EFFECT OF 3D SIMULATED WELDING INDUCED HAZ, RESIDUAL STRESS AND DISTORTION FIELDS ON ULTIMATE STRENGTH OF ALUMINUM STIFFENED PLATES

Abstract

A finite element study was conducted to investigate the friction stir welding (FSW) and metal inert gas welding (MIG) induced residual stresses, distortions and heat affected zone (HAZ) and their effects on the behaviour and strength of aluminum ship hull structures. A three-dimensional, two-step thermo-mechanical finite element model was developed to predict the residual stress and distortion fields as well as the heat affected zone for both welding methods. Following the verification of the model, the load vs. shortening curves of stiffened plates of varying geometries were obtained incorporating the welding induced imperfections from both welding methods. The load vs. shortening curves were then used to obtain the moment vs. curvature relationship for a modelled aluminum hull girder. The effect of stiffened plate geometry, and fabrication methods and associated welding induced imperfections on the behaviour, ultimate strength, and failure mode of stiffened plates as well as of the hull girder were presented and discussed. For the welding induced imperfection study of two aluminum alloy 6061-T6 plates being welded together, results indicated that in comparison with the MIG welding, the FSW process resulted in up to 40% and 60% less tensile residual stresses and HAZ extent and negligible distortions. Increasing the clamping area in FSW process resulted in a decrease in the maximum tensile residual stress by 40%. For the load vs. shortening response study of tee-bar stiffened aluminum plates of various geometries, the result revealed that the presence of the residual stress and HAZ can reduce the ultimate strength of aluminum stiffened plates under the compressive loading by as much as 16.5% and 10%, respectively. The comparison of extruded stiffened plates welded by either FSW or MIG showed that the ultimate strength of the stiffened plates as affected by two welding methods was dependent on the plate slenderness. The higher magnitude of residual stress was shown to be beneficial for the ultimate strength of plates with high plate slenderness. For an ultimate strength study of a modelled hull girder, results showed that the model with extruded elements welded by FSW can have as much as 28% higher ultimate moment capacity than that with non-extruded elements welded by MIG welding. Within the models joined by MIG, the butt welding model resulted in 17% higher moment capacity than the fillet welding model.