| Resumo : |
This doctorate thesis aims to investigate the effect of the resistance spot welding (RSW) process on the integrity of Twinning-Induced Plasticity (TWIP) steel weld spots and also to investigate the changes in the joint integrity when a manufacturing chain is considered. Firstly, a RSW parametrization study was performed. Weld spots were produced by varying the three main RSW parameters: welding current, welding time and electrode compression force. Experimental investigations were conducted to study the influence of these parameters on the mechanical and metallurgical characteristics of the weld spots. Subsequently, x-ray diffraction (XRD) analysis were carried out to investigate the distribution of residual stresses along the weld spots and how they are affected by the variations in the welding parameters. Similar to the observed for the microstructure and mechanical properties, the XRD data suggested that the magnitude and gradient of residual stresses are sensible to the variations in each of the main RSW parameters. The experimental findings also evidenced that there are positive correlations between the variations in the RSW parameters, the material features, the distribution of residual stresses and the mechanical reliability of the weld spots. Lastly, investigations on the evolution of the residual stresses along two different manufacturing routes were carried out. The first route encompassed a plastic deformation prior to the welding stage, whereas the second involved the spot welding followed by a bake treatment. A convergent approach was adopted to isolate the effects of the first and final manufacturing steps. The findings evidenced that the integrity of TWIP steel weld spots in the as-welded condition does not reflect the real integrity of the component, as other individual manufacturing steps may promote material changes, which came accompanied by changes in the residual stresses state. |