Papers by Author: Mohammad Jahazi

Abstract: As a relatively new solid-state joining process, friction stir welding (FSW) may provide a feasible approach to join dissimilar materials such as Mg to Al alloys. In this work, the effects of selected process parameters including work-piece placement, pin tilting angle, and pin location on the quality of dissimilar AA 2024-T3 to AZ31B-H24 butt joints were investigated for the first time. Sound butt joints with low distortion and no solidification cavities or cracks were successfully obtained indicating the potential of FSW to join dissimilar Al to Mg alloys.

Abstract: The effects of post-weld heat treatment on 3.05-mm thick Ti-6Al-4V alloy were investigated using a 4 kW Nd:YAG laser. Two main defects, underfill and porosity, were observed. The use of filler wire reduced underfill defects but slightly increased porosity. No cracks were detected. The as-welded and stress-relief annealed welds had very similar microstructures, hardness, and tensile properties. However, the post-weld solution heat treatment and aging transformed the martensite in the fusion zone into a coarse interlamellar α-β structure, causing a decrease in ductility but a more homogeneous distribution in the hardness of the welds.

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: Friction stir welding (FSWing) induces residual stresses and distortions in welded structures. Such residual stresses reduce the fatigue life of welded components, while the induced distortions prevent the welding of large or thin components. In the present study, needle peening was used to induce additional residual stresses in 2.3-mm thick (FSWed) aluminum alloy (AA) 2024-T3 sheets. This was done with the objective to counterbalance the welding-induced stresses and thus reduce the overall stresses and distortions. The needle peening process, which stems from shot peening, consists of hammering a surface using cylindrical spherical ended shots sliding back and forth in a treatment head. An instrumented needle peening machine was used to carry out peening on as-received (or bare) and bead-on-plate FSWed AA2024-T3 material. In both cases, the width of the peening area corresponded to that of a typical weld. The influence of the peening process parameters such as needle size, applied power and travel speed on the surface quality and magnitude of the induced distortions were evaluated. The results indicate that, by increasing the needle diameter from 1.2 mm to 2.0 mm, the peening-induced deflection on bare sheet material increased by an average value of 27% while the roughness average, Ra, decreased by an average value of 47%. It was also found that a surface finish qualitatively similar to that of conventional shot peening could be obtained by using appropriate needle peening trajectories. Finally, needle peening with an applied power of 10% was sufficient for eliminating 37% of the welding-induced transverse curvature and 82% of the welding-induced longitudinal curvature.

Abstract: The recrystallization behaviour of near-alpha titanium alloy IMI834, which is primarily used for high temperature aerospace compressor disc applications, has been investigated at hot working temperatures. The latest results of a finite element model, developed using the commercial code DEFORM-3D with constitutive equations adapted from available literature, will be presented. Model development and validation involved the hot compression of specimens with an initial bimodal alpha+beta microstructure at temperatures of 1000°C-1100°C, strain rates of 0.01s 1-1s 1, and varied post-deformation annealing times. The characterization of microstructure through quantitative metallography revealed beta grain refinement achieved primarily through static/metadynamic recrystallization. The beta recrystallization kinetics were subsequently predicted through an Avrami-type relationship.

Abstract: The linear friction welding (LFW) behavior of Ti-6Al-4V, a commercial α + β titanium alloy, was investigated using oscillation frequencies ranging from 30-70 Hz and axial pressures from 50-110 MPa. LFW samples were examined using electron backscattered diffraction (EBSD) to relate the texture to the welding parameters and to the estimated strain and strain rate. Characterization of the welds included analysis of the microstructure of the weld and of the thermomechanically affected zones (TMAZ) in relation to the parent material.

Abstract: Ni-based superalloys are extensively used in the manufacture of aircraft engine components because of their excellent heat-resistant and corrosion-resistant properties. The principal joining processes for Ni-based superalloys are TIG welding, MIG welding, submerged arc welding, electron beam welding, and CO2 laser welding. In this investigation, a robotic 4-kW continuous-wave Nd:YAG laser system was used to identify the optimal laser welding process for 2.0 mm thick Inconel (IN) 718 sheets. The effect of various processing parameters, which included power input, welding speed, weld geometry and filler wire, was studied using the Taguchi design of experiment (DOE) methodology. The DOE methodology enabled the evaluation of the relationship between the process parameters and the quality of the welded joints, from which the optimal Nd:YAG laser welding process was developed for IN718 alloy. Joint quality was examined by tensile and nondestructive testing methods. Using the optimal process established in this research, mechanically-sound welds with narrow fusion and heat-affected zones were produced. The outcome of this research demonstrates the feasibility of the application of Nd:YAG laser in the joining of IN718 sheets for the manufacture of aircraft engine components.

Abstract: The use of electron beam technology for freeforming 321 SS was investigated using 347 SS solid wire and BNi-2 brazing paste as filler materials. The electron beam freeforming (EBFF) studies involved examining the effect of processing parameters on the characteristics of the line build-ups. Specifically, the effective growth rate and the dimensional features (height-to-width ratio) of the build-ups were found to be dependent on the beam energy and the filler material conditions (e.g. wire feed rate and the number of re-melting passes). The EBFF work indicated that build-ups with either filler material could be deposited on 321 SS using an optimized processing window that resulted in properties comparable to technical data available for 347 SS and BNi-2.

Abstract: The feasibility of laser repair was studied based on single-/two-layer multi-bead cladding using a continuous wave (CW) 4 kW Nd:YAG laser system and 1.6-mm EZ33A-T5 filler rods for 22.5-mm ZE41A-T5 magnesium alloy sand castings in the machined surface conditions. The results demonstrate the potentiality of repairing magnesium castings using laser surface cladding by wire. Similarly to weld joints, three distinct regions, i.e. clad, partially melted and heat affected zones are observed in the laser-clad beads. The microstructures in these zones and their evolutions are discussed in details.

Abstract: There has been an increasing tendency to determine liquid metal quality using pressure filtration methods such as Prefil Footprinter tests. The scientific understanding of this method, however, is emerging only recently with the introduction of derivative methods. Based on the investigation of flow behavior over the pressure filtration test, classic filtration theory has been successfully applied to understand the filtrate weight versus filtration time curve. The “clean” and “effective” resistance of filter medium, overall filtration resistance, cake resistance, pressure loss and distribution can be quantitatively calculated now. In the present work, a typical weight versus time curve from the Prefil Footprinter test of a cast Al-Si alloy is analyzed and discussed in details. The method developed in the work can provide new insights into the filtration behavior.