asymmetric rolling facilitates the formation of ultra-fine grain structure (1-2µm grain size) by shear strain promoted continuous recrystallization, which is a process of extended recovery and subgrain/grain growth. The ultra-fine grain structure is not thermally stable, and when the annealing temperature or time increases, the grain size eventually grows to its Zener limit. The deformation texture is similar to the typical f.c.c. cold rolling texture but rotated about the transverse direction. Along with the formation of an ultra-fine grain structure and subsequent grain growth, the deformation texture is retained.
Authors: Robert B. Wagstaff, David J. Lloyd, Todd F. Bischoff
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.
Authors: S. Esmaeili, Warren J. Poole, David J. Lloyd
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