Relationship between FSW parameters and hardness of the ferritic steel joints: Modeling and optimization

Abstract

The response surface method was used to study the influence of friction stir welding on the hardness of the ferritic carbon steel joints. The input parameters were tool rotational speed, tool traverse speed, and tool axial force. In addition, FSW parameters were optimized to obtain the highest value of the hardness. Microstructure and hardness of the joints were characterized using optical microscopy, orientation imaging microscopy, transmission electron microscopy, Vickers hardness, and tensile test. The results showed that among the parameters, axial force had the most effect on the hardness of the joints. The best situation for FSW was obtained at rotational speed, traverse speed, and the axial force of 980.4 rpm, 66.6 mm/min, and 2.95 kN, correspondingly. The optimum condition during FSW caused a maximum hardness of 197 Hv. The most important result of this study was the fact that the hardness of the joints does not follow the Hall-Petch relationship, which was due to the change of ferrite morphology and formation of the pearlite phase in the joints at higher heat input conditions. The lower heat input condition caused higher dislocation densities. Moreover, the influence of residual gas contamination is disclosed for the joints welded with and without Ar shielding gas.