Papers by Author: Michael A. Gharghouri

Abstract: In-situ neutron diffraction experiments were employed to investigate the micromechanical behavior of solid-solution-strengthened Mg-1wt.%Al alloy. Two starting textures were used: 1) as-extruded then solutionized texture, T1, in which the basal poles of most grains are tilted around 70~85° from the extrusion axis, and 2) a reoriented texture, T2, in which the basal poles of most grains are tilted around 10~20° from the extrusion axis. Lattice strains and diffraction peak intensity variations were measured in situ during loading-unloading cycles in uniaxial tension. Twinning activities and stress states for various grain orientations were revealed. The results show that the soft grain orientations, favorably oriented for either extension twinning or basal slip, exhibit stress relaxation, resulting in compressive residual strain after unloading. On the other hand, the hard grain orientations, unfavorably oriented for both extension twinning and basal slip, carry more applied load, leading to much higher lattice strains during loading followed by tensile residual strains upon unloading.

Abstract: A detailed study of the complex triaxial residual stress distribution of the double-pass friction stir welded (FSW) lap-joint between two different high strength aluminum alloy sheet materials was conducted. A non-destructive technique known as neutron diffraction was used to measure the internal residual stress distribution in the three principal direction of the lap-joint in the as-welded and hammer peened configurations to determine effects of hammer peening on redistribution of residual stresses across the weld. The residual stress variation across the weld in the transverse direction contained the highest values of tensile stress in all three principal directions. The residual stress in the hammer peened test specimen was in most cases reduced in all three principal directions.

Abstract: Friction Stir Welding (FSW) induces thermal residual stresses resulting in distortions in thin-walled structures. In order to understand and quantify this phenomenon, simulations and experiments of FSW on aluminium alloy (AA) 2024-T3 have been performed using different rotational and welding speeds. A sequentially coupled finite element (FE) model was used to study the residual stresses caused by the thermal cycling induced from FSW. The 3D FE model used temperature-dependent mechanical and thermophysical material properties. The predicted longitudinal stresses peaked at ~300 MPa and had a ‘‘W’’ profile with tensile stress peaks in the weld and compressive stresses outside the weld. In the FE model, the influence of process parameters on residual stress distribution was studied. The application of ‘hot’ welding conditions, i.e. low welding speed and high rotational speed, increased the residual stresses significantly, mainly in the transverse direction. Conversely, ‘cold’ welding conditions resulted in lower residual stresses. The magnitude and distribution of the residual stresses predicted by the FE model were validated by neutron diffraction. The results indicate a good agreement between the measured and predicted residual stresses in AA2024-T3.

Abstract: Important activities in the aluminum industry are the development of new alloys, and the optimization of thermo-mechanical treatments to obtain desired performance. The strength and formability of aluminum alloys depend on the distribution and scale of precipitating phases, on the grain size and grain orientation distribution, on the distribution and scale of flaws, and on the presence of residual stresses. Thus it is useful to have detailed quantitative data on the crystal structures and volume fractions of phases that form during thermomechanical treatment, on the kinetics of solid state reactions, on the distribution of grain orientations, and on the stresses that develop during mechanical testing and forming. Neutron scattering is a powerful tool that can provide unique data to guide the development of improved materials and processes. Of particular interest are in-situ experiments: such experiments are uniquely suited to neutron diffraction because of the high penetrating power of neutrons, which allows data to be collected from materials subjected to realistic conditions (load, temperature, atmosphere) in specialized sample environments. In this presentation, we discuss several examples of neutron scattering studies, including residual strain mapping, in-situ loading experiments, texture analysis, powder diffraction, and tomography.