Simulation of the Effectiveness of Dynamic Cooling for Controlling Residual Stresses in Friction Stir Welds

An FE model has been used to study the effect of localised dynamic cooling on the residual stresses developed during friction stir welding. The main aim of the work was to see if the cooling power and source positions required, to achieve significant residual stress reductions in friction stir welds, were compatible with the FSW process and recent developments in CO2 cooling systems. Comparisons were made between welds produced with a single cold spot placed over the weld line, either ahead or behind the tool, or with cold spots both leading and trailing the tool. Simulations revealed that a large reduction in the residual stresses can be obtained, particularly at the weld line, but the benefits are very dependent on the size and positioning of the cooling sinks. Cold spots placed behind the heat source have the greatest effect in reducing the build up of trailing tensile stresses, by counteracting the compressive plastic misfit of the hot soft weld metal. Overall, it is shown that the application of local dynamic cooling has potential as a practical solution for controlling residual stresses in industry.