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This paper deals with analysis and comparison of the equivalent plastic strain and temperature fields in the aluminium alloy 2024 T3 (AA2024 T3) welded joint, with macro/microstructure appearance and hardness profile. In the alloys hardened by heat treatment, grain size and particle size of the precipitate are functions of equivalent plastic strain, strain rate and temperature. By analysing the equivalent plastic strain fields and temperature fields it is possible, to some extent, to capture the effect of welding parameters and thermo-mechanical conditions on grain structure, and therefore hardness and strength in the welded joint. A coupled thermo-mechanical model is applied to study the material behaviour during the linear welding stage of friction stir welding. The 3-D finite element model has been created in ABAQUS/EXPLICIT software using the Johnson-Cook material law. The values of thermo-mechanical quantities during the welding stage are obtained from the numerical model and shown as distributions across the joint. The obtained values of these quantities are related to the microstructure of the joint zones and hardness distribution, and this relation is discussed. [ABSTRACT FROM AUTHOR]

The influence of friction stir welding parameters on thermo-mechanical behavior of the material during welding is analyzed. An aluminum alloy is considered (Al 2024 T351), and different rotating and welding speeds are applied. The finite element model consists of the working plate (Al alloy), backing plate and welding tool. The influence of the welding conditions on material behavior is taken into account the application of the Johnson-Cook material model. The rotation speed of the tool affects the results. If increased, it contributes to an increase of friction-generated heat intensity. The other component of the generated heat, which stems from the plastic deformation of the material, is negligibly changed. When the welding speed, i.e. tool translation speed, is increased, the intensity of friction-generated heat decreases, while the heat generation due to plastic deforming is becoming more pronounced. Summed, this leads to rather small change of the total generation. The changes of the heat generation influence both the temperature field and reaction force. Also, the inadequate selection of welding parameters resulted in occurrence of the defects (pores) in the model. [ABSTRACT FROM AUTHOR]

Cleavage fracture of the V and Ti-V microalloyed forging steels was investigated by the four-point bending testing of the notched specimens of Griffith-Owen's type at −196 °C, in conjunction with the finite element analysis and the fractographic examination by scanning electron microscopy. To assess the mixed microstructure consisting mostly of the acicular ferrite, alongside proeutectoid ferrite grains and pearlite, the samples were held at 1250 °C for 30 min and subsequently cooled instill air. Cleavage fracture was initiated in the matrix under the high plastic strains near the notch root of the four-point bending specimens without the participation of the second phase particles in the process. Estimated values of the effective surface energy for the V and the Ti-V microalloyed steel of 37 Jm−2 and 74 Jm−2, respectively, and the related increase of local critical fracture stress were attributed to the increased content of the acicular ferrite. It was concluded that the observed increase of the local stress for cleavage crack propagation through the matrix was due to the increased number of the high angle boundaries, but also that the acicular ferrite affects the cleavage fracture mechanism by its characteristic stress–strain response with relatively low yield strength and considerable ductility at −196 °C. [ABSTRACT FROM AUTHOR]