Improving reliability of heat transfer and materials flow calculations during friction stir welding of dissimilar aluminum alloys

The temperature fields, cooling rates, torque on the tool, stir zone geometry, and the magnesium concentration profiles were examined experimentally and theoretically for the friction stir welding (FSW) of AA 1200 and AA 6061 dissimilar aluminum alloys. The thermal cycles, torque on the tool, and the magnesium concentration profiles were experimentally determined for various welding conditions. A heat, momentum, and solute transport model based on a rectangular fixed grid finite difference method was developed. Four important parameters, friction coefficient, the extent of sticking, heat transfer coefficient at the bottom surface, and the extent of viscous dissipation converted to heat significantly affected both the temperature fields and the torque on the tool. The reliability of the model predictions was improved by optimizing these four parameters that cannot be prescribed either from the welding conditions or from fundamental principles, using a genetic algorithm and measured thermal cycles and torques for different welding conditions. The magnesium concentration profiles showed that the plasticized materials moved in layers without significant diffusive interlayer mixing. The computed results showed that the extent of viscous dissipation converted to heat was fairly low consistent with limited atomic mixing. The optimized values of the extent of slip between the tool and the workpiece indicated extensive sticking for various welding conditions. The approach to determine values of uncertain parameters led to reliable model predictions as evidenced by good agreement between both the experimentally measured and computed torque values and thermal cycles.