Heat generation, plastic deformation and residual stresses in friction stir welding of aluminium alloy Omar S. Salih, Hengan Ou, Wei Sun International Journal of Mechanical Sciences, 2023 The interactions among thermal history, plastic deformation and residual stresses in the friction stir welding (FSW) process under different welding parameters have been widely considered a crucial issue and still not fully understood. In the present study, a novel three-dimensional fully coupled thermo-mechanical finite element (FE) model based on Coupled Eulerian-Lagrangian approach (CEL) has been developed to simulate the FSW process of aluminium alloy AA 6082-T6 and to analyse the thermo-mechanical interaction mechanisms under different welding conditions. The numerical model successfully simulates the plunge, dwell, and welding steps in FSW and captures the evolution of temperature, plastic deformation, and residual stresses in the welded joint. The obtained results were validated by experimental testing with observed cross-weld thermal history, optical macrography and residual stress measurement using the neutron diffraction technique. The results reveal that the tool rotation speed governs the temperature evolution; the peak temperature increased from 740 to 850 °K when the tool rotation speed rose from 800 to 1100 rpm. The rotational speed also affected the plastic deformation, material flow, and the volume of material being stirred during the welding process. Higher plastic deformation is formed in the stirring zone by increasing the tool angular velocity. This behaviour led to an increase in the stirring effect of the welding tool, reduction of the tunnel defect size and enhancing the quality of weldments. The distribution of residual stresses in different zones of the FSW joints has been found to have an M-shaped profile. A significant tensile residual stress is characterised in the edge of the nugget zone in both longitudinal and transverse directions, balanced by compressive stresses in the thermo-mechanically affected zone, heat-affected zone and base metal. The presented FE modelling provides a reliable insight into the effects of the welding parameters on the weld quality of FSW joints and process optimisation with minimised experimental trials.
Pinless friction stir spot welding of aluminium alloy with copper interlayer Balsam H. Abed, Omar S. Salih, Khalid M. Sowoud Open Engineering, 2020 Spot welding joints of Al-Mg-Si alloy (AA6061-T6) were produced with and without the addition of copper interlayer using pinless friction stir spot welding (P-FSSW). To investigate the effects of welding parameters on the metallurgical and mechanical properties of the weldment, various tool plunge depth and dwell time were used. Optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS) have been used for microstructural characterisation.Meanwhile, the mechanical characterisation of the welded joints was evaluated by tensile-shear test. The experimental results showed that a larger bonding area and sound joint were achieved with the addition of Cu interlayer due to the improvement in thermal distribution. Also, an alloying reaction took place between the aluminium substrate and Cu interlayer during P-FSSW, forming intermetallic compounds layer in the interface through the diffusion process. The increasing of dwell time and plunge depth to some extent were beneficial to the formation of the joint and diffusion process, and thus increasing the tensile-shear load of the joints. The observed fracture mode of the joint was either completely shear off in the interface or complete nugget pullout.
Influence of process parameters on the microstructural evolution and mechanical characterisations of friction stir welded Al-Mg-Si alloy Omar S. Salih, Nigel Neate, Hengan Ou, W. Sun Journal of Materials Processing Technology, 2020 Heat generation and plastic deformation during Friction Stir Welding (FSW) produce profound changes in the microstructure and structural properties of welded joints. Strengthening precipitate, grain size and crystallographic texture evolution are the most important microstructural changes in the case of welding aluminium alloys. An interaction relationship has been developed in this study to understand the evolution of microstructure during FSW of Al-Mg-Si alloy for a wide range of welding temperatures and plastic deformations by controlling two important process parameters (tool rotation and welding speeds). Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) have been used for microstructural characterisation. The mechanical properties were evaluated using microhardness, tensile and low cycle fatigue tests. In all cases, fine recrystallised equiaxed grains with a partial fibre texture was evolved in the nugget zone. Microstructural development was found to be significantly influenced by the weld pitch (welding speed/rotation speed), as it is controlled the heat input, cooling rate, exposure time and plastic deformations. The strength of the FSW joints is improved by increasing the welding speed because of grain refinement, incomplete dissolution or re-precipitate of strengthening precipitates M g 5 S i 6 ( β ) ´ ´ and the introduction of edge dislocations. The fatigue performance of FSW joints is dependent on grain size with a remarkable improvement in the fatigue life observed for smaller grain sizes.
Microstructure and mechanical properties of friction stir welded AA6092/SiC metal matrix composite Omar S. Salih, Hengan Ou, Xingguo Wei, W. Sun Materials Science and Engineering A, 2019 There is a need for improved understanding on the effects of friction stir welding (FSW) on the metallurgical and mechanical properties of aluminium matrix composite (AMC). In this study, AA6092/SiC/17.5p-T6 AMC joints were produced by using FSW with varying tool rotation and traverse speeds. The microstructural characterisation by scanning electron microscopy equipped with electron backscattered diffraction (EBSD) system revealed a substantial grain refinement and a homogeneous distribution of reinforcement particles in the nugget zone. The grain size of the nugget zone was greatly influenced by weld pitch, as a key indicator to control the amount of heat input, exposure time and cooling rate. Vickers microhardness profile across the welding zone revealed a significant difference in microhardness among the base metal, heat affected zone, thermo-mechanically affected zone and nugget zone. The tensile strength of the cross-weld specimens showed a high joint efficiency of about 75% of the base metal combined with relatively high ductility. Low-cycle fatigue properties were investigated in the axial total strain-amplitude control mode (from 0.3% to 0.5%) with R = e min / e max = − 1 . The results indicate that the fatigue life of the cross-welded joints varies with grain size in the nugget zone and it is lower than that of the base metal. A significant improvement of fatigue life is found to be related to the finer equiaxed grains dominated by high angle grain boundaries in the nugget zone.
A review of friction stir welding of aluminium matrix composites Omar S. Salih, Hengan Ou, W. Sun, D.G. McCartney Materials and Design, 2015 As a solid state joining process, friction stir welding (FSW) has proven to be a promising approach for joining aluminium matrix composites (AMCs). However, challenges still remain in using FSW to join AMCs even with considerable progress having been made in recent years. This review paper provides an overview of the state of-the-art of FSW of AMC materials. Specific attention and critical assessment have been given to: (a) the macrostructure and microstructure of AMC joints, (b) the evaluation of mechanical properties of joints, and (c) the wear of FSW tools due to the presence of reinforcement materials in aluminium matrices. This review concludes with recommendations for future research directions.
