Michael Gharghouri

Effect of texture on acoustic emission produced by slip and twinning in AZ31B magnesium alloy—part II: clustering and neural network analysis

The mechanical behaviour of extruded AZ31B magnesium alloy with two different textures was studied. Acoustic emission (AE) was used to record the signals to detect the various deformation processes occurring during the mechanical test. Hierarchical clustering and Kohonens self-organising neural network map were used to process the data from AE and to develop a relationship in terms of the deformation modes. The signals produced from the two different deformation mechanisms (i.e. slip and twinning) were successfully identified. The developed method could be efficiently used for the classification of AE data.

Effect of texture on acoustic emission produced by slip and twinning in AZ31B magnesium alloy

The mechanical behaviour of extruded AZ31B magnesium alloy with two different textures subjected to uniaxial tensile loading was studied. Monotonic and cyclic tensile tests were carried out at ambient temperature, and acoustic emission (AE) signals were recorded to detect various deformation processes occurring during the test. The results of mechanical tests and AE are discussed with respect to the crystallographic texture and the microstructure in terms of the orientation dependent activation of different deformation mechanisms, such as twinning and slip.

Prediction and Measurements of Thermal Residual Stresses in AA2024-T3 Friction Stir Welds as a Function of Welding Parameters

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.

Fabrication of magnetic shape memory alloy/polymer composites

NiMnGa-based magnetic shape memory (MSM) alloys have attained magnetic-field-induced strains up to approximately 10%, making them very attractive for a variety of applications. However, for applications that require the use of an alternating magnetic field, eddy current losses can be significant. Also, NiMnGa-based MSM alloys fracture toughness is relatively low. Using these materials in the form of particles embedded in a polymer matrix composite could mitigate these limitations. Since the MSM effect is anisotropic, the crystallographic texture of the particles in the composites is of great interest. In this work, a procedure for fabricating NiMnGa-based MSMA/elastomer composites is described. Processing routes for optimizing the crystallographic texture in the composites are considered.

Distributions of residual stresses in stiffened plates with one and two stiffeners

This study was undertaken for a better understanding of the residual stress distributions associated with welds typically found in ship hulls. Specimens that represent small sections of an actual ship hull were built and tested using the neutron diffraction method at the Canadian Neutron Beam Centre in the Chalk River Laboratories. The specimens comprised 9.5 mm thick steel plates stiffened by L127 × 76 × 9.5 steel angles. This paper presents one- and three-dimensional distributions of all three components of residual stress created from the production of the steel plate and from the welding of one and two stiffeners onto the parent steel plate. Subsequently, the longitudinal stress in the transverse direction of the stiffened plate specimens was compared with the Faulkner model. It was found that the Faulkner model is able to predict the general distributions of this stress; however, it was unable to predict the stress values correctly.

Plastic Deformation of Magnesium Alloy Subjected to Compression-First Cyclic Loading

In-situ neutron diffraction has been employed to study the deformation mechanisms in a precipitation-hardened and extruded Mg-8.5wt.% Al alloy subjected to compression followed by reverse tension. The starting texture is such that the basal poles of most grains are oriented normal to the extrusion axis and a small portion of grains are oriented with the basal pole parallel to the extrusion axis. Diffraction peak intensities for several grain orientations monitored in-situ during deformation show that deformation twinning plays an important role in the elastic-plastic transition and subsequent plastic deformation behavior. Significant non-linear behavior is observed during unloading after compression and appears to be due to detwinning. This effect is much stronger after compressive loading than after tensile loading.

Neutron diffraction techniques for alloy characterization and development

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.

In-Situ Neutron Diffraction Study of the Behavior of AL6XN Stainless Steel Under Biaxial Loading

In–situ neutron diffraction has been used to measure lattice strains parallel to two principal stress directions in biaxially-loaded AL6XN stainless steel. A new fixture was developed for loading thin-walled tubular specimens through combinations of internal pressure and axial loading. Under these conditions, the principal directions (σ zz and σ θθ in a cylindrical r, θ, z coordinate system) remain constant with respect to the initial crystallographic texture regardless of the level of biaxiality, a distinct advantage for diffraction experiments over the traditional tension/torsion tests for which this condition does not hold. Specimens were first pressurized to the level required to obtain a chosen value of σ θθ . The axial load was then increased to reach the yield surface at different σ θθ /σ zz ratios, ranging from uniaxial to balanced biaxial loading (0, 0.4, 0.7, 1 according to Tresca). The {200}, {220}, {222}, and {311} reflections were measured in the axial and hoop directions as a function of axial load. A sequence of axial loading/unloading episodes was applied for different levels of plastic deformation. Under uniaxial tension, the {200} reflection showed the highest axial strains, followed by the {311}, and {220}/{222} reflections. With increasing internal pressure (biaxiality), the axial lattice strains corresponding to a given axial stress tended to decrease, and the responses of the various reflections tended to merge.

High-temperature plasticity of polycrystalline Galfenol (Fe-Ga)

Galfenol (Fe-Ga) is a promising and mechanically robust magnetostrictive actuator material. However, due to its high conductivity, it needs to be in thin sheet form to avoid excessive eddy current losses. Work is underway to develop conventional rolling processes to produce large quantities of thin Galfenol sheet, while retaining a preferred <100> crystallographic texture to optimize magnetostrictive performance. Knowledge of high temperature polycrystalline plasticity is crucial to understanding formability and crystallographic texture evolution during rolling. The deformation behavior of polycrystalline Galfenol at high temperatures was studied. Preliminary results suggest that significant dynamic recovery and/or recrystallization occur during deformation, resulting in a random texture. In-situ neutron diffraction experiments are being developed to obtain qualitative and quantitative information on the high temperature plane strain deformation of Galfenol. These experiments will be used to identify the slip systems that contribute to plastic deformation, and their dependence on temperature. Simultaneously, models of large-scale polycrystal plasticity are being developed to predict internal strains and texture evolution during deformation, which will be validated against the data obtained from the neutron diffraction experiments. Ultimately, the models will be used to develop thermo-mechanical treatments to optimize texture evolution during rolling.

The Investigation of the Triaxial Residual Stress in the Friction Stir Welded Lap Joint Using Neutron Diffraction

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.